| Crystalline silicon solar cell is the main support of the modern photovoltaic power system and p type silicon solar cell assumes absolute superiority due to its traditional materials and process advantages. However, n-type crystalline silicon solar cell has gained the recognition because of its inherent prominent advantages in recent years. It will become the development direction of crystalline silicon solar cell technology in the future. One of the chief obstacles of the n type silicon solar cell is the formation of the pn junction since it is difficult to obtain p type doped layer on n type silicon substrate. The Al doped silicon layer grown from Al-Si melt on n type silicon substrate is a promising solution. It has low cost. In fact, the principle of this method has been successfully applied in the formation of the back surface fields(BSF)by alloying of the screen-printed aluminum pastes. However, the unfortunate reality is that Al-doped P+ emitter exhibit serious inhomogeneity phenomenon by screen-printed method. In addition, compared with aluminum ingots used in epitaxial growth melt, the cost of the aluminum paste is much higher. Therefore we studied the growth of the p-type doped crystalline silicon on Si(001) substrate by means of low temperature liquid phase epitaxy from Al-Si melt. The study focued on the epitaxy of the(001)silicon substrate, since monocrystalline silicon solar cells all used(001) silicon wafers as materials.A vertical dipping device with Ar circulated as ambient gas was used. Both isothermal growth and continuous cooling growth with melt-back treatment were examined. The factors concerned were substrate surface states, melt composition which decided to liquid-solid equilibrium temperature, super-cooling degree, growth time, temperature range, substrate preheating temperature and so on. The morphology and structure of the epitaxial silicon thin layers were examined. And the open-circuit voltage of the pn junctions formed were measured. Furthermore, the influence factors were analyzed by computational simulation. The main results were as follows:Epitaxy in the Al-Si melt can be realized. The morphology of the liquid phase epitaxial layer performed pyramid-like island-pattern at different kinds of surface states in the isothermal growth from Al-Si alloy melt. The island formed by {111} surfaces and finally shaped in pyramid or roof-type silicon crystal. This result was caused by surface energy minimization. Only when the islands silicon crystal contacted with each other could reach the basic requirements of pn junction, so high and uniform nucleation density should be the direction of process optimization. From the experiment, the substrate surface states could influence the size and density of the islands: smooth surface was conducive to improves the density and uniformity of the nucleation and the QJ201 Al-flux can improve the nucleation rate; the extensions of the growth time, the decrease of the super-cooling degree, the promotion of the silica content and the initial substrate temperature all were beneficial to the uniform distribution of the island. With the extension of epitaxial growth time, pyramid shape gradually was smoothing.Under the continuous cooling growth mode, the in situ continuous cooling epitaxial growth after the substrate melt back at the temperature slightly higher than the solid-liquid equilibrium temperature can be realized. The result showed that substrate melt back could obtain very smooth and clear surface. After substrate melt back, continued epitaxial film could be obtained with small cooling range, followed by island-pattern growth when the cooling range enlarged, indicating a Stranski-Krastanov growth mode.Compared to the isothermal growth mode, the open-circuit had been greatly improved in continuous cooling growth mode.and continuous homogeneous epitaxial layer fromed in the early stage. The island growth after formation of the continuous epitaxial film remarkably reduced the open circuit voltage. Higher growth rate leaded to slightly lower open circuit voltage of the continuously grown pn junctions.The simulation results showed that, the average value of the pn junction fromed by p type epitaxial layer and the silicon substrate using the substrate melt back followed by in situ continuous cooling growth method was 524 m V. The max value was 529 mV. This had been very close to the theoretical calculation values(538 mV). The computational simulation also showed that, as long as the p-type epitaxial layer thickness was less than 10μm, then minority carrier lifetime was just more than 1μs. It could be able ensure the p-n junction open circuit voltage at a high level. Therefore, the development foreground of n-type silicon solar cell of which the pn junction formed by low temperature liquid phase epitaxy was very optimistic. |
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