High-Performance Flexible Organic Solar Cells exceeding 12% Enabled by High-Purity Transfer-Free Graphene Electrode produced via Direct Polyimide Integration

Abstract

The performance of organic solar cells (OSCs) has been steadily increasing, surpassing 15% power conversion efficiency (PCE) with advances in photoactive and charge transport materials. While most high-efficiency OSCs are fabricated based on a rigid glass and brittle indium tin oxide (ITO) electrode, flexible devices, which represent an important platform for future electronics, are also being actively developed. Nonetheless, the device performance of flexible OSCs still falls behind that of their rigid counterparts, and thus the flexibility of organic materials, an important advantage of OSCs, is not fully utilized. Furthermore, commonly used plastic substrates, polyethylene terephthalate and polyethylene naphthalate, are vulnerable to high-temperature annealing processes due to their low glass transition temperatures. Herein, we introduce a highly flexible and durable transparent electrode with thermal stability achieved through direct integration of polyimide (PI) possessing dual functionalities (-CF3 and -SO2-) on graphene. The PI-assisted graphene electrode is completely free of polymer residues and exhibits an ultra-clean surface, unlike conventional polymer-assisted transferred graphene, together with a desirable optical transmittance exceeding 92%, a sheet resistance of 83 Ω/sq, and high thermal stability. Moreover, direct integration of PI on graphene sheet improved the durability of the graphene electrode by inhibiting delamination of the graphene film under mechanical stress. Using the PI-assisted graphene as a transparent electrode, a high-efficiency flexible OSC with a PCE of 12.2% was obtained with outstanding mechanical robustness. The proposed highly functional PI-assisted graphene electrode is potentially promising for use in various next-generation optoelectronic devices requiring high efficiency and flexibility.