@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{BenduhnTvingstedtPiersimonietal.2017, author = {Benduhn, Johannes and Tvingstedt, Kristofer and Piersimoni, Fortunato and Ullbrich, Sascha and Fan, Yeli and Tropiano, Manuel and McGarry, Kathryn A. and Zeika, Olaf and Riede, Moritz K. and Douglas, Christopher J. and Barlow, Stephen and Marder, Seth R. and Neher, Dieter and Spoltore, Donato and Vandewal, Koen}, title = {Intrinsic non-radiative voltage losses in fullerene-based organic solar cells}, series = {Nature Energy}, volume = {2}, journal = {Nature Energy}, publisher = {Nature Publ. Group}, address = {London}, issn = {2058-7546}, doi = {10.1038/nenergy.2017.53}, pages = {6}, year = {2017}, abstract = {Organic solar cells demonstrate external quantum efficiencies and fill factors approaching those of conventional photovoltaic technologies. However, as compared with the optical gap of the absorber materials, their open-circuit voltage is much lower, largely due to the presence of significant non-radiative recombination. Here, we study a large data set of published and new material combinations and find that non-radiative voltage losses decrease with increasing charge-transfer-state energies. This observation is explained by considering non-radiative charge-transfer-state decay as electron transfer in the Marcus inverted regime, being facilitated by a common skeletal molecular vibrational mode. Our results suggest an intrinsic link between non-radiative voltage losses and electron-vibration coupling, indicating that these losses are unavoidable. Accordingly, the theoretical upper limit for the power conversion efficiency of single-junction organic solar cells would be reduced to about 25.5\% and the optimal optical gap increases to (1.45-1.65) eV, that is, (0.2-0.3) eV higher than for technologies with minimized non-radiative voltage losses.}, language = {en} } @article{VandewalBenduhnSchellhammeretal.2017, author = {Vandewal, Koen and Benduhn, Johannes and Schellhammer, Karl Sebastian and Vangerven, Tim and R{\"u}ckert, Janna E. and Piersimoni, Fortunato and Scholz, Reinhard and Zeika, Olaf and Fan, Yeli and Barlow, Stephen and Neher, Dieter and Marder, Seth R. and Manca, Jean and Spoltore, Donato and Cuniberti, Gianaurelio and Ortmann, Frank}, title = {Absorption Tails of Donor}, series = {Journal of the American Chemical Society}, volume = {139}, journal = {Journal of the American Chemical Society}, number = {4}, publisher = {American Chemical Society}, address = {Washington}, issn = {0002-7863}, doi = {10.1021/jacs.6b12857}, pages = {1699 -- 1704}, year = {2017}, abstract = {In disordered organic semiconductors, the transfer of a rather localized charge carrier from one site to another triggers a deformation of the molecular structure quantified by the intramolecular relaxation energy. A similar structural relaxation occurs upon population of intermolecular charge-transfer (CT) states formed at organic electron donor (D)-acceptor (A) interfaces. Weak CT absorption bands for D A complexes occur at photon energies below the optical gaps of both the donors and the C-60 acceptor as a result of optical transitions from the neutral ground state to the ionic CT state. In this work, we show that temperature-activated intramolecular vibrations of the ground state play a major role in determining the line shape of such CT absorption bands. This allows us to extract values for the relaxation energy related to the geometry change from neutral to ionic CT complexes. Experimental values for the relaxation energies of 20 D:C-60 CT complexes correlate with values calculated within density functional theory. These results provide an experimental method for determining the polaron relaxation energy in solid-state organic D-A blends and show the importance of a reduced relaxation energy, which we introduce to characterize thermally activated CT processes.}, language = {en} } @article{BenduhnPiersimoniLondietal.2018, author = {Benduhn, Johannes and Piersimoni, Fortunato and Londi, Giacomo and Kirch, Anton and Widmer, Johannes and Koerner, Christian and Beljonne, David and Neher, Dieter and Spoltore, Donato and Vandewal, Koen}, title = {Impact of triplet excited states on the open-circuit voltage of organic solar cells}, series = {dvanced energy materials}, volume = {8}, journal = {dvanced energy materials}, number = {21}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1614-6832}, doi = {10.1002/aenm.201800451}, pages = {7}, year = {2018}, abstract = {The best organic solar cells (OSCs) achieve comparable peak external quantum efficiencies and fill factors as conventional photovoltaic devices. However, their voltage losses are much higher, in particular those due to nonradiative recombination. To investigate the possible role of triplet states on the donor or acceptor materials in this process, model systems comprising Zn- and Cu-phthalocyanine (Pc), as well as fluorinated versions of these donors, combined with C-60 as acceptor are studied. Fluorination allows tuning the energy level alignment between the lowest energy triplet state (T-1) and the charge-transfer (CT) state, while the replacement of Zn by Cu as the central metal in the Pcs leads to a largely enhanced spin-orbit coupling. Only in the latter case, a substantial influence of the triplet state on the nonradiative voltage losses is observed. In contrast, it is found that for a large series of typical OSC materials, the relative energy level alignment between T-1 and the CT state does not substantially affect nonradiative voltage losses.}, 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{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} } @article{SchubertCollinsMangoldetal.2014, author = {Schubert, Marcel and Collins, Brian A. and Mangold, Hannah and Howard, Ian A. and Schindler, Wolfram and Vandewal, Koen and Roland, Steffen and Behrends, Jan and Kraffert, Felix and Steyrleuthner, Robert and Chen, Zhihua and Fostiropoulos, Konstantinos and Bittl, Robert and Salleo, Alberto and Facchetti, Antonio and Laquai, Frederic and Ade, Harald W. and Neher, Dieter}, title = {Correlated donor/acceptor crystal orientation controls photocurrent generation in all-polymer solar cells}, series = {Advanced functional materials}, volume = {24}, journal = {Advanced functional materials}, number = {26}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1616-301X}, doi = {10.1002/adfm.201304216}, pages = {4068 -- 4081}, year = {2014}, abstract = {New polymers with high electron mobilities have spurred research in organic solar cells using polymeric rather than fullerene acceptors due to their potential of increased diversity, stability, and scalability. However, all-polymer solar cells have struggled to keep up with the steadily increasing power conversion efficiency of polymer: fullerene cells. The lack of knowledge about the dominant recombination process as well as the missing concluding picture on the role of the semi-crystalline microstructure of conjugated polymers in the free charge carrier generation process impede a systematic optimization of all-polymer solar cells. These issues are examined by combining structural and photo-physical characterization on a series of poly(3-hexylthiophene) (donor) and P(NDI2OD-T2) (acceptor) blend devices. These experiments reveal that geminate recombination is the major loss channel for photo-excited charge carriers. Advanced X-ray and electron-based studies reveal the effect of chloronaphthalene co-solvent in reducing domain size, altering domain purity, and reorienting the acceptor polymer crystals to be coincident with those of the donor. This reorientation correlates well with the increased photocurrent from these devices. Thus, effi cient split-up of geminate pairs at polymer/polymer interfaces may necessitate correlated donor/acceptor crystal orientation, which represents an additional requirement compared to the isotropic fullerene acceptors.}, language = {en} } @article{VandewalAlbrechtHokeetal.2014, author = {Vandewal, Koen and Albrecht, Steve and Hoke, Eric T. and Graham, Kenneth R. and Widmer, Johannes and Douglas, Jessica D. and Schubert, Marcel and Mateker, William R. and Bloking, Jason T. and Burkhard, George F. and Sellinger, Alan and Frechet, Jean M. J. and Amassian, Aram and Riede, Moritz K. and McGehee, Michael D. and Neher, Dieter and Salleo, Alberto}, title = {Efficient charge generation by relaxed charge-transfer states at organic interfaces}, series = {Nature materials}, volume = {13}, journal = {Nature materials}, number = {1}, publisher = {Nature Publ. Group}, address = {London}, issn = {1476-1122}, doi = {10.1038/NMAT3807}, pages = {63 -- 68}, year = {2014}, abstract = {carriers on illumination. Efficient organic solar cells require a high yield for this process, combined with a minimum of energy losses. Here, we investigate the role of the lowest energy emissive interfacial charge-transfer state (CT1) in the charge generation process. We measure the quantum yield and the electric field dependence of charge generation on excitation of the charge-transfer (CT) state manifold viaweakly allowed, low-energy optical transitions. For a wide range of photovoltaic devices based on polymer: fullerene, small-molecule:C-60 and polymer: polymer blends, our study reveals that the internal quantum efficiency (IQE) is essentially independent of whether or not D, A or CT states with an energy higher than that of CT1 are excited. The best materials systems show an IQE higher than 90\% without the need for excess electronic or vibrational energy.}, language = {en} }