Study On Wide Band-gap Amorphous Silicon Oxide Materials And Solar Cells

The results from theoretical modeling show that it is necessary to develop wide band-gap materials as the absorber layer in the top cell to improve the open-circuit voltage (Voc) and then the conversion efficiency of silicon based tandem solar cells. The amorphous hydrogenated silicon oxide (a-SiOx:H) is one of the most promising candidates. In this thesis, the systematic studies on the properties of a-SiOx:H and its application as the absorber in solar cells have been performed by plasma enhanced chemical vapor deposition (PECVD) methods using CO2as the oxygen source.Firstly, as the wide band-gap absorber in the solar cells, the increase of band gap while maintaining the relatively low defect densities in a-SiOx:H materials is mainly focused. It shows that the high ratio of CO2/SiH4(fo=[CO2]/[SiH4]), low SiH4-CO2concentration (SOC=([SiH4]+[CO2])/([H2]+[SiH4]+[CO2]), high power density and low deposition pressure could improve the oxygen and hydrogen concentration in the a-SiOx:H materials, and then lead to higher optical band gap. Among them, the fo plays the most important role on the band gap of the a-SiOx:H materials. By analyzing the structural and electrical properties of a-SiOx:H materials, it indicates that the variation of the microstructural factor R*(the ratio of2100cm-1to the sum of all Si-H stretching modes) in the FTIR spectra shows the similar trend as that of the defect densities in the a-SiOx:H materials depending on the variation of deposition parameters. The high deposition temperature, suitable deposition pressure and the SOC in the range of7%-10%are beneficial to reduce the defect densities of a-SiOx:H materials. But the increase of oxygen concentration causes inevitable increase of defect densities in the materials. Especially, the doping effects of oxygen is observed even when the oxygen concentration is relatively high and the improvement of the optical gap is the dominate effect of oxygen. This n type doping effect of oxygen strongly affects the electrical properties of a-SiOx:H materials, such as the dark and photo-conductivities and the mobility-lifetime products of electrons. With increasing the fo, the doping effect of oxygen firstly increases and then decreases. This doping effect could also be reduced by a low SOC. Secondly, the effects of oxygen incorporation on the cell performance with a-SiOA:H absorber layers have been studied. It shows the Voc cannot be improved by the wide band-gap absorber layers with the insufficiently good doped layers. The short circuit current density (Jsc) and the fill factor (FF) of the cells are degraded by the incorporation of oxygen in the absorber layer. The n-type nature of a-SiOx:H layers has been proved on the device level. The hole transport properties show much stronger degradation than that of the electrons with increasing the oxygen concentration in the a-SiOx:H absorber. It is believed to be the main reason for the deterioration of the cell performance. This n-type nature of a-SiO/.H layers also reforms the electrical field in the cell. The enhanced electrical field near the p/i interface improves the external quantum efficiency (EQE) in the short wavelength region. Meanwhile, the Voc of the cells is more sensitive to the recombination at the p/i interface. By increasing the band gap or the thickness of a-Si:H buffer layer, the Voc can be improved because of the reduced recombination at the p/i interface. On the other hand, the property at the i/n interface has little effect on Voc.Thirdly, the doped layers used have been studied and optimized to improve the Voc of the a-SiOx:H cells. A parameter of AEp=Egp-Eap has been proposed and effectively confirmed to estimate the effect of p layer on the built-in potential and Voc. In contrast, the Voc is not sensitive to the conductivity of n layer. Compared with the results from computer simulation, it is concluded that the Voc of a-SiOx:H cell is limited by the insufficient built-in potential provided by the non-ideal doped layers. Additionally, the effects of the properties of the p layer on Jsc, FF and the recombination at the p/i interface in a-SiOx:H cells have been also discussed in detail. Finally, without the ZnO back reflector, the single junction a-SiO,::H cell (with the thickness of absorber layer of200nm) with the best Voc of951mV (η=6.69%,Jsc=10.9mA/cm2, FF=64.7%) and the best efficiency of7.86%(Voc=940mV, Jsc=12.8mA/cm2, FF=65.2%) have been obtained respectively.Finally, the a-SiOx:H cells has been used as the top cell in the a-SiOA:H/μc-Si tandem cells. Compared to the traditional a-Si:H/μc-Si tandem cells, the Voc is improved and the re-arrangement of EQE response of the tandem cell is realized. The EQE response in the short wavelength region and the integrated current over the whole spectrum is enhanced. In the end, the a-SiOx:H/μc-Si with the best efficiency of9.9%(V0C=1423mV, Jsc=9.86mA/cm2, FF=70.6%) has been obtained without the ZnO back reflector. It also indicates the good potential of usage in triple junction solar cells.