Molecular Diffusion-Driven Graded Polymer Hole-Transport Layers Enabling Efficient and Durable Organic Photovoltaics

High-performance and durable organic photovoltaics (OPVs) require interlayers that enable efficient carrier extraction and long-term stability while remaining compatible with scalable and printing-based processing. In this study, we demonstrate a molecular diffusion-driven graded polymer hole-transport layer (g-HTL) for efficient and thermally durable OPVs. The g-HTL is fabricated by water-transfer printing a thin PM6 layer onto a non-fullerene acceptor (NFA)-rich PM6:L8-BO active layer, followed by surface treatment with phosphomolybdic acid (PMA). The limited penetration of PMA produces a vertically graded p+–p–i configuration within the printed polymer layer, while preserving its laterally correlated surface morphology. OPVs incorporating a g-HTL achieve a power conversion efficiency of 17.18%, compared with 16.31% for conventional MoO3-based devices, owing to the improved carrier transport and collection. The printed PM6-based graded layer exhibits high morphological stability under thermal stress. Together with the thermally robust NFA-rich active layer, this morphologically stable g-HTL enables OPVs to retain nearly 90% of their initial efficiency after thermal aging at 85 °C for 15 days, compared with approximately 70% for MoO3-based devices. These results demonstrate that molecular diffusion engineering of printed polymer interlayers is an effective strategy for improving the efficiency and thermal durability of polymer-based photovoltaic devices.