Photonic and plasmonic crystal based enhancement of solar cells — Theory of overcoming the Lambertian limit

We develop a new conformal nc-Si solar cell architecture that has a periodic plasmonic back-reflector of tapered nano-pillars. Using rigorous scattering matrix simulations, we show that the optimized solar architecture has absorption that can exceed the classical 4n2 limit, or the Lambertian limit of absorption by a randomly roughened back-reflector, for common thicknesses of the nc-Si absorber layer upto 1500 nm in thickness. The optimized solar architecture has pitch between 500 and 900 nm and tapered pillar heights of ~ 200 nm. The conformal solar cell geometry minimizes the reflection loss at the top surface. The enhancement over the classical limit is provided by light concentration and waveguide modes or diffraction resonances within the absorber layer. Nc-Si cells of 1000 nm thickness may generate short circuit currents in excess of 32 mA/cm2, where the enhancement is more than 60% relative to a flat silver back-reflector. Sources of losses in solar cells and are identified.

► We develop a conformal nano-silicon solar cell architecture with a plasmonic back-reflector
► Absorption exceeds the classical 4n2 absorption limit
► The enhanced absorption occurs from plasmonic light concentration and diffraction
► Enhancement exceeds 60% over flat cells, with potential for > 32 mA/cm2 current