Blade-Coating (100)-Oriented α-FAPbI Perovskite Films via Crystal Surface Energy Regulation for Efficient and Stable Inverted Perovskite Photovoltaics.

Photoactive black-phase formamidinium lead triiodide (α-FAPbI) perovskite has dominated the prevailing high-performance perovskite solar cells (PSCs), normally for those spin-coated, conventional n-i-p structured devices. Unfortunately, α-FAPbI has not been made full use of its advantages in inverted p-i-n structured PSCs fabricated via blade-coating techniques owing to uncontrollable crystallization kinetics and complicated phase evolution of FAPbI perovskites during film formation. Herein, a customized crystal surface energy regulation strategy has been innovatively developed by incorporating 0.5 mol % of N-aminoethylpiperazine hydroiodide (NAPI) additive into α-FAPbI crystal-derived perovskite ink, which enabled the formation of highly-oriented α-FAPbI films. We deciphered the phase transformation mechanisms and crystallization kinetics of blade-coated α-FAPbI perovskite films via combining a series of in-situ characterizations and theoretical calculations. Interestingly, the strong chemical interactions between the NAPI and inorganic Pb-I framework help to reduce the surface energy of (100) crystal plane by 42 %, retard the crystallization rate and lower the formation energy of α-FAPbI. Benefited from multifaceted advantages of promoted charge extraction and suppressed non-radiative recombination, the resultant blade-coated inverted PSCs based on (100)-oriented α-FAPbI perovskite films realized promising efficiencies up to 24.16 % (~26.5 % higher than that of the randomly-oriented counterparts), accompanied by improved operational stability. This result represented one of the best performances reported to date for FAPbI-based inverted PSCs fabricated via scalable deposition methods.