Silicon solar cells cannot use about 20% of the energy contained in sunlight, because photons with energy lower than the band gap are not absorbed in silicon. Upconversion makes these lower-energy photons also useful. In this process, two photons with low energy are combined to generate one photon with higher energy. By improving the solar cells and materials used, a significant increase in current due to upconversion was measured for the first time at Fraunhofer ISE in concentrator modules with silicon solar cells and upconverters.
In an optimal system consisting of a solar cell and an upconverter behind it, it is necessary not only to absorb as much light as possible in the silicon (just as in a simple solar cell), but also as many low-energy photons as possible must pass through the solar cell to the upconverter. To this purpose, special bifacial silicon solar cells with a high efficiency value were developed at Fraunhofer ISE, which are ideally suited to further increase the efficiency by upconversion. They feature not only a double-layer anti-reflective front coating but also an anti-reflective coating of the back surface. This also ensures that the light emitted by the upconverter is coupled in well.
The upconverter on the back of the solar cell (Fig. 1) consists of erbium-doped sodium yttrium tetrafluoride, which was synthesised at the University of Berne. This microcrystalline powder was then encapsulated in a polymer by the Heriot- Watt University in Edinburgh.
The system of upconverter and solar cell was integrated into a concentrator module (Fig. 2). The gain due to upconversion increases with the sunlight concentration factor (Fig. 3). Although the best results for upconversion to date were measured after the optimisation, the relative benefit for the solar cell is still slight. In order to change this, different photonic concepts are being investigated, with which the upconversion efficiency can be increased further.