Traditionally, the power conversion efficiency of solution-processed copper indium gallium sulfur diselenide (CIGSSe) solar cells is significantly lower than that achieved by expensive, vacuum-based fabrication methods. Moreover, solution-based methods rely on solvents that are hazardous. The researchers’ new process overcomes these problems.

Solution-processed CIGSSe cells have generated significant interest due to their excellent photovoltaic properties, such as high absorption of visible light, stability, and tunable bandgap. However, large-scale, practical applications for these cells are limited by a two-fold challenge. First, solution-based CIGSSe fabrication yields very low power conversion efficiency and often uses solvents that are not environmentally friendly. Secondly, to achieve higher power conversion efficiency, fabrication methods rely on an expensive vacuum environment that leads to substantial material loss. 

The researchers used aqueous spray deposition in an air environment and developed a CIGSSe solar cell with power conversion efficiency greater than 17 percent. 

“For the spray solution, we used deionized water, which is eco-friendly and the cheapest solvent to date,” explains professor JunHo Kim, Ph.D., of the Global Energy Research Center for Carbon Neutrality at Incheon National University. Moreover, conventional solution-based fabrication processes rely on environmentally hazardous, cadmium-based buffers for the optimization of thin-film solar cells. In contrast, the researchers used an indium sulfide-based buffer that is a cadmium free, eco-friendly alternative.

The researchers further investigated the alloying effects of zirconium on indium sulfide buffers. Remarkably, the team found that zirconium alloying increases the electron concentration in the buffer. Moreover, this method “passivates” or reduces defect states in the CIGSSe absorber, optimizing the charge transfer between various interfaces, leading to enhanced power conversion efficiency. The fabricated cell also has an optimum bandgap for high efficiency applications such as a bottom cell or a tandem cell.

The researchers’ technique is cost-effective and easily scalable, since it does not require a vacuum environment. “We carried out spray deposition in an air environment without using any high vacuum facility, which significantly reduces fabrication cost and thus makes the fabrication technique more practical and competitive in the industry sector,” says Kim.