太陽能多晶矽的晶癖控制之研究

Abstract

在現今熱門的太陽光電產業中，多晶矽由於其高效率及低成本，成為太陽能電池市場的主流。然而，主導電池效率的因素不僅在於電池製程本身，多晶矽片本身的品質也扮演極關鍵的角色。在本論文中，主要的目的就是以提升太陽能多晶矽的品質，進而提升多晶矽太陽能電池的效率。 在本論文中，吾人提出兩種方法以提升攣生晶界在多晶矽晶體內的比例，已達到提升太陽能電池效率之目標。其一為在坩堝底部設置凹槽，其二為設置均勻過冷裝置於坩堝下方。 在凹槽實驗中，擁有15mm開口的凹槽可以成功使晶粒於凹槽內競爭，進而在凹槽附近生長出大範圍的{112}晶粒，並且伴隨著大量的孿生晶界以及較高的少數載子壽命。 在均勻過冷實驗中，以氣體流速10L/min所控制的晶體可以得到高比例的孿生晶界，但是可能由於晶體內部雜質濃度之影響，無法看出少數載子壽命的提升。

A novel idea for grain control during directional solidification by using a special crucible with notches at the bottom for mc-Si solar materials was proposed. It was observed that with a proper notch size, the initial grain competition could be controlled with a proper cooling rate. The notch could enlarge the grains induced by the spot cooling method and the grains became dominant for the later stage of the solidification. Furthermore, the crystals grown from the notch showed a higher minority carrier lifetime and a larger area of twin boundaries, with a much lower dislocation density as well. The electron back scattered diffraction (EBSD) analysis for the controlled crystals further indicated that the region near the notch had almost the same <112> orientation from the notch. The proposed method could be easily implemented in a commercial ingot production. The effects of supercooling during directional solidification on the quality of multi-crystalline silicon ingots for solar cells was also studied. An active uniform-undercooling setup at the crucible bottom was proposed, and the properties of the grown grains under different initial supercoolings were measured. It was found that the percentage of the twin boundaries were affected significantly by the supercooling.