Application Of Liquid Deposition SiO 2 Subcompression Thin Films On Solar Cells

Solar photovoltaics (PV) technology is developing rapidly in recent decades since solid-state solar cells are a promising green power for easing the issues of energy shortage and environmental pollution. So far, cystalline silicon solar cells remain a predominant share in photovoltaics industry. Nevertheless, large-scale utilization and commercialization of solar cells is largely hindered by its limited photoelectric conversion efficiency and high cost with the use of high-purity cystallinesilicon materials. Therefore, investigation on the measures to reduce the cost of Si wafers and increase the photoelectric conversion efficiency is of great significance to promote the development of Si-based solar cells. In order to increase the cost-effective, the use of thinner crystalline Si wafer in solar cells without compromising their photoelectric conversion efficiency is a growing trend in current mass production of these cells. Along this line, single-sided wet texturing has a great advantage over double-sided texturing in terms of reducing Si wafer thickness losses. Traditionally, two methods consisting of PECVD and wafer floating are used for single-sided texturing, which are rather complicated and expensive.On the other hand, a wet-chemistry process called liquid-phase-deposition (LPD) has been widely used to grow various functional coatings including SiO2, TiO2 and metal-oxide films for applications in microelectronics, photocatalysis and chemical sensing. The LPD method possesses distinguishing advantages such as low temperature, high selectivity, large area, simplicity and support of mass production. In the present work, the LPD method has successfully been extended to grow a fluorine-doped SiO2 film on monocrystalline Si wafers and initially explore the new application of this film as a sacrificial mask against corrosive etching of the underlying Si in basic texturing solutions. We report a new all-wet circulatory single-sided texturing alternative tactics that enables the single-sided texturing of monocrystalline Si wafers and favors the follow-up mass production of Si-wafer-based solar cells, aiming to save monocrystalline Si material and to improve the photoelectric conversion efficiency.The main results of the research work are summarized as follows:The mechanism of liquid phase deposition of a silicon oxide film is that saturated H2SiF6 hydrolyzes in an aqueous solution with the assistance of H3BO3. A dense SiO2 film is formed along with the doping of the Fluorine as a result of the binding interaction among the -OH groups on Si substrate and the hydrolysate Si(OH)nF4-n in aqueous solution. The experiment results indicate that growth rate of the LPD-SiO2 film increased monotonically with an increase of the deposition temperature up to 60℃ for a given precursor solution. LPD-SiO2 films exhibit varying colors and tolerances to an alkaline solution (1.5wt%,75℃) depending on the thicknesses. A>120 nm-thick silicon oxide film is grown on a (100)-oriented Si wafer through liquid-phase deposition (LPD) as a protective mask of the wafer’s rear side in order to chemically texture the wafer’s unprotected front side in a basic etching bath. Field-emission scanning electron microscopy (FE-SEM) indicates that a pyramidal surface texture forms on the front side in the chemical texturing bath, whereas the underlying Si surface on the rear side remains intact. As a result, the average reflectivity for incident light over 450-850 nm is decreased to 11.1% on the front side, and a 5.8 μm thick Si surface on the rear side is saved per wafer.We apply the single-sided textured square Si wafers with a larger size of 15.6 cm X 15.6 cm2 to the solar cell fabrication process. However, the growth rate of the silicon oxide film on a larger silicon wafer decreases due to the relatively enhanced self-deposition of the Si(OH)nF4-n occurred in precursor solution. Four groups of Si wafers with different thicknesses that were subjected to the above LPD-SiO2 processes are used for the construction of solar cells and the subsequent examination of photo-electric efficiency. It is revealed that the thickness losses of Si wafers due to texturing could be also lessened with the protection of the LPD-SiO2 films thinner than 120 nm. Notably, if the rough damage on silicon wafer surface is not be removed preliminary, the efficiency of the cells would decrease under a complete protection of LPD-SiO2 mask. This is because remained rough damage on the rear side surface of the silicon wafer forms the centers of minority carrier recombination, which would be adverse to the collection of minority carrier and efficiency for cells. Aiming those problems above, we further propose an alternative tactics in terms of improved LPD device and modified wet single-sided texturing method, which is promise for a lower cost mass production of a higher efficiency solar cell.