@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{DeschlerNeherSchmidtMende2019, author = {Deschler, Felix and Neher, Dieter and Schmidt-Mende, Lukas}, title = {Perovskite semiconductors for next generation optoelectronic applications}, series = {APL Materials}, volume = {7}, journal = {APL Materials}, number = {8}, publisher = {American Institute of Physics}, address = {Melville}, issn = {2166-532X}, doi = {10.1063/1.5119744}, pages = {3}, year = {2019}, language = {en} } @article{KniepertPaulkePerdigonToroetal.2019, author = {Kniepert, Juliane and Paulke, Andreas and Perdigon-Toro, Lorena and Kurpiers, Jona and Zhang, Huotian and Gao, Feng and Yuan, Jun and Zou, Yingping and Le Corre, Vincent M. and Koster, Lambert Jan Anton and Neher, Dieter}, title = {Reliability of charge carrier recombination data determined with charge extraction methods}, series = {Journal of applied physics}, volume = {126}, journal = {Journal of applied physics}, number = {20}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0021-8979}, doi = {10.1063/1.5129037}, pages = {15}, year = {2019}, abstract = {Charge extraction methods are popular for measuring the charge carrier density in thin film organic solar cells and to draw conclusions about the order and coefficient of nongeminate charge recombination. However, results from such studies may be falsified by inhomogeneous steady state carrier profiles or surface recombination. Here, we present a detailed drift-diffusion study of two charge extraction methods, bias-assisted charge extraction (BACE) and time-delayed collection field (TDCF). Simulations are performed over a wide range of the relevant parameters. Our simulations reveal that both charge extraction methods provide reliable information about the recombination order and coefficient if the measurements are performed under appropriate conditions. However, results from BACE measurements may be easily affected by surface recombination, in particular for small active layer thicknesses and low illumination densities. TDCF, on the other hand, is more robust against surface recombination due to its transient nature but also because it allows for a homogeneous high carrier density to be inserted into the active layer. Therefore, TDCF is capable to provide meaningful information on the order and coefficient of recombination even if the model conditions are not exactly fulfilled. We demonstrate this for an only 100 nm thick layer of a highly efficient nonfullerene acceptor (NFA) blend, comprising the donor polymer PM6 and the NFA Y6. TDCF measurements were performed as a function of delay time for different laser fluences and bias conditions. The full set of data could be consistently fitted by a strict second order recombination process, with a bias- and fluence-independent bimolecular recombination coefficient k(2) = 1.7 x 10(-17)m(3) s(-1). BACE measurements performed on the very same layer yielded the identical result, despite the very different excitation conditions. This proves that recombination in this blend is mostly through processes in the bulk and that surface recombination is of minor importance despite the small active layer thickness. Published under license by AIP Publishing.}, language = {en} } @article{WangMosconiWolffetal.2019, author = {Wang, Qiong and Mosconi, Edoardo and Wolff, Christian Michael and Li, Junming and Neher, Dieter and De Angelis, Filippo and Suranna, Gian Paolo and Grisorio, Roberto and Abate, Antonio}, title = {Rationalizing the molecular design of hole-selective contacts to improve charge extraction in Perovskite solar cells}, series = {dvanced energy materials}, volume = {9}, journal = {dvanced energy materials}, number = {28}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1614-6832}, doi = {10.1002/aenm.201900990}, pages = {9}, year = {2019}, abstract = {Two new hole selective materials (HSMs) based on dangling methylsulfanyl groups connected to the C-9 position of the fluorene core are synthesized and applied in perovskite solar cells. Being structurally similar to a half of Spiro-OMeTAD molecule, these HSMs (referred as FS and DFS) share similar redox potentials but are endowed with slightly higher hole mobility, due to the planarity and large extension of their structure. Competitive power conversion efficiency (up to 18.6\%) is achieved by using the new HSMs in suitable perovskite solar cells. Time-resolved photoluminescence decay measurements and electrochemical impedance spectroscopy show more efficient charge extraction at the HSM/perovskite interface with respect to Spiro-OMeTAD, which is reflected in higher photocurrents exhibited by DFS/FS-integrated perovskite solar cells. Density functional theory simulations reveal that the interactions of methylammonium with methylsulfanyl groups in DFS/FS strengthen their electrostatic attraction with the perovskite surface, providing an additional path for hole extraction compared to the sole presence of methoxy groups in Spiro-OMeTAD. Importantly, the low-cost synthesis of FS makes it significantly attractive for the future commercialization of perovskite solar cells.}, language = {en} } @article{SchwarzeSchellhammerOrtsteinetal.2019, author = {Schwarze, Martin and Schellhammer, Karl Sebastian and Ortstein, Katrin and Benduhn, Johannes and Gaul, Christopher and Hinderhofer, Alexander and Toro, Lorena Perdigon and Scholz, Reinhard and Kublitski, Jonas and Roland, Steffen and Lau, Matthias and Poelking, Carl and Andrienko, Denis and Cuniberti, Gianaurelio and Schreiber, Frank and Neher, Dieter and Vandewal, Koen and Ortmann, Frank and Leo, Karl}, title = {Impact of molecular quadrupole moments on the energy levels at organic heterojunctions}, series = {Nature Communications}, volume = {10}, journal = {Nature Communications}, publisher = {Nature Publ. Group}, address = {London}, issn = {2041-1723}, doi = {10.1038/s41467-019-10435-2}, pages = {9}, year = {2019}, abstract = {The functionality of organic semiconductor devices crucially depends on molecular energies, namely the ionisation energy and the electron affinity. Ionisation energy and electron affinity values of thin films are, however, sensitive to film morphology and composition, making their prediction challenging. In a combined experimental and simulation study on zinc-phthalocyanine and its fluorinated derivatives, we show that changes in ionisation energy as a function of molecular orientation in neat films or mixing ratio in blends are proportional to the molecular quadrupole component along the p-p-stacking direction. We apply these findings to organic solar cells and demonstrate how the electrostatic interactions can be tuned to optimise the energy of the charge-transfer state at the donor-acceptor interface and the dissociation barrier for free charge carrier generation. The confirmation of the correlation between interfacial energies and quadrupole moments for other materials indicates its relevance for small molecules and polymers.}, language = {en} } @article{UllbrichBenduhnJiaetal.2019, author = {Ullbrich, Sascha and Benduhn, Johannes and Jia, Xiangkun and Nikolis, Vasileios C. and Tvingstedt, Kristofer and Piersimoni, Fortunato and Roland, Steffen and Liu, Yuan and Wu, Jinhan and Fischer, Axel and Neher, Dieter and Reineke, Sebastian and Spoltore, Donato and Vandewal, Koen}, title = {Emissive and charge-generating donor-acceptor interfaces for organic optoelectronics with low voltage losses}, series = {Nature materials}, volume = {18}, journal = {Nature materials}, number = {5}, publisher = {Nature Publ. Group}, address = {London}, issn = {1476-1122}, doi = {10.1038/s41563-019-0324-5}, pages = {459 -- 464}, year = {2019}, abstract = {Intermolecular charge-transfer states at the interface between electron donating (D) and accepting (A) materials are crucial for the operation of organic solar cells but can also be exploited for organic light-emitting diodes(1,2). Non-radiative charge-transfer state decay is dominant in state-of-the-art D-A-based organic solar cells and is responsible for large voltage losses and relatively low power-conversion efficiencies as well as electroluminescence external quantum yields in the 0.01-0.0001\% range(3,4). In contrast, the electroluminescence external quantum yield reaches up to 16\% in D-A-based organic light-emitting diodes(5-7). Here, we show that proper control of charge-transfer state properties allows simultaneous occurrence of a high photovoltaic and emission quantum yield within a single, visible-light-emitting D-A system. This leads to ultralow-emission turn-on voltages as well as significantly reduced voltage losses upon solar illumination. These results unify the description of the electro-optical properties of charge-transfer states in organic optoelectronic devices and foster the use of organic D-A blends in energy conversion applications involving visible and ultraviolet photons(8-11).}, language = {en} } @article{LiBenduhnQiaoetal.2019, author = {Li, Tian-yi and Benduhn, Johannes and Qiao, Zhi and Liu, Yuan and Li, Yue and Shivhare, Rishi and Jaiser, Frank and Wang, Pei and Ma, Jie and Zeika, Olaf and Neher, Dieter and Mannsfeld, Stefan C. B. and Ma, Zaifei and Vandewal, Koen and Leo, Karl}, title = {Effect of H- and J-Aggregation on the Photophysical and Voltage Loss of Boron Dipyrromethene Small Molecules in Vacuum-Deposited Organic Solar Cells}, series = {The journal of physical chemistry letters}, volume = {10}, journal = {The journal of physical chemistry letters}, number = {11}, publisher = {American Chemical Society}, address = {Washington}, issn = {1948-7185}, doi = {10.1021/acs.jpclett.9b01222}, pages = {2684 -- 2691}, year = {2019}, abstract = {An understanding of the factors limiting the open-circuit voltage (V-oc) and related photon energy loss mechanisms is critical to increase the power conversion efficiency (PCE) of small-molecule organic solar cells (OSCs), especially those with near-infrared (NIR) absorbers. In this work, two NIR boron dipyrromethene (BODIPY) molecules are characterized for application in planar (PHJ) and bulk (BHJ) heterojunction OSCs. When two H atoms are substituted by F atoms on the peripheral phenyl rings of the molecules, the molecular aggregation type in the thin film changes from the H-type to J-type. For PHJ devices, the nonradiative voltage loss of 0.35 V in the J-aggregated BODIPY is lower than that of 0.49 V in the H-aggregated device. In BHJ devices with a nonradiative voltage loss of 0.35 V, a PCE of 5.5\% is achieved with an external quantum efficiency (EQE) maximum of 68\% at 700 nm.}, language = {en} } @article{ZuWolffRalaiarisoaetal.2019, author = {Zu, Fengshuo and Wolff, Christian Michael and Ralaiarisoa, Maryline and Amsalem, Patrick and Neher, Dieter and Koch, Norbert}, title = {Unraveling the Electronic Properties of Lead Halide Perovskites with Surface Photovoltage in Photoemission Studies}, series = {ACS applied materials \& interfaces}, volume = {11}, journal = {ACS applied materials \& interfaces}, number = {24}, publisher = {American Chemical Society}, address = {Washington}, issn = {1944-8244}, doi = {10.1021/acsami.9b05293}, pages = {21578 -- 21583}, year = {2019}, abstract = {The tremendous success of metal-halide perovskites, especially in the field of photovoltaics, has triggered a substantial number of studies in understanding their optoelectronic properties. However, consensus regarding the electronic properties of these perovskites is lacking due to a huge scatter in the reported key parameters, such as work function (Φ) and valence band maximum (VBM) values. Here, we demonstrate that the surface photovoltage (SPV) is a key phenomenon occurring at the perovskite surfaces that feature a non-negligible density of surface states, which is more the rule than an exception for most materials under study. With ultraviolet photoelectron spectroscopy (UPS) and Kelvin probe, we evidence that even minute UV photon fluxes (500 times lower than that used in typical UPS experiments) are sufficient to induce SPV and shift the perovskite Φ and VBM by several 100 meV compared to dark. By combining UV and visible light, we establish flat band conditions (i.e., compensate the surface-state-induced surface band bending) at the surface of four important perovskites, and find that all are p-type in the bulk, despite a pronounced n-type surface character in the dark. The present findings highlight that SPV effects must be considered in all surface studies to fully understand perovskites' photophysical properties.}, language = {en} } @article{KegelmannTockhornWolffetal.2019, author = {Kegelmann, Lukas and Tockhorn, Philipp and Wolff, Christian Michael and M{\´a}rquez, Jos{\´e} A. and Caicedo D{\´a}vila, Sebasti{\´a}n and Korte, Lars and Unold, Thomas and Loevenich, Wilfried and Neher, Dieter and Rech, Bernd and Albrecht, Steve}, title = {Mixtures of Dopant-Free Spiro-OMeTAD and Water-Free PEDOT as a Passivating Hole Contact in Perovskite Solar Cells}, series = {ACS applied materials \& interfaces}, volume = {11}, journal = {ACS applied materials \& interfaces}, number = {9}, publisher = {American Chemical Society}, address = {Washington}, issn = {1944-8244}, doi = {10.1021/acsami.9b01332}, pages = {9172 -- 9181}, year = {2019}, abstract = {Doped spiro-OMeTAD at present is the most commonly used hole transport material (HTM) in n-i-p-type perovskite solar cells, enabling high efficiencies around 22\%. However, the required dopants were shown to induce nonradiative recombination of charge carriers and foster degradation of the solar cell. Here, in a novel approach, highly conductive and inexpensive water-free poly(3,4-ethylenedioxythiophene) (PEDOT) is used to replace these dopants. The resulting spiro-OMeTAD/PEDOT (SpiDOT) mixed films achieve higher lateral conductivities than layers of doped spiro-OMeTAD. Furthermore, combined transient and steady-state photoluminescence studies reveal a passivating effect of PEDOT, suppressing nonradiative recombination losses at the perovskite/HTM interface. This enables excellent quasi-Fermi level splitting values of up to 1.24 eV in perovskite/SpiDOT layer stacks and high open-circuit voltages (V-OC) up to 1.19 V in complete solar cells. Increasing the amount of dopant-free spiro-OMeTAD in SpiDOT layers is shown to enhance hole extraction and thereby improves the fill factor in solar cells. As a consequence, stabilized efficiencies up to 18.7\% are realized, exceeding cells with doped spiro-OMeTAD as a HTM in this study. Moreover, to the best of our knowledge, these results mark the lowest nonradiative recombination loss in the V-OC (140 mV with respect to the Shockley-Queisser limit) and highest efficiency reported so far for perovskite solar cells using PEDOT as a HTM.}, language = {en} } @article{ColladoFregosoPuglieseWojciketal.2019, author = {Collado-Fregoso, Elisa and Pugliese, Silvina N. and Wojcik, Mariusz and Benduhn, Johannes and Bar-Or, Eyal and Toro, Lorena Perdigon 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} }