Lattice Dynamics and Electron–Phonon Coupling in Lead-Free Cs₂AgIn_1–xBi_xCl₆ Double Perovskite Nanocrystals

Abstract

Recently, lead free all-inorganic double perovskites have revolutionized photovoltaic research, showing promising light emitting efficiency and tunability via modification of inherent structural and chemical properties. Here, we report a combined experimental and theoretical study on the variation of carrier–lattice interaction and optoelectronic properties of Cs2AgIn1–x Bi x Cl6 double perovskite nanocrystals with varying alloying concentrations. Our UV–vis study confirms the parity allowed first direct transition for x ≤ 0.25. Using a careful analysis of Raman spectra assisted with first-principles simulations, we assign the possible three types of active modes to intrinsic atomic vibrations; 2 T2g modes (one for translational motion of “Cs” and other for octahedral breathing), 1 Eg and 1 A1g mode for various stretching of Ag–Cl octahedra. Ab-initio simulation reveals dominant carrier-phonon scattering via Fröhlich mechanism near room temperature, with longitudinal optical phonons being effectively activated around 230 K. We observe a noticeable increase in hole mobility (∼4 times) with small Bi alloying, attributed to valence band (VB) maxima acquiring Bi-s orbital characteristics, thus resulting in a dispersive VB. We believe that our results should help to gain a better understanding of the intrinsic electronic and lattice dynamical properties of these compounds and provide a base toward improving the overall performance of double perovskite nanocrystals.

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

“…[ 151 ] Work by Manna et al. [ 152 ] combined Raman measurements with density functional theory (DFT) calculations, finding Fröhlich coupling constants in the range α F = 2−5 for Cs 2 AgIn 1− x Bi x Cl 6 nanocrystals, and calculations of charge‐carrier mobilities using the Feynman/Hellwarth polaron model suggested upper limits of 2−52 cm 2 V −1 s −1 in nanocrystals across the range of x . [ 152 ]…”

Lead‐Free Halide Double Perovskites: Structure, Luminescence, and Applications

“…[ 143,144 ] The aforementioned theoretical analysis gives us a chance to tune the bandgap of HDPs through precise component engineering and unexpected optoelectronic properties will be uncovered. [ 68,145–147 ]…”

“…(b) Reproduced with permission. [ 145 ] Copyright 2017, American Chemical Society. (c) Reproduced with permission.…”

“…Figure 7b shows the Lorentzian fit for the Raman spectra of pure Cs 2 AgBiCl 6 and Cs 2 AgInCl 6 NCs. [ 145 ] The Raman active modes are marked in the corresponding positions. Theoretical analysis indicates the e.g., mode is related to the asymmetric stretching vibrations of Cl around B‐site atoms (mainly contributed from ).…”

Lead‐Free Halide Double Perovskite Nanocrystals for Light‐Emitting Applications: Strategies for Boosting Efficiency and Stability

“…Furthermore, Manna et al. [ 150 ] studied the band structure of Bi 3+ doped Cs 2 AgInCl 6 nanocrystals in detail. They observed that the width of bandgap reduced and the VBM composed of hybridization between the Ag 4d and Cl 3p transformed into Bi 6p orbital with increasing substitution ratio of Bi 3+ (Figure 8c).…”