Effects of Ga Compositional Grading on CIGS Electronic Properties Relevant to Solar Cell Performance

Abstract

AbstractTransient photocurrent (TPI) and photocapacitance (TPC) spectroscopy have been applied to a set of compositional graded CuIn1-xGaxSe2 (CIGS) solar cell devices deposited by the vacuum co-evaporation method at the National Renewable Energy Laboratory. These measurements provide a spectral map of the optically induced release of carriers for photon energies from below 1 eV to 2 eV. By comparing the two types of spectra one can distinguish majority from minority carrier processes and they clearly reveal a higher degree of minority carrier collection for devices in which the Ga fraction increased monotonically with distance from the junction. This agrees with notions of how compositional grading improves overall cell performance. Minority carrier collection was even more strongly enhanced in sample devices incorporating v-shaped Ga-grading. Spatial profiles of the free hole carrier densities and deep acceptor concentrations were examined using drive-level capacitance profiling (DLCP). In the compositionally graded sample devices we found that the free carrier density decreased and that defect density increased with increasing Ga fraction toward back contact.