@article{StolterfohtCaprioglioWolffetal.2019, author = {Stolterfoht, Martin and Caprioglio, Pietro and Wolff, Christian Michael and Marquez, Jose A. and Nordmann, Joleik and Zhang, Shanshan and Rothhardt, Daniel and H{\"o}rmann, Ulrich and Amir, Yohai and Redinger, Alex and Kegelmann, Lukas and Zu, Fengshuo and Albrecht, Steve and Koch, Norbert and Kirchartz, Thomas and Saliba, Michael and Unold, Thomas and Neher, Dieter}, title = {The impact of energy alignment and interfacial recombination on the internal and external open-circuit voltage of perovskite solar cells}, series = {Energy \& environmental science}, volume = {12}, journal = {Energy \& environmental science}, number = {9}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {1754-5692}, doi = {10.1039/c9ee02020a}, pages = {2778 -- 2788}, year = {2019}, abstract = {Charge transport layers (CTLs) are key components of diffusion controlled perovskite solar cells, however, they can induce additional non-radiative recombination pathways which limit the open circuit voltage (V-OC) of the cell. In order to realize the full thermodynamic potential of the perovskite absorber, both the electron and hole transport layer (ETL/HTL) need to be as selective as possible. By measuring the photoluminescence yield of perovskite/CTL heterojunctions, we quantify the non-radiative interfacial recombination currents in pin- and nip-type cells including high efficiency devices (21.4\%). Our study comprises a wide range of commonly used CTLs, including various hole-transporting polymers, spiro-OMeTAD, metal oxides and fullerenes. We find that all studied CTLs limit the V-OC by inducing an additional non-radiative recombination current that is in most cases substantially larger than the loss in the neat perovskite and that the least-selective interface sets the upper limit for the V-OC of the device. Importantly, the V-OC equals the internal quasi-Fermi level splitting (QFLS) in the absorber layer only in high efficiency cells, while in poor performing devices, the V-OC is substantially lower than the QFLS. Using ultraviolet photoelectron spectroscopy and differential charging capacitance experiments we show that this is due to an energy level mis-alignment at the p-interface. The findings are corroborated by rigorous device simulations which outline important considerations to maximize the V-OC. This work highlights that the challenge to suppress non-radiative recombination losses in perovskite cells on their way to the radiative limit lies in proper energy level alignment and in suppression of defect recombination at the interfaces.}, language = {en} } @article{NikolisMischokSiegmundetal.2019, author = {Nikolis, Vasileios C. and Mischok, Andreas and Siegmund, Bernhard and Kublitski, Jonas and Jia, Xiangkun and Benduhn, Johannes and H{\"o}rmann, Ulrich and Neher, Dieter and Gather, Malte C. and Spoltore, Donato and Vandewal, Koen}, title = {Strong light-matter coupling for reduced photon energy losses in organic photovoltaics}, series = {Nature Communications}, volume = {10}, journal = {Nature Communications}, publisher = {Nature Publ. Group}, address = {London}, issn = {2041-1723}, doi = {10.1038/s41467-019-11717-5}, pages = {8}, year = {2019}, abstract = {Strong light-matter coupling can re-arrange the exciton energies in organic semiconductors. Here, we exploit strong coupling by embedding a fullerene-free organic solar cell (OSC) photo-active layer into an optical microcavity, leading to the formation of polariton peaks and a red-shift of the optical gap. At the same time, the open-circuit voltage of the device remains unaffected. This leads to reduced photon energy losses for the low-energy polaritons and a steepening of the absorption edge. While strong coupling reduces the optical gap, the energy of the charge-transfer state is not affected for large driving force donor-acceptor systems. Interestingly, this implies that strong coupling can be exploited in OSCs to reduce the driving force for electron transfer, without chemical or microstructural modifications of the photoactive layer. Our work demonstrates that the processes determining voltage losses in OSCs can now be tuned, and reduced to unprecedented values, simply by manipulating the device architecture.}, language = {en} } @article{HoermannZeiskePiersimonietal.2018, author = {H{\"o}rmann, Ulrich and Zeiske, Stefan and Piersimoni, Fortunato and Hoffmann, Lukas and Schlesinger, Raphael and Koch, Norbert and Riedl, Thomas and Andrienko, Denis and Neher, Dieter}, title = {Stark effect of hybrid charge transfer states at planar ZnO/organic interfaces}, series = {Physical review : B, Condensed matter and materials physics}, volume = {98}, journal = {Physical review : B, Condensed matter and materials physics}, number = {15}, publisher = {American Physical Society}, address = {College Park}, issn = {2469-9950}, doi = {10.1103/PhysRevB.98.155312}, pages = {9}, year = {2018}, abstract = {We investigate the bias dependence of the hybrid charge transfer state emission at planar heterojunctions between the metal oxide acceptor ZnO and three donor molecules. The electroluminescence peak energy linearly increases with the applied bias, saturating at high fields. Variation of the organic layer thickness and deliberate change of the ZnO conductivity through controlled photodoping allow us to confirm that this bias-induced spectral shift relates to the internal electric field in the organic layer rather than the filling of states at the hybrid interface. We show that existing continuum models overestimate the hole delocalization and propose a simple electrostatic model in which the linear and quadratic Stark effects are explained by the electrostatic interaction of a strongly polarizable molecular cation with its mirror image.}, 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{ColladoFregosoPuglieseWojciketal.2019, author = {Collado-Fregoso, Elisa and Pugliese, Silvina N. and Wojcik, Mariusz and Benduhn, Johannes and Bar-Or, Eyal and Perdigon-Toro, Lorena and H{\"o}rmann, Ulrich and Spoltore, Donato and Vandewal, Koen and Hodgkiss, Justin M. and Neher, Dieter}, title = {Energy-gap law for photocurrent generation in fullerene-based organic solar cells}, series = {Journal of the American Chemical Society}, volume = {141}, journal = {Journal of the American Chemical Society}, number = {6}, publisher = {American Chemical Society}, address = {Washington}, issn = {0002-7863}, doi = {10.1021/jacs.8b09820}, pages = {2329 -- 2341}, year = {2019}, abstract = {The involvement of charge-transfer (CT) states in the photogeneration and recombination of charge carriers has been an important focus of study within the organic photovoltaic community. In this work, we investigate the molecular factors determining the mechanism of photocurrent generation in low-donor-content organic solar cells, where the active layer is composed of vacuum-deposited C-60 and small amounts of organic donor molecules. We find a pronounced decline of all photovoltaic parameters with decreasing CT state energy. Using a combination of steady-state photocurrent measurements and time-delayed collection field experiments, we demonstrate that the power conversion efficiency, and more specifically, the fill factor of these devices, is mainly determined by the bias dependence of photocurrent generation. By combining these findings with the results from ultrafast transient absorption spectroscopy, we show that blends with small CT energies perform poorly because of an increased nonradiative CT state decay rate and that this decay obeys an energy-gap law. Our work challenges the common view that a large energy offset at the heterojunction and/or the presence of fullerene clusters guarantee efficient CT dissociation and rather indicates that charge generation benefits from high CT state energies through a slower decay to the ground state.}, language = {en} } @article{HoermannZeiskeParketal.2019, author = {H{\"o}rmann, Ulrich and Zeiske, Stefan and Park, Soohyung and Schultz, Thorsten and Kickhoefel, Sebastian and Scherf, Ullrich and Blumstengel, Sylke and Koch, Norbert and Neher, Dieter}, title = {Direct observation of state-filling at hybrid tin oxide/organic interfaces}, series = {Applied physics letters}, volume = {114}, journal = {Applied physics letters}, number = {18}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0003-6951}, doi = {10.1063/1.5082704}, pages = {5}, year = {2019}, abstract = {Electroluminescence (EL) spectra of hybrid charge transfer states at metal oxide/organic type-II heterojunctions exhibit bias-induced spectral shifts. The reasons for this phenomenon have been discussed controversially and arguments for either electric field-induced effects or the filling of trap states at the oxide surface have been put forward. Here, we combine the results of EL and photovoltaic measurements to eliminate the unavoidable effect of the series resistance of inorganic and organic components on the total voltage drop across the hybrid device. For SnOx combined with the conjugated polymer [ladder type poly-(para-phenylene)], we find a one-to-one correspondence between the blue-shift of the EL peak and the increase of the quasi-Fermi level splitting at the hybrid heterojunction, which we unambiguously assign to state filling. Our data are resembled best by a model considering the combination of an exponential density of states with a doped semiconductor. Published under license by AIP Publishing.}, language = {en} }