TY - JOUR A1 - Pranav, Manasi A1 - Hultzsch, Thomas A1 - Musiienko, Artem A1 - Sun, Bowen A1 - Shukla, Atul A1 - Jaiser, Frank A1 - Shoaee, Safa A1 - Neher, Dieter T1 - Anticorrelated photoluminescence and free charge generation proves field-assisted exciton dissociation in low-offset PM6:Y5 organic solar cells JF - APL materials : high impact open access journal in functional materials science N2 - Understanding the origin of inefficient photocurrent generation in organic solar cells with low energy offset remains key to realizing high-performance donor-acceptor systems. Here, we probe the origin of field-dependent free-charge generation and photoluminescence in wnon-fullereneacceptor (NFA)-based organic solar cells using the polymer PM6 and the NFA Y5-a non-halogenated sibling to Y6, with a smaller energetic offset to PM6. By performing time-delayed collection field (TDCF) measurements on a variety of samples with different electron transport layers and active layer thickness, we show that the fill factor and photocurrent are limited by field-dependent free charge generation in the bulk of the blend. We also introduce a new method of TDCF called m-TDCF to prove the absence of artifacts from non-geminate recombination of photogenerated and dark charge carriers near the electrodes. We then correlate free charge generation with steady-state photoluminescence intensity and find perfect anticorrelation between these two properties. Through this, we conclude that photocurrent generation in this low-offset system is entirely controlled by the field-dependent dissociation of local excitons into charge-transfer states. (c) 2023 Author(s). Y1 - 2023 U6 - https://doi.org/10.1063/5.0151580 SN - 2166-532X VL - 11 IS - 6 PB - AIP Publishing CY - Melville ER - TY - JOUR A1 - Poelking, Carl A1 - Benduhn, Johannes A1 - Spoltore, Donato A1 - Schwarze, Martin A1 - Roland, Steffen A1 - Piersimoni, Fortunato A1 - Neher, Dieter A1 - Leo, Karl A1 - Vandewal, Koen A1 - Andrienko, Denis T1 - Open-circuit voltage of organic solar cells BT - interfacial roughness makes the difference JF - Communications physics N2 - 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. Y1 - 2022 U6 - https://doi.org/10.1038/s42005-022-01084-x SN - 2399-3650 VL - 5 IS - 1 PB - Nature portfolio CY - Berlin ER - TY - JOUR A1 - Vollbrecht, Joachim A1 - Tokmoldin, Nurlan A1 - Sun, Bowen A1 - Brus, Viktor V. A1 - Shoaee, Safa A1 - Neher, Dieter T1 - Determination of the charge carrier density in organic solar cells BT - a tutorial JF - Journal of applied physics N2 - The increase in the performance of organic solar cells observed over the past few years has reinvigorated the search for a deeper understanding of the loss and extraction processes in this class of device. A detailed knowledge of the density of free charge carriers under different operating conditions and illumination intensities is a prerequisite to quantify the recombination and extraction dynamics. Differential charging techniques are a promising approach to experimentally obtain the charge carrier density under the aforementioned conditions. In particular, the combination of transient photovoltage and photocurrent as well as impedance and capacitance spectroscopy have been successfully used in past studies to determine the charge carrier density of organic solar cells. In this Tutorial, these experimental techniques will be discussed in detail, highlighting fundamental principles, practical considerations, necessary corrections, advantages, drawbacks, and ultimately their limitations. Relevant references introducing more advanced concepts will be provided as well. Therefore, the present Tutorial might act as an introduction and guideline aimed at new prospective users of these techniques as well as a point of reference for more experienced researchers. Published under an exclusive license by AIP Publishing. KW - Electrical properties and parameters KW - Organic semiconductors KW - Solar cells KW - Photoconductivity KW - Capacitance spectroscopy Y1 - 2022 U6 - https://doi.org/10.1063/5.0094955 SN - 0021-8979 SN - 1089-7550 SN - 1520-8850 VL - 131 IS - 22 PB - American Institute of Physics CY - Melville, NY ER - TY - JOUR A1 - Sun, Bowen A1 - Sandberg, Oskar A1 - Neher, Dieter A1 - Armin, Ardalan A1 - Shoaee, Safa T1 - Wave optics of differential absorption spectroscopy in thick-junction organic solar cells BT - optical artifacts and correction strategies JF - Physical review applied / The American Physical Society N2 - Differential absorption spectroscopy techniques serve as powerful techniques to study the excited species in organic solar cells. However, it has always been challenging to employ these techniques for characterizing thick-junction organic solar cells, especially when a reflective top contact is involved. In this work, we present a detailed and systematic study on how a combination of the presence of the interference effect and a nonuniform charge-distribution profile, severely manipulates experimental spectra and the decay dynamics. Furthermore, we provide a practical methodology to correct these optical artifacts in differential absorption spectroscopies. The results and the proposed correction method generally apply to all kinds of differential absorption spectroscopy techniques and various thin-film systems, such as organics, perovskites, kesterites, and two-dimensional materials. Notably, it is found that the shape of differential absorption spectra can be strongly distorted, starting from 150-nm active-layer thickness; this matches the thickness range of thick-junction organic solar cells and most perovskite solar cells and needs to be carefully considered in experiments. In addition, the decay dynamics of differential absorption spectra is found to be disturbed by optical artifacts under certain conditions. With the help of the proposed correction formalism, differential spectra and the decay dynamics can be characterized on the full device of thin-film solar cells in transmission mode and yield accurate and reliable results to provide design rules for further progress. Y1 - 2022 U6 - https://doi.org/10.1103/PhysRevApplied.17.054016 SN - 2331-7019 VL - 17 IS - 5 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Fritsch, Tobias A1 - Kurpiers, Jona A1 - Roland, Steffen A1 - Tokmoldin, Nurlan A1 - Shoaee, Safa A1 - Ferron, Thomas A1 - Collins, Brian A. A1 - Janietz, Silvia A1 - Vandewal, Koen A1 - Neher, Dieter T1 - On the interplay between CT and singlet exciton emission in organic solar cells with small driving force and its impact on voltage loss JF - Advanced energy materials N2 - 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. KW - external quantum efficiency KW - organic photovoltaics KW - ternary blends KW - voltage losses Y1 - 2022 U6 - https://doi.org/10.1002/aenm.202200641 SN - 1614-6832 SN - 1614-6840 VL - 12 IS - 31 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Yuan, Jun A1 - Zhang, Chujun A1 - Qiu, Beibei A1 - Liu, Wei A1 - So, Shu Kong A1 - Mainville, Mathieu A1 - Leclerc, Mario A1 - Shoaee, Safa A1 - Neher, Dieter A1 - Zou, Yingping T1 - Effects of energetic disorder in bulk heterojunction organic solar cells JF - Energy & environmental science N2 - 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. Y1 - 2022 U6 - https://doi.org/10.1039/d2ee00271j SN - 1754-5692 SN - 1754-5706 VL - 15 IS - 7 SP - 2806 EP - 2818 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Kroh, Daniel A1 - Eller, Fabian A1 - Schötz, Konstantin A1 - Wedler, Stefan A1 - Perdigón-Toro, Lorena A1 - Freychet, Guillaume A1 - Wei, Qingya A1 - Dörr, Maximilian A1 - Jones, David A1 - Zou, Yingping A1 - Herzig, Eva M. A1 - Neher, Dieter A1 - Köhler, Anna T1 - Identifying the signatures of intermolecular interactions in blends of PM6 with Y6 and N4 using absorption spectroscopy JF - Advanced functional materials N2 - 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. KW - charge-transfer states KW - Frank-Condon analysis KW - morphology KW - organic solar cells Y1 - 2022 U6 - https://doi.org/10.1002/adfm.202205711 SN - 1616-301X SN - 1616-3028 VL - 32 IS - 44 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Tockhorn, Philipp A1 - Sutter, Johannes A1 - Cruz Bournazou, Alexandros A1 - Wagner, Philipp A1 - Jäger, Klaus A1 - Yoo, Danbi A1 - Lang, Felix A1 - Grischek, Max A1 - Li, Bor A1 - Li, Jinzhao A1 - Shargaieva, Oleksandra A1 - Unger, Eva A1 - Al-Ashouri, Amran A1 - Köhnen, Eike A1 - Stolterfoht, Martin A1 - Neher, Dieter A1 - Schlatmann, Rutger A1 - Rech, Bernd A1 - Stannowski, Bernd A1 - Albrecht, Steve A1 - Becker, Christiane T1 - Nano-optical designs for high-efficiency monolithic perovskite-silicon tandem solar cells JF - Nature nanotechnology N2 - Designing gentle sinusoidal nanotextures enables the realization of high-efficiency perovskite-silicon solar cells
Perovskite-silicon tandem solar cells offer the possibility of overcoming the power conversion efficiency limit of conventional silicon solar cells. Various textured tandem devices have been presented aiming at improved optical performance, but optimizing film growth on surface-textured wafers remains challenging. Here we present perovskite-silicon tandem solar cells with periodic nanotextures that offer various advantages without compromising the material quality of solution-processed perovskite layers. We show a reduction in reflection losses in comparison to planar tandems, with the new devices being less sensitive to deviations from optimum layer thicknesses. The nanotextures also enable a greatly increased fabrication yield from 50% to 95%. Moreover, the open-circuit voltage is improved by 15 mV due to the enhanced optoelectronic properties of the perovskite top cell. Our optically advanced rear reflector with a dielectric buffer layer results in reduced parasitic absorption at near-infrared wavelengths. As a result, we demonstrate a certified power conversion efficiency of 29.80%. Y1 - 2022 U6 - https://doi.org/10.1038/s41565-022-01228-8 SN - 1748-3387 SN - 1748-3395 VL - 17 IS - 11 SP - 1214 EP - 1221 PB - Nature Publishing Group CY - London [u.a.] ER - TY - JOUR A1 - Perdigón-Toro, Lorena A1 - Le Quang Phuong, A1 - Eller, Fabian A1 - Freychet, Guillaume A1 - Saglamkaya, Elifnaz A1 - Khan, Jafar A1 - Wei, Qingya A1 - Zeiske, Stefan A1 - Kroh, Daniel A1 - Wedler, Stefan A1 - Koehler, Anna A1 - Armin, Ardalan A1 - Laquai, Frederic A1 - Herzig, Eva M. A1 - Zou, Yingping A1 - Shoaee, Safa A1 - Neher, Dieter T1 - Understanding the role of order in Y-series non-fullerene solar cells to realize high open-circuit voltages JF - Advanced energy materials N2 - Non-fullerene acceptors (NFAs) as used in state-of-the-art organic solar cells feature highly crystalline layers that go along with low energetic disorder. Here, the crucial role of energetic disorder in blends of the donor polymer PM6 with two Y-series NFAs, Y6, and N4 is studied. By performing temperature-dependent charge transport and recombination studies, a consistent picture of the shape of the density of state distributions for free charges in the two blends is developed, allowing an analytical description of the dependence of the open-circuit voltage V-OC on temperature and illumination intensity. Disorder is found to influence the value of the V-OC at room temperature, but also its progression with temperature. Here, the PM6:Y6 blend benefits substantially from its narrower state distributions. The analysis also shows that the energy of the equilibrated free charge population is well below the energy of the NFA singlet excitons for both blends and possibly below the energy of the populated charge transfer manifold, indicating a down-hill driving force for free charge formation. It is concluded that energetic disorder of charge-separated states has to be considered in the analysis of the photovoltaic properties, even for the more ordered PM6:Y6 blend. KW - energetic disorder KW - non-fullerene acceptors KW - open-circuit voltage KW - organic solar cells Y1 - 2022 U6 - https://doi.org/10.1002/aenm.202103422 SN - 1614-6832 SN - 1614-6840 VL - 12 IS - 12 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Brinkmann, Kai Oliver A1 - Becker, Tim A1 - Zimmermann, Florian A1 - Kreusel, Cedric A1 - Gahlmann, Tobias A1 - Theisen, Manuel A1 - Haeger, Tobias A1 - Olthof, Selina A1 - Tückmantel, Christian A1 - Günster, M. A1 - Maschwitz, Timo A1 - Göbelsmann, Fabian A1 - Koch, Christine A1 - Hertel, Dirk A1 - Caprioglio, Pietro A1 - Peña-Camargo, Francisco A1 - Perdigón-Toro, Lorena A1 - Al-Ashouri, Amran A1 - Merten, Lena A1 - Hinderhofer, Alexander A1 - Gomell, Leonie A1 - Zhang, Siyuan A1 - Schreiber, Frank A1 - Albrecht, Steve A1 - Meerholz, Klaus A1 - Neher, Dieter A1 - Stolterfoht, Martin A1 - Riedl, Thomas T1 - Perovskite-organic tandem solar cells with indium oxide interconnect JF - Nature N2 - Multijunction solar cells can overcome the fundamental efficiency limits of single-junction devices. The bandgap tunability of metal halide perovskite solar cells renders them attractive for multijunction architectures(1). Combinations with silicon and copper indium gallium selenide (CIGS), as well as all-perovskite tandem cells, have been reported(2-5). Meanwhile, narrow-gap non-fullerene acceptors have unlocked skyrocketing efficiencies for organic solar cells(6,7). Organic and perovskite semiconductors are an attractive combination, sharing similar processing technologies. Currently, perovskite-organic tandems show subpar efficiencies and are limited by the low open-circuit voltage (V-oc) of wide-gap perovskite cells(8) and losses introduced by the interconnect between the subcells(9,10). Here we demonstrate perovskite-organic tandem cells with an efficiency of 24.0 per cent (certified 23.1 per cent) and a high V-oc of 2.15 volts. Optimized charge extraction layers afford perovskite subcells with an outstanding combination of high V-oc and fill factor. The organic subcells provide a high external quantum efficiency in the near-infrared and, in contrast to paradigmatic concerns about limited photostability of non-fullerene cells(11), show an outstanding operational stability if excitons are predominantly generated on the non-fullerene acceptor, which is the case in our tandems. The subcells are connected by an ultrathin (approximately 1.5 nanometres) metal-like indium oxide layer with unprecedented low optical/electrical losses. This work sets a milestone for perovskite-organic tandems, which outperform the best p-i-n perovskite single junctions(12) and are on a par with perovskite-CIGS and all-perovskite multijunctions(13). Y1 - 2022 U6 - https://doi.org/10.1038/s41586-022-04455-0 SN - 0028-0836 SN - 1476-4687 VL - 604 IS - 7905 SP - 280 EP - 286 PB - Nature Research CY - Berlin ER -