Polarons and Charge Localization in Metal‐Halide Semiconductors for Photovoltaic and Light‐Emitting Devices

Abstract

Metal‐halide semiconductors have shown excellent performance in optoelectronic applications such as solar cells, light‐emitting diodes, and detectors. In this review the role of charge–lattice interactions and polaron formation in a wide range of these promising materials, including perovskites, double perovskites, Ruddlesden–Popper layered perovskites, nanocrystals, vacancy‐ordered, and other novel structures, is summarized. The formation of Fröhlich‐type “large” polarons in archetypal bulk metal‐halide ABX3 perovskites and its dependence on A‐cation, B‐metal, and X‐halide composition, which is now relatively well understood, are discussed. It is found that, for nanostructured and novel metal‐halide materials, a larger variation in the strengths of polaronic effects is reported across the literature, potentially deriving from variations in potential barriers and the presence of interfaces at which lattice relaxation may be enhanced. Such findings are further discussed in the context of different experimental approaches used to explore polaronic effects, cautioning that firm conclusions are often hampered by the presence of alternate processes and interactions giving rise to similar experimental signatures. Overall, a complete understanding of polaronic effects will prove essential given their direct influence on optoelectronic properties such as charge‐carrier mobilities and emission spectra, which are critical to the performance of energy and optoelectronic applications.

Chemical Control of the Dimensionality of the Octahedral Network of Solar Absorbers from the CuI–AgI–BiI3 Phase Space by Synthesis of 3D CuAgBiI5

“… 46 Given the high trap density that is apparent from the other spectroscopic measurements, it is possible that a reduction in trap density, along with enhanced crystallinity and reduced energetic disorder, could lead to an improvement in the charge-carrier mobilities for CuAgBiI 5 , although the low values for charge-carrier mobilities 2 , 48 , 49 and fast charge-carrier recombination 39 , 49 reported across a variety of silver–bismuth compositions could be indicative of more fundamental limitations to charge-carrier transport in these materials that require further chemical tuning to improve. 50 …”

Charge-Carrier Mobility and Localization in Semiconducting Cu2AgBiI6 for Photovoltaic Applications

“…48 These interactions can be revealed by clear signals in spectroscopic studies investigating charge-carrier energetics, dynamics, and mobilities. 45,47,49 Charge-carrier self-trapping has been reported across materials including all-inorganic KMgF 3 , 50 in the bismuth-based systems Cs 2 AgBiBr 6 , 19,20,51 Cs 3 Bi 2 Br 9 , 27 and Rb 4 Ag 2 BiBr 9 , 28 in layered perovskites, 30−34 and in metal-halide materials such as PbBr 2 and a number of alkali halides. 52−55 As we have noted elsewhere, 47 the strength of charge-carrier localization in a given material depends on its chemical composition, ease of structural distortion, and structural and electronic dimensionality.…”

“…45,47,49 Charge-carrier self-trapping has been reported across materials including all-inorganic KMgF 3 , 50 in the bismuth-based systems Cs 2 AgBiBr 6 , 19,20,51 Cs 3 Bi 2 Br 9 , 27 and Rb 4 Ag 2 BiBr 9 , 28 in layered perovskites, 30−34 and in metal-halide materials such as PbBr 2 and a number of alkali halides. 52−55 As we have noted elsewhere, 47 the strength of charge-carrier localization in a given material depends on its chemical composition, ease of structural distortion, and structural and electronic dimensionality. Further, it appears that in double perovskite or silver−bismuth materials, although the crystal structure can be three-dimensional, the electronic band structure is of lower dimensionality, 56 making charge-carrier localization more likely.…”

Double perovskite Cs2AgBiBr6 radiation sensor: synthesis and characterization of single crystals

“…A weakness in the electrical performance of Cs 2 AgBiBr 6 has its origin in the electrical isolation of the two types of octahedra resulting in zero electronic dimensionality and consequently larger effective masses of charge carriers [16]. In addition, owing to the crystal structure and composition, a stronger charge carrier-lattice coupling compared to that in the MAPbX 3 can cause selftrapped charges and inevitably lead to limited l [17][18][19]. In contrast, suppressed ion migration arising from the ordered crystal structure, stronger electrostatic interaction between the Cs ?…”