@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{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{VollbrechtBrus2020,
  author    = {Vollbrecht, Joachim and Brus, Viktor V.},
  title     = {On charge carrier density in organic solar cells obtained via capacitance spectroscopy},
  series = {Advanced electronic materials},
  volume    = {6},
  journal   = {Advanced electronic materials},
  number    = {10},
  publisher = {Wiley},
  address   = {Hoboken},
  issn      = {2199-160X},
  doi       = {10.1002/aelm.202000517},
  pages     = {9},
  year      = {2020},
  abstract  = {The determination of the voltage-dependent density of free charge carriers via capacitance spectroscopy is considered an important step in the analysis of emerging photovoltaic technologies, such as organic and perovskite solar cells. In particular, an intimate knowledge of the density of free charge carriers is required for the determination of crucial parameters such as the effective mobility, charge carrier lifetime, nongeminate recombination coefficients, average extraction times, and competition factors. Hence, it is paramount to verify the validity of the commonly employed approaches to obtain the density of free charge carriers. The advantages, drawbacks, and limitations of the most common approaches are investigated in detail and strategies to mitigate misleading values are explored. To this end, two types of nonfullerene organic solar cells based on a PTB7-Th:ITIC-2F blend and a PM6:Y6 blend, respectively, are used as a case study to assess how subsequent analyses of the nongeminate recombination dynamics depend on the chosen approach to calculate the density of free charge carriers via capacitance spectroscopy.},
  language  = {en}
}
@article{VollbrechtBrus2020,
  author    = {Vollbrecht, Joachim and Brus, Viktor V.},
  title     = {On the recombination order of surface recombination under open circuit conditions},
  series = {Organic electronics : physics, materials and applications},
  volume    = {86},
  journal   = {Organic electronics : physics, materials and applications},
  publisher = {Elsevier Science},
  address   = {Amsterdam [u.a.]},
  issn      = {1566-1199},
  doi       = {10.1016/j.orgel.2020.105905},
  pages     = {7},
  year      = {2020},
  abstract  = {Understanding the recombination dynamics of organic and perovskite solar cells has been a crucial prerequisite in the steadily increasing performance of these promising new types of photovoltaics. Surface recombination in particular has turned out to be one of the last remaining roadblocks, which specifically reduces the open circuit voltage. In this study, the relationship between the rate of surface recombination and the density of charge carriers is analyzed, revealing a cubic dependence between these two parameters. This hypothesis is then tested and verified with the recombination dynamics of an organic solar cell known to exhibit significant surface recombination and a high energy proton irradiated CH3NH3PbI3 pemvskite solar cell during white light illumination. Incidentally, these results can also explain recombination orders exceeding the commonly known threshold for bimolecular recombination that have been observed in some studies without the need for a charge carrier dependent bimolecular recombination coefficient.},
  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}
}