Low non-radiative recombination loss in organic solar cells with 5,6-Difluorobenzo[c][1,2,5]thiadiazole based donor polymers

Abstract

Non-radiative recombination loss (ΔE_nr) constitutes the primary component of energy loss (E_loss) in organic solar cells (OSCs). Reducing E_loss is critical for enhancing OSC performance, yet developing simple, cost-effective polymer donors capable of achieving high-efficiency OSCs with minimal E_loss remains a significant challenge. In this study, four 5,6-difluorobenzo[c][1,2,5]thiadiazole (ffBT)-based copolymer donors are successfully synthesized via straightforward reactions. By modulating the molar feed ratio of ffBT units during polymerization, the solubility and crystallinity of the polymers are optimized. When the molar feed ratio is set to 25 %, the terpolymer D2 achieves an optimal balance between solubility and crystallinity, leading to a better film morphology in the active layer. Consequently, the D2:Y6-based devices demonstrate efficient charge transfer, effective charge generation, and suppressed charge recombination, yielding a highest short-circuit current density of 23.03 mA cm⁻² and power conversion efficiency of 12.79 % among four OSCs. Remarkably, devices fabricated with these donors and the Y6 acceptor all exhibit a low ΔE_nr of approximately 0.190 eV. This work highlights that combining ffBT with terpolymerization offers a promising strategy for designing simple yet high-performance polymer donors, while providing critical insights into minimizing ΔE_nr in OSCs.