N. Tokmoldin, J. Vollbrecht, S. M. Hosseini, B. Sun, L. Perdigón-Toro, H. Y. Woo, Y. Zou, D. Neher, S. Shoaee* Explaining the Fill-Factor and Photocurrent Losses of Nonfullerene Acceptor-Based Solar Cells by Probing the Long-Range Charge Carrier Diffusion and Drift Lengths. Advanced Energy Materials. 10 2100804 (2021) https://doi.org/10.1002/aenm.202100804
Organic solar cells (OSC) nowadays match their inorganic competitors in terms of current production but lag behind with regards to their open-circuit voltage loss and fill-factor, with state-of-the-art OSCs rarely displaying fill-factor of 80% and above. The fill-factor of transport-limited solar cells, including organic photovoltaic devices, is affected by material and device-specific parameters, whose combination is represented in terms of the established figures of merit, such as θ and α. Herein, it is demonstrated that these figures of merit are closely related to the long-range carrier drift and diffusion lengths. Further, a simple approach is presented to devise these characteristic lengths using steady-state photoconductance measurements. This yields a straightforward way of determining θ and α in complete cells and under operating conditions. This approach is applied to a variety of photovoltaic devices—including the high efficiency nonfullerene acceptor blends—and show that the diffusion length of the free carriers provides a good correlation with the fill-factor. It is, finally, concluded that most state-of-the-art organic solar cells exhibit a sufficiently large drift length to guarantee efficient charge extraction at short circuit, but that they still suffer from too small diffusion lengths of photogenerated carriers limiting their fill factor.
N. Gasparini, F. V. A. Camargo, S. Frühwald, T. Nagahara, A.Classen, S. Roland, A. Wadsworth, V.G. Gregoriou, C. L. Chochos, D. Neher, M. Salvador, D-Baran, I. McCulloch, A. Görling, L. Lüer, G. Cerullo & C. J. Brabec J. Benduhn, M. Nyman, S. M. Hosseini, J. Kublitski, S. Shoaee, D. Neher, K. Leo, D. Spoltore* Adjusting the energy of interfacial states in organic photovoltaics for maximum efficiency. Nature Communications. 12, Article number: 1772 (2021). https://doi.org/10.1038/s41467-021-22032-3
A critical bottleneck for improving the performance of organic solar cells (OSC) is minimising non-radiative losses in the interfacial charge-transfer (CT) state via the formation of hybrid energetic states. This requires small energetic offsets often detrimental for high external quantum efficiency (EQE). Here, we obtain OSC with both non-radiative voltage losses (0.24 V) and photocurrent losses (EQE > 80%) simultaneously minimised. The interfacial CT states separate into free carriers with ≈40-ps time constant. We combine device and spectroscopic data to model the thermodynamics of charge separation and extraction, revealing that the relatively high performance of the devices arises from an optimal adjustment of the CT state energy, which determines how the available overall driving force is efficiently used to maximize both exciton splitting and charge separation. The model proposed is universal for donor:acceptor (D:A) with low driving forces and predicts which D:A will benefit from a morphology optimization for highly efficient OSC.
M. Pranav, J. Benduhn, M. Nyman, S. M. Hosseini, J. Kublitski, S. Shoaee, D. Neher, K. Leo, D. Spoltore* Enhanced Charge Selectivity via Anodic-C60 Layer Reduces Nonradiative Losses in Organic Solar Cells. ACS Appl. Mater. Interfaces. 2021, 13, 10, 12603–12609. https://doi.org/10.1021/acsami.1c00049.
Interfacial layers in conjunction with suitable charge-transport layers can significantly improve the performance of optoelectronic devices by facilitating efficient charge carrier injection and extraction. This work uses a neat C60 interlayer on the anode to experimentally reveal that surface recombination is a significant contributor to nonradiative recombination losses in organic solar cells. These losses are shown to proportionally increase with the extent of contact between donor molecules in the photoactive layer and a molybdenum oxide (MoO3) hole extraction layer, proven by calculating voltage losses in low- and high-donor-content bulk heterojunction device architectures. Using a novel in-device determination of the built-in voltage, the suppression of surface recombination, due to the insertion of a thin anodic-C60 interlayer on MoO3, is attributed to an enhanced built-in potential. The increased built-in voltage reduces the presence of minority charge carriers at the electrodes—a new perspective on the principle of selective charge extraction layers. The benefit to device efficiency is limited by a critical interlayer thickness, which depends on the donor material in bilayer devices. Given the high popularity of MoO3 as an efficient hole extraction and injection layer and the increasingly popular discussion on interfacial phenomena in organic optoelectronic devices, these findings are relevant to and address different branches of organic electronics, providing insights for future device design.
E. Gutierrez-Partida, H. Hempel, S. Caicedo-Dávila, M. Raoufi, F. Peña-Camargo, M. Grischek, R. Gunder, J. Diekmann, P. Caprioglio, K. O. Brinkmann, H. Köbler, S. Albrecht, T. Riedl, A. Abate, D. Abou-Ras, T. Unold, D. Neher and M. Stolterfoht*Large-Grain Double Cation Perovskites with 18 μs Lifetime and High Luminescence Yield for Efficient Inverted Perovskite Solar Cells. ACS Energy Lett. 2021, 6, XXX, 1045-1054. pubs.acs.org/doi/10.1021/acsenergylett.0c02642.
Recent advancements in perovskite solar cell performance were achieved by stabilizing the α-phase of FAPbI3 in nip-type architectures. However, these advancements could not be directly translated to pin-type devices. Here, we fabricated a high-quality double cation perovskite (MA0.07FA0.93PbI3) with low bandgap energy (1.54 eV) using a two-step approach on a standard polymer (PTAA). The perovskite films exhibit large grains (∼1 μm), high external photoluminescence quantum yields of 20%, and outstanding Shockley–Read–Hall carrier lifetimes of 18.2 μs without further passivation. The exceptional optoelectronic quality of the neat material was translated into efficient pin-type cells (up to 22.5%) with improved stability under illumination. The low-gap cells stand out by their high fill factor (∼83%) due to reduced charge transport losses and short-circuit currents >24 mA cm–2. Using intensity-dependent quasi-Fermi level splitting measurements, we quantify an implied efficiency of 28.4% in the neat material, which can be realized by minimizing interfacial recombination and optical losses.
L. Perdigón-Toro, L. Q. Phuong, S. Zeiske, K. Vandewal, A. Armin, S. Shoaee, D. Neher, "Excitons Dominate the Emission from PM6:Y6 Solar Cells, but This Does Not Help the Open-Circuit Voltage of the Device", ACS Energy letters 557-564, (2021), DOI: 10.1021/acsenergylett.0c02572
Non-fullerene acceptors (NFAs) are far more emissive than their fullerene-based counterparts. Here, we study the spectral properties of photocurrent generation and recombination of the blend of the donor polymer PM6 with the NFA Y6. We find that the radiative recombination of free charges is almost entirely due to the re-occupation and decay of Y6 singlet excitons, but that this pathway contributes less than 1% to the total recombination. As such, the open-circuit voltage of the PM6:Y6 blend is determined by the energetics and kinetics of the charge-transfer (CT) state. Moreover, we find that no information on the energetics of the CT state manifold can be gained from the low-energy tail of the photovoltaic external quantum efficiency spectrum, which is dominated by the excitation spectrum of the Y6 exciton. We, finally, estimate the charge-separated state to lie only 120 meV below the Y6 singlet exciton energy, meaning that this blend indeed represents a high-efficiency system with a low energetic offset.
P. Caprioglio, S. Caicedo-Dávila, T. C-J. Yang, C. M. Wolff, F. Peña-Camargo, P. Fiala, B. Rech, C. Ballif, D. Abou-Ras, M. Stolterfoht, S. Albrecht, Q. Jeangros, D. Neher, "Nano-emitting heterostructures Violate Optical Reciprocity and Enable Efficient Photoluminescence in Halide-Segregated Methylammonium-Free Wide Bandgap Perovskites", ACS Energy letters 419-428, (2021), DOI: 10.1021/acsenergylett.0c02270
This work investigates halide segregation in methylammonium-free wide bandgap perovskites by photoluminescence quantum yield (PLQY) and advanced electron microscopy techniques. Our study reveals how the formation of nano-emitting low-energy domains embedded in a wide bandgap matrix, located at surfaces and grain boundaries, enables a PLQY up to 25%. Intensity-dependent PLQY measurement and PL excitation spectroscopy revealed efficient charge funnelling and the failure of optical reciprocity between absorption and emission, limiting the use of PLQY data to determine the quasi-Fermi level splitting (QFLS) in these layers. Concomitantly, the small spectral overlap between emission and absorption reduces photon re-absorption. We demonstrate that phase segregation and charge funnelling, although harmful for the radiative efficiency of the mixed phase, are essential for achieving high PLQYs, selectively at low energies, otherwise not achievable in non-segregated perovskites with a similar bandgap. This promotes the applicability of this phenomenon in thermally stable high-efficiency emitting devices and color-conversion heterostructures.
L. Q. Phuong, S. M. Hosseini, O. J. Sandberg, Y. Zou, H. Y. Woo, D. Neher, S. Shoaee, “Quantifying Quasi-Fermi Level Splitting and Open-Circuit Voltage Losses in Highly Efficient Nonfullerene Organic Solar Cells”, Solar RRL 2000649, (2020), DOI: 10.1002/solr.202000649
The power conversion efficiency (PCE) of state-of-the-art organic solar cells is still limited by significant open-circuit voltage (VOC) losses, partly due to the excitonic nature of organic materials and partly due to ill-designed architectures. Thus, quantifying different contributions of the VOC losses is of importance to enable further improvements in the performance of organic solar cells. Herein, the spectroscopic and semiconductor device physics approaches are combined to identify and quantify losses from surface recombination and bulk recombination. Several state-of-the-art systems that demonstrate different VOC losses in their performance are presented. By evaluating the quasi-Fermi level splitting (QFLS) and the VOC as a function of the excitation fluence in nonfullerene-based PM6:Y6, PM6:Y11, and fullerene-based PPDT2FBT:PCBM devices with different architectures, the voltage losses due to different recombination processes occurring in the active layers, the transport layers, and at the interfaces are assessed. It is found that surface recombination at interfaces in the studied solar cells is negligible, and thus, suppressing the non-radiative recombination in the active layers is the key factor to enhance the PCE of these devices. This study provides a universal tool to explain and further improve the performance of recently demonstrated high-open-circuit-voltage organic solar cells.
S. Samson, J. Rech, L. Perdigón-Toro, Z. Peng, S. Shoaee, H. Ade, D. Neher, M. Stolterfoht, W. You, “Organic Solar Cells with Large Insensitivity to Donor Polymer Molar Mass across All Acceptor Classes”, ACS Applied Polymer Materials 2, 5300–5308 (2020), DOI: 10.1021/acsapm.0c01041
Donor polymer number-average molar mass (Mn) has long been known to influence organic photovoltaic (OPV) performance via changes in both the polymer properties and the resulting bulk heterojunction morphology. The exact nature of these Mn effects varies from system to system, although there is generally some intermediate Mn that results in optimal performance. Interestingly, our earlier work with the difluorobenzotriazole (FTAZ)-based donor polymer, paired with either N2200 (polymer acceptor) or PC61BM (fullerene acceptor), demonstrated <10% variation in power conversion efficiency and a consistent morphology over a large span of Mn (30 kg/mol to over 100 kg/mol). Would such insensitivity to polymer Mn still hold true when prevailing small molecular acceptors were used with FTAZ? To answer this question, we explored the impact of FTAZ Mn on OPVs with ITIC, a high-performance small-molecule fused-ring electron acceptor (FREA). By probing the photovoltaic characteristics of the resulting OPVs, we show that a similar FTAZ Mn insensitivity is also found in the FTAZ:ITIC system. This study highlights a single-donor polymer which, when paired with an archetypal fullerene, polymer, and FREA, results in systems that are largely insensitive to donor Mn. Our results may have implications in polymer batch-to-batch reproducibility, in particular, relaxing the need for tight Mn control during synthesis.
S. M. Hosseini, N. Tokmoldin, Y. W. Lee, Y. Zou, H. Y. Woo, D. Neher, S. Shoaee, “Putting Order into PM6:Y6 Solar Cells to Reduce the Langevin Recombination in 400 nm Thick Junction”, Solar RRL 4, 2000498 (2020), DOI: 10.1002/solr.202000498
Increasing the active layer thickness without sacrificing the power conversion efficiency (PCE) is one of the great challenges faced by organic solar cells (OSCs) for commercialization. Recently, PM6:Y6 as an OSC based on a non-fullerene acceptor (NFA) has excited the community because of its PCE reaching as high as 15.9%; however, by increasing the thickness, the PCE drops due to the reduction of the fill factor (FF). This drop is attributed to change in mobility ratio with increasing thickness. Furthermore, this work demonstrates that by regulating the packing and the crystallinity of the donor and the acceptor, through volumetric content of chloronaphthalene (CN) as a solvent additive, one can improve the FF of a thick PM6:Y6 device (≈400 nm) from 58% to 68% (PCE enhances from 12.2% to 14.4%). The data indicate that the origin of this enhancement is the reduction of the structural and energetic disorders in the thick device with 1.5% CN compared with 0.5% CN. This correlates with improved electron and hole mobilities and a 50% suppressed bimolecular recombination, such that the non-Langevin reduction factor is 180 times. This work reveals the role of disorder on the charge extraction and bimolecular recombination of NFA-based OSCs.
M. Arvind, C. E. Tait, M. Guerrini, J. Krumland, A. M. Valencia, C. Cocchi, A. E. Mansour, N. Koch, S. Barlow, S. R. Marder, J. Behrends, D. Neher, “Quantitative Analysis of Doping-Induced Polarons and Charge-Transfer Complexes of Poly(3-hexylthiophene) in Solution”, The Journal of Physical Chemistry B 124, 7694–7708 (2020), DOI: 10.1021/acs.jpcb.0c03517
The mechanism and the nature of the species formed by molecular doping of the model polymer poly(3-hexylthiophene) (P3HT) in its regioregular (rre-) and regiorandom (rra-) forms in solution are investigated for three different dopants: the prototypical π-electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), the strong Lewis acid tris(pentafluorophenyl)borane (BCF), and the strongly oxidizing complex molybdenum tris[1-(methoxycarbonyl)-2-(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd-CO2Me)3). In a combined optical and electron paramagnetic resonance study, we show that the doping of rreP3HT in solution occurs by integer charge transfer, resulting in formation of P3HT radical cations (polarons) for all of the dopants considered here. Remarkably, despite the different chemical nature of the dopants and dopant-polymer interaction, the formed polarons exhibit essentially identical optical absorption spectra. The situation is very different for the doping of rraP3HT, where we observe formation of a charge-transfer complex with F4TCNQ and of a "localized" P3HT polaron on nonaggregated chains upon doping with BCF, while there is no indication of dopant-induced species in the case of Mo(tfd-CO2Me)3. We estimate the ionization efficiency of the respective dopants for the two polymers in solution and report the molar extinction coefficient spectra of the three different species. Finally, we observe increased spin delocalization in regioregular compared to regiorandom P3HT by electron nuclear double resonance, suggesting that the ability of the charge to delocalize on aggregates of planarized polymer backbones plays a significant role in determining the doping mechanism.
S. Zhang, P. E. Shaw, G. Zhang, H. Jin, M. Tai, H. Lin, P. Meredith, P. L. Burn, D. Neher, M. Stolterfoht, “Defect/Interface Recombination Limited Quasi-Fermi Level Splitting and Open-Circuit Voltage in Mono- and Triple-Cation Perovskite Solar Cells”, ACS Applied Materials & Interfaces 12, 37647–37656 (2020), DOI: 10.1021/acsami.0c02960
Multication metal-halide perovskites exhibit desirable performance and stability, compared to their monocation counterparts. However, the study of the photophysical properties and the nature of defect states in these materials is still a challenging and ongoing task. Here, we study bulk and interfacial energy loss mechanisms in solution-processed MAPbI3 (MAPI) and (CsPbI3)0.05[(FAPbI3)0.83(MAPbBr3)0.17]0.95 (triple cation) perovskite solar cells using absolute photoluminescence (PL) measurements. In neat MAPI films, we find a significantly smaller quasi-Fermi level splitting than for the triple cation perovskite absorbers, which defines the open-circuit voltage of the MAPI cells. PL measurements at low temperatures (20 K) on MAPI films demonstrate that emissive subgap states can be effectively reduced using different passivating agents, which lowers the nonradiative recombination loss at room temperature. We conclude that while triple cation perovskite cells are limited by interfacial recombination, the passivation of surface trap states within the MAPI films is the primary consideration for device optimization.
F. Peña-Camargo, P. Caprioglio, F. Zu, E. Gutierrez-Partida, C. M. Wolff, K. Brinkmann, S. Albrecht, T. Riedl, N. Koch, D. Neher, M. Stolterfoht, “Halide Segregation versus Interfacial Recombination in Bromide-Rich Wide-Gap Perovskite Solar Cells”, ACS Energy Letters 5, 2728–2736 (2020), DOI: 10.1021/acsenergylett.0c01104
Perovskites offer exciting opportunities to realize efficient multijunction photovoltaic devices. This requires high-VOC and often Br-rich perovskites, which currently suffer from halide segregation. Here, we study triple-cation perovskite cells over a wide bandgap range (∼1.5-1.9 eV). While all wide-gap cells (≥1.69 eV) experience rapid phase segregation under illumination, the electroluminescence spectra are less affected by this process. The measurements reveal a low radiative efficiency of the mixed halide phase which explains the VOC losses with increasing Br content. Photoluminescence measurements on nonsegregated partial cell stacks demonstrate that both transport layers (PTAA and C60) induce significant nonradiative interfacial recombination, especially in Br-rich (>30%) samples. Therefore, the presence of the segregated iodide-rich domains is not directly responsible for the VOC losses. Moreover, LiF can only improve the VOC of cells that are primarily limited by the n-interface (≤1.75 eV), resulting in 20% efficient 1.7 eV bandgap cells. However, a simultaneous optimization of the p-interface is necessary to further advance larger bandgap (≥1.75 eV) pin-type cells.
Q. Wang, F. Zu, P. Caprioglio, C. M. Wolff, M. Stolterfoht, M. Li, S.-H. Turren-Cruz, N. Koch, D. Neher, A. Abate, “Large Conduction Band Energy Offset Is Critical for High Fill Factors in Inorganic Perovskite Solar Cells”, ACS Energy Letters 5, 2343–2348 (2020), DOI: 10.1021/acsenergylett.0c00980
Although SnO2 has been reported to give high efficiencies of over 20% for organic-inorganic perovskite solar cells and has been frequently used in perovskite tandem solar cells, very few contributions have explored its feasibility in inorganic perovskite solar cells (IPSCs). Inorganic perovskites with a wide bandgap tunable from 1.7 to 2.0 eV are promising candidates for top cells in tandem structures; development of IPSCs based on SnO2 will greatly benefit their integration into tandem solar cells. We examined SnO2 in comparison to the prevalent TiO2. We found that although SnO2 had a good energy alignment with the inorganic perovskite and exhibited slower nonradiative recombination, the relatively low conduction band minimum energy offset restricted efficient charge extraction. In contrast, TiO2 that had a large energy offset of ∼400 meV led to a high fill factor of 78.7% and a state-of-the-art efficiency of 14.2% for IPSCs with a bandgap of 1.93 eV.
P. Caprioglio, C. M. Wolff, O. J. Sandberg, A. Armin, B. Rech, S. Albrecht, D. Neher, M. Stolterfoht, “On the Origin of the Ideality Factor in Perovskite Solar Cells”, Advanced Energy Materials 10, 2000502 (2020), DOI: 10.1002/aenm.202000502
The measurement of the ideality factor (nid) is a popular tool to infer the dominant recombination type in perovskite solar cells (PSC). However, the true meaning of its values is often misinterpreted in complex multilayered devices such as PSC. In this work, the effects of bulk and interface recombination on the nid are investigated experimentally and theoretically. By coupling intensity-dependent quasi-Fermi level splitting measurements with drift diffusion simulations of complete devices and partial cell stacks, it is shown that interfacial recombination leads to a lower nid compared to Shockley–Read–Hall (SRH) recombination in the bulk. As such, the strongest recombination channel determines the nid of the complete cell. An analytical approach is used to rationalize that nid values between 1 and 2 can originate exclusively from a single recombination process. By expanding the study over a wide range of the interfacial energy offsets and interfacial recombination velocities, it is shown that an ideality factor of nearly 1 is usually indicative of strong first-order non-radiative interface recombination and that it correlates with a lower device performance. It is only when interface recombination is largely suppressed and bulk SRH recombination dominates that a small nid is again desirable.
Q. Wang, J. A. Smith, D. Skroblin, J. A. Steele, C. M. Wolff, P. Caprioglio, M. Stolterfoht, H. Köbler, M. Li, S.-H. Turren-Cruz, C. Gollwitzer, D. Neher, A. Abate, “Managing Phase Purities and Crystal Orientation for High‐Performance and Photostable Cesium Lead Halide Perovskite Solar Cells”, Solar RRL 4, 2000213 (2020), DOI: 10.1002/solr.202000213
Inorganic perovskites with cesium (Cs+) as the cation have great potential as photovoltaic materials if their phase purity and stability can be addressed. Herein, a series of inorganic perovskites is studied, and it is found that the power conversion efficiency of solar cells with compositions CsPbI1.8Br1.2, CsPbI2.0Br1.0, and CsPbI2.2Br0.8 exhibits a high dependence on the initial annealing step that is found to significantly affect the crystallization and texture behavior of the final perovskite film. At its optimized annealing temperature, CsPbI1.8Br1.2 exhibits a pure orthorhombic phase and only one crystal orientation of the (110) plane. Consequently, this allows for the best efficiency of up to 14.6% and the longest operational lifetime, TS80, of ≈300 h, averaged of over six solar cells, during the maximum power point tracking measurement under continuous light illumination and nitrogen atmosphere. This work provides essential progress on the enhancement of photovoltaic performance and stability of CsPbI3 − xBrx perovskite solar cells.
P. S. C. Schulze, A. J. Bett, M. Bivour, P. Caprioglio, F. M. Gerspacher, Ö. Ş. Kabaklı, A. Richter, M. Stolterfoht, Q. Zhang, D. Neher, M. Hermle, H. Hillebrecht, S. W. Glunz, J. C. Goldschmidt, “25.1% High‐Efficiency Monolithic Perovskite Silicon Tandem Solar Cell with a High Bandgap Perovskite Absorber”, Solar RRL 4, 2000152 (2020), DOI: 10.1002/solr.202000152
Monolithic perovskite silicon tandem solar cells can overcome the theoretical efficiency limit of silicon solar cells. This requires an optimum bandgap, high quantum efficiency, and high stability of the perovskite. Herein, a silicon heterojunction bottom cell is combined with a perovskite top cell, with an optimum bandgap of 1.68 eV in planar p–i–n tandem configuration. A methylammonium-free FA0.75Cs0.25Pb(I0.8Br0.2)3 perovskite with high Cs content is investigated for improved stability. A 10% molarity increase to 1.1 m of the perovskite precursor solution results in ≈75 nm thicker absorber layers and 0.7 mA cm−2 higher short-circuit current density. With the optimized absorber, tandem devices reach a high fill factor of 80% and up to 25.1% certified efficiency. The unencapsulated tandem device shows an efficiency improvement of 2.3% (absolute) over 5 months, showing the robustness of the absorber against degradation. Moreover, a photoluminescence quantum yield analysis reveals that with adapted charge transport materials and surface passivation, along with improved antireflection measures, the high bandgap perovskite absorber has the potential for 30% tandem efficiency in the near future.
F. Zu, T. Schultz, C. M. Wolff, D. Shin, L. Frohloff, D. Neher, P. Amsalem, N. Koch, “Position-locking of volatile reaction products by atmosphere and capping layers slows down photodecomposition of methylammonium lead triiodide perovskite”, RSC Advances 10, 17534–17542 (2020), DOI: 10.1039/D0RA03572F
The remarkable progress of metal halide perovskites in photovoltaics has led to the power conversion efficiency approaching 26%. However, practical applications of perovskite-based solar cells are challenged by the stability issues, of which the most critical one is photo-induced degradation. Bare CH3NH3PbI3 perovskite films are known to decompose rapidly, with methylammonium and iodine as volatile species and residual solid PbI2 and metallic Pb, under vacuum under white light illumination, on the timescale of minutes. We find, in agreement with previous work, that the degradation is non-uniform and proceeds predominantly from the surface, and that illumination under N2 and ambient air (relative humidity 20%) does not induce substantial degradation even after several hours. Yet, in all cases the release of iodine from the perovskite surface is directly identified by X-ray photoelectron spectroscopy. This goes in hand with a loss of organic cations and the formation of metallic Pb. When CH3NH3PbI3 films are covered with a few nm thick organic capping layer, either charge selective or non-selective, the rapid photodecomposition process under ultrahigh vacuum is reduced by more than one order of magnitude, and becomes similar in timescale to that under N2 or air. We conclude that the light-induced decomposition reaction of CH3NH3PbI3, leading to volatile methylammonium and iodine, is largely reversible as long as these products are restrained from leaving the surface. This is readily achieved by ambient atmospheric pressure, as well as a thin organic capping layer even under ultrahigh vacuum. In addition to explaining the impact of gas pressure on the stability of this perovskite, our results indicate that covalently “locking” the position of perovskite components at the surface or an interface should enhance the overall photostability.
N. Tokmoldin, S. M. Hosseini, M. Raoufi, L. Q. Phuong, O. J. Sandberg, H. Guan, Y. Zou, D. Neher, S. Shoaee, “Extraordinarily long diffusion length in PM6:Y6 organic solar cells”, Journal of Materials Chemistry A 8, 7854–7860 (2020), DOI: 10.1039/D0TA03016C
The PM6:Y6 bulk-heterojunction (BHJ) blend system achieves high short-circuit current (JSC) values in thick photovoltaic junctions. Here we analyse these solar cells to understand the observed independence of the short-circuit current upon photoactive layer thickness. We employ a range of optoelectronic measurements and analyses, including Mott–Schottky analysis, CELIV, photoinduced absorption spectroscopy, mobility measurements and simulations, to conclude that, the invariant photocurrent for the devices with different active layer thicknesses is associated with the Y6's diffusion length exceeding 300 nm in case of a 300 nm thick cell. This is despite unintentional doping that occurs in PM6 and the associated space-charge effect, which is expected to be even more profound upon photogeneration. This extraordinarily long diffusion length – which is an order of magnitude larger than typical values for organics – dominates transport in the flat-band region of thick junctions. Our work suggests that the performance of the doped PM6:Y6 organic solar cells resembles that of inorganic devices with diffusion transport playing a pivotal role. Ultimately, this is expected to be a key requirement for the fabrication of efficient, high-photocurrent, thick organic solar cells.
M. Stolterfoht, M. Grischek, P. Caprioglio, C. M. Wolff, E. Gutierrez‐Partida, F. Peña‐Camargo, D. Rothhardt, S. Zhang, M. Raoufi, J. Wolansky, M. Abdi‐Jalebi, S. D. Stranks, S. Albrecht, T. Kirchartz, D. Neher, “How To Quantify the Efficiency Potential of Neat Perovskite Films: Perovskite Semiconductors with an Implied Efficiency Exceeding 28%”, Advanced Materials 32, 2000080 (2020), DOI: 10.1002/adma.202000080
Perovskite photovoltaic (PV) cells have demonstrated power conversion efficiencies (PCE) that are close to those of monocrystalline silicon cells; however, in contrast to silicon PV, perovskites are not limited by Auger recombination under 1-sun illumination. Nevertheless, compared to GaAs and monocrystalline silicon PV, perovskite cells have significantly lower fill factors due to a combination of resistive and non-radiative recombination losses. This necessitates a deeper understanding of the underlying loss mechanisms and in particular the ideality factor of the cell. By measuring the intensity dependence of the external open-circuit voltage and the internal quasi-Fermi level splitting (QFLS), the transport resistance-free efficiency of the complete cell as well as the efficiency potential of any neat perovskite film with or without attached transport layers are quantified. Moreover, intensity-dependent QFLS measurements on different perovskite compositions allows for disentangling of the impact of the interfaces and the perovskite surface on the non-radiative fill factor and open-circuit voltage loss. It is found that potassium-passivated triple cation perovskite films stand out by their exceptionally high implied PCEs > 28%, which could be achieved with ideal transport layers. Finally, strategies are presented to reduce both the ideality factor and transport losses to push the efficiency to the thermodynamic limit.
O. J. Sandberg, J. Kurpiers, M. Stolterfoht, D. Neher, P. Meredith, S. Shoaee, A. Armin, “On the Question of the Need for a Built‐In Potential in Perovskite Solar Cells”, Advanced Materials Interfaces 7, 2000041 (2020), DOI: 10.1002/admi.202000041
Perovskite semiconductors as the active materials in efficient solar cells exhibit free carrier diffusion lengths on the order of microns at low illumination fluxes and many hundreds of nanometers under 1 sun conditions. These lengthscales are significantly larger than typical junction thicknesses, and thus the carrier transport and charge collection should be expected to be diffusion controlled. A consensus along these lines is emerging in the field. However, the question as to whether the built-in potential plays any role is still of matter of some conjecture. This important question using phase-sensitive photocurrent measurements and theoretical device simulations based upon the drift-diffusion framework is addressed. In particular, the role of the built-in electric field and charge-selective transport layers in state-of-the-art p–i–n perovskite solar cells comparing experimental findings and simulation predictions is probed. It is found that while charge collection in the junction does not require a drift field per se, a built-in potential is still needed to avoid the formation of reverse electric fields inside the active layer, and to ensure efficient extraction through the charge transport layers.
I. García-Benito, C. Quarti, V. I. E. Queloz, Y. J. Hofstetter, D. Becker-Koch, P. Caprioglio, D. Neher, S. Orlandi, M. Cavazzini, G. Pozzi, J. Even, M. K. Nazeeruddin, Y. Vaynzof, G. Grancini, “Fluorination of Organic Spacer Impacts on the Structural and Optical Response of 2D Perovskites”, Frontiers in Chemistry 7, 1–11 (2020), DOI: 10.3389/fchem.2019.00946
Low-dimensional hybrid perovskites have triggered significant research interest due to their intrinsically tunable optoelectronic properties and technologically relevant material stability. In particular, the role of the organic spacer on the inherent structural and optical features in two-dimensional (2D) perovskites is paramount for material optimization. To obtain a deeper understanding of the relationship between spacers and the corresponding 2D perovskite film properties, we explore the influence of the partial substitution of hydrogen atoms by fluorine in an alkylammonium organic cation, resulting in (Lc)2PbI4 and (Lf)2PbI4 2D perovskites, respectively. Consequently, optical analysis reveals a clear 0.2 eV blue-shift in the excitonic position at room temperature. This result can be mainly attributed to a band gap opening, with negligible effects on the exciton binding energy. According to Density Functional Theory (DFT) calculations, the band gap increases due to a larger distortion of the structure that decreases the atomic overlap of the wavefunctions and correspondingly bandwidth of the valence and conduction bands. In addition, fluorination impacts the structural rigidity of the 2D perovskite, resulting in a stable structure at room temperature and the absence of phase transitions at a low temperature, in contrast to the widely reported polymorphism in some non-fluorinated materials that exhibit such a phase transition. This indicates that a small perturbation in the material structure can strongly influence the overall structural stability and related phase transition of 2D perovskites, making them more robust to any phase change. This work provides key information on how the fluorine content in organic spacer influence the structural distortion of 2D perovskites and their optical properties which possess remarkable importance for future optoelectronic applications, for instance in the field of light-emitting devices or sensors.
C. M. Wolff, L. Canil, C. Rehermann, N. Ngoc Linh, F. Zu, M. Ralaiarisoa, P. Caprioglio, L. Fiedler, M. Stolterfoht, S. Kogikoski, I. Bald, N. Koch, E. L. Unger, T. Dittrich, A. Abate, D. Neher, “Perfluorinated Self-Assembled Monolayers Enhance the Stability and Efficiency of Inverted Perovskite Solar Cells”, ACS Nano 14, 1445–1456 (2020), DOI: 10.1021/acsnano.9b03268
Perovskite solar cells are among the most exciting photovoltaic systems as they combine low recombination losses, ease of fabrication, and high spectral tunability. The Achilles heel of this technology is the device stability due to the ionic nature of the perovskite crystal, rendering it highly hygroscopic, and the extensive diffusion of ions especially at increased temperatures. Herein, we demonstrate the application of a simple solution-processed perfluorinated self-assembled monolayer (p-SAM) that not only enhances the solar cell efficiency, but also improves the stability of the perovskite absorber and, in turn, the solar cell under increased temperature or humid conditions. The p–i–n-type perovskite devices employing these SAMs exhibited power conversion efficiencies surpassing 21%. Notably, the best performing devices are stable under standardized maximum power point operation at 85 °C in inert atmosphere (ISOS-L-2) for more than 250 h and exhibit superior humidity resilience, maintaining ∼95% device performance even if stored in humid air in ambient conditions over months (∼3000 h, ISOS-D-1). Our work, therefore, demonstrates a strategy towards efficient and stable perovskite solar cells with easily deposited functional interlayers.
Y. Zhong, M. Causa’, G. J. Moore, P. Krauspe, B. Xiao, F. Günther, J. Kublitski, R. Shivhare, J. Benduhn, E. BarOr, S. Mukherjee, K. M. Yallum, J. Réhault, S. C. B. Mannsfeld, D. Neher, L. J. Richter, D. M. DeLongchamp, F. Ortmann, K. Vandewal, E. Zhou, N. Banerji, “Sub-picosecond charge-transfer at near-zero driving force in polymer:non-fullerene acceptor blends and bilayers”, Nature Communications 11, 833 (2020), DOI: 10.1038/s41467-020-14549-w
Organic photovoltaics based on non-fullerene acceptors (NFAs) show record efficiency of 16 to 17% and increased photovoltage owing to the low driving force for interfacial charge-transfer. However, the low driving force potentially slows down charge generation, leading to a tradeoff between voltage and current. Here, we disentangle the intrinsic charge-transfer rates from morphology-dependent exciton diffusion for a series of polymer:NFA systems. Moreover, we establish the influence of the interfacial energetics on the electron and hole transfer rates separately. We demonstrate that charge-transfer timescales remain at a few hundred femtoseconds even at near-zero driving force, which is consistent with the rates predicted by Marcus theory in the normal region, at moderate electronic coupling and at low re-organization energy. Thus, in the design of highly efficient devices, the energy offset at the donor:acceptor interface can be minimized without jeopardizing the charge-transfer rate and without concerns about a current-voltage tradeoff.
A. E. Mansour, D. Lungwitz, T. Schultz, M. Arvind, A. M. Valencia, C. Cocchi, A. Opitz, D. Neher, N. Koch, “The optical signatures of molecular-doping induced polarons in poly(3-hexylthiophene-2,5-diyl): Individual polymer chains versus aggregates”, Journal of Materials Chemistry C 8, 2870–2879 (2020), DOI: 10.1039/c9tc06509a
Optical absorption spectroscopy is a key method to investigate doped conjugated polymers and to characterize the doping-induced charge carriers, i.e., polarons. For prototypical poly(3-hexylthiophene-2,5-diyl) (P3HT), the absorption intensity of molecular dopant induced polarons is widely used to estimate the carrier density and the doping efficiency, i.e., the number of polarons formed per dopant molecule. However, the dependence of the polaron-related absorption features on the structure of doped P3HT, being either aggregates or separated individual chains, is not comprehensively understood in contrast to the optical absorption features of neutral P3HT. In this work, we unambiguously differentiate the optical signatures of polarons on individual P3HT chains and aggregates in solution, notably the latter exhibiting the same shape as aggregates in solid thin films. This is enabled by employing tris(pentafluorophenyl)borane (BCF) as dopant, as this dopant forms only ion pairs with P3HT and no charge transfer complexes, and BCF and its anion have no absorption in the spectral region of P3HT polarons. Polarons on individual chains exhibit absorption peaks at 1.5 eV and 0.6 eV, whereas in aggregates the high-energy peak is split into a doublet 1.3 eV and 1.65 eV, and the low-energy peak is shifted below 0.5 eV. The dependence of the fraction of solvated individual chains versus aggregates on absolute solution concentration, dopant concentration, and temperature is elucidated, and we find that aggregates predominate in solution under commonly used processing conditions. Aggregates in BCF-doped P3HT solution can be effectively removed upon simple filtering. From varying the filter pore size (down to 200 nm) and thin film morphology characterization with scanning force microscopy we reveal the aggregates' size dependence on solution absolute concentration and dopant concentration. Furthermore, X-ray photoelectron spectroscopy shows that the dopant loading in aggregates is higher than for individual P3HT chains. The results of this study help understanding the impact of solution pre-aggregation on thin film properties of molecularly doped P3HT, and highlight the importance of considering such aggregation for other doped conjugated polymers in general.
J. C. Brauer, D. Tsokkou, S. Sanchez, N. Droseros, B. Roose, E. Mosconi, X. Hua, M. Stolterfoht, D. Neher, U. Steiner, F. De Angelis, A. Abate, N. Banerji, “Comparing the excited-state properties of a mixed-cation–mixed-halide perovskite to methylammonium lead iodide”, The Journal of Chemical Physics 152, 104703 (2020), DOI: 10.1063/1.5133021
Organic–inorganic perovskites are one of the most promising photovoltaic materials for the design of next generation solar cells. The lead-based perovskite prepared with methylammonium and iodide was the first in demonstrating high power conversion efficiency, and it remains one of the most used materials today. However, perovskites prepared by mixing several halides and several cations systematically yield higher efficiencies than “pure” methylammonium lead iodide (MAPbI3) devices. In this work, we unravel the excited-state properties of a mixed-halide (iodide and bromide) and mixed-cation (methylammonium and formamidinium) perovskite. Combining time-resolved photoluminescence, transient absorption, and optical-pump–terahertz-probe experiments with density functional theory calculations, we show that the population of higher-lying excited states in the mixed material increases the lifetime of photogenerated charge carriers upon well above-bandgap excitation. We suggest that alloying different halides and different cations reduces the structural symmetry of the perovskite, which partly releases the selection rules to populate the higher-energy states upon light absorption. Our investigation thus shows that mixed halide perovskites should be considered as an electronically different material than MAPbI3, paving the way toward further materials optimization and improved power conversion efficiency of perovskite solar cells.
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) 217, 1900876 (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.
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 32, 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.
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.