Stabilizing Donor/Acceptor Interfaces with Ordered Polymer Layers in Planar‐Heterojunction Organic Solar Cells Under Thermal Stress

Abstract As the efficiency of single‐junction organic solar cells (OSCs) is about to break 20%, more research effort is needed to achieve long‐term device stability for commercialization. The complex active layer microstructure of bulk‐heterojunctions challenges the degradation mechanism study, especially for the morphological changes under thermal stress. In this work, this issue can be overcome by employing planar‐heterojunction (PHJ) OSCs with well‐defined bi‐layered structures, which enables effective control of the microstructure of polymer donor layers. The evolution of photovoltaic parameters of the PHJ OSCs under thermal stress is specifically revealed, which is strongly related to the inter diffusion of small molecular acceptors into the amorphous phases of the polymer donors. Increasing the crystallinity of polymer donors can effectively stabilize the donor/acceptor interfaces and the photovoltaic performances. The work puts forward effective strategies to improve the morphological stability of OSCs and alternative approaches for mechanistic studies on the thermal stability of OSCs.