Crystalline silicon thin film growth by ECR plasma CVD for solar cells

This thesis describes the background, motivation and work carried out towards this PhD programme entitled 'Crystalline Silicon Thin Film Growth by ECR Plasma CVD for Solar Cells'. The fundamental principles of silicon solar cells are introduced with a review of silicon thin film and bulk solar cells. The development and prospects for thin film silicon solar cells are described. Some results of a modelling study on thin film single crystalline solar cells are given which has been carried out using a commercially available solar cell simulation package (PC-1D). This is followed by a description of thin film deposition techniques. These include Chemical Vapour Deposition (CVD) and Plasma-Assisted CVD (PACVD). The basic theory and technology of the emerging technique of Electron Cyclotron Resonance (ECR) PACVD, which was used in this research, are introduced and the potential advantages summarised. Some of the basic methods of material and cell characterisation are briefly described, together with the work carried out in this research. The growth by ECR PACVD at temperatures < 700 deg. C of undoped, n-type and p-type crystalline silicon films is discussed in detail. The influence of process parameters such as growth temperature, silane to hydrogen gas ratio, microwave power and gas pressure on film properties, for films grown on silicon and a variety of metal layers, was systematically investigated and is described. The transition from microcrystalline to epitaxial growth was studied. N-type and p-type epitaxial growth was demonstrated at temperatures of ∼470 deg. C and ∼680 deg. C, respectively. Also described is silicon nitride growth at low temperatures (230-500 deg. C) by ECR PACVD. The relations between process parameters (e.g. temperature and silane to nitrogen gas ratio) and film index, growth rate and composition were investigated and are outlined. The application of these films as antireflection coatings (ARC) for thin solar cells is demonstrated. These cells were fabricated on ∼ 15μm thick, p-type base layers epitaxially grown on (100) p+ substrates by conventional CVD. Crystalline, n+ emitter layers were then formed by thermal diffusion of phosphorus or by the ECR PACVD process with phosphine as the doping gas. A description of cell performance as a function of emitter doping and microstructure is given. A conversion efficiency of up to 12.99% was achieved for the diffused emitter structures under 100 mW/cm2 illumination. The best efficiency in the ECR grown structures was 13.76% using an epitaxial emitter. Cell performance was analysed in detail and the factors controlling performance identified by fitting self-consistently the fight and dark current-voltage and spectral response data using PC-1D. Finally, the conclusions for this research and suggestions for further work are outlined. (author)