@article{YuanZhangQiuetal.2022,
  author    = {Yuan, Jun and Zhang, Chujun and Qiu, Beibei and Liu, Wei and So, Shu Kong and Mainville, Mathieu and Leclerc, Mario and Shoaee, Safa and Neher, Dieter and Zou, Yingping},
  title     = {Effects of energetic disorder in bulk heterojunction organic solar cells},
  series = {Energy \& environmental science},
  volume    = {15},
  journal   = {Energy \& environmental science},
  number    = {7},
  publisher = {Royal Society of Chemistry},
  address   = {Cambridge},
  issn      = {1754-5692},
  doi       = {10.1039/d2ee00271j},
  pages     = {2806 -- 2818},
  year      = {2022},
  abstract  = {Organic solar cells (OSCs) have progressed rapidly in recent years through the development of novel organic photoactive materials, especially non-fullerene acceptors (NFAs). Consequently, OSCs based on state-of-the-art NFAs have reached significant milestones, such as similar to 19\% power conversion efficiencies (PCEs) and small energy losses (less than 0.5 eV). Despite these significant advances, understanding of the interplay between molecular structure and optoelectronic properties lags significantly behind. For example, despite the theoretical framework for describing the energetic disorder being well developed for the case of inorganic semiconductors, the question of the applicability of classical semiconductor theories in analyzing organic semiconductors is still under debate. A general observation in the inorganic field is that inorganic photovoltaic materials possessing a polycrystalline microstructure exhibit suppressed disorder properties and better charge carrier transport compared to their amorphous analogs. Accordingly, this principle extends to the organic semiconductor field as many organic photovoltaic materials are synthesized to pursue polycrystalline-like features. Yet, there appears to be sporadic examples that exhibit an opposite trend. However, full studies decoupling energetic disorder from aggregation effects have largely been left out. Hence, the potential role of the energetic disorder in OSCs has received little attention. Interestingly, recently reported state-of-the-art NFA-based devices could achieve a small energetic disorder and high PCE at the same time; and interest in this investigation related to the disorder properties in OSCs was revived. In this contribution, progress in terms of the correlation between molecular design and energetic disorder is reviewed together with their effects on the optoelectronic mechanism and photovoltaic performance. Finally, the specific challenges and possible solutions in reducing the energetic disorder of OSCs from the viewpoint of materials and devices are proposed.},
  language  = {en}
}
@article{KrohEllerSchoetzetal.2022,
  author    = {Kroh, Daniel and Eller, Fabian and Sch{\"o}tz, Konstantin and Wedler, Stefan and Perdig{\´o}n-Toro, Lorena and Freychet, Guillaume and Wei, Qingya and D{\"o}rr, Maximilian and Jones, David and Zou, Yingping and Herzig, Eva M. and Neher, Dieter and K{\"o}hler, Anna},
  title     = {Identifying the signatures of intermolecular interactions in blends of PM6 with Y6 and N4 using absorption spectroscopy},
  series = {Advanced functional materials},
  volume    = {32},
  journal   = {Advanced functional materials},
  number    = {44},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1616-301X},
  doi       = {10.1002/adfm.202205711},
  pages     = {14},
  year      = {2022},
  abstract  = {In organic solar cells, the resulting device efficiency depends strongly on the local morphology and intermolecular interactions of the blend film. Optical spectroscopy was used to identify the spectral signatures of interacting chromophores in blend films of the donor polymer PM6 with two state-of-the-art nonfullerene acceptors, Y6 and N4, which differ merely in the branching point of the side chain. From temperature-dependent absorption and luminescence spectroscopy in solution, it is inferred that both acceptor materials form two types of aggregates that differ in their interaction energy. Y6 forms an aggregate with a predominant J-type character in solution, while for N4 molecules the interaction is predominantly in a H-like manner in solution and freshly spin-cast film, yet the molecules reorient with respect to each other with time or thermal annealing to adopt a more J-type interaction. The different aggregation behavior of the acceptor materials is also reflected in the blend films and accounts for the different solar cell efficiencies reported with the two blends.},
  language  = {en}
}
@article{FritschKurpiersRolandetal.2022,
  author    = {Fritsch, Tobias and Kurpiers, Jona and Roland, Steffen and Tokmoldin, Nurlan and Shoaee, Safa and Ferron, Thomas and Collins, Brian A. and Janietz, Silvia and Vandewal, Koen and Neher, Dieter},
  title     = {On the interplay between CT and singlet exciton emission in organic solar cells with small driving force and its impact on voltage loss},
  series = {Advanced energy materials},
  volume    = {12},
  journal   = {Advanced energy materials},
  number    = {31},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1614-6832},
  doi       = {10.1002/aenm.202200641},
  pages     = {11},
  year      = {2022},
  abstract  = {The interplay between free charge carriers, charge transfer (CT) states and singlet excitons (S-1) determines the recombination pathway and the resulting open circuit voltage (V-OC) of organic solar cells. By combining a well-aggregated low bandgap polymer with different blend ratios of the fullerenes PCBM and ICBA, the energy of the CT state (E-CT) is varied by 130 meV while leaving the S-1 energy of the polymer (ES1\[{E_{{{\rm{S}}_1}}}\]) unaffected. It is found that the polymer exciton dominates the radiative properties of the blend when ECT\[{E_{{\rm{CT}}}}\] approaches ES1\[{E_{{{\rm{S}}_1}}}\], while the V-OC remains limited by the non-radiative decay of the CT state. It is concluded that an increasing strength of the exciton in the optical spectra of organic solar cells will generally decrease the non-radiative voltage loss because it lowers the radiative V-OC limit (V-OC,V-rad), but not because it is more emissive. The analysis further suggests that electronic coupling between the CT state and the S-1 will not improve the V-OC, but rather reduce the V-OC,V-rad. It is anticipated that only at very low CT state absorption combined with a fairly high CT radiative efficiency the solar cell benefit from the radiative properties of the singlet excitons.},
  language  = {en}
}
@article{GrischekCaprioglioZhangetal.2022,
  author    = {Grischek, Max and Caprioglio, Pietro and Zhang, Jiahuan and Pena-Camargo, Francisco and Sveinbjornsson, Kari and Zu, Fengshuo and Menzel, Dorothee and Warby, Jonathan H. and Li, Jinzhao and Koch, Norbert and Unger, Eva and Korte, Lars and Neher, Dieter and Stolterfoht, Martin and Albrecht, Steve},
  title     = {Efficiency Potential and Voltage Loss of Inorganic CsPbI2Br Perovskite Solar Cells},
  series = {Solar RRL},
  volume    = {6},
  journal   = {Solar RRL},
  number    = {11},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {2367-198X},
  doi       = {10.1002/solr.202200690},
  pages     = {12},
  year      = {2022},
  abstract  = {Inorganic perovskite solar cells show excellent thermal stability, but the reported power conversion efficiencies are still lower than for organic-inorganic perovskites. This is mainly caused by lower open-circuit voltages (V(OC)s). Herein, the reasons for the low V-OC in inorganic CsPbI2Br perovskite solar cells are investigated. Intensity-dependent photoluminescence measurements for different layer stacks reveal that n-i-p and p-i-n CsPbI2Br solar cells exhibit a strong mismatch between quasi-Fermi level splitting (QFLS) and V-OC. Specifically, the CsPbI2Br p-i-n perovskite solar cell has a QFLS-e center dot V-OC mismatch of 179 meV, compared with 11 meV for a reference cell with an organic-inorganic perovskite of similar bandgap. On the other hand, this study shows that the CsPbI2Br films with a bandgap of 1.9 eV have a very low defect density, resulting in an efficiency potential of 20.3\% with a MeO-2PACz hole-transporting layer and 20.8\% on compact TiO2. Using ultraviolet photoelectron spectroscopy measurements, energy level misalignment is identified as a possible reason for the QFLS-e center dot V-OC mismatch and strategies for overcoming this V-OC limitation are discussed. This work highlights the need to control the interfacial energetics in inorganic perovskite solar cells, but also gives promise for high efficiencies once this issue is resolved.},
  language  = {en}
}
@article{PenaCamargoThiesbrummelHempeletal.2022,
  author    = {Pena-Camargo, Francisco and Thiesbrummel, Jarla and Hempel, Hannes and Musiienko, Artem and Le Corre, Vincent M. and Diekmann, Jonas and Warby, Jonathan and Unold, Thomas and Lang, Felix and Neher, Dieter and Stolterfoht, Martin},
  title     = {Revealing the doping density in perovskite solar cells and its impact on device performance},
  series = {Applied physics reviews},
  volume    = {9},
  journal   = {Applied physics reviews},
  number    = {2},
  publisher = {AIP Publishing},
  address   = {Melville},
  issn      = {1931-9401},
  doi       = {10.1063/5.0085286},
  pages     = {11},
  year      = {2022},
  abstract  = {Traditional inorganic semiconductors can be electronically doped with high precision. Conversely, there is still conjecture regarding the assessment of the electronic doping density in metal-halide perovskites, not to mention of a control thereof. This paper presents a multifaceted approach to determine the electronic doping density for a range of different lead-halide perovskite systems. Optical and electrical characterization techniques, comprising intensity-dependent and transient photoluminescence, AC Hall effect, transfer-length-methods, and charge extraction measurements were instrumental in quantifying an upper limit for the doping density. The obtained values are subsequently compared to the electrode charge per cell volume under short-circuit conditions ( CUbi/eV), which amounts to roughly 10(16) cm(-3). This figure of merit represents the critical limit below which doping-induced charges do not influence the device performance. The experimental results consistently demonstrate that the doping density is below this critical threshold 10(12) cm(-3), which means << CUbi / e V) for all common lead-based metal-halide perovskites. Nevertheless, although the density of doping-induced charges is too low to redistribute the built-in voltage in the perovskite active layer, mobile ions are present in sufficient quantities to create space-charge-regions in the active layer, reminiscent of doped pn-junctions. These results are well supported by drift-diffusion simulations, which confirm that the device performance is not affected by such low doping densities.},
  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{TaitReckwitzArvindetal.2021,
  author    = {Tait, Claudia E. and Reckwitz, Anna and Arvind, Malavika and Neher, Dieter and Bittl, Robert and Behrends, Jan},
  title     = {Spin-spin interactions and spin delocalisation in a doped organic semiconductor probed by EPR spectroscopy},
  series = {Physical chemistry, chemical physics : PCCP ; a journal of European chemical societies},
  volume    = {23},
  journal   = {Physical chemistry, chemical physics : PCCP ; a journal of European chemical societies},
  number    = {25},
  publisher = {Royal Society of Chemistry},
  address   = {Cambridge},
  issn      = {1463-9076},
  doi       = {10.1039/d1cp02133h},
  pages     = {13827 -- 13841},
  year      = {2021},
  abstract  = {The enhancement and control of the electrical conductivity of organic semiconductors is fundamental for their use in optoelectronic applications and can be achieved by molecular doping, which introduces additional charge carriers through electron transfer between a dopant molecule and the organic semiconductor. Here, we use Electron Paramagnetic Resonance (EPR) spectroscopy to characterise the unpaired spins associated with the charges generated by molecular doping of the prototypical organic semiconductor poly(3-hexylthiophene) (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F(4)TCNQ) and tris(pentafluorophenyl)borane (BCF). The EPR results reveal the P3HT radical cation as the only paramagnetic species in BCF-doped P3HT films and show evidence for increased mobility of the detected spins at high doping concentrations as well as formation of antiferromagnetically coupled spin pairs leading to decreased spin concentrations at low temperatures. The EPR signature for F(4)TCNQ-doped P3HT is found to be determined by spin exchange between P3HT radical cations and F(4)TCNQ radical anions. Results from continuous-wave and pulse EPR measurements suggest the presence of the unpaired spin on P3HT in a multitude of environments, ranging from free P3HT radical cations with similar properties to those observed in BCF-doped P3HT, to pairs of dipolar and exchange-coupled spins on P3HT and the dopant anion. Characterisation of the proton hyperfine interactions by ENDOR allowed quantification of the extent of spin delocalisation and revealed reduced delocalisation in the F(4)TCNQ-doped P3HT films.},
  language  = {en}
}
@article{PerdigonToroLeQuangPhuongZeiskeetal.2021,
  author    = {Perdig{\´o}n-Toro, Lorena and Le Quang Phuong, and Zeiske, Stefan and Vandewal, Koen and Armin, Ardalan and Shoaee, Safa and Neher, Dieter},
  title     = {Excitons dominate the emission from PM6},
  series = {ACS energy letters / American Chemical Society},
  volume    = {6},
  journal   = {ACS energy letters / American Chemical Society},
  number    = {2},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {2380-8195},
  doi       = {10.1021/acsenergylett.0c02572},
  pages     = {557 -- 564},
  year      = {2021},
  abstract  = {Non-fullerene acceptors (NFAs) are far more emissive than their fullerene-based counterparts. Here, we study the spectral properties of photocurrent generation and recombination of the blend of the donor polymer PM6 with the NFA Y6. We find that the radiative recombination of free charges is almost entirely due to the re-occupation and decay of Y6 singlet excitons, but that this pathway contributes less than 1\% to the total recombination. As such, the open-circuit voltage of the PM6:Y6 blend is determined by the energetics and kinetics of the charge-transfer (CT) state. Moreover, we find that no information on the energetics of the CT state manifold can be gained from the low-energy tail of the photovoltaic external quantum efficiency spectrum, which is dominated by the excitation spectrum of the Y6 exciton. We, finally, estimate the charge-separated state to lie only 120 meV below the Y6 singlet exciton energy, meaning that this blend indeed represents a high-efficiency system with a low energetic offset.},
  language  = {en}
}
@article{PranavBenduhnNymanetal.2021,
  author    = {Pranav, Manasi and Benduhn, Johannes and Nyman, Mathias and Hosseini, Seyed Mehrdad and Kublitski, Jonas and Shoaee, Safa and Neher, Dieter and Leo, Karl and Spoltore, Donato},
  title     = {Enhanced charge selectivity via anodic-C60 layer reduces nonradiative losses in organic solar cells},
  series = {ACS applied materials \& interfaces},
  volume    = {13},
  journal   = {ACS applied materials \& interfaces},
  number    = {10},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {1944-8244},
  doi       = {10.1021/acsami.1c00049},
  pages     = {12603 -- 12609},
  year      = {2021},
  abstract  = {Interfacial layers in conjunction with suitable charge-transport layers can significantly improve the performance of optoelectronic devices by facilitating efficient charge carrier injection and extraction. This work uses a neat C-60 interlayer on the anode to experimentally reveal that surface recombination is a significant contributor to nonradiative recombination losses in organic solar cells. These losses are shown to proportionally increase with the extent of contact between donor molecules in the photoactive layer and a molybdenum oxide (MoO3) hole extraction layer, proven by calculating voltage losses in low- and high-donor-content bulk heterojunction device architectures. Using a novel in-device determination of the built-in voltage, the suppression of surface recombination, due to the insertion of a thin anodic-C-60 interlayer on MoO3, is attributed to an enhanced built-in potential. The increased built-in voltage reduces the presence of minority charge carriers at the electrodes-a new perspective on the principle of selective charge extraction layers. The benefit to device efficiency is limited by a critical interlayer thickness, which depends on the donor material in bilayer devices. Given the high popularity of MoO3 as an efficient hole extraction and injection layer and the increasingly popular discussion on interfacial phenomena in organic optoelectronic devices, these findings are relevant to and address different branches of organic electronics, providing insights for future device design.},
  language  = {en}
}
@article{GarciaBenitoQuartiQuelozetal.2020,
  author    = {Garc{\´i}a-Benito, In{\´e}s and Quarti, Claudio and Queloz, Valentin I. E. and Hofstetter, Yvonne J. and Becker-Koch, David and Caprioglio, Pietro and Neher, Dieter and Orlandi, Simonetta and Cavazzini, Marco and Pozzi, Gianluca and Even, Jacky and Nazeeruddin, Mohammad Khaja and Vaynzof, Yana and Grancini, Giulia},
  title     = {Fluorination of organic spacer impacts on the structural and optical response of 2D perovskites},
  series = {Frontiers in Chemistry},
  volume    = {7},
  journal   = {Frontiers in Chemistry},
  publisher = {Frontiers Media},
  address   = {Lausanne},
  issn      = {2296-2646},
  doi       = {10.3389/fchem.2019.00946},
  pages     = {1 -- 11},
  year      = {2020},
  abstract  = {Low-dimensional hybrid perovskites have triggered significant research interest due to their intrinsically tunable optoelectronic properties and technologically relevant material stability. In particular, the role of the organic spacer on the inherent structural and optical features in two-dimensional (2D) perovskites is paramount for material optimization. To obtain a deeper understanding of the relationship between spacers and the corresponding 2D perovskite film properties, we explore the influence of the partial substitution of hydrogen atoms by fluorine in an alkylammonium organic cation, resulting in (Lc)(2)PbI4 and (Lf)(2)PbI4 2D perovskites, respectively. Consequently, optical analysis reveals a clear 0.2 eV blue-shift in the excitonic position at room temperature. This result can be mainly attributed to a band gap opening, with negligible effects on the exciton binding energy. According to Density Functional Theory (DFT) calculations, the band gap increases due to a larger distortion of the structure that decreases the atomic overlap of the wavefunctions and correspondingly bandwidth of the valence and conduction bands. In addition, fluorination impacts the structural rigidity of the 2D perovskite, resulting in a stable structure at room temperature and the absence of phase transitions at a low temperature, in contrast to the widely reported polymorphism in some non-fluorinated materials that exhibit such a phase transition. This indicates that a small perturbation in the material structure can strongly influence the overall structural stability and related phase transition of 2D perovskites, making them more robust to any phase change. This work provides key information on how the fluorine content in organic spacer influence the structural distortion of 2D perovskites and their optical properties which possess remarkable importance for future optoelectronic applications, for instance in the field of light-emitting devices or sensors.},
  language  = {en}
}