Research On Key Technology Of Poly-Si And Cdte Thin Film Photovoltaic Material

Photovoltaics has the potential to become a major source of energy and to have a significant and beneficial effect on the global environment. In order for photovoltaics to realize that potential, PV must become standardized products that are inexpensive, durable, and efficient. Until today only two kinds of material have shown a definite potential as the primary material used for PV power generation:poly-Si thin film and CdTe thin film. Poly-Si thin film has both advantages of amorphous silicon and mono-crystalline silicon. Its low cost of and high mobility and great stability give its possibility of application in large area solar cell. And CdTe has an energy gap of 1.45 eV, very well suited to absorb the solar light spectrum. The energy gap is direct, resulting in an absorption coefficient for visible light of>10-5 cm-1, so that the absorber layer need only be a few micon thick to absorb 90% of light above the band gap. By now numerous low-cost deposition technologies could prepare large area CdTe solar cells.However, there are some limit in preparing poly-si thin film and CdTe thin film solar cells.To begin with, as for CSG poly-si solar cells, poly-Si thin films always have grain boundary defects and intra-grain defects after they crystillized, which severely affect the performances and stabilities of the devices made of such poly-Si. The hydrogen passivation is one of the most effective methods to passivate the defects in poly-Si devices. However, by now, although Hydrogen Passivation was applied abroad, the passivation mechanism was not clear completely yet. In this dissertation, we studied the hydrogen passivation of different poly-Si thin films which were prepared by metal induced crystallization (MIC) or solid phase crystallization (SPC) using different amorphous silicon thin films as precursors. We investigated in-situ optical emission spectroscopy (OES) of hydrogen plasma during passivation, and characterized the corresponding electrical and optical properties of passivated poly-Si thin films. Based on the measurement results of OES and the electrical and optical properties of poly-Si, it was found that different hydrogen plasma radicals acted in different roles in hydrogen passivation. We found that the reaction between hydrogen radicals in plasma and poly-Si depend on the defect type in crystallized poly-Si. For LP-SPC poly-Si, in which the major defects were intra-grain defects, higher energy radicals Hp and Hγwere needed to perform the passivation effectively. For PE-SPC poly-Si, in which the major defects were the dangling bond defects in grain boundaries, low energy radicals Ha can passivate them effectively. For LP-MIC poly-Si, which contains many defects relative to Ni impurity, H* radicals with middle energy were suitable to passivate these kind of defects.We also optimized the hydrogen passivation process. After passivation 20 minutes at reaction pressure of 800 millitorr, RF active power of 10w and substrate temperature of 550℃, the hall mobility of PE-SPC poly-Si increased by 84.9%; At reaction pressure of 300 millitorr, RF active power of 10w and substrate temperature of 550℃, the hall mobility of LP-MIC poly-Si increased by 50.1%; At reaction pressure of 1200 millitorr, RF active power of lOw and substrate temperature of 550℃, the hall mobility of LP-SPC poly-Si increased by 50.1%.In addition, as for AIC poly-si solar cells, poly-Si thin films always have Al impurity as recombination center after crystillization, which affect the performances and stabilities of the devices made of such poly-Si. So we developed solution-based aluminum-induced crystallization technology and aluminum induced crystallization assisted by hydrogen plasma technology which had never been reported. The former technology could control Al content by controling aluminum concentration in salt solution, and the latter put the annealing process of AIC in hydogen plasma to reduce Al impurity. Furthermore, both technology were low-cost. The former technology could aviod vacuum pocess, and the latter could integrate the crystallization and passivation into one process. What's more, analyzed by Raman spectroscopy, SIMS and Hall mobility, we found that it could not only reduce the annealing time of AIC but also enhance the performance of crystallized poly-Si.Finally, as for CdTe solar cells, the difficulty in making ohmic contact to p-CdTe is one of the greatest obstacles to achieve high efficiency. And maximum performance of CdTe solar cells can often be achieved only after CdCl2 treament. So we developed graphite-forming back contact technology. Analyzed by XRD and SIMS, we found this technology could not only form CuxTe layer but also lead to interdiffusion of CdTe and CdS simultaneously. In addition, this technology could reduce Cu impurity in CdTe, improving the performance of CdTe solar cells. We did't use acid and vaccum in the process. This simple technology has never been reported before either.