11.6% Efficient Pure Sulfide Cu(In,Ga)S₂ Solar Cell through a Cu-Deficient and KCN-Free Process

Pure sulfide chalcopyrite Cu(In,Ga)S2 (CIGS) is a promising large-band-gap, light-absorbing material for a single-junction cell or the upper cell of tandem solar cells. Current processes for producing such large-band-gap CIGS cells of over 10% efficiency involve use of either toxic cyanide solutions for Cu-rich processes or hazardous H2S gas during sulfurization for Cu-deficient processes. Herein, we report the fabrication of an over 11% efficient CIGS solar cell by a Cu-deficient and cyanide-free process using sulfur pellets for sulfurization. The Cu content (Cu/(In+Ga)), as one of the most critical factors for Cu-deficient processes, has been carefully adjusted from 0.90 to 0.99 and the effects of Cu content on the quality of the film and on device performance are compared. CIGS with a lower Cu content shows In-rich secondary phases, a rough surface and associated low PL yield, and a short minority carrier lifetime. The sample with a Cu content of Cu/(In+Ga) = 0.96 seems to be the optimal content in this batch; it demonstrates lower nonradiative recombination with an effective minority lifetime of up to 1.28 ns. By composition optimization, CIGS with 11.6% efficiency and 827 mV open-circuit voltage has been achieved, showing excellent potential from this cyanide/H2S-free process.