Advanced Light Management Techniques for Two-terminal Hybrid Tandem Solar Cells

Making a delicate tandem cell work as one device

In this work, we explored how a tandem solar cell can make better use of sunlight by combining two thin-film materials with different strengths. Our device pairs a CIGS bottom cell with a hydrogenated amorphous-silicon top cell. Together, they can absorb different parts of the solar spectrum and potentially deliver more voltage and efficiency than a single-junction cell. The promise is attractive, but the layers have to work together electrically and physically before that promise becomes real.

We focused on two problems that repeatedly limit this kind of two-terminal design. The first is the naturally rough surface of the CIGS cell. If the amorphous-silicon layer grows over sharp features, defects and electrical shunts can form. We therefore investigated mechanical polishing of an intermediate zinc-oxide layer to give the upper cell a smoother foundation. The second problem is current matching. Because both sub-cells are connected in series, the whole tandem device is held back by the cell that produces the lower current.

Using texture where it helps, smoothing where it protects

To help the top cell capture more light, we textured its transparent ZnO:Al front electrode through wet etching. This might sound like the opposite of smoothing the surface, but the two actions serve different parts of the device. At the interface between the bottom and top cells, a gentler surface protects electrical quality. At the front electrode, carefully created texture can scatter light more effectively into the current-limiting top cell.

One of the useful things we found is that this texture does not have to be accepted as a fixed feature of a material. By adjusting the growth conditions of the ZnO:Al layer, we could influence the form of its random texture. That gives researchers another handle for balancing optical behaviour, electrical performance and the realities of making the cells.

For us, the wider point is that higher-efficiency solar cells are built through many small, connected choices. New absorber materials matter, but so do the interfaces and surfaces that determine where light travels and where electrical losses appear. This study shows practical routes for tackling rough interfaces and current mismatch in a hybrid thin-film tandem cell, while keeping the focus on a device that must ultimately be manufacturable as well as efficient.