Optimization Investigation On Nanocrystalline Silicon Thin Film Solar Cells

Solar cell based-silicon has been dominated in the rapidly growing photovoltaic technology field. In particular, monocrystalline silicon solar cells and polycrystalline silicon solar cells are known as mainstream products of solar cells. But due to their high cost and their material shortage, they cannot meet the needs of the rapid economic and social development. At present, silicon thin film solar cells which are called as the second generation silicon solar cells have been becoming the mainstream product of application and development of solar cells. Because amorphous silicon thin film solar cells, polycrystalline silicon thin film solar cell and microcrystalline silicon thin film solar cells are unlikely to have "multiple stimulating" effect, so they cannot break through the solar cells Shockley Queisser limit efficiency (32.8%) in theory and practice. With the rise and development of the nano science and technology and nano photonics technology, it is recognized that using nano materials which have many unique properties to design and develop solar cells is the only way to break through the battery bottleneck of the solar cells with "low conversion efficiency, high manufacturing costs, short life and low stability". Although a lot of the investigations on nanocrystalline silicon thin films have been carried out, the consolidate cognition is not established on the deposition mechanism, microstructure, optical property, electrical property and mechanical property of nanocrystalline silicon thin films, which seriously hinders the research and development of nanocrystalline silicon thin films and its solar cells.The theoretical investigation and experimental investigation on the main window layer and the light absorption layer of nanocrystalline silicon thin film solar cell are carried out based silicon films materials. In this paper, the plasma chemical vapor deposition system is improved by the direct current bias and a series of nanocrystalline silicon films have been prepared with different paraments. And the deposition mechanism is discussed systematically. The surface morphology and micro structural features of these films were investigated by Atomic Force Microscopy, Raman spectroscopy and X-ray diffraction respectively; the configuration element of intrinsic nanocrystalline silicon films were studied by Fourier transform Infrared Spectroscopy; their optical, electrical and mechanical characteristics were gained by Transmission spectroscopy, Four probe resistance tester and nano Indentation systematically. The main research contents and research results are included as followed:Firstly, it is found that the nucleation and crystalline process of the reactive group during the normal growth of nanocrystalline silicon thin film is restrained by boron doped, as a result, there is a large of incomplete fine silicon grains formed in films. With increasing the concentration ratio of borane and silane from 2% to 5%, the average grain size and crystalline volume fraction of B-doped nanocrystalline silicon thin film decrease 40.% and 4.7 nm to 32.0% and 2.3 nm respectively, both them showed the downward trend and they indicate that the microstructure transited from crystalline state to amorphous state. After annealing treatment, the grain distribution in films becomes more orderly, and these films showed high conductivity and wide optical band gap properties. Therefore, B-doped nanocrystalline silicon film will be a kind of alternative materials, which used as the window layer of silicon film solar cells.Secondly, based on the characteristics of plasma chemical vapor deposition technology, the effects of the substrate temperature, radio frequency power, bias current bias voltage and the silane concentration on the average grain size and the crystalline volume fraction of intrinsic nanocrystalline silicon films were analyzed. Hydrogen atoms can be saturated dangling bonds in films and they will form Si-H bonds with silicon atoms. The density, the bond distortion and the number of dangling bonds decrease with the increase of the crystalline volume fraction, resulting part of the band tail states and the band gap states are transformed into extended states or disappear. And the change of number of Si-H bond and the different level effluence of hydrogen atoms is caused by the change of the crystalline volume fraction, further, leading to that the optical band gap showed a downward trend with the increase of the crystalline volume fraction.In addition, because Si-Si bond in films is affected by hydrogen atoms, causing that the optical band gap transformed from indirect band gap into direct band gap. This indicates that there is a closely relation between the optical band gap and Si-H bond model.Thirdly, there are kinds of Si-H bond model in nanocrystalline silicon films, these Si-H band models correspond to different vibrational mode peaks of Fourier transform transmission spectra. As the contribution of SiH, SiH2 bonding mode are only considered in the bimodal Gaussian fitting method, while S1H3 bonding mode is ignored, there is a certain error when hydrogen content, the structure factor and force constant of nanocrystalline silicon film are calculated. Then in this paper, three peaks Gaussian fitting method is proposed. Based on three peaks Gaussian fitting method, the impact of the chemical composition on the optical band gap is analyzed and the reasons were concluded.Fourthly, the optical model of nanocrystalline silicon is established by the equivalent theory. According this model, the absorption coefficient and the optical band gap are calculated. The calculated results are compared with the experimental results, and some reasonable explanations for their difference are given. In theory, an analysis expression between the crystalline volume fraction, the average grain size and the optical band gap is concluded.Finally, hydrogen dilution ratio is one of the key factors, which affects the surface morphology and mechanical properties. If we properly control hydrogen dilution ratio, it would promote the nucleation rate, and after the mobile hydrogen atoms etched silicon atom in the film surface, the rigid crystalline structure formed, therefore, it could reduce the film roughness and improve the elastic modus and hardness values as well, effectively. Nanocrystalline silicon film growth process is also a continuous improvement process of the crystalline volume fraction. Therefore, the greater the depth of the internal pressure into film is, the smaller the elastic modulus and the hardness values that we got are.The above investigations not only make a consolidated cognition on the deposition mechanism, microstructure, optical properties, electrical properties and mechanical properties, but also provided theoretical foundation and basic data for the application of nanocrystalline silicon solar cells.