@article{CaprioglioZuWolffetal.2019, author = {Caprioglio, Pietro and Zu, Fengshuo and Wolff, Christian Michael and Prieto, Jose A. Marquez and Stolterfoht, Martin and Becker, Pascal and Koch, Norbert and Unold, Thomas and Rech, Bernd and Albrecht, Steve and Neher, Dieter}, title = {High open circuit voltages in pin-type perovskite solar cells through strontium addition}, series = {Sustainable Energy \& Fuels}, volume = {3}, journal = {Sustainable Energy \& Fuels}, number = {2}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {2398-4902}, doi = {10.1039/c8se00509e}, pages = {550 -- 563}, year = {2019}, abstract = {The incorporation of even small amounts of strontium (Sr) into lead-base hybrid quadruple cation perovskite solar cells results in a systematic increase of the open circuit voltage (V-oc) in pin-type perovskite solar cells. We demonstrate via absolute and transient photoluminescence (PL) experiments how the incorporation of Sr significantly reduces the non-radiative recombination losses in the neat perovskite layer. We show that Sr segregates at the perovskite surface, where it induces important changes of morphology and energetics. Notably, the Sr-enriched surface exhibits a wider band gap and a more n-type character, accompanied with significantly stronger surface band bending. As a result, we observe a significant increase of the quasi-Fermi level splitting in the neat perovskite by reduced surface recombination and more importantly, a strong reduction of losses attributed to non-radiative recombination at the interface to the C-60 electron-transporting layer. The resulting solar cells exhibited a V-oc of 1.18 V, which could be further improved to nearly 1.23 V through addition of a thin polymer interlayer, reducing the non-radiative voltage loss to only 110 meV. Our work shows that simply adding a small amount of Sr to the precursor solutions induces a beneficial surface modification in the perovskite, without requiring any post treatment, resulting in high efficiency solar cells with power conversion efficiency (PCE) up to 20.3\%. Our results demonstrate very high V-oc values and efficiencies in Sr-containing quadruple cation perovskite pin-type solar cells and highlight the imperative importance of addressing and minimizing the recombination losses at the interface between perovskite and charge transporting layer.}, 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{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{HosseiniRolandKurpiersetal.2019, author = {Hosseini, Seyed Mehrdad and Roland, Steffen and Kurpiers, Jona and Chen, Zhiming and Zhang, Kai and Huang, Fei and Armin, Ardalan and Neher, Dieter and Shoaee, Safa}, title = {Impact of Bimolecular Recombination on the Fill Factor of Fullerene and Nonfullerene-Based Solar Cells}, series = {The journal of physical chemistry : C, Nanomaterials and interfaces}, volume = {123}, journal = {The journal of physical chemistry : C, Nanomaterials and interfaces}, number = {11}, publisher = {American Chemical Society}, address = {Washington}, issn = {1932-7447}, doi = {10.1021/acs.jpcc.8b11669}, pages = {6823 -- 6830}, year = {2019}, abstract = {Power conversion efficiencies of donor/acceptor organic solar cells utilizing nonfullerene acceptors have now increased beyond the record of their fullerene-based counterparts. There remain many fundamental questions regarding nanomorphology, interfacial states, charge generation and extraction, and losses in these systems. Herein, we present a comparative study of bulk heterojunction solar cells composed of a recently introduced naphthothiadiazole-based polymer (NT812) as the electron donor and two different acceptor molecules, namely, [6,6]-phenyl-C71-butyric acid methyl ester (PCBM)[70] and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC). A comparison between the photovoltaic performance of these two types of solar cells reveals that the open-circuit voltage (Voc) of the NT812:ITIC-based solar cell is larger, but the fill factor (FF) is lower than that of the NT812:PCBM[70] device. We find the key reason behind this reduced FF in the ITIC-based device to be faster nongeminate recombination relative to the NT812:PCBM[70] system.}, language = {en} } @article{RolandKniepertLoveetal.2019, author = {Roland, Steffen and Kniepert, Juliane and Love, John A. and Negi, Vikas and Liu, Feilong and Bobbert, Peter and Melianas, Armantas and Kemerink, Martijn and Hofacker, Andreas and Neher, Dieter}, title = {Equilibrated Charge Carrier Populations Govern Steady-State Nongeminate Recombination in Disordered Organic Solar Cells}, series = {The journal of physical chemistry letters}, volume = {10}, journal = {The journal of physical chemistry letters}, number = {6}, publisher = {American Chemical Society}, address = {Washington}, issn = {1948-7185}, doi = {10.1021/acs.jpclett.9b00516}, pages = {1374 -- 1381}, year = {2019}, abstract = {We employed bias-assisted charge extraction techniques to investigate the transient and steady-state recombination of photogenerated charge carriers in complete devices of a disordered polymer-fullerene blend. Charge recombination is shown to be dispersive, with a significant slowdown of the recombination rate over time, consistent with the results from kinetic Monte Carlo simulations. Surprisingly, our experiments reveal little to no contributions from early time recombination of nonequilibrated charge carriers to the steady-state recombination properties. We conclude that energetic relaxation of photogenerated carriers outpaces any significant nongeminate recombination under application-relevant illumination conditions. With equilibrated charges dominating the steady-state recombination, quasi-equilibrium concepts appear suited for describing the open-circuit voltage of organic solar cells despite pronounced energetic disorder.}, 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{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} } @article{CaprioglioStolterfohtWolffetal.2019, author = {Caprioglio, Pietro and Stolterfoht, Martin and Wolff, Christian Michael and Unold, Thomas and Rech, Bernd and Albrecht, Steve and Neher, Dieter}, title = {On the relation between the open-circuit voltage and quasi-fermi level splitting in efficient perovskite solar cells}, series = {advanced energy materials}, volume = {9}, journal = {advanced energy materials}, number = {33}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1614-6832}, doi = {10.1002/aenm.201901631}, pages = {10}, year = {2019}, abstract = {Today's perovskite solar cells (PSCs) are limited mainly by their open-circuit voltage (VOC) due to nonradiative recombination. Therefore, a comprehensive understanding of the relevant recombination pathways is needed. Here, intensity-dependent measurements of the quasi-Fermi level splitting (QFLS) and of the VOC on the very same devices, including pin-type PSCs with efficiencies above 20\%, are performed. It is found that the QFLS in the perovskite lies significantly below its radiative limit for all intensities but also that the VOC is generally lower than the QFLS, violating one main assumption of the Shockley-Queisser theory. This has far-reaching implications for the applicability of some well-established techniques, which use the VOC as a measure of the carrier densities in the absorber. By performing drift-diffusion simulations, the intensity dependence of the QFLS, the QFLS-VOC offset and the ideality factor are consistently explained by trap-assisted recombination and energetic misalignment at the interfaces. Additionally, it is found that the saturation of the VOC at high intensities is caused by insufficient contact selectivity while heating effects are of minor importance. It is concluded that the analysis of the VOC does not provide reliable conclusions of the recombination pathways and that the knowledge of the QFLS-VOC relation is of great importance.}, language = {en} } @misc{PisoniStolterfohtLockingeretal.2019, author = {Pisoni, Stefano and Stolterfoht, Martin and Lockinger, Johannes and Moser, Thierry and Jiang, Yan and Caprioglio, Pietro and Neher, Dieter and Buecheler, Stephan and Tiwari, Ayodhya N.}, title = {On the origin of open-circuit voltage losses in flexible n-i-p perovskite solar cells}, series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, number = {1110}, issn = {1866-8372}, doi = {10.25932/publishup-45961}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-459617}, pages = {12}, year = {2019}, abstract = {The possibility to manufacture perovskite solar cells (PSCs) at low temperatures paves the way to flexible and lightweight photovoltaic (PV) devices manufactured via high-throughput roll-to-roll processes. In order to achieve higher power conversion efficiencies, it is necessary to approach the radiative limit via suppression of non-radiative recombination losses. Herein, we performed a systematic voltage loss analysis for a typical low-temperature processed, flexible PSC in n-i-p configuration using vacuum deposited C-60 as electron transport layer (ETL) and two-step hybrid vacuum-solution deposition for CH3NH3PbI3 perovskite absorber. We identified the ETL/absorber interface as a bottleneck in relation to non-radiative recombination losses, the quasi-Fermi level splitting (QFLS) decreases from similar to 1.23 eV for the bare absorber, just similar to 90 meV below the radiative limit, to similar to 1.10 eV when C-60 is used as ETL. To effectively mitigate these voltage losses, we investigated different interfacial modifications via vacuum deposited interlayers (BCP, B4PyMPM, 3TPYMB, and LiF). An improvement in QFLS of similar to 30-40 meV is observed after interlayer deposition and confirmed by comparable improvements in the open-circuit voltage after implementation of these interfacial modifications in flexible PSCs. Further investigations on absorber/hole transport layer (HTL) interface point out the detrimental role of dopants in Spiro-OMeTAD film (widely employed HTL in the community) as recombination centers upon oxidation and light exposure. [GRAPHICS] .}, 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 Perdigon-Toro, Lorena 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} }