Metal layer for enhanceng optical reflection in passivation layers in thin-film solar cells

Abstract

Thin film, CIGS solar cells were able to achieve efficiency values higher than multicristalline silicon solar cells (22.9 % vs 22.3 %). However, a possible In scarcity, may increase its production costs, hence decreasing the CIGS thickness is of upmost importance for the technology to stay cost-competitive. However, when decreasing the CIGS thickness two main efficiency limitations arise; rear contact recombination and optical losses due to incomplete light absorption. The main focus of this thesis is to increase the rear optical reflection of an ultrathin CIGS solar cell. For that, six different metals (Ag, Au, Cu, Ta, Ti and Zn) and two metallic alloys (AlSiCu and TiW) were tested as possible candidates for increasing the rear optical reflection and a nanopattern point structure was employed with Al2O3 as the passivation layer. Optical simulations were conducted to verify which metals gives the highest enhancement at increasing the light absorption in the CIGS layer. The solar cells fabricated were heavily affected by the reaction of the metal layer with the CIGS. As such, an alternative solar cell architecture was developed to prevent such reaction by depositing a Mo layer to cover the point contacts. In the end, the solar cell that incorporate a TiW layer, managed to achieve a 3.69 % (absolute) enhancement in efficiency values over the reference solar cell.