Role of the energy offset in the charge photogeneration and voltage loss of nonfullerene acceptor-based organic solar cells

The trade-off between short-circuit current density (JSC) and open-circuit voltage (VOC) has been one of the largest challenges in improving the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Although the energy offset between the excited and charge transfer (CT) states should remain minimal to achieve a high VOC, a very small energy offset typically leads to degradation of JSC, even when novel nonfullerene acceptors (NFAs), such as Y6, are used. Therefore, understanding the limit to what extent the energy offset can be minimized and the physics underlying the trade-off relationship is important to optimize the design of new materials and further improve the PCEs. This study provides a threshold energy that can ensure high charge photogeneration quantum efficiencies for Y-series NFA-based OSCs and discusses the role of the energy offset in device performances. We found that an insufficient energy offset led to not only slow hole transfer at the donor:acceptor interfaces, but also inefficient long-range spatial dissociation of the CT states and degradation of the fill factor (FF). This study also discusses the interplay of the energy levels of the two NFAs that constitute ternary blend OSCs. We found that, by introducing a low-efficiency NFA into a high-efficiency donor:acceptor blend, the voltage loss can be reduced while maintaining a high charge photogeneration quantum efficiency. Our findings highlight the importance of overcoming the trade-off between FF and VOC for further improving the PCE

Delocalization suppresses nonradiative charge recombination in polymer solar cells

Suppressing nonradiative deactivation of charge transfer (CT) states is pivotal to realizing further improvements in the power conversion efficiencies of polymer solar cells (PSCs). According to the energy gap law, the nonradiative decay rate constant knr scales exponentially with decreasing CT state energy ECT; thereby, as long as knr is governed by the energy gap law, a decrease in ECT will inevitably increase nonradiative deactivation of CT states and hence decrease the power conversion efficiency. Here, we report the nonradiative decay dynamics of CT states generated in various nonfullerene-acceptor-based PSCs by using transient absorption spectroscopy. The absence of a strong correlation between knr and ECT indicates that the energy gap law is not valid for these PSCs and that parameters other than ECT contribute significantly to knr. We found that knr decreased with an increase in materials’ crystallinities, indicating that increasing crystallinity leads to CT state delocalization, which in turn mitigates the nonradiative deactivation of CT states

Role of the energy offset on the charge photogeneration and voltage loss of nonfullerene acceptor-based organic solar cells

The trade-off between the short-circuit current density (JSC) and open-circuit voltage (VOC) has been one of the largest challenges in improving the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Although energy offset between the optical bandgap and the charge transfer (CT) states should be maintained as small as possible to improve VOC, a very small energy offset typically leads to degradation of JSC, even when novel nonfullerene acceptors (NFAs), such as Y6, are used. Therefore, understanding the limit to what extent the energy offset can be minimized and the underlying physics behind the trade-off relationship is important to optimize the design of new materials and further improve the PCEs. This study provides a threshold energy that can ensure high charge photogeneration quantum efficiencies for Y-series NFA-based OSCs and discusses the role of the energy offset in device performances. We found that an insufficient energy offset led to not only slow hole transfer at the donor:acceptor interfaces, but also inefficient long-range spatial dissociation of the CT states and degradation of the fill factor (FF). This study also discusses the interplay of the energy levels of two NFAs that constitute ternary blend OSCs. We found that combining two NFAs enables to reduce the voltage loss, while maintaining a high charge photogeneration quantum efficiency. Our findings highlight the importance of overcoming the trade-off between FF and VOC for further improving the PCE

Cascaded energy landscape as a key driver for slow yet efficient charge separation with small energy offset in organic solar cells

Recent studies have shown that efficient free carrier (FC) generation with a small voltage loss can be achieved in organic solar cells (OSCs); however, the photophysical insights underpinning this remain unclear. Herein, we examined the mechanisms underlying the FC generation in a state-of-the-art OSC consisting of PM6 and Y6 as an electron donor and electron acceptor, respectively, wherein the energy offset between the lowest excited singlet state and the charge transfer state is as small as ~0.1 eV. We used transient absorption spectroscopy to track the time evolution of electroabsorption caused by electron–hole pairs generated at donor/acceptor interfaces. Upon photoexcitation of the lower-bandgap Y6, we observed slow yet efficient spatial charge dissociation on a time scale of picoseconds. Based on temperature dependence measurements, we found that this slow yet efficient FC generation is driven by downhill energy relaxation of charges through the energy cascade generated near the interfaces

Cascaded energy landscape as a key driver for slow yet efficient charge separation with small energy offset in organic solar cells

【研究成果】プラスチック太陽電池の発電機構を解明 --オフセットが小さくても光電変換効率が高い理由を分析--. 京都大学プレスリリース. 2022-02-24.Recent studies have shown that efficient free carrier (FC) generation with a small voltage loss can be achieved in organic solar cells (OSCs); however, the photophysical insights underpinning this remain unclear. Herein, we examined the mechanisms underlying the FC generation in a state-of-the-art OSC consisting of PM6 and Y6 as electron donor and acceptor, respectively, wherein the energy offset between the lowest excited singlet state and the charge transfer state is as small as ∼0.12 eV. We used transient absorption spectroscopy to track the time evolution of electroabsorption caused by electron–hole pairs generated at donor/acceptor interfaces. After hole transfer from Y6 to PM6, we observed slow yet efficient spatial charge dissociation on a time scale of picoseconds. Based on temperature-dependence measurements, we found that this slow yet efficient FC generation is driven by downhill energy relaxation of charges through the energy cascade generated near the interfaces. We provide here direct experimental evidence for the FC generation mechanism in the very topical PM6/Y6 blend system

Singlet and Triplet Excited-State Dynamics of a Nonfullerene Electron Acceptor Y6

Understanding the excited-state dynamics of nonfullerene electron acceptors is essential for further improvement of organic solar cells as they are responsible for near-IR light absorption. Herein, we investigated the singlet and triplet excited-state dynamics in Y6, a novel nonfullerene acceptor, using transient absorption spectroscopy. We found that, even at low excitation fluences, pristine Y6 films show biphasic singlet exciton decay kinetics with decay constants of ∼220 ps and ∼1200 ps. The majority of the Y6 singlet excitons decayed with the faster (∼220 ps) component, whereas a clear photoluminescence with the slower (∼1200 ps) component was observed, which is the origin of the large discrepancies in the previously reported exciton lifetimes of Y6 in the solid state. At high excitation fluences, on the other hand, Y6 singlet excitons are more likely to decay via singlet–singlet exciton annihilation due to fast exciton diffusion in crystalline domains, after which we observed ultrafast triplet formation, assigned to singlet fission from higher excited singlet states

Singlet and triplet excited-state dynamics of a nonfullerene electron acceptor Y6

Understanding the excited-state dynamics of nonfullerene electron acceptors is essential for further improvement of organic solar cells. Herein, we investigated the singlet and triplet excited-state dynamics in Y6, a novel nonfullerene acceptor, using transient absorption spectroscopy. We found that pristine Y6 films show biphasic singlet exciton decay kinetics with decay constants of ~220 ps and ~1200 ps, which is the origin of the large discrepancies in the previously reported exciton lifetimes in the solid state. The majority of the Y6 singlet excitons decayed with the faster (~220 ps) component, whereas a clear photoluminescence with the slower (~1200 ps) component was observed. Y6 singlet excitons undergo fast diffusion in the crystalline domains, resulting in fast singlet–singlet exciton annihilation, after which ultrafast triplet formation, assigned to singlet fission from higher excited singlet states, is observed