The Research Of Structure And Performance On Multicrystalline Silicon By Directional Solidification

Multicrystalline Silicon (mc-Si) is one of the main raw materials used for solar cells in the photovoltaic market. The quality of silicon wafer significantly affects the conversion efficiency of solar cells. Therefore, it’s important to optimize the crystal growth technique to reduce the defect formation during the process of crystal growth. In this paper, directional solidification method was applied to develop the mc-Si ingot. The microstructure, crystal orientation, electrical properties and grain boundary type were investigated using metalloscope, microwave photoconductive decay method (μ-PCD), resistivity tester and electron back scattering diffraction (EBSD). The following works were done.1. The effect of solidification rate was investigated via the mc-Si ingot casting under 5 dm/s,10 dm/s,15 dm/s, and 20 dm/s. The results showed that the optimized rate was 15 dm/s. With this rate, an obviously grown columnar crystal, uniform grain size distribution, lower dislocation density between 5.5～7.98×103 cm-2 and better electrical properties were obtained in the prepared mc-Si. The electrical resistivity fluctuated between 0.56-0.76 Ω·m, and the minority carrier lifetime reached up to 0.91～1.72μ.s. The crystal preferred orientations occurred from the bottom to the top as: (101)â†' (110)â†' (111).The twin boundaries appeared easily on the crystal planes, and corresponding sigma three boundaries ratio was as high as 45.5%,37.48%, and 49.56% in the bottom, middles, and top of silicon slice, respectively.2. The effect of melting temperature was investigated via the mc-Si ingot casting under 1420℃,1470℃,1520℃,1570 ℃, and 1620 ℃. The results showed that the axial growth was more obvious and the resistivity increased with increasing casting temperature. However, when the casting temperature increased further, the bottom of the ingot would remelt and the top of the ingot would crack. Therefore,1520 ℃ is suitable for the manufacture of mc-Si ingot under which better removal of impurities can be obtained due to gathered grain growth, and coincidence site lattice boundaries was can be easily produced due to the preferred (111) orientation. The mc-Si ingot casting under 1520 ℃ had resistivity that ranged in 0.65-0.81 Ω·cm and minority carrier lifetime that fluctuated between 0.75～1.19μs.3. The effect of crucible bottom shape was investigated via the mc-Si ingot casting in different crucibles with flat bottom, cone bottom, concave bottom, and convex bottom. The results indicated that conical crucible bottom could provide better nucleation growth conditions during the initial stage of solidification. A very small undercooling was needed for crystal nucleation and growth. In addition, the conical bottom structure was facilitating to enrich the impurities to the edge, so that few dislocations would occur in the middle area of mc-Si ingot. Ingot casting in crucible with conical bottom had resistivity that ranged in 0.62-0.81 Ω·cm and minority carrier life that reached 1.1～1.75 μs. Contrarily, when the convex crucible was used, the least favored mc-Si ingot would be obtained with electrical resistivity fluctuating in the range of 0.12～0.53 Ωcm and minority carrier life so low as 0.3～0.7 us.