| 摘要: | 近十年來科技發展快速，能源的需求日益提升，但傳統能源如石油並非取之不盡，因此再生能源的發展一直是人們所關心的，其中無污染的太陽能電池更是指標性的再生能源。眾多太陽能電池中，矽基太陽能電池俱有低成本，成熟技術的優點，目前市占比高達90 %，2015異質接面太陽能電池在結合背電極技術後效率已突破25 ％，仍然是太陽能電池市場中的主流，也受到各研究團隊的矚目。在此論文中，我們以自主性開發且具有高密度電漿之電子迴旋共振化學氣相沈積法開發矽、鍺、碳化矽等新式薄膜並應用其開發新結構矽基太陽能電池及新創三五族堆疊矽太陽能電池，以新材料為基礎提出矽基太陽能電池轉換效率之方法與概念。首先我們完成了矽薄膜相圖驗證電子迴旋共振化學氣相沈積法高密度電漿的特性。接著根據此結果進一步調整製程參數沈積碳化矽及奈米晶矽薄膜應用於矽基太陽能電池之開發。除此之外， 180度超低溫成長40 nm薄型且平坦之磊晶鍺更是本論文之一大亮點，該成果突破了傳統高溫製程，開創了新創之三五族堆疊矽太陽能電池的應用，經由理論計算，該類性電池之理論效率可超過30 ％。
本研究使用之電子迴旋共振化學氣相沈積法除了俱有高密度電漿的特性，更因為其是自主開發之系統，因此有非常高度的彈性調整以利發展新式材料。在此研究的第一部分我們完成基本矽薄膜的製備與研究，並製作出薄膜的成長相圖，本方法可在較低的氫稀釋比(H2/SiH4= 1)及低溫(180 ˚C)下沈積高結晶率之矽薄膜以驗證此方法俱有高密度電漿、高解離率的特性。接著我們沈積並研究碳化矽及奈米晶矽薄膜特性並應用其於新型矽基太陽能電池。本質碳化矽薄膜相較傳統非晶矽薄膜有較低的吸收係數，可降低入光面的光學損失，同時該薄膜俱有極佳的鈍化效果，表面復合速率達21 cm/sec、有效載子生命週期達680 sec的水準；也因為高密度電漿的特性，低溫製程符合異質接面太陽能電池的需求。在硼摻雜奈米晶矽的成長方面，奈米晶矽的形成可提升薄膜的導電率以降低其與透明導電膜之接觸電阻，更重要的是其非晶態的存在使其仍俱有鈍化的效果。最後製作銀/氧化銦錫/p型奈米晶矽/i型碳化矽/n型矽基板/銀之新型矽基異質接面太陽能電池，於此簡單平面的結構轉換效率可達13.66 % （開路電壓＝518 mV, 短路電流＝ 37.95 mA/cm2, 填充因子＝69.5 %）。
矽基磊晶鍺薄膜多元的應用如：光偵測器、雷射、太陽能電池、及積體化三五族元件於矽基板上引起了多方的深入研究探討。一般傳統磊晶製程如分子束磊晶法或是超高真空化學氣相沉積法往往需要超過600 ˚C的高溫及200 nm的厚度。但是過高的溫度會造成整合性及高熱預算等重重限制。而高密度電漿之電子迴旋共振化學氣相沈積法則可解決此問題。我們首先成長高導電率之硼摻雜鍺薄膜以了解其成長機制，接著以180˚C超低溫開發僅有40 nm磊晶鍺於矽基板上，其除了有不錯的磊晶結構（X射線半高寬＝683 arcsec），更有著非常平坦之表面粗糙度（粗糙度＝0.342 nm）。可解決三五族與矽材料間晶格不匹配的問題以開發新創三五族堆疊矽太陽能電池，更重要的是其極薄的厚度可避免本身的吸收造成的光損失。本結果突破傳統製程高溫、厚膜的限制，開創了矽基磊晶鍺薄膜在太陽能電池元件中的新應用。經由理論計算，此型新創三五族堆疊矽太陽能轉換效率可超過30 %（開路電壓＝2.19 V, 短路電流＝ 14.78 mA/cm2, 填充因子＝93 %），而40 nm的鍺緩衝層則會造成6.36 mA/cm2的短路電流損失。
;With the rapid development of science and technology, the sustainable energies get more and more attention, and the photovoltaic without pollutions is regard as the most important sustainable energy in the future. In the many kinds of solar cells, Si based solar cell has advantages of low cost and mature technology, and the Si hetero-junction (SHJ) solar cell can achieve the highest efficiency of 25.6 % in 2015. In this dissertation, we use homemade electron cyclotron resonance chemical vapor deposition (ECR-CVD) method with high plasma density characteristic to develop several films such as: amorphous hydrogenated silicon carbide (a-Si1-xCx:H), boron doped hydrogenated nanocrystalline silicon (B-dopd nc-Si:H), and epitaxial germanium (epi-Ge) for SHJ solar cell application and novel III-V/Si tandem solar cell development. At first, we complete Si:H phased diagram to demonstrate the high plasma density of ECR-CVD, which is a significant result to further adjust growth parameters to develop new thin films. Then we grow and investigate structural, optical, electrical, and passivation qualities of a-Si1-xCx:H and B-dopd nc-Si:H. We provide another choice of passivation and emitter layers for a SHJ solar cell. Furthermore, the Ge films development opens the newly created application for III-V/Si solar cell development. Epitaxial 40 nm thick Ge on Si with smooth surface can be grown at an ultra-low temperature of 180 ˚C. Such a thin Ge on Si is suitable being the buffer layer for III-V/Si tandem solar cell with theoretical conversion efficiency above 30 %.
Si:H film phase diagram has been completed by ECR-CVD at a low temperature of 180 ˚C. The amorphous to microcrystalline phase transition occurs at a lower hydrogen dilution ratio (H2/SiH4= 1) due to the high plasma density of ECR-CVD, compare with conventional PECVD process with higher dilution ratio (H2/SiH4~ 10). Then we introduce the CH4 and B2H6 gases to further deposit a-Si1-xCx:H and B-doped nc-Si:H for SHJ solar cell, respectively. The typical structure of SHJ solar cell is used a-Si:H as passivation and emitter layers grown at a low temperature (< 200 ˚C) by PECVD. However, the bandgap of a-Si:H is about 1.85 eV which will reduce photo generation at the short wavelength. The a-Si1-xCx:H film has a higher optical bandgap is a good solution, but it usually needs a high temperature above 300 ˚C to achieve a high quality by PECVD. We can use low temperature of 180 ˚C, satisfied SHJ solar cell requirement, to obtain an a-Si1-xCx:H film for a passivation layer. The film, having an excellent passivation quality (Lifetime= 680 sec, surface recombination velocity= 21 cm/sec) and lower absorption coefficient than a-Si:H, can keep the Voc performance and reduce the optical loss in a passivation layer meanwhile. In the other hand, the a-Si:H has a poor conductive property so that the SHJ solar cell needs a highly conductive transparent conductive oxide layer to reduce resistance loss. For the B-doped nc-Si:H deposited by ECR-CVD, the formation of nanocrystal results a high conductivity for resistance reduction, and the a-Si:H matrix passivates the p/i interface for Voc preservation. We also apply them to fabricate planar Ag grid/ITO/nc-Si:H(p)/a-Si1-xCx:H (i)/c-Si(n)/Ag SHJ solar cell without back surface field and passivation layer at rear side, and the conversion efficiency is 13.66 % (Voc= 518 mV, Jsc= 37.95 mA/cm2, FF= 69.5 %).
Ge on Si is attracted attention recently because of several applications like: infrared photodetector, Ge laser, solar cell, and monolithic integration of III-V semiconductor devices on c-Si. Usually, the growth of epi-Ge on Si needs high growth temperature (> 600 ˚C) and thick thickness (> 200 nm) by conventional epitaxy process like ultra-high vacuum chemical vapor deposition (UHV-CVD) or molecular beam epitaxy (MBE). However, the high temperature process limits the materials or devices integration, and enhances the thermal budget. ECR-CVD method with high plasma density can solve these problems. We first grow highly conductive B-doped Ge:H films to realize the growth mechanism of it, and then successfully grow a 40 nm thick Ge on Si at an ultra-low temperature of 180 ˚C. The film, having a good crystal quality (XRD FWHM= 683 arcsec) and smooth surface (RMS roughness= 0.342 nm), is suitable being a buffer layer for III-V/Si tandem solar cell to solve lattice mismatched between III-V and Si materials. More importantly, the thin thickness of 40 nm can reduce the optical loss in a buffer layer, that the Ge has a lower bandgap 0.66 eV than Si. Different to the conventional thick Ge on Si application, the thin Ge on Si developed by ECR-CVD with high plasma density starts a new concept of III-V/Si tandem solar cell. By theoretically calculation, the efficiency can achieve above 30 % (Voc= 2.19 V, Jsc= 14.78 mA/cm2, FF= 93 %). |
