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

Abstract

2D halide perovskites feature a versatile structure, which not only enables the fine‐tuning of their optoelectronic properties but also makes them appealing as model systems to investigate the fundamental properties of hybrid perovskites. In this study, the authors analyze the changes in the optical absorption of 2D Dion‐Jacobson mixed halide perovskite thin films (encapsulated) based on (PDMA)Pb(I0.5Br0.5)4 (PDMA: 1,4‐phenylenedimethanammonium spacer) exposed to a constant illumination. It is demonstrated that these 2D mixed‐halide perovskites undergo photo‐induced phase segregation, where the pristine mixed‐phase de‐mixes into iodide‐rich and bromide‐rich phases (photo de‐mixing). The de‐mixed state is largely maintained in the dark at room temperature for several months, while at higher temperatures it shows complete reversibility to the mixed‐phase in terms of optical and structural properties (dark re‐mixing). The authors further investigate temperature‐dependent absorption measurements under light to extract the photo de‐mixed compositions and to map the photo‐miscibility‐gap. This work thereby reveals that reversible photo de‐mixing occurs in Dion‐Jacobson 2D hybrid perovskites and provides strategies to address the role of light in the thermodynamic properties of these materials.

Photoinduced Segregation Behavior in 2D Mixed Halide Perovskite: Effects of Light and Heat

“…The trace was fit with a monoexponential function; the pseudo-first-order rate constant ( k ) of 3.04 × 10 –4 s –1 was obtained. The obtained rate constant is comparable to the value obtained with the PEA 2 Pb­(Br 0.5 I 0.5 ) 4 film (<4 × 10 –4 s –1 at 100 mW cm –2 and 295 K), and it is an order of magnitude smaller than that with the (PDMA)­Pb­(Br 0.5 I 0.5 ) 4 film (6 × 10 –2 s –1 at 1.5 mW cm –2 and 313 K); however, neither of them showed obvious segregation in the given conditions. Thus, it should be noted that the degree of segregation cannot be revealed by the rate of reaction.…”

“…In addition, while PHS in 3D perovskites is widely investigated, limited knowledge exists for PHS and ion migration in layered Ruddlesden–Popper (RP) and/or Dion–Jacobson (DJ) perovskites, − which have a general formula of L 2 A n –1 B n X 3 n +1 (RP) or LA n –1 B n X 3 n +1 (DJ), where L is the bulky organic spacer cation and n is the number of octahedral lead halide layers sandwiched between spacer cation bilayers. Similar to 3D perovskites, the divergence between different PHS models persists in these materials as well, and its interpretation is further aggravated by the effect that the choice of the spacer cation has on the PHS, which is absent in 3D perovskites.…”

“…For example, the activation energy of PHS in PEA 2 Pb­(Br 0.5 I 0.5 ) 4 was 21.7 kJ mol –1 , which is smaller than that in MAPb­(Br 0.5 I 0.5 ) 3 (28.9 kJ mol –1 ), , which would indicate that PHS in PEA 2 Pb­(Br 0.5 I 0.5 ) 4 would be more pronounced compared to that in MAPb­(Br 0.5 I 0.5 ) 3 , different from experimental observations. Recently, a study on the mixing–demixing process in (PDMA)­Pb­(Br 0.5 I 0.5 ) 4 (PDMA: 1,4-phenylenedimethanammonium) thin film revealed a miscibility gap under illumination, which supports the idea that the process is governed by thermodynamics rather than kinetics . This implies that the segregation resistance cannot be evaluated by comparing the activation energies in different MHPs due to different experimental conditions, such as sample preparation methods (which would affect the grain sizes and defect levels in the samples and consequently ion migration) and illumination intensities.…”

“…Recently, a study on the mixing−demixing process in (PDMA)Pb(Br 0.5 I 0.5 ) 4 (PDMA: 1,4-phenylenedimethanammonium) thin film revealed a miscibility gap under illumination, which supports the idea that the process is governed by thermodynamics rather than kinetics. 27 This implies that the segregation resistance cannot be evaluated by comparing the activation energies in different MHPs due to different experimental conditions, such as sample preparation methods (which would affect the grain sizes and defect levels in the samples and consequently ion migration) and illumination intensities. It has been well recognized that there are still several unanswered questions concerning PHS and that the analysis is complicated by conflicting data in the literature.…”

Intrinsic Halide Immiscibility in 2D Mixed-Halide Ruddlesden–Popper Perovskites

“…The wide selection of spacer cations and relaxed structural tolerances compared to 3D LHPs present a vast landscape of chemical and structural tunability rarely seen in high-performance semiconductors, − and the relative hydrophobicity of organic spacer cations improves the environmental resilience of RPPs relative to 3D LHPs . Photoinduced phase separation is understood to be kinetically slowed in mixed-halide RPPs (and in similar layered Dion–Jacobson mixed-halide perovskites), , and the ion migration kinetics is further influenced by n and the choice of spacer cation. − Existing studies of mixed-halide RPPs mostly presume that they are fully miscible under standard conditions and that phase separation only occurs under perturbations like light or heat, − with most attention turned toward the influence of structural parameters on phase separation kinetics.…”