Silver-Promoted High-Performance (Ag,Cu)(In,Ga)Se-2 Thin-Film Solar Cells Grown at Very Low Temperature

Achieving high power conversion efficiencies with Cu(In,Ga)Se-2 (CIGS) solar cells grown at low temperature is challenging because of insufficient thermal energy for grain growth and defect annihilation, resulting in poor crystallinity, higher defect concentration, and degraded device performance. Herein, the possibilities for high-performing devices produced at very low temperatures (<= 450 degrees C) are explored. By alloying CIGS with Ag by the precursor layer method, (Ag,Cu)(In,Ga)Se-2 (ACIGS) solar cells grown at about 450 degrees C reach an efficiency of 20.1%. Only a small efficiency degradation (0.5% and 1.6% absolute) is observed for ACIGS absorbers deposited at 60 and 110 degrees C lower substrate temperature. CIGS devices exhibit a stronger efficiency degradation, driven by a decrease in the open-circuit voltage (V-oc). The root cause of the V-oc difference between ACIGS and CIGS devices is investigated by advanced characterization techniques, which show improved morphology, reduced tail states, and higher doping density in ACIGS absorbers. The proposed approach offers several benefits in view of depositions on temperature-sensitive substrates. Increased Cu diffusion promoted by Ag allows end-point detection in the three-stage process at the substrate temperatures below 300 degrees C. The modified process requires minimal modification of existing processes and equipment and shows the potential for the use of different flexible substrates and device architectures.