Crack‐Free Monolayer Graphene Interlayer for Improving Perovskite Crystallinity and Energy Level Alignment in Efficient Inverted Perovskite Solar Cells

Inverted planar perovskite solar cells (PSCs) have intrigued great promise in negative hysteresis, simple fabrication process, and flexible substrate implementation, in which the shared hole transport materiel is NiO x . However, the low‐temperature solution processing of the NiO x film is usually accompanied by defect formation, which deteriorates the perovskite quality and device performance. Meanwhile, the energy‐level offset between the NiO x and perovskite films is relatively large, limiting the interfacial charge transport. To suppress those setbacks, a defect‐free monolayer graphene sheet is transferred onto the NiO x film surface as a template for van der Waals epitaxial growth of perovskite films for the first time, leading to enhancing crystallinity with a large grain size of perovskite layer, 0.20 eV energy level offset drop, and accelerating charge transfer for the devices. Finally, the power conversion efficiency of 19.21% without hysteresis is achieved, exceeding 18.35% of the control device.