To Investigate the Influence of i-a-Si:H to Heterojunction Solar Cells with Pyramid Textured Structure

在本篇論文中，主要是研究非晶矽/單晶矽組合而成的異質接面太陽能電池的轉換效率變化；我們改變矽晶圓的表面結構，探討表面結構的變化對於反射率的關係。在n型矽晶圓上，以極高頻電漿輔助化學系統沉積本質氫化非晶矽(i-layer)以及p型氫化非晶矽薄膜(p-layer)，並針對本質氫化非晶矽薄膜作調變，製作成異質接面太陽能電池，經由I-V量測結果的電性參數做討論與分析。 在改變矽晶圓表面結構上，使用氫氧化鉀(KOH)對矽晶圓表面進行非等向性蝕刻，由於晶體(100)面和(111)面有不同蝕刻速率的關係，蝕刻後將產生金字塔狀的表面結構。利用氫氧化鉀與異丙醇(IPA)的混合溶液，搭配不同KOH蝕刻液濃度、反應時間進行蝕刻，可將拋光矽晶圓表面蝕刻成粗糙的金字塔狀，達到光封存效果，使反射率下降。 我們採用極高頻(40.68 MHz)電漿輔助化學氣相沉積系統，沉積本質氫化非晶矽薄膜和p型氫化非晶矽薄膜；其優點為沉積薄膜時，可使電漿內產生較高的離子密度以及降低離子轟擊對薄膜造成的損傷，比一般射頻(13.56 MHz)電漿輔助化學氣相沉積系統具有更高的氫原子解離率，可以獲得良好的薄膜品質。在實驗中，我們以不同氫稀釋比、不同製程溫度去分析其薄膜的結構和品質，並利用以上的調變參數，以及不同厚度調變，製作成異質接面太陽能電池。 使用氫氧化鉀濕蝕刻的矽晶圓，製作成異質接面太陽能電池，可發現短路電流密度明顯提升，這是金字塔結構發揮光封存的效果，增加載子的產生，但同時，開路電壓卻有下降的趨勢，這是因為經過濕蝕刻的矽晶圓表面有較多缺陷，經過RCA 清洗步驟後可能無法完全去除這些缺陷，形成較差的氫化非晶矽與單晶矽接面，造成開路電壓下降。將氫氧化鉀濕蝕刻後的矽晶圓進行乾氧化處理，再浸泡於氫氟酸(HF)中，以去除氧化層，來改善矽晶圓表面的缺陷及金屬離子殘留問題，隨即沉積氫化非晶矽薄膜，製成太陽能電池，經計算後的各項電性參數: 轉換效率為8.28 %，開路電壓為0.449 V，短路電流密度為28.84 mA/cm2，填充因子為63.9 %，串聯電阻為3.78 Ω-cm2，分流電阻為2272 Ω-cm2。In this thesis, the conversion efficiency of amorphous / crystalline Si heterojunction solar cell was studied. First, we changed the surface structures of the Si wafer and investigated the reflectance of these samples. Then, various intrinsic hydrogenated amorphous-Si thin films and p-type hydrogenated amorphous-Si thin films were deposited on the Si wafer by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD). Finally, the heterojunction solar cells were fabricated and the electrical characteristics were analyzed. To change surface structures of the Si wafer, we used the potassium hydroxide (KOH) solution to etch the polished ones. And the pyramid structures were shown on the Si wafer due to the different etching rate between the (100) and (111) orientations. In the experiments, the KOH and IPA mixture solutions with different concentrations and various etching times were carried out, and the pyramid structure could be formed on the polished wafer. The average reflectivity of the etched samples decreased, resulting from the effects of light trapping. VHF-PECVD (40.68 MHz) was used to deposit the intrinsic and p-type hydrogenated amorphous Si thin films because this system could produce the high density and low ion energy plasma which would enhance the deposition rate and reduce ion bombardment. As compared with the radio-frequency plasma enhanced chemical vapor deposition (RF-PECVD, 13.56 MHz), VHF-PECVD could achieve better quality of thin films because of higher generation rate of hydrogen atoms. In the experiments, the H2 dilution ratio, process temperature and film thickness were changed and the structure and quality of thin films were analyzed. The heterojunction solar cells were fabricated by depositing the hydrogenated amorphous-Si thin films on the textured Si wafer. And the short circuit current density of the solar cells increased obviously, resulting from the pyramid structure of Si wafer to enhance the light trapping. However, the open circuit voltage decreased, due to the high density of defects and the residues of metal ions, coming from the wet etching process. Even after RCA cleaning procedures, they could not be fully removed from the surface of the Si wafer. Therefore, the interface of hydrogenated amorphous-Si / crystalline Si became poor. In order to solve this issue, the textured wafers were oxidized or annealed in the dry and low temperature ambients. And then, the very thin SiO2 layer was removed by diluted HF solution. The electrical parameters of the heterojunction solar cells could achieve the conversion efficiency of 8.28 %, open circuit voltage of 0.449 V, short circuit current density of 28.84 mA/cm2, fill factor of 63.9 %, series resistance of 3.78 Ω-cm2 and shun resistance of 2272 Ω-cm2.誌謝 i 摘要 ii Abstract iii 目錄 v 圖目錄 vii 表目錄 ix 第一章 簡介 1 1.1 前言 1 1.2 動機與目的 3 1.3 文獻探討 4 1.3.1 HIT太陽能電池文獻探討 4 1.3.2 金字塔結構抗反射文獻探討 6 1.4 論文架構 7 第二章 太陽能電池 8 2.1 太陽光譜 8 2.2 氫化非晶矽薄膜特性 9 2.3 太陽能電池基本原理 10 2.4 太陽能電池的轉換效率 14 2.5 太陽能電池的寄生電阻 17 2.6 單晶矽的非等向性濕式蝕刻 20 2.6.1 KOH非等向性濕式蝕刻理論 20 2.6.2 非等向性濕蝕刻的反應機制與影響因素 22 2.7 太陽能電池的結構 23 第三章 實驗方法、流程及實驗機台介紹 26 3.1 矽晶圓表面粗糙化實驗流程 27 3.2 本質氫化非晶矽薄膜材料實驗流程 29 3.2.1 試片清洗 29 3.2.2 實驗流程 30 3.3 矽晶圓低溫乾氧化處理流程 32 3.3.1 實驗流程 32 3.4 太陽能電池製作流程 33 3.4.1 HIT太陽能電池製作實驗流程 33 3.4.2 具有金字塔結構HIT太陽能電池製作實驗流程 37 3.5 極高頻電漿輔助化學氣相沉積系統 39 3.6 濺鍍機 41 3.7 熱蒸鍍機 42 3.8 高溫爐管 42 3.9 傅立葉轉換紅外線光譜儀 43 3.10 拉曼光譜儀 43 3.11 紫外光可見光光譜儀 44 3.12 少數載子生命週期量測儀器 45 第四章 結果與討論 46 4.1 矽晶圓非等向性蝕刻 46 4.1.1 不同蝕刻參數的矽晶圓表面型態 46 4.1.2 不同蝕刻參數的矽晶圓反射率光學量測 52 4.2 本質氫化非晶矽薄膜特性探討與對太陽能電池特性影響 55 4.2.1 不同氫稀釋比對本質氫化非晶矽薄膜的影響 55 4.2.2 不同氫稀釋比本質氫化非晶矽薄膜對HIT太陽能電池特性量測 62 4.2.3 不同製程溫度對本質氫化非晶矽薄膜的影響 64 4.2.4 不同製程溫度的本質氫化非晶矽薄膜對HIT太陽能電池特性量測 67 4.2.5 改變本質氫化非晶矽薄膜厚度對HIT太陽能電池特性量測 69 4.3 矽晶圓表面型態變化對HIT太陽能電池特性影響 70 4.3.1 不同矽晶圓表面型態對HIT太陽能電池特性量測 70 4.3.2 矽晶圓表面低溫氧化機制對少數載子生命週期的影響 72 4.3.3 矽晶圓表面氧化機制對HIT太陽能電池特性量測 74 第五章 結論 77 參考文獻 7