Nanoparticle/metal rear reflectors for low- and high-temperature silicon solar cells

Metallic rear electrodes in silicon solar cells with textured surfaces absorb infrared light parasitically, stealing current from the cells. The loss can be mitigated, however, with the insertion of a low-refractive-index dielectric layer between the metal and absorber that is at least as thick as the penetration depth of the evanescent wave that results when light arrives outside the escape cone at the rear interface. We investigate silicon nanoparticle coatings with tunable porosity-and thus refractive index-in this role. We show that a nanoparticle/Ag rear reflector achieves an internal reflectance exceeding 99.5%, and, when used in a silicon heterojunction solar cell, boosts the infrared short-circuit current density by 0.4 mA/cm2 compared to a state-of-the-art ITO/Ag reflector. In addition, the nanoparticle films survive annealing at temperatures up to 800 °C without severe changes in their optical properties, suggesting that they may be used in high-temperature silicon solar cells too.