Researchers around the world are working to constantly improve the efficiency of solar cells. But using these devices in the real world represents an extra challenge. For example, solar cells need to be incorporated into modules that can protect sensitive materials from harsh environments. These modules can reduce the power conversion efficiency, losing the performance gains.
KAUST researchers have developed a solar-cell module that mitigates cell-to-module losses, following a rethink of the module’s optical design and how it should be stacked.
The solar cells used by the team were made of a combination of two light-absorbing semiconductors – one silicon and the other made from a perovskite material. Silicon is, by far, the most common and well-established semiconductor material used in solar cells, while perovskites are an emerging material. Adding a thin layer on top of the silicon has already been shown to improve performance with an acceptable increase in cost.
These perovskite-silicon tandem solar cells have previously exhibited efficiencies in optical-to-electrical power conversion as high as 30%, and theoretical modeling has indicated it could go as high as 45%.
But when the KAUST team put their tandem solar cells into a module, they found the efficiency dropped from 28.9% to 25.7%. Their module was made by sandwiching the solar cells between two glass sheets, with the inside filled with thermoplastic polyurethane to encapsulate the solar cells.
Researchers believe that the reduction in efficiency is due to a refractive index mismatch after the introduction of glass and polyurethane directly on solar cells without cell-to-module optimization. This results in increased reflection of the incoming light.
And so, the team decided to reduce this front reflection loss by an optical redesign of the module through refractive-index engineering. By moving a film of magnesium fluoride from the top of the cell to the top of the front glass, they reduced the refractive index mismatch, thus achieving efficient light in-coupling.
“This simple optimization effectively enables the highest short circuit current density – related to the maximum current that can be drawn from the device – that is reported in the literature for monolithic perovskite/silicon tandem solar modules, resulting in a power conversion efficiency increase from 25.7% to 26.2%,” says KAUST researcher Lujia Xu, who worked on the research. “We now hope to explore how different materials and texturing the material surface could reduce the current losses from cells to modules even further.”