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M. Raoufi, U. Hörmann, G. Ligorio, J. Hildebrandt, M. Pätzel, T. Schultz, L. Perdigon, N. Koch, E. List-Kratochvil, S. Hecht, D. Neher, “Simultaneous Effect of UV‐Radiation and Surface Modification on the Work Function and Hole Injection Properties of ZnO Thin Films”, physica status solidi (a) (2020), DOI: 10.1002/pssa.201900876

We systematically investigated the combined effect of UV light soaking and self‐assembled monolayer deposition on the work function of thin ZnO layers and on the efficiency of hole injection into the prototypical conjugated polymer P3HT. We show that the work function and with this the injection efficiency depends strongly on the history of UV light exposure. Proper treatment of the ZnO layer enables ohmic hole injection into P3HT, demonstrating ZnO as a potential anode material for organic optoelectronic devices. The results also suggest that valid conclusions on the energy level alignment at the ZnO/organic interfaces may only be drawn if the illumination history is precisely known and controlled. This is inherently problematic when comparing electronic data from UPS measurements which were carried out under different or ill‐defined illumination conditions.

Image: The Authors
Image: The Authors

L. Perdigón-Toro, H. Zhang, A. Markina, J. Yuan, S. M. Hosseini, C. M. Wolff, G. Zuo, M. Stolterfoht, Y. Zou, F. Gao, D. Andrienko, S. Shoaee, D. Neher, “Barrierless Free Charge Generation in the High-Performance PM6:Y6 Bulk Heterojunction Non-Fullerene Solar Cell”, Advanced Materials 1906763 (2020), DOI: 10.1002/adma.201906763

Organic solar cells are currently experiencing a second golden age thanks to the development of novel non‐fullerene acceptors (NFAs). Surprisingly, some of these blends exhibit high efficiencies despite a low energy offset at the heterojunction. Herein, free charge generation in the high‐performance blend of the donor polymer PM6 with the NFA Y6 is thoroughly investigated as a function of internal field, temperature and excitation energy. Results show that photocurrent generation is essentially barrierless with near‐unity efficiency, regardless of excitation energy. Efficient charge separation is maintained over a wide temperature range, down to 100 K, despite the small driving force for charge generation. Studies on a blend with a low concentration of the NFA, measurements of the energetic disorder, and theoretical modeling suggest that CT state dissociation is assisted by the electrostatic interfacial field which for Y6 is large enough to compensate the Coulomb dissociation barrier.

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Image: The Authors

L. Krückemeier, U. Rau, M. Stolterfoht, T. Kirchartz, “How to Report Record Open‐Circuit Voltages in Lead‐Halide Perovskite Solar Cells”, Advanced Energy Materials 10, 1902573 (2020), DOI: 10.1002/aenm.201902573

Open‐circuit voltages of lead‐halide perovskite solar cells are improving rapidly and are approaching the thermodynamic limit. Since many different perovskite compositions with different bandgap energies are actively being investigated, it is not straightforward to compare the open‐circuit voltages between these devices as long as a consistent method of referencing is missing. For the purpose of comparing open‐circuit voltages and identifying outstanding values, it is imperative to use a unique, generally accepted way of calculating the thermodynamic limit, which is currently not the case. Here a meta‐analysis of methods to determine the bandgap and a radiative limit for open‐circuit voltage is presented. The differences between the methods are analyzed and an easily applicable approach based on the solar cell quantum efficiency as a general reference is proposed.

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Image: The Authors

A. Chandrasekharan, H. Jin, M. Stolterfoht, E. Gann, C.R. McNeill, M. Hambsch, P.L. Burn, ”9,9′-Bifluorenylidene-diketopyrrolopyrrole donors for non-polymeric solution processed solar cells”, Synthetic Metals 250, 79–87 (2019), DOI: 10.1016/j.synthmet.2019.02.015

We have synthesised new materials comprised of 9,9′-bifluorenylidene and dithienodiketopyrrolopyrrole units. While 9,9′-bifluorenylidene has been primarily used in non-fullerene acceptors, when used in combination with the diketopyrrolopyrrole moiety it can form donor materials that can be used in conjunction with fullerene acceptors. The compounds differ in the substituents on the 9,9′-bifluorenylidene (protonated = 1A and dimethoxy = 1B) moiety. The structure of both the neat and blend films with PC70BM were found to be strongly dependent on the processing solvent used. In particular, Grazing Incidence Wide Angle X-ray Scattering measurements of films of 1A or 1B blended with PC70BM prepared from chloroform or chloroform with o-dichlorobenzene as an additive showed that the donor material had no particular ordering. However, when 1,8-diiodooctane was added to the processing solvent the 1A blends showed liquid crystalline ordering while 1B formed well-defined crystallites with three-dimensional ordering. The difference in film structure had a profound effect on the device properties. For 1A the optimised blend ratio with PC70BM was 1:4, but when 1,8-diiodooctane was used as the additive the best ratio of 1A to fullerene was 1:1. The films containing the well-defined crystallites of 1B all performed poorly, which was ascribed to a lack of a percolation pathway for hole extraction. The best performing device was comprised of a 1:1 blend of 1A and PC70BM, which had a power conversion efficiency of 2.6%.

Image: The Authors
Image: The Authors

A. J. L. Hofmann, S. Züfle, K. Shimizu, M. Schmid, V. Wessels, L. Jäger, S. Altazin, K. Ikegami, M. R. Khan, D. Neher, H. Ishii, B. Ruhstaller, W. Brütting, “Dipolar Doping of Organic Semiconductors to Enhance Carrier Injection”, Physical Review Applied 12, 064052 (2019), DOI: 10.1103/PhysRevApplied.12.064052

If not oriented perfectly isotropically, the strong dipole moment of polar organic semiconductor materials such as tris-(8-hydroxyquinolate)aluminum (Alq3) will lead to the buildup of a giant surface potential (GSP) and thus to a macroscopic dielectric polarization of the organic film. Despite this having been a known fact for years, the implications of such high potentials within an organic layer stack have only been studied recently. In this work, the influence of the GSP on hole injection into organic layers is investigated. Therefore, we apply a concept called dipolar doping to devices consisting of the prototypical organic materials N,N’-Di(1-naphthyl)-N,N’-diphenyl-(1,1’-biphenyl)-4,4’-diamine (NPB) as nonpolar host and Alq3 as dipolar dopant with different mixing ratios to tune the GSP. The mixtures are investigated in single-layer monopolar devices as well as bilayer metal/insulator/semiconductor structures. Characterization is done electrically using current-voltage (I-V) characteristics, impedance spectroscopy, and charge extraction by linearly increasing voltage and time of flight, as well as with ultraviolet photoelectron spectroscopy. We find a maximum in device performance for moderate to low doping concentrations of the polar species in the host. The observed behavior can be described on the basis of the Schottky effect for image-force barrier lowering, if the changes in the interface dipole, the carrier mobility, and the GSP induced by dipolar doping are taken into account.

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Image: The Authors

M. Stolterfoht, V. M. Le Corre, M. Feuerstein, P. Caprioglio, L. J. A. Koster, D. Neher, “Voltage-Dependent Photoluminescence and How It Correlates with the Fill Factor and Open-Circuit Voltage in Perovskite Solar Cells”, ACS Energy Letters 4, 2887–2892 (2019), DOI: 10.1021/acsenergylett.9b02262

Optimizing the photoluminescence (PL) yield of a solar cell has long been recognized as a key principle to maximize the power conversion efficiency. While PL measurements are routinely applied to perovskite films and solar cells under open-circuit conditions (VOC), it remains unclear how the emission depends on the applied voltage. Here, we performed PL(V) measurements on perovskite cells with different hole transport layer thicknesses and doping concentrations, resulting in remarkably different fill factors (FFs). The results reveal that PL(V) mirrors the current–voltage (JV) characteristics in the power-generating regime, which highlights an interesting correlation between radiative and nonradiative recombination losses. In particular, high FF devices show a rapid quenching of PL(V) from open-circuit to the maximum power point. We conclude that, while the PL has to be maximized at VOC, at lower biases < VOC, the PL must be rapidly quenched as charges need to be extracted prior to recombination.

Image: The Authors
Image: The Authors

C. M. Wolff, P. Caprioglio, M. Stolterfoht, D. Neher, “Nonradiative Recombination in Perovskite Solar Cells: The Role of Interfaces”, Advanced Materials 31, 1902762 (2019), DOI: 10.1002/adma.201902762

Perovskite solar cells combine high carrier mobilities with long carrier lifetimes and high radiative efficiencies. Despite this, full devices suffer from significant nonradiative recombination losses, limiting their VOC to values well below the Shockley–Queisser limit. Here, recent advances in understanding nonradiative recombination in perovskite solar cells from picoseconds to steady state are presented, with an emphasis on the interfaces between the perovskite absorber and the charge transport layers. Quantification of the quasi‐Fermi level splitting in perovskite films with and without attached transport layers allows to identify the origin of nonradiative recombination, and to explain the VOC of operational devices. These measurements prove that in state‐of‐the‐art solar cells, nonradiative recombination at the interfaces between the perovskite and the transport layers is more important than processes in the bulk or at grain boundaries. Optical pump‐probe techniques give complementary access to the interfacial recombination pathways and provide quantitative information on transfer rates and recombination velocities. Promising optimization strategies are also highlighted, in particular in view of the role of energy level alignment and the importance of surface passivation. Recent record perovskite solar cells with low nonradiative losses are presented where interfacial recombination is effectively overcome—paving the way to the thermodynamic efficiency limit.

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Image: The Authors

S. Shoaee, A. Armin, M. Stolterfoht, S. M. Hosseini, J. Kurpiers, D. Neher, “Decoding Charge Recombination through Charge Generation in Organic Solar Cells”, Solar RRL 3, 1900184 (2019), DOI: 10.1002/solr.201900184

The in‐depth understanding of charge carrier photogeneration and recombination mechanisms in organic solar cells is still an ongoing effort. In donor:acceptor (bulk) heterojunction organic solar cells, charge photogeneration and recombination are inter‐related via the kinetics of charge transfer states—being singlet or triplet states. Although high‐charge‐photogeneration quantum yields are achieved in many donor:acceptor systems, only very few systems show significantly reduced bimolecular recombination relative to the rate of free carrier encounters, in low‐mobility systems. This is a serious limitation for the industrialization of organic solar cells, in particular when aiming at thick active layers. Herein, a meta‐analysis of the device performance of numerous bulk heterojunction organic solar cells is presented for which field‐dependent photogeneration, charge carrier mobility, and fill factor are determined. Herein, a “spin‐related factor” that is dependent on the ratio of back electron transfer of the triplet charge transfer (CT) states to the decay rate of the singlet CT states is introduced. It is shown that this factor links the recombination reduction factor to charge‐generation efficiency. As a consequence, it is only in the systems with very efficient charge generation and very fast CT dissociation that free carrier recombination is strongly suppressed, regardless of the spin‐related factor.

Image: The Authors
Image: The Authors

V. M. Le Corre, M. Stolterfoht, L. Perdigón Toro, M. Feuerstein, C. Wolff, L. Gil-Escrig, H. J. Bolink, D. Neher, L. J. A. Koster, “Charge Transport Layers Limiting the Efficiency of Perovskite Solar Cells: How To Optimize Conductivity, Doping, and Thickness”, ACS Applied Energy Materials 2, 6280–6287 (2019), DOI: 10.1021/acsaem.9b00856

Perovskite solar cells (PSCs) are one of the main research topics of the photovoltaic community; with efficiencies now reaching up to 24%, PSCs are on the way to catching up with classical inorganic solar cells. However, PSCs have not yet reached their full potential. In fact, their efficiency is still limited by nonradiative recombination, mainly via trap-states and by losses due to the poor transport properties of the commonly used transport layers (TLs). Indeed, state-of-the-art TLs (especially if organic) suffer from rather low mobilities, typically within 10–5 and 10–2 cm2 V–1 s–1, when compared to the high mobilities, 1–10 cm2 V–1 s–1, measured for perovskites. This work presents a comprehensive analysis of the effect of the mobility, thickness, and doping density of the transport layers based on combined experimental and modeling results of two sets of devices made of a solution-processed high-performing triple-cation (PCE ≈ 20%). The results are also cross-checked on vacuum-processed MAPbI3 devices. From this analysis, general guidelines on how to optimize a TL are introduced and especially a new and simple formula to easily calculate the amount of doping necessary to counterbalance the low mobility of the TLs.

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F. Deschler, D. Neher, L. Schmidt-Mende, “Perovskite semiconductors for next generation optoelectronic applications”, APL Materials 7, 080401 (2019), DOI: 10.1063/1.5119744

Since 2012, when two groups reported for the first time power conversion efficiencies exceeding numbers with only one digit for metal-halide perovskite solar cells, the field has progressed rapidly. Tremendous efforts have brought a lot of advancement in the field of perovskite semiconductors and their applications in optoelectronic devices. Record efficiencies of over 24% have been reported for solar cells, but we have seen also strong developments in perovskite-based optoelectronic devices such as LEDs and photodetectors. We have now a much better understanding about this fascinating semiconductor material. Nevertheless, there are still numerous aspects to be addressed, and despite the progress in the field, we find new questions arising after answering others. This special issue on perovskite semiconductors is devoted to the next generation of optoelectronic applications. We want to summarize some important aspects concerning interface engineering, device lifetime, novel applications, and physical understanding of the processes in perovskite semiconductor applications. In this article, we will only pick selected examples to demonstrate not only the progress of the field but also the open questions and interesting research questions ahead of us.

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S. Pisoni, M. Stolterfoht, J. Löckinger, T. Moser, Y. Jiang, P. Caprioglio, D. Neher, S. Buecheler, A. N. Tiwari, “On the origin of open-circuit voltage losses in flexible n-i-p perovskite solar cells”, Science and Technology of Advanced Materials 20, 786–795 (2019), DOI: 10.1080/14686996.2019.1633952

The possibility to manufacture perovskite solar cells (PSCs) at low temperatures paves the way to flexible and lightweight photovoltaic (PV) devices manufactured via high-throughput roll-to-roll processes. In order to achieve higher power conversion efficiencies, it is necessary to approach the radiative limit via suppression of non-radiative recombination losses. Herein, we performed a systematic voltage loss analysis for a typical low-temperature processed, flexible PSC in n-i-p configuration using vacuum deposited C60 as electron transport layer (ETL) and two-step hybrid vacuum-solution deposition for CH3NH3PbI3 perovskite absorber. We identified the ETL/absorber interface as a bottleneck in relation to non-radiative recombination losses, the quasi-Fermi level splitting (QFLS) decreases from ~1.23 eV for the bare absorber, just ~90 meV below the radiative limit, to ~1.10 eV when C60 is used as ETL. To effectively mitigate these voltage losses, we investigated different interfacial modifications via vacuum deposited interlayers (BCP, B4PyMPM, 3TPYMB, and LiF). An improvement in QFLS of ~30–40 meV is observed after interlayer deposition and confirmed by comparable improvements in the open-circuit voltage after implementation of these interfacial modifications in flexible PSCs. Further investigations on absorber/hole transport layer (HTL) interface point out the detrimental role of dopants in Spiro-OMeTAD film (widely employed HTL in the community) as recombination centers upon oxidation and light exposure.

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Image: The Authors

V. C. Nikolis, A. Mischok, B. Siegmund, J. Kublitski, X. Jia, J. Benduhn, U. Hörmann, D. Neher, M. C. Gather, D. Spoltore, K. Vandewal, “Strong light-matter coupling for reduced photon energy losses in organic photovoltaics”, Nature Communications 10,  3706 (2019), DOI: 10.1038/s41467-019-11717-5

Strong light-matter coupling can re-arrange the exciton energies in organic semiconductors. Here, we exploit strong coupling by embedding a fullerene-free organic solar cell (OSC) photo-active layer into an optical microcavity, leading to the formation of polariton peaks and a red-shift of the optical gap. At the same time, the open-circuit voltage of the device remains unaffected. This leads to reduced photon energy losses for the low-energy polaritons and a steepening of the absorption edge. While strong coupling reduces the optical gap, the energy of the charge-transfer state is not affected for large driving force donor-acceptor systems. Interestingly, this implies that strong coupling can be exploited in OSCs to reduce the driving force for electron transfer, without chemical or microstructural modifications of the photo-active layer. Our work demonstrates that the processes determining voltage losses in OSCs can now be tuned, and reduced to unprecedented values, simply by manipulating the device architecture.

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Image: The Authors

Q. Wang, E. Mosconi, C. M. Wolff, J. Li, D. Neher, F. De Angelis, G. P. Suranna, R. Grisorio, A. Abat, “Rationalizing the Molecular Design of Hole‐Selective Contacts to Improve Charge Extraction in Perovskite Solar Cells”,  Advanced Energy Materials 9, 1900990 (2019), DOI: 10.1002/aenm.201900990

Two new hole selective materials (HSMs) based on dangling methylsulfanyl groups connected to the C‐9 position of the fluorene core are synthesized and applied in perovskite solar cells. Being structurally similar to a half of Spiro‐OMeTAD molecule, these HSMs (referred as FS and DFS) share similar redox potentials but are endowed with slightly higher hole mobility, due to the planarity and large extension of their structure. Competitive power conversion efficiency (up to 18.6%) is achieved by using the new HSMs in suitable perovskite solar cells. Time‐resolved photoluminescence decay measurements and electrochemical impedance spectroscopy show more efficient charge extraction at the HSM/perovskite interface with respect to Spiro‐OMeTAD, which is reflected in higher photocurrents exhibited by DFS/FS‐integrated perovskite solar cells. Density functional theory simulations reveal that the interactions of methylammonium with methylsulfanyl groups in DFS/FS strengthen their electrostatic attraction with the perovskite surface, providing an additional path for hole extraction compared to the sole presence of methoxy groups in Spiro‐OMeTAD. Importantly, the low‐cost synthesis of FS makes it significantly attractive for the future commercialization of perovskite solar cells.

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Image: The Authors

M. Stolterfoht, P. Caprioglio, C. M. Wolff, J. A. Márquez, J. Nordmann, S. Zhang, D. Rothhardt, U. Hörmann, Y. Amir, A. Redinger, L. Kegelmann, F. Zu, S. Albrecht, N. Koch, T. Kirchartz, M. Saliba, T. Unold, D. Neher, “The impact of energy alignment and interfacial recombination on the internal and external open-circuit voltage of perovskite solar cells”, Energy & Environmental Science 12, 2778-2788 (2019), DOI: 10.1039/C9EE02020A

Charge transport layers (CTLs) are key components of diffusion controlled perovskite solar cells, however, they can induce additional non-radiative recombination pathways which limit the open circuit voltage (VOC) of the cell. In order to realize the full thermodynamic potential of the perovskite absorber, both the electron and hole transport layer (ETL/HTL) need to be as selective as possible. By measuring the photoluminescence yield of perovskite/CTL heterojunctions, we quantify the non-radiative interfacial recombination currents in pin- and nip-type cells including high efficiency devices (21.4%). Our study comprises a wide range of commonly used CTLs, including various hole-transporting polymers, spiro-OMeTAD, metal oxides and fullerenes. We find that all studied CTLs limit the VOC by inducing an additional non-radiative recombination current that is in most cases substantially larger than the loss in the neat perovskite and that the least-selective interface sets the upper limit for the VOC of the device. Importantly, the VOC equals the internal quasi-Fermi level splitting (QFLS) in the absorber layer only in high efficiency cells, while in poor performing devices, the VOC is substantially lower than the QFLS. Using ultraviolet photoelectron spectroscopy and differential charging capacitance experiments we show that this is due to an energy level mis-alignment at the p-interface. The findings are corroborated by rigorous device simulations which outline important considerations to maximize the VOC. This work highlights that the challenge to suppress non-radiative recombination losses in perovskite cells on their way to the radiative limit lies in proper energy level alignment and in suppression of defect recombination at the interfaces.

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Image: The Authors

P. Caprioglio, M. Stolterfoht, C. M. Wolff, T. Unold, B. Rech, S. Albrecht, D. Neher, “On the Relation between the Open‐Circuit Voltage and Quasi‐Fermi Level Splitting in Efficient Perovskite Solar Cells”, Advanced Energy Materials 9, 1901631 (2019), DOI: 10.1002/aenm.201901631

Today's perovskite solar cells (PSCs) are limited mainly by their open‐circuit voltage (VOC) due to nonradiative recombination. Therefore, a comprehensive understanding of the relevant recombination pathways is needed. Here, intensity‐dependent measurements of the quasi‐Fermi level splitting (QFLS) and of the VOC on the very same devices, including pin‐type PSCs with efficiencies above 20%, are performed. It is found that the QFLS in the perovskite lies significantly below its radiative limit for all intensities but also that the VOC is generally lower than the QFLS, violating one main assumption of the Shockley‐Queisser theory. This has far‐reaching implications for the applicability of some well‐established techniques, which use the VOC as a measure of the carrier densities in the absorber. By performing drift‐diffusion simulations, the intensity dependence of the QFLS, the QFLS‐VOC offset and the ideality factor are consistently explained by trap‐assisted recombination and energetic misalignment at the interfaces. Additionally, it is found that the saturation of the VOC at high intensities is caused by insufficient contact selectivity while heating effects are of minor importance. It is concluded that the analysis of the VOC does not provide reliable conclusions of the recombination pathways and that the knowledge of the QFLS‐VOC relation is of great importance.

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Image: The Authors

L. Kegelmann, P. Tockhorn, C.M. Wolff, J.A. Márquez, S. Caicedo-Dávila, L. Korte, T. Unold, W. Lövenich, D. Neher, B.  Rech, S. Albrecht, “Mixtures of Dopant-Free Spiro-OMeTAD and Water-Free PEDOT as a Passivating Hole Contact in  Perovskite Solar Cells”, ACS Applied Materials & Interfaces 11, 9172-9181 (2019)  DOI: 10.1021/acsami.9b01332

Doped spiro-OMeTAD at present is the most commonly used hole transport material (HTM) in n–i–p-type perovskite solar cells, enabling high efficiencies around 22%. However, the required dopants were shown to induce nonradiative recombination of charge carriers and foster degradation of the solar cell. Here, in a novel approach, highly conductive and inexpensive water-free poly(3,4-ethylenedioxythiophene) (PEDOT) is used to replace these dopants. The resulting spiro-OMeTAD/PEDOT (SpiDOT) mixed films achieve higher lateral conductivities than layers of doped spiro-OMeTAD. Furthermore, combined transient and steady-state photoluminescence studies reveal a passivating effect of PEDOT, suppressing nonradiative recombination losses at the perovskite/HTM interface. This enables excellent quasi-Fermi level splitting values of up to 1.24 eV in perovskite/SpiDOT layer stacks and high open-circuit voltages (VOC) up to 1.19 V in complete solar cells. Increasing the amount of dopant-free spiro-OMeTAD in SpiDOT layers is shown to enhance hole extraction and thereby improves the fill factor in solar cells. As a consequence, stabilized efficiencies up to 18.7% are realized, exceeding cells with doped spiro-OMeTAD as a HTM in this study. Moreover, to the best of our knowledge, these results mark the lowest nonradiative recombination loss in the VOC (140 mV with respect to the Shockley–Queisser limit) and highest efficiency reported so far for perovskite solar cells using PEDOT as a HTM.

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Image: The Authors

U. Hörmann, S. Zeiske, S. Park, T. Schultz, S. Kickhöfel, U. Scherf, S. Blumstengel, N. Koch, D. Neher, “Direct observation of state-filling at hybrid tin oxide/organic interfaces”, Applied Physics Letters 114, 183301 (2019)  DOI: 10.1063/1.508270

Electroluminescence (EL) spectra of hybrid charge transfer states at metal oxide/organic type-II heterojunctions exhibit bias-induced spectral shifts. The reasons for this phenomenon have been discussed controversially and arguments for either electric field-induced effects or the filling of trap states at the oxide surface have been put forward. Here, we combine the results of EL and photovoltaic measurements to eliminate the unavoidable effect of the series resistance of inorganic and organic components on the total voltage drop across the hybrid device. For SnOx combined with the conjugated polymer [ladder type poly-(para-phenylene)], we find a one-to-one correspondence between the blueshift of the EL peak and the increase of the quasi-Fermi level splitting at the hybrid heterojunction, which we unambiguously assign to state filling. Our data are resembled best by a model considering the combination of an exponential density of states with a doped semiconductor.

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Image: The Authors

M. Schwarze, K.S. Schellhammer, K. Ortstein, J. Benduhn, C. Gaul, A. Hinderhofer, L. Perdigón Toro, R. Scholz, J. Kublitski, S. Roland, M. Lau, C. Poelking, D. Andrienko, G. Cuniberti, F. Schreiber, D. Neher, K. Vandewal, F. Ortmann,  K. Leo, “Impact of molecular quadrupole moments on the energy levels at organic heterojunctions”, Nature  Communications 10, 2466 (2019), DOI: 10.1038/s41467-019-10435-2

The functionality of organic semiconductor devices crucially depends on molecular energies, namely the ionisation energy and the electron affinity. Ionisation energy and electron affinity values of thin films are, however, sensitive to film morphology and composition, making their prediction challenging. In a combined experimental and simulation study on zinc-phthalocyanine and its fluorinated derivatives, we show that changes in ionisation energy as a function of molecular orientation in neat films or mixing ratio in blends are proportional to the molecular quadrupole component along the π-π-stacking direction. We apply these findings to organic solar cells and demonstrate how the electrostatic interactions can be tuned to optimise the energy of the charge-transfer state at the donor−acceptor interface and the dissociation barrier for free charge carrier generation. The confirmation of the correlation between interfacial energies and quadrupole moments for other materials indicates its relevance for small molecules and polymers.

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Image: The Authors

T. Li, J. Benduhn, Z. Qiao, Y. Liu, Y. Li, R. Shivhare, F. Jaiser, P. Wang, J. Ma, O. Zeika, D. Neher, S.C.B. Mannsfeld, Z. Ma, K. Vandewal, K. Leo, “Effect of H- and J-Aggregation on the Photophysical and Voltage Loss of Boron Dipyrromethene Small Molecules in Vacuum-Deposited Organic Solar Cells”, Journal of Physical Chemistry Letters 10, 2684-2691 (2019), DOI: 10.1021/acs.jpclett.9b01222

An understanding of the factors limiting the open-circuit voltage (Voc) and related photon energy loss mechanisms is critical to increase the power conversion efficiency (PCE) of small-molecule organic solar cells (OSCs), especially those with near-infrared (NIR) absorbers. In this work, two NIR boron dipyrromethene (BODIPY) molecules are characterized for application in planar (PHJ) and bulk (BHJ) heterojunction OSCs. When two H atoms are substituted by F atoms on the peripheral phenyl rings of the molecules, the molecular aggregation type in the thin film changes from the H-type to J-type. For PHJ devices, the nonradiative voltage loss of 0.35 V in the J-aggregated BODIPY is lower than that of 0.49 V in the H-aggregated device. In BHJ devices with a nonradiative voltage loss of 0.35 V, a PCE of 5.5% is achieved with an external quantum efficiency (EQE) maximum of 68% at 700 nm.

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Image: The Authors

S. Zhang, S.M. Hosseini, R. Gunder, A. Petsiuk, P. Caprioglio, C.M. Wolff, S. Shoaee, P. Meredith, S. Schorr, T. Unold, P.L. Burn, D. Neher, M. Stolterfoht, “The Role of Bulk and Interface Recombination in High‐Efficiency Low‐Dimensional Perovskite Solar Cells”,  Advanced Materials 31, 1901090 (2019), DOI: 10.1002/adma.201901090

2D Ruddlesden–Popper perovskite (RPP) solar cells have excellent environmental stability. However, the power conversion efficiency (PCE) of RPP cells remains inferior to 3D perovskite‐based cells. Herein, 2D (CH3(CH2)3NH3)2(CH3NH3)n−1PbnI3n+1 perovskite cells with different numbers of [PbI6]4− sheets (n = 2–4) are analyzed. Photoluminescence quantum yield (PLQY) measurements show that nonradiative open‐circuit voltage (VOC) losses outweigh radiative losses in materials with n > 2. The n = 3 and n = 4 films exhibit a higher PLQY than the standard 3D methylammonium lead iodide perovskite although this is accompanied by increased interfacial recombination at the top perovskite/C60 interface. This tradeoff results in a similar PLQY in all devices, including the n = 2 system where the perovskite bulk dominates the recombination properties of the cell. In most cases the quasi‐Fermi level splitting matches the device VOC within 20 meV, which indicates minimal recombination losses at the metal contacts. The results show that poor charge transport rather than exciton dissociation is the primary reason for the reduction in fill factor of the RPP devices. Optimized n = 4 RPP solar cells had PCEs of 13% with significant potential for further improvements.

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Image: The Authors

U. Würfel, L. Perdigón-Toro, J. Kurpiers, C.M. Wolff, P. Caprioglio, J.J. Rech, J. Zhu, X. Zhan, W. You, S. Shoaee, D. Neher, M. Stolterfoht, “Recombination between Photogenerated and Electrode-Induced Charges Dominates the Fill Factor Losses in Optimized Organic Solar Cells”, Journal of Physical Chemistry Letters 10, 3473-3480 (2019), DOI: 10.1021/acs.jpclett.9b01175

Charge extraction in organic solar cells (OSCs) is commonly believed to be limited by bimolecular recombination of photogenerated charges. Yet, the fill factor of OSCs is usually almost entirely governed by recombination processes that scale with the first-order of the light intensity. This linear loss was often interpreted to be a consequence of geminate or trap-assisted recombination. Numerical simulations show that this linear dependence is a direct consequence of the large amount of excess dark charge near the contact. The first-order losses increase with decreasing mobility of minority carriers and we discuss the impact of several material and device parameters on this loss mechanism. This work highlights that OSCs are especially vulnerable to injected charges as a result of their poor charge transport properties. This implicates that dark charges need to be better accounted for when interpreting electro-optical measurements and charge collection based on simple figures of merit.

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Image: The Authors

F. Zu, C. M. Wolff, M. Ralaiarisoa, P. Amsalem, D. Neher, N. Koch, “Unraveling the Electronic Properties of Lead Halide Perovskites with Surface Photovoltage in Photoemission Studies”, ACS Applied Materials & Interfaces 11, 21578-21583 (2019), DOI: 10.1021/acsami.9b05293

The tremendous success of metal-halide perovskites, especially in the field of photovoltaics, has triggered a substantial number of studies in understanding their optoelectronic properties. However, consensus regarding the electronic properties of these perovskites is lacking due to a huge scatter in the reported key parameters, such as work function (Φ) and valence band maximum (VBM) values. Here, we demonstrate that the surface photovoltage (SPV) is a key phenomenon occurring at the perovskite surfaces that feature a non-negligible density of surface states, which is more the rule than an exception for most materials under study. With ultraviolet photoelectron spectroscopy (UPS) and Kelvin probe, we evidence that even minute UV photon fluxes (500 times lower than that used in typical UPS experiments) are sufficient to induce SPV and shift the perovskite Φ and VBM by several 100 meV compared to dark. By combining UV and visible light, we establish flat band conditions (i.e., compensate the surface-state-induced surface band bending) at the surface of four important perovskites, and find that all are p-type in the bulk, despite a pronounced n-type surface character in the dark. The present findings highlight that SPV effects must be considered in all surface studies to fully understand perovskites’ photophysical properties.

Image: The Authors
Image: The Authors

F. Zu, P. Amsalem, D. A. Egger, R. Wang, C. M. Wolff, H. Fang, M. A. Loi, D. Neher, L. Kronik, S. Duhm, and N. Koch, “Constructing the Electronic Structure of CH3NH3PbI3andCH3NH3PbBr3Perovskite Thin Films from Single-Crystal BandStructure Measurements”, Journal of Physical Chemistry Letters 10, 601−609 (2019), DOI: 10.1021/acs.jpclett.8b03728

Photovoltaic cells based on halide perovskites, possessing remarkably high power conversion efficiencies have been reported. To push the development of such devices further, a comprehensive and reliable understanding of their electronic properties is essential but presently not available. To provide a solid foundation for understanding the electronic properties of polycrystalline thin films, we employ single-crystal band structure data from angle-resolved photoemission measurements. For two prototypical perovskites (CH3NH3PbBr3 and CH3NH3PbI3), we reveal the band dispersion in two high-symmetry directions and identify the global valence band maxima. With these benchmark data, we construct “standard” photoemission spectra from polycrystalline thin film samples and resolve challenges discussed in the literature for determining the valence band onset with high reliability. Within the framework laid out here, the consistency of relating the energy level alignment in perovskite-based photovoltaic and optoelectronic devices with their functional parameters is substantially enhanced.

Image: The Authors
Image: The Authors

S. Ullbrich , J. Benduhn  , X. Jia , V. C. Nikolis, K. Tvingstedt  , F. Piersimoni, S. Roland, Y. Liu, J. Wu, A. Fischer, D. Neher, S. Reineke, D. Spoltore, K. Vandewal, “Emissive and charge-generating donor–acceptor interfaces for organic optoelectronics with low voltage losses”, Nature Materials 18, 459-464 (2019) , DOI: 10.1038/s41563-019-0324-5

Intermolecular charge-transfer states at the interface between electron donating (D) and accepting (A) materials are crucial for the operation of organic solar cells but can also be exploited for organic light-emitting diodes. Non-radiative charge-transfer state decay is dominant in state-of-the-art D–A-based organic solar cells and is responsible for large voltage losses and relatively low power-conversion efficiencies as well as electroluminescence external quantum yields in the 0.01–0.0001% range. In contrast, the electroluminescence external quantum yield reaches up to 16% in D–A-based organic light-emitting diodes. Here, we show that proper control of charge-transfer state properties allows simultaneous occurrence of a high photovoltaic and emission quantum yield within a single, visible-light-emitting D–A system. This leads to ultralow-emission turn-on voltages as well as significantly reduced voltage losses upon solar illumination. These results unify the description of the electro-optical properties of charge-transfer states in organic optoelectronic devices and foster the use of organic D–A blends in energy conversion applications involving visible and ultraviolet photons.

Image: The Authors
Image: The Authors

S. Roland, J. Kniepert, J. A. Love, V. Negi, F. Liu, P. Bobbert, A. Melianas, M. Kemerink, A. Hofacker,  D. Neher, “Equilibrated Charge Carrier Populations Govern Steady-State Nongeminate Recombination in Disordered Organic Solar Cells”, Journal of Physical Chemistry Letters 10, 1374-1381 (2019), DOI: 10.1021/acs.jpclett.9b00516

We employed bias-assisted charge extraction techniques to investigate the transient and steady-state recombination of photogenerated charge carriers in complete devices of a disordered polymer−fullerene blend. Charge recombination is shown to be dispersive, with a significant slowdown of the recombination rate over time, consistent with the results from kinetic Monte Carlo simulations. Surprisingly, our experiments reveal little to no contributions from early time recombination of nonequilibrated charge carriers to the steady-state recombination properties. We conclude that energetic relaxation of photogenerated carriers outpaces any significant nongeminate recombination under application-relevant illumination conditions. With equilibrated charges dominating the steady-state recombination, quasi-equilibrium concepts appear suited for describing the open-circuit voltage of organic solar cells despite pronounced energetic disorder.

Image: The Authors
Image: The Authors

S. M. Hosseini, S. Roland, J. Kurpiers, Z. Chen, K. Zhang, F. Huang, A. Armin, D. Neher, S. Shoaee, “Impact of Bimolecular Recombination on the Fill Factor of Fullerene and Nonfullerene-Based Solar Cells: A Comparative Study of Charge Generation and Extraction”, The Journal of Physical Chemistry C 123, 6823-6830 (2019), DOI: 10.1021/acs.jpcc.8b11669 

Power conversion efficiencies of donor/acceptor organic solar cells utilizing nonfullerene acceptors have now increased beyond the record of their fullerene-based counterparts. There remain many fundamental questions regarding nanomorphology, interfacial states, charge generation and extraction, and losses in these systems. Herein, we present a comparative study of bulk heterojunction solar cells composed of a recently introduced naphthothiadiazole-based polymer (NT812) as the electron donor and two different acceptor molecules, namely, [6,6]-phenyl-C71-butyric acid methyl ester (PCBM)[70] and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC). A comparison between the photovoltaic performance of these two types of solar cells reveals that the open-circuit voltage (Voc) of the NT812:ITIC-based solar cell is larger, but the fill factor (FF) is lower than that of the NT812:PCBM[70] device. We find the key reason behind this reduced FF in the ITIC-based device to be faster nongeminate recombination relative to the NT812:PCBM[70] system.

Image: The Authors
Image: The Authors

E. Collado-Fregoso, S. N. Pugliese, M. Wojcik, J. Benduhn, E. Bar-Or, L. Perdigón Toro, U. Hörmann, D. Spoltore, K. Vandewal, J. M. Hodgkiss,  D. Neher, “Energy-Gap Law for Photocurrent Generation in Fullerene-Based Organic Solar Cells: The Case of Low-Donor-Content Blends”, Journal of the American Chemical Society 141, 2329–2341 (2019), DOI: 10.1021/jacs.8b09820

The involvement of charge-transfer (CT) states in the photogeneration and recombination of charge carriers has been an important focus of study within the organic photovoltaic community. In this work, we investigate the molecular factors determining the mechanism of photocurrent generation in low-donor-content organic solar cells, where the active layer is composed of vacuum-deposited C60 and small amounts of organic donor molecules. We find a pronounced decline of all photovoltaic parameters with decreasing CT state energy. Using a combination of steady-state photocurrent measurements and time-delayed collection field experiments, we demonstrate that the power conversion efficiency, and more specifically, the fill factor of these devices, is mainly determined by the bias dependence of photocurrent generation. By combining these findings with the results from ultrafast transient absorption spectroscopy, we show that blends with small CT energies perform poorly because of an increased nonradiative CT state decay rate and that this decay obeys an energy-gap law. Our work challenges the common view that a large energy offset at the heterojunction and/or the presence of fullerene clusters guarantee efficient CT dissociation and rather indicates that charge generation benefits from high CT state energies through a slower decay to the ground state.

Image: The Authors
Image: The Authors

P. Caprioglio, F. Zu, C. M. Wolff, J. A. Márquez Prieto, M. Stolterfoht, P. Becker, N. Koch, T. Unold, B. Rech, S. Albrecht, D. Neher, “High Open Circuit Voltages in pin-Type Perovskite Solar Cells through Strontium Addition”, Sustainable Energy & Fuels 3, 550-563 (2019), DOI:  10.1039/c8se00509e 

The incorporation of even small amounts of strontium (Sr) into lead-based quadruple cation hybrid perovskite solar cells results in a systematic increase of the open circuit voltage (Voc) in pin-type perovskite solar cells. We demonstrate via absolute and transient photoluminescence (PL) experiments how the incorporation of Sr significantly reduces the non-radiative recombination losses in the neat perovskite layer. We show that Sr segregates at the perovskite surface, where it induces important changes of morphology and energetics. Notably, the Sr-enriched surface exhibits a wider band gap and a more n-type character, accompanied with significantly stronger surface band bending. As a result, we observe a significant increase of the quasi-Fermi level splitting in the neat perovskite by reduced surface recombination and more importantly, a strong reduction of losses attributed to non-radiative recombination at the interface to the C60 electron-transporting layer. The resulting solar cells exhibited a Voc of 1.18 V, which could be further improved to nearly 1.23 V through addition of a thin polymer interlayer, bringing the non-radiative voltage loss to only 110 meV. Our work shows that simply adding a small amount of Sr to the precursor solutions induces a beneficial surface modification in the perovskite, without requiring any post treatment, resulting in high efficiency solar cells with power conversion efficiency (PCE) up to 20.3%. Our results demonstrate very high Voc values and efficiencies in Sr-containing quadruple cation perovskite pin solar cells and highlight the imperative importance of addressing and minimizing the recombination losses at the interface between perovskite and charge transporting layer.

Image: The Authors


T. Li, J. Benduhn, Y. Li, F. Jaiser, D. Spoltore, O. Zeika, Z. Ma, D. Neher, K. Vandewal, K. Leo, “Boron dipyrromethene (BODIPY) with meso -perfluorinated alkyl substituents as near infrared donors in organic solar cells”, Journal of Materials Chemistry A 6, 18583–18591 (2018), DOI: 10.1039/C8TA06261G

Three furan-fused BODIPYs were synthesized with perfluorinated alkyl substitutes on the meso -C. As NIR absorbers, a PCE of 6.4% was achieved in a single junction organic solar cell with relatively low energy losses.

R. Shivhare, T. Erdmann, U. Hörmann, E. Collado-Fregoso, S. Zeiske, J. Benduhn, S. Ullbrich, R. Hübner, M. Hambsch, A. Kiriy, B. Voit, D. Neher, K. Vandewal, S. C. B. Mannsfeld, “Alkyl Branching Position in Diketopyrrolopyrrole Polymers: Interplay between Fibrillar Morphology and Crystallinity and Their Effect on Photogeneration and Recombination in Bulk-Heterojunction Solar Cells”, Chemistry of Materials 30, 6801–6809 (2018), DOI: 10.1021/acs.chemmater.8b02739

Diketopyrrolopyrrole (DPP)-based donor−acceptor copolymers have gained a significant amount of research interest in the organic electronics community because of their high charge carrier mobilities in organic field-effect transistors (OFETs) and their ability to harvest near-infrared (NIR) photons in solar cells. In this study, we have synthesized four DPP-based donor−acceptor copolymers with variations in the donor unit and the branching point of the solubilizing alkyl chains (at the second or sixth carbon position). Grazing incidence wide-angle X-ray scattering (GIWAXS) results suggest that moving the branching point further away from the polymer backbone increases the tendency for aggregation and yields polymer phases with a higher degree of crystallinity (DoC). The polymers were blended with PC 70 BM and used as active layers in solar cells. A careful analysis of the energetics of the neat polymer and blend films reveals that the charge-transfer state energy (E CT) of the blend films lies exceptionally close to the singlet energy of the donor (E D*), indicating near zero electron transfer losses. The difference between the optical gap and open-circuit voltage (V OC) is therefore determined to be due to rather high nonradiative (≈ 418 ± 13 mV) and unavoidable radiative voltage losses (≈ 255 ± 8 mV). Even though the four materials have similar optical gaps, the short-circuit current density (J SC) covers a vast span from 7 to 18 mA cm −2 for the best performing system. Using photoluminescence (PL) quenching and transient charge extraction techniques, we quantify geminate and nongeminate losses and find that fewer excitons reach the donor−acceptor interface in polymers with further away branching points due to larger aggregate sizes. In these material systems, the photogeneration is therefore mainly limited by exciton harvesting efficiency.

M. Stolterfoht, C. M. Wolff, J. A. Márquez, S. Zhang, C. J. Hages, D. Rothhardt, S. Albrecht, P. L. Burn, P. Meredith, T. Unold, D. Neher, “Visualization and suppression of interfacial recombination for high-efficiency large-area pin perovskite solar cells”, Nature Energy 3, 847–854 (2018), DOI: 10.1038/s41560-018-0219-8

The performance of perovskite solar cells is predominantly limited by non-radiative recombination, either through trap-assisted recombination in the absorber layer or via minority carrier recombination at the perovskite/transport layer interfaces. Here, we use transient and absolute photoluminescence imaging to visualize all non-radiative recombination pathways in planar pin-type perovskite solar cells with undoped organic charge transport layers. We find significant quasi-Fermi-level splitting losses (135 meV) in the perovskite bulk, whereas interfacial recombination results in an additional free energy loss of 80 meV at each individual interface, which limits the open-circuit voltage (VOC) of the complete cell to ~1.12 V. Inserting ultrathin interlayers between the perovskite and transport layers leads to a substantial reduction of these interfacial losses at both the p and n contacts. Using this knowledge and approach, we demonstrate reproducible dopant-free 1 cm2 perovskite solar cells surpassing 20% efficiency (19.83% certified) with stabilized power output, a high VOC (1.17 V) and record fill factor (>81%).

U. Hörmann, S. Zeiske, F. Piersimoni, L. Hoffmann, R. Schlesinger, N. Koch, T. Riedl, D. Andrienko, D. Neher, “Stark Effect of Hybrid Charge Transfer States at Planar ZnO/Organic Interfaces”, Physical Review B 98, 155312 (2018), DOI: 10.1103/PhysRevB.98.155312

We investigate the bias-dependence of the hybrid charge transfer state emission at planar heterojunctions between the metal oxide acceptor ZnO and three donor molecules. The electroluminescence peak energy linearly increases with the applied bias, saturating at high fields. Variation of the organic layer thickness and deliberate change of the ZnO conductivity through controlled photo-doping allow us to confirm that this bias-induced spectral shifts relate to the internal electric field in the organic layer rather than the filling of states at the hybrid interface. We show that existing continuum models overestimate the hole delocalization and propose a simple electrostatic model in which the linear and quadratic Stark effects are explained by the electrostatic interaction of a strongly polarizable molecular cation with its mirror image.

S. Braunger, L. E. Mundt, C. M. Wolff, M. Mews, C. Rehermann, M. Jošt, A. Tejada, D. Eisenhauer, C. Becker, J. A. Guerra, E. Unger, L. Korte, D. Neher, M. C. Schubert, B. Rech, S. Albrecht, “CsxFA1–xPb(I1–yBry)3 Perovskite Compositions: the Appearance of Wrinkled Morphology and its Impact on Solar Cell Performance”, The Journal of Physical Chemistry C 122, 17123–17135 (2018), DOI: 10.1021/acs.jpcc.8b06459

We report on the formation of wrinkle-patterned surface morphologies in cesium formamidinium-based CsxFA1–xPb(I1–yBry)3 perovskite compositions with x = 0–0.3 and y = 0–0.3 under various spin-coating conditions. By varying the Cs and Br contents, the perovskite precursor solution concentration and the spin-coating procedure, the occurrence and characteristics of the wrinkle-shaped morphology can be tailored systematically. Cs0.17FA0.83Pb(I0.83Br0.17)3 perovskite layers were analyzed regarding their surface roughness, microscopic structure, local and overall composition, and optoelectronic properties. Application of these films in p–i–n perovskite solar cells (PSCs) with indium-doped tin oxide/NiOx/perovskite/C60/bathocuproine/Cu architecture resulted in up to 15.3 and 17.0% power conversion efficiency for the flat and wrinkled morphology, respectively. Interestingly, we find slightly red-shifted photoluminescence (PL) peaks for wrinkled areas and we are able to directly correlate surface topography with PL peak mapping. This is attributed to differences in the local grain size, whereas there is no indication for compositional demixing in the films. We show that the perovskite composition, crystallization kinetics, and layer thickness strongly influence the formation of wrinkles which is proposed to be related to the release of compressive strain during perovskite crystallization. Our work helps us to better understand film formation and to further improve the efficiency of PSCs with widely used mixed-perovskite compositions.

M. Saliba, J.-P. Correa-Baena, C. M. Wolff, M. Stolterfoht, N. Phung, S. Albrecht, D. Neher, A. Abate, “How to Make over 20% Efficient Perovskite Solar Cells in Regular ( n–i–p ) and Inverted ( p–i–n ) Architectures”, Chemistry of Materials 30, 4193–4201 (2018), DOI: 10.1021/acs.chemmater.8b00136

Perovskite solar cells (PSCs) are currently one of the most promising photovoltaic technologies for highly efficient and cost-effective solar energy production. In only a few years, an unprecedented progression of preparation procedures and material compositions delivered lab-scale devices that have now reached record power conversion efficiencies (PCEs) higher than 20%, competing with most established solar cell materials such as silicon, CIGS, and CdTe. However, despite a large number of researchers currently involved in this topic, only a few groups in the world can reproduce >20% efficiencies on a regular nip architecture. In this work, we present detailed protocols for preparing PSCs in regular (nip) and inverted (pin) architectures with ≥20% PCE. We aim to provide a comprehensive, reproducible description of our device fabrication protocols. We encourage the practice of reporting detailed and transparent protocols that can be more easily reproduced by other laboratories. A better reporting standard may, in turn, accelerate the development of perovskite solar cells and related research fields.

S. Zhang, M. Stolterfoht, A. Armin, Q. Lin, F. Zu, J. Sobus, H. Jin, N. Koch, P. Meredith, P. L. Burn, D. Neher, “Interface Engineering of Solution-Processed Hybrid Organohalide Perovskite Solar Cells”, ACS Applied Materials & Interfaces 10, 21681–21687 (2018), DOI: 10.1021/acsami.8b02503

Engineering the interface between the perovskite absorber and the charge-transporting layers has become an important method for improving the charge extraction and open-circuit voltage (VOC) of hybrid perovskite solar cells. Conjugated polymers are particularly suited to form the hole-transporting layer, but their hydrophobicity renders it difficult to solution-process the perovskite absorber on top. Herein, oxygen plasma treatment is introduced as a simple means to change the surface energy and work function of hydrophobic polymer interlayers for use as p-contacts in perovskite solar cells. We find that upon oxygen plasma treatment, the hydrophobic surfaces of different prototypical p-type polymers became sufficiently hydrophilic to enable subsequent perovskite junction processing. In addition, the oxygen plasma treatment also increased the ionization potential of the polymer such that it became closer to the valance band energy of the perovskite. It was also found that the oxygen plasma treatment could increase the electrical conductivity of the p-type polymers, facilitating more efficient charge extraction. On the basis of this concept, inverted MAPbI3 perovskite devices with different oxygen plasma-treated polymers such as P3HT, P3OT, polyTPD, or PTAA were fabricated with power conversion efficiencies of up to 19%.

M. Saliba, M. Stolterfoht, C. M. Wolff, D. Neher, A. Abate, “Measuring Aging Stability of Perovskite Solar Cells”, Joule 2, 1019–1024 (2018), DOI: 10.1016/j.joule.2018.05.005

Perovskite semiconductors are among the most promising emerging semiconductor materials. They have multiple outstanding optoelectronic and material properties such as defect tolerance, a sharp band edge, and a tunable band gap in the entire visible and near-infrared range. Remarkably, perovskites can be processed by almost all standard techniques from solution processing to thermal evaporation, providing easy access for diverse scientific communities from material science, physics, chemistry, process engineering, semiconductor devices, etc. This gave rise to the notion of perovskites being a “wonder material,” currently at the center stage of semiconductor research.

J. Kurpiers, T. Ferron, S. Roland, M. Jakoby, T. Thiede, F. Jaiser, S. Albrecht, S. Janietz, B. A. Collins, I. A. Howard, D. Neher, “Probing the pathways of free charge generation in organic bulk heterojunction solar cells”, Nature Communications 9, 2038 (2018), DOI: 10.1038/s41467-018-04386-3

The fact that organic solar cells perform efficiently despite the low dielectric constant of most photoactive blends initiated a long-standing debate regarding the dominant pathways of free charge formation. Here, we address this issue through the accurate measurement of the activation energy for free charge photogeneration over a wide range of photon energy, using the method of time-delayed collection field. For our prototypical low bandgap polymer:fullerene blends, we find that neither the temperature nor the field dependence of free charge generation depend on the excitation energy, ruling out an appreciable contribution to free charge generation though hot carrier pathways. On the other hand, activation energies are on the order of the room temperature thermal energy for all studied blends. We conclude that charge generation in such devices proceeds through thermalized charge transfer states, and that thermal energy is sufficient to separate most of these states into free charges.

J. Benduhn, F. Piersimoni, G. Londi, A. Kirch, J. Widmer, C. Koerner, D. Beljonne, D. Neher, D. Spoltore, K. Vandewal, “Impact of Triplet Excited States on the Open‐Circuit Voltage of Organic Solar Cells”,  Advanced Energy Materials, 1800451 (2018), DOI: 10.1002/aenm.201800451

The best organic solar cells (OSCs) achieve comparable peak external quantum efficiencies and fill factors as conventional photovoltaic devices. However, their voltage losses are much higher, in particular those due to nonradiative recombination. To investigate the possible role of triplet states on the donor or acceptor materials in this process, model systems comprising Zn‐ and Cu‐phthalocyanine (Pc), as well as fluorinated versions of these donors, combined with C60 as acceptor are studied. Fluorination allows tuning the energy level alignment between the lowest energy triplet state (T1) and the charge‐transfer (CT) state, while the replacement of Zn by Cu as the central metal in the Pcs leads to a largely enhanced spin–orbit coupling. Only in the latter case, a substantial influence of the triplet state on the nonradiative voltage losses is observed. In contrast, it is found that for a large series of typical OSC materials, the relative energy level alignment between T1 and the CT state does not substantially affect nonradiative voltage losses.

S. Shoaee, M. Stolterfoht, D. Neher, “The Role of Mobility on Charge Generation, Recombination, and Extraction in Polymer‐Based Solar Cells”, Advanced Energy Materials, 1703355 (2018), DOI: 10.1002/aenm.201703355

Organic semiconductors are of great interest for a broad range of optoelectronic applications due to their solution processability, chemical tunability, highly scalable fabrication, and mechanical flexibility. In contrast to traditional inorganic semiconductors, organic semiconductors are intrinsically disordered systems and therefore exhibit much lower charge carrier mobilities-the Achilles heel of organic photovoltaic cells. In this progress review, the authors discuss recent important developments on the impact of charge carrier mobility on the charge transfer state dissociation, and the interplay of free charge extraction and recombination. By comparing the mobilities on different timescales obtained by different techniques, the authors highlight the dispersive nature of these materials and how this reflects on the key processes defining the efficiency of organic photovoltaics.

O. Alqahtani, M. Babics, J. Gorenflot, V. Savikhin, T. Ferron, A. H. Balawi, A. Paulke, Z. Kan, M. Pope, A. J. Clulow, J. Wolf, P. L. Burn, I. R. Gentle, D. Neher, M. F. Toney, F. Laquai, P. M. Beaujuge, B. A. Collins, “Mixed Domains Enhance Charge Generation and Extraction in Bulk‐Heterojunction Solar Cells with Small‐Molecule Donors”, Advanced Energy Materials, 1702941 (2018), DOI: 10.1002/aenm.201702941

The interplay between nanomorphology and efficiency of polymer‐fullerene bulk‐heterojunction (BHJ) solar cells has been the subject of intense research, but the generality of these concepts for small‐molecule (SM) BHJs remains unclear. Here, the relation between performance; charge generation, recombination, and extraction dynamics; and nanomorphology achievable with two SM donors benzo[1,2‐b:4,5‐b]dithiophene‐pyrido[3,4‐b]‐pyrazine BDT(PPTh2)2, namely SM1 and SM2, differing by their side‐chains, are examined as a function of solution additive composition. The results show that the additive 1,8‐diiodooctane acts as a plasticizer in the blends, increases domain size, and promotes ordering/crystallinity. Surprisingly, the system with high domain purity (SM1) exhibits both poor exciton harvesting and severe charge trapping, alleviated only slightly with increased crystallinity. In contrast, the system consisting of mixed domains and lower crystallinity (SM2) shows both excellent exciton harvesting and low charge recombination losses. Importantly, the onset of large, pure crystallites in the latter (SM2) system reduces efficiency, pointing to possible differences in the ideal morphologies for SM‐based BHJ solar cells compared with polymer‐fullerene devices. In polymer‐based systems, tie chains between pure polymer crystals establish a continuous charge transport network, whereas SM‐based active layers may in some cases require mixed domains that enable both aggregation and charge percolation to the electrodes.

G. Sini, M. Schubert, C. Risko, S. Roland, O. P. Lee, Z. Chen, T. V. Richter, D. Dolfen, V. Coropceanu, S. Ludwigs, U. Scherf, A. Facchetti, J. M. J. Fréchet, D. Neher, “On the Molecular Origin of Charge Separation at the Donor–Acceptor Interface”, Advanced Energy Materials, 1702232 (2018), DOI: 10.1002/aenm.201702232

Fullerene-based acceptors have dominated organic solar cells for almost two decades. It is only within the last few years that alternative acceptors rival their dominance, introducing much more flexibility in the optoelectronic properties of these material blends. However, a fundamental physical understanding of the processes that drive charge separation at organic heterojunctions is still missing, but urgently needed to direct further material improvements. Here a combined experimental and theoretical approach is used to understand the intimate mechanisms by which molecular structure contributes to exciton dissociation, charge separation, and charge recombination at the donor–acceptor (D–A) interface. Model systems comprised of polythiophene-based donor and rylene diimide-based acceptor polymers are used and a detailed density functional theory (DFT) investigation is performed. The results point to the roles that geometric deformations and direct-contact intermolecular polarization play in establishing a driving force (energy gradient) for the optoelectronic processes taking place at the interface. A substantial impact for this driving force is found to stem from polymer deformations at the interface, a finding that can clearly lead to new design approaches in the development of the next generation of conjugated polymers and small molecules.


N. A. Ran, J. A. Love, M. C. Heiber, X. Jiao, M. P. Hughes, A. Karki, M. Wang, V. V. Brus, H. Wang, D. Neher, H. Ade, G. C. Bazan, T.-Q. Nguyen, “Charge Generation and Recombination in an Organic Solar Cell with Low Energetic Offsets”, Advanced Energy Materials 8, 1701073 (2018), DOI: 10.1002/aenm.201701073

Organic bulk heterojunction (BHJ) solar cells require energetic offsets between the donor and acceptor to obtain high short-circuit currents (JSC) and fill factors (FF). However, it is necessary to reduce the energetic offsets to achieve high open-circuit voltages (VOC). Recently, reports have highlighted BHJ blends that are pushing at the accepted limits of energetic offsets necessary for high efficiency. Unfortunately, most of these BHJs have modest FF values. How the energetic offset impacts the solar cell characteristics thus remains poorly understood. Here, a comprehensive characterization of the losses in a polymer:fullerene BHJ blend, PIPCP:phenyl-C61-butyric acid methyl ester (PC61BM), that achieves a high VOC (0.9 V) with very low energy losses (Eloss = 0.52 eV) from the energy of absorbed photons, a respectable JSC (13 mA cm−2), but a limited FF (54%) is reported. Despite the low energetic offset, the system does not suffer from field-dependent generation and instead it is characterized by very fast nongeminate recombination and the presence of shallow traps. The charge-carrier losses are attributed to suboptimal morphology due to high miscibility between PIPCP and PC61BM. These results hold promise that given the appropriate morphology, the JSC, VOC, and FF can all be improved, even with very low energetic offsets.

J. Gorenflot, A. Paulke, F. Piersimoni, J. Wolf, Z. Kan, F. Cruciani, A. El Labban, D. Neher, P. M. Beaujuge, F. Laquai, “From Recombination Dynamics to Device Performance: Quantifying the Efficiency of Exciton Dissociation, Charge Separation, and Extraction in Bulk Heterojunction Solar Cells with Fluorine-Substituted Polymer Donors”, Advanced Energy Materials 8, 1701678 (2018), DOI: 10.1002/aenm.201701678

An original set of experimental and modeling tools is used to quantify the yield of each of the physical processes leading to photocurrent generation in organic bulk heterojunction solar cells, enabling evaluation of materials and processing condition beyond the trivial comparison of device performances. Transient absorption spectroscopy, “the” technique to monitor all intermediate states over the entire relevant timescale, is combined with time-delayed collection field experiments, transfer matrix simulations, spectral deconvolution, and parametrization of the charge carrier recombination by a two-pool model, allowing quantification of densities of excitons and charges and extrapolation of their kinetics to device-relevant conditions. Photon absorption, charge transfer, charge separation, and charge extraction are all quantified for two recently developed wide-bandgap donor polymers: poly(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-3,4-difluorothiophene) (PBDT[2F]T) and its nonfluorinated counterpart poly(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-3,4-thiophene) (PBDT[2H]T) combined with PC71BM in bulk heterojunctions. The product of these yields is shown to agree well with the devices' external quantum efficiency. This methodology elucidates in the specific case studied here the origin of improved photocurrents obtained when using PBDT[2F]T instead of PBDT[2H]T as well as upon using solvent additives. Furthermore, a higher charge transfer (CT)-state energy is shown to lead to significantly lower energy losses (resulting in higher VOC) during charge generation compared to P3HT:PCBM.

A. Hofacker, D. Neher, “Dispersive and steady-state recombination in organic disordered semiconductors”, Physical Review B 96, 245204 (2017), DOI: 10.1103/PhysRevB.96.245204

Charge carrier recombination in organic disordered semiconductors is strongly influenced by the thermalization of charge carriers in the density of states (DOS). Measurements of recombination dynamics, conducted under transient or steady-state conditions, can easily be misinterpreted when a detailed understanding of the interplay of thermalization and recombination is missing. To enable adequate measurement analysis, we solve the multiple-trapping problem for recombining charge carriers and analyze it in the transient and steady excitation paradigm for different DOS distributions. We show that recombination rates measured after pulsed excitation are inherently time dependent since recombination gradually slows down as carriers relax in the DOS. When measuring the recombination order after pulsed excitation, this leads to an apparent high-order recombination at short times. As times goes on, the recombination order approaches an asymptotic value. For the Gaussian and the exponential DOS distributions, this asymptotic value equals the recombination order of the equilibrated system under steady excitation. For a more general DOS distribution, the recombination order can also depend on the carrier density, under both transient and steady-state conditions. We conclude that transient experiments can provide rich information about recombination in and out of equilibrium and the underlying DOS occupation provided that consistent modeling of the system is performed.

R. Di Pietro, T. Erdmann, J. H. Carpenter, N. Wang, R. R. Shivhare, P. Formanek, C. Heintze, B. Voit, D. Neher, H. Ade, A. Kiriy, “Synthesis of High-Crystallinity DPP Polymers with Balanced Electron and Hole Mobility”, Chemistry of Materials 29, 10220–10232 (2017), DOI: 10.1021/acs.chemmater.7b04423

We review the Stille coupling synthesis of P(DPP2OD-T) (Poly[[2,5-di(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-3,6-diyl]-alt-[2,2′:5′,2″-terthiophene-5,5″-diyl]]) and show that high-quality, high molecular weight polymer chains are already obtained after as little as 15 min of reaction time. The results of UV–vis spectroscopy, grazing incidence wide-angle X-ray scattering (GIWAXS), and atomic force microscopy show that longer reaction times are unnecessary and do not produce any improvement in film quality. We achieve the best charge transport properties with polymer batches obtained from short reaction times and demonstrate that the catalyst washing step is responsible for the introduction of charge-trapping sites for both holes and electrons. These trap sites decrease the charge injection efficiency, strongly reducing the measured currents. The careful tuning of the synthesis allows us to reduce the reaction time by more than 100 times, achieving a more environmentally friendly, less costly process that leads to high and balanced hole and electron transport, the latter being the best reported for an isotropic, spin-coated DPP polymer.

J. Kniepert, D. Neher, “Effect of the RC time on photocurrent transients and determination of charge carrier mobilities”, Journal of Applied Physics 122, 195501 (2017), DOI: 10.1063/1.4999278

We present a closed analytical model to describe time dependent photocurrents upon pulsed illumination in the presence of an external RC circuit. In combination with numerical drift diffusion simulations, it is shown that the RC time has a severe influence on the shape of the transients. In particular, the maximum of the photocurrent is delayed due to a delayed recharging of the electrodes. This delay increases with the increasing RC constant. As a consequence, charge carrier mobilities determined from simple extrapolation of the initial photocurrent decay will be in general too small and feature a false dependence on the electric field. Here, we present a recipe to correct charge carrier mobilities determined from measured photocurrent transients by taking into account the RC time of the experimental set-up. We also demonstrate how the model can be used to more reliably determine the charge carrier mobility from experimental data of a typical polymer/fullerene organic solar cell. It is shown that further aspects like a finite rising time of the pulse generator and the current contribution of the slower charger carriers influence the shape of the transients and may lead to an additional underestimation of the transit time.

J. A. Love, M. Feuerstein, C. M. Wolff, A. Facchetti, D. Neher, “Lead Halide Perovskites as Charge Generation Layers for Electron Mobility Measurement in Organic Semiconductors”, ACS Applied Materials & Interfaces 9, 42011–42019 (2017), DOI: 10.1021/acsami.7b10361

Hybrid lead halide perovskites are introduced as charge generation layers (CGLs) for the accurate determination of electron mobilities in thin organic semiconductors. Such hybrid perovskites have become a widely studied photovoltaic material in their own right, for their high efficiencies, ease of processing from solution, strong absorption, and efficient photogeneration of charge. Time-of-flight (ToF) measurements on bilayer samples consisting of the perovskite CGL and an organic semiconductor layer of different thickness are shown to be determined by the carrier motion through the organic material, consistent with the much higher charge carrier mobility in the perovskite. Together with the efficient photon-to-electron conversion in the perovskite, this high mobility imbalance enables electron-only mobility measurement on relatively thin application-relevant organic films, which would not be possible with traditional ToF measurements. This architecture enables electron-selective mobility measurements in single components as well as bulk-heterojunction films as demonstrated in the prototypical polymer/fullerene blends. To further demonstrate the potential of this approach, electron mobilities were measured as a function of electric field and temperature in an only 127 nm thick layer of a prototypical electron-transporting perylene diimide-based polymer, and found to be consistent with an exponential trap distribution of ca. 60 meV. Our study furthermore highlights the importance of high mobility charge transporting layers when designing perovskite solar cells.

E. Collado-Fregoso, S. N. Hood, S. Shoaee, B. C. Schroeder, I. McCulloch , I. Kassal, D. Neher, J. R. Durrant, “Intercalated vs Nonintercalated Morphologies in Donor–Acceptor Bulk Heterojunction Solar Cells: PBTTT:Fullerene Charge Generation and Recombination Revisited”, The Journal of Physical Chemistry Letters 8, 4061–4068 (2017), DOI: 10.1021/acs.jpclett.7b01571

In this Letter, we study the role of the donor:acceptor interface nanostructure upon charge separation and recombination in organic photovoltaic devices and blend films, using mixtures of PBTTT and two different fullerene derivatives (PC70BM and ICTA) as models for intercalated and nonintercalated morphologies, respectively. Thermodynamic simulations show that while the completely intercalated system exhibits a large free-energy barrier for charge separation, this barrier is significantly lower in the nonintercalated system and almost vanishes when energetic disorder is included in the model. Despite these differences, both femtosecond-resolved transient absorption spectroscopy (TAS) and time-delayed collection field (TDCF) exhibit extensive first-order losses in both systems, suggesting that geminate pairs are the primary product of photoexcitation. In contrast, the system that comprises a combination of fully intercalated polymer:fullerene areas and fullerene-aggregated domains (1:4 PBTTT:PC70BM) is the only one that shows slow, second-order recombination of free charges, resulting in devices with an overall higher short-circuit current and fill factor. This study therefore provides a novel consideration of the role of the interfacial nanostructure and the nature of bound charges and their impact upon charge generation and recombination.

Z. Chen, A. Savateev, S. Pronkin, V. Papaefthimiou, C. Wolff, M. G. Willinger, E. Willinger, D. Neher, M. Antonietti, D. Dontsova, “‘The Easier the Better’ Preparation of Efficient Photocatalysts—Metastable Poly(heptazine imide) Salts”, Advanced Materials 29, 1700555 (2017), DOI: 10.1002/adma.201700555

Cost-efficient, visible-light-driven hydrogen production from water is an attractive potential source of clean, sustainable fuel. Here, it is shown that thermal solid state reactions of traditional carbon nitride precursors (cyanamide, melamine) with NaCl, KCl, or CsCl are a cheap and straightforward way to prepare poly(heptazine imide) alkali metal salts, whose thermodynamic stability decreases upon the increase of the metal atom size. The chemical structure of the prepared salts is confirmed by the results of X-ray photoelectron and infrared spectroscopies, powder X-ray diffraction and electron microscopy studies, and, in the case of sodium poly(heptazine imide), additionally by atomic pair distribution function analysis and 2D powder X-ray diffraction pattern simulations. In contrast, reactions with LiCl yield thermodynamically stable poly(triazine imides). Owing to the metastability and high structural order, the obtained heptazine imide salts are found to be highly active photocatalysts in Rhodamine B and 4-chlorophenol degradation, and Pt-assisted sacrificial water reduction reactions under visible light irradiation. The measured hydrogen evolution rates are up to four times higher than those provided by a benchmark photocatalyst, mesoporous graphitic carbon nitride. Moreover, the products are able to photocatalytically reduce water with considerable reaction rates, even when glycerol is used as a sacrificial hole scavenger.

N. A. Ran, S. Roland, J. A. Love, V. Savikhin, C. J. Takacs, Y.-T. Fu, H. Li, V. Coropceanu, X. Liu, J.-L. Brédas, G. C. Bazan, M. F. Toney, D. Neher, T.-Q. Nguyen, “Impact of interfacial molecular orientation on radiative recombination and charge generation efficiency”, Nature Communications 8, 79 (2017), DOI: 10.1038/s41467-017-00107-4

A long standing question in organic electronics concerns the effects of molecular orientation at donor/acceptor heterojunctions. Given a well-controlled donor/acceptor bilayer system, we uncover the genuine effects of molecular orientation on charge generation and recombination. These effects are studied through the point of view of photovoltaics—however, the results have important implications on the operation of all optoelectronic devices with donor/acceptor interfaces, such as light emitting diodes and photodetectors. Our findings can be summarized by two points. First, devices with donor molecules face-on to the acceptor interface have a higher charge transfer state energy and less non-radiative recombination, resulting in larger open-circuit voltages and higher radiative efficiencies. Second, devices with donor molecules edge-on to the acceptor interface are more efficient at charge generation, attributed to smaller electronic coupling between the charge transfer states and the ground state, and lower activation energy for charge generation.

V. C. Nikolis, J. Benduhn, F. Holzmueller, F. Piersimoni, M. Lau, O. Zeika, D. Neher, C. Koerner, D. Spoltore, K. Vandewal, "Reducing Voltage Losses in Cascade Organic Solar Cells while Maintaining High External Quantum Efficiencies", Advanced Energy Materials 7, 1700855 (2017), DOI: 10.1002/aenm.201700855

High photon energy losses limit the open-circuit voltage (VOC) and power conversion efficiency of organic solar cells (OSCs). In this work, an optimization route is presented which increases the VOC by reducing the interfacial area between donor (D) and acceptor (A). This optimization route concerns a cascade device architecture in which the introduction of discontinuous interlayers between alpha-sexithiophene (α-6T) (D) and chloroboron subnaphthalocyanine (SubNc) (A) increases the VOC of an α-6T/SubNc/SubPc fullerene-free cascade OSC from 0.98 V to 1.16 V. This increase of 0.18 V is attributed solely to the suppression of nonradiative recombination at the D–A interface. By accurately measuring the optical gap (Eopt) and the energy of the charge-transfer state (ECT) of the studied OSC, a detailed analysis of the overall voltage losses is performed. Eopt – qVOC losses of 0.58 eV, which are among the lowest observed for OSCs, are obtained. Most importantly, for the VOC-optimized devices, the low-energy (700 nm) external quantum efficiency (EQE) peak remains high at 79%, despite a minimal driving force for charge separation of less than 10 meV. This work shows that low-voltage losses can be combined with a high EQE in organic photovoltaic devices.

C. M. Wolff, F. Zu, A. Paulke, L. Perdigón-Toro, N. Koch, D. Neher, “Reduced Interface-Mediated Recombination for High Open-Circuit Voltages in CH3NH3PbI3 Solar Cells”, Advanced Materials 29, 1700159 (2017),  DOI: 10.1002/adma.201700159

Perovskite solar cells with all-organic transport layers exhibit efficiencies rivaling their counterparts that employ inorganic transport layers, while avoiding high-temperature processing. Herein, it is investigated how the choice of the fullerene derivative employed in the electron-transporting layer of inverted perovskite cells affects the open-circuit voltage (VOC). It is shown that nonradiative recombination mediated by the electron-transporting layer is the limiting factor for the VOC in the cells. By inserting an ultrathin layer of an insulating polymer between the active CH3NH3PbI3 perovskite and the fullerene, an external radiative efficiency of up to 0.3%, a VOC as high as 1.16 V, and a power conversion efficiency of 19.4% are realized. The results show that the reduction of nonradiative recombination due to charge-blocking at the perovskite/organic interface is more important than proper level alignment in the search for ideal selective contacts toward high VOC and efficiency.

J. Benduhn, K. Tvingstedt, F. Piersimoni, S. Ullbrich, Y. Fan, M. Tropiano, K. A. McGarry, O. Zeika, M. K. Riede, C. J. Douglas, S. Barlow, S. R. Marder, D. Neher, D. Spoltore & K. Vandewal, “Intrinsic non-radiative voltage losses in fullerene-based organic solar cells”, Nature Energy 2, 17053 (2017), DOI: 10.1038/nenergy.2017.53 

Organic solar cells demonstrate external quantum efficiencies and fill factors approaching those of conventional photovoltaic technologies. However, as compared with the optical gap of the absorber materials, their open-circuit voltage is much lower, largely due to the presence of significant non-radiative recombination. Here, we study a large data set of published and new material combinations and find that non-radiative voltage losses decrease with increasing charge-transfer-state energies. This observation is explained by considering non-radiative charge-transfer-state decay as electron transfer in the Marcus inverted regime, being facilitated by a common skeletal molecular vibrational mode. Our results suggest an intrinsic link between non-radiative voltage losses and electron-vibration coupling, indicating that these losses are unavoidable. Accordingly, the theoretical upper limit for the power conversion efficiency of single-junction organic solar cells would be reduced to about 25.5% and the optimal optical gap increases to 1.45–1.65 eV, that is, 0.2–0.3 eV higher than for technologies with minimized non-radiative voltage losses.

L. Kegelmann, C. M. Wolff, C. A. Omondi, F. Lang, E. L. Unger, L. Korte, T. Dittrich, D. Neher, B. Rech, S. Albrecht, “It takes two to tango – double-layer selective contacts in perovskite solar cells for improved device performance and reduced hysteresis”, ACS Applied Materials & Interfaces 9, 17245-17255 (2017), DOI: 10.1021/acsami.7b00900

Solar cells made from inorganic-organic perovskites gradually approach market requirements as efficiency and stability improved tremendously in recent years. Planar low-temperature processed perovskite solar cells are advantageous for a possible large-scale production but are more prone to exhibit photocurrent hysteresis, especially in the regular n-i-p structure. Here, a systematic characterization of different electron selective contacts with a variety of chemical and electrical properties in planar n-i-p devices processed below 180 °C is presented. The inorganic metal oxides TiO2 and SnO2, the organic fullerene derivatives C60, PCBM and ICMA as well as double-layers with a metal oxide/PCBM structure are used as electron transport materials (ETMs). Perovskite layers deposited atop the different ETMs with the herein applied fabrication method show a similar morphology according to scanning electron microscopy. Further, surface photovoltage spectroscopy measurements indicate comparable perovskite absorber qualities on all ETMs except TiO2 showing a more prominent influence of defect states. Transient photoluminescence studies together with current-voltage scans over a broad range of scan speeds reveal faster charge extraction, less pronounced hysteresis effects and higher efficiencies for devices with fullerene compared to metal oxide ETMs. Beyond this, only double-layer ETM structures substantially diminish hysteresis effects for all performed scan speeds and strongly enhance the power conversion efficiency up to a champion stabilized value of 18.0 %. The results indicate reduced recombination losses for a double layer TiO2/PCBM contact design: First a reduction of shunt paths through the fullerene to the ITO layer. Second, an improved hole blocking by the wide band-gap metal oxide. Third, decreased transport losses due to an energetically more favorable contact as implied by photoelectron spectroscopy measurements. The herein demonstrated improvements of multi-layer selective contacts may serve as a general design guideline for perovskite solar cells.

S. Roland, L. Yan, Q. Zhang, X. Jiao, A. Hunt, M. Ghasemi, H. Ade, W. You, D. Neher, “Charge Generation and Mobility-Limited Performance of Bulk Heterojunction Solar Cells with a Higher Adduct Fullerene”, The Journal of Physical Chemistry C 121, 10305-10316 (2017), DOI: 10.1021/acs.jpcc.7b02288

Alternative electron acceptors are being actively explored in order to advance the development of bulk-heterojunction (BHJ) organic solar cells (OSCs). The indene–C60 bisadduct (ICBA) has been regarded as a promising candidate, as it provides high open-circuit voltage in BHJ solar cells; however, the photovoltaic performance of such ICBA-based devices is often inferior when compared to cells with the omnipresent PCBM electron acceptor. Here, by pairing the high performance polymer (FTAZ) as the donor with either PCBM or ICBA as the acceptor, we explore the physical mechanism behind the reduced performance of the ICBA-based device. Time delayed collection field (TDCF) experiments reveal reduced, yet field-independent free charge generation in the FTAZ:ICBA system, explaining the overall lower photocurrent in its cells. Through the analysis of the photoluminescence, photogeneration, and electroluminescence, we find that the lower generation efficiency is neither caused by inefficient exciton splitting, nor do we find evidence for significant energy back-transfer from the CT state to singlet excitons. In fact, the increase in open circuit voltage when replacing PCBM by ICBA is entirely caused by the increase in the CT energy, related to the shift in the LUMO energy, while changes in the radiative and nonradiative recombination losses are nearly absent. On the other hand, space charge limited current (SCLC) and bias-assisted charge extraction (BACE) measurements consistently reveal a severely lower electron mobilitiy in the FTAZ:ICBA blend. Studies of the blends with resonant soft X-ray scattering (R-SoXS), grazing incident wide-angle X-ray scattering (GIWAXS), and scanning transmission X-ray microscopy (STXM) reveal very little differences in the mesoscopic morphology but significantly less nanoscale molecular ordering of the fullerene domains in the ICBA based blends, which we propose as the main cause for the lower generation efficiency and smaller electron mobility. Calculations of the JV curves with an analytical model, using measured values, show good agreement with the experimentally determined JV characteristics, proving that these devices suffer from slow carrier extraction, resulting in significant bimolecular recombination losses. Therefore, this study highlights the importance of high charge carrier mobility for newly synthesized acceptor materials, in addition to having suitable energy levels.

M. Jošt, S. Albrecht, L. Kegelmann, C. M. Wolff, F. Lang, B. Lipovšek, J. Krč, L. Korte, D. Neher, B. Rech, M. Topič, “Efficient Light Management by Textured Nanoimprinted Layers for Perovskite Solar Cells”, ACS Photonics 4, 1232-1239 (2017), DOI: 10.1021/acsphotonics.7b00138

Inorganic–organic perovskites like methylammonium-lead-iodide have proven to be an effective class of materials for fabricating efficient solar cells. To improve their performance, light management techniques using textured surfaces, similar to those used in established solar cell technologies, should be considered. Here, we apply a light management foil created by UV nanoimprint lithography on the glass side of an inverted (p-i-n) perovskite solar cell with 16.3% efficiency. The obtained 1 mA/cm² increase in the short-circuit current density translates to a relative improvement in cell performance of 5%, which results in a power conversion efficiency of 17.1%. Optical 3D simulations based on experimentally obtained parameters were used to support the experimental findings. A good match between the simulated and experimental data was obtained, validating the model. Optical simulations reveal that the main improvement in device performance is due to a reduction in total reflection and that relative improvement in the short-circuit current density of up to 10% is possible for large-area devices. Therefore, our results present the potential of light management foils for improving the device performance of perovskite solar cells and pave the way for further use of optical simulations in the field of perovskite solar cells.

M. A. Kelly, S. Roland, Q. Zhang, Y. Lee, B. Kabius, Q. Wang, E. D. Gomez, D. Neher, W. You, “Incorporating Fluorine Substitution into Conjugated Polymers for Solar Cells: Three Different Means , Same Results”, The Journal of Physical Chemistry C 121, 2059–2068 (2017), DOI: 10.1021/acs.jpcc.6b10993

Fluorinating conjugated polymers is a proven strategy for creating high performance materials in polymer solar cells, yet few studies have investigated the importance of the fluorination method. We compare the performance of three fluorinated systems: a poly(benzodithieno-dithienyltriazole) (PBnDT-XTAZ) random copolymer where 50% of the acceptor units are difluorinated, PBnDT-mFTAZ where every acceptor unit is monofluorinated, and a 1:1 physical blend of the difluorinated and nonfluorinated polymer. All systems have the same degree of fluorination (50%) yet via different methods (chemically vs physically, random vs regular). We show that these three systems have equivalent photovoltaic behavior:,similar to 5.2% efficiency with a short-circuit current (Jsc) at ~ 11 mA/cm2, an open-circuit voltage (Voc) at 0.77 V, and a fill factor (FF) of ~ 60%. Further investigation of these three systems demonstrates that the charge generation, charge extraction, and charge transfer state are essentially identical for the three studied systems. Transmission electron microscopy shows no significant differences in the morphologies. All these data illustrate that it is possible to improve performance not only via regular or random fluorination but also by physical addition via a ternary blend. Thus, our results demonstrate the versatility of incorporating fluorine in the active layer of polymer solar cells to enhance device performance.

K. Vandewal, J. Benduhn, K. S. Schellhammer, T. Vangerven, J. E. Rückert, F. Piersimoni, R. Scholz, O. Zeika, Y. Fan, S. Barlow, D. Neher, S. R. Marder, J. Manca, D. Spoltore, G. Cuniberti, F. Ortmann, “Absorption Tails of Donor:C60 Blends Provide Insight into Thermally Activated Charge-Transfer Processes and Polaron Relaxation”, Journal of the American Chemical Society 139, 1699–1704 (2017), DOI: 10.1021/jacs.6b12857

In disordered organic semiconductors, the transfer of a rather localized charge carrier from one site to another triggers a deformation of the molecular structure quantified by the intramolecular relaxation energy. A similar structural relaxation occurs upon population of intermolecular charge-transfer (CT) states formed at organic electron donor (D)–acceptor (A) interfaces. Weak CT absorption bands for D–A complexes occur at photon energies below the optical gaps of both the donors and the C60 acceptor as a result of optical transitions from the neutral ground state to the ionic CT state. In this work, we show that temperature-activated intramolecular vibrations of the ground state play a major role in determining the line shape of such CT absorption bands. This allows us to extract values for the relaxation energy related to the geometry change from neutral to ionic CT complexes. Experimental values for the relaxation energies of 20 D:C60 CT complexes correlate with values calculated within density functional theory. These results provide an experimental method for determining the polaron relaxation energy in solid-state organic D–A blends and show the importance of a reduced relaxation energy, which we introduce to characterize thermally activated CT processes.

M. Stolterfoht, C. M. Wolff, Y. Amir, A. Paulke, L. Perdigón-Toro, P. Caprioglio, D. Neher, “Approaching the fill factor Shockley–Queisser limit in stable, dopant-free triple cation perovskite solar cells”, Energy & Environmental Science 10, 1530-1539 (2017), DOI: 10.1039/C7EE00899F

Perovskite solar cells now compete with their inorganic counterparts in terms of power conversion efficiency, not least because of their small open-circuit voltage (VOC) losses. A key to surpass traditional thin-film solar cells is the fill factor (FF). Therefore, more insights into the physical mechanisms that define the bias dependence of the photocurrent are urgently required. In this work, we studied charge extraction and recombination in efficient triple cation perovskite solar cells with undoped organic electron/hole transport layers (ETL/HTL). Using integral time of flight we identify the transit time through the HTL as the key figure of merit for maximizing the fill factor (FF) and efficiency. Complementarily, intensity dependent photocurrent and VOC measurements elucidate the role of the HTL on the bias dependence of non-radiative and transport-related loss channels. We show that charge transport losses can be completely avoided under certain conditions, yielding devices with FFs of up to 84%. Optimized cells exhibit power conversion efficiencies of above 20% for 6 mm² sized pixels and 18.9% for a device area of 1 cm². These are record efficiencies for hybrid perovskite devices with dopant-free transport layers, highlighting the potential of this device technology to avoid charge-transport limitations and to approach the Shockley-Queisser limit.

M. Schubert, J. Frisch, S. Allard, E. Preis, U. Scherf, N. Koch, D. Neher, “Tuning Side Chain and Main Chain Order in a Prototypical Donor-Acceptor Copolymer: Implications for Optical, Electronic, and Photovoltaic Characteristics” in Elem. Process. Org. Photovoltaics (Ed.: K. Leo), Springer-Verlag, Berlin (2017), pp. 243–26, DOI: 10.1007/978-3-319-28338-8_10

The recent development of donor-acceptor copolymers has led to an enormous improvement in the performance of organic solar cells and organic field-effect transistors. Here we describe the synthesis, detailed characterisation, and application of a series of structurally modified copolymers to investigate fundamental structure-property relationships in this class of conjugated polymers. The interplay between chemical structure and optoelectronic properties is investigated. These are further correlated to the charge transport and solar cell performance, which allows us to link their chemical structure to the observed physical properties.

F. Laquai, D. Andrienko, C. Deibel, D. Neher, “Charge Carrier Generation, Recombination, and Extraction in Polymer-Fullerene Bulk Heterojunction Organic Solar Cells” in Elem. Process. Org. Photovoltaics (Ed.: Leo, K), Springer-Verlag, Berlin (2017), pp. 267–29, DOI: 10.1007/978-3-319-28338-8_11

In this chapter we review the basic principles of photocurrent generation in bulk heterojunction organic solar cells, discuss the loss channels limiting their efficiency, and present case studies of several polymer-fullerene blends. Using steady-state and transient, optical, and electrooptical techniques, we create a precise picture of the fundamental processes that ultimately govern solar cell efficiency.

A. Savateev, S. Pronkin, J. D. Epping, M. G. Willinger, C. Wolff, D. Neher, M. Antonietti, D. Dontsova, “Potassium Poly(heptazine imides) from Aminotetrazoles: Shifting Band Gaps of Carbon Nitride-like Materials for More Efficient Solar Hydrogen and Oxygen Evolution”, ChemCatChem 9, 167–174 (2017), DOI: 10.1002/cctc.201601165

Potassium poly(heptazine imide) (PHI) is a photocatalytically active carbon nitride material that was recently prepared from substituted 1,2,4-triazoles. Here, we show that the more acidic precursors, such as commercially available 5-aminotetrazole, upon pyrolysis in LiCl/KCl salt melt yield PHI with the greatly improved structural order and thermodynamic stability. Tetrazole-derived PHIs feature long-range crystallinities and unconventionally small layer stacking distances, leading to the altered electronic band structures as shown by Mott–Schottky analyses. Under the optimized synthesis conditions, visible-light driven hydrogen evolution rates reach twice the rate provided by the previous gold standard, mesoporous graphitic carbon nitride, which has a much higher surface area. More interestingly, the up to 0.7 V higher valence band potential of crystalline PHI compared with ordinary carbon nitrides makes it an efficient water oxidation photocatalyst, which works even in the absence of any metal-based co-catalysts under visible light. To our knowledge, this is the first case of metal-free oxygen liberation from water.


M. Stolterfoht, A. Armin, B. Philippa, D. Neher, “The Role of Space Charge Effects on the Competition between Recombination and Extraction in Solar Cells with Low-Mobility Photoactive Layers”, The Journal of Physical Chemistry Letters 7, 4716–4721 (2016), DOI: 10.1021/acs.jpclett.6b02106

The competition between charge extraction and nongeminate recombination critically determines the current–voltage characteristics of organic solar cells (OSCs) and their fill factor. As a measure of this competition, several figures of merit (FOMs) have been put forward; however, the impact of space charge effects has been either neglected, or not specifically addressed. Here we revisit recently reported FOMs and discuss the role of space charge effects on the interplay between recombination and extraction. We find that space charge effects are the primary cause for the onset of recombination in so-called non-Langevin systems, which also depends on the slower carrier mobility and recombination coefficient. The conclusions are supported with numerical calculations and experimental results of 25 different donor/acceptor OSCs with different charge transport parameters, active layer thicknesses or composition ratios. The findings represent a conclusive understanding of bimolecular recombination for drift dominated photocurrents and allow one to minimize these losses for given device parameters.

R. Di Pietro, I. Nasrallah, J. Carpenter, E. Gann, L.S. Kölln, L. Thomsen, D. Venkateshvaran, K. O'Hara, A. Sadhanala, M. Chabinyc, C.R. McNeill, A. Facchetti, H. Ade, H. Sirringhaus & D. Neher, “Coulomb Enhanced Charge Transport in Semicrystalline Polymer Semiconductors”, Advanced Functional Materials 26, 8011-8022 (2016), 10.1002/adfm.201602080

Polymer semiconductors provide unique possibilities and flexibility in tailoring their optoelectronic properties to match specific application demands. The recent development of semicrystalline polymers with strongly improved charge transport properties forces a review of the current understanding of the charge transport mechanisms and how they relate to the polymer's chemical and structural properties. Here, the charge density dependence of field effect mobility in semicrystalline polymer semiconductors is studied. A simultaneous increase in mobility and its charge density dependence, directly correlated to the increase in average crystallite size of the polymer film, is observed. Further evidence from charge accumulation spectroscopy shows that charges accumulate in the crystalline regions of the polymer film and that the increase in crystallite size affects the average electronic orbitals delocalization. These results clearly point to an effect that is not caused by energetic disorder. It is instead shown that the inclusion of short range coulomb repulsion between charge carriers on nanoscale crystalline domains allows describing the observed mobility dependence in agreement with the structural and optical characterization. The conclusions that are extracted extend beyond pure transistor characterization and can provide new insights into charge carrier transport for regimes and timescales that are relevant to other optoelectronic devices.

L. Fang, F. Holzmueller, T. Matulaitis, A. Baasner, C. Hauenstein, J. Benduhn, M. Schwarze, A. Petrich, F. Piersimoni, R. Scholz, O. Zeika, C. Koerner, D. Neher, K. Vandewal & K. Leo, “Fluorine-containing low-energy-gap organic dyes with low voltage losses for organic solar cells”, Synthetic Metals 222, Part B, 232-239 (2016), 10.1016/j.synthmet.2016.10.025

Fluorine-containing donor molecules TFTF, CNTF and PRTF are designed and isomer selectively synthesized for application in vacuum‍-deposited organic solar cells. These molecules comprise a donor‍-‍acceptor molecular architecture incorporating thiophene and benzothiadiazole derivatives as the electron-‍donating and electron-‍withdrawing moieties, respectively. As opposed to previously reported materials from this class, PRTF can be purified by vacuum sublimation at moderate to high yields because of its higher volatility and better stabilization due to a stronger intramolecular hydrogen bond, as compared to TFTF and CNTF. The UV-‍vis absorption spectra of the three donors show an intense broadband absorption between 500 nm and 800 nm with similar positions of their frontier energy levels. The photophysical properties of the three donor molecules are thoroughly tested and optimized in bulk heterojunction solar cells with C60 as acceptor. PRTF shows the best performance, yielding power conversion efficiencies of up to 3.8%. Moreover, the voltage loss for the PRTF device due to the non-‍radiative recombination of free charge carriers is exceptionally low (0.26 V) as compared to typical values for organic solar cells (> 0.34 V).

R. Di Pietro, T. Erdmann, N. Wang, X. Liu, D. Gräfe, J. Lenz, J. Brandt, D. Kasemann, K. Leo, M. Al-Hussein, K. L. Gerasimov, D. Doblas, D. A. Ivanov, B. Voit, D. Neher, A. Kiriy, “The impact of molecular weight, air exposure and molecular doping on the charge transport properties and electronic defects in dithienyl-diketopyrrolopyrrole-thieno[3,2-b]thiophene copolymers”, Journal of Materials Chemistry C 4, 10827–10838 (2016), 10.1039/C6TC03545K

We performed an in-depth study of high molecular weight poly[3,6-(dithiophene-2-yl)-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-thieno[3,2-b]thiophene] P(DPP2OD-TT) synthesized through the Stille coupling polycondensation in order to understand the correlation between molecular weight, processing conditions and charge transport. We observed a rapid increase in its aggregation in solution with increasing molecular weight which strongly limits the solubility and processability for weight average molecular weights beyond 200 kg mol-1. This results in severe limitation in the charge transport properties of the polymer. We further observe the presence of bulk electronic defects in all different polymer batches that severely limit the current flow and manifest themselves in organic field effect transistors as apparent charge density dependence of the mobility. These defects are passivated by exposure to an ambient atmosphere, as confirmed by an increase in current and mobility that is no more charge density dependent. This is further confirmed by the result of chemical doping using 2,2-(perfluoronaphthalene-2,6-diylidene)dimalononitrile, F6TCNNQ, which leads to the filling of the trap states and a higher charge density independent mobility of up to 1 cm2V-1s-1.

G. Lu, R.D. Pietro, L.S. Kölln, I. Nasrallah, L. Zhou, S. Mollinger, S. Himmelberger, N. Koch, A. Salleo & D. Neher, “Dual-Characteristic Transistors Based on Semiconducting Polymer Blends”, Advanced Electronic Materials 2, 1600267 (2016), 10.1002/aelm.201600267

A dual-characteristic polymer field-effect transistor has markedly different characteristics in low and high voltage operations. In the low-voltage range (<5 V) it shows sharp subthreshold slopes (0.3–0.4 V/dec), using which a low-voltage inverter with gain 8 is realized, while high-voltage (>5 V) induces symmetric current with regard to drain and gate voltages, leading to discrete differential (trans) conductances.

T. Hahn, S. Tscheuschner, C. Saller, P. Strohriegl, P. Boregowda, T. Mukhopadhyay, S. Patil, D. Neher, H. Bässler & A. Köhler, “Role of Intrinsic Photogeneration in Single Layer and Bilayer Solar Cells with C60 and PCBM”, The Journal of Physical Chemistry C 120, 25083-25091 (2016), 10.1021/acs.jpcc.6b08471

In an endeavor to examine how optical excitation of C60 and PCBM contribute to the photogeneration of charge carriers in organic solar cells, we investigated stationary photogeneration in single-layer C60 and PCBM films over a broad spectrum as a function of the electric field. We find that intrinsic photogeneration starts at a photon energy of about 2.25 eV, i.e., about 0.4 eV above the first singlet excited state. It originates from charge transfer type states that can autoionize before relaxing to the lower-energy singlet S1 state, in the spirit of Onsager’s 1938 theory. We analyze the internal quantum efficiency as a function of electric field and photon energy to determine (1) the Coulombic binding and separation of the electron–hole pairs, (2) the value of the electrical gap, and (3) which fraction of photoexcitations can fully separate at a given photon energy. The latter depends on the coupling between the photogenerated charge transfer states and the eventual charge transporting states. It is by a factor of 3 lower in PCBM. Close to the threshold energy for intrinsic photoconduction (2.25 eV), the generating entity is a photogenerated electron–hole pair with roughly 2 nm separation. At higher photon energy, more expanded pairs are produced incoherently via thermalization.

P. Pingel, M. Arvind, L. Kölln, R. Steyrleuthner, F. Kraffert, J. Behrends, S. Janietz & D. Neher, “p-Type Doping of Poly(3-hexylthiophene) with the Strong Lewis Acid Tris(pentafluorophenyl)borane”, Advanced Electronic Materials 2, 1600204 (2016), 10.1002/aelm.201600204

State-of-the-art p-type doping of organic semiconductors is usually achieved by employing strong π-electron acceptors, a prominent example being tetrafluorotetracyanoquinodimethane (F4TCNQ). Here, doping of the semiconducting model polymer poly(3-hexylthiophene), P3HT, using the strong Lewis acid tris(pentafluorophenyl)borane (BCF) as a dopant, is investigated by admittance, conductivity, and electron paramagnetic resonance measurements. The electrical characteristics of BCF- and F4TCNQ-doped P3HT layers are shown to be very similar in terms of the mobile hole density and the doping efficiency. Roughly 18% of the employed dopants create mobile holes in either F4TCNQ- or BCF-doped P3HT, while the majority of doping-induced holes remain strongly Coulomb-bound to the dopant anions. Despite similar hole densities, conductivity and hole mobility are higher in BCF-doped P3HT layers than in F4TCNQ-doped samples. This and the good solubility in many organic solvents render BCF very useful for p-type doping of organic semiconductors.

M. Zerson, M. Neumann, R. Steyrleuthner, D. Neher & R. Magerle, “Surface Structure of Semicrystalline Naphthalene Diimide–Bithiophene Copolymer Films Studied with Atomic Force Microscopy”, Macromolecules 49, 6549-6557 (2016), DOI: 10.1021/acs.macromol.6b00988

The crystallization behavior, the surface structure, and the nanomechanical properties of a semiconducting polymer play a crucial role in understanding the charge injection process, the transport of the charge carriers and the processability of the material. Here we study the semiconducting copolymer poly([N,N′-bis(2-octyldodecyl)-11-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-12 bithiophene)) (P(NDI2OD-T2)) and investigate the influence of annealing conditions on its surface structure through intermittent contact mode atomic force microscopy (AFM) and AFM-based measurements of amplitude–phase–distance (APD) curves. For spin-cast thin films as well as for films annealed at temperatures up to 320 °C, we find that the edges of crystalline lamellae are exposed at the surface. A 1.2 nm thick layer of alkyl side chains covers the film surface as indicated by the tip indentation. This suggests that charge injection into P(NDI2OD-T2) films is not hindered by a surface layer of amorphous material. In 5 nm thick films, corresponding to two monolayers of P(NDI2OD-T2), after annealing at 320 °C, crystalline lamella also orient perpendicular to the film plane with the (100) surfaces oriented parallel to the film plane. The lamellae form ∼100 nm large areas (terraces) with uniform lamella height. The step height between adjacent terraces is 2.2 nm, and we attribute it to monomolecular steps between the molecular thin layers of edge-on-oriented polymer chains. This well-defined molecular conformation at the film surface with the chain backbone and the π-stacking direction oriented in the film plane is presumably an important factor contributing to the exceptional performance of P(NDI2OD-T2) in bottom-gate organic field-effect transistors.

L. Müller, D. Nanova, T. Glaser, S. Beck, A. Pucci, A.K. Kast, R.R. Schröder, E. Mankel, P. Pingel, D. Neher, W. Kowalsky & R. Lovrincic, “Charge-Transfer–Solvent Interaction Predefines Doping Efficiency in p-Doped P3HT Films”, Chemistry of Materials 28, 4432-4439 (2016), DOI: 10.1021/acs.chemmater.6b01629

Efficient electrical doping of organic semiconductors is a necessary prerequisite for the fabrication of high performance organic electronic devices. In this work, we study p-type doping of poly(3-hexylthiophene) (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) spin-cast from two different solvents. Using electron diffraction, we find strong dopant-induced π–π-stacking for films from the solvent chloroform, but not from chlorobenzene. This image is confirmed and expanded by the analysis of vibrational features of P3HT and polaron absorptions using optical spectroscopy. Here, a red-shifted polaron absorption is found in doped films from chloroform, caused by a higher conjugation length of the polymer backbone. These differences result in a higher conductivity of films from chloroform. We use optical spectroscopy on the corresponding blend solutions to shed light on the origin of this effect and propose a model to explain why solutions of doped P3HT reveal more aggregation of charged molecules in chlorobenzene, whereas more order is finally observed in dried films from chloroform. Our study emphasizes the importance of solvent parameters exceeding the bare solubility of pure dopant and host material for the preparation of highly conductive doped films.

J. Kurpiers & D. Neher, “Dispersive Non-Geminate Recombination in an Amorphous Polymer:Fullerene Blend”, Scientific Reports 6, 26832 (2016), DOI: 10.1038/srep26832

Recombination of free charge is a key process limiting the performance of solar cells. For low mobility materials, such as organic semiconductors, the kinetics of non-geminate recombination (NGR) is strongly linked to the motion of charges. As these materials possess significant disorder, thermalization of photogenerated carriers in the inhomogeneously broadened density of state distribution is an unavoidable process. Despite its general importance, knowledge about the kinetics of NGR in complete organic solar cells is rather limited. We employ time delayed collection field (TDCF) experiments to study the recombination of photogenerated charge in the high-performance polymer:fullerene blend PCDTBT:PCBM. NGR in the bulk of this amorphous blend is shown to be highly dispersive, with a continuous reduction of the recombination coefficient throughout the entire time scale, until all charge carriers have either been extracted or recombined. Rapid, contact-mediated recombination is identified as an additional loss channel, which, if not properly taken into account, would erroneously suggest a pronounced field dependence of charge generation. These findings are in stark contrast to the results of TDCF experiments on photovoltaic devices made from ordered blends, such as P3HT:PCBM, where non-dispersive recombination was proven to dominate the charge carrier dynamics under application relevant conditions.

D. Neher, J. Kniepert, A. Elimelech & L.J.A. Koster, “A New Figure of Merit for Organic Solar Cells with Transport-limited Photocurrents”, Scientific Reports 6, 24861 (2016), DOI: 10.1038/srep24861

Compared to their inorganic counterparts, organic semiconductors suffer from relatively low charge carrier mobilities. Therefore, expressions derived for inorganic solar cells to correlate characteristic performance parameters to material properties are prone to fail when applied to organic devices. This is especially true for the classical Shockley-equation commonly used to describe current-voltage (JV)-curves, as it assumes a high electrical conductivity of the charge transporting material. Here, an analytical expression for the JV-curves of organic solar cells is derived based on a previously published analytical model. This expression, bearing a similar functional dependence as the Shockley-equation, delivers a new figure of merit α to express the balance between free charge recombination and extraction in low mobility photoactive materials. This figure of merit is shown to determine critical device parameters such as the apparent series resistance and the fill factor.

A. Paulke, S.D. Stranks, J. Kniepert, J. Kurpiers, C.M. Wolff, N. Schön, H.J. Snaith, T.J.K. Brenner & D. Neher, “Charge carrier recombination dynamics in perovskite and polymer solar cells”, Applied Physics Letters 108, 113505 (2016), DOI: 10.1063/1.4944044

Time-delayed collection field experiments are applied to planar organometal halide perovskite (CH3NH3PbI3) based solar cells to investigate charge carrier recombination in a fully working solar cell at the nanosecond to microsecond time scale. Recombination of mobile (extractable) charges is shown to follow second-order recombination dynamics for all fluences and time scales tested. Most importantly, the bimolecular recombination coefficient is found to be time-dependent, with an initial value of ca. 10−9 cm3/s and a progressive reduction within the first tens of nanoseconds. Comparison to the prototypical organic bulk heterojunction device PTB7:PC71BM yields important differences with regard to the mechanism and time scale of free carrier recombination.

J. Kurpiers, D.M. Balazs, A. Paulke, S. Albrecht, I. Lange, L. Protesescu, M.V. Kovalenko, M.A. Loi & D. Neher, “Free carrier generation and recombination in PbS quantum dot solar cells”, Applied Physics Letters 108, 103102 (2016), DOI: 10.1063/1.4943379

Time Delayed Collection Field and Bias Assisted Charge Extraction (BACE) experiments are used to investigate the charge carrier dynamics in PbS colloidal quantum dot solar cells. We find that the free charge carrier creation is slightly field dependent, thus providing an upper limit to the fill factor. The BACE measurements reveal a rather high effective mobility of 2×10-3 cm2/Vs, meaning that charge extraction is efficient. On the other hand, a rather high steady state non-geminate recombination coefficient of 3×10-10 cm3/s is measured. We, therefore, propose a rapid free charge recombination to constitute the main origin for the limited efficiency of the PbS colloidal quantum dots cells.

G. Ligorio, M.V. Nardi, R. Steyrleuthner, D. Ihiawakrim, N. Crespo-Monteiro, M. Brinkmann, D. Neher & N. Koch, “Metal nanoparticle mediated space charge and its optical control in an organic hole-only device”, Applied Physics Letters 108, 153302 (2016), DOI: 10.1063/1.4945710

We reveal the role of localized space charges in hole-only devices based on an organic semiconductor with embedded metal nanoparticles (MNPs). MNPs act as deep traps for holes and reduce the current density compared to a device without MNPs by a factor of 104 due to the build-up of localized space charge. Dynamic MNPs charged neutrality can be realized during operation by electron transfer from excitons created in the organic matrix, enabling light sensing independent of device bias. In contrast to the previous speculations, electrical bistability in such devices was not observed.

A.M. Anton, R. Steyrleuthner, W. Kossack, D. Neher & F. Kremer, “Spatial Orientation and Order of Structure-Defining Subunits in Thin Films of a High Mobility n-Type Copolymer”, Macromolecules 49, 1798-1806 (2016), DOI: 10.1021/acs.macromol.5b02420

Orientation and order of distinct molecular subunits in solid layers of the high mobility n-type copolymer poly[N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalenediimide-2,6-diyl]-5,5′-(2,2′-bithiophene) P(NDI2OD-T2) are investigated by means of infrared transition moment orientational analysis. This novel spectroscopic technique based on concurrent absorbance measurements of structure-specific bands in dependence on inclination and polarization of the incoming light enables to determine the complete tensor of absorption independently for each transition moment. As a result, for nanometer thin films pronounced in-plane anisotropy arising from self-aggregated order is detected, which, however, is no longer discernible for micrometer thick samples. In contrast, the out-of-plane orientation (inclination of molecular subunits) is retained irrespective of the widely varying layer thicknesses (150 nm vs 1.4 μm). Thus, the conception of the sample morphology occurs as stratification of slightly misaligned layers of oriented polymers; with increasing film thickness the macroscopic in-plane order diminishes, whereas the out-of-plane orientation is preserved.


G. Lu, N. Koch & D. Neher, “In-situ tuning threshold voltage of field-effect transistors based on blends of poly(3-hexylthiophene) with an insulator electret”, Applied Physics Letters 107, 063301 (2015), DOI: 10.1063/1.4928554

Blending the conjugated polymer poly(3-hexylthiophene) (P3HT) with the insulating electret polystyrene (PS), we show that the threshold voltage Vt of organic field-effect transistors (OFETs) can be easily and reversely tuned by applying a gate bias stress at 130°C. It is proposed that this phenomenon is caused by thermally activated charge injection from P3HT into PS matrix, and that this charge is immobilized within the PS matrix after cooling down to room temperature. Therefore, room-temperature hysteresis-free FETs with desired Vt can be easily achieved. The approach is applied to reversely tune the OFET mode of operation from accumulation to depletion, and to build inverters.

J. Xu, M. Shalom, F. Piersimoni, M. Antonietti, D. Neher & T.J.K. Brenner, “Color-Tunable Photoluminescence and NIR Electroluminescence in Carbon Nitride Thin Films and Light-Emitting Diodes”, Advanced Optical Materials 3, 913-917 (2015), DOI: 10.1002/adom.201500019

Color-tunable photoluminescence from continuous graphitic carbon nitride (g-C3N4) thin films is demonstrated. The films are employed in light-emitting diodes. A strong redshift of the electroluminescence with respect to photo­luminescence is observed and a model for this uncommon behavior is provided.

D. Bartesaghi, I.d.C. Perez, J. Kniepert, S. Roland, M. Turbiez, D. Neher & L.J.A. Koster, “Competition between recombination and extraction of free charges determines the fill factor of organic solar cells”, Nature Communications 6, 7083 (2015), DOI: 10.1038/ncomms8083

Among the parameters that characterize a solar cell and define its power-conversion efficiency, the fill factor is the least well understood, making targeted improvements difficult. Here we quantify the competition between charge extraction and recombination by using a single parameter [theta], and we demonstrate that this parameter is directly related to the fill factor of many different bulk-heterojunction solar cells. Our finding is supported by experimental measurements on 15 different donor:acceptor combinations, as well as by drift-diffusion simulations of organic solar cells in which charge-carrier mobilities, recombination rate, light intensity, energy levels and active-layer thickness are all varied over wide ranges to reproduce typical experimental conditions. The results unify the fill factors of several very different donor:acceptor combinations and give insight into why fill factors change so much with thickness, light intensity and materials properties. To achieve fill factors larger than 0.8 requires further improvements in charge transport while reducing recombination.

A.M. Anton, R. Steyrleuthner, W. Kossack, D. Neher & F. Kremer, “Infrared Transition Moment Orientational Analysis on the Structural Organization of the Distinct Molecular Subunits in Thin Layers of a High Mobility n-Type Copolymer”, Journal of the American Chemical Society 137, 6034-6043 (2015), DOI: 10.1021/jacs.5b01755

The IR-based method of infrared transition moment orientational analysis (IR-TMOA) is employed to unravel molecular order in thin layers of the semiconducting polymer poly[N,N'-bis(2-octyldodecyl)-1,4,5,8-naphthalenediimide-2,6-diyl]-alt-5,5'-(2,2'-bithiophene) (P(NDI2OD-T2)). Structure-specific vibrational bands are analyzed in dependence on polarization and inclination of the sample with respect to the optical axis. By that the molecular order parameter tensor for the respective molecular moieties with regard to the sample coordinate system is deduced. Making use of the specificity of the IR spectral range, we are able to determine separately the orientation of atomistic planes defined through the naphthalenediimide (NDI) and bithiophene (T2) units relative to the substrate, and hence, relative to each other. A pronounced solvent effect is observed: While chlorobenzene causes the T2 planes to align preferentially parallel to the substrate at an angle of 29°, using a 1:1 chloronaphthalene:xylene mixture results in a reorientation of the T2 units from a face on into an edge on arrangement. In contrast the NDI unit remains unaffected. Additionally, for both solvents evidence is observed for the aggregation of chains in accord with recently published results obtained by UV-vis absorption spectroscopy.

U. Würfel, D. Neher, A. Spies & S. Albrecht, “Impact of charge transport on current-voltage characteristics and power-conversion efficiency of organic solar cells”, Nature Communications 6, 6951 (2015), DOI: 10.1038/ncomms7951

This work elucidates the impact of charge transport on the photovoltaic properties of organic solar cells. Here we show that the analysis of current-voltage curves of organic solar cells under illumination with the Shockley equation results in values for ideality factor, photocurrent and parallel resistance, which lack physical meaning. Drift-diffusion simulations for a wide range of charge-carrier mobilities and illumination intensities reveal significant carrier accumulation caused by poor transport properties, which is not included in the Shockley equation. As a consequence, the separation of the quasi Fermi levels in the organic photoactive layer (internal voltage) differs substantially from the external voltage for almost all conditions. We present a new analytical model, which considers carrier transport explicitly. The model shows excellent agreement with full drift-diffusion simulations over a wide range of mobilities and illumination intensities, making it suitable for realistic efficiency predictions for organic solar cells.

J. Kniepert, I. Lange, J. Heidbrink, J. Kurpiers, T.J.K. Brenner, L.J.A. Koster & D. Neher, “Effect of Solvent Additive on Generation, Recombination, and Extraction in PTB7:PCBM Solar Cells: A Conclusive Experimental and Numerical Simulation Study”, The Journal of Physical Chemistry C 119, 8310-8320 (2015), DOI: 10.1021/jp512721e

Time-delayed collection field (TDCF), bias-assisted charge extraction (BACE), and space charge-limited current (SCLC) measurements are combined with complete numerical device simulations to unveil the effect of the solvent additive 1,8-diiodooctane (DIO) on the performance of PTB7:PCBM bulk heterojunction solar cells. DIO is shown to increase the charge generation rate, reduce geminate and bimolecular recombination, and increase the electron mobility. In total, the reduction of loss currents by processing with the additive raises the power conversion efficiency of the PTB7:PCBM blend by a factor of almost three. The lower generation rates and higher geminate recombination losses in devices without DIO are consistent with a blend morphology comprising large fullerene clusters embedded within a PTB7-rich matrix, while the low electron mobility suggests that these fullerene clusters are themselves composed of smaller pure fullerene aggregates separated by disordered areas. Our device simulations show unambiguously that the effect of the additive on the shape of the current–voltage curve (JV) cannot be ascribed to the variation of only the mobility, the recombination, or the field dependence of generation. It is only when the changes of all three parameters are taken into account that the simulation matches the experimental JV characteristics under all illumination conditions and for a wide range of voltages.

L. Lysyakova, N. Lomadze, D. Neher, K. Maximova, A.V. Kabashin & S. Santer, “Light-Tunable Plasmonic Nanoarchitectures Using Gold Nanoparticle–Azobenzene-Containing Cationic Surfactant Complexes”, The Journal of Physical Chemistry C 119, 3762-3770 (2015), DOI: 10.1021/jp511232g

When arranged in a proper nanoaggregate architecture, gold nanoparticles can offer controllable plasmon-related absorption/scattering, yielding distinct color effects that depend critically on the relative orientation and distance between nanoparticle constituents. Herein, we report on the implementation of novel plasmonic nanoarchitectures based on complexes between gold nanoparticles and an azobenzene-modified cationic surfactant that can exhibit a light-tunable plasmonic response. The formation of such complexes becomes possible through the use of strongly negatively charged bare gold nanoparticles (~10 nm diameter) prepared by the method of laser ablation in deionized water. Driven by electrostatic interactions, the cationic surfactant molecules attach and form a shell around the negatively charged nanoparticles, resulting in neutralization of the particle charge or even overcompensation beyond which the nanoparticles become positively charged. At low and high surfactant concentrations, Au nanoparticles are negatively and positively charged, respectively, and are represented by single species due to electric repulsion effects having absorption peaks around 523–527 nm, whereas at intermediate concentrations, the Au nanoparticles become neutral, forming nanoscale 100-nm clusterlike aggregates and exhibiting an additional absorption peak at λ > 600 nm and a visible change in the color of the solution from red to blue. Because of the presence of the photosensitive azobenzene unit in the surfactant tail that undergoes trans-to-cis isomerization under irradiation with UV light, we then demonstrate a light-controlled nanoclustering of nanoparticles, yielding a switch in the plasmonic absorption band and a related change in the solution color. The formed hybrid architectures with a light-controlled plasmonic response could be important for a variety of tasks, including biomedical, surface-enhanced Raman spectroscopy (SERS), data transmission, and storage applications.

F. Piersimoni, R. Schlesinger, J. Benduhn, D. Spoltore, S. Reiter, I. Lange, N. Koch, K. Vandewal & D. Neher, “Charge Transfer Absorption and Emission at ZnO/Organic Interfaces”, The Journal of Physical Chemistry Letters 6, 500-504 (2015), DOI: 10.1021/jz502657z

We investigate hybrid charge transfer states (HCTS) at the planar interface between α-NPD and ZnO by spectrally resolved electroluminescence (EL) and external quantum efficiency (EQE) measurements. Radiative decay of HCTSs is proven by distinct emission peaks in the EL spectra of such bilayer devices in the NIR at energies well below the bulk α-NPD or ZnO emission. The EQE spectra display low energy contributions clearly red-shifted with respect to the α-NPD photocurrent and partially overlapping with the EL emission. Tuning of the energy gap between the ZnO conduction band and α-NPD HOMO level (Eint) was achieved by modifying the ZnO surface with self-assembled monolayers based on phosphonic acids. We find a linear dependence of the peak position of the NIR EL on Eint, which unambiguously attributes the origin of this emission to radiative recombination between an electron on the ZnO and a hole on α-NPD. In accordance with this interpretation, we find a strictly linear relation between the open-circuit voltage and the energy of the charge state for such hybrid organic/inorganic interfaces.

I. Lange, S. Reiter, J. Kniepert, F. Piersimoni, M. Pätzel, J. Hildebrandt, T. Brenner, S. Hecht & D. Neher, “Zinc oxide modified with benzylphosphonic acids as transparent electrodes in regular and inverted organic solar cell structures”, Applied Physics Letters 106, 113302 (2015), DOI: 10.1063/1.4916182

An approach is presented to modify the work function of solution-processed sol-gel derived zinc oxide (ZnO) over an exceptionally wide range of more than 2.3 eV. This approach relies on the formation of dense and homogeneous self-assembled monolayers based on phosphonic acids with different dipole moments. This allows us to apply ZnO as charge selective bottom electrodes in either regular or inverted solar cell structures, using poly(3-hexylthiophene):phenyl-C71-butyric acid methyl ester as the active layer. These devices compete with or even surpass the performance of the reference on indium tin oxide/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate. Our findings highlight the potential of properly modified ZnO as electron or hole extracting electrodes in hybrid optoelectronic devices.

S. Roland, S. Neubert, S. Albrecht, B. Stannowski, M. Seger, A. Facchetti, R. Schlatmann, B. Rech & D. Neher, “Hybrid Organic/Inorganic Thin-Film Multijunction Solar Cells Exceeding 11% Power Conversion Efficiency”, Advanced Materials 27, 1262-1267 (2015), DOI: 10.1002/adma.201404698

Hybrid multijunction solar cells comprising hydrogenated amorphous silicon and an organic bulk heterojunction are presented, reaching 11.7% power conversion efficiency. The benefits of merging inorganic and organic subcells are pointed out, the optimization of the cells, including optical modeling predictions and tuning of the recombination contact are described, and an outlook of this technique is given.

F.S.U. Fischer, D. Trefz, J. Back, N. Kayunkid, B. Tornow, S. Albrecht, K.G. Yager, G. Singh, A. Karim, D. Neher, M. Brinkmann & S. Ludwigs, “Highly Crystalline Films of PCPDTBT with Branched Side Chains by Solvent Vapor Crystallization: Influence on Opto-Electronic Properties”, Advanced Materials 27, 1223-1228 (2015), DOI: 10.1002/adma.201403475

PCPDTBT, a marginally crystallizable polymer, is crystallized into a new crystal structure using solvent-vapor annealing. Highly ordered areas with three different polymer-chain orientations are identified using TEM/ED, GIWAXS, and polarized Raman spectroscopy. The optical and structural properties differ significantly from films prepared by standard device preparation protocols. Bilayer solar cells, however, show similar performance.

F. Ghani, A. Opitz, P. Pingel, G. Heimel, I. Salzmann, J. Frisch, D. Neher, A. Tsami, U. Scherf & N. Koch, “Charge transfer in and conductivity of molecularly doped thiophene-based copolymers”, Journal of Polymer Science Part B: Polymer Physics 53, 58-63 (2015), DOI: 10.1002/polb.23631

The electrical conductivity of organic semiconductors can be enhanced by orders of magnitude via doping with strong molecular electron acceptors or donors. Ground-state integer charge transfer and charge-transfer complex formation between organic semiconductors and molecular dopants have been suggested as the microscopic mechanisms causing these profound changes in electrical materials properties. Here, we study charge-transfer interactions between the common molecular p-dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane and a systematic series of thiophene-based copolymers by a combination of spectroscopic techniques and electrical measurements. Subtle variations in chemical structure are seen to significantly impact the nature of the charge-transfer species and the efficiency of the doping process, underlining the need for a more detailed understanding of the microscopic doping mechanism in organic semiconductors to reliably guide targeted chemical design.