Towards Lambertian internal light scattering in solar cells using coupled plasmonic and dielectric nanoparticles as back reflector

We present a novel approach that opens the route to exceeding the 4n² light trapping limit. White paint, usually based on titanium dioxide (TiO₂) nanoparticles, is well known for its high reflectance and excellent light scattering properties. The angular intensity distribution (AID) is close to the ideal Lambertian cos(φ) distribution when light is scattered into air. White paint therefore seems to be the ideal back reflector for solar cell applications. However, when white paint scatters light into c-Si, the AID is refracted into a narrower cone, leading to a large deviation from the Lambertian scattering. Here we present an effective approach to avoid this refraction effect and significantly improve light trapping. Our approach is based on combining dielectric TiO₂ nanoparticles with plasmonic silver nanoparticles. The dielectric particles efficiently couple the incident light to the plasmonic particles which in turn couple the light into the absorber layer. Because plasmonic nanoparticles can couple the light beyond the critical angle, the resulting AID inside the absorber is much broader than without the plasmonic particles. We show that this leads to improved light trapping and solar cell performance.