**Significance Statement**

Spectrally selective beam splitting or filtering of radiation can be applied in various types of systems that convert solar energy into electricity. In thermophotovoltaics it was proposed to apply a filter between a heat source and a long-wavelength solar cell transmitting only thermal radiation in a narrow spectral window with high conversion efficiency in the solar cell. Multilayer interference structures for spectrally selective beam splitting are of interest for increasing the conversion efficiency of systems combining a number of solar cells, being efficient at various wavelengths.

A type of system of special interest in which spectrally selective beam splitting is utilized is in a hybrid system consisting of a beamsplitter, a PV cell (solar cell), and a thermoelectric (TE) generator. Here the basic idea is to spectrally split the sunlight incident on the beamsplitter, such that the short wavelengths are reflected onto the solar cell. The longer wavelengths that cannot be converted in the solar cell are instead transmitted onto the TE generator, and in this way the efficiency of the hybrid system can exceed that of a single solar cell. Previous work on this type of beam splitting system assumed an ideal beamsplitter that reflects all the short wavelengths onto the solar cell and transmits all the long wavelengths onto the TE generator with a perfectly sharp cutoff and no losses. Under this assumption, theoretical studies have shown a potential for increasing the efficiency of the hybrid system beyond that of a single solar cell. The optimal cut-off wavelength has been studied in order to maximize the total efficiency, but the question of how to actually construct the beamsplitter has not been addressed, nor has the study of how close it is possible to get to the ideal beamsplitter using a constructed beamsplitter.

Therefore Associate professor Thomas Søndergaard and his master student Enok Skjølstrup from Aalborg University in Denmark have now examined how to actually construct a beamsplitter in practice. It is designed for maximizing the output of the hybrid system taking into account the spectral efficiency of realistic solar cells based on amorphous (a-Si) or microcrystalline (mc-Si) silicon, efficiencies of the TE generator of either 4% or 8%, and the AM1.5 solar spectrum. They have constructed the beamsplitter using thin-film layers of silicon nitride (Si_{3}N_{4}) and silicon dioxide (SiO_{2}) deposited on N-BK7 glass. As an initial design, the beamsplitter consists of different blocks, where each block consists of a certain number of unit cells serving as a layered bandgap structure with large reflection in a certain wavelength interval. When combining several blocks appropriately the structure reflects almost 100% within a much larger wavelength interval than a single block would. For a given TE generator and solar cell, and a given solar radiation spectrum, the conversion efficiency of the hybrid system is a function of all the layer thicknesses of the beamsplitter, which here can vary from app. 20 to 200. Thus the authors have optimized a function of a large number of variables, where the initial block structure is used as the initial guess in the optimization procedure.

The beamsplitter is optimized for both the a-Si and mc-Si solar cells, and in both cases the efficiency of the hybrid system is found to remarkably exceed that of a single solar cell, and the number of layers required to construct a useful beamsplitter is determined. The efficiency of the hybrid system is found to scale approximately linear with the TE efficiency, and the relative improvement compared to a single solar cell is found to be 21.4% for the a-Si solar cell. The beamsplitter is optimized for an incident angle of 45^{o} but the efficiency as a function of incident angle is found to be relative insensitive to the angle of light incidence from 35-55^{o}. Furthermore, due to higher order band gaps it was found impossible to construct a useful beamsplitter for the reverse configuration with the TE generator and solar cell interchanged.

**Reference**

Enok J.H. Skjølstrup and Thomas Søndergaard. **Design and optimization of spectral beam-splitter for hybrid thermoelectric-photovoltaic concentrated solar energy devices**. Solar Energy, volume 139 (2016), pages 149-156.

Department of Physics and Nanotechnology, Aalborg University, Skjernvej 4A, DK-9220 Aalborg East, Denmark.

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