Vertically Oriented Perovskites with Minimized Intrinsic Boundaries for Efficient Photovoltaics.

Intrinsic boundaries formed by grain stacks of randomly oriented perovskite crystallites seriously restrict charge transport in the resultant photovoltaic devices, whereas direct passivation of these defects remains unexplored, and it is desirable to modulate perovskite growth with uniform orientation. Herein, we report a simple but effective method to regulate perovskite crystallization by employing a volatile and polymerizable monomer of hydroxyethyl methacrylate (HEMA), which can simultaneously interact with both FA⁺ and Pb²⁺ via hydrogen and coordination bonding, respectively, to seed perovskite crystallization with accelerated nucleation and retarded crystal growth. Upon thermal annealing, the gradual volatilization and partial self-condensation of the HEMA drive the perovskite growth perpendicularly to the substrate, leading to largely suppressed defect states, improved crystallinity, and a reduced Young's modulus of the perovskite film. As a result, champion efficiencies exceeding 24 and 22% with improved operational and mechanical stability of the optimized perovskite solar cells based on rigid and flexible substrates were finally achieved.