Design of multijunction solar cells based on thin film silicon

Abstract

Multijunction devices based on thin film silicon are useful for a range of applications due to their low costs, better performance at higher temperatures, potential for flexibility, and light weight. One particularly interesting application of such devices is in solar-to-fuel conversion. In light of this, the goal of this thesis was to suggest ways to better design multijunction devices based on thin film silicon. Three main research objectives were identified.
The first objective was the development of a high VOC top junction. Cells based on a-Si and a-SiOX were optimized by varying gas flows during the deposition of the intrinsic absorber layer. A maximum VOC of around 890 mV was obtained from cells based on both absorbers. Based on these results, it was possible to identify unique advantages of using either a-Si or a-SiOX as the top junction. For the second research objective, solar cells based on a-SiGe were studied. The impact of varying the bandgap profile in a-SiGe absorbers on the device performance was investigated with a set of experiments. Subsequently, a set of semi-empirical equations were motivated and fitted to the experimental data. These relations could approximately show how bandgap profiling affects device performance in a more general way. The presented approach may be used to expedite the design of a-SiGe based cells for use in different types of multijunction devices. Finally, a systematic optimization of tunnel recombination junctions (TRJ) was conducted. Experiments were done to study the impact of using different TRJs in two types of tandem devices: a-Si/a-SiGe and a-SiGe/nc-Si. Different features of the TRJ were varied separately: p-doped layer(s) used, thicknesses of the p-doped layer(s), and the n-doped layer(s) used. The best performing p-doped layers was identified among the studied combinations. For the n-layer(s), the experimental data suggested the different roles played by n-a-Si, n-nc-SiOX, and n-nc-Si in the TRJ. This observation could be used to design an optimized combination of n-layers for the TRJ. Collectively, the work done as part of each research objective aims to contribute to the better design of multijunction devices based on thin film silicon alloys.