Overcoming the carrier transport limitation in Ruddlesden–Popper perovskite films by using lamellar nickel oxide substrates

Abstract

Quasi two-dimensional (2D) Ruddlesden–Popper perovskites films fabricated by a solution process are composed of multiple 2D perovskite phases, and carrier transportation is limited by low-n-value 2D perovskite phases (n = 1 and n = 2). Systematic characterization shows that BA based 2D perovskites do not form low-n-value phases, while PEA and FPEA based 2D perovskites have strong n = 1, 2 signals when excited from the bottom of the perovskite films. To solve this problem, a lamellar nickel oxide substrate (lamellar-NiO_x) fabricated by the hydrothermal method is employed as a hole transporting layer, forming a vertical charge transport pathway at the bottom of perovskite films, thus bypassing the charge trapping and recombination centers of the low-n-value regions of PEA and FPEA based 2D perovskites. As a result, both fill factor and open-circuit voltage are greatly enhanced for PEA and FPEA based 2D perovskite solar cells and a champion power conversion efficiency of 15.2% is achieved with a composition of FPEA₂MA₃Pb₄I₁₃. Furthermore, 2D perovskite films on lamellar-NiO_x show enlarged grain size, improved out-of-plane orientation, and enhanced carrier transport efficiency between different n-value 2D perovskite phases. These findings suggest that lamellar-NiO_x is an ideal substrate candidate to help overcome the carrier transport limitation of low-n-value 2D perovskite phases near the bottom of perovskite films.