Design of ZnO:Al films with optimized surface texture for silicon thin-film solar cells

This study addresses the design of radio frequency (rf) magnetron sputtered aluminum doped zinc oxide (ZnO:Al) front contacts for silicon thin-film solar cells. Optimized films comprise high conductivity and transparency, as well as a surface topography trapping the light within the photovoltaically active layers. We have investigated the influence of the doping level of the target as well as the substrate temperature during sputter deposition on the ZnO:Al properties. The aluminum content in the target influences the transmission in the near infrared (NIR), the conductivity as well as the film growth of the ZnO:Al layer. The latter affects the surface topography which develops during wet-chemical etching in diluted hydrochloric acid. Depending on aluminum content in the target and heater temperature three different regimes of etching behavior have been identified. We have applied the ZnO:Al films as front contacts in thin-film silicon solar cells to study their light trapping ability. While high transparency is a prerequisite, the light trapping has been improved using front contacts with a surface topography consisting of relatively uniformly dispersed craters. We have identified low amount of target doping and high substrate temperatures as sputter parameters enabling high cell currents. Short-circuit current densities of up to 26.8 mA/cm(2) have been realized in mu c-Si:H single junction cell with absorber layer thickness of 1.9 mu m.