A Universal Strategy for Quasiplanar Heterojunction Architecture Using Polymer Buffer Layer Enables 19.8% Efficiency and Prolonged Stability

Abstract The sequential spin‐coating of donor and acceptor using orthogonal solvents to construct quasiplanar heterojunction (Q‐PHJ) devices has been proven to be an effective strategy to achieve bilayer‐dominated device architecture and optimize the device lifetime. Herein, polyacrylonitrile (PAN) is introduced with a specific solvent as a buffer layer, spin‐coated between the donor and acceptor layers, which can avoid the requirement of orthogonal solvents for donor and accepter. The incorporation of this buffer layer yields a more distinct bilayer structure, resulting in an ideal Q‐PHJ morphology and a high power conversion efficiency (PCE) of 19.83% using PM6 and BTP‐eC9 as active materials. More importantly, the T 80 lifetime of the devices under heating at 65 °C is extended from 480 to 1306 h, and under continuous light‐soaking conditions, from 1207 to 2025 h. Additionally, this strategy demonstrates strong universality across multiple donor–acceptor systems and solvent combinations. This work highlights the potential of using PAN with tailored solubility as an effective buffer layer to realize ideal Q‐PHJ device architecture without orthogonal solvents distinct in processing, which opens a new window for the development of Q‐PHJ technology with long‐term stability in organic solar cells.