Abstract
Global solar cell production topped 29.5 GW, up from 23 GW in 2010. Crystalline silicon based (including mono-crystalline silicon made by CZ pulling and multi-crystalline silicon made by directional solidification casting) solar cell production made up 89 percent of that. In particular, multi-crystalline silicon solar cells have been the mainstream of the market. Due to cheaper silicon oxide crucible, lower price graphite hot zone parts and lower multi-crystalline silicon crystal growth power consumption than mono-crystalline silicon crystal growth, the cost of multi-crystalline silicon crystal growth by DSC (directional solidification casting) is much lower than CZ mono-crystalline silicon. In recent year, multi-crystalline silicon solar cells accounted for the proportion of silicon solar cells has been growing trend. However, there are two main problems limited multi-crystalline silicon solar cell development, first of all is the grow-in defect of multi-crystalline silicon that will impact the voltage of open circuit (Voc) and solar cell conversion efficiency, the second important issue is the grains on multi-crystalline silicon wafer with different crystallographic orientation which cannot be made the pyramid structure on wafer surface for anti-reflection purpose by using alkali etching, that will also impact the current of short circuit (Isc) and the final solar cell efficiency. If we can overcome these two problems of multi-crystalline silicon, generally believed that the conversion efficiency of solar cell will improve or even very closely to the mono-crystalline silicon solar cell. This dissertation focused on these two problems of multi-crystalline silicon and proposed an approached method to grow low defect density multi-crystalline silicon and a nano-texturing method to improve the uniformity of antireflection on multi-crystalline silicon wafer. The defect growth speed is fast from bottom to top of casting ingot by using conventional multi-crystalline silicon directional solidification casting. It will cause the getting lower efficiency of solar cell from bottom to top of ingot. The new approached crystal growth method of this dissertation can control the defect growth speed and get uniform high quality multi-crystalline silicon from bottom to top ingot. The average of solar cell efficiency of a whole multi-crystalline ingot can meet 17.5 % and the highest efficiency even can touch 17.9 %. The output power of the solar panel which made by 60 pcs with 17.9 % high efficiency solar cell can be higher than 260W. That means the high output power panel of multi-crystalline silicon solar cell can meet the panel requirement of high output power between 250 W per panel to 270 W per panel made by mono-crystalline silicon solar cell. A new approach with high uniform antireflection of different grain orientation on multi-crystalline silicon wafer has been proposed by nano-texturing. Different kinds of silicon crystal orientation, including (111) and (110) and (100) are demonstrated with the same etching rate and then demonstrated on multi-crystalline silicon to get the low reflection and high uniform antireflection.