Surface Functionalization of a Graphene Cathode to Facilitate ALD Growth of an Electron Transport Layer and Realize High-Performance Flexible Perovskite Solar Cells

Graphene is a promising electrode for the flexible perovskite solar cells (PSCs), whereas electron transport layer (ETL) oxides, prepared by atomic layer deposition (ALD), render high performance in PSCs. However, the growth of ALD oxides on surface-inert graphene remains a challenging task. Herein, we demonstrate that a viscous precursor of molecular layer deposition (MLD) ethylene glycol (EG) endows active sites to the graphene surface by the strong van der Waals forces and realizes the ALD of high-performance oxides for next-generation flexible PSCs. The EG-functionalized graphene surface is utilized to deposit an ALD-ZnO film, which renders high transmittance, low-energy levels, and superior electron injection capability. Moreover, the power conversion efficiency (PCE) of EG-graphene/ZnO-based PSCs is 133.9% higher than graphene/ZnO-based PSCs, and they retained 92.2% of the initial PCE after 1000 bending cycles. This method opens up alternate avenues to improve the flexible PSCs and other flexible optical electronics.