Vitamin A as a simple, dual-role agent for the band bending-induced passivation of perovskite solar cells

Abstract

Vitamin A (vit A), in the form of retinyl acetate, was used as a passivation material owing to its interesting structural properties: the polar acetate group of vitamin A serves as a linking functional group on perovskite surfaces inducing band-bending, and the non-polar alkyl chain with a π-conjugation system facilitates charge extraction. In this study, solutions of retinyl acetate dissolved in chlorobenzene with varying concentrations were spin-coated on an annealed perovskite film. An improvement of power conversion efficiency (PCE) from 16.3% (control) to 17.6% was observed on the device passivated with a vit A concentration of 2 mg mL⁻¹. The improvement in the overall performance can be attributed to the increase in the fill factor. X-ray photoelectron spectroscopy (XPS) confirmed the interaction between the perovskite surface and vitamin A molecules. Further XPS depth profile analysis revealed a Fermi level shift on the passivated surface relative to the bulk, exhibiting a stronger p-type behavior on the surface compared to the control sample. This resulted in a band bending, driving a more efficient hole-extraction from the perovskite absorber to the spiro-OMeTAD hole transport layer and reducing trap-assisted non-radiative recombination, as supported by time-resolved photoluminescence spectroscopy and ideality factor estimation. Finally, better device stability was observed, with 2 mg mL⁻¹ vit A passivated devices maintaining more than 90% of their initial PCE, compared to less than 60% for control devices, for a span of around 800 hours.