Alkali Metal Doping Engineering for Copper Indium Gallium Selenium (CIGS) Thin Film Solar Cells

Copper indium gallium selenium (CIGS) solar cells have attracted intense interest because of their high light absorption coefficient, high photoelectric energy conversion efficiency, good stability and low production costs. Researchers have discovered that alkali metal doping engineering can significantly improve the performance of CIGS solar cells since the 1990s, and a lot of research on alkali metal doping has led to a series of breakthroughs. Based on the results of the research group, the major technological breakthroughs caused by alkali metal doping engineering were introduced in this paper. Three methods of alkali metal doping including the front doping, the middle doping and the post doping methods were presented briefly. Among them, the CIGS solar cells prepared by the post doping method showed highest conversion efficiency. The roles of alkali metal doping were adjusting the band gap of the CIGS absorption layer, passivating the defects, increasing the hole density, and suppressing the carrier recombination in the CIGS bulk and at the interface. These functions finally increased the open circuit voltage and fill factor of the solar cell. Alkali metals also affected the element distribution and micro-morphology of the absorption layer, although its effect on the device performance remained controversial.