Structural and electronic properties of nanocrystalline silicon thin film solar cells fabricated by hot wire and ECR-plasma CVD techniques
MUTHUKRISHNAN, KAMAL KUMAR (2007) Structural and electronic properties of nanocrystalline silicon thin film solar cells fabricated by hot wire and ECR-plasma CVD techniques. PhD thesis, Iowa State University.
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Nanocrystalline silicon has become the material of interest recently, for solar cell applications and also in the fabrication of thin film transistors. The material contains crystalline grains surrounded by amorphous tissues and when used as intrinsic layer in solar cell devices, greatly enhances the device stability against the light induced degradation which is a critical problem with amorphous silicon solar cells. The conventional PECVD techniques used for the deposition of high efficiency devices have a major drawback of very low growth rates. This project deals with a systematic study of structural and electronic properties of nanocrystalline Si:H films and devices fabricated using a relatively new technique called the Hot Wire CVD (HWCVD). In addition, we study the influence of ions on the crystalline ratio, grain size and orientation of the nanocrystalline films. Our apparatus allows us to add plasma ions separately from the primary growth process, which is growth using only radicals that are generated by the thermal dissociation of silane and hydrogen at the hot wire. In this way, we have also deposited the first ever nanocrystalline silicon solar cells by the combined HWCVD and Electron Cyclotron Resonance (ECR) PECVD technique. While hot wire deposition of nanocrsytalline Si:H has been studied in the past, virtually all previous work utilized a close-proximity hot wire deposition condition that creates a varying temperature profile during deposition because of the intense heating of the growing film due to radiation from the filament. In contrast, in this work, we use a remote filament to minimize sample heating, a conclusion verified by experimental measurements of surface temperatures during growth conditions. We have found that low energy ion bombardment, by either inert (helium) or reactive (hydrogen) ions significantly helps in crystallization of the film. We also systematically study the influence of hydrogen dilution on grain size and grain orientation of the film. It is found that higher hydrogen dilution suppresses the <220> grains and leads to more random nucleation. It is found that <220> orientation is the thermodynamically preferred growth direction and <111> grains are created due to random nucleation which is enhanced by increasing the ion bombardment from the plasma source. We have also studied the fragmentation pattern of silane in ECR PECVD using a quadruple mass spectrometer. The study revealed the dominant radicals in both nc-Si and a-Si depositions for varying power and chamber pressures. In the second part of this work, we focus primarily on the fabrication and analysis of the electronic properties of solar cells using nanocrystalline intrinsic layers. Apart from measuring the regular I-V characteristics and quantum efficiency, we investigate the critical device properties such as the defect densities in the intrinsic layer and the diffusion length of the minority carriers. By correlating the device results with the structural properties of the films, we are able to conclude that the maximum diffusion length and the minimum defect density can only be attained by depositing the intrinsic layers that are close to the transition to amorphous phase. Although few studies have been done on this transition regime of the deposited films, most of them have concentrated only on the film properties such as conductivity ratios and crystalline fractions. This work clearly describes why transition region is ideal for the fabrication of high efficiency solar cells and what are the critical deposition parameters that are involved in their design.
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