Gettering Technology And Analysis Method Of Neural Network For Porous Silicon In Solar Cells Based On Solar-Grade Silicon

Development and application of solar cells are efficient ways to resolve the problems of energy crisis and environment. Use of Solar-Grade (SOG) Silicon (Si) material is a valid measure to reduce the production costs of Si photovoltaic solar cells. But due to the large amount of impurities and structural defects in SOG-Si substrates, the transform efficiency of solar cells is largely reduced, which limits the further improvement of solar cells based on SOG-Si materials. In the thesis, the photon-generated current density of Si solar cells with single-junction was numerically simulated. The relationship between recombination loss and performance parameters of solar cells was studied. The effect of increase in the effective lifetime (τeff) of minority carriers in SOG-Si material on improving the performances of solar cells was certificated. Combined theory with experiments, the gettering technology using porous silicon layer (PSL) to improve performances of SOG-Si material and solar cells was discussed in detail. The new gettering methods were put forward and applied in fabrication process of solar cells. The possibility of application of Neural Network (NN) method in analyzing the PSL gettering method was discussed. The PSL gettering effect prediction model based on Back-propagation Neural Network (BPNN) was constructed, and sensitivity of technological parameters of PSL gettering process was analyzed. The studying work of this dissertation has been carried together with the scientific research item entitled "Development and industrialize of new-type mono-crystalline Silicon Solar Cells" (07-2-46), which is sponsored by Lanzhou Science and Technology Committee. The key points of the thesis are listed below:The problems of internal and external energy crisis and energy change, history and present situation of the development for solar cells and the conventional gettering methods used in fabrication of SOG-Si solar cells were elaborated so that the necessary of studying on gettering technology of PSL in SOG-Si solar cells was presented. The present application situation of BPNN in semiconductor materials and devices was summed up and the possibility of application of BPNN method in PSL gettering technology was demonstrated.From microcosmic semiconductor energy theory to macroscopic output characteristic of Si solar cells, along with the center p-n junction,formation,mathematic description and photo-electricity transform theory of p-n junction were introduced, the working principle of solar cells with p-n junction was thoroughly analyzed, by analogue program designed by ourselves, the photo-generated current density of Si solar cells with single-junction was simulated, and the influence of parameters related to recombination loss on photovoltaic characteristics of solar cells was researched in detail, experimental results confirmed the effect of increase inτeff of minority carriers in SOG-Si material on improving the performances of solar cells.Impurities and structural defects in SOG-Si material along with gettering mechanism of conventional gettering methods used in fabrication of SOG-Si solar cells has been thoroughly deliberated. PSL with good surface morphology has been fabricated by stain-etching technique. Gettering technology using PSL and its application in SOG-Si solar cells were studied in detail. On the basis of research into normal heat treatment and Phosphorus Diffusion Gettering at Constant Temperature(CTPDG) by PSL, two new gettering methods—Short Duration Interval Duplicating Gettering(SDIDG) and Continuous Variable Temperature Phosphorus Diffusion Gettering(CVTPDG) of PSL have been put forward. Experimental results show that PSL thermally treated under O2 atmosphere can effectively get unwanted impurities about 40μm beneath surface of the SOG-Si. A few addition of chloride in O2 can enhance the gettering effect of PSL. There is a saturation state for conventional thermal treatment gettering of PSL, the gettering effect will not be obvious beyond the saturation state. SDIDG(PSL) method can overcome the saturation state of conventional thermal treatment gettering method of PSL. SDIDG(PSL) with the sequence t1 and duplicating number m has better gettering effect than the usual thermal treatment gettering of PSL with the same gettering temperature and gettering time mt1; Within a certain time range, using PSL as a sacrificed layer can improve gettering effect of PDG at the same gettering temperature. The optimum condition for CTPGD(PSL) is 900℃/90min. Because of an appropriate decrease in gettering time at high temperature and an addition of a gettering step at low temperature, the deterioration in performances of PSL limiting the gettering efficiency, such as increase in the mean pore size and decrease in the coarseness, is weakened and the segregation coefficient of impurities between gettering regions and SOG-Si substrates is improved. So, CVTPDG(PSL) under some conditions has more obvious gettering effect than CTPG (PSL) under the optimum gettering conditions.Technological parameters of CVTPDG(PSL) were optimized by means of orthogonal experiments. The CVTPDG(PSL) gettering method under the optimum conditions was used in fabrication SOG-Si solar cells and performances of solar cells was improved obviously.For the first time, the NN method was applied to analyzing in CVTPDG(PSL) process. The prediction model for gettering effect of CVTPDG(PSL) process was constructed. The model is based on BPNN and has high precision. The sensitivity of technological parameters of CVTPDG(PSL) process was analyzed, which provides basis for importance judgment and optimization, adjustment as well as estimation of each technological parameters.