Hindered Formation of Photoinactive δ-FAPbI3 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 δ-FAPbI 3 and hysteresis behavior in planar heterojunction perovskite solar cells based on K x (MA 0.17 FA 0.83 ) 1– x PbI 2.5 Br 0.5 (0≤ x ≤ 0.1) through incorporation of potassium ions (K + ). X-ray diffraction patterns demonstrate formation of photoinactive δ-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.