RGS- and Tri-silicon: Alternative wafer materials in photovoltaics - Characterisation and solar cell processing

The worldwide increase in solar cell production may lead to a shortage of silicon material in the foreseeable future. To overcome this problem, new silicon wafer materials with potentially lower production costs and material losses have been developed. Two of these materials are RGS (Ribbon Growth on Substrate)- and Tri-silicon. This thesis addresses the development of solar cell processes optimised to the specific properties of these two silicon materials. In addition to the development of the solar cell processes, a detailed analysis of the specific properties of both materials is presented.Firstly, a cell process is defined that uses simple process sequences but nevertheless has an eye on high efficiencies. This standard process is then used as a basis for optimisations on the specific properties of both materials. Furthermore, additional process steps and all characterisation methods used in this work are explained briefly.Due to the importance of hydrogen passivation for RGS-silicon, several parameters of an already existing microwave induced remote hydrogen plasma (MIRHP) passivation system are optimised. Theoretical considerations concerning the concentration profile of atomic hydrogen in the reactor show good agreement with experimental results. In addition, the identification of the appropriate distance between plasma and sample and different gas compositions are studied.The first material examined in detail is RGS-silicon. SIMS measurements show the concentration profile of deuteron after MIRDP passivation. Comparing the results with theoretical models leads to the conclusion that the effective diffusion of hydrogen (deuteron) is a combination of Fickian diffusion and hindered diffusion (multiple trapping).To reveal the influence of the high defect density in RGS-silicon on minority charge carriers, a new technique is developed to determine the mobilities of the minorities in silicon materials with low diffusion lengths. The mobilities are shown to be reduced by a factor of 2-3 compared to CZ silicon.By implementing the H-passivation and a front surface texturing in the solar cell process and by depositing a double antireflection coating (DARC) on the front surface, a new record efficiency of 13.2% was obtained for a 4 cm² solar cell. This is the first time ever that an efficiency over 13% was achieved with RGS-silicon material.The second material dealt with in this thesis is Tri-silicon (Tri-Si). A breakage experiment is designed and reveals a higher stability as well as a higher stiffness for Tri-Si compared to CZ silicon, so that thinner wafers can be used for solar cell production without losses in yield. However, after applying an acidic front surface texturing this advantage vanishes.Before optimising the cell process for Tri-Si, simulations are performed that demonstrate a high dependency of the efficiency of a Tri-Si solar cell on the recombination rate at the back surface. Therefore, an aluminium back surface field is implemented, based on screen printed aluminium used in industrial processes rather than evaporated aluminium used in the standard process. Additionally, the galvanisation of the front grid and a front surface texture are implemented in the cell process. After applying a DARC, record efficiencies of 18.8% with a cell area of 4 cm² were reached