Photoinduced Halide Segregation in Ruddlesden–Popper 2D Mixed Halide Perovskite Films

Abstract

Abstract2D lead halide perovskites, which exhibit bandgap tunability and increased chemical stability, have been found to be useful for designing optoelectronic devices. Reducing dimensionality with decreasing number of layers (n = 10–1) also imparts resistance to light‐induced ion migration as seen from the halide ion segregation and dark recovery in mixed halide (Br:I = 50:50) perovskite films. The light‐induced halide ion segregation efficiency, as determined from difference absorbance spectra, decreases from 20% to <1% as the dimensionality is decreased for 2D perovskite film from n = 10 to 1. The segregation rate constant (ksegregation), which decreases from 5.9 × 10−3 s−1 (n = 10) to 3.6 × 10−4 s−1 (n = 1), correlates well with nearly an order of magnitude decrease observed in charge‐carrier lifetime (τaverage = 233 ps for n = 10 vs τavg = 27 ps for n = 1). The tightly bound excitons in 2D perovskites make charge separation less probable, which in turn decreases the halide mobility and resulting phase segregation. The importance of controlling the dimensionality of the 2D architecture in suppressing halide ion mobility is discussed.

Photo De‐Mixing in Dion‐Jacobson 2D Mixed Halide Perovskites

“…From the analysis of the temperature-dependent half-time we obtain an activation energy of 0.39 eV (Figure S31, Supporting Information). This value is slightly larger than the value reported for photo de-mixing in 3D (0.27 and 0.30 eV) [13,40] and 2D RP case (0.22 eV) [31] mixed halide perovskites.…”

“…[ 18 ] According to such model, the energies related to defect interaction with electronic charge carriers, and specifically the difference in hole trapping behavior for iodide and bromide‐rich phases, would contribute to the driving force for de‐mixing. However, unlike 3D systems, the large exciton binding energy in 2D perovskites [ 44 ] investigated here may also have influence on the charge carrier dynamics [ 31 ] and the interaction between the electronic charges and ionic defects. In addition, the peculiar shape of the photo‐miscibility‐gap on the Br‐rich side may be related to a more complex mechanism during de‐mixing, such as ordering effects in the Br‐rich phase.…”

“…[ 29 ] Recently, photo de‐mixing in 2D RP mixed halide perovskites was reported, also emphasizing the role of the spacer and of the n value in dictating the occurrence and kinetics of the photo‐induced phase segregation respectively. [ 30,31 ] A number of remaining questions related to photo de‐mixing in 2D mixed halide perovskites require clarification, including the degree of reversibility of this process, the evaluation of the compositions of the de‐mixed phases, and the effect of bifunctional organic spacers relevant to DJ systems. More in general, the understanding of the phase behavior of 2D halide perovskites would shed light on fundamental questions related to the role of confinement in the determination of their ionic properties.…”

Halide Mixing Inhibits Exciton Transport in Two-dimensional Perovskites Despite Phase Purity

“… 31 Interestingly, phase segregation appears to be absent in 2D perovskites. 12 , 32 − 36 This may be attributed to the increased flexibility of the 2D lattice, which is more tolerant to local strain.…”

“…This is consistent with recent reports by Kamat and co-workers, who showed that PEA-based 2D perovskites are resilient to light-induced phase segregation. 35 , 36 …”