Carrier recombination mechanism and photovoltage deficit in 1.7-eV band gap near-stoichiometric Cu(In,Ga) S2

Abstract

$\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{S}}_{2}$ is a promising semiconductor that offers excellent prospects for photovoltaics. However, the performance has been plagued mostly due to large photovoltage deficit. Here, we investigate defects and optoelectronic properties of 1.7-eV band gap near-stoichiometric $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{S}}_{2}$ (CIGS). We have estimated quasi-Fermi-level splitting of 921 meV from steady state photoluminescence (PL) measurements at 1 sun. Detailed analysis of temperature and excitation dependent PL reveals the behavior of a strongly compensated semiconductor. We show that spatially varying energetic disorder, described by electrostatic potential fluctuations, causes band-tail recombination and strongly affects the carrier recombination in compensated CIGS. Apart from band-tail transitions, we also observe two deep defects at about 0.3 and 0.45 eV below the band edge. The defects are also discerned by admittance measurements. Temperature dependent current-voltage measurements show that the open-circuit voltage is further limited by interface recombination. Thus, the performance improvement lies in the mitigation of deep defect states and absorber/buffer interface optimization.