Plasmonic degradation and the importance of over-coating metal nanoparticles for a plasmonic solar cell

Plasmonics is an emerging area of study for photovoltaic applications. The last decade has seen an increasing interest in the application of metal nanoparticles (MNP) on the surface of solar cells to scatter light and subsequently achieve light trapping. In this study we investigate the optical effects of single and double layer rear light trapping reflectors – planar or scattering; direct or detached, which are fabricated on the 400 μm front-planar silicon solar cells. As a single layer reflecting scheme, we find that Ag metal nanoparticles (Ag MNP) as a scattering reflector outperforms all other single layer reflectors with a maximum current enhancement of 11% (calculated from 900 nm to 1200 nm) compared to metal back reflectors. However over time we notice that the scattering properties are dampened due to tarnishing of the silver nanoparticles. A double layer reflecting scheme using optimised Ag nanoparticles as the first layer and the second layer of evaporated Ag (E Ag) separated via MgF₂ is introduced to overcome this issue together with additional light trapping. We find that no EQE or current degradation is observed over time. More importantly, after optimising the over-coating MgF₂ thickness, the best-performing double layer reflector structure improves EQE by 4.5-fold at 1160 nm and enhances photocurrent by 25.6% (calculated from 900 nm to 1200 nm), compared to cells with metal back reflectors. We also conclude that the optimum thickness of the over-coating layer is dependent on the wavelength to be optimised, the angular distribution of the plasmonic nanoparticles scattering within the layer, and the type of adjacent metal reflector used. It performs best when condition for constructive interference or enhanced electric field at the rear Si interface is satisfied. An enhancement of effective optical path length factor Z of around 9-fold compared to an Al reflector is achieved by using the optimised double layer reflector, higher than the 6-fold enhancement reported previously.