Diffusion coefficients and I–V curves of solid state devices based on single crystals of the Cu–Ag–In–Se system with ionic motion

Abstract

An In-rich chalcopyrite single crystal of the Cu–Ag–In–Se (CAIS) system shows a mixed ionic and electronic conduction. To study the influence of the ionic motion in the I–V relations, three slices of this single crystal with different composition have been chosen, and an electrode/CAIS/electrode solid state device has been formed with each one. The current, I, and the potential drop across the sample have been measured as a function of time for different external applied voltages. The I(t) curves can be only fully understood by using a three mobile ion framework. The ionic diffusion coefficients have been computed from σ(1/t) curves and assigned to AgCux, CuCux and InCu•. The I–V relations shows non-linear shapes and hysteresis effects associated with the presence of a remaining voltage at the CAIS/electrode interface due to the ionic accumulation from previous analysis. In addition, the I–V relations are not reproducible, since the initial sample state is not fully recovered by ionic redistribution after 20 days of recovering time. To fully explain the results, the number of available vacancies, the Ag/Cu atomic ratio and/or the apparition or disappearance of (InCu•+VCu/) defect pairs must be taken into account. A relation between the sample stoichiometry and the net ionic charge accumulated at the CAIS/electrode interface and its remaining time upon removing the electric field has been established. The remaining interface potential has been calculated. It shows a linear dependence with the sample potential drop. The ionic motion could produce stoichiometric deviations along the active film of the solar cell that would vary its resistivity. Likewise, the interface potential could be responsible of the significant benefit reported in the literature on CuInSe2/Cu-poor solar cells.