Improvement of a-Si:H devices by analysis, simulations and experiment

Abstract

Due increasing energy use per capita and increasing world population, the world faces an energy-crisis. However, the energy required to solve it is abundantly available: solar energy. The technology to convert this energy to electricity, solar cell technology, is well-developed. The technical issue is how to manufacture solar cells at such a large scale that they can produce a significant part of the global energy demand. For that purpose the cost of solar electricity must be reduced. Solar cells based on hydrogenated amorphous silicon (a-Si:H) offer the potential of low-cost solar electricity, mainly because of two reasons. Firstly, a-Si:H solar cells are manufactured at low temperatures, thus saving energy. Secondly, the costs for raw materials are significantly reduced as the solar cells are very thin (C 300 nm). Cost can further be reduced by enhancing the conversion efficiency. That is the topic of this thesis. Before we can study how to improve the performance, first we require a good understanding of the loss factors in a-Si:H solar cells. In chapter 2 we perform a systematic study of the electrical losses in the a-Si:H material itself, i.e. the recombination of charge carriers, and investigate the effect of different recombination paths on the theoretical maximal performance of a solar cell based on a-Si:H. We find that recombination processes limit the open circuit voltage of these solar cells. In actual a-Si:H solar cells, the recombination at the p-i interface is a significant loss-factor. Simulations in chapter 3 show that the charge carrier density at the pi interface is very high. By inserting a thin, high band-gap material with suitable electrical properties at this interface we show that the charge carrier density and consequently the losses at this interface can be reduced, leading to a higher open circuit voltage and efficiency of the solar cell. In the second part of chapter 3 we search for a material that has these properties by experiments on proto-crystalline silicon (pc-Si:H), hydrogenated amorphous silicon carbide (a-SiC:H) and hydrogenated amorphous silicon nitride (a-SiN:H). Consecutively, these materials are tested in solar cells. We find that a-SiN:H is unsuitable and that a-SiC:H gives better performance than pc-Si:H. In the final chapter we optimize the deposition process of a-Si:H material for use in an industrial flexible solar cell product. For that reason a material study is performed. The relations between the measurements of the hydrogen content, band gap, Urbach energy and defect density are explained in terms of a novel model for a-Si:H, the disordered network with hydrogenated vacancies (DNHV). Next lab-scale solar cells on glass substrate have been optimized. The results are transferred to the industrial proces on flexible substrate.