Enhanced device performance through optimization of acceptor layer thickness relative to exciton diffusion length and ionization energy offset in bilayer organic solar cells

Abstract

The recent advancements in power conversion efficiency (PCE) of bilayer organic solar cells (b-OSCs) utilizing non-fullerene acceptors (NFAs) can be attributed to the synergistic effects of long-range exciton diffusion and enhanced charge generation. However, a critical analysis is essential for optimizing the NFA layer thickness and selecting the appropriate donor polymer to maximize performance metrics. In this study, we demonstrate that the simultaneous optimization of NFA layer thickness relative to exciton diffusion length and ionization energy (IE) offset is crucial for enhancing the performance of bilayer OSCs. The NFA acceptor used, COi8DFIC, exhibits one of the longest exciton diffusion lengths reported, 40 nm. Systematic device optimization using COi8DFIC as the NFA, along with a series of polymer donors (PTB7-Th, PBDB-T, and PBDB-T-2F), reveals that the highest PCE is achieved with an acceptor layer thickness of approximately 40 nm, which matches the exciton diffusion length in COi8DFIC. In conjunction with the tailored acceptor layer thickness, the optimal IE offset leads to maximum PCE in PTB7-Th/COi8DFIC devices. This dual optimization approach facilitates rapid charge carrier extraction (≈690 ns) in PTB7-Th/COi8DFIC devices, significantly enhancing both the short-circuit current and the fill factor. Furthermore, the high charge carrier density and comparably low bimolecular recombination loss both contribute to the overall device performance enhancement. This study presents a rational framework for designing NFA-based bilayer OSCs that promote enhanced charge generation and extraction.