Haoyu Zhao1,Nathaniel Prine1,Guorong Ma1,Andrew Bates1,Xiaodan Gu1
The University of Southern Mississippi1
Haoyu Zhao1,Nathaniel Prine1,Guorong Ma1,Andrew Bates1,Xiaodan Gu1
The University of Southern Mississippi1
The performance of conjugated polymer-based organic photovoltaic 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) is above 18%, 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 crystallization dominates the process at 110 °C. And the isothermal crystallization of donor further accelerated for Y6 in the blend. As the blend is maintained at elevated temperatures, the crystallization of the Y6 domains was observed to induce phase separation with donors, as evidenced by the AFM-IR characterizations. Finally, we systematically examined the impacts of operational conditions (temperature and time) on the blend film morphology, which could suggest a pathway to suppress the crystallization to improve the device stability in future.