Shoaee, S., Luong, H. M., Song, J., Zou, Y., Nguyen, T.-Q., Neher, D., What We have Learnt from PM6:Y6. Adv. Mater. 2023, 2302005. DOI: 10.1002/adma.202302005

Over the past three years, remarkable advancements in organic solar cells (OSCs) have emerged, propelled by the introduction of Y6—an innovative A-DA'D-A type small molecule non-fullerene acceptor (NFA). This review provides a critical discussion of the current knowledge about the structural and physical properties of the PM6:Y6 material combination in relation to its photovoltaic performance. The design principles of PM6 and Y6 are discussed, covering charge transfer, transport, and recombination mechanisms. Then, the authors delve into blend morphology and degradation mechanisms before considering commercialization. The current state of the art is presented, while also discussing unresolved contentious issues, such as the blend energetics, the pathways of free charge generation, and the role of triplet states in recombination. As such, this review aims to provide a comprehensive understanding of the PM6:Y6 material combination and its potential for further development in the field of organic solar cells. By addressing both the successes and challenges associated with this system, this review contributes to the ongoing research efforts toward achieving more efficient and stable organic solar cells.

Sağlamkaya E, Musiienko A, Shadabroo MS, et al. What is special about Y6; the working mechanism of neat Y6 organic solar cells. Mater Horizons. 2023;10(5):1825-1834, DOI: 10.1039/d2mh01411d

Non-fullerene acceptors (NFAs) have delivered advancement in bulk heterojunction organic solar cell efficiencies, with a significant milestone of 20% now in sight. However, these materials challenge the accepted wisdom of how organic solar cells work. In this work we present a neat Y6 device with an efficiency above 4.5%. We thoroughly investigate mechanisms of charge generation and recombination as well as transport in order to understand what is special about Y6. Our data suggest that Y6 generates bulk free charges, with ambipolar mobility, which can be extracted in the presence of transport layers.

Yang F, MacQueen RW, Menzel D, et al. Rubidium Iodide Reduces Recombination Losses in Methylammonium‐Free Tin‐Lead Perovskite Solar Cells. Adv Energy Mater. 2023;13(19). DOI: 10.1002/aenm.202204339

Outstanding optoelectronic properties of mixed tin-lead perovskites are the cornerstone for the development of high-efficiency all-perovskite tandems. However, recombination losses in Sn-Pb perovskites still limit the performance of these perovskites, necessitating more fundamental research. Here, rubidium iodide is employed as an additive for methylammonium-free Sn-Pb perovskites. It is first investigated the effect of the RbI additive on the perovskite composition, crystal structure, and element distribution. Quasi-Fermi level splitting and transient photoluminescence measurements reveal that the RbI additive reduces recombination losses and increases carrier lifetime of the perovskite films. This finding is attributed to an approximately ten-fold reduction in the defect density following RbI treatment, as probed using constant final state yield photoelectron spectroscopy. Additionally, the concentration of Sn vacancies is also reduced, and the perovskite film becomes less p-type both in the bulk and at the interface towards the electron contact. Thus, the conductivity for electrons increases, improving carrier extraction. As a result, the open-circuit voltage of RbI-containing solar cells improves by 61 mV on average, with the best efficiency >20%. This comprehensive study demonstrates that RbI is effective at reducing recombination losses and carrier trapping, paving way for a new approach to Sn-Pb perovskite solar cell research.

Diekmann J, Peña-Camargo F, Tokmoldin N, et al. Determination of Mobile Ion Densities in Halide Perovskites via Low-Frequency Capacitance and Charge Extraction Techniques. J Phys Chem Lett. 2023;14(18):4200-4210. DOI: 10.1021/acs.jpclett.3c00530

Mobile ions in perovskite photovoltaic devices can hinder performance and cause degradation by impeding charge extraction and screening the internal field. Accurately quantifying mobile ion densities remains a challenge and is a highly debated topic. We assess the suitability of several experimental methodologies for determining mobile ion densities by using drift-diffusion simulations. We found that charge extraction by linearly increasing voltage (CELIV) underestimates ion density, but bias-assisted charge extraction (BACE) can accurately reproduce ionic lower than the electrode charge. A modified Mott–Schottky (MS) analysis at low frequencies can provide ion density values for high excess ionic densities, typical for perovskites. The most significant contribution to capacitance originates from the ionic depletion layer rather than the accumulation layer. Using low-frequency MS analysis, we also demonstrate light-induced generation of mobile ions. These methods enable accurate tracking of ionic densities during device aging and a deeper understanding of ionic losses.

Caprioglio P, Smith JA, Oliver RDJ, et al. Open-circuit and short-circuit loss management in wide-gap perovskite p-i-n solar cells. Nat Commun. 2023;14(1):932. DOI: 10.1038/s41467-023-36141-8

In this work, we couple theoretical and experimental approaches to understand and reduce the losses of wide bandgap Br-rich perovskite pin devices at open-circuit voltage (V_OC) and short-circuit current (J_SC) conditions. A mismatch between the internal quasi-Fermi level splitting (QFLS) and the external V_OC is detrimental for these devices. We demonstrate that modifying the perovskite top-surface with guanidinium-Br and imidazolium-Br forms a low-dimensional perovskite phase at the n-interface, suppressing the QFLS-V_OC mismatch, and boosting the V_OC. Concurrently, the use of an ionic interlayer or a self-assembled monolayer at the p-interface reduces the inferred field screening induced by mobile ions at J_SC, promoting charge extraction and raising the J_SC. The combination of the n- and p-type optimizations allows us to approach the thermodynamic potential of the perovskite absorber layer, resulting in 1 cm² devices with performance parameters of V_OCs up to 1.29 V, fill factors above 80% and J_SCs up to 17 mA/cm², in addition to a thermal stability T₈₀ lifetime of more than 3500 h at 85 °C.

Raoufi M, Chandrabose S, Wang R, et al. Influence of the Energy Level Alignment on Charge Transfer and Recombination at the Monolayer-MoS 2 /Organic Hybrid Interface. J Phys Chem C. 2023;127(12):5866-5875. DOI: 10.1021/acs.jpcc.2c08186

Monolayer (ML) transition-metal dichalcogenides (TMDCs) exhibit numerous unique optoelectronic features. This motivates recent efforts to combine TMDCs with organic semiconductors to form heterostructures with tailorable properties that feature the advantages of both materials. Here, we study the photoinduced charge transfer across hybrid interfaces of ML-MoS₂ and a series of organic semiconductors─often used as hole transport materials─where we systematically tune the offsets of the frontier energy levels. Steady-state photoluminescence and ultrafast transient absorption spectroscopy reveal that a larger energy level offset causes a lower efficiency of photoinduced charge transfer but also a longer lifetime of the charge separated state. Both observations are explained in the framework of Marcus’ theory of electron transfer. In fact, our observations question direct electron–hole recombination across the hybrid interface as the main decay pathway for photogenerated carriers in the considered systems. Instead, back transfer of holes to ML-MoS₂ is suggested as the key decay channel. Adding a 1 nm LiF interlayer causes a significant slowdown of interfacial carrier recombination while not suppressing free carrier formation. This strategy serves as a guideline for optimizing further hybrid systems toward high-performance ML-TMDC/organic-based optoelectronic devices.

Vollbrecht J, Tokmoldin N, Sun B, et al. On the relationship of the effective mobility and photoconductance mobility in organic solar cells. Energy Adv. 2023;2(9):1390-1398. DOI: 10.1039/D3YA00125C

The efficiency of organic solar cells has increased significantly in the recent years due to the continued improvement in material properties, including the charge carrier mobilities within the bulk heterojunction. However, common strategies to measure the mobility of electrons and holes, such as the space-charge-limited-current approach, rely on purpose-made single carrier diodes, which are operated in the injection regime. Alternatively, impedance spectroscopy measurements can yield an effective mobility as well as a photoconductance mobility for solar cells under realistic operating conditions. There exist various theoretical interpretations that relate the experimentally determined values of the effective mobility with the mobility of the individual charge carriers (i.e. electrons and holes). Furthermore, the relationship between the effective and photoconductance mobility has not been clarified yet. This study shows how the effective and photoconductance mobilities can be combined in a system of equations to calculate the individual mobilities of the faster and slower carriers. Finally, these considerations are applied to determine individual carrier mobilities in several blend systems, including fullerene-based P3HT:PC₆₀BM solar cells, as well as non-fullerene devices based on PM6:Y11-N4, PM6:Y5, PPDT2FBT:Y6, PM6:Y11, PM6:N4, and PM6:Y6. These results were validated with mobility values obtained via the space-charge-limited-current approach.

Yu, J., Shen, Z., Lu, W., Zhu, Y., Liu, Y., Neher, D., Koch, N. and Lu, G. (2023), Composition Waves in Solution-Processed Organic Films and Its Propagations from Kinetically Frozen Surface Mesophases. Adv. Funct. Mater. 2302089.  DOI: 10.1002/adfm.202302089

Organic thin films deposited from solution attract wide interest for next-generation (opto-)electronic and energy applications. During solvent evaporation, the phase evolution dynamics for different components at different locations are not synchronic within the incrementally concentrated liquid films, determining the final anisotropic morphology and performance. Herein, by examining tens of widely investigated optoelectronic organic films, the general existence of composition wave propagating along the surface-normal direction upon solidification is identified. The composition wave is initiated by a few nanometers thick surface mesophase kinetically forming at the foremost stage of phase transition, and afterward propagates toward the substrate during solvent evaporation. The composition waves exhibit well-defined wave properties, including spatial wavelength, period, amplitude, and propagation velocity. These wave properties are closely correlated with the evaporation rate and the diffusion rate of organic molecules, which determines the dynamically varied local composition gradient along the surface-normal direction. Such composition waves are commonly found for more than 80% of randomly examined solution-processed thin films for high-performance organic electronic devices including photovoltaic cells and field-effect transistors.

Thiesbrummel, J., Peña-Camargo, F., Brinkmann, K. O., Gutierrez-Partida, E., Yang, F., Warby, J., Albrecht, S., Neher, D., Riedl, T., Snaith, H. J., Stolterfoht, M., Lang, F., Understanding and Minimizing V_OC Losses in All-Perovskite Tandem Photovoltaics. Adv. Energy Mater. 2023, 13, 2202674. DOI: 10.1002/aenm.202202674

Understanding performance losses in all-perovskite tandem photovoltaics is crucial to accelerate advancements toward commercialization, especially since these tandem devices generally underperform in comparison to what is expected from isolated layers and single junction devices. Here, the individual sub-cells in all-perovskite tandem stacks are selectively characterized to disentangle the various losses. It is found that non-radiative losses in the high-gap subcell dominate the overall recombination in the baseline system, as well as in the majority of literature reports. Through a multi-faceted approach, the open-circuit voltage (V_OC) of the high-gap perovskite subcell is enhanced by 120 mV. Employing a novel (quasi) lossless indium oxide interconnect, this enables all-perovskite tandem solar cells with 2.00 V V_OC and 23.7% stabilized efficiency. Reducing transport losses as well as imperfect energy-alignments boosts efficiencies to 25.2% and 27.0% as identified via subcell selective electro- and photo-luminescence. Finally, it is shown how, having improved the V_OC, improving the current density of the low-gap absorber pushes efficiencies even further, reaching 25.9% efficiency stabilized, with an ultimate potential of 30.0% considering the bulk quality of both absorbers measured using photo-luminescence. These insights not only show an optimization example but also a generalizable evidence-based optimization strategy utilizing optoelectronic sub-cell characterization.

He G, Mayberry B, Pranav M, et al. Performance-Limiting Factors in Ultralow-Bandgap PTB7-Th:COTIC-4F-Based Organic Solar Cells. ACS Energy Lett. Published online August 31, 2023:3980-3988. DOI: 10.1021/acsenergylett.3c01444

Understanding performance-limiting factors of organic solar cells (OSCs) with very small optical bandgaps is crucial for the development of tandem photovoltaics. Here, we investigate ultralow-bandgap (1.15 eV) OSCs based on the blend of the donor polymer PTB7-Th with the nonfullerene acceptor COTIC-4F. In a conventional device structure, we reach record PCE values of 8.55%. However, this PCE still falls behind that of higher-bandgap OSCs. To guide further improvements, we investigate various loss and recombination processes. Complemented with optical and electrical simulations, we show that J_SC loss can be largely attributed to inefficient exciton dissociation and geminate recombination of the charge transfer state. Further, we identify a high bimolecular recombination coefficient as the main reason for the poor performance, while surface recombination predominantly affects V_OC. Finally, our simulations show an efficiency potential >15% upon simultaneous reduction of bimolecular, surface, exciton, and charge transfer recombination in the PTB7-Th:COTIC-4F system.

Tokmoldin N, Sun B, Moruzzi F, et al. Elucidating How Low Energy Offset Matters to Performance of Nonfullerene Acceptor-Based Solar Cells. ACS Energy Lett. 2023;8(6):2552-2560. DOI: 10.1021/acsenergylett.3c00572

The energetic offset between the highest occupied molecular orbitals of the donor and acceptor components of organic photovoltaic blends is well-known to affect the device efficiency. It is well-established that a decreasing offset increases the open-circuit voltage but reduces the short-circuit current, which has been explained by insufficient exciton dissociation. However, the impact of the offset on the fill factor and underlying processes is less clear. Here, we study free charge generation and recombination in three different nonfullerene acceptors, Y6, ITIC, and o-IDBTR, blended with the same donor polymer PM6. We demonstrate that a diminishing offset results in field-dependent charge generation related to field-assisted exciton dissociation. On the other hand, reformation of excitons from free charges is identified as an additional channel for charge recombination, which goes along with a substantial rise in the bimolecular recombination coefficient. In combination of these two effects, the fill factor drops considerably with a decreasing energy offset. Using the comparison between PM6:ITIC and PM6:o-IDBTR, we show that bulk properties such as morphology and carrier mobilities can not fully explain the observed difference in performance, highlighting the importance of interfacial kinetics and thermodynamics in controlling the device efficiency, both through generation and recombination of charge carriers.

Pranav M, Hultzsch T, Musiienko A, et al. Anticorrelated photoluminescence and free charge generation proves field-assisted exciton dissociation in low-offset PM6:Y5 organic solar cells. APL Mater. 2023;11(6). DOI: 10.1063/5.0151580

Understanding the origin of inefficient photocurrent generation in organic solar cells with low energy offset remains key to realizing high-performance donor-acceptor systems. Here, we probe the origin of field-dependent free-charge generation and photoluminescence in non-fullereneacceptor (NFA)-based organic solar cells using the polymer PM6 and the NFA Y5—a non-halogenated sibling to Y6, with a smaller energetic offset to PM6. By performing time-delayed collection field (TDCF) measurements on a variety of samples with different electron transport layers and active layer thickness, we show that the fill factor and photocurrent are limited by field-dependent free charge generation in the bulk of the blend. We also introduce a new method of TDCF called m-TDCF to prove the absence of artifacts from non-geminate recombination of photogenerated and dark charge carriers near the electrodes. We then correlate free charge generation with steady-state photoluminescence intensity and find perfect anticorrelation between these two properties. Through this, we conclude that photocurrent generation in this low-offset system is entirely controlled by the field-dependent dissociation of local excitons into charge-transfer states.

Sun, B., Tokmoldin, N., Alqahtani, O., Patterson, A., De Castro, C. S. P., Riley, D. B., Pranav, M., Armin, A., Laquai, F., Collins, B. A., Neher, D., Shoaee, S., Toward More Efficient Organic Solar Cells: A Detailed Study of Loss Pathway and Its Impact on Overall Device Performance in Low-Offset Organic Solar Cells. Adv. Energy Mater. 2023, 13, 2300980. DOI: 10.1002/aenm.202300980

Low-offset organic solar cell systems have attracted great interest since nonfullerene acceptors came into the picture. While numerous studies have focused on the charge generation process in these low-offset systems, only a few studies have focused on the details of each loss channel in the charge generation process and their impact on the overall device performance. Here, several nonfullerene acceptors are blended with the same polymer donor to form a series of low-offset organic solar cell systems where significant variation in device performance is observed. Through detailed analyses of loss pathways, it is found that: i) the donor:acceptor interfaces of PM6:Y6 and PM6:TPT10 are close to the optimum energetic condition, ii) energetics at the donor:acceptor interface are the most important factor to the overall device performance, iii) exciton dissociation yield can be field-dependent owing to the sufficiently small energetic offset at the donor:acceptor interface, and iv) the change in substituents in the terminal group of Y-series acceptors in this work mainly affects energetics at the donor:acceptor interface instead of the interface density in the active layer. In general, this work presents a path toward more efficient organic solar cells.

Zhang S, Ye F, Wang X, et al. Minimizing buried interfacial defects for efficient inverted perovskite solar cells. Science. 2023;380(6643):404-409. DOI: 10.1126/science.adg3755

Controlling the perovskite morphology and defects at the buried perovskite-substrate interface is challenging for inverted perovskite solar cells. In this work, we report an amphiphilic molecular hole transporter, (2-(4-(bis(4-methoxyphenyl)amino)phenyl)-1-cyanovinyl)phosphonic acid, that features a multifunctional cyanovinyl phosphonic acid group and forms a superwetting underlayer for perovskite deposition, which enables high-quality perovskite films with minimized defects at the buried interface. The resulting perovskite film has a photoluminescence quantum yield of 17% and a Shockley-Read-Hall lifetime of nearly 7 microseconds and achieved a certified power conversion efficiency (PCE) of 25.4% with an open-circuit voltage of 1.21 volts and a fill factor of 84.7%. In addition, 1–square centimeter cells and 10–square centimeter minimodules show PCEs of 23.4 and 22.0%, respectively. Encapsulated modules exhibited high stability under both operational and damp heat test conditions.

Raoufi, M., Rühl, S., Chandrabose, S., Shukla, A., Sun, B., Kratochvil, E., Blumstengel, S. and Neher, D. (2023), Fast Photoresponse from Hybrid Monolayer MoS₂/Organic Photodetector. Phys. Status Solidi A 2300107. DOI: 10.1002/pssa.202300107

As a direct-bandgap transition semiconductor with high carrier mobility, monolayer (ML) transition metal dichalcogenides (TMDCs) have attracted significant attention as a promising class of material for photodetection. It is reported that these layers exhibit a persistent photoconductance (PPC) effect, which is assigned to long-lasting hole capture by deep traps. Therefore, TMDCs-based photodetectors show a high photoresponse but also a slow response. Herein, intensity-modulated photocurrent spectroscopy (IMPS) with steady-state background illumination is performed to investigate the photoresponse dynamics in a hybrid photodetector based on ML MoS₂ covered with an ultrathin layer of phthalocyanine (H₂Pc) molecules. The results demonstrate that adding the H₂Pc layer speeds up the photoresponse of the neat ML-MoS₂ photodetector by almost two orders of magnitude without deteriorating its responsivity. The origin of these improvements is revealed by applying the Hornbeck–Haynes model to the photocarrier dynamics in the IMPS experiment. It is shown that the improved response speed of the hybrid device arises mostly from a faster detrapping of holes in the presence of the H₂Pc layer, while the trap densities remain rather unchanged. Meanwhile, the additional absorption of photons in the H₂Pc layer contributes to photocarrier generation, resulting in an enlarged responsivity of the hybrid device.

D. Kroh, F. Eller, K. Schötz, S. Wedler, L. Perdigón-Toro, G. Freychet, Q. Wei, M. Dörr, D. Jones, Y. Zou, E. M. Herzig, D. Neher, and A. Köhler, Identifying the Signatures of Intermolecular Interactions in Blends of PM6 with Y6 and N4 Using Absorption Spectroscopy. Adv. Funct. Mater. 2022, 2205711  DOI: 10.1002/adfm.20220571

In organic solar cells, the resulting device efficiency depends strongly on the local morphology and intermolecular interactions of the blend film. Optical spectroscopy was used to identify the spectral signatures of interacting chromophores in blend films of the donor polymer PM6 with two state-of-the-art nonfullerene acceptors, Y6 and N4, which differ merely in the branching point of the side chain. From temperature-dependent absorption and luminescence spectroscopy in solution, it is inferred that both acceptor materials form two types of aggregates that differ in their interaction energy. Y6 forms an aggregate with a predominant J-type character in solution, while for N4 molecules the interaction is predominantly in a H-like manner in solution and freshly spin-cast film, yet the molecules reorient with respect to each other with time or thermal annealing to adopt a more J-type interaction. The different aggregation behavior of the acceptor materials is also reflected in the blend films and accounts for the different solar cell efficiencies reported with the two blends.

Fritsch, T., Kurpiers, J., Roland, S., Tokmoldin, N., Shoaee, S., Ferron, T., Collins, B. A., Janietz, S., Vandewal, K., Neher, D., On the Interplay between CT and Singlet Exciton Emission in Organic Solar Cells with Small Driving Force and Its Impact on Voltage Loss. Adv. Energy Mater. 2022, 2200641. 

https://doi.org/10.1002/aenm.202200641

The interplay between free charge carriers, charge transfer (CT) states and sin-glet excitons (S1) determines the recombination pathway and the resulting open circuit voltage (VOC) of organic solar cells. By combining a well-aggregated low bandgap polymer with different blend ratios of the fullerenes PCBM and ICBA, the energy of the CT state (ECT) is varied by 130 meV while leaving the S1 energy of the polymer (ES1) unaffected. It is found that the polymer exciton dominates the radiative properties of the blend when ECT approaches ES1, while the VOCremains limited by the non-radiative decay of the CT state. It is concluded that an increasing strength of the exciton in the optical spectra of organic solar cells will generally decrease the non-radiative voltage loss because it lowers the radiative VOC limit (VOC,rad), but not because it is more emissive. The analysis further suggests that electronic coupling between the CT state and the S1 will not improve the VOC, but rather reduce the VOC,rad. It is anticipated that only at very low CT state absorption combined with a fairly high CT radiative efficiency the solar cell benefit from the radiative properties of the singlet excitons.

Joachim Vollbrecht, Nurlan Tokmoldin, Bowen Sun, Viktor V. Brus, Safa Shoaee, and Dieter Neher , "Determination of the charge carrier density in organic solar cells: A tutorial", Journal of Applied Physics 131, 221101 (2022)

 https://doi.org/10.1063/5.0094955

The increase in the performance of organic solar cells observed over the past few years has reinvigorated the search for a deeper understanding of the loss and extraction processes in this class of device. A detailed knowledge of the density of free charge carriers under different operating conditions and illumination intensities is a prerequisite to quantify the recombination and extraction dynamics. Differential charging techniques are a promising approach to experimentally obtain the charge carrier density under the aforementioned conditions. In particular, the combination of transient photovoltage and photocurrent as well as impedance and capacitance spectroscopy have been successfully used in past studies to determine the charge carrier density of organic solar cells. In this Tutorial, these experimental techniques will be discussed in detail, highlighting fundamental principles, practical considerations, necessary corrections, advantages, drawbacks, and ultimately their limitations. Relevant references introducing more advanced concepts will be provided as well. Therefore, the present Tutorial might act as an introduction and guideline aimed at new prospective users of these techniques as well as a point of reference for more experienced researchers.

Bowen Sun, Oskar J. Sandberg, Dieter Neher, Ardalan Armin, and Safa Shoaee, Wave Optics of Differential Absorption Spectroscopy in Thick-Junction Organic Solar Cells: Optical Artifacts and Correction Strategies, Phys. Rev. Applied 17, 054016

https://doi.org/10.1103/PhysRevApplied.17.054016

Differential absorption spectroscopy techniques serve as powerful techniques to study the excited species in organic solar cells. However, it has always been challenging to employ these techniques for characterizing thick-junction organic solar cells, especially when a reflective top contact is involved. In this work, we present a detailed and systematic study on how a combination of the presence of the interference effect and a nonuniform charge-distribution profile, severely manipulates experimental spectra and the decay dynamics. Furthermore, we provide a practical methodology to correct these optical artifacts in differential absorption spectroscopies. The results and the proposed correction method generally apply to all kinds of differential absorption spectroscopy techniques and various thin-film systems, such as organics, perovskites, kesterites, and two-dimensional materials. Notably, it is found that the shape of differential absorption spectra can be strongly distorted, starting from 150-nm active-layer thickness; this matches the thickness range of thick-junction organic solar cells and most perovskite solar cells and needs to be carefully considered in experiments. In addition, the decay dynamics of differential absorption spectra is found to be disturbed by optical artifacts under certain conditions. With the help of the proposed correction formalism, differential spectra and the decay dynamics can be characterized on the full device of thin-film solar cells in transmission mode and yield accurate and reliable results to provide design rules for further progress.

Le Corre, V.M., Diekmann, J., Peña-Camargo, F., Thiesbrummel, J., Tokmoldin, N., Gutierrez-Partida, E., Peters, K.P., Perdigón-Toro, L., Futscher, M.H., Lang, F., Warby, J., Snaith, H.J., Neher, D. and Stolterfoht, M. (2022), Quantification of Efficiency Losses Due to Mobile Ions in Perovskite Solar Cells via Fast Hysteresis Measurements. Sol. RRL, 6: 2100772. 

https://doi.org/10.1002/solr.202100772  

Perovskite semiconductors differ from most inorganic and organic semicon-ductors due to the presence of mobile ions in the material. Although the phe-nomenon is intensively investigated, important questions such as the exactimpact of the mobile ions on the steady-state power conversion efficiency (PCE)and stability remain. Herein, a simple method is proposed to estimate theefficiency loss due to mobile ions via“fast-hysteresis”measurements by pre-venting the perturbation of mobile ions out of their equilibrium position at fastscan speeds (1000 V s1). The“ion-free”PCE is between 1% and 3% higherthan the steady-state PCE, demonstrating the importance of ion-induced losses,even in cells with low levels of hysteresis at typical scan speeds (100 mV s1).The hysteresis over many orders of magnitude in scan speed provides importantinformation on the effective ion diffusion constant from the peak hysteresisposition. The fast-hysteresis measurements are corroborated by transient chargeextraction and capacitance measurements and numerical simulations, whichconfirm the experimentalfindings and provide important insights into the chargecarrier dynamics. The proposed method to quantify PCE losses due tofieldscreening induced by mobile ions clarifies several important experimentalobservations and opens up a large range of future experiments.

Warby, J., Zu, F., Zeiske, S., Gutierrez-Partida, E., Frohloff, L., Kahmann, S., Frohna, K., Mosconi, E., Radicchi, E., Lang, F., Shah, S., Peña-Camargo, F., Hempel, H., Unold, T., Koch, N., Armin, A., De, F., Stranks, S. D., Neher, D., Stolterfoht, M., Understanding Performance Limiting Interfacial Recombination in pin Perovskite Solar Cells. Adv. Energy Mater. 2022, 12, 2103567. 

https://doi.org/10.1002/aenm.202103567

Perovskite semiconductors are an attractive option to overcome the limita-tions of established silicon based photovoltaic (PV) technologies due to their exceptional opto-electronic properties and their successful integration into multijunction cells. However, the performance of single- and multijunc-tion cells is largely limited by significant nonradiative recombination at the perovskite/organic electron transport layer junctions. In this work, the cause of interfacial recombination at the perovskite/C60 interface is revealed via a combination of photoluminescence, photoelectron spectroscopy, and first-principle numerical simulations. It is found that the most significant con-tribution to the total C60-induced recombination loss occurs within the first monolayer of C60, rather than in the bulk of C60 or at the perovskite surface. The experiments show that the C60 molecules act as deep trap states when in direct contact with the perovskite. It is further demonstrated that by reducing the surface coverage of C60, the radiative efficiency of the bare perovskite layer can be retained. The findings of this work pave the way toward overcoming one of the most critical remaining performance losses in perovskite solar cells.

Perdigón-Toro, L., Phuong, L. Q., Eller, F., Freychet, G., Saglamkaya, E., Khan, J. I., Wei, Q., Zeiske, S., Kroh, D., Wedler, S., Köhler, A., Armin, A., Laquai, F., Herzig, E. M., Zou, Y., Shoaee, S., Neher, D., Understanding the Role of Order in Y-Series Non-Fullerene Solar Cells to Realize High Open-Circuit Voltages. Adv. Energy Mater. 2022, 12, 2103422. 

https://doi.org/10.1002/aenm.202103422

Non-fullerene acceptors (NFAs) as used in state-of-the-art organic solar cells feature highly crystalline layers that go along with low energetic disorder. Here, the crucial role of energetic disorder in blends of the donor polymer PM6 with two Y-series NFAs, Y6, and N4 is studied. By performing tempera-ture-dependent charge transport and recombination studies, a consistent picture of the shape of the density of state distributions for free charges in the two blends is developed, allowing an analytical description of the dependence of the open-circuit voltage VOC on temperature and illumination intensity. Disorder is found to influence the value of the VOC at room temperature, but also its progression with temperature. Here, the PM6:Y6 blend benefits sub-stantially from its narrower state distributions. The analysis also shows that the energy of the equilibrated free charge population is well below the energy of the NFA singlet excitons for both blends and possibly below the energy of the populated charge transfer manifold, indicating a down-hill driving force for free charge formation. It is concluded that energetic disorder of charge-separated states has to be considered in the analysis of the photovoltaic properties, even for the more ordered PM6:Y6 blend.

Robert D. J. Oliver, ‡a Pietro Caprioglio, ‡*a Francisco Pen˜a-Camargo, Leonardo R. V. Buizza, Fengshuo Zu, Alexandra J. Ramadan, Silvia G. Motti, Suhas Mahesh, Melissa M. McCarthy,  Jonathan H. Warby,b Yen-Hung Lin, Norbert Koch, Steve Albrecht, Laura M. Herz, Michael B. Johnston, Dieter Neher, Martin Stolterfoht and Henry J. Snaith * Understanding and suppressing non-radiative losses in methylammonium-free wide-bandgap perovskite solar cells, Energy Environ. Sci., 2022, 15, 714

https://doi.org/10.1039/D1EE02650J

With power conversion efficiencies of perovskite-on-silicon and all-perovskite tandem solar cells increasing at rapid pace, wide bandgap (>1.7 eV) metal-halide perovskites (MHPs) are becoming a major focus of academic and industrial photovoltaic research. Compared to their lower bandgap (≤1.6 eV) counterparts, these types of perovskites suffer from higher levels of non-radiative losses in both the bulk material and in device configurations, constraining their efficiencies far below their thermodynamic potential. In this work, we investigate the energy losses in methylammonium (MA) free high-Br-content wide bandgap perovskites by using a combination of THz spectroscopy, steady-state and time-resolved photoluminescence, coupled with drift-diffusion simulations. The investigation of this system allows us to study charge-carrier recombination in these materials and devices in the absence of halide segregation due to the photostabilty of formamidinium-cesium based lead halide perovskites. We find that these perovskites are characterised by large non-radiative recombination losses in the bulk material and that the interfaces with transport layers in solar cell devices strongly limit their open-circuit voltage. In particular, we discover that the interface with the hole transport layer performs particularly poorly, in contrast to 1.6 eV bandgap MHPs which are generally limited by the interface with the electron-transport layer. To overcome these losses, we incorporate and investigate the recombination mechanisms present with perovskites treated with the ionic additive 1-butyl-1-methylpipiderinium tetrafluoroborate. We find that this additive not only improves the radiative efficiency of the bulk perovskite, but also reduces the non-radiative recombination at both the hole and electron transport layer interfaces of full photovoltaic devices. In addition to unravelling the beneficial effect of this specific treatment, we further optimise our solar cells by introducing an additional LiF interface treatment at the electron transport layer interface. Together these treatments enable MA-free 1.79 eV bandgap perovskite solar cells with open-circuit voltages of 1.22 V and power conversion efficiencies approaching 17%, which is among the highest reported for this material system.

Wolff, C. M., Bourelle, S. A., Phuong, L. Q., Kurpiers, J., Feldmann, S., Caprioglio, P., Marquez, J. A., Wolansky, J., Unold, T., Stolterfoht, M., Shoaee, S., Deschler, F., Neher, D., Orders of Recombination in Complete Perovskite Solar Cells – Linking Time-Resolved and Steady-State Measurements. Adv. Energy Mater. 2021, 11, 2101823. 

https://doi.org/10.1002/aenm.202101823 

Ideally, the charge carrier lifetime in a solar cell is limited by the radiative free carrier recombination in the absorber which is a second-order process. Yet, real-life cells suffer from severe nonradiative recombination in the bulk of the absorber, at interfaces, or within other functional layers. Here, the dynamics of photogenerated charge carriers are probed directly in pin-type mixed halide perovskite solar cells with an efficiency >20%, using time-resolved optical absorption spectroscopy and optoelectronic techniques. The charge carrier dynamics in complete devices is fully consistent with a superposition of first-, second-, and third-order recombination processes, with no admixture of recombination pathways with non-integer order. Under solar illumination, recombination in the studied solar cells proceeds predominantly through nonradiative first-order recombination with a lifetime of 250 ns, which competes with second-order free charge recombination which is mostly if not entirely radiative. Results from the transient experiments are further employed to successfully explain the steady-state solar cell properties over a wide range of illumination intensities. It is concluded that improving carrier lifetimes to >3 μs will take perovskite devices into the radiative regime, where their performance will benefit from photon-recycling

Felix Lang, Eike Köhnen, Jonathan Warby, Ke Xu, Max Grischek, Philipp Wagner, Dieter Neher, Lars Korte, Steve Albrecht, and Martin Stolterfoht* Revealing Fundamental Efficiency Limits of Monolithic Perovskite/Silicon Tandem Photovoltaics through Subcell Characterization, ACS Energy Letters 2021 6 (11), 3982-3991

https://doi.org/10.1021/acsenergylett.1c01783

Perovskite/silicon tandem photovoltaics (PVs) promise to accelerate the decarbonization of our energy systems. Here, we present a thorough subcell diagnosis methodology to reveal deep insights into the practical efficiency limitations of state-of-the-art perovskite/silicon tandem PVs. Our subcell selective intensity-dependent photoluminescence (PL) and injection-dependent electroluminescence (EL) measurements allow independent assessment of pseudo-V_OC and power conversion efficiencies (PCEs) for both subcells. We reveal identical metrics from PL and EL, which implies well-aligned energy levels throughout the entire cell. Relatively large ideality factors and insufficient charge extraction, however, cause each a fill factor penalty of about 6% (absolute). Using partial device stacks, we then identify significant losses in standard perovskite subcells due to bulk and interfacial recombination. Lastly, we present strategies to minimize these losses using triple halide (CsFAPb(IBrCl)₃) based perovskites. Our results give helpful feedback for device development and lay the foundation toward advanced perovskite/silicon tandem PVs capable of exceeding 33% PCE.

Lukas Schmidt-Mende, Vladimir Dyakonov, Selina Olthof, Feray Ünlü, Khan Moritz Trong Lê, Sanjay Mathur, Andrei D. Karabanov, Doru C. Lupascu, Laura M. Herz, Alexander Hinderhofer, Frank Schreiber, Alexey Chernikov, David A. Egger, Oleksandra Shargaieva, Caterina Cocchi, Eva Unger, Michael Saliba, Mahdi Malekshahi Byranvand, Martin Kroll, Frederik Nehm, Karl Leo, Alex Redinger, Julian Höcker, Thomas Kirchartz, Jonathan Warby, Emilio Gutierrez-Partida, Dieter Neher, Martin Stolterfoht, Uli Würfel, Moritz Unmüssig, Jan Herterich, Clemens Baretzky, John Mohanraj, Mukundan Thelakkat, Clément Maheu, Wolfram Jaegermann, Thomas Mayer, Janek Rieger, Thomas Fauster, Daniel Niesner, Fengjiu Yang, Steve Albrecht, Thomas Riedl, Azhar Fakharuddin, Maria Vasilopoulou, Yana Vaynzof, Davide Moia, Joachim Maier, Marius Franckevičius, Vidmantas Gulbinas, Ross A. Kerner, Lianfeng Zhao, Barry P. Rand, Nadja Glück, Thomas Bein, Fabio Matteocci, Luigi Angelo Castriotta, Aldo Di Carlo, Matthias Scheffler, and Claudia Draxl    Roadmap on organic–inorganic hybrid perovskite semiconductors and devices", APL Materials 9, 109202 (2021) 

https://doi.org/10.1063/5.0047616

Metal halide perovskites are the first solution processed semiconductors that can compete in their functionality with conventional semiconductors, such as silicon. Over the past several years, perovskite semiconductors have reported breakthroughs in various optoelectronic devices, such as solar cells, photodetectors, light emitting and memory devices, and so on. Until now, perovskite semiconductors face challenges regarding their stability, reproducibility, and toxicity. In this Roadmap, we combine the expertise of chemistry, physics, and device engineering from leading experts in the perovskite research community to focus on the fundamental material properties, the fabrication methods, characterization and photophysical properties, perovskite devices, and current challenges in this field. We develop a comprehensive overview of the current state-of-the-art and offer readers an informed perspective of where this field is heading and what challenges we have to overcome to get to successful commercialization.

Thiesbrummel, J., Le, V. M., Peña-Camargo, F., Perdigón-Toro, L., Lang, F., Yang, F., Grischek, M., Gutierrez-Partida, E., Warby, J., Farrar, M. D., Mahesh, S., Caprioglio, P., Albrecht, S., Neher, D., Snaith, H. J., Stolterfoht 2101447, M., Universal Current Losses in Perovskite Solar Cells Due to Mobile Ions. Adv. Energy Mater. 2021, 11, 2101447. 

https://doi.org/10.1002/aenm.202101447 

Efficient mixed metal lead-tin halide perovskites are essential for the develop-ment of all-perovskite tandem solar cells, however they are currently limited by significant short-circuit current losses despite their near optimal bandgap (≈1.25 eV). Herein, the origin of these losses is investigated, using a combination of voltage dependent photoluminescence (PL) timeseries and various charge extraction measurements. It is demonstrated that the Pb/Sn-perovskite devices suffer from a reduction in the charge extraction efficiency within the first few seconds of operation, which leads to a loss in current and lower maximum power output. In addition, the emitted PL from the device rises on the exact same time-scales due to the accumulation of electronic charges in the active layer. Using transient charge extraction measurements, it is shown that these observations cannot be explained by doping-induced electronic charges but by the movement of mobile ions toward the perovskite/transport layer interfaces, which inhibits charge extraction due to band flattening. Finally, these findings are generalized to lead-based perovskites, showing that the loss mechanism is universal. This elucidates the negative role mobile ions play in perovskite solar cells and paves a path toward understanding and mitigating a key loss mechanism

Jonas Diekmann,Pietro Caprioglio,Moritz H. Futscher,Vincent M. Le Corre,Sebastian Reichert,Frank Jaiser,Malavika Arvind,Lorena Perdigón Toro,Emilio Gutierrez-Partida,Francisco Peña-Camargo,Carsten Deibel,Bruno Ehrler,Thomas Unold,Thomas Kirchartz,Dieter Neher,Martin Stolterfoht. Pathways toward 30% Efficient Single-Junction Perovskite Solar Cells and the Role of Mobile Ions 

https://doi.org/10.1002/solr.202100219

Perovskite semiconductors have demonstrated outstanding external luminescence quantum yields, enabling high power conversion efficiencies (PCEs). However, the precise conditions to advance to an efficiency regime above monocrystalline silicon cells are not well understood. Herein, a simulation model that describes efficient p–i–n-type perovskite solar cells well and a range of different experiments is established. Then, important device and material parameters are studied and it is found that an efficiency regime of 30% can be unlocked by optimizing the built-in voltage across the perovskite layer using either highly doped (10¹⁹ cm⁻³) transport layers (TLs), doped interlayers or ultrathin self-assembled monolayers. Importantly, only parameters that have been reported in recent literature are considered, that is, a bulk lifetime of 10 μs, interfacial recombination velocities of 10 cm s⁻¹, a perovskite bandgap (Egap) of 1.5 eV, and an external quantum efficiency (EQE) of 95%. A maximum efficiency of 31% is predicted for a bandgap of 1.4 eV. Finally, it is demonstrated that the relatively high mobile ion density does not represent a significant barrier to reach this efficiency regime. The results of this study suggest continuous PCE improvements until perovskites may become the most efficient single-junction solar cell technology in the near future.

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.

Albertus A. Sutanto, Pietro Caprioglio, Nikita Drigo, Yvonne J. Hofstetter, Ines Garcia-Benito, Valentin IE Queloz, Dieter Neher, Mohammad Khaja Nazeeruddin, Martin Stolterfoht, Yana Vaynzof, Giulia Grancini. 2D/3D perovskite engineering eliminates interfacial recombination losses in hybrid perovskite solar cells . Chem, Volume 7, Issue 7, 2021, Pages 1903-1916,
https://doi.org/10.1016/j.chempr.2021.04.002 

Interface engineering and design is paramount in the optimization of a multilayer device stack. This stands true for multi-dimensional (2D/3D) perovskite-based solar cells, in which high efficiency can be combined with promising device durability. However, the complex function of the 2D/3D device interfaces remains vague. Here, we provide the exact knowledge on the interface energetics and demonstrate that the 2D/3D perovskite interface forms a p-n junction that is capable of reducing the electron density at the hole transport layer interface and ultimately suppresses interfacial recombination. As a consequence, we demonstrate photovoltaic devices with an enhanced fill factor (FF) and open-circuit voltage (V_OC) of 1.19 V, which approaches the potential internal quasi-Fermi level splitting (QFLS) voltage of the perovskite absorber, nullifying the interfacial losses. We thus identify the essential parameters and energetic alignment scenario required for 2D/3D perovskite systems to surpass the current limitations of hybrid perovskite solar cell performances.

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.