Interstitial Occupancy by Extrinsic Alkali Cations in Perovskites and Its Impact on Ion Migration

Abstract

Recent success in achieving highly stable Rb-containing organolead halide perovskites has indicated the possibility of incorporating small monovalent cations, which cannot fit in the lead-halide cage with an appropriate tolerance factor, into the perovskite lattice while maintaining a pure stable "black" phase. In this study, through a combined experimental and theoretical investigation by density functional theory (DFT) calculations on the incorporation of extrinsic alkali cations (Rb , K , Na , and Li ) in perovskite materials, the size-dependent interstitial occupancy of these cations in the perovskite lattice is unambiguously revealed. Interestingly, DFT calculations predict the increased ion migration barriers in the lattice after the interstitial occupancy. To verify this prediction, ion migration behavior is characterized through hysteresis analysis of solar cells, electrical poling, temperature-dependent conductivity, and time-dependent photoluminescence measurements. The results collectively point to the suppression of ion migration after lattice interstitial occupancy by extrinsic alkali cations. The findings of this study provide new material design principles to manipulate the structural and ionic properties of multication perovskite materials.

Alkali Chlorides for the Suppression of the Interfacial Recombination in Inverted Planar Perovskite Solar Cells

“…Since the ion migration is affected by the presence of native defects, it is expected that the films prepared on alkali halide modified surfaces would exhibit lower rates of ion migration. It should also be pointed out that the presence of interstitial alkali cations in perovskites can suppress the ion migration . Although Na and K are mainly located within NiOx and at the NiOx perovskite interface, there is a low presence of Na or K in the perovskite films and therefore the possible effect of Na i (K i ) cannot be entirely excluded.…”

Efficient Passivation with Lead Pyridine‐2‐Carboxylic for High‐Performance and Stable Perovskite Solar Cells

“…The high n value indicates the severe monomolecular recombination in the control and incorporating PbPyA 2 benefits for decreasing traps density and suppressing charge recombination, corresponding to the PL measurements. [48] A V oc can be detected by illuminated the poled devices and the magnitude of V oc is essentially determined by the amount of polarized ions toward two sides. Based on the Nernst equation, ions migration with low E A can easily occur when exposed to the thermal or operational condition and lead to the rapid degradation of PSCs.…”

Detrimental Effect of Unreacted PbI2 on the Long‐Term Stability of Perovskite Solar Cells

“…The certified power conversion efficiency (PCE) of PSCs has quickly climbed to 25.2% [ 1 ] since the first discovery of PSCs in 2009 by Miyasaka et al [ 2 ] To reach this high efficiency, “mixed” perovskites with different anions (I − and Br − ) and cations [methylammonium (MA + ), formamidinium (FA + ), and alkali metals, such as Cs + , K + , and Rb + ] have been employed in PSCs. [ 3–12 ] Besides high PCE, high operational stability is important for commercialization. [ 13 ] Among reported mixed perovskites, cesium containing triple cation mixed perovskites (CsFAMA) with the composition of Cs 0.05 (FA 1− x MA x ) 0.95 Pb(I 1− y Br y ) 3 is known to have excellent solar power conversion and long‐term stability.…”