Back reflectors with periodic gratings for light trapping in a-SiGe:H solar cells

In the present study, we introduce a new method for preparing textured reflectors for thin film solar cells: this novel design of textured surfaces increases light trapping in tandem solar cells. The structure under study comprises periodic nanoislands fabricated using self-assembly silica spheres, which are applicable to large area fabrication with a low cost. First we focus this study on the fabrication of textured back reflector and their optical properties. The periodic nanoislands increase the diffuse reflectivity of the back reflector. The optical results are compared to industrial solutions currently used in tandem solar cells. Optical simulation was also performed in order to design the best diffusive textured back reflectors. In a second time we applied this textured back reflector on a-SiGe:H low band gap solar cells. This cell was only selected in order to be a reference for demonstrating the efficiency of textured back reflector in bottom cell (low band gap) of tandem structures. External Quantum Efficiency (EQE) and electrical characteristics (J(V) experiments under AM1.5 illumination) are measured on a-SiGe:H solar cells with an optimized textured back reflector and a standard flat silver reflector. An improvement of 10% in current density with this low cost solution compared to standard silver back reflector is obtained. Moreover, the other electrical parameters are not damaged by applying this process of texturation which leads to a 15% enhancement of the solar cell efficiency.