@article{MansourLungwitzSchultzetal.2020, author = {Mansour, Ahmed E. and Lungwitz, Dominique and Schultz, Thorsten and Arvind, Malavika and Valencia, Ana M. and Cocchi, Caterina and Opitz, Andreas and Neher, Dieter and Koch, Norbert}, title = {The optical signatures of molecular-doping induced polarons in poly(3-hexylthiophene-2,5-diyl)}, series = {Journal of materials chemistry : C, Materials for optical and electronic devices}, volume = {8}, journal = {Journal of materials chemistry : C, Materials for optical and electronic devices}, number = {8}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {2050-7526}, doi = {10.1039/c9tc06509a}, pages = {2870 -- 2879}, year = {2020}, abstract = {Optical absorption spectroscopy is a key method to investigate doped conjugated polymers and to characterize the doping-induced charge carriers, i.e., polarons. For prototypical poly(3-hexylthiophene-2,5-diyl) (P3HT), the absorption intensity of molecular dopant induced polarons is widely used to estimate the carrier density and the doping efficiency, i.e., the number of polarons formed per dopant molecule. However, the dependence of the polaron-related absorption features on the structure of doped P3HT, being either aggregates or separated individual chains, is not comprehensively understood in contrast to the optical absorption features of neutral P3HT. In this work, we unambiguously differentiate the optical signatures of polarons on individual P3HT chains and aggregates in solution, notably the latter exhibiting the same shape as aggregates in solid thin films. This is enabled by employing tris(pentafluorophenyl)borane (BCF) as dopant, as this dopant forms only ion pairs with P3HT and no charge transfer complexes, and BCF and its anion have no absorption in the spectral region of P3HT polarons. Polarons on individual chains exhibit absorption peaks at 1.5 eV and 0.6 eV, whereas in aggregates the high-energy peak is split into a doublet 1.3 eV and 1.65 eV, and the low-energy peak is shifted below 0.5 eV. The dependence of the fraction of solvated individual chains versus aggregates on absolute solution concentration, dopant concentration, and temperature is elucidated, and we find that aggregates predominate in solution under commonly used processing conditions. Aggregates in BCF-doped P3HT solution can be effectively removed upon simple filtering. From varying the filter pore size (down to 200 nm) and thin film morphology characterization with scanning force microscopy we reveal the aggregates' size dependence on solution absolute concentration and dopant concentration. Furthermore, X-ray photoelectron spectroscopy shows that the dopant loading in aggregates is higher than for individual P3HT chains. The results of this study help understanding the impact of solution pre-aggregation on thin film properties of molecularly doped P3HT, and highlight the importance of considering such aggregation for other doped conjugated polymers in general.}, language = {en} } @article{RaoufiHoermannLigorioetal.2020, author = {Raoufi, Meysam and H{\"o}rmann, Ulrich and Ligorio, Giovanni and Hildebrandt, Jana and P{\"a}tzel, Michael and Schultz, Thorsten and Perdigon-Toro, Lorena and Koch, Norbert and List-Kratochvil, Emil and Hecht, Stefan and Neher, Dieter}, title = {Simultaneous effect of ultraviolet radiation and surface modification on the work function and hole injection properties of ZnO thin films}, series = {Physica Status Solidi. A , Applications and materials science}, volume = {217}, journal = {Physica Status Solidi. A , Applications and materials science}, number = {5}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1862-6300}, doi = {10.1002/pssa.201900876}, pages = {1 -- 6}, year = {2020}, abstract = {The combined effect of ultraviolet (UV) light soaking and self-assembled monolayer deposition on the work function (WF) of thin ZnO layers and on the efficiency of hole injection into the prototypical conjugated polymer poly(3-hexylthiophen-2,5-diyl) (P3HT) is systematically investigated. It is shown that the WF and injection efficiency depend strongly on the history of UV light exposure. Proper treatment of the ZnO layer enables ohmic hole injection into P3HT, demonstrating ZnO as a potential anode material for organic optoelectronic devices. The results also suggest that valid conclusions on the energy-level alignment at the ZnO/organic interfaces may only be drawn if the illumination history is precisely known and controlled. This is inherently problematic when comparing electronic data from ultraviolet photoelectron spectroscopy (UPS) measurements carried out under different or ill-defined illumination conditions.}, 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} } @misc{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 = {Zweitver{\"o}ffentlichungen der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, journal = {Zweitver{\"o}ffentlichungen der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, number = {1}, issn = {1866-8372}, doi = {10.25932/publishup-57001}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-570018}, pages = {8}, 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{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{ZuSchultzWolffetal.2020, author = {Zu, Fengshuo and Schultz, Thorsten and Wolff, Christian Michael and Shin, Dongguen and Frohloff, Lennart and Neher, Dieter and Amsalem, Patrick and Koch, Norbert}, title = {Position-locking of volatile reaction products by atmosphere and capping layers slows down photodecomposition of methylammonium lead triiodide perovskite}, series = {RSC Advances}, volume = {10}, journal = {RSC Advances}, number = {30}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {2046-2069}, doi = {10.1039/d0ra03572f}, pages = {17534 -- 17542}, year = {2020}, abstract = {The remarkable progress of metal halide perovskites in photovoltaics has led to the power conversion efficiency approaching 26\%. However, practical applications of perovskite-based solar cells are challenged by the stability issues, of which the most critical one is photo-induced degradation. Bare CH3NH3PbI3 perovskite films are known to decompose rapidly, with methylammonium and iodine as volatile species and residual solid PbI2 and metallic Pb, under vacuum under white light illumination, on the timescale of minutes. We find, in agreement with previous work, that the degradation is non-uniform and proceeds predominantly from the surface, and that illumination under N-2 and ambient air (relative humidity 20\%) does not induce substantial degradation even after several hours. Yet, in all cases the release of iodine from the perovskite surface is directly identified by X-ray photoelectron spectroscopy. This goes in hand with a loss of organic cations and the formation of metallic Pb. When CH3NH3PbI3 films are covered with a few nm thick organic capping layer, either charge selective or non-selective, the rapid photodecomposition process under ultrahigh vacuum is reduced by more than one order of magnitude, and becomes similar in timescale to that under N-2 or air. We conclude that the light-induced decomposition reaction of CH3NH3PbI3, leading to volatile methylammonium and iodine, is largely reversible as long as these products are restrained from leaving the surface. This is readily achieved by ambient atmospheric pressure, as well as a thin organic capping layer even under ultrahigh vacuum. In addition to explaining the impact of gas pressure on the stability of this perovskite, our results indicate that covalently "locking" the position of perovskite components at the surface or an interface should enhance the overall photostability.}, language = {en} } @article{SamsonRechPerdigonToroetal.2020, author = {Samson, Stephanie and Rech, Jeromy and Perdigon-Toro, Lorena and Peng, Zhengxing and Shoaee, Safa and Ade, Harald and Neher, Dieter and Stolterfoht, Martin and You, Wei}, title = {Organic solar cells with large insensitivity to donor polymer molar mass across all acceptor classes}, series = {ACS applied polymer materials}, volume = {2}, journal = {ACS applied polymer materials}, number = {11}, publisher = {American Chemical Society}, address = {Washington}, issn = {2637-6105}, doi = {10.1021/acsapm.0c01041}, pages = {5300 -- 5308}, year = {2020}, abstract = {Donor polymer number-average molar mass (M-n) has long been known to influence organic photovoltaic (OPV) performance via changes in both the polymer properties and the resulting bulk heterojunction morphology. The exact nature of these M-n effects varies from system to system, although there is generally some intermediate M-n that results in optimal performance. Interestingly, our earlier work with the difluorobenzotriazole (FTAZ)-based donor polymer, paired with either N2200 (polymer acceptor) or PC61BM (fullerene acceptor), PcBm demonstrated <10\% variation in power conversion efficiency and a consistent morphology over a large span of M-n (30 kg/mol to over 100 kg/mol). Would such insensitivity to polymer M-n still hold true when prevailing small molecular acceptors were used with FTAZ? To answer this question, we explored the impact of FTAZ on OPVs with ITIC, a high-performance small-molecule fused-ring electron acceptor (FREA). By probing the photovoltaic characteristics of the resulting OPVs, we show that a similar FTAZ mn insensitivity is also found in the FTAZ:ITIC system. This study highlights a single-donor polymer which, when paired with an archetypal fullerene, polymer, and FREA, results in systems that are largely insensitive to donor M. Our results may have implications in polymer batch-to-batch reproducibility, in particular, relaxing the need for tight M-n control during synthesis.}, language = {en} } @article{SandbergKurpiersStolterfohtetal.2020, author = {Sandberg, Oskar J. and Kurpiers, Jona and Stolterfoht, Martin and Neher, Dieter and Meredith, Paul and Shoaee, Safa and Armin, Ardalan}, title = {On the question of the need for a built-in potential in Perovskite solar cells}, series = {Advanced materials interfaces}, volume = {7}, journal = {Advanced materials interfaces}, number = {10}, publisher = {Wiley}, address = {Hoboken}, issn = {2196-7350}, doi = {10.1002/admi.202000041}, pages = {8}, year = {2020}, abstract = {Perovskite semiconductors as the active materials in efficient solar cells exhibit free carrier diffusion lengths on the order of microns at low illumination fluxes and many hundreds of nanometers under 1 sun conditions. These lengthscales are significantly larger than typical junction thicknesses, and thus the carrier transport and charge collection should be expected to be diffusion controlled. A consensus along these lines is emerging in the field. However, the question as to whether the built-in potential plays any role is still of matter of some conjecture. This important question using phase-sensitive photocurrent measurements and theoretical device simulations based upon the drift-diffusion framework is addressed. In particular, the role of the built-in electric field and charge-selective transport layers in state-of-the-art p-i-n perovskite solar cells comparing experimental findings and simulation predictions is probed. It is found that while charge collection in the junction does not require a drift field per se, a built-in potential is still needed to avoid the formation of reverse electric fields inside the active layer, and to ensure efficient extraction through the charge transport layers.}, language = {en} } @misc{WangSmithSkroblinetal.2020, author = {Wang, Qiong and Smith, Joel A. and Skroblin, Dieter and Steele, Julian A. and Wolff, Christian Michael and Caprioglio, Pietro and Stolterfoht, Martin and K{\"o}bler, Hans and Turren-Cruz, Silver-Hamill and Li, Meng and Gollwitzer, Christian and Neher, Dieter and Abate, Antonio}, title = {Managing phase purities and crystal orientation for high-performance and photostable cesium lead halide perovskite solar cells}, series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, number = {9}, issn = {1866-8372}, doi = {10.25932/publishup-52537}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-525374}, pages = {11}, year = {2020}, abstract = {Inorganic perovskites with cesium (Cs+) as the cation have great potential as photovoltaic materials if their phase purity and stability can be addressed. Herein, a series of inorganic perovskites is studied, and it is found that the power conversion efficiency of solar cells with compositions CsPbI1.8Br1.2, CsPbI2.0Br1.0, and CsPbI2.2Br0.8 exhibits a high dependence on the initial annealing step that is found to significantly affect the crystallization and texture behavior of the final perovskite film. At its optimized annealing temperature, CsPbI1.8Br1.2 exhibits a pure orthorhombic phase and only one crystal orientation of the (110) plane. Consequently, this allows for the best efficiency of up to 14.6\% and the longest operational lifetime, T-S80, of approximate to 300 h, averaged of over six solar cells, during the maximum power point tracking measurement under continuous light illumination and nitrogen atmosphere. This work provides essential progress on the enhancement of photovoltaic performance and stability of CsPbI3 - xBrx perovskite solar cells.}, language = {en} } @article{WangSmithSkroblinetal.2020, author = {Wang, Qiong and Smith, Joel A. and Skroblin, Dieter and Steele, Julian A. and Wolff, Christian Michael and Caprioglio, Pietro and Stolterfoht, Martin and K{\"o}bler, Hans and Turren-Cruz, Silver-Hamill and Li, Meng and Gollwitzer, Christian and Neher, Dieter and Abate, Antonio}, title = {Managing phase purities and crystal orientation for high-performance and photostable cesium lead halide perovskite solar cells}, series = {Solar RRL}, volume = {4}, journal = {Solar RRL}, number = {9}, publisher = {WILEY-VCH}, address = {Weinheim}, pages = {9}, year = {2020}, abstract = {Inorganic perovskites with cesium (Cs+) as the cation have great potential as photovoltaic materials if their phase purity and stability can be addressed. Herein, a series of inorganic perovskites is studied, and it is found that the power conversion efficiency of solar cells with compositions CsPbI1.8Br1.2, CsPbI2.0Br1.0, and CsPbI2.2Br0.8 exhibits a high dependence on the initial annealing step that is found to significantly affect the crystallization and texture behavior of the final perovskite film. At its optimized annealing temperature, CsPbI1.8Br1.2 exhibits a pure orthorhombic phase and only one crystal orientation of the (110) plane. Consequently, this allows for the best efficiency of up to 14.6\% and the longest operational lifetime, T-S80, of approximate to 300 h, averaged of over six solar cells, during the maximum power point tracking measurement under continuous light illumination and nitrogen atmosphere. This work provides essential progress on the enhancement of photovoltaic performance and stability of CsPbI3 - xBrx perovskite solar cells.}, language = {en} }