Scaffold-Guided Crystallization of Oriented α-FAPbI₃ Nanowire Arrays for Solar Cells

Perovskite nanowire arrays with large surface areas for efficient charge transfer and continuous highly crystalline domains for efficient charge transport exhibit ideal morphologies for solar-cell active layers. Here, we introduce a room temperature two-step method to grow dense, vertical nanowire arrays of formamidinium lead iodide (FAPbI₃). PbI₂ nanocrystals embedded in the cylindrical nanopores of anodized titanium dioxide scaffolds were converted to FAPbI₃ by immersion in a FAI solution for a period of 0.5-30 min. During immersion, FAPbI₃ crystals grew vertically from the scaffold surface as nanowires with diameters and densities determined by the underlying scaffold. The presence of butylammonium cations during nanowire growth stabilized the active α polymorph of FAPbI₃, precluding the need for a thermal annealing step. Solar cells comprising α-FAPbI₃ nanowire arrays exhibited maximum solar conversion efficiencies of >14%. Short-circuit current densities of 22-23 mA cm^-2 were achieved, on par with those recorded for the best-performing FAPbI₃ solar cells reported to date. Such large photocurrents are attributed to the single-crystalline, low-defect nature of the nanowires and increased interfacial area for photogenerated charge transfer compared with thin films.