Functional Third Components in Nonfullerene Acceptor-Based Ternary Organic Solar Cells

Abstract

ConspectusNonfullerene acceptors (NFAs) represent the latest breakthrough in organic solar cells (OSCs), which significantly outperform the long benchmark OSCs with fullerene derivative acceptors. In addition to binary bulk heterojunction (BHJ) OSCs, multicomponent, in particular ternary BHJ strategy has attracted tremendous interests due to the realization of broadened coverage of solar spectrum while maintaining easy processing compared to tandem OSCs. Subsequently, ternary OSCs containing NFA(s) have provided exciting opportunities to the promising field and achieved great progress in power conversion efficiencies (PCEs) over 17%. Inspired by the recent progress, in this account, we focus on the ternary OSCs containing NFA(s), share our view of the topic and aim to provide guidelines for material selections in designing successful ternary OSCs—toward the cornerstone of 20% PCE in the near future.In this Account, we first discuss the principle of OSCs—working mechanisms, including charge transfer and energy transfer that govern the charge dynamics. Moreover, the promising functional roles of the third components in NFA OSCs are highlighted at the following the sequence: (1) Absorption sensitizing—the additional component plays a key role in broadening the solar cell absorption window and thus promising in improving the photocurrent in the ternary active layer, especially when employing a near-infrared material NFA as the third component. (2) Morphology manipulation approaches and models—the ternary blend nanostructure is complex and not trivial to understand. Parallel and alloy nanomorphology models are discussed on the basis of the simultaneous consideration of the intrinsic properties among the involved materials, such as electronic properties and interplay (miscibility) among the host–guest materials. (3) Energy loss management—the addition of donor or acceptor with small energy loss, especially low bandgap material, as the third component holds the promises to reduce the energy loss in ternary OSCs. On the other hand, due to the advantage of the alloyed microstructure in precisely tuning VOC, applying a second donor with deeper HOMO level or a second acceptor with shallower LUMO level is expected to increase VOC and thus reduce the energy loss in ternary OSCs. (4) Stability boosting agent—from the thermodynamic point of view, the stable BHJ microstructure is beneficial and critical in enhancing the OSCs’ long-term thermal and light stability. However, most material combinations suffer from the poor miscibility between the donor phase and acceptor phase, leading to demixing of two phases and thus the performance degradation. On the basis of this concept, rational selection of the third component could facilitate the formation of glassy nanostructure by vitrifying the pure donor phase or acceptor phase. Also it worth noting that suppressed crystallization of the small molecular acceptors by mixing a miscible component is effective to achieve a stable morphology. At the end of the Account, we share our view on the issues that limit the performance of ternary OSCs and outline their perspectives.