太陽能多晶矽生長之晶粒與界面控制之研究

Abstract

本論文中將提供一個簡易的長晶機制來控制晶粒生長與改善生長界面，以得到高品質的太陽能多晶矽。吾人使用active spot cooling的技術在坩堝底部控制晶粒誘導生長，並且配合熱場的側保溫使得生長界面呈現微凸狀，讓晶粒可以隨著生長高度逐漸放大。Active spot cooling是由一個導熱極佳的石墨棒，兩端接在坩堝與冷卻載台之間所組成，其冷卻載台有冷卻循環水流通且可以調控流量，進而控制冷卻速率。多晶矽晶粒因不同的冷卻速率而有不同的生長晶癖，藉由active spot cooling調控吾人能夠誘導生長Σ3孿生晶界。Σ3晶界是屬於一種電鈍性晶界，當光電轉換所展生的電子與電洞不易在此處產生再結合的效果，因而可以提高電池的轉換效率。另外，此裝置還可以變化冷卻速率，除了初始的快速冷卻生長，還可以後段降低冷卻速率減少缺陷與新的成核點產生。在生長機制的改良下，晶粒大小、缺陷密度分佈，還有少數載子壽命都能獲得改善。相較於沒有受控制的晶體，藉由冷卻控制可以得到較高比例的Σ3晶界，在太陽能電池應用可以獲得明顯的光電轉換效率提升。

We report simple ideas to control the grain orientation and solidification front to get high quality multi-crystalline silicon for solar cells. The method employed an active cooling spot to induce initial dendrite growth, and the solidification front was controlled to be slightly convex through crucible insulation. This active spot cooling employs a graphite rod at the bottom of crucible which served as an active cooling spot. According to silicon grain growth behavior, the undercooling caused by cooling spot at the initial crystal growth can induce grains with Σ3 grain boundary. The Σ3 grain boundary is electrically inactive so that it does not act as recombination center for solar cell. In addition, the cooling spot can control cooling rate to reduce new nucleation formation at the grain boundary during crystal growth. That is, it helps induced dendrite growth from the bottom to the whole crystal. As compared with non-controlled crystal, it was found that there were more twins and slip bands in controlled one. These areas often had high lifetime and low EPD density. Furthermore, grain size and lifetime of controlled crystal also enhance along the growth direction. As the results, the solar cell conversion efficiency is significantly improved.