Study of nano-crystalline silicon deposited by vhf-pecvd for solar cell devices
Sharma, Puneet / PS (2005) Study of nano-crystalline silicon deposited by vhf-pecvd for solar cell devices. PhD thesis, Iowa State University.
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Nanocrystalline Silicon (nc-Si: H) is an important material for photovoltaic energy conversion devices and for thin film transistors. The material consists of small grains of Si, of the order of 10-20 nm, with a significant amorphous phase and bonded Hydrogen (H) interspersed between the grains which through passivation leads to good electronic transport in the device. In this work, a systematic study is carried out of midgap defect densities and minority carrier diffusion lengths in Nanocrystalline p+nn+ devices by changing the doping and defect densities with different techniques. The devices and films were deposited in a VHF diode plasma discharge with varying deposition parameters. Defect densities were measured and the energetic location of the traps was found to be ~0.5 to 0.35 eV below the conduction band. A surprising finding was that there was a 1:1 correlation between the deep defect states which were responsible for carrier capture, and the donor density. The diffusion length and its excellent correlation with the deep defect density, verified SRH recombination model. Excellent hole and electron mobilities from n+nn+ devices using SCLC technique were obtained (some measurements by other students) with good lifetimes from reverse recovery technique. Linear relationship between hole lifetimes against the inverse of the defect densities measured showed approximate linear correlation, thus verifying the SRH recombination model for very first time. Also Dark I-V current studies were done and their relationship with crystallinity was studied, backed by QE measurements. Two techniques of graded TMB and H2 were used to improve hole transport in these devices. A final device innovation was the use of a thin a-Si: H buffer layer at n-p+ interface which significantly improved the Voc, presumably by reducing the recombination at the interface states. Thus, important fundamental material properties, electron and hole mobility, defect density, minority carrier lifetime and minority carrier diffusion length, capture coefficients were all measured in the same or very similar samples. This is the first time ever that such systematic measurements have been made in this material system.
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