Preparation Of Si-based Films By Hot-wire CVD And Simulation Of Related Solar Cells

Si-based films have good optical and electrical properties. Bandgap of them could be adjusted by changing depositing paremeters. These advantages make them one of the focuses of the third generation tandem solar cells. Many Si-based films and devices prepared by Hot-wire CVD have shown better properties compared with those deposited by PECVD. But, as a new method, there are still many unknown things about hot-wire CVD that need to be found out and concluded. The main purpose of this thesis is to study the effects of process parameters on properties of Si-based films prepared by hot-wire CVD and to simulate and optimize related Solar cells. The main content and conclusions are as follows:For Si films, effects of the process parameters, such as, hot-wire temperature, ratios of source gases flow, depositing pressure and substrate type on properties of intrinsic, p- and n-Si films have been studied. According to the results, different kinds of silicon films (such as amorphous silicon, micro-crystallized silicon, et al) could be designed and prepared for solar cells. Highly doped p- and n-Si with the lowest resistanceρ=0.036Ω·cm and 0.064Ω·cm have been prepared. High quality epitaxial silicon film on Si(111) substrate has been deposited at low substrate temperature T_s=200℃.For borone-doped p-Ge films, with the ratio of B₂H₆/GeH₄ (R_B/G) increasing, their crystallization quality declines while Ge(220) preferential orientation is enhanced. Resistivity of the films decreases with R_B/G increasing. Crystallization quality is enhanced while the doping efficiency declines with hydrogen dilution increasing. High quality epitaxial Ge film is prepared on Si(111) substrate and the film with the Raman peak at 300.3cm^-1 is (220) preferrentially orientated. For phosphor-doped n-Ge films, the conductivity of the films increases with the ratio of PH3/GeH4 or hydrogen dilution ratio decreasing. The lowest resistivity of the p- and n-Ge films prepared are 0.024Ω·cm and 0.10Ω·cm, respectively.Hot-wire temperature (signified by hot-wire current, I_w) and SiH4/GeH4 ratio (R_S/G) both have great effects on properties of SiGe films. With I_w increasing, the deposition rate increases almost linearly and the relative content of Ge-Si bond increases while relative contents of the hydrogen bonds (Ge-H, Si-H, et al) have no obvious changes. The bandgap of the films increases to 1.39eV at I_w =24A and then declines. The increase of R_S/G is beneficial to the formation of Ge-Si, Si-Si and Si-H in the films, compared with the formation of Ge-Ge, Ge-H and Ge-H₂. The relationship between bandgap and R_S/G is indicated as the function: E_g=0.19R_S/G+1.14. With hydrogen dilition ratio increasing, the films are better crystallized, and proportions of SiH increases while proportions of the other hydrogen bonds decrease, and Eg of the films decreases.One-dimensional model of solar cell with pin structure is used to study the effects of the properties of each layer on the performance of the device. Analytical solution of the relationship between the performance of the device and the parameters of each layer is deduced from Poisson's equations, current equations and transport equations. For single junction solar cell, thinner doped layers are beneficial to improve the performance of the device. The thickness of the i-layer and concentration of the carrier in p- and n-layers should be set in a suitable range and the limit for different materials is different.For double junction tandem solar cells, it is suggested that the bandgaps of both sub-cell matched each other is very important to improve the performance of the device. Performance of the tandem solar cell composed with different bandgap materials is better than that of the single junction solar cell composed with the same thickness and one of the same materials. Tandem solar cells with sub-cells composed of the same material also have higher fill-factor and transform efficiency than the single junction solar cells with the same material and thickness. The conclusion that tandem structure is beneficial to enhance the performance of the solar cells has also been proved by the experimental study on solar cells.