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  • 學位論文

非晶微晶矽薄膜太陽電池照光穩定度與軟性電子之研製與分析

Fabrication and Analysis of Amorphous Microcrystalline Silicon Thin Film Solar Cells under Long-Term Illumination and Flexible Electronics

指導教授 : 李嗣涔

摘要


非晶矽材料已經被廣泛的使用於薄膜太陽電池中,相較於傳統矽晶圓太陽電池具有大面積低成本的優點,然而非晶矽導電度的照光衰退效應,將嚴重降低太陽電池於長時間照光使用後的效率。一般而言,薄膜的材料與結構即會明顯影響太陽電池的穩定度。因此,我們討論許多種的適用於太陽電池的薄膜材料。首先,我們以傳統電漿輔助化學氣相沈積法製備非晶矽碳、非晶矽鍺、非晶矽鍺碳薄膜,並利用拉曼光譜及傅立葉轉換紅外光譜,分析加入碳、鍺後對非晶結構的影響。此外我們也討論薄膜於長時間照光下的導電度變化。結果顯示當碳比例Xg(C)=0.1時可得到較佳的穩定度。這樣的改進可能是導因於矽碳晶格與矽氫懸垂鍵之間產生了交互作用,使矽氫懸垂鍵的震動能量不致累積,得以釋放。 另一方面,我們也利用SiF4+SiH4+H2混合氣體來沈積微晶矽膜。薄膜的表面形貌與結晶結構則利用拉曼光譜、原子力顯微鏡、以及穿透式電子顯微鏡來觀察。發現當加入少量的氟化矽烷可同時增加結晶率與成長速度,此微晶矽膜結晶率可達86 %、成長速度3.5 nm/min,其單晶矽顆粒可達20-30 nm。同時我們也藉由表面形貌分析氟化矽烷對結晶結構的影響。我們也利用熱燈絲化學氣相沈積法製作非晶矽及微晶矽薄膜。藉由高溫熱燈絲提高氣體解離率,藉此提高非晶矽與微晶矽薄膜的沈積速率同時維持品質。我們利用光暗電流、傅立葉紅外線光譜儀、拉曼頻譜、紫外/可見光頻譜儀等技術分析薄膜特性,探討一系列製程條件:燈絲溫度、氣體流量、氣體混合比例、等對矽薄膜的影響。更進一步比較薄膜摻雜硼或磷後的結晶變化,發現在高壓條件下少量的硼會與矽氫的產生交互作用使的晶粒更容易形成。我們也找出非晶矽開始結晶成微晶矽的轉移點。 我們以電漿輔助化學氣相沈積法或熱燈絲化學氣相沈積法製備的各種材料來製作太陽電池,並探討太陽電池長時間照光下的衰退行為。首先我們比較了非晶矽、非晶矽碳、非晶矽鍺碳太陽電池。雖然三種材料在材料結構上稍有不同,但是三者製作成太陽電池後其衰退效應是很相似的。此外我們也以熱燈絲化學氣相沈積法製作了p-i-n以及n-i-p基板式太陽電池,並藉由梯度摻雜的硼與磷來控制吸收層中的電場分布。由於梯度摻雜磷於電池上層區形成較強電場,因此n-i-p太陽電池具有較好的初始效率與穩定度。之後,我們也將退化後的太陽電池以退火方式恢復其電性,並研究退火後穩定度的改變。發現受到硼摻雜的p-i-n電池在退火恢復後穩定度有顯著的提升。 除了光照衰退的問題外,薄膜太陽電池的另一缺點就是轉換效率太低。因此,利用微結構來增加光在薄膜介面中的散射路徑,進而提升電池的光吸收率以及光電轉換效率是目前普遍使用的方法。然而要在軟性基板上製作微結構,則受限於軟性材料的不耐高溫且抗酸鹼度低的特性。因此,我們研究在軟性聚亞醯胺基板上,利用轉印方式將硬質模板上的微結構複製到軟性基板表面,將適當的隔離層加入硬質基板與聚亞醯胺間,成功調整兩者間的附著力,使得轉印時不會因附著力過強造成脫模失敗。利用此軟性微結構基板,我們成功製作出軟性非晶矽薄膜太陽電池,相較於一般太陽電池其效率提升比率達17.8 %。之後我們將上述技術製作出的軟性太陽電池元件分別在拉伸/壓縮載台上測量其電性變化。具微結構的太陽電池不論在拉伸或壓縮應力下,於曲率半徑1公分的情況下皆呈現約25-40 %的效率下降,而無微結構的太陽電池則在受壓縮應力有較明顯的降低。

並列摘要


The hydrogenated amorphous silicon (a-Si:H) has been widely applied in thin film solar cells. The benefit was large area and low cost process than single crystal silicon. However, the light induced degradation in the conductivity of a-Si:H would seriously lower the efficiency of solar cell after long-term illumination. Typically, the material and structure of film would significantly affect the stability of solar cell. Therefore, some kinds of thin film materials are discussed. Firstly, the a-SiC, a-SiGe, and a-SiGeC layers are deposited by conventional plasma enhanced chemical vapor deposition. The effect of Ge and C on the amorphous structure is deduced by Raman spectroscopy and fourier transform infrared spectroscopy. In addition, the variation of the conductivity of the films under long-term illumination is discussed. A better stability of a-SiC layers is obtained when Xg(C)=0.1. The improvement of stability might be attributed to the coupling of Si-C network and Si-H dangling bonds and the accumulated energy of Si-H sites can be easily released. On the other hand, the microcrystalline silicon (μc-Si) is deposited using SiF4+SiH4+H2 gas mixture. The film morphologies and crystalline structure were studied by Raman spectroscopy, atomic force microscope (AFM) and transmission electron microscopy (TEM). It is found that a subtle SiF4 could significantly improve both the crystalline fraction (85 %) and deposition rate (3.5 nm/min) of microcrystalline silicon. The grain size is 20-30 nm. The surface morphology is also investigated to deduce the influence of adding SiF4 on crystalline structure. Hot-wire chemical vapor deposition (HWCVD) is also used to deposit a-Si:H and μc-Si films. The extremely high temperature filament would significantly improve the gas dissociation. The a-Si:H and μc-Si films could thus be deposited in a high rate with good quality. Some methods including photo/dark-conductivity, Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, UV-visible spectroscopy were used to investigate the film quality. The influence of deposition parameter: filament temperature, gas flow rate and gas-mixture ratio were discussed. Furthermore, the crystallinity of boron and phosphorous doped μc-Si is studied. It is found that under low substrate temperature, the interaction between boron and Si-H would improve the crystal formation. The condition of a-Si:H transit to μc-Si is also investigated. Numerous materials deposited by PECVD or HWCVD are used in the solar cell fabrication. The degradation behaviors of the solar cells under long-term illumination are systematically investigated. Firstly, a comparison of the degradation behaviors between a-Si:H, a-SiC and a-SiGeC solar cells fabricated by PECVD are reported. Even though there are some different in the structural and electrical properties, however, the degradation behaviors of the three solar cell is very similar. In addition, the p-i-n and n-i-p substrate-type thin film solar cells were fabricated by HWCVD. Graded B- and P-doping was used to modify the electric filed distribution in the i-layer and thus significantly affects the efficiency and stability of solar cells. The graded P-doping causes a stronger electric field near the upper region. Therefore the n-i-p cell has better initial efficiency and stability than the p-i-n cell. Furthermore, the degraded solar cells were annealed to restore the light-induced degradation. Through light-soaking and annealing, the stability of p-i-n solar cell is dramatically improved and the cell would suitable for long-term usage. Besides the light-induced degradation, another drawback of thin film solar cells is the conversion efficiency is typically low due to the amorphous structure. Surface texturing is thus widely employed on thin film solar cells to enhance the light trapping effect. Several methods have been reported to fabricate textured surface, however, some of processes would damage the plastic substrate. Therefore, a new method is developed to form the textured PI substrate by copying the surface morphology of a rigid substrate as a template. A buffer layer is introduced to moderate the adhesion between rigid substrate and polyimide. The polyimide could therefore be peeled off without damage. A flexible solar cell is fabricated on such textured polyimide substrate. The conversion efficiency is higher than the conventional solar cell in a fraction 17.8 %. In addition, the electrical characteristics of the solar cells are measured under different bending conditions and both tensile and compress stress are applied. When the radius of curvature is equal to 1 cm, the efficiencies of textured solar cells degrade apparently 25-40 %, whereas the non-textured solar cells display a larger degradation under compress stress then tensile stress.

參考文獻


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