@article{ArminChenJinetal.2018,
  author    = {Armin, Ardalan and Chen, Zhiming and Jin, Yaocheng and Zhang, Kai and Huang, Fei and Shoaee, Safa},
  title     = {A Shockley-Type polymer},
  series = {Advanced energy materials},
  volume    = {8},
  journal   = {Advanced energy materials},
  number    = {7},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1614-6832},
  doi       = {10.1002/aenm.201701450},
  pages     = {9},
  year      = {2018},
  abstract  = {Charge extraction rate in solar cells made of blends of electron donating/accepting organic semiconductors is typically slow due to their low charge carrier mobility. This sets a limit on the active layer thickness and has hindered the industrialization of organic solar cells (OSCs). Herein, charge transport and recombination properties of an efficient polymer (NT812):fullerene blend are investigated. This system delivers power conversion efficiency of >9\% even when the junction thickness is as large as 800 nm. Experimental results indicate that this material system exhibits exceptionally low bimolecular recombination constant, 800 times smaller than the diffusion-controlled electron and hole encounter rate. Comparing theoretical results based on a recently introduced modified Shockley model for fill factor, and experiments, clarifies that charge collection is nearly ideal in these solar cells even when the thickness is several hundreds of nanometer. This is the first realization of high-efficiency Shockley-type organic solar cells with junction thicknesses suitable for scaling up.},
  language  = {en}
}
@article{ArminDurrantShoaee2017,
  author    = {Armin, Ardalan and Durrant, James R. and Shoaee, Safa},
  title     = {Interplay Between Triplet-, Singlet-Charge Transfer States and Free Charge Carriers Defining Bimolecular Recombination Rate Constant of Organic Solar Cells},
  series = {The journal of physical chemistry : C, Nanomaterials and interfaces},
  volume    = {121},
  journal   = {The journal of physical chemistry : C, Nanomaterials and interfaces},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {1932-7447},
  doi       = {10.1021/acs.jpcc.7b04825},
  pages     = {13969 -- 13976},
  year      = {2017},
  abstract  = {Despite the myriad of organic donor:acceptor materials, only few systems have emerged in the life of organic solar cells to, exhibit considerable reduced bimolecular recombination, with respect to the random encounter rate given by the Langevin equation. Monte Carlo simulations have revealed that the rate constant of the formation of electron-hole bound states depends on the random encounter of opposite charges and is nearly given by the Langevin equation for the domain sizes relevant to efficient bulk heterojunction systems. Recently, three studies :suggested that charge transfer states dissociating much faster than their decay rate to the ground state, can result in reduced bimolecular recombination by lowering the recombination rate to the ground state as a loss pathway. A separate study identified another loss pathway and suggested that forbidden back electron transfer from triplet charge transfer states to triplet excitons is a key to achieving reduced recombination. Herein we further explain the reduced bimolecular recombination by investigating the limitations of these two proposals. By solving kinetic rate equations for a BHJ system with realistic rates, we show that both of these previously presented conditions must only be held at the same time fora system to exhibit non-Langevin behavior. We demonstrate that suppression of both of the parallel loss channels of singlet and triplet states can be achieved through increasing the dissociation rate of the charge transfer states; a crucial requirement to achieve a high charge carrier extraction efficiency.},
  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{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{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{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{LeQuangPhuongHosseiniKohetal.2019,
  author    = {Le Quang Phuong, and Hosseini, Seyed Mehrdad and Koh, Chang Woo and Woo, Han Young and Shoaee, Safa},
  title     = {Measuring Competing Recombination Losses in a Significantly Reduced Langevin System by Steady-State Photoinduced Absorption and Photocurrent Spectroscopy},
  series = {The journal of physical chemistry : C, Nanomaterials and interfaces},
  volume    = {123},
  journal   = {The journal of physical chemistry : C, Nanomaterials and interfaces},
  number    = {45},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {1932-7447},
  doi       = {10.1021/acs.jpcc.9b08901},
  pages     = {27417 -- 27422},
  year      = {2019},
  abstract  = {Understanding and disentangling photophysical properties of long-lived photoexcitations in bulk heterojunction (BHJ) solar cells, which contribute mostly to photocurrent, provide essential guidelines to their improvement. However, to construct improved physical models, their rational design relies on reliable measurement techniques for charge recombination. Here, we combine photocurrent and photoinduced absorption spectroscopy (PCPIA) to directly probe the free carrier concentration and investigate loss mechanisms of long-lived excitations in nearly 10\% efficient PPDT2FBT/PC70BM BHJ solar cells under steady-state operational conditions. From the PCPIA data obtained under open- circuit and short-circuit conditions, the absorption cross section and the concentration of photoexcitations are obtained. This material system exhibits an exceptionally low bimolecular recombination rate, about 300 times smaller than the diffusion-controlled electron and hole encounter rate. Furthermore, we observe that the fill factor is limited by losses originating from long-lived photoexcitations undergoing dispersive bimolecular recombination.},
  language  = {en}
}
@article{MarinBeloquiZhangGuoetal.2022,
  author    = {Marin-Beloqui, Jose and Zhang, Guanran and Guo, Junjun and Shaikh, Jordan and Wohrer, Thibaut and Hosseini, Seyed Mehrdad and Sun, Bowen and Shipp, James and Auty, Alexander J. and Chekulaev, Dimitri and Ye, Jun and Chin, Yi-Chun and Sullivan, Michael B. and Mozer, Attila J. and Kim, Ji-Seon and Shoaee, Safa and Clarke, Tracey M.},
  title     = {Insight into the origin of trapping in polymer/fullerene blends with a systematic alteration of the fullerene to higher adducts},
  series = {Journal of physical chemistry C},
  volume    = {126},
  journal   = {Journal of physical chemistry C},
  number    = {5},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {1932-7447},
  doi       = {10.1021/acs.jpcc.1c10378},
  pages     = {2708 -- 2719},
  year      = {2022},
  abstract  = {The bimolecular recombination characteristics of conjugated polymer poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(2,5-bis 3-tetradecylthiophen-2-y1 thiazolo 5,4-d thiazole)-2,5diy1] (PDTSiTTz) blended with the fullerene series PC60BM, ICMA, ICBA, and ICTA have been investigated using microsecond and femtosecond transient absorption spectroscopy, in conjunction with electroluminescence measurements and ambient photoemission spectroscopy. The non-Langevin polymer PDTSiTTz allows an inspection of intrinsic bimolecular recombination rates uninhibited by diffusion, while the low oscillator strengths of fullerenes allow polymer features to dominate, and we compare our results to those of the well-known polymer Si-PCPDTBT. Using mu s-TAS, we have shown that the trap -limited decay dynamics of the PDTSiTTz polaron becomes progressively slower across the fullerene series, while those of Si-PCPDTBT are invariant. Electroluminescence measurements showed an unusual double peak in pristine PDTSiTTz, attributed to a low energy intragap charge transfer state, likely interchain in nature. Furthermore, while the pristine PDTSiTTz showed a broad, low-intensity density of states, the ICBA and ICTA blends presented a virtually identical DOS to Si-PCPDTBT and its blends. This has been attributed to a shift from a delocalized, interchain highest occupied molecular orbital (HOMO) in the pristine material to a dithienosilole-centered HOMO in the blends, likely a result of the bulky fullerenes increasing interchain separation. This HOMO localization had a side effect of progressively shifting the polymer HOMO to shallower energies, which was correlated with the observed decrease in bimolecular recombination rate and increased "trap" depth. However, since the density of tail states remained the same, this suggests that the traditional viewpoint of "trapping" being dominated by tail states may not encompass the full picture and that the breadth of the DOS may also have a strong influence on bimolecular recombination.},
  language  = {en}
}
@article{NeusserSunTanetal.2022,
  author    = {Neusser, David and Sun, Bowen and Tan, Wen Liang and Thomsen, Lars and Schultz, Thorsten and Perdigon-Toro, Lorena and Koch, Norbert and Shoaee, Safa and McNeill, Christopher R. and Neher, Dieter and Ludwigs, Sabine},
  title     = {Spectroelectrochemically determined energy levels of PM6:Y6 blends and their relevance to solar cell performance},
  series = {Journal of materials chemistry : C, Materials for optical and electronic devices},
  volume    = {10},
  journal   = {Journal of materials chemistry : C, Materials for optical and electronic devices},
  number    = {32},
  publisher = {Royal Society of Chemistry},
  address   = {Cambridge},
  issn      = {2050-7526},
  doi       = {10.1039/d2tc01918c},
  pages     = {11565 -- 11578},
  year      = {2022},
  abstract  = {Recent advances in organic solar cell performance have been mainly driven forward by combining high-performance p-type donor-acceptor copolymers (e.g.PM6) and non-fullerene small molecule acceptors (e.g.Y6) as bulk-heterojunction layers. A general observation in such devices is that the device performance, e.g., the open-circuit voltage, is strongly dependent on the processing solvent. While the morphology is a typically named key parameter, the energetics of donor-acceptor blends are equally important, but less straightforward to access in the active multicomponent layer. Here, we propose to use spectral onsets during electrochemical cycling in a systematic spectroelectrochemical study of blend films to access the redox behavior and the frontier orbital energy levels of the individual compounds. Our study reveals that the highest occupied molecular orbital offset (Delta E-HOMO) in PM6:Y6 blends is similar to 0.3 eV, which is comparable to the binding energy of Y6 excitons and therefore implies a nearly zero driving force for the dissociation of Y6 excitons. Switching the PM6 orientation in the blend films from face-on to edge-on in bulk has only a minor influence on the positions of the energy levels, but shows significant differences in the open circuit voltage of the device. We explain this phenomenon by the different interfacial molecular orientations, which are known to affect the non-radiative decay rate of the charge-transfer state. We compare our results to ultraviolet photoelectron spectroscopy data, which shows distinct differences in the HOMO offsets in the PM6:Y6 blend compared to neat films. This highlights the necessity to measure the energy levels of the individual compounds in device-relevant blend films.},
  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}
}