@article{PiersimoniSchlesingerBenduhnetal.2015,
  author    = {Piersimoni, Fortunato and Schlesinger, Raphael and Benduhn, Johannes and Spoltore, Donato and Reiter, Sina and Lange, Ilja and Koch, Norbert and Vandewal, Koen and Neher, Dieter},
  title     = {Charge Transfer Absorption and Emission at ZnO/Organic Interfaces},
  series = {The journal of physical chemistry letters},
  volume    = {6},
  journal   = {The journal of physical chemistry letters},
  number    = {3},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {1948-7185},
  doi       = {10.1021/jz502657z},
  pages     = {500 -- 504},
  year      = {2015},
  abstract  = {We investigate hybrid charge transfer states (HCTS) at the planar interface between a-NPD and ZnO by spectrally resolved electroluminescence (EL) and external quantum efficiency (EQE) measurements. Radiative decay of HCTSs is proven by distinct emission peaks in the EL spectra of such bilayer devices in the NIR at energies well below the bulk a-NPD or ZnO emission. The EQE spectra display low energy contributions clearly red-shifted with respect to the a-NPD photocurrent and partially overlapping with the EL emission. Tuning of the energy gap between the ZnO conduction band and a-NPD HOMO level (E-int) was achieved by modifying the ZnO surface with self-assembled monolayers based on phosphonic acids. We find a linear dependence of the peak position of the NIR EL on E-int, which unambiguously attributes the origin of this emission to radiative recombination between an electron on the ZnO and a hole on a-NPD. In accordance with this interpretation, we find a strictly linear relation between the open-circuit voltage and the energy of the charge state for such hybrid organicinorganic interfaces.},
  language  = {en}
}
@article{XuShalomPiersimonietal.2015,
  author    = {Xu, Jingsan and Shalom, Menny and Piersimoni, Fortunato and Antonietti, Markus and Neher, Dieter and Brenner, Thomas J. K.},
  title     = {Color-Tunable Photoluminescence and NIR Electroluminescence in Carbon Nitride Thin Films and Light-Emitting Diodes},
  series = {Advanced optical materials},
  volume    = {3},
  journal   = {Advanced optical materials},
  number    = {7},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {2195-1071},
  doi       = {10.1002/adom.201500019},
  pages     = {913 -- 917},
  year      = {2015},
  language  = {en}
}
@article{LangeReiterKniepertetal.2015,
  author    = {Lange, Ilja and Reiter, Sina and Kniepert, Juliane and Piersimoni, Fortunato and Paetzel, Michael and Hildebrandt, Jana and Brenner, Thomas J. K. and Hecht, Stefan and Neher, Dieter},
  title     = {Zinc oxide modified with benzylphosphonic acids as transparent electrodes in regular and inverted organic solar cell structures},
  series = {Applied physics letters},
  volume    = {106},
  journal   = {Applied physics letters},
  number    = {11},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0003-6951},
  doi       = {10.1063/1.4916182},
  pages     = {5},
  year      = {2015},
  abstract  = {An approach is presented to modify the work function of solution-processed sol-gel derived zinc oxide (ZnO) over an exceptionally wide range of more than 2.3 eV. This approach relies on the formation of dense and homogeneous self-assembled monolayers based on phosphonic acids with different dipole moments. This allows us to apply ZnO as charge selective bottom electrodes in either regular or inverted solar cell structures, using poly(3-hexylthiophene): phenyl-C71-butyric acid methyl ester as the active layer. These devices compete with or even surpass the performance of the reference on indium tin oxide/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate. Our findings highlight the potential of properly modified ZnO as electron or hole extracting electrodes in hybrid optoelectronic devices. (C) 2015 AIP Publishing LLC.},
  language  = {en}
}
@article{FangHolzmuellerMatulaitisetal.2016,
  author    = {Fang, Lijia and Holzmueller, Felix and Matulaitis, Tomas and Baasner, Anne and Hauenstein, Christoph and Benduhn, Johannes and Schwarze, Martin and Petrich, Annett and Piersimoni, Fortunato and Scholz, Reinhard and Zeika, Olaf and Koerner, Christian and Neher, Dieter and Vandewal, Koen and Leo, Karl},
  title     = {Fluorine-containing low-energy-gap organic dyes with low voltage losses for organic solar cells},
  series = {Synthetic metals : the journal of electronic polymers and electronic molecular materials},
  volume    = {222},
  journal   = {Synthetic metals : the journal of electronic polymers and electronic molecular materials},
  publisher = {Elsevier},
  address   = {Lausanne},
  issn      = {0379-6779},
  doi       = {10.1016/j.synthmet.2016.10.025},
  pages     = {232 -- 239},
  year      = {2016},
  abstract  = {Fluorine-containing donor molecules TFTF, CNTF and PRTF are designed and isomer selectively synthesized for application in vacuum-deposited organic solar cells. These molecules comprise a donor acceptor molecular architecture incorporating thiophene and benzothiadiazole derivatives as the electron-donating and electron-withdrawing moieties, respectively. As opposed to previously reported materials from this class, PRTF can be purified by vacuum sublimation at moderate to high yields because of its higher volatility and better stabilization due to a stronger intramolecular hydrogen bond, as compared to TFTF and CNTF. The UV-vis absorption spectra of the three donors show an intense broadband absorption between 500 nm and 800 nm with, similar positions of their frontier energy levels. The photophysical properties of the three donor molecules are thoroughly tested and optimized in bulk heterojunction solar cells with C-60 as acceptor. PRTF shows the best performance, yielding power conversion efficiencies of up to 3.8\%. Moreover, the voltage loss for the PRTF device due to the non radiative recombination of free charge carriers is exceptionally low (0.26 V) as compared to typical values for organic solar cells (>0.34V). (C) 2016 Published by Elsevier B.V.},
  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{NikolisBenduhnHolzmuelleretal.2017,
  author    = {Nikolis, Vasileios C. and Benduhn, Johannes and Holzmueller, Felix and Piersimoni, Fortunato and Lau, Matthias and Zeika, Olaf and Neher, Dieter and Koerner, Christian and Spoltore, Donato and Vandewal, Koen},
  title     = {Reducing Voltage Losses in Cascade Organic Solar Cells while Maintaining High External Quantum Efficiencies},
  series = {dvanced energy materials},
  volume    = {7},
  journal   = {dvanced energy materials},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1614-6832},
  doi       = {10.1002/aenm.201700855},
  pages     = {122 -- 136},
  year      = {2017},
  abstract  = {High photon energy losses limit the open-circuit voltage (V-OC) and power conversion efficiency of organic solar cells (OSCs). In this work, an optimization route is presented which increases the V-OC by reducing the interfacial area between donor (D) and acceptor (A). This optimization route concerns a cascade device architecture in which the introduction of discontinuous interlayers between alpha-sexithiophene (alpha-6T) (D) and chloroboron subnaphthalocyanine (SubNc) (A) increases the V-OC of an alpha-6T/SubNc/SubPc fullerene-free cascade OSC from 0.98 V to 1.16 V. This increase of 0.18 V is attributed solely to the suppression of nonradiative recombination at the D-A interface. By accurately measuring the optical gap (E-opt) and the energy of the charge-transfer state (E-CT) of the studied OSC, a detailed analysis of the overall voltage losses is performed. E-opt - qV(OC) losses of 0.58 eV, which are among the lowest observed for OSCs, are obtained. Most importantly, for the V-OC-optimized devices, the low-energy (700 nm) external quantum efficiency (EQE) peak remains high at 79\%, despite a minimal driving force for charge separation of less than 10 meV. This work shows that low-voltage losses can be combined with a high EQE in organic photovoltaic devices.},
  language  = {en}
}
@article{GorenflotPaulkePiersimonietal.2018,
  author    = {Gorenflot, Julien and Paulke, Andreas and Piersimoni, Fortunato and Wolf, Jannic and Kan, Zhipeng and Cruciani, Federico and El Labban, Abdulrahman and Neher, Dieter and Beaujuge, Pierre M. and Laquai, Frederic},
  title     = {From recombination dynamics to device performance},
  series = {dvanced energy materials},
  volume    = {8},
  journal   = {dvanced energy materials},
  number    = {4},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1614-6832},
  doi       = {10.1002/aenm.201701678},
  pages     = {12},
  year      = {2018},
  abstract  = {An original set of experimental and modeling tools is used to quantify the yield of each of the physical processes leading to photocurrent generation in organic bulk heterojunction solar cells, enabling evaluation of materials and processing condition beyond the trivial comparison of device performances. Transient absorption spectroscopy, "the" technique to monitor all intermediate states over the entire relevant timescale, is combined with time-delayed collection field experiments, transfer matrix simulations, spectral deconvolution, and parametrization of the charge carrier recombination by a two-pool model, allowing quantification of densities of excitons and charges and extrapolation of their kinetics to device-relevant conditions. Photon absorption, charge transfer, charge separation, and charge extraction are all quantified for two recently developed wide-bandgap donor polymers: poly(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-3,4-difluorothiophene) (PBDT[2F]T) and its nonfluorinated counterpart poly(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-3,4-thiophene) (PBDT[2H]T) combined with PC71BM in bulk heterojunctions. The product of these yields is shown to agree well with the devices' external quantum efficiency. This methodology elucidates in the specific case studied here the origin of improved photocurrents obtained when using PBDT[2F]T instead of PBDT[2H]T as well as upon using solvent additives. Furthermore, a higher charge transfer (CT)-state energy is shown to lead to significantly lower energy losses (resulting in higher VOC) during charge generation compared to P3HT:PCBM.},
  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{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{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{PoelkingBenduhnSpoltoreetal.2022,
  author    = {Poelking, Carl and Benduhn, Johannes and Spoltore, Donato and Schwarze, Martin and Roland, Steffen and Piersimoni, Fortunato and Neher, Dieter and Leo, Karl and Vandewal, Koen and Andrienko, Denis},
  title     = {Open-circuit voltage of organic solar cells},
  series = {Communications physics},
  volume    = {5},
  journal   = {Communications physics},
  number    = {1},
  publisher = {Nature portfolio},
  address   = {Berlin},
  issn      = {2399-3650},
  doi       = {10.1038/s42005-022-01084-x},
  pages     = {7},
  year      = {2022},
  abstract  = {Organic photovoltaics (PV) is an energy-harvesting technology that offers many advantages, such as flexibility, low weight and cost, as well as environmentally benign materials and manufacturing techniques. Despite growth of power conversion efficiencies to around 19 \% in the last years, organic PVs still lag behind inorganic PV technologies, mainly due to high losses in open-circuit voltage. Understanding and improving open circuit voltage in organic solar cells is challenging, as it is controlled by the properties of a donor-acceptor interface where the optical excitations are separated into charge carriers. Here, we provide an electrostatic model of a rough donor-acceptor interface and test it experimentally on small molecule PV materials systems. The model provides concise relationships between the open-circuit voltage, photovoltaic gap, charge-transfer state energy, and interfacial morphology. In particular, we show that the electrostatic bias generated across the interface reduces the photovoltaic gap. This negative influence on open-circuit voltage can, however, be circumvented by adjusting the morphology of the donor-acceptor interface. Organic solar cells, despite their high power conversion efficiencies, suffer from open circuit voltage losses making them less appealing in terms of applications. Here, the authors, supported with experimental data on small molecule photovoltaic cells, relate open circuit voltage to photovoltaic gap, charge-transfer state energy, and donor-acceptor interfacial morphology.},
  language  = {en}
}