Photovoltaic applications of Si and Ge thin films deposited by PECVD

Abstract

This thesis represents a systematic study of amorphous silicon microcrystalline silicon and germanium thin films, and a-Si:H thin film solar cells fabricated using an OPT plasma lab 100 RF PECVD system carried out with a view to studying novel lighttrapping structures for thin film solar cells and novel IR photovoltaic cells. The work includes the optimisation of amorphous based single layers by optical and electrical characterisation, their doping and the fabrication and optimisation of single junction solar cells. These developments were extended to include deposition of microcrystalline and germanium films with the aim of developing a range of multijunction and single junction research devices. The optical characterisation of intrinsic amorphous based layers shows that device-grade layer fabrications are achievable with more than 90% absorption in the 450 to 550 nm wavelength range which can be deposited at 4-5?A/s with good thickness uniformity. The bandgap of intrinsic amorphous layers can be tuned from 1.4 to 1.7 eV. Secondary Ion Mass Spectroscopy (SIMS) depth profile characterisation has verified that doping levels in p-type and n type are in the range of 1021 atoms/cm3 which can maintain high open circuit voltage of 0.83V in the single junction device. Systematic single layers well as in-device optimisations lead to the best single junction devices fabricated at a temperature of 250oC and at a pressure of 350 mT and of initially 8.22% efficiency. Initial quantum efficiency (QE) measurements show 75% photon absorption at 550 nm wavelength. A novel technique of wavelength and angle resolved scattering (WARS) measurements have been used to analyse the effects of textured TCOs on light-trapping in single junction device. Showing Asahi-U to be the best substrate with regards to light-trapping, although thicker films benefited more from TEC8 . Deposition of microcrystalline silicon and germanium have also been reported. More focus has been given to optical characterisation of germanium films whose absorption has reached more than 70% in 400 to 1600 nm wavelength range with successful n-type and p-type doping. Ohmic contacts with a low resistivity of 0.029-cm for p-type Ge with the usage of Ni have been achieved. The overall working capacity of the OPT PECVD tool has been analysed and it was concluded that chamber design modification are essential for the system to work in a multidisciplinary field to avoid serious chamber contamination and 10% efficient a:Si:H benchmarks.

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