Ageing effects on plasmonic properties for solar cell applications

Metal nanoparticles are known for their unique optical properties due to surface plasmon excitations. The far field and near field effects from these metal particles have been captured to enhance efficiency of thin film solar cells by way of light trapping. Our group has extensively studied the different design parameters for a plasmon enhanced solar cell like effect of metal size/shape, location, effect of dielectric layer thickness and also the effect of plasmons on the electrical properties like passivation of cells. Whilst identifying and minimising parasitic absorption losses in these metal particles are important and is attracting lot of attention, we choose to look at a more interesting issue of ageing effects. Plasmonics at the moment promises efficiency enhancements exceeding 30% including associated losses in metals. However as is needed for solar cells, the technologies incorporated have to stand the test of time. In this work we look at the age effects on the plasmon performance by analysing our cells over time. Our preliminary results show that plasmons supported by silver metal nanoparticles can degrade by upto 10% with time. Metal nanoparticles when exposed to air can get tarnished easily causing degradation of the plasmonic properties. This will result in weakening of the scattering and reduce light trapping effects. We also look at ways of minimizing the ageing losses by overcoating. MgF2 is used as the dielectric film to overcoat metal nanoparticles preventing degradation and also to isolate MNP layer from the back surface reflector of cells. Our results show that such a rear scheme brings an additional current enhancement over interested wavelength region improving over time.