利用SiO2/ITO薄膜作為外偏壓式MOS結構高效率矽太陽能電池透明電極及抗反射層之研究

Performance Characterization of Silicon MOS-structure Solar Cell using The Biasing ITO Transparent Electrode and SiO2/ITO AR-coating

吳佳穎(Jia-Ying Wu)

指導教授 ： 何文章

國立臺北科技大學/電資學院/光電工程系研究所/碩士(2013年)

摘要

本論文為了提升在單晶矽太陽能電池轉換效率，將氧化銦錫(ITO)/二氧化矽(SiO2)/矽半導體(Si)製作成金屬/氧化物/半導體(MOS)結構太陽能電池，並在ITO電極上蒸鍍一層二氧化矽形成雙層抗反射層。當在ITO透明電極上外加偏壓時，使得p-n半導體內空乏區增加，而有效增加在半導體之光吸收體積。另外在ITO薄膜上製作SiO2薄膜形成雙層抗反射層，使較寬波段之入射光反射損失再降低，增強光在半導體內吸收，提升太陽能電池短路電流及轉換效率。
本實驗將磷擴散源以旋轉塗佈(Spin-on film, SOF)方法在p--Si表面，進行900℃磷擴散後在正面形成表面濃度1020 cm-3及深度為0.42μm之n+-Si Emitter。再以電子槍系統蒸鍍Al與Ti/Al於p-Si與n+-Si表面形成背面與正面電極，完成單晶矽太陽能電池(裸電池)。然後，利用電子槍系統蒸鍍SiO2 厚度為20 nm以及在溫度250℃熱濺鍍厚度為50 nm 之ITO透明電極於裸電池上，完成MOS結構太陽能電池製作。最後，將再使用電子槍系統蒸鍍SiO2厚度為47 nm製作抗反射層於MOS太陽能電池之表面。
本文於不同製程階段進行量測及分析，其結果如下：
(1)ITO透明電極製作：厚度為50 nm時，在可見光範圍穿透率達80%以上與片電阻約為253.79 Ω/□。
(2)在ITO結構上製作SiO2抗反射層及反射率光譜量測，厚度為47 nm抗反射效果較佳。
(3)無照光下裸電池及MOS電池之理想因子(n)介於1.5~2之間與逆向飽和電流(Io)介於10-9~10-10 A之間，MOS電池因表面被覆SiO2層，n及Io均較低。
(4)在AM1.5G太陽光下：(a)裸電池(Bare solar cell)之轉換效率(η)為9.36%。(b)蒸鍍SiO2/ ITO透明電極於裸電池形成MOS太陽能電池後，其 η上升到10.50%。(c)在MOS太陽能電池表面蒸鍍SiO2最佳化抗反射層，其η提高到12.25%。(d) MOS太陽能電池之ITO電極加上順向偏壓2.5 V時， 其η上升至16.47%。
(5)MOS太陽能電池進行電容-電壓(C-V)量測分析，以驗證p-n接面空乏區隨ITO電極偏壓變化，光載子之產生與收集也隨偏壓增加而變大。

關鍵字

單晶矽太陽能電池 ； ITO薄膜電極 ； 抗反射層 ； 金屬氧化物半導體

並列摘要

In this study, a Metal/Oxide/Semiconductor (MOS) structure solar cell by using the indium tin oxide (ITO) metal/SiO2/silicon semiconductor was fabricated and demonstrated which indicate significantly increased in photocurrent and conversion efficiency as applying the bias voltage on the ITO electrode. Because the depletion width of p-n junction under the ITO transparent electrode would be extended more deeply and obtained more large volume of absorber when biased voltage increased. As the MOS-structure solar cell coated with a SiO2 layer on ITO surface (double-layer AR-coating), the cell with broadband low reflectance is achieved. Thus the short-circuit current and conversion efficiency of MOS solar cell are further enhanced.
The first, we used the Spin-on film (SOF) diffusion technology, which spun the phosphorus (P) diffusion source on the front side of p-Si wafer and annealed treatment at 900℃ by RTA. The surface concentration and depth of n+-Si emitter of 1020 cm-3 and 0.42 μm were obtained. Secondly, Al electrode on p-Si and Ti/Al electrode on n+-Si were evaporated by electron-beam system and annealed in the RTA chamber to obtain good ohmic-contact electrode. Therefore, the single crystal silicon bare-type solar cell was created after isolation etching. Thirdly, the 20-nm-thick SiO2 layer upon the surface of bare solar cell was deposited by electron beam, and the 50-nm-thick indium tin oxide (ITO) upon SiO2 layer was deposited by a thermally sputtering at temperature above 250℃. Thus, the MOS-structure solar cell was formed. Finally, a SiO2 layer was deposited on the surface of MOS solar cell for anti-reflection by electron beam evaporation.
The characteristics of the fabricated cells were measured in different stages as shown in the following.
(1) The average transmittance of 80% and sheet resistance of 253.79 Ω/□ were obtained from the ITO film obtained.
(2) The optimum anti-reflection condition of SiO2 layer upon ITO film was the thickness of 47 nm.
(3) The dark I-V characteristics of a bare solar cell and MOS-structure solar cell show that reverse saturation current (Io) between 10-9A and 10-10 A and ideality factor (n) of 1.5~2 at room temperature are obtained.
(4) Under AM1.5G illumination at temperatures of 25℃, (a) The conversion efficiency (η) of 9.36% was presented in bare solar cell. (b) Bare solar cell with SiO2 layer and ITO film electrode (MOS-structure solar cell)：η of 10.50% was obtained. (c) The SiO2 layer deposited upon the surface of MOS solar cell：η of 12.25% was presented. (d) ITO electrode biased at 2.5 V, the conversion efficiency of 16.47% is achieved.
(5) The capacitance-voltage (C-V) of MOS-structure silicon solar cell was measured and characterized.

並列關鍵字

Single crystal silicon solar cells ； ITO electrode ； anti-reflection coating ； Metal Oxide Semiconductor

參考文獻

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