Optimized Optics for Highly Efficient Photovoltaic Devices

Luminescence extraction has played a tremendous role in improving the efficiency of solar photovoltaics. This has been assisted by the development of a highly reflective rear mirror. At the same time, thermophotovoltaic efficiency has shown to benefit from a rear mirror. The low-energy, unabsorbed photons in the thermophotovoltaic cells can be reflected by the mirror, thereby assisting in photon regeneration. In both cases, it has been shown that device efficiency has an increasing return with improvement rear mirror reflectivity. However, the realization of broadband mirrors with reflectivity >98% has remained elusive. Here, we demonstrate a mirror that can achieve broadband 99.3% reflectivity, for thermophotovoltaic cells. The design concept can be equally extended to photovoltaic cells as well, thereby enhancing the efficiencies closer to the thermodynamic limits.