High‐Quality van der Waals Epitaxial CsPbBr3 Film Grown on Monolayer Graphene Covered TiO2 for High‐Performance Solar Cells.

Two‐dimensional materials have been widely used to tune the growth and energy‐level alignment of perovskites. However, their incomplete passivation and chaotic usage amounts are not conducive to the preparation of high‐quality perovskite films. Herein, we succeeded in obtaining higher‐quality CsPbBr3 films by introducing large‐area monolayer graphene as a stable physical overlay on top of TiO2 substrates. Benefiting from the inert and atomic smooth graphene surface, the CsPbBr3 film grown on top by the van der Waal epitaxy has higher crystallinity, improved (100) orientation, and an average domain size of up to 1.22 μm. Meanwhile, a strong downward band bending is observed at the graphene/perovskite interface, improving the electron extraction to the electron transport layers (ETL). As a result, perovskite film grown on graphene has lower photoluminescence (PL) intensity, shorter carrier lifetime, and fewer defects. Finally, a photovoltaic device based on epitaxy CsPbBr3 film is fabricated, exhibiting power conversion efficiency (PCE) of up to 10.64% and stability over 2000 h in the air. [ABSTRACT FROM AUTHOR]