Multi-Cavity Dielectric Mirrors for Spectral-Splitting Photovoltaic Applications

Dielectric optical mirrors are designed, fabricated and tested, to be used as spectral-splitting mirrors in four-terminal photovoltaic modules. Mirrors are conceived as 1D-Photonics Bandgap Materials and designed as coupled multiple Bragg reflectors. Spectral tunability, transition steepness and in-band flatness are set by tuning a whole set of parameters, namely: the number and thickness of layers for the single cavity, the number of cavities and their sequence and the adequate refractive index contrast. The final design is optimized by machine-learning. In the present case, the final structure is made of the cascade of 5 Bragg reflectors, each one formed by 4 pairs of alternated layers of SiO2/TiO2, or SiO2/HfO2 as a whole summing up to 41 layers. Multilayers fabrication is by Radio-Frequency sputtering deposition. Theoretical and experimental spectral curves well match. Preliminary results confirm the suitability of the fabrication technique to perform on flexible substrates, both of thin silica glass and of polyurethane resist.