Abstract
Microcrystalline silicon (μc-Si:H) is a material that is promising for application in solar cells and that has interesting material properties. This thesis reports on the study of the plasma properties in the growth process, the optoelectronic material properties, and the device application of both doped and intrinsic μc-Si:H.
Microcrystalline silicon
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p- and n-type doped layers have been developed by layer-by-layer (LbL) deposition using very high frequency chemical vapor deposition (VHF PECVD) at high temperature. The LbL deposition consists of alternating amorphous silicon depositions and crystallization by hydrogen plasma treatments. Device quality μc-Si:H p- and n-type doped thin layers have been developed in a temperature range from 250 to 400 ºC, whereas in standard continuous PECVD the addition of a dopant gas reduces the crystallinity. Etching, abstraction and hydrogen diffusion are analyzed and it is concluded that our observations support the nucleation model that is based on hydrogen diffusion in the growth zone. Test solar cells demonstrate the temperature stability and the resistance to high atomic hydrogen fluxes of these μc-Si:H doped layers.
Plasma conditions involving high pressures and high RF powers (high-pressure depletion (HPD) regime) for high-rate deposition by VHF PECVD of μc-Si:H are explored by studying the optical emission spectra (OES) of the plasma. With an adapted electrode configuration with a shower head gas inlet and a small electrode distance, μc-Si:H can be deposited at a high rate. Correlations between the material properties and the optical emission spectra of the plasma were found. Source gas depletion is found to be favorable to reduce the loss of atomic hydrogen to abstraction reactions in the gas phase at high pressures.
The optoelectronic properties of microcrystalline silicon layers with different crystalline fractions have been studied with the aim to develop an intrinsic material for i-layer application in p-i-n solar cells. It is observed that μc-Si:H deposited close to the transition to a-Si:H shows the best intrinsic behavior and the highest photo-to-dark conductivity ratio. At higher crystalline fractions, the dark conductivity increases while the dark conductivity activation energy decreases. The oxygen concentration in the layers is around 1019 cm-3.
Microcrystalline silicon based p-i-n solar cells are deposited by VHF PECVD under HPD conditions using a shower head cathode. The i-layers made near the transition from amorphous to crystalline are optimized in solar cells. It was found that the performance is very sensitive to the TCO morphology. At an i-layer deposition rate of 0.45 nm/s an efficiency of 9.9% is obtained (Voc = 0.52 V, FF = 0.73) on texture-etched ZnO:Al; the performance is stable under light soaking. In spite of the presence of oxygen contamination a good infrared response is obtained. The i-layer deposition rate was increased up to 4.5 nm/s by increasing the RF power and the total gas flow such that the depletion condition remains similar. This resulted in an efficiency of 6.4% on texture-etched ZnO:Al. It was observed that the performance of cells deposited at these high rates improves upon light soaking.
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