11.6% Efficient Pure Sulfide Cu(In,Ga)S<sub>2</sub> Solar Cell through a Cu-Deficient and KCN-Free Process
Guojun He, Chang Yan, Jianjun Li, Xiaojie Yuan, Kaiwen Sun, Jialiang Huang, Heng Sun, Mingrui He, Yuanfang Zhang, John A. Stride, Martin A. Green, Xiaojing Hao
Abstract
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.