Scientists in Japan have developed new processes for CIGS solar cell manufacturing, entirely eliminating the use of cadmium and replacing a waste-intensive wet chemical stage. With a thin film deposited on a flexible steel substrate, the group achieved 16.7% efficiency, which it says is a record for a cell of this type produced using scalable fabrication processes.
Among various materials for thin-film solar cells, copper-indium-gallium-selenide (CIGS) is one of the most developed, having achieved efficiencies rivaling those of crystalline silicon. Though commercial manufacturing of CIGS solar products has faltered in recent years, the technology has retained plenty of interest among researchers, particularly for its potential in flexible or multijunction devices.
A group of scientists led by Ritsumeikan University in Japan investigated one problem for CIGS manufacturing; the use of a buffer layer of cadmium sulfide, a highly toxic material. “In the conventional process, cadmium is deposited on the CIGSSe layer via a chemical bath deposition process,” explained Jakapan Chantana, a professor at Ritsumeikan. “By eliminating this step, we have created a completely dry manufacturing process that generates less waste.”
The group was able to replace the cadmium-containing buffer layer by oxidizing the surface of the CIGS film, forming a “native” buffer layer. This approach has been successfully demonstrated before, however, in some previous research the oxidation could take as long as several months. Here, it is reduced to a six-hour air-annealing process at 130 C – which could potentially be implemented in large-scale manufacturing.
The academics described the process in full in the paper Formation of Native Inx(O,S)y Buffer through Surface Oxidation of Cu(In,Ga)(S,Se)2 Absorber for Significantly Enhanced Conversion Efficiency of Flexible and Cd-Free Solar Cell by All-Dry Process, published in Solar RRL. Depositing a commercially produced CIGS film on a flexible steel substrate, the group demonstrated 16.7% efficiency.
While higher efficiencies have been reported for cadmium-free CIGS cells in the past, the group states that what it has developed is the first with potential for large-scale applications, and also that the all-dry processing could lead to significant waste reduction and cost savings.