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 $1–3\mu \mathrm{m}$ . 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 $(0.5–1\mathrm{wt}\%)$ and relatively high substrate temperatures (about $350–450°\mathrm{C}$ as sputter parameters enabling short-circuit current densities as high as $26.8\mathrm{mA}∕{\mathrm{cm}}^2$ in $\mu c\text{-}\mathrm{Si}:\mathrm{H}$ pin cells with an $i$ -layer thickness of $1.9\mu \mathrm{m}$ . Limitations on further improvements of light-trapping ability are discussed in comparison with the theoretical limitations and Monte Carlo simulations presented in the literature.

This study addresses the electrical and optical properties of radio frequency magnetron sputtered aluminum doped zinc oxide (ZnO:Al) films. The main focus was on the improvement in carrier mobility μ to achieve simultaneously high transparency for visible and particularly near-infrared light and low resistivity. The influence of Al concentration in the target, film thickness, sputter power, deposition pressure, and substrate temperature on material properties was investigated. The structural, compositional, electrical and optical properties were studied using x-ray diffraction, secondary ion mass spectrometry (SIMS), room temperature Hall effect measurements and spectral photometry, respectively. All ZnO:Al films were polycrystalline and preferentially oriented along [002]. The grain size along the direction of growth increased with higher Al doping and with increasing film thickness. The SIMS measurements revealed that the Al concentration in the film was nearly the same as in the target. Carrier concentration $N$ and mobility μ are determined by the target Al concentration. In addition μ is influenced by the film thickness and the sputter pressure. For each Al concentration, the highest μ was generally observed at low deposition pressures. By using a target with low ${\mathrm{Al}}_2{\mathrm{O}}_3$ concentration of 0.5 wt %, μ could be improved up to $\text{44.2\hspace{0.17em}}{\mathrm{cm}}^2/\mathrm{V s}$ while maintaining the electrical resistivity ρ as low as ${\text{3.8×10}}^{\text{−4}}\mathrm{\hspace{0.17em}\Omega }\mathrm{cm}.$ For these films the transparency in the near-infrared wavelength range strongly improved which makes them particularly interesting for the application in optoelectronic devices like thin-film solar cells. The $\mathrm{\mu -N}$ dependence for films deposited under diverse conditions was studied to identify a practical limit for μ.

A postdeposition thermal treatment has been applied to sputtered Al-doped zinc oxide films and shown to strongly decrease the resistivity of the films. While high temperature annealing usually leads to deterioration of electrical transport properties, a silicon capping layer successfully prevented the degradation of carrier concentration during the annealing step. The effect of annealing time and temperature has been studied in detail. A mobility increase from values of around $40{\text{cm}}^2/\text{V}\text{s}$ up to $67{\text{cm}}^2/\text{V}\text{s}$ , resulting in a resistivity of $1.4\times {10}^{-4}\Omega \text{cm}$ has been obtained for annealing at temperatures of $650°\text{C}$ . The high mobility increase is most likely obtained by reduced grain boundary scattering. Changes in carrier concentration in the films caused by the thermal treatment are the result of two competing processes. For short annealing procedures we observed an increase in carrier concentration that we attribute to hydrogen diffusing into the zinc oxide film from a silicon nitride barrier layer between the zinc oxide and the glass substrate and the silicon capping layer on top of the zinc oxide. Both are hydrogen-rich if deposited by plasma-enhanced chemical vapor deposition. For longer annealing times a decrease in carrier concentration can occur if a thin capping layer is used. This can be explained by the deteriorating effect of oxygen during thermal treatments which is well known from annealing of uncapped zinc oxide films. The reduction in carrier concentration can be prevented by the use of capping layers with thicknesses of 40 nm or more.