The effect of front ZnO:Al surface texture and optical transparency on efficient light trapping in silicon thin-film solar cells

This study addresses the material properties of magnetron-sputtered aluminum-doped zinc oxide (ZnO:Al) films and their application as front contacts in silicon thin-film solar cells. Optimized films exhibit high conductivity and transparency, as well as a surface topography with adapted light-scattering properties to induce efficient light trapping in silicon thin-film solar cells. We investigated the influence on the ZnO:Al properties of the amount of alumina in the target as well as the substrate temperature during sputter deposition. The alumina content in the target influences the carrier concentration leading to different conductivity and free carrier absorption in the near infrared. Additionally, a distinct influence on the film growth of the ZnO:Al layer was found. The latter affects the surface topography which develops during wet-chemical etching in diluted hydrochloric acid. Depending on alumina content in the target and heater temperature, three different regimes of etching behavior have been identified. Low amounts of target doping and low heater temperatures result in small and irregular features in the postetching surface topography, which does not scatter the light efficiently. At higher substrate temperatures and target doping levels, more regularly distributed craters evolve with mean opening angles between 120° and 135° and lateral sizes of 1–3μm. These layers are very effective in light scattering. In the third regime—at very high substrate temperatures and high doping levels—the postetching surface is rather flat and almost no light scattering is observed. We 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, light trapping was improved by using front contacts with a surface topography consisting of relatively uniformly dispersed craters. We have identified a low amount of target doping (0.5–1wt%) and relatively high substrate temperatures (about 350–450°C as sputter parameters enabling short-circuit current densities as high as 26.8mA∕cm2 in μc-Si:H pin cells with an i-layer thickness of 1.9μm. Limitations on further improvements of light-trapping ability are discussed in comparison with the theoretical limitations and Monte Carlo simulations presented in the literature.