Study Of Crystaline Silicon Films Fabrication And Its Application In Solar Cells

The finite supply of the fossil fuels and the pollution caused by the consumption of these fuelsmake it urgent to develop the clean renewable energy. Photovoltaic (PV) technology is a promisingtechnology which can convert the solar radiation directly into electricity. The dominant products inthe PV industry are the c-Si wafer based solar cells. However, high cost of the c-Si wafers blocks thedevelopment of the PV industry. Replacing the c-Si wafer by crystalline silicon films is an alternativeway to reduce the cost of the solar cells.In this paper, we promoted some novel methods to fabricate high-quality crystalline silicon filmswith lower fabrication cost than the commonly used fabrication technologies. We have studied theinfluence of the fabrication parameters on the property of crystalline silicon films and discussed itsmechanism in detail. Based on the property optimization of the crystalline silicon films, we fabricatedcrystalline silicon thin film solar cells with relative high conversion efficiency.The influence of the temperature of hot-wires and substrates, H2dilution ratio, and type ofsubstrates on the properties of the Si films deposited by hot-wire chemical vapor deposition (HWCVD)was systematically studied. We found that properly high temperature of the hot-wires was beneficialto the enhancement of the crystallinity of the Si films. However, too high temperature of hot-wireswill deteriorate the crystallinity. The H2dilution ratio exhibits similar tendency, proper high H2dilution ratio is helpful for enhancing the crystallinity of the Si films, but the deposition rate isrelatively low. As the increasing of the temperature of the substrate, the crystallinity of the Si filmswas gradually improved. By optimizing the parameters, we fabricated the silicon film p-n junctions.We found that the highest working temperature of the junction was as high as280oC. Different type ofsubstrates also has an effective impact on the crystallinity of the poly-Si films. We found that thepoly-Si films deposited on Si (111) obtained the best crystallinity, while the following were on the Si(100), AZO and glass substrates. This may be caused by the substrate induction effect.An effective method to fabricate the poly-Si films is to crystallize the a-Si:H films, which alwaysrefers to solid phase crystallization (SPC) or metal induced crystallization (MIC). However, a quitelong duration which always took several or several tens hours is usually necessary for these processes.In this paper, the rapid thermal annealing was applied to crystallize the a-Si:H films. Influence ofdifferent parameters on the properties of the poly-Si films was studied. We have found that poly-Sifilms with high crystalline fraction (>95%) were obtained in a very short time (<20s). Based on the optimization above, p-n junctions have been fabricated by this technique. The working temperaturewas as high as300oC, which was greatly higher than that of silicon wafer based junctions. Byoptimizing the fabrication process, the micro-cracks and the peeling-off phenomenons of whichappeared in the fabrication of micrometer-order poly-Si films were effectively avoided.Through the double-layer porous silicon layer transfer technique, thin film based HIT(Heterojunction with Intrinsic Thin layer) solar cells was fabricated. The double-layer porous siliconwas formed on the whole area of the Si wafer by electrochemical etching process in the self-designedetch tank. This double-layer porous silicon can act as the template for the epitaxial growth ofhigh-quality crystalline silicon film by LPCVD. After the formation of this film, the emitter layer ofthe solar cell was deposited by HWCVD at low substrate temperature. By using the high-quanlitycrystalline film as the absorber layer, the consumption of the Si material can be effectively reduced.Besides, the fabricated solar cells can be attached to the cost-effective substrates and the Si wafer forfabricating double-layer porous silicon can be reused, which can also reduce the cost of the solar cell.Morever, compared to the trainditional emitter fabrication process, emitter layers fabricated byHWCVD have the advantages of low temperature and high growth rate (only50s in this paper), whichcan further reduce the cost of the solar cells. By optimizing the parameters during the fabricationprocess, solar cell with the conversion efficiency of9.6%was obtained.After mastering the parameters for depositing different type of Si films by HWCVD, a dc substratebias was introduced into the HWCVD system to investigate the influence of substrate bias on theproperties of the deposited Si films. The results show that the substrate bias can effectively improvethe crystallinity of the films deposited at low H2dilution ratio. And this improvement is moreeffective for the films deposited on stainless steel (SS) than that on glass, which make it possible todeposited high crystallinity microcrystalline Si (μc-Si:H) films at high growth rate. On the other hand,however, at high H2dilution ratio, the crystallinity of the Si films deposited on SS was graduallydeteriorated as the increasing of the negative substrate bias, while that on glass was firstly improvedand then deteriorated. The mechanism of the influence of substrate bias on the crystallinity of theμc-Si:H films is related to the electrons emitted from the hot wires. These electrons are accelerated inthe electrical field and collide with the reactant radicals make the deposition process different with thetraditional HWCVD. The differences of the crystallinity of the films deposited on SS and glass aremostly attributed to the different electrical potentials between these two kinds of substrates. Afteroptimizing the properties of the μc-Si:H films deposited with substrate bias by HWCVD, solar cellswith the n+nipp+structure were fabricated on SS. By optimizing the parameters during its fabrication,the solar cell with a conversion efficiency of6.07%was obtained. Compared to that fabricated without the applying substrate bias, which attained the conversion efficiency of3.86%, the solar celldeposited with applying substrate bias showed a57.25%higher efficiency.