Hindered Formation of Photoinactive δ-FAPbI₃ Phase and Hysteresis-Free Mixed-Cation Planar Heterojunction Perovskite Solar Cells with Enhanced Efficiency via Potassium Incorporation

Abstract

Organic-inorganic hybrid lead halide perovskite solar cells have demonstrated competitive power conversion efficiency over 22%; nevertheless, critical issues such as unsatisfactory device stability, serious current-voltage hysteresis, and formation of photo nonactive perovskite phases are obstacles for commercialization of this photovoltaics technology. Herein we report a facial yet effective method to hinder formation of photoinactive δ-FAPbI₃ and hysteresis behavior in planar heterojunction perovskite solar cells based on K _x(MA_0.17FA_0.83)_{1- x}PbI_2.5Br_0.5 (0≤ x ≤ 0.1) through incorporation of potassium ions (K⁺). X-ray diffraction patterns demonstrate formation of photoinactive δ-FAPbI₃ was almost completely suppressed after K⁺ incorporation. Density functional theory calculation shows K⁺ prefers to enter the interstitial sites of perovskite lattice, leading to chemical environmental change in the crystal structure. Ultrafast transient absorption spectroscopy has revealed that K⁺ incorporation leads to enhanced carrier lifetime by 50%, which is also confirmed by reduced trap-assisted recombination of the perovskite solar cells containing K⁺ in photovoltage decay. Ultraviolet photoelectron spectroscopy illustrates that K⁺ incorporation results in a significant rise of conduction band minimum of the perovskite material by 130 meV, leading to a more favorable energy alignment with electron transporting material. At the optimal content of 3% K⁺ (molar ratio, relative to the total monovalent cations), nearly hysteresis-free, enhanced power conversion efficiencies from 15.72% to 17.23% were obtained in this solar cell.