多晶矽控制表面絨化以降低黑絲絨

Abstract

近年太陽能產業經歷歐債風暴，景氣低迷，各公司擴產，造成業界產能過多，多晶矽太陽能電池面臨嚴峻的考驗，降低成本及提升品質為首要目標。因此一、提升電池的轉換效率，二、良率提升，及三、消費者在多晶矽太陽能電池外觀視覺更顯重要。 本論文主要是討論： 多晶矽（MC-Si）太陽能電池在長晶過程會產生熱應力，形成差排（dislocation）晶格缺陷。表面有差排突出位置在製絨製程(Texture)中的化學反應速率比正常多晶矽快，因此在多晶矽表面產生腐蝕坑(Etch pits)，蝕刻時間久就會產生黑絲絨(表面黑色塊狀不規則形狀)，成為外觀異常而被客訴，而單晶矽並無此現象 。 由本研究得知一、蝕刻時間愈長則黑絲絨的腐蝕坑越深。二、不是每個差排都會跑進雜質，而影響逆電流。三、由光激發螢光頻譜可以預先測知黑絲絨的缺陷位置。四、蝕刻後黑絲絨少的其反射率最佳。 經反覆實驗獲得最佳製絨製程為蝕刻槽的氫氟酸和硝酸的蝕刻時間95~100秒、蝕刻溫度攝氏6度及氫氟酸和硝酸的體積百分濃度比例0.26。如此化學反應最佳參數可降低的多晶矽在差排位置產生黑絲絨的腐蝕坑，實驗結果黑絲絨發生率由66%下降到約1%，達到提升產品的競爭力及降低外觀異常品產出(外觀異常品需降價賣出及庫存品)。

In recent years solar industry has experienced European debt storm, economic slowdown, companies expansion, resulting in the industry's excess capacity. Multi-crystalline silicon(MC-Si) solar cell industry faces a severe competition, therefore reducing costs and improving quality become the primary goal . These include the enhancements of conversion efficiency and production yield, and the improvement of the visual appearance of multi-crystalline silicon solar cells. This paper mainly discusses the reduction of unpleasant black velvets in the texturing process of microcrystalline Si solar cell fabrication .During the multi-crystalline silicon (MC-Si) crystal growth process, due to thermal stresses, lattice defects such as line dislocations are generated. Dislocations in multi-crystalline silicon affect the texturing process from an increased chemical reaction rate, therefore etch pits are generated. Long etching time will produce feature of black velvets, which are massive black surface irregularities. This abnormal appearance process does not happen in mono-crystalline silicon. We have observed the following phenomena: 1. with long etching time the black velvet etch pits penetrate deeper, 2. dislocations will not always trap into impurity or affecting reverse current, 3. photoluminescence spectrum can reveal in advance the locations of the black velvet defect positions, and 4. lower density of black velvet reduces the reflectivity significantly. We have found the following texturing process: 1. using hydrofluoric acid and nitric acid etching bath, 2. etching time of 95 ~ 100 s, 3. etching temperature of 6 ℃, 4. volume ratio of hydrofluoric acid and nitric acid 0.26, are the best condition to reduce the etch pits produced in black velvet dislocation position. Experimental results showed that the black velvet generation rate decreased from 66% to about 1%. This enhanced the competitiveness of these products with significant reduction the output of abnormal products.