| 摘要: | 對於傳統的矽晶太陽電池，通常使用擴散製程進行硼原子或磷原子的摻雜且其擴散深度往往厚於0.5 μm，導致介面不明顯及使光電流降低。因此在本論文中，擁有超淺摻雜陡峭接面的摻雜磊晶矽薄膜將被開發研究。此外，為了加以了解矽晶鈍化機制及降低設備成本考量，我們使用國內自主生產的電漿輔助化學氣相沈積法沉積鈍化薄膜並進行其鈍化成效研究。因此，在本論文中主要的目標為開發並優化矽基薄膜並更進一步的在低製程溫度中應用於新發展的超淺接面矽晶太陽電池與矽異質接面太陽電池，我們使用電子迴旋共振化學氣相沈積法沉積的高品質硼摻雜磊晶矽薄膜作為超淺接面矽晶太陽電池之射極層以能獲得高短路電流，因得助於電子迴旋共振化學氣相沈積法擁有以下幾項優點及特性（1）屬高密度電漿；（2）低製程壓力；（3）高效氣體使用率。此外，亦將使用國內所生產的電漿輔助化學氣相沈積系統進行優質鈍化薄膜製程與矽異質接面電池之研究中。
為了實現如同傳統矽晶同質接面紫外太陽電池高藍光頻譜量子響應之目標，我們使用電子迴旋共振化學氣相沈積法沉積低吸收係數與陡峭超淺摻雜輪廓之高品質硼摻雜磊晶矽薄膜，由於高階游離率的特性因此電子迴旋共振化學氣相沈積法相當適合用於沉積磊晶薄膜，使得能在低溫140 °C時藉由沉積高品質硼摻雜磊晶矽薄膜實現超淺摻雜陡峭接面（< 30 nm），此高品質硼摻雜磊晶矽薄膜已經由高解析度穿透式電子顯微鏡與二次離子質譜儀已做確認，在超薄厚度（25 nm）下可得低薄膜片電阻（< 304 Ω/sq）。對於超淺接面矽晶太陽電池而言，我們製作其結構為Ag/Ti/ITO/epi-Si(p)/c-Si(n)/μc-Si:H(n)/ ITO/Ti/Ag的太陽電池於n型CZ矽基板上，研究中我們發現到電池的開路電壓強烈的受薄膜內載子摻雜濃度所影響，在沒有進一步的鈍化製程使用下，電池的開路電壓界於560 mV至600 mV之間，但最重要的一點是，相當優異的高短路電流密度（41.35 mA/cm2）能在低成長溫度且簡單製程條件下被實現，此超淺接面矽晶太陽電池研究成果中，在n型粗糙化CZ矽基板（200 μm）可獲得轉換效率17.16 %、填充因子74.74 %、開路電壓582 mV、短路電流密度39.44 mA/cm2之最佳太陽電池。此新型態超淺接面矽晶太陽電池在與國際其他研究團隊之比較下，本團隊的研究結果亦表現相當突出。
另一方面日本松下公司在2014年已成功達成其轉換效率高達24.7 %之矽異質接面太陽電池，儘管各研究者都明白矽晶表面鈍化技術是達成高效矽異質接面電池之關鍵技術之一，但其表面鈍化之機制與如何能有效沉積優異的高品質鈍化薄膜依舊呈現相對困難。因此，為了能進一步的了解與發展薄膜鈍化技術，我們與國內的設備廠商合作並使用其自主生產的電漿輔助化學氣相沈積系統進行本質氫化非晶矽薄膜之沉積與研究。根據我們的研究結果，優異的高品質鈍化薄膜將出現於薄膜非晶相轉變為薄膜結晶相中間的過渡區間內，在此過度區間內之高品質鈍化薄膜擁有著低微結構參數、豐富的氫含量、與較高的光敏度等特性。研究中我們能獲得優異的有效載子生命週期4.7 ms、低表面複合速率2.98 cm/s、高暗喻開路電壓725 mV。對矽異質接面太陽電池而言，我們製作其結構為Ag/Ti/ITO/a-Si:H(p)/i/c-Si(n)/i/a-Si:H(n)/ITO/Ti/Ag的太陽電池於n型CZ矽基板上，並在完成太陽電池前，對電池前驅結構p/i/c-Si/i/n、n/i/c-Si/i/n、p/i/c-Si/i/p進行有效載子生命週期之量測監控，研究中發現使用電子迴旋共振化學氣相沈積法所沉積之硼摻雜射極層將導致有效載子生命週期衰減，因此改採用漸變式沉積功率法（150 W~1800 W）以防止有效載子生命週期之衰減與同時維持射極層之電性需求，除了進行優化矽異質接面太陽電池外，同時我們亦導入超薄型矽基板（50 μm）進行相關研究。於矽異質接面太陽電池研究成果中，在n型粗糙化CZ矽基板（200 μm）可獲得轉換效率17.26 %、填充因子71.75 %、開路電壓660 mV、短路電流密度36.71 mA/cm2之最佳太陽電池；在n型平面CZ矽基板（50 μm）可獲得轉換效率12.46 %、填充因子65.40 %、開路電壓651 mV、短路電流密度29.28 mA/cm2之最佳太陽電池。比較於國內學術界中相關矽異質接面太陽電池之研究，本團隊的研究結果亦表現相當突出。
此外，除了上述所提及之硼摻雜磊晶矽薄膜與本質氫化非晶矽薄膜外，為了進一步達到高效太陽電池之目的，擁有著低吸收係數與寬能隙之替代性新材料硼摻雜微晶氫化氧化矽薄膜與本質氫化非晶氧化矽薄膜在本論文中亦將被研究與開發，研究成果中可獲得寬能隙1.83 eV之低吸收係數硼摻雜微晶氫化氧化矽與有效載子生命週期達800 μs之低吸收係數本質氫化非晶氧化矽薄膜。
;For conventional c-Si solar cells, the doping profile of boron or phosphorous is usually more than 0.5 μm depth by diffusion process, hence the doped epi-Si thin films with abrupt ultra-shallow doping profile were investigated in this thesis. Furthermore, in order to understanding the mechanism of passivation technique, passivation thin films deposited by domestic PECVD was developed. Therefore, the main focus of this thesis is the development and optimization of silicon-based thin films for further applications in newly developed ultra-shallow junction (USJ) Si solar cells and silicon heterojunction (SHJ) solar cells at low substrate temperature. The high Jsc can be obtained by the new type USJ Si solar cells with high quality boron-doped epitaxial silicon (epi-Si) emitter deposited by ECRCVD because the ECRCVD has advantages such as (i) high plasma density of 1012cm−3, (ii) low working pressure, and (iii) efficient gas utilization due to high degree of dissociation. Furthermore, the domestic PECVD is involved in the excellent passivation process for the fabrication of SHJ solar cells.
In order to achieve the objective as same as the conventional c-Si homojunction solar cells with high blue response of the quantum efficiency, the high quality boron-doped epi-Si thin films with low absorption coefficient and abrupt ultra-shallow doping profile were prepared by ECRCVD. The ECRCVD is appropriate for the deposition of epitaxial thin films due to the high degree of dissociation. For conventional c-Si solar cells, the doping profile of boron or phosphorous usually is more than 0.5 μm depth by high temperature diffusion process. Compare with the conventional diffusion process, we can obtain the abrupt ultra-shallow doping profile (< 30 nm) by depositing the high quality boron-doped epi-Si thin film at low temperature (140 °C). The high quality boron-doped epi-Si thin film had been confirmed by HRTEM and SIMS. And the low sheet resistances (< 304 Ω/sq) for quite low thicknesses (25 nm) had been demonstrated. For USJ Si solar cells, the solar cell structure of Ag/Ti/ITO/epi-Si(p)/c-Si(n)/μc-Si:H(n)/ ITO/Ti/Ag was fabricated on n-type c-Si substrate. We find that the Voc is strongly affected by carrier concentration. The Voc of solar cells was obtained in the range between 560 mV and 600 mV without further passivation treatment. Most important of all, outstanding performance of high Jsc 41.35 mA/cm2 can be obtained with this simple process and low deposition temperature. Our best USJ Si solar cells on 200 μm n-type textured CZ c-Si wafer was obtained with an efficiency of 17.16 %, FF = 74.74 %, Voc = 582 mV, and Jsc = 39.44 mA/cm2. These results of newly developed USJ Si solar cells are outstanding comparing with the academic community in the world.
In 2014, the high conversion efficiency 24.7 % was achieved using SHJ solar cell by Panasonic. Although the c-Si surface passivation technology is a key process for achieving high efficiency SHJ solar cells as be known to all, the mechanism of passivation technique and how to prepare thin film with excellent passivation quality are still hard. Therefore, in order to further understand and develop the thin film passivation technique, cooperating with domestic PECVD to deposit intrinsic hydrogenated amorphous (a-Si:H) thin film was performed. In this thesis, the material of a-Si:H thin film was main investigated. It should be noted that the a-Si:H thin films with excellent passivation quality were prepared by domestic PECVD. According to our experimental results, the best passivation quality results from films are consistent with the low microstructure parameter, abundant hydrogen content, and high photosensitivity. The high passivation quality can be obtained at the onset of the amorphous–crystalline transition, while the thin film remains in the amorphous phase to form an abrupt interface with c-Si. The excellent effective lifetime, low surface recombination velocity (SRV), and high implied Voc have achieved 4.7 ms, 2.98 cm/s, and 725 mV, respectively. For SHJ solar cells with the structure of Ag/Ti/ITO/a-Si:H(p)/i/c-Si(n)/i/a-Si:H(n)/ ITO/Ti/Ag, the effective lifetime was verified of the unfinished cell structure p/i/c-Si/i/n, n/i/c-Si/i/n, and p/i/c-Si/i/p. Due to the passivation reduction with the boron-doped emitter deposited by ECRCVD result in the reduction of lifetime, hence the graded power (150 W~1800 W) was introduced to improve the lifetime and requirement of conductivity. In addition to optimize the SHJ solar cells, the ultra-thin (50 μm) Si substrates were also used. Above all, the SHJ solar cell on 200 μm n-type textured CZ c-Si wafer was obtained with an efficiency of 17.26 %, FF = 71.75 %, Voc = 660 mV, and Jsc = 36.71 mA/cm2. And the ultra-thin SHJ solar cell on 50 μm n-type planar CZ c-Si wafer was obtained with an efficiency of 12.46 %, FF = 65.40 %, Voc = 651 mV, and Jsc = 29.28 mA/cm2. These results of SHJ solar cells are outstanding comparing with the academic community in Taiwan.
In addition, except for the material of boron-doped epi-Si thin films and intrinsic a-Si:H thin films, the new material of boron-doped microcrystalline silicon oxide (μc-SiO:H) thin films and the intrinsic hydrogenated silicon oxide (a-SiO:H) passivation layer with low absorption coefficient and wide band gap were also developed for the high efficiency solar cells. The boron-doped μc-SiO:H thin film with high band gap (1.83 eV) and low absorption coefficient was achieved. And the a-SiO:H passivation layer with effective lifetime (800 μs) and low absorption coefficient was also achieved. |
