@article{PhuongHosseiniSandbergetal.2020,
  author    = {Phuong, Le Quang and Hosseini, Seyed Mehrdad and Sandberg, Oskar J. and Zou, Yingping and Woo, Han Young and Neher, Dieter and Shoaee, Safa},
  title     = {Quantifying quasi-fermi level splitting and open-circuit voltage losses in highly efficient nonfullerene organic solar cells},
  series = {Solar RRL},
  volume    = {5},
  journal   = {Solar RRL},
  number    = {1},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {2367-198X},
  doi       = {10.1002/solr.202000649},
  pages     = {6},
  year      = {2020},
  abstract  = {The power conversion efficiency (PCE) of state-of-the-art organic solar cells is still limited by significant open-circuit voltage (V-OC) losses, partly due to the excitonic nature of organic materials and partly due to ill-designed architectures. Thus, quantifying different contributions of the V-OC 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 V-OC losses in their performance are presented. By evaluating the quasi-Fermi level splitting (QFLS) and the V-OC 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.},
  language  = {en}
}
@article{TokmoldinHosseiniRaoufietal.2020,
  author    = {Tokmoldin, Nurlan and Hosseini, Seyed Mehrdad and Raoufi, Meysam and Phuong, Le Quang and Sandberg, Oskar J. and Guan, Huilan and Zou, Yingping and Neher, Dieter and Shoaee, Safa},
  title     = {Extraordinarily long diffusion length in PM6:Y6 organic solar cells},
  series = {Journal of materials chemistry : A, materials for energy and sustainability},
  volume    = {8},
  journal   = {Journal of materials chemistry : A, materials for energy and sustainability},
  number    = {16},
  publisher = {Royal Society of Chemistry},
  address   = {Cambridge},
  issn      = {2050-7488},
  doi       = {10.1039/d0ta03016c},
  pages     = {7854 -- 7860},
  year      = {2020},
  abstract  = {The PM6:Y6 bulk-heterojunction (BHJ) blend system achieves high short-circuit current (J(SC)) 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.},
  language  = {en}
}
@article{ZuoShoaeeKemerinketal.2021,
  author    = {Zuo, Guangzheng and Shoaee, Safa and Kemerink, Martijn and Neher, Dieter},
  title     = {General rules for the impact of energetic disorder and mobility on nongeminate recombination in phase-separated organic solar cells},
  series = {Physical review applied},
  volume    = {16},
  journal   = {Physical review applied},
  number    = {3},
  publisher = {American Physical Society},
  address   = {College Park},
  issn      = {2331-7019},
  doi       = {10.1103/PhysRevApplied.16.034027},
  pages     = {19},
  year      = {2021},
  abstract  = {State-of-the-art organic solar cells exhibit power conversion efficiencies of 18\% and above. These devices benefit from the suppression of free charge recombination with regard to the Langevin limit of charge encounter in a homogeneous medium. It is recognized that the main cause of suppressed free charge recombination is the reformation and resplitting of charge-transfer (CT) states at the interface between donor and acceptor domains. Here, we use kinetic Monte Carlo simulations to understand the interplay between free charge motion and recombination in an energetically disordered phase-separated donor-acceptor blend. We identify conditions for encounter-dominated and resplitting-dominated recombination. In the former regime, recombination is proportional to mobility for all parameters tested and only slightly reduced with respect to the Langevin limit. In contrast, mobility is not the decisive parameter that determines the nongeminate recombination coefficient, k(2), in the latter case, where k2 is a sole function of the morphology, CT and charge-separated (CS) energetics, and CT-state decay properties. Our simulations also show that free charge encounter in the phase-separated disordered blend is determined by the average mobility of all carriers, while CT reformation and resplitting involves mostly states near the transport energy. Therefore, charge encounter is more affected by increased disorder than the resplitting of the CT state. As a consequence, for a given mobility, larger energetic disorder, in combination with a higher hopping rate, is preferred. These findings have implications for the understanding of suppressed recombination in solar cells with nonfullerene acceptors, which are known to exhibit lower energetic disorder than that of fullerenes.},
  language  = {en}
}
@misc{PhuongHosseiniSandbergetal.2020,
  author    = {Phuong, Le Quang and Hosseini, Seyed Mehrdad and Sandberg, Oskar J. and Zou, Yingping and Woo, Han Young and Neher, Dieter and Shoaee, Safa},
  title     = {Quantifying quasi-fermi level splitting and open-circuit voltage losses in highly efficient nonfullerene organic solar cells},
  series = {Zweitver{\"o}ffentlichungen der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  journal   = {Zweitver{\"o}ffentlichungen der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  number    = {1},
  issn      = {1866-8372},
  doi       = {10.25932/publishup-57001},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-570018},
  pages     = {8},
  year      = {2020},
  abstract  = {The power conversion efficiency (PCE) of state-of-the-art organic solar cells is still limited by significant open-circuit voltage (V-OC) losses, partly due to the excitonic nature of organic materials and partly due to ill-designed architectures. Thus, quantifying different contributions of the V-OC 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 V-OC losses in their performance are presented. By evaluating the quasi-Fermi level splitting (QFLS) and the V-OC 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.},
  language  = {en}
}
@article{HosseiniTokmoldinLeeetal.2020,
  author    = {Hosseini, Seyed Mehrdad and Tokmoldin, Nurlan and Lee, Young Woong and Zou, Yingping and Woo, Han Young and Neher, Dieter and Shoaee, Safa},
  title     = {Putting order into PM6:Y6 solar cells to reduce the langevin recombination in 400 nm thick junction},
  series = {Solar RRL},
  volume    = {4},
  journal   = {Solar RRL},
  number    = {11},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {2367-198X},
  doi       = {10.1002/solr.202000498},
  pages     = {7},
  year      = {2020},
  abstract  = {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 (approximate to 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.},
  language  = {en}
}
@article{VollbrechtTokmoldinSunetal.2022,
  author    = {Vollbrecht, Joachim and Tokmoldin, Nurlan and Sun, Bowen and Brus, Viktor V. and Shoaee, Safa and Neher, Dieter},
  title     = {Determination of the charge carrier density in organic solar cells},
  series = {Journal of applied physics},
  volume    = {131},
  journal   = {Journal of applied physics},
  number    = {22},
  publisher = {American Institute of Physics},
  address   = {Melville, NY},
  issn      = {0021-8979},
  doi       = {10.1063/5.0094955},
  pages     = {18},
  year      = {2022},
  abstract  = {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. Published under an exclusive license by AIP Publishing.},
  language  = {en}
}
@article{SunSandbergNeheretal.2022,
  author    = {Sun, Bowen and Sandberg, Oskar and Neher, Dieter and Armin, Ardalan and Shoaee, Safa},
  title     = {Wave optics of differential absorption spectroscopy in thick-junction organic solar cells},
  series = {Physical review applied / The American Physical Society},
  volume    = {17},
  journal   = {Physical review applied / The American Physical Society},
  number    = {5},
  publisher = {American Physical Society},
  address   = {College Park},
  issn      = {2331-7019},
  doi       = {10.1103/PhysRevApplied.17.054016},
  pages     = {12},
  year      = {2022},
  abstract  = {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.},
  language  = {en}
}
@article{FritschKurpiersRolandetal.2022,
  author    = {Fritsch, Tobias and Kurpiers, Jona and Roland, Steffen and Tokmoldin, Nurlan and Shoaee, Safa and Ferron, Thomas and Collins, Brian A. and Janietz, Silvia and Vandewal, Koen and Neher, Dieter},
  title     = {On the interplay between CT and singlet exciton emission in organic solar cells with small driving force and its impact on voltage loss},
  series = {Advanced energy materials},
  volume    = {12},
  journal   = {Advanced energy materials},
  number    = {31},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1614-6832},
  doi       = {10.1002/aenm.202200641},
  pages     = {11},
  year      = {2022},
  abstract  = {The interplay between free charge carriers, charge transfer (CT) states and singlet excitons (S-1) determines the recombination pathway and the resulting open circuit voltage (V-OC) 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 (E-CT) is varied by 130 meV while leaving the S-1 energy of the polymer (ES1\[{E_{{{\rm{S}}_1}}}\]) unaffected. It is found that the polymer exciton dominates the radiative properties of the blend when ECT\[{E_{{\rm{CT}}}}\] approaches ES1\[{E_{{{\rm{S}}_1}}}\], while the V-OC remains 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 V-OC limit (V-OC,V-rad), but not because it is more emissive. The analysis further suggests that electronic coupling between the CT state and the S-1 will not improve the V-OC, but rather reduce the V-OC,V-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.},
  language  = {en}
}
@article{YuanZhangQiuetal.2022,
  author    = {Yuan, Jun and Zhang, Chujun and Qiu, Beibei and Liu, Wei and So, Shu Kong and Mainville, Mathieu and Leclerc, Mario and Shoaee, Safa and Neher, Dieter and Zou, Yingping},
  title     = {Effects of energetic disorder in bulk heterojunction organic solar cells},
  series = {Energy \& environmental science},
  volume    = {15},
  journal   = {Energy \& environmental science},
  number    = {7},
  publisher = {Royal Society of Chemistry},
  address   = {Cambridge},
  issn      = {1754-5692},
  doi       = {10.1039/d2ee00271j},
  pages     = {2806 -- 2818},
  year      = {2022},
  abstract  = {Organic solar cells (OSCs) have progressed rapidly in recent years through the development of novel organic photoactive materials, especially non-fullerene acceptors (NFAs). Consequently, OSCs based on state-of-the-art NFAs have reached significant milestones, such as similar to 19\% power conversion efficiencies (PCEs) and small energy losses (less than 0.5 eV). Despite these significant advances, understanding of the interplay between molecular structure and optoelectronic properties lags significantly behind. For example, despite the theoretical framework for describing the energetic disorder being well developed for the case of inorganic semiconductors, the question of the applicability of classical semiconductor theories in analyzing organic semiconductors is still under debate. A general observation in the inorganic field is that inorganic photovoltaic materials possessing a polycrystalline microstructure exhibit suppressed disorder properties and better charge carrier transport compared to their amorphous analogs. Accordingly, this principle extends to the organic semiconductor field as many organic photovoltaic materials are synthesized to pursue polycrystalline-like features. Yet, there appears to be sporadic examples that exhibit an opposite trend. However, full studies decoupling energetic disorder from aggregation effects have largely been left out. Hence, the potential role of the energetic disorder in OSCs has received little attention. Interestingly, recently reported state-of-the-art NFA-based devices could achieve a small energetic disorder and high PCE at the same time; and interest in this investigation related to the disorder properties in OSCs was revived. In this contribution, progress in terms of the correlation between molecular design and energetic disorder is reviewed together with their effects on the optoelectronic mechanism and photovoltaic performance. Finally, the specific challenges and possible solutions in reducing the energetic disorder of OSCs from the viewpoint of materials and devices are proposed.},
  language  = {en}
}
@article{PranavBenduhnNymanetal.2021,
  author    = {Pranav, Manasi and Benduhn, Johannes and Nyman, Mathias and Hosseini, Seyed Mehrdad and Kublitski, Jonas and Shoaee, Safa and Neher, Dieter and Leo, Karl and Spoltore, Donato},
  title     = {Enhanced charge selectivity via anodic-C60 layer reduces nonradiative losses in organic solar cells},
  series = {ACS applied materials \& interfaces},
  volume    = {13},
  journal   = {ACS applied materials \& interfaces},
  number    = {10},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {1944-8244},
  doi       = {10.1021/acsami.1c00049},
  pages     = {12603 -- 12609},
  year      = {2021},
  abstract  = {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 C-60 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-C-60 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.},
  language  = {en}
}
@article{HosseiniRolandKurpiersetal.2019,
  author    = {Hosseini, Seyed Mehrdad and Roland, Steffen and Kurpiers, Jona and Chen, Zhiming and Zhang, Kai and Huang, Fei and Armin, Ardalan and Neher, Dieter and Shoaee, Safa},
  title     = {Impact of Bimolecular Recombination on the Fill Factor of Fullerene and Nonfullerene-Based Solar Cells},
  series = {The journal of physical chemistry : C, Nanomaterials and interfaces},
  volume    = {123},
  journal   = {The journal of physical chemistry : C, Nanomaterials and interfaces},
  number    = {11},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {1932-7447},
  doi       = {10.1021/acs.jpcc.8b11669},
  pages     = {6823 -- 6830},
  year      = {2019},
  abstract  = {Power conversion efficiencies of donor/acceptor organic solar cells utilizing nonfullerene acceptors have now increased beyond the record of their fullerene-based counterparts. There remain many fundamental questions regarding nanomorphology, interfacial states, charge generation and extraction, and losses in these systems. Herein, we present a comparative study of bulk heterojunction solar cells composed of a recently introduced naphthothiadiazole-based polymer (NT812) as the electron donor and two different acceptor molecules, namely, [6,6]-phenyl-C71-butyric acid methyl ester (PCBM)[70] and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC). A comparison between the photovoltaic performance of these two types of solar cells reveals that the open-circuit voltage (Voc) of the NT812:ITIC-based solar cell is larger, but the fill factor (FF) is lower than that of the NT812:PCBM[70] device. We find the key reason behind this reduced FF in the ITIC-based device to be faster nongeminate recombination relative to the NT812:PCBM[70] system.},
  language  = {en}
}
@article{ColladoFregosoHoodShoaeeetal.2017,
  author    = {Collado-Fregoso, Elisa and Hood, Samantha N. and Shoaee, Safa and Schr{\"o}der, Bob C. and McCulloch, Iain and Kassal, Ivan and Neher, Dieter and Durrant, James R.},
  title     = {Intercalated vs Nonintercalated Morphologies in Donor-Acceptor Bulk Heterojunction Solar Cells: PBTTT:Fullerene Charge Generation and Recombination Revisited},
  series = {The journal of physical chemistry letters},
  volume    = {8},
  journal   = {The journal of physical chemistry letters},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {1948-7185},
  doi       = {10.1021/acs.jpclett.7b01571},
  pages     = {4061 -- 4068},
  year      = {2017},
  abstract  = {In this Letter, we study the role of the donor:acceptor interface nanostructure upon charge separation and recombination in organic photovoltaic devices and blend films, using mixtures of PBTTT and two different fullerene derivatives (PC70BM and ICTA) as models for intercalated and nonintercalated morphologies, respectively. Thermodynamic simulations show that while the completely intercalated system exhibits a large free-energy barrier for charge separation, this barrier is significantly lower in the nonintercalated system and almost vanishes when energetic disorder is included in the model. Despite these differences, both femtosecond-resolved transient absorption spectroscopy (TAS) and time-delayed collection field (TDCF) exhibit extensive first-order losses in both systems, suggesting that geminate pairs are the primary product of photoexcitation. In contrast, the system that comprises a combination of fully intercalated polymer:fullerene areas and fullerene-aggregated domains (1:4 PBTTT:PC70BM) is the only one that shows slow, second-order recombination of free charges, resulting in devices with an overall higher short-circuit current and fill factor. This study therefore provides a novel consideration of the role of the interfacial nanostructure and the nature of bound charges and their impact upon charge generation and recombination.},
  language  = {en}
}
@article{PranavHultzschMusiienkoetal.2023,
  author    = {Pranav, Manasi and Hultzsch, Thomas and Musiienko, Artem and Sun, Bowen and Shukla, Atul and Jaiser, Frank and Shoaee, Safa and Neher, Dieter},
  title     = {Anticorrelated photoluminescence and free charge generation proves field-assisted exciton dissociation in low-offset PM6:Y5 organic solar cells},
  series = {APL materials : high impact open access journal in functional materials science},
  volume    = {11},
  journal   = {APL materials : high impact open access journal in functional materials science},
  number    = {6},
  publisher = {AIP Publishing},
  address   = {Melville},
  issn      = {2166-532X},
  doi       = {10.1063/5.0151580},
  pages     = {8},
  year      = {2023},
  abstract  = {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 wnon-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. (c) 2023 Author(s).},
  language  = {en}
}
@article{TokmoldinVollbrechtHosseinietal.2021,
  author    = {Tokmoldin, Nurlan and Vollbrecht, Joachim and Hosseini, Seyed Mehrdad and Sun, Bowen and Perdig{\´o}n-Toro, Lorena and Woo, Han Young and Zou, Yingping and Neher, Dieter and Shoaee, Safa},
  title     = {Explaining the fill-factor and photocurrent losses of nonfullerene acceptor-based solar cells by probing the long-range charge carrier diffusion and drift lengths},
  series = {Advanced energy materials},
  volume    = {11},
  journal   = {Advanced energy materials},
  number    = {22},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1614-6840},
  doi       = {10.1002/aenm.202100804},
  pages     = {9},
  year      = {2021},
  abstract  = {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 theta and alpha. 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 theta and alpha 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.},
  language  = {en}
}
@article{ZhangHosseiniGunderetal.2019,
  author    = {Zhang, Shanshan and Hosseini, Seyed Mehrdad and Gunder, Rene and Petsiuk, Andrei and Caprioglio, Pietro and Wolff, Christian Michael and Shoaee, Safa and Meredith, Paul and Schorr, Susan and Unold, Thomas and Burn, Paul L. and Neher, Dieter and Stolterfoht, Martin},
  title     = {The Role of Bulk and Interface Recombination in High-Efficiency Low-Dimensional Perovskite Solar Cells},
  series = {Advanced materials},
  volume    = {31},
  journal   = {Advanced materials},
  number    = {30},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {0935-9648},
  doi       = {10.1002/adma.201901090},
  pages     = {11},
  year      = {2019},
  abstract  = {2D Ruddlesden-Popper perovskite (RPP) solar cells have excellent environmental stability. However, the power conversion efficiency (PCE) of RPP cells remains inferior to 3D perovskite-based cells. Herein, 2D (CH3(CH2)(3)NH3)(2)(CH3NH3)(n-1)PbnI3n+1 perovskite cells with different numbers of [PbI6](4-) sheets (n = 2-4) are analyzed. Photoluminescence quantum yield (PLQY) measurements show that nonradiative open-circuit voltage (V-OC) losses outweigh radiative losses in materials with n > 2. The n = 3 and n = 4 films exhibit a higher PLQY than the standard 3D methylammonium lead iodide perovskite although this is accompanied by increased interfacial recombination at the top perovskite/C-60 interface. This tradeoff results in a similar PLQY in all devices, including the n = 2 system where the perovskite bulk dominates the recombination properties of the cell. In most cases the quasi-Fermi level splitting matches the device V-OC within 20 meV, which indicates minimal recombination losses at the metal contacts. The results show that poor charge transport rather than exciton dissociation is the primary reason for the reduction in fill factor of the RPP devices. Optimized n = 4 RPP solar cells had PCEs of 13\% with significant potential for further improvements.},
  language  = {en}
}
@article{WuerfelPerdigonToroKurpiersetal.2019,
  author    = {W{\"u}rfel, Uli and Perdig{\´o}n-Toro, Lorena and Kurpiers, Jona and Wolff, Christian Michael and Caprioglio, Pietro and Rech, Jeromy James and Zhu, Jingshuai and Zhan, Xiaowei and You, Wei and Shoaee, Safa and Neher, Dieter and Stolterfoht, Martin},
  title     = {Recombination between Photogenerated and Electrode-Induced Charges Dominates the Fill Factor Losses in Optimized Organic Solar Cells},
  series = {The journal of physical chemistry letters},
  volume    = {10},
  journal   = {The journal of physical chemistry letters},
  number    = {12},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {1948-7185},
  doi       = {10.1021/acs.jpclett.9b01175},
  pages     = {3473 -- 3480},
  year      = {2019},
  abstract  = {Charge extraction in organic solar cells (OSCs) is commonly believed to be limited by bimolecular recombination of photogenerated charges. However, the fill factor of OSCs is usually almost entirely governed by recombination processes that scale with the first order of the light intensity. This linear loss was often interpreted to be a consequence of geminate or trap-assisted recombination. Numerical simulations show that this linear dependence is a direct consequence of the large amount of excess dark charge near the contact. The first-order losses increase with decreasing mobility of minority carriers, and we discuss the impact of several material and device parameters on this loss mechanism. This work highlights that OSCs are especially vulnerable to injected charges as a result of their poor charge transport properties. This implies that dark charges need to be better accounted for when interpreting electro-optical measurements and charge collection based on simple figures of merit.},
  language  = {en}
}
@article{ShoaeeArminStolterfohtetal.2019,
  author    = {Shoaee, Safa and Armin, Ardalan and Stolterfoht, Martin and Hosseini, Seyed Mehrdad and Kurpiers, Jona and Neher, Dieter},
  title     = {Decoding Charge Recombination through Charge Generation in Organic Solar Cells},
  series = {Solar RRL},
  volume    = {3},
  journal   = {Solar RRL},
  number    = {11},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {2367-198X},
  doi       = {10.1002/solr.201900184},
  pages     = {8},
  year      = {2019},
  abstract  = {The in-depth understanding of charge carrier photogeneration and recombination mechanisms in organic solar cells is still an ongoing effort. In donor:acceptor (bulk) heterojunction organic solar cells, charge photogeneration and recombination are inter-related via the kinetics of charge transfer states-being singlet or triplet states. Although high-charge-photogeneration quantum yields are achieved in many donor:acceptor systems, only very few systems show significantly reduced bimolecular recombination relative to the rate of free carrier encounters, in low-mobility systems. This is a serious limitation for the industrialization of organic solar cells, in particular when aiming at thick active layers. Herein, a meta-analysis of the device performance of numerous bulk heterojunction organic solar cells is presented for which field-dependent photogeneration, charge carrier mobility, and fill factor are determined. Herein, a "spin-related factor" that is dependent on the ratio of back electron transfer of the triplet charge transfer (CT) states to the decay rate of the singlet CT states is introduced. It is shown that this factor links the recombination reduction factor to charge-generation efficiency. As a consequence, it is only in the systems with very efficient charge generation and very fast CT dissociation that free carrier recombination is strongly suppressed, regardless of the spin-related factor.},
  language  = {en}
}
@article{PerdigonToroZhangMarkinaetal.2020,
  author    = {Perdig{\´o}n-Toro, Lorena and Zhang, Huotian and Markina, Anastaa si and Yuan, Jun and Hosseini, Seyed Mehrdad and Wolff, Christian Michael and Zuo, Guangzheng and Stolterfoht, Martin and Zou, Yingping and Gao, Feng and Andrienko, Denis and Shoaee, Safa and Neher, Dieter},
  title     = {Barrierless free charge generation in the high-performance PM6:Y6 bulk heterojunction non-fullerene solar cell},
  series = {Advanced materials},
  volume    = {32},
  journal   = {Advanced materials},
  number    = {9},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {0935-9648},
  doi       = {10.1002/adma.201906763},
  pages     = {9},
  year      = {2020},
  abstract  = {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.},
  language  = {en}
}
@article{SandbergKurpiersStolterfohtetal.2020,
  author    = {Sandberg, Oskar J. and Kurpiers, Jona and Stolterfoht, Martin and Neher, Dieter and Meredith, Paul and Shoaee, Safa and Armin, Ardalan},
  title     = {On the question of the need for a built-in potential in Perovskite solar cells},
  series = {Advanced materials interfaces},
  volume    = {7},
  journal   = {Advanced materials interfaces},
  number    = {10},
  publisher = {Wiley},
  address   = {Hoboken},
  issn      = {2196-7350},
  doi       = {10.1002/admi.202000041},
  pages     = {8},
  year      = {2020},
  abstract  = {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.},
  language  = {en}
}
@article{SamsonRechPerdigonToroetal.2020,
  author    = {Samson, Stephanie and Rech, Jeromy and Perdig{\´o}n-Toro, Lorena and Peng, Zhengxing and Shoaee, Safa and Ade, Harald and Neher, Dieter and Stolterfoht, Martin and You, Wei},
  title     = {Organic solar cells with large insensitivity to donor polymer molar mass across all acceptor classes},
  series = {ACS applied polymer materials},
  volume    = {2},
  journal   = {ACS applied polymer materials},
  number    = {11},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {2637-6105},
  doi       = {10.1021/acsapm.0c01041},
  pages     = {5300 -- 5308},
  year      = {2020},
  abstract  = {Donor polymer number-average molar mass (M-n) 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 M-n effects varies from system to system, although there is generally some intermediate M-n 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), PcBm demonstrated <10\% variation in power conversion efficiency and a consistent morphology over a large span of M-n (30 kg/mol to over 100 kg/mol). Would such insensitivity to polymer M-n still hold true when prevailing small molecular acceptors were used with FTAZ? To answer this question, we explored the impact of FTAZ 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 M. Our results may have implications in polymer batch-to-batch reproducibility, in particular, relaxing the need for tight M-n control during synthesis.},
  language  = {en}
}
@misc{ShoaeeStolterfohtNeher2018,
  author    = {Shoaee, Safa and Stolterfoht, Martin and Neher, Dieter},
  title     = {The Role of Mobility on Charge Generation, Recombination, and Extraction in Polymer-Based Solar Cells},
  series = {dvanced energy materials},
  volume    = {8},
  journal   = {dvanced energy materials},
  number    = {28},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1614-6832},
  doi       = {10.1002/aenm.201703355},
  pages     = {20},
  year      = {2018},
  abstract  = {Organic semiconductors are of great interest for a broad range of optoelectronic applications due to their solution processability, chemical tunability, highly scalable fabrication, and mechanical flexibility. In contrast to traditional inorganic semiconductors, organic semiconductors are intrinsically disordered systems and therefore exhibit much lower charge carrier mobilities-the Achilles heel of organic photovoltaic cells. In this progress review, the authors discuss recent important developments on the impact of charge carrier mobility on the charge transfer state dissociation, and the interplay of free charge extraction and recombination. By comparing the mobilities on different timescales obtained by different techniques, the authors highlight the dispersive nature of these materials and how this reflects on the key processes defining the efficiency of organic photovoltaics.},
  language  = {en}
}
@article{PerdigonToroLeQuangPhuongElleretal.2022,
  author    = {Perdig{\´o}n-Toro, Lorena and Le Quang Phuong, and Eller, Fabian and Freychet, Guillaume and Saglamkaya, Elifnaz and Khan, Jafar and Wei, Qingya and Zeiske, Stefan and Kroh, Daniel and Wedler, Stefan and Koehler, Anna and Armin, Ardalan and Laquai, Frederic and Herzig, Eva M. and Zou, Yingping and Shoaee, Safa and Neher, Dieter},
  title     = {Understanding the role of order in Y-series non-fullerene solar cells to realize high open-circuit voltages},
  series = {Advanced energy materials},
  volume    = {12},
  journal   = {Advanced energy materials},
  number    = {12},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1614-6832},
  doi       = {10.1002/aenm.202103422},
  pages     = {13},
  year      = {2022},
  abstract  = {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 temperature-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 V-OC on temperature and illumination intensity. Disorder is found to influence the value of the V-OC at room temperature, but also its progression with temperature. Here, the PM6:Y6 blend benefits substantially 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.},
  language  = {en}
}
@article{PerdigonToroLeQuangPhuongZeiskeetal.2021,
  author    = {Perdig{\´o}n-Toro, Lorena and Le Quang Phuong, and Zeiske, Stefan and Vandewal, Koen and Armin, Ardalan and Shoaee, Safa and Neher, Dieter},
  title     = {Excitons dominate the emission from PM6},
  series = {ACS energy letters / American Chemical Society},
  volume    = {6},
  journal   = {ACS energy letters / American Chemical Society},
  number    = {2},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {2380-8195},
  doi       = {10.1021/acsenergylett.0c02572},
  pages     = {557 -- 564},
  year      = {2021},
  abstract  = {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.},
  language  = {en}
}
@misc{YeZhangWarbyetal.2022,
  author    = {Ye, Fangyuan and Zhang, Shuo and Warby, Jonathan and Wu, Jiawei and Gutierrez-Partida, Emilio and Lang, Felix and Shah, Sahil and Saglamkaya, Elifnaz and Sun, Bowen and Zu, Fengshuo and Shoaee, Safa and Wang, Haifeng and Stiller, Burkhard and Neher, Dieter and Zhu, Wei-Hong and Stolterfoht, Martin and Wu, Yongzhen},
  title     = {Overcoming C₆₀-induced interfacial recombination in inverted perovskite solar cells by electron-transporting carborane},
  series = {Zweitver{\"o}ffentlichungen der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  journal   = {Zweitver{\"o}ffentlichungen der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  number    = {1317},
  issn      = {1866-8372},
  doi       = {10.25932/publishup-58770},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-587705},
  pages     = {12},
  year      = {2022},
  abstract  = {Inverted perovskite solar cells still suffer from significant non-radiative recombination losses at the perovskite surface and across the perovskite/C₆₀ interface, limiting the future development of perovskite-based single- and multi-junction photovoltaics. Therefore, more effective inter- or transport layers are urgently required. To tackle these recombination losses, we introduce ortho-carborane as an interlayer material that has a spherical molecular structure and a three-dimensional aromaticity. Based on a variety of experimental techniques, we show that ortho-carborane decorated with phenylamino groups effectively passivates the perovskite surface and essentially eliminates the non-radiative recombination loss across the perovskite/C₆₀ interface with high thermal stability. We further demonstrate the potential of carborane as an electron transport material, facilitating electron extraction while blocking holes from the interface. The resulting inverted perovskite solar cells deliver a power conversion efficiency of over 23\% with a low non-radiative voltage loss of 110 mV, and retain >97\% of the initial efficiency after 400 h of maximum power point tracking. Overall, the designed carborane based interlayer simultaneously enables passivation, electron-transport and hole-blocking and paves the way toward more efficient and stable perovskite solar cells.},
  language  = {en}
}
@article{YeZhangWarbyetal.2022,
  author    = {Ye, Fangyuan and Zhang, Shuo and Warby, Jonathan and Wu, Jiawei and Gutierrez-Partida, Emilio and Lang, Felix and Shah, Sahil and Saglamkaya, Elifnaz and Sun, Bowen and Zu, Fengshuo and Shoaee, Safa and Wang, Haifeng and Stiller, Burkhard and Neher, Dieter and Zhu, Wei-Hong and Stolterfoht, Martin and Wu, Yongzhen},
  title     = {Overcoming C-60-induced interfacial recombination in inverted perovskite solar cells by electron-transporting carborane},
  series = {Nature Communications},
  volume    = {13},
  journal   = {Nature Communications},
  number    = {1},
  publisher = {Nature Publishing Group},
  address   = {London},
  issn      = {2041-1723},
  doi       = {10.1038/s41467-022-34203-x},
  pages     = {12},
  year      = {2022},
  abstract  = {Inverted perovskite solar cells still suffer from significant non-radiative recombination losses at the perovskite surface and across the perovskite/C-60 interface, limiting the future development of perovskite-based single- and multi-junction photovoltaics. Therefore, more effective inter- or transport layers are urgently required. To tackle these recombination losses, we introduce ortho-carborane as an interlayer material that has a spherical molecular structure and a three-dimensional aromaticity. Based on a variety of experimental techniques, we show that ortho-carborane decorated with phenylamino groups effectively passivates the perovskite surface and essentially eliminates the non-radiative recombination loss across the perovskite/C-60 interface with high thermal stability. We further demonstrate the potential of carborane as an electron transport material, facilitating electron extraction while blocking holes from the interface. The resulting inverted perovskite solar cells deliver a power conversion efficiency of over 23\% with a low non-radiative voltage loss of 110mV, and retain >97\% of the initial efficiency after 400h of maximum power point tracking. Overall, the designed carborane based interlayer simultaneously enables passivation, electron-transport and hole-blocking and paves the way toward more efficient and stable perovskite solar cells. Effective transport layers are essential to suppress non-radiative recombination losses. Here, the authors introduce phenylamino-functionalized ortho-carborane as an interfacial layer, and realise inverted perovskite solar cells with efficiency of over 23\% and operational stability of T97=400h.},
  language  = {en}
}