Sodium Induced Beneficial Effects in Wide Bandgap Cu(In,Ga)S2 Solar Cell With 15.7% Efficiency

Abstract

This study underscores the pivotal role of sodium (Na) supply in optimizing the optoelectronic properties of wide bandgap (~1.6 eV) Cu(In,Ga)S₂ (CIGS) thin-film absorbers for high-efficiency solar cells. Our findings demonstrate that the synergistic use of Na from the glass substrate, in conjunction with in situ sodium fluoride (NaF) co-evaporation, significantly enhances the structural and optoelectronic properties of the CIGS. CIGS grown under either Na-deficient or excess conditions exhibit inferior microstructural and optoelectronic properties, whereas an optimal Na supply leads to enhanced photovoltaic performance. Optimal Na incorporation minimizes vertical gallium fluctuations and improves the grain size and crystallinity. An absolute 1 sun calibrated photoluminescence (PL) measurement reveals a substantial suppression of bulk defects and a reduction in non-radiative losses, resulting in a high quasi-Fermi level splitting (ΔE_F) of 1.07 eV, 93 meV higher than in Na-deficient CIGS with the same bandgap. Optimal Na supply further increases excited carrier decay time, as revealed from time-resolved PL, and hole doping density. Cross-sectional hyperspectral cathodoluminescence mapping reveals that optimal Na supply significantly reduces defect density near the surface, thereby effectively translating ΔE_F to open-circuit voltage (V_OC). As a result, a champion wide bandgap CIGS solar cell with a cadmium-free ZnSnO_x buffer layer achieved an impressive V_OC of 971 mV and an active area power conversion efficiency of 15.7%, highlighting its potential for advancing tandem photovoltaic technologies with a stable inorganic top cell.