Unraveling Vibrational Energies of Chemical Bonds in Silver-Containing Chalcopyrite Compounds (Ag,Cu)InSe2 and Ag(In,Ga)Se2 by Low-Temperature EXAFS Analysis

Abstract

Understanding the diffusion properties of constituent atoms in chalcopyrite-type Ag-containing Cu(In,Ga)Se₂-based semiconductors is important for the design of highly efficient photovoltaic devices. Herein, we experimentally evaluated the vibrational energy (Einstein frequency and temperature) of individual bonds in (Ag,Cu)InSe₂ and Ag(In,Ga)Se₂ powders using Debye–Waller factors derived from extended X-ray absorption fine structure (EXAFS) data at low temperatures. We found that the Einstein temperature and frequency increased in the order of Ag–Se < Cu–Se < In–Se < Ga–Se. This order correlates with the theoretical activation energy of atomic migration calculated for Ag(In,Ga)Se₂ and Cu(In,Ga)Se₂ with Ag or Cu defect sites. In the chalcopyrite compounds, the Ag atom is the easiest to diffuse, owing to the smallest force constant and the largest reduced mass of the Ag–Se bond. The bond length varied with the sample composition, and the force constant’s dependence on bond length further suggests that activation energy for atomic diffusion can be modulated through compositional adjustments. The low-temperature EXAFS study provides beneficial information for the design of multicomponent materials including bond length variations and Einstein frequencies of individual bonds. The Einstein frequencies could be an indicator of the atomic diffusion properties, reflecting the influence of force constant and reduced mass, to understand the elemental gradients in chalcopyrite semiconductors for highly efficient photovoltaic devices.