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-FAPbI and hysteresis behavior in planar heterojunction perovskite solar cells based on K (MAFA)PbIBr (0≤ x ≤ 0.1) through incorporation of potassium ions (K). X-ray diffraction patterns demonstrate formation of photoinactive δ-FAPbI 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.

Potassium Doping to Enhance Green Photoemission of Light‐Emitting Diodes Based on CsPbBr3 Perovskite Nanocrystals

“…In addition, there is an ongoing debate regarding the position of K + in the perovskite crystal and its beneficial mechanism in scientific community. Our previous research showed that K + energetically prefers to stay in the interstitial sites of MA 0.17 FA 0.83 PbI 2.5 Br 0.5 perovskite lattice proved by density functional theory (DFT) calculation, [ 13 ] which is consistent with other reports by Park and co‐workers and Jung and co‐workers on different composition of perovskite. [ 21,22 ] Park and co‐workers, on the other hand, claimed that K dopant can replace the A‐site cation in the CsPbI 2 Br perovskite to improve the phase stability.…”

“…This is consistent with previous report where theoretical simulation shows interstitial doping is the most favorable site energetically. [ 13,21 ] It is also noted that the change of unit cell volume is not significant for different K + doping concentration and the maximum volume was obtained with 2.5% K + . This indicates that the limit for potassium doping is around 2.5%.…”

“…In particular, several researches have shown that incorporation of potassium to perovskite can significantly tailor material properties. K + incorporation was reported to help eliminate hysteresis of perovskite solar cells through suppression of ion migration in bulk perovskite such as methylammonium lead iodide (MAPbI 3 ), formamidinium lead iodide (FAPbI 3 ), or mixed cation perovskite materials, [ 11,12 ] and/or suppression of formation of secondary phases such as photoinactive δ‐phase FAPbI 3 in mixed cation perovskite [ 13 ] or into CsPbI 3 solar cell to enhance phase stability. [ 14 ] In contrast to the use of potassium in perovskite solar cells, the study on effect of K + on photoemission of PeNCs for optoelectronic application is much limited.…”

Tuning the Amount of Oxygen Vacancies in Sputter‐Deposited SnOx films for Enhancing the Performance of Perovskite Solar Cells

“…As observed in Table , the capacitance of the device containing SnO x deposited at 250 °C was much lower than that of the device containing SnO x deposited at RT, indicating lower charge accumulation at the ETL/perovskite interface and lower hysteresis for the PSCs. Furthermore, according to the combined results of UPS and UV/Vis spectroscopy, the PSC prepared with the SnO x ETL deposited at 250 °C had a better conduction band alignment with the adjacent perovskite film, which resulted in better flow of charge compare to the SnO x ETL deposited at RT . Furthermore, as shown in the EIS results (Figure ) the recombination resistance increased two‐fold for the film deposited at 250 °C.…”

Triggering the Passivation Effect of Potassium Doping in Mixed‐Cation Mixed‐Halide Perovskite by Light Illumination

“…[9] Further doping of triple cation CsFAMA perovskite by oxidation-stable rubidium cation (Rb + ) leads to low-voltage-loss PSCs with efficiencies of up to 21.6% and long-term device stability at elevated temperature. [13,14] The incorporation of K + effectively increases the grain size and reduces the interfacial defect density in the perovskite layer, leading to hysteresis-free, stable, and high PCE (20.56%) quadruple-cation PSCs. [13,14] The incorporation of K + effectively increases the grain size and reduces the interfacial defect density in the perovskite layer, leading to hysteresis-free, stable, and high PCE (20.56%) quadruple-cation PSCs.…”