The main motivation for developing solar energy is laid on the desire to get away from fossil fuels with their adverse effect on the environment, and to reduce it to the limited amount. The major reason for the low penetration of PV (Photovoltaic) today is the high cost, and it’s also the main reason why the solar energy has not yet been used widely in most countries. Silicon is one of the most important fundamental materials in modern semiconductor industry. It is also very useful as the substrate material for solar cells. Conventional methods of the fabrication of Si wafer are either the slicing of Si crystals cast as bars or the direct growing of Si sheet from the molten Si. However these methods have some economical disadvantages resulting from slicing loss, limited wafer size, and difficulty of growing process. In this study, the new direct fabrication method of Si wafer for the solar cell substrate was proposed, which is the vacuum casting method.
Silicon was melted in vacuum induction furnace. Several mold and crucible forms were tested to obtain a proper filling of molten silicon for vacuum casting by a pressure difference. The target dimension of Si wafer was 50×50×0.5(∼1.5)㎣. The casting parameters such as pressure, temperature and filling method were investigated. For proper filling, the pressure difference and the mold temperature were very important. When the pressure was too high, the molten Si overflowed, and vice versa. So it was essential to achieve the proper pressure difference in this sense. It was also important to prevent the gas leakage through the parting line of the mold for the perfect filling of Si melt. Therefore optimal the graphite crucible and mold were designed and made to pressurize the Si melt only. In this work, the mold positioned under the crucible to apply proper over pressure only on top of the melt. The mold cavity with 1.0mm thickness was filled with molten Si by 30 torr over pressure.
The mold temperature is important as well. In the case of a high temperature of the mold such as 1420℃, the molten Si reacted with the BN coated graphite mold and stuck together strongly. It was impossible to release the Si wafer. However at the mold temperature below 1280℃, the molten Si reacted little with the mold. It was easy to detach the Si wafer from the mold wall.
Because of silicon’s reactive characteristic, the reaction between mold and molten silicon must be prevented by using proper mold or mold coating materials. Various coating materials, for example, BN, $Si_3N_4$, $CaCl_2$, and oxides $(Al_2O_3$, $SiO_2$, $Y_2O_3$, $ZrO_2)$ were used in this research. The BN coating was regarded as the most durable and non-wetting properties in molten silicon. And in that case the grain size of cast Si wafer showed larger than any other coatings. The reaction between molten silicon and mold (or mold coating materials) was investigated in the study as well. The Si wafer cast by vacuum casting method was polycrystalline, which had a lot of dislocations with various directions.
A computational simulation was done for investigation of fluid flow, heat transfer, and solidification phenomena. By $EASYFAST^\circledR$ the fluid flow was simulated with pressure difference. And also the heat transfer and solidification were simulated with the variation of mold temperature by $ProCAST6^\circledR$. The results of computational simulations were consistent with the experimental results.
After casting poly crystalline Si solar cell was fabricated on this Si wafer. It was $n^+p$ type solar cell. The photovoltaic efficiency$(\eta)$ was measured in AM1.5 condition under the solar simulator(WXS-105H). The solar cell using the commercial single and poly crystalline Si wafer with large grain size were fabricated. Its efficiency was measured to compare with that of the solar cell fabricated on the vacuum cast Si wafer. The Si solar cell using vacuum cast Si wafer showed very low short-circuit current$(I_{sc}=4.625mA)$, open circuit voltage$(V_{oc}=280mV)$ and efficiency$(\leta=0.475%)$. The severe over-doping (>$10^{17}$/㎤) of Si-wafer from BN-coating or other impurities, such as metallic elements or carbon, would be the main reason. The BN coating might act as the dopant of Si-wafer during casting. The small grain size can be one of reasons of its inefficiency. The grain size was about 1mm. It was about 10 times smaller than that of commercial polycrystalline Si-wafer. This grain boundary provides the recombination center of electron-hole pair, and reduce the efficiency of Si solar cell.
The vacuum casting method can have the proper economical potential to reduce the fabrication cost of solar cell. The graphite mold was reused. No cutting process was involved during the entire process of a wafer fabrication. It could enable high material yield because there is no slicing loss. The automatic system for mold change, for example to rotate the molds in a multi-stage system, will make the production rate higher than that of this small scale experiment.