Enabling Low-Temperature Deposition of High-Efficiency CIGS Solar Cells with a Modified Three-Stage Co-Evaporation Process

CuIn1–xGaxSe2 (CIGS) absorber layers with good crystallinity and large grain sizes deposited at low substrate temperatures are essential to the development of CIGS solar cells. The main challenges are the poor crystallinity and the deficiency in Na diffusion from the soda-lime substrates. The poor crystallinity of the CIGS films deposited at low substrate temperatures is attributed to the low interdiffusivity of In and Ga at low temperatures. In addition, Na diffusion from the substrate is reported to improve the crystallinity of the CIGS films and the performances of the device. However, when the substrate temperature is below 500 °C, Na diffusion from the substrate is limited, and thus postdeposition alkali elemental treatment is essential for the fabrication of high-efficiency CIGS solar cells. In this work, we develop a modified three-stage co-evaporation process which incorporates an additional Cu-rich phase deposition after stage II. Such a process enables the low-temperature growth of large CIGS grains with good crystallinity even without the alkali elemental postdeposition treatment. The efficiency of the CIGS solar cells deposited at 470 °C is comparable to those deposited at a high temperature of 540 °C.