Hindered Formation of Photoinactive delta-FAPbl(3) Phase and Hysteresis Free Mixed-Cation Planar Heterojunction Perovskite Solar Cells with Enhanced Efficiency via Potassium Incorporation

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 delta-FAPbI(3) and hysteresis behavior in planar heterojunction perovskite solar cells based on K-x(MA(0.17)FA(0.83))(1-x)PbI2.5Br0.5 (0 <= x <= 0.1) through incorporation of potassium ions (K+). X-ray diffraction patterns demonstrate formation of photoinactive delta-FAPbI(3) 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.