Advanced light trapping designs for high efficiency thin film silicon solar cells

State-of-the-art optical trapping designs for tandem thin film silicon solar cells in the superstrate configuration commonly feature two elements: a textured transparent conductive oxide front contact and a low refractive index interlayer between top and bottom cell. We investigate more advanced super light trapping schemes that have been designed and implemented in thin film tandem junctions at different levels in the solar cells to further enhance light trapping capabilities. Regularly patterned nano-imprinted substrates offer an additional capability to tune the substrate morphology and optimize it for enhanced solar cell performance. We show that the onset of defect formation in thin film layers can be controlled and that the spectral response in the infra-red part of the spectrum is increased. Intermediate reflectors with ultra-low refractive indeces and plasmonic properties lead to increased light confinement in top cells. Results show that reducing the refractive index below 1.5 still leads to a substantial current increase in the top cell. These innovative designs improve the output current in amorphous/microcrystalline tandem devices with thin photo-active layers. In addition, they surprisingly enhance the electrical parameters of the tandem cells with a record open circuit voltage of 1.47 V. Plasmonic effects of metal nano-particles embedded in interlayers do not compromise the electrical solar cell parameters and similarly strengthen light confinement in the top cell, however detrimental effects are observed in the bottom cell current output.