Enhancing long-term morphological stability in BHJ organic solar cells through thermocleavable sidechains under continuous thermal stress.

Organic solar cells (OSCs) have recently achieved power conversion efficiencies (PCEs) exceeding 20%; however, their long-term operational stability remains a significant barrier to commercialization. A key factor in ensuring sustained device performance is the morphological stability of the bulk heterojunction (BHJ) layer, which is closely linked to the mobilities of the donor and acceptor materials. In this study, we introduce a strategy to enhance morphological and thermal stability by incorporating thermocleavable side chains into both the donor and acceptor components of the active layer. Specifically, we employed a thermocleavable side-chain (TCS) containing conjugated polymer donor, PffBT-4T-TCS, and a similarly functionalized fullerene acceptor, C60-TCS. Upon complete thermal cleavage of the alkyl side chains, the backbone glass transition temperature of PffBT-4T-TCS increased by 60 °C. To assess the morphological stability of the thermocleaved BHJ, we utilized atomic force microscopy coupled with infrared spectroscopy (AFM-IR) to analyze films before and after thermal treatment. Our results demonstrate that the BHJ morphology remains remarkably stable after cleavage, maintaining consistent donor/acceptor composition and surface roughness for over 10 weeks at 85 °C thermal stress conditions. Normalized PCE measurements further confirm that this thermocleavable side-chain strategy offers a promising route to achieve both high efficiency and long-term stability in OSC devices.