Light Trapping in Thin Film a-Si:H Solar Cells with Reflective and Plasmonic Nanostructured Back Contacts Nanorod synthesis combined with FDTD simulations and optimizations for nanostructured solar cells

Abstract

Solar energy is a promising renewable energy resource. Solar cells can convert solar energy into electrical energy. In order to make solar cells widely economically competitive, it is required to decrease the production cost. Thin film solar cells are developed to reduce the cost by the use of less and low cost material. One of the opportunities to increase the absorption of thin film solar cells, is a nanostructured back reflector. A recent development is to place plasmonic nanostructures at the rear side. By plasmonic light trapping, the absorption can be enhanced.

In this thesis, two structures of a-Si:H solar cells with (plasmonic) nanostructured back reflectors are investigated: a nanorod and a nanohole back reflector. The benefit of a nanorod structured solar cell is the orthogonalization of the light travel path and the carrier transport path. In addition, the nanorod system demonstrates an anti-reflection effect at the top surface and a strong light scattering inside the cell. In this thesis first a wet chemical method for the synthesis of ZnO nanorods on polyethylene naphtalane is described, serving as a base for the metal nanorod back reflector. The growth parameters including reactant concentration, growth time and seed layer thickness on the morphology of ZnO nanorods are investigated and optimized. Secondly, Finite-Difference Time Domain (FDTD) simulations are performed on the nanorod solar cell to study the absorption in the active a-Si:H i-layer. From the simulations, the origin of the absorption is investigated and an optimal nanorod solar cell design is proposed.

The second studied solar cell has a back contact with nanoholes. This is an interesting plasmonic nanostructure, since it can as well propagate surface plasmons and as scattering resonance plasmons can be generated. The absorption of this nanostructured is studied by the use of FDTD simulations. The results give an insight on the origin of the enhanced absorption, by which it can be determined which absorption enhancements are caused by a plasmonic effect.