@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{PerdigonToroZhangMarkinaetal.2020, author = {Perdigon-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{ShoaeeSannaSforazzini2021, author = {Shoaee, Safa and Sanna, Anna Laura and Sforazzini, Giuseppe}, title = {Elucidating charge generation in green-solvent processed organic solar cells}, series = {Molecules : a journal of synthetic chemistry and natural product chemistry / Molecular Diversity Preservation International}, volume = {26}, journal = {Molecules : a journal of synthetic chemistry and natural product chemistry / Molecular Diversity Preservation International}, number = {24}, publisher = {MDPI}, address = {Basel}, issn = {1420-3049}, doi = {10.3390/molecules26247439}, pages = {13}, year = {2021}, abstract = {Organic solar cells have the potential to become the cheapest form of electricity. Rapid increase in the power conversion efficiency of organic solar cells (OSCs) has been achieved with the development of non-fullerene small-molecule acceptors. Next generation photovoltaics based upon environmentally benign "green solvent" processing of organic semiconductors promise a step-change in the adaptability and versatility of solar technologies and promote sustainable development. However, high-performing OSCs are still processed by halogenated (non-environmentally friendly) solvents, so hindering their large-scale manufacture. In this perspective, we discuss the recent progress in developing highly efficient OSCs processed from eco-compatible solvents, and highlight research challenges that should be addressed for the future development of high power conversion efficiencies devices.}, language = {en} } @article{VollbrechtBrus2021, author = {Vollbrecht, Joachim and Brus, Viktor V.}, title = {Effects of recombination order on open-circuit voltage decay measurements of organic and perovskite solar cells}, series = {Energies : open-access journal of related scientific research, technology development and studies in policy and management / Molecular Diversity Preservation International (MDPI)}, volume = {14}, journal = {Energies : open-access journal of related scientific research, technology development and studies in policy and management / Molecular Diversity Preservation International (MDPI)}, number = {16}, publisher = {MDPI}, address = {Basel}, issn = {1996-1073}, doi = {10.3390/en14164800}, pages = {16}, year = {2021}, abstract = {Non-geminate recombination, as one of the most relevant loss mechanisms in organic and perovskite solar cells, deserves special attention in research efforts to further increase device performance. It can be subdivided into first, second, and third order processes, which can be elucidated by the effects that they have on the time-dependent open-circuit voltage decay. In this study, analytical expressions for the open-circuit voltage decay exhibiting one of the aforementioned recombination mechanisms were derived. It was possible to support the analytical models with experimental examples of three different solar cells, each of them dominated either by first (PBDBT:CETIC-4F), second (PM6:Y6), or third (irradiated CH3NH3PbI3) order recombination. Furthermore, a simple approach to estimate the dominant recombination process was also introduced and tested on these examples. Moreover, limitations of the analytical models and the measurement technique itself were discussed.}, language = {en} } @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{ZhangChenArminetal.2017, author = {Zhang, Kai and Chen, Zhiming and Armin, Ardalan and Dong, Sheng and Xia, Ruoxi and Yip, Hin-Lap and Shoaee, Safa and Huang, Fei and Cao, Yong}, title = {Efficient large area organic solar cells processed by blade-coating with single-component green solvent}, series = {Solar Rrl}, volume = {2}, journal = {Solar Rrl}, number = {1}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {2367-198X}, doi = {10.1002/solr.201700169}, pages = {9}, year = {2017}, abstract = {While the performance of laboratory-scale organic solar cells (OSCs) continues to grow, development of high efficiency large area OSCs remains a big challenge. Although a few attempts to produce large area organic solar cells (OSCs) have been reported, there are still challenges on the way to realizing efficient module devices, such as the low compatibility of the thickness-sensitive active layer with large area coating techniques, the frequent need for toxic solvents and tedious optimization processes used during device fabrication. In this work, highly efficient thickness-insensitive OSCs based on PTB7-Th:PC71BM that processed with single-component green solvent 2-methylanisole are presented, in which both junction thickness limitation and solvent toxicity issues are simultaneously addressed. Careful investigation reveals that this green solvent prevents the evolution of PC71BM into large area clusters resulting in reduced charge carrier recombination, and largely eliminates trapping centers, and thus improves the thickness tolerance of the films. These findings enable us to address the scalability and solvent toxicity issues and to fabricate a 16 cm(2) OSC with doctor-blade coating with a state-of-the-art power conversion efficiency of 7.5\% using green solvent.}, language = {en} } @article{RanLoveHeiberetal.2018, author = {Ran, Niva A. and Love, John A. and Heiber, Michael C. and Jiao, Xuechen and Hughes, Michael P. and Karki, Akchheta and Wang, Ming and Brus, Viktor V. and Wang, Hengbin and Neher, Dieter and Ade, Harald and Bazan, Guillermo C. and Thuc-Quyen Nguyen,}, title = {Charge generation and recombination in an organic solar cell with low energetic offsets}, series = {dvanced energy materials}, volume = {8}, journal = {dvanced energy materials}, number = {5}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1614-6832}, doi = {10.1002/aenm.201701073}, pages = {12}, year = {2018}, abstract = {Organic bulk heterojunction (BHJ) solar cells require energetic offsets between the donor and acceptor to obtain high short-circuit currents (J(SC)) and fill factors (FF). However, it is necessary to reduce the energetic offsets to achieve high open-circuit voltages (V-OC). Recently, reports have highlighted BHJ blends that are pushing at the accepted limits of energetic offsets necessary for high efficiency. Unfortunately, most of these BHJs have modest FF values. How the energetic offset impacts the solar cell characteristics thus remains poorly understood. Here, a comprehensive characterization of the losses in a polymer:fullerene BHJ blend, PIPCP:phenyl-C61-butyric acid methyl ester (PC61BM), that achieves a high V-OC (0.9 V) with very low energy losses (E-loss = 0.52 eV) from the energy of absorbed photons, a respectable J(SC) (13 mA cm(-2)), but a limited FF (54\%) is reported. Despite the low energetic offset, the system does not suffer from field-dependent generation and instead it is characterized by very fast nongeminate recombination and the presence of shallow traps. The charge-carrier losses are attributed to suboptimal morphology due to high miscibility between PIPCP and PC61BM. These results hold promise that given the appropriate morphology, the J(SC), V-OC, and FF can all be improved, even with very low energetic offsets.}, language = {en} } @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{YazmaciyanStolterfohtBurnetal.2018, author = {Yazmaciyan, Aren and Stolterfoht, Martin and Burn, Paul L. and Lin, Qianqian and Meredith, Paul and Armin, Ardalan}, title = {Recombination losses above and below the transport percolation threshold in bulk heterojunction organic solar cells}, series = {Advanced energy materials}, volume = {8}, journal = {Advanced energy materials}, number = {18}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1614-6832}, doi = {10.1002/aenm.201703339}, pages = {8}, year = {2018}, abstract = {Achieving the highest power conversion efficiencies in bulk heterojunction organic solar cells requires a morphology that delivers electron and hole percolation pathways for optimized transport, plus sufficient donor:acceptor contact area for near unity charge transfer state formation. This is a significant structural challenge, particularly in semiconducting polymer:fullerene systems. This balancing act in the model high efficiency PTB7:PC70BM blend is studied by tuning the donor:acceptor ratio, with a view to understanding the recombination loss mechanisms above and below the fullerene transport percolation threshold. The internal quantum efficiency is found to be strongly correlated to the slower carrier mobility in agreement with other recent studies. Furthermore, second-order recombination losses dominate the shape of the current density-voltage curve in efficient blend combinations, where the fullerene phase is percolated. However, below the charge transport percolation threshold, there is an electric-field dependence of first-order losses, which includes electric-field-dependent photogeneration. In the intermediate regime, the fill factor appears to be limited by both first- and second-order losses. These findings provide additional basic understanding of the interplay between the bulk heterojunction morphology and the order of recombination in organic solar cells. They also shed light on the limitations of widely used transport models below the percolation threshold.}, language = {en} } @article{AlqahtaniBabicsGorenflotetal.2018, author = {Alqahtani, Obaid and Babics, Maxime and Gorenflot, Julien and Savikhin, Victoria and Ferron, Thomas and Balawi, Ahmed H. and Paulke, Andreas and Kan, Zhipeng and Pope, Michael and Clulow, Andrew J. and Wolf, Jannic and Burn, Paul L. and Gentle, Ian R. and Neher, Dieter and Toney, Michael F. and Laquai, Frederic and Beaujuge, Pierre M. and Collins, Brian A.}, title = {Mixed Domains Enhance Charge Generation and Extraction in Bulk-Heterojunction Solar Cells with Small-Molecule Donors}, series = {Advanced energy materials}, volume = {8}, journal = {Advanced energy materials}, number = {19}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1614-6832}, doi = {10.1002/aenm.201702941}, pages = {16}, year = {2018}, abstract = {The interplay between nanomorphology and efficiency of polymer-fullerene bulk-heterojunction (BHJ) solar cells has been the subject of intense research, but the generality of these concepts for small-molecule (SM) BHJs remains unclear. Here, the relation between performance; charge generation, recombination, and extraction dynamics; and nanomorphology achievable with two SM donors benzo[1,2-b:4,5-b]dithiophene-pyrido[3,4-b]-pyrazine BDT(PPTh2)(2), namely SM1 and SM2, differing by their side-chains, are examined as a function of solution additive composition. The results show that the additive 1,8-diiodooctane acts as a plasticizer in the blends, increases domain size, and promotes ordering/crystallinity. Surprisingly, the system with high domain purity (SM1) exhibits both poor exciton harvesting and severe charge trapping, alleviated only slightly with increased crystallinity. In contrast, the system consisting of mixed domains and lower crystallinity (SM2) shows both excellent exciton harvesting and low charge recombination losses. Importantly, the onset of large, pure crystallites in the latter (SM2) system reduces efficiency, pointing to possible differences in the ideal morphologies for SM-based BHJ solar cells compared with polymer-fullerene devices. In polymer-based systems, tie chains between pure polymer crystals establish a continuous charge transport network, whereas SM-based active layers may in some cases require mixed domains that enable both aggregation and charge percolation to the electrodes.}, language = {en} }