TY - JOUR A1 - Vollbrecht, Joachim A1 - Brus, Viktor V. T1 - Effects of recombination order on open-circuit voltage decay measurements of organic and perovskite solar cells JF - Energies : open-access journal of related scientific research, technology development and studies in policy and management / Molecular Diversity Preservation International (MDPI) N2 - 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. KW - organic solar cells KW - perovskite solar cells KW - non-geminate recombination KW - recombination order KW - open-circuit voltage decay Y1 - 2021 U6 - https://doi.org/10.3390/en14164800 SN - 1996-1073 VL - 14 IS - 16 PB - MDPI CY - Basel ER - TY - JOUR A1 - Vollbrecht, Joachim A1 - Brus, Viktor V. T1 - On charge carrier density in organic solar cells obtained via capacitance spectroscopy JF - Advanced electronic materials N2 - 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. KW - bulk-heterojunction solar cells KW - capacitance spectroscopy KW - charge KW - carrier density KW - impedance spectroscopy KW - organic photovoltaics Y1 - 2020 U6 - https://doi.org/10.1002/aelm.202000517 SN - 2199-160X VL - 6 IS - 10 PB - Wiley CY - Hoboken ER - TY - JOUR A1 - Vollbrecht, Joachim A1 - Brus, Viktor V. T1 - On the recombination order of surface recombination under open circuit conditions JF - Organic electronics : physics, materials and applications N2 - 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. KW - surface recombination KW - recombination order KW - organic photovoltaics KW - Perovskite solar cells KW - charge carrier density KW - Shockley-Read-Hall KW - statistics Y1 - 2020 U6 - https://doi.org/10.1016/j.orgel.2020.105905 SN - 1566-1199 SN - 1878-5530 VL - 86 PB - Elsevier Science CY - Amsterdam [u.a.] ER - TY - JOUR A1 - Vollbrecht, Joachim A1 - Tokmoldin, Nurlan A1 - Sun, Bowen A1 - Brus, Viktor V. A1 - Shoaee, Safa A1 - Neher, Dieter T1 - Determination of the charge carrier density in organic solar cells BT - a tutorial JF - Journal of applied physics N2 - 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. KW - Electrical properties and parameters KW - Organic semiconductors KW - Solar cells KW - Photoconductivity KW - Capacitance spectroscopy Y1 - 2022 U6 - https://doi.org/10.1063/5.0094955 SN - 0021-8979 SN - 1089-7550 SN - 1520-8850 VL - 131 IS - 22 PB - American Institute of Physics CY - Melville, NY ER - TY - JOUR A1 - Wiebeler, Christian A1 - Vollbrecht, Joachim A1 - Neuba, Adam A1 - Kitzerow, Heinz A1 - Schumacher, Stefan T1 - Unraveling the electrochemical and spectroscopic properties of neutral and negatively charged perylene tetraethylesters JF - Scientific reports N2 - A detailed investigation of the energy levels of perylene-3,4,9,10-tetracarboxylic tetraethylester as a representative compound for the whole family of perylene esters was performed. It was revealed via electrochemical measurements that one oxidation and two reductions take place. The bandgaps determined via the electrochemical approach are in good agreement with the optical bandgap obtained from the absorption spectra via a Tauc plot. In addition, absorption spectra in dependence of the electrochemical potential were the basis for extensive quantum-chemical calculations of the neutral, monoanionic, and dianionic molecules. For this purpose, calculations based on density functional theory were compared with post-Hartree-Fock methods and the CAM-B3LYP functional proved to be the most reliable choice for the calculation of absorption spectra. Furthermore, spectral features found experimentally could be reproduced with vibronic calculations and allowed to understand their origins. In particular, the two lowest energy absorption bands of the anion are not caused by absorption of two distinct electronic states, which might have been expected from vertical excitation calculations, but both states exhibit a strong vibronic progression resulting in contributions to both bands. Y1 - 2021 U6 - https://doi.org/10.1038/s41598-021-95551-0 SN - 2045-2322 VL - 11 IS - 1 PB - Macmillan Publishers Limited, part of Springer Nature CY - London ER - TY - JOUR A1 - Tokmoldin, Nurlan A1 - Vollbrecht, Joachim A1 - Hosseini, Seyed Mehrdad A1 - Sun, Bowen A1 - Perdigón-Toro, Lorena A1 - Woo, Han Young A1 - Zou, Yingping A1 - Neher, Dieter A1 - Shoaee, Safa T1 - Explaining the fill-factor and photocurrent losses of nonfullerene acceptor-based solar cells by probing the long-range charge carrier diffusion and drift lengths JF - Advanced energy materials N2 - 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. KW - diffusion length KW - drift length KW - figure of merit KW - lifetime‐ mobility product KW - steady‐ state photoconductance Y1 - 2021 U6 - https://doi.org/10.1002/aenm.202100804 SN - 1614-6840 VL - 11 IS - 22 PB - Wiley-VCH CY - Weinheim ER -