Arbitrarily-Wide-Band Dielectric Mirrors and Their Applications to SiGe Solar Cells

The dielectric mirror is an important optical component for optoelectronic devices, passive photonic devices, and solar cells. Unfortunately, the reflection bandwidth of distributed Bragg reflectors (DBRs) and high-index contrast mirrors (HCGs) are limited by the index contrast of the material system used. Here, an aperiodic design for dielectric mirrors is proposed, and it is shown that for a fixed index contrast, the bandwidth of the reflection band can be arbitrarily widened by simply incorporating more dielectric layers. This is pronouncedly different from the fixed bandwidth of HCGs and DBRs. The physics behind the broadband reflection for the aperiodic stacking is identified as the photonic bandgap widening due to the annihilation of the quasi-guided modes in nonperiodic structures. This observation applies very well to aperiodic auto-cloned 3-D photonic crystal reflectors, to aperiodic DBRs, and even to diffuse dielectric mirrors that have recently emerged to be very promising for solar cells due to their zero plasmonic absorption nature. Experimentally, the white paint diffuse medium reflectors are applied to SiGe solar cells to confirm their high reflectance and the feasibility of enhancing solar cell efficiency.