Lattice Dynamics and Electron–Phonon Coupling in Lead-Free Cs₂AgIn_1–<i>x</i>Bi_<i>x</i>Cl₆ 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 Cs₂AgIn_1-<i>x</i>Bi_<i>x</i>Cl₆ double perovskite nanocrystals with varying alloying concentrations. Our UV-vis study confirms the parity allowed first direct transition for <i>x</i> ≤ 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 T_2g modes (one for translational motion of "Cs" and other for octahedral breathing), 1 E_g and 1 A_1g 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.