@article{NeherKniepertElimelechetal.2016, author = {Neher, Dieter and Kniepert, Juliane and Elimelech, Arik and Koster, L. Jan Anton}, title = {A New Figure of Merit for Organic Solar Cells with Transport-limited Photocurrents}, series = {Scientific reports}, volume = {6}, journal = {Scientific reports}, publisher = {Nature Publishing Group}, address = {London}, issn = {2045-2322}, doi = {10.1038/srep24861}, pages = {9}, year = {2016}, abstract = {Compared to their inorganic counterparts, organic semiconductors suffer from relatively low charge carrier mobilities. Therefore, expressions derived for inorganic solar cells to correlate characteristic performance parameters to material properties are prone to fail when applied to organic devices. This is especially true for the classical Shockley-equation commonly used to describe current-voltage (JV)-curves, as it assumes a high electrical conductivity of the charge transporting material. Here, an analytical expression for the JV-curves of organic solar cells is derived based on a previously published analytical model. This expression, bearing a similar functional dependence as the Shockley-equation, delivers a new figure of merit α to express the balance between free charge recombination and extraction in low mobility photoactive materials. This figure of merit is shown to determine critical device parameters such as the apparent series resistance and the fill factor.}, language = {en} } @misc{NeherKniepertElimelechetal.2016, author = {Neher, Dieter and Kniepert, Juliane and Elimelech, Arik and Koster, L. Jan Anton}, title = {A New Figure of Merit for Organic Solar Cells with Transport-limited Photocurrents}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-91414}, pages = {9}, year = {2016}, abstract = {Compared to their inorganic counterparts, organic semiconductors suffer from relatively low charge carrier mobilities. Therefore, expressions derived for inorganic solar cells to correlate characteristic performance parameters to material properties are prone to fail when applied to organic devices. This is especially true for the classical Shockley-equation commonly used to describe current-voltage (JV)-curves, as it assumes a high electrical conductivity of the charge transporting material. Here, an analytical expression for the JV-curves of organic solar cells is derived based on a previously published analytical model. This expression, bearing a similar functional dependence as the Shockley-equation, delivers a new figure of merit α to express the balance between free charge recombination and extraction in low mobility photoactive materials. This figure of merit is shown to determine critical device parameters such as the apparent series resistance and the fill factor.}, language = {en} } @article{NeherKniepertElimelechetal.2016, author = {Neher, Dieter and Kniepert, Juliane and Elimelech, Arik and Koster, L. Jan Anton}, title = {A New Figure of Merit for Organic Solar Cells with Transport-limited Photocurrents}, series = {Scientific reports}, volume = {6}, journal = {Scientific reports}, publisher = {Nature Publ. Group}, address = {London}, issn = {2045-2322}, doi = {10.1038/srep24861}, pages = {E2348 -- E2349}, year = {2016}, abstract = {Compared to their inorganic counterparts, organic semiconductors suffer from relatively low charge carrier mobilities. Therefore, expressions derived for inorganic solar cells to correlate characteristic performance parameters to material properties are prone to fail when applied to organic devices. This is especially true for the classical Shockley-equation commonly used to describe current-voltage (JV)-curves, as it assumes a high electrical conductivity of the charge transporting material. Here, an analytical expression for the JV-curves of organic solar cells is derived based on a previously published analytical model. This expression, bearing a similar functional dependence as the Shockley-equation, delivers a new figure of merit a to express the balance between free charge recombination and extraction in low mobility photoactive materials. This figure of merit is shown to determine critical device parameters such as the apparent series resistance and the fill factor.}, language = {en} } @article{LeCorreStolterfohtPerdigonToroetal.2019, author = {Le Corre, Vincent M. and Stolterfoht, Martin and Perdigon Toro, Lorena and Feuerstein, Markus and Wolff, Christian Michael and Gil-Escrig, Lidon and Bolink, Henk J. and Neher, Dieter and Koster, L. Jan Anton}, title = {Charge Transport Layers Limiting the Efficiency of Perovskite Solar Cells: How To Optimize Conductivity, Doping, and Thickness}, series = {ACS Applied Energy Materials}, volume = {2}, journal = {ACS Applied Energy Materials}, number = {9}, publisher = {American Chemical Society}, address = {Washington}, issn = {2574-0962}, doi = {10.1021/acsaem.9b00856}, pages = {6280 -- 6287}, year = {2019}, abstract = {Perovskite solar cells (PSCs) are one of the main research topics of the photovoltaic community; with efficiencies now reaching up to 24\%, PSCs are on the way to catching up with classical inorganic solar cells. However, PSCs have not yet reached their full potential. In fact, their efficiency is still limited by nonradiative recombination, mainly via trap-states and by losses due to the poor transport properties of the commonly used transport layers (TLs). Indeed, state-of-the-art TLs (especially if organic) suffer from rather low mobilities, typically within 10(-5) and 10(-2) cm(-2) V-1 s(-1), when compared to the high mobilities, 1-10 cm(-2) V-1 s(-1), measured for perovskites. This work presents a comprehensive analysis of the effect of the mobility, thickness, and doping density of the transport layers based on combined experimental and modeling results of two sets of devices made of a solution-processed high-performing triple-cation (PCE approximate to 20\%). The results are also cross-checked on vacuum-processed MAPbI(3) devices. From this analysis, general guidelines on how to optimize a TL are introduced and especially a new and simple formula to easily calculate the amount of doping necessary to counterbalance the low mobility of the TLs.}, language = {en} } @article{KniepertLangevanderKaapetal.2014, author = {Kniepert, Juliane and Lange, Ilja and van der Kaap, Niels J. and Koster, L. Jan Anton and Neher, Dieter}, title = {A conclusive view on charge generation, recombination, and extraction in As-prepared and annealed P3HT:PCBM blends: combined experimental and simulation work}, series = {dvanced energy materials}, volume = {4}, journal = {dvanced energy materials}, number = {7}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1614-6832}, doi = {10.1002/aenm.201301401}, pages = {11}, year = {2014}, abstract = {Time-delayed collection field (TDCF) and bias-amplified charge extraction (BACE) are applied to as-prepared and annealed poly(3-hexylthiophene):[6,6]-phenyl C-71 butyric acid methyl ester (P3HT:PCBM) blends coated from chloroform. Despite large differences in fill factor, short-circuit current, and power conversion efficiency, both blends exhibit a negligible dependence of photogeneration on the electric field and strictly bimolecular recombination (BMR) with a weak dependence of the BMR coefficient on charge density. Drift-diffusion simulations are performed using the measured coefficients and mobilities, taking into account bimolecular recombination and the possible effects of surface recombination. The excellent agreement between the simulation and the experimental data for an intensity range covering two orders of magnitude indicates that a field-independent generation rate and a density-independent recombination coefficient describe the current-voltage characteristics of the annealed P3HT: PCBM devices, while the performance of the as-prepared blend is shown to be limited by space charge effects due to a low hole mobility. Finally, even though the bimolecular recombination coefficient is small, surface recombination is found to be a negligible loss mechanism in these solar cells.}, language = {en} } @article{KniepertLangeHeidbrinketal.2015, author = {Kniepert, Juliane and Lange, Ilja and Heidbrink, Jan and Kurpiers, Jona and Brenner, Thomas J. K. and Koster, L. Jan Anton and Neher, Dieter}, title = {Effect of Solvent Additive on Generation, Recombination, and Extraction in PTB7:PCBM Solar Cells: A Conclusive Experimental and Numerical Simulation Study}, series = {The journal of physical chemistry : C, Nanomaterials and interfaces}, volume = {119}, journal = {The journal of physical chemistry : C, Nanomaterials and interfaces}, number = {15}, publisher = {American Chemical Society}, address = {Washington}, issn = {1932-7447}, doi = {10.1021/jp512721e}, pages = {8310 -- 8320}, year = {2015}, abstract = {Time-delayed collection field (TDCF), bias-assisted charge extraction (BACE), and space charge-limited current (SCLC) measurements are combined with complete numerical device simulations to unveil the effect of the solvent additive 1,8-diiodooctane (DIO) on the performance of PTB7:PCBM bulk heterojunction solar cells. DIO is shown to increase the charge generation rate, reduce geminate and bimolecular recombination, and increase the electron mobility. In total, the reduction of loss currents by processing with the additive raises the power conversion efficiency of the PTB7:PCBM blend by a factor of almost three. The lower generation rates and higher geminate recombination losses in devices without DIO are consistent with a blend morphology comprising large fullerene clusters embedded within a PTB7-rich matrix, while the low electron mobility suggests that these fullerene clusters are themselves composed of smaller pure fullerene aggregates separated by disordered areas. Our device simulations show unambiguously that the effect of the additive on the shape of the currentvoltage curve (J-V) cannot be ascribed to the variation of only the mobility, the recombination, or the field dependence of generation. It is only when the changes of all three parameters are taken into account that the simulation matches the experimental J-V characteristics under all illumination conditions and for a wide range of voltages.}, language = {en} } @article{BartesaghiPerezKniepertetal.2015, author = {Bartesaghi, Davide and Perez, Irene del Carmen and Kniepert, Juliane and Roland, Steffen and Turbiez, Mathieu and Neher, Dieter and Koster, L. Jan Anton}, title = {Competition between recombination and extraction of free charges determines the fill factor of organic solar cells}, series = {Nature Communications}, volume = {6}, journal = {Nature Communications}, publisher = {Nature Publ. Group}, address = {London}, issn = {2041-1723}, doi = {10.1038/ncomms8083}, pages = {10}, year = {2015}, abstract = {Among the parameters that characterize a solar cell and define its power-conversion efficiency, the fill factor is the least well understood, making targeted improvements difficult. Here we quantify the competition between charge extraction and recombination by using a single parameter theta, and we demonstrate that this parameter is directly related to the fill factor of many different bulk-heterojunction solar cells. Our finding is supported by experimental measurements on 15 different donor: acceptor combinations, as well as by drift-diffusion simulations of organic solar cells in which charge-carrier mobilities, recombination rate, light intensity, energy levels and active-layer thickness are all varied over wide ranges to reproduce typical experimental conditions. The results unify the fill factors of several very different donor: acceptor combinations and give insight into why fill factors change so much with thickness, light intensity and materials properties. To achieve fill factors larger than 0.8 requires further improvements in charge transport while reducing recombination.}, language = {en} }