Boron-doped Hydrogenated Nanocrystalline Silicon Prepared By HWCVD And Enhanced Spectra Response Of Silicon Thin Film Solar Cells By Ag Nanoparticles

Silicon thin film solar cells mainly include hydrogenated amorphous silicon (a-Si:H) solar cells, hydrogenated nanocrystalline silicon (nc-Si:H) solar cells and tandem or triple junction solar cells made of a-Si:H and nc-Si:H. Because of abundant raw material, less material and energy consumed, non-toxic, low cost and easy to deposit in large area, there is a great potential to realize the mass production of silicon thin film solar cells. This thesis contains two aspects of contents, one is boron doped hydrogenated nanocrystalline silicon prepared by hot-wire chemical vapor deposition (HWCVD) and another is enhanced spectra response of silicon thin film solar cells by Ag nanoparticles.In silicon thin film solar cells, it is crucial to have highly conductive p-type nc-Si:H with high crystallinity to improve the cell efficiency. Compared with common plasma enhanced chemical vapor deposition (PECVD), HWCVD appears to be a promising deposition method because of absence of powder formation and ion bombardment. In recent years, there have been a great of fruits of intrinsic layer deposited by HWCVD in silicon thin film solar cells. While there are few and short of systematical researches on boron-doped nc-Si:H prepared by HWCVD. In this thesis, it is successful to obtain boron-doped silicon thin films transited from a-Si:H to highly crystallized nc-Si:H by widely adjusting the deposition parameters of HWCVD. The microstructural, electrical, doping properties of the samples and the relationship of them are systemically investigated by Raman spectra, infrared absorption spectra, and special precise Hall effects measurements and second ion mass spectra (SIMS). It is showing that the highest conductive boron-doped nc-Si:H don't have commonly believed the highest crystallinity, but have moderate crystalline volume fraction. Second, HWCVD realized the boron-doped nc-Si:H with high crystallinity, high boron concentration, high doping efficiency and high carrier concentration which is a long time existed problem of the common PECVD method. Finally, the p-type nc-Si:H deposited by HWCVD other than PECVD can have higher conductivity. Theses results exhibit the merits of HWCVD compared with the common PECVD in preparing boron-doped nc-Si:H with high cyrstallinity and high conductivity. This work has instructive significance and application value to further improve the efficiency of silicon thin film solar cells.On the other hand, in order to improve the conversion efficiency of silicon thin film solar cells it is very important to adopt a suitable light trapping method to enhance the light absorption because of low absorption coefficiency of a-Si:H and nc-Si:H in long wavelengths range and limited thickness of solar cells. A common used light trapping method is texturing transparent conductive oxide (TCO) which makes the incident light scattered at the interface between TCO and silicon film. In this case the light path is prolonged inside solar cells, and thereby the light absorption is enhanced. Recently, it has been reported a novel enhancing light absorption method by small (the diameter smaller than 100 nm) metallic nanoparticles which originates from the light induced metallic nanoparticles localized surface plasmon resonance (LSPR). This novel method was applied in organic and dye-sensitized solar cells and the spectra response were corresponding enhanced. However, it was not observed enhanced spectra response in silicon thin film solar cells by metallic nanoparticels even though a huge improved light absorption and surface enhanced Raman scattering were reported. In this thesis, small Ag nanoparticles are prepared by thermal evaporation which is easy to deposit in large area. And it is observed enhanced red and near-infrared spectra responses when the Ag nanoparticles are innovatively integrated in special structural a-Si:H solar cells. Meanwhile, it is deeply discussed the mechanism and influence factors of enhancing spectra responses of silicon thin film solar cells by small Ag nanoparticles. This work establishes fundaments of enhancing light absorption and spectra response for standard silicon thin film solar cells. It has important significance in theory and practice.Besides enhanced light absorption by small metallic nanoparticles, large metallic nanoparticles (diameter larger than 100 nm) have an attractive LSPR enhancing light scattering effect. Different from geometry light scattering the light scattering of large metallic nanoparticles are wavelength dependent. It has been reported that light absorption and spectra response were enhanced by employing large metallic nanoparticles on top of crystalline silicon and a-Si:H solar cells. In this thesis, large Ag nanoparticles and Ag nanostructure (large Ag nanoparticles connected to each other) prepared by thermal evaporation are integrated inside n-i-p nc-Si:H and a-Si:H solar cells, and the light absorption and spectra response in long wavelengths are distinctly enhanced. Results show that there are surface plasmon resonance light absorption losses of nanoparticles and nanostructure when they directly connect with n-layer. Such light absorption losses can be restrained by covering a thin layer of medium, which has smaller refractive index in comparison to silicon film, on tope of nanoparticles and nanostructure to blue shift resonance absorption losses. This work provides a new way and method to increase photocurrent and reduce light absorption losses in silicon thin film solar cells. It has significant application value.