Silver nanoparticles for plasmonic light trapping in A-Si:H solar cells

We investigate the use of silver nanoparticles as light scattering elements to improve light trapping in amorphous silicon solar cells. Simulations presented in literature show that these nanoparticles can scatter light very efficiently due to plasmon resonance. However, our previous experimental work showed that light trapping does not improve when silver nanoparticles, fabricated by annealing a thin silver film, are embedded in a-Si:H solar cells. To shed some light on this we investigate the optical properties of these nanoparticles in more detail. Silver nanoparticle films with a mass thickness ranging from 3 to 18 nm were fabricated, resulting in films with average particle sizes ranging from 20 to 120 nm. We found that in all cases less than 10% of the incident light is scattered. The undesired absorption of light by the silver nanoparticles is at least three times stronger than the desired light scattering effect. We tentatively attribute this to the wide size distribution and high surface coverage inherent to this particle fabrication technique. We use an effective medium approach to incorporate the experimentally obtained optical properties of the nanoparticle films into our opto-electrical device simulator. This allows us to use realistic optical properties in solar cell simulations. We focus on a solar cell design with the silver nanoparticles embedded in a transparent conductive oxide layer at the rear of the a-Si:H layer. The solar cell simulations show that light trapping does not improve as long as absorption dominates over scattering. The simulated quantum efficiency curves are in agreement with experimental results.