Hard X-ray photoelectron spectroscopy (HAXPES) is used to identify chemical interactions (such as elemental redistribution) at the buried silicon/aluminum-doped zinc oxide thin-film solar cell interface. Expanding our study of the interfacial oxidation of silicon upon its solid-phase crystallization (SPC), in which we found zinc oxide to be the source of oxygen, in this investigation we address chemical interaction processes involving zinc and aluminum. In particular, we observe an increase of zinc- and aluminum-related HAXPES signals after SPC of the deposited amorphous silicon thin films. Quantitative analysis suggests an elemental redistribution in the proximity of the silicon/aluminum-doped zinc oxide interface - more pronounced for aluminum than for zinc - as explanation. Based on these insights the complex chemical interface structure is discussed. (C) 2013 Elsevier B.V. All rights reserved.

Polycrystalline silicon (poly-Si) thin-films have the potential to overcome the limits of today's silicon thin-film solar cell technology because of the chance to grow high-quality material by low-cost deposition techniques. As poly-Si thin-film fabrication is independent of rather slow and costly PECVD processes, it is possible to switch to physical high-rate deposition methods such as electron-beam (e-beam) evaporation exhibiting a strong cost-saving potential. In this contribution, the challenges and opportunities of two different poly-Si solar cell preparation techniques using e-beam evaporated silicon are investigated: Thermal solid phase crystallization (SPC) of initially amorphous silicon layers, and direct growth of poly-Si films at elevated temperatures >500 °C. For both approaches, attention is given to the interplay between substrate texture, structure of the grown silicon and the final solar cell performance. Poly-Si thin film solar cells with 7.8% conversion efficiency were prepared on a smooth substrate in cooperation with CSG solar, demonstrating the equality of e-beam evaporation as deposition technique compared to PECVD. However since e-beam evaporation leads to a non-conformal deposition of rough surfaces, the implementation of a textured substrate for light trapping is highly desired but still challenging. A different behavior is observed for poly-Si thin films directly grown at higher temperatures. For the best photovoltaic performance, a certain substrate microstructure is even necessary and can be optimized by the use of glass coated by differently textured ZnO films as substrate.

Thermal post deposition treatments are applied to DC-sputtered aluminum-doped zinc oxide (ZnO:Al) films and lead to a significant improvement of the electrical properties. Protective layers of amorphous silicon are used to protect the films from degradation during the high temperature treatment. Annealing for 6 hours at 500°C leads to a carrier mobility of 48cm2/Vs at a carrier concentration of 5.5·1020cm−3. Furthermore, improvements in the optical as well as in the electrical properties are possible at the same time compared to the as-deposited film. This is achieved by carrying out two thermal treatments to the ZnO:Al film, one prior to the capping with the protective layer and one afterwards. A series of samples with different carrier concentrations allows us to draw conclusions on the specific electrical transport properties.

The kinetics of crystal nucleation in high-rate electron beam evaporated amorphous Si for polycrystalline thin film solar cells was systematically studied on SiN and selected ZnO:Al-coated glass substrates with dissimilar surface topographies by employing Raman spectroscopy, transmission electron microscopy, and optical microscopy. The influence of the surface topography of the substrate and the disorder of the deposited amorphous Si could be correlated to the respective characteristics of the transient and steady state regime of the nucleation rate. The steady state nucleation rate Iss, its corresponding activation energy EIss, and consequently the size of the grains in the crystallized Si were found to be governed by the interplay between the surface roughness and the deposition temperature. The steady state nucleation rate Iss increased gradually upon increasing the substrate roughness, while lowering the deposition temperature of the amorphous Si on rough textures resulted in a decline of Iss. The time-lag τ, which represents a distinctive parameter for the transient regime, was only slightly affected by the substrate topography. The deposition temperature, however, had a significant influence on τ, with τ increasing by a factor of 8 upon lowering the deposition temperature from 300 to 200 °C for all substrate topographies. These characteristics could be correlated with the increasing structural disorder of the deposited a-Si upon decreasing the deposition temperature. Based on this analysis, we could determine design rules for the controlled preparation of large-grained poly-Si in minimized processing time on any of the used substrate types by individually adjusting the deposition temperature and implementing nucleation layers.

The chemical structure of the interface between silicon thin films and the transparent conductive oxide ZnO:Al has been investigated by hard x-ray photoelectron spectroscopy. By varying the excitation energy between 2010 and 8040 eV, we were able to probe the Si/ZnO interface buried below 12 nm Si. This allowed for the identification of changes induced by solid phase crystallization (SPC). Based on in-situ SPC annealing experiments, we find clear indications that the formation of Si–O bonds takes place at the expense of Zn–O bonds. Hence, the ZnO:Al acts as the oxygen source for the interfacial Si oxidation.

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