In‐Depth Investigation of Morphology Evolution in Highly Efficient Pseudo‐Planar Heterojunction All‐polymer Organic Photovoltaics

Abstract Pseudo‐planar heterojunction (PPHJ) structure using sequential deposition is an effective method for achieving high‐efficiency organic photovoltaics (OPVs). The crystallization and phase separation behavior during film‐forming process is intricately related to morphology with PPHJ structure. Despite this, the relationship between these two factors remains ambiguous, thereby impeding further improvements in performance. To tackle this challenge, PM6 and PY‐DT all‐polymer systems as models are utilized, and combine theoretical calculations, in situ spectroscopy, and morphological characterization to elucidate the aforementioned relationship. Thermodynamically, PPHJ film‐forming process can effectively suppress van der Waals forces, increase the crystallinity of PM6, and promote matching crystallinity. Kinetically, the crystallization rate is notably faster and more sustained, facilitating the development of an interpenetrating network structure with enhanced crystallinity. Furthermore, PPHJ film‐forming process is driven by crystallization‐induced‐phase separation, characterized by minimal intermixed phase content and substantial domain size. Concurrently, PM6 layer swells to a limited extent, impeding the downward diffusion of PY‐DT, which promotes the formation of a vertical phase separation structure. Ultimately, the favorable morphology is achieved by optimizing PPHJ film‐forming process, resulting in the power conversion efficiency (PCE) of 18.08%, which is the highest PCEs reported for binary all‐polymer OPVs based on PPHJ structure to date.