Suppressing Molecular Aggregation Enables Efficient and Thermally Stable Ternary Bilayer Organic Solar Cells

Bilayer organic solar cells (OSCs) have been actively investigated due to more ideal photoactive layer structures and possibly better device stability compared to those of bulk‐heterojunction OSCs. Here, we introduce a binary non‐fullerene acceptor layer composed of IDIC and IT‐4F into bilayer OSCs and investigate its effects on device performance and thermal stability. We found that IT‐4F suppresses IDIC aggregation, which provides ideal interface morphologies between acceptor and electron transport layers, resulting in improved power conversion efficiency. In addition, due to suppressed IDIC aggregation, PM6/IDIC:IT‐4F films showed robust thermal durability upon various pre‐annealing test conditions. Moreover, PM6/IDIC:IT‐4F bilayer OSCs exhibited improved stability compared to that of PM6/IDIC devices under continuous thermal annealing conditions. Whereas all photovoltaic parameters deteriorated upon thermal annealing in PM6/IDIC devices, only open‐circuit voltages were reduced in PM6/IDIC:IT‐4F devices, possibly due to a minor aggregation of IDIC. Nevertheless, binary acceptor layer reduced non‐radiative voltage losses in bilayer OSCs, suggesting that there is more room to achieve better device stability by proper material selection. This work demonstrates that introducing a binary acceptor layer into bilayer OSCs can be an effective strategy to achieve both high efficiency and improved stability, especially when the given NFA tends to aggregate strongly.