2019
In this work, we investigated how optical design can compensate for the light lost when a CIGS solar-cell absorber is made thinner. CIGS uses copper, indium, gallium and selenium to absorb light. Reducing the absorber thickness can save material and lower production costs, but it also means the cell naturally captures less of the incoming spectrum. We developed and optimised a wideband anti-reflection coating based on porous magnesium fluoride, while also looking at how the rear of the cell could return more light to the absorber.
We used three-dimensional optical modelling for a reference cell with a 750-nanometre CIGS layer. At the front, we described the porous magnesium-fluoride coating with an effective-medium approximation. The result was not a single simple film, but a porous layer on top of a compact layer. Using reflectance as our target, sequential nonlinear programming identified the geometry that gave the best light coupling across a wide range of wavelengths.
The porous-on-compact coating increased photocurrent density by 6.8 percent compared with the reference cell without an anti-reflection coating. We also tested different angles of incoming light, since solar modules are not illuminated at one fixed laboratory angle. Under those changing conditions, the double-layer coating performed better than a conventional compact magnesium-fluoride single layer.
The method also made the design process much more manageable. The effective-medium approximation cut computer memory use by a factor of thirty and reduced the simulation time from four days to about one hour. At the rear of the cell, we combined a point-contacted magnesium-fluoride/aluminium-oxide reflector with the front coating. Together, those strategies improved photocurrent density by 11.3 percent over the reference configuration.
For us, the key finding is that this combined light management can more than compensate for the losses expected from a much thinner absorber, even compared with a 1,600-nanometre CIGS cell without light management. The work offers practical guidance for coatings that tolerate variation in thickness and porosity and could be manufactured at scale. Careful control of light, we show, can let a material-efficient solar cell perform far beyond what its thickness alone suggests.