Origin of Temperature-Driven Morphology Changes in High-Performance Bulk Heterojunction Active Layer of Organic Solar Cell Device

The performance of conjugated polymer-based organic solar cell (OSC) device relies on the bulk heterojunction morphology of the electron donor and acceptor blend. The morphology including the average domain size, crystallinity, and phase purity of donor/acceptor blend determines the device performance. Although the power conversion efficiency (PCE) of OSC is above 19%, the origin of the instable performance over longer times remains poorly understood. In this work, we conducted multiple characterization techniques to explore the dynamic, temperature-dependent morphology of a state-of-the-art donor polymer (PM6) blended with a non-fullerene small-molecule acceptor (Y6). Particularly, we focused on the thermal analysis of the donor and acceptor using fast scanning calorimetry (Flash DSC) to understand the crystallization kinetics. Combined with the assistance of atomic-force microscopy paired with infrared microscopy (AFM-IR) and X-ray scattering, we concluded the origin of PCE loss can be attributed to the severe phase separation caused by acceptor diffusional crystallization. The pure Y6 showed growing crystallization as annealing temperature increased. And the isothermal crystallization growth of blend was further accelerated due to diffusions of Y6. As the blend is maintained at elevated temperatures, the formation of Y6 crystalline domains induced phase separation with donors, as evidenced by the domain purities via AFM-IR characterizations. The influences of small molecules crystallization were characterized by the performance changes for solar cell device. This study will contribute to the development of more reliable, efficient and commercially viable OSC in the future.