（State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China）
The bulk heterojunction (BHJ) morphology of organic solar cells (OSCs) is critically important for achieving high device performance. While the microstructure optimization in a single casting process is extremely complicated due to the coupling of crystallization and phase separation of the photoactive materials. By contrast, the sequential blade coated P-i-N structure of the donor and acceptor materials can lead to a more favorable morphology and nanostructure, which is an efficient method to circumvent the existing issue in single-solvent BHJ device fabrication. However, the underlying mechanism of morphology optimization in sequential blade coating process and how it impacts the device performance are still mysterious, which limit the application of this method in the manufacture of high-efficiency large-area devices. Herein, the inverted P-i-N device based on PM6/Y6 system was fabricated in ambient environment and the mechanism of improved device performance based on morphology optimization was studied in detail. Compared to the single-solvent method, sequential blade coating strategy successfully controlled the crystallization and phase separation of active layer, and the gradient distribution of donor-rich phase/mixing phase/acceptor-rich phase from anode to cathode was formed. More importantly, this strategy eliminated the interaction between donor and acceptor molecules, thus effectively enhanced the crystallinity of donor and acceptor. Based on the simultaneous optimization of vertical phase separation and crystallinity, more balanced hole/electron mobility and reduced carrier recombination were achieved, leading to the highest power conversion efficiency (PCE) of 15.84% in P-i-N device (15.16% for the BHJ control device). It is worth noting that by further optimizing the large-area sequential blade coating process, the morphologic defects of large-area active layer were effectively reduced, resulting in higher PCE of large-area P-i-N device. This indicates the sequential blade coating strategy provides an important guideline for the optimization of large-area fabrication in OSCs.