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

Abstract

Halide mixing is one of the most powerful techniques to tune the optical bandgap of metal-halide perovskites. However, halide mixing has commonly been observed to result in phase segregation, which reduces excited-state transport and limits device performance. While the current emphasis lies on the development of strategies to prevent phase segregation, it remains unclear how halide mixing may affect excited-state transport even if phase purity is maintained. Here, we study exciton transport in phase pure mixed-halide 2D perovskites of (PEA) 2 Pb(I 1– x Br x ) 4 . Using transient photoluminescence microscopy, we show that, despite phase purity, halide mixing inhibits exciton transport. We find a significant reduction even for relatively low alloying concentrations. By performing Brownian dynamics simulations, we are able to reproduce our experimental results and attribute the decrease in diffusivity to the energetically disordered potential landscape that arises due to the intrinsic random distribution of alloying sites.

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

“…The low-temperature gradient and mild supersaturation conditions used for crystal growth in this study suggest that halide domain separation is the thermodynamically favored outcome room temperature for RPPs, and such phase immiscibility explains the recent success in creating sharp halide heterojunctions in n = 1 and n = 2 RPPs. 44,53 However, homogeneous halide alloys may still be attainable and may have already been attained in some reported mixedhalide RPPs, 38,40,54 but such results are likely accessible through kinetically controlled growth pathways that depend on synthesis conditions. The intrinsic phase separation of halide domains in mixed-halide RPPs revealed in this study demands a shift in approach from 3D to 2D as more mixedhalide RPPs are utilized in optoelectronic devices, while also painting a unique halide phase landscape that offers intrinsic multihalide speciation previously inaccessible to 3D mixedhalide LHPs.…”

Yb3+‐Doped Phenylethylammonium Lead Bromide 2D Layered Hybrid Perovskite for Near‐Infrared Emission

“…This inhomogeneity in platelet thickness will not change the band gap of the material since the electron and hole are confined in one atomically thin inorganic layer (Figure 1a) with excitonic Bohr radius of ~0.7 nm. [35] So thinner (~3.5 nm) nanoplatelets do not impose any additional quantum confinement effect compared to the already existing quantum well structure of bulk (PEA) 2 PbBr 4 . However, the platelets, particularly thicker ones, scatter enough light even below the band gap.…”

“…Note that in the previous section we mentioned about the existence of both few‐nm thick nanoplatelets and significantly thicker platelets. This inhomogeneity in platelet thickness will not change the band gap of the material since the electron and hole are confined in one atomically thin inorganic layer (Figure 1a) with excitonic Bohr radius of ∼0.7 nm [35] . So thinner (∼3.5 nm) nanoplatelets do not impose any additional quantum confinement effect compared to the already existing quantum well structure of bulk (PEA) 2 PbBr 4 .…”

Broadband-tunable spectral response of perovskite-on-paper photodetectors using halide mixing

“…The decremental device efficiencies present for the halide mixtures are attributed to a disordered electronic structure, as previously suggested for 3D and 2D perovskites, which can lead to electron-hole recombination. 49,54…”