利用不同厚度之SiO2和TiO2空間層及ITO透明電極製作MOS結構矽太陽能電池之研究

Performance Characterization of Silicon MOS-structure Solar Cell Based on Using ITO Transparent Electrode on the Thin TiO2 and SiO2 Space Layer

黃旻駿(Min-Chun Huang)

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(2014年)

摘要

本論文是利用金屬/氧化物/半導體(MOS)結構的概念導入到單晶矽太陽能電池製作，以提升光電流及轉換效率之研究。利用SiO2和TiO2兩種不同的介電層去探討其對於MOS太陽能電池光學和電學的影響，再利用濺鍍並將基板加熱的方式將氧化銦錫(ITO)濺鍍在介電層上作為透明金屬電極，本研究之ITO金屬/氧化物結構就相當於一層抗反射層，並在可見光波段有很低的反射率及高穿透率。當ITO透明電極加偏壓後會使底層pn半導體之空乏區增大，如此可增加其有效光吸收體積，太陽能電池的短路電流及轉換效率也隨之提高。
本論文首先利用旋轉塗佈(Spin-on film, SOF)方法，將磷(P)擴散源塗佈在p-Si的正面，在高溫爐進行擴散製程後，製作出正面n+-Si Emitter層。其次，利用電子束蒸鍍系統將Al蒸鍍於p-Si表面形成背面電極，再經由熱退火處理形成良好歐姆接觸。再將Ti/Al蒸鍍在n+-Si表面形成正面電極，完成單晶矽太陽能裸電池結構製作。接下來利用電子槍蒸鍍 SiO2或TiO2兩種材料當作空間層，並分別探討不同厚度下空間層對太陽能電池特性的影響。再以射頻磁控濺鍍系統，先將基板加熱至250℃，在氬氣環境下濺鍍ITO透明導電薄膜於裸電池表面。本文所獲得之ITO透明電極平均穿透率約80%以上(350~1100 nm)及導電率約為158.73(Ω．Cm)-1。
在MOS結構中空間層材料以及厚度的選擇上，不管是在反射率、漏電流或是加偏壓後效率的提升，均是TiO2 (20 nm)的表現最佳。在AM1.5G、25℃照光條件下，量測所完成之最佳單晶矽太陽電池於不同製程階段之特性參數如下：(1)裸電池(電池遮蔽率為10.14%，電池面積為0.16 cm2)：開路電壓(Open circuit voltage, Voc)為0.55V，短路電流密度(Short circuit current density, Jsc)為25.40 mA/cm2，轉換效率(Conversion efficiency, η)為10.68%；(2)經蒸鍍完TiO2 (20 nm)與熱濺鍍ITO透明電極(MOS結構)後：Voc為0.56V，Jsc為33.31 mA/cm2，η為14.06%；(3)ITO閘電極加順向偏壓 +4V時，太陽能電池的Voc提升至0.57V，Jsc提升至47.19 mA/cm2，η提升至19.68%。最後以多頻LCR測試儀對電池進行電容-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，其電容在0 V時為4.79 nF，ITO偏壓為4 V時，電容下降到2.25 nF，由C-V量測結果得知p-n接面之感應空乏區隨ITO偏壓增加而增加。

關鍵字

單晶矽太陽能電池 ； ITO電極 ； 背面電場 ； 金屬氧化物半導體

並列摘要

In this study, a Metal/Oxide/Semiconductor (MOS) structure solar cell was fabricated and demonstrated. Significantly increased in photocurrent and conversion efficiency were obtained when the cell with a biasing voltage. The optical and electrical properties for MOS solar cell using SiO2 and TiO2 as a dielectric space layer are investigated. The transparent indium tin oxide (ITO) electrode was deposited upon the dielectric layer by a thermally sputtering. The Metal/Oxide structure also presented a good anti-reflection (AR) properties and has broadband low reflectance on visible wavelength. 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. Thus, the short-circuit current and conversion efficiency of MOS solar cell are further enhanced.
The first, the liquid type phosphorus (P) source was spun upon the front-side of p--Si wafer, and then anneal by RTA at 900°C. Then the n+-Si emitter layer on the front side was formed. Secondly, Al film was evaporated by electron-beam system on p-Si and annealed in the RTA chamber to obtain good ohmic-contact electrode. Next Ti/Al film was evaporated by electron-beam system on n+-Si. After isolation etching, the single crystal silicon bare-type solar cell was created. Then, the SiO2 and TiO2 layer was deposited upon the surface of bare solar cell by electron-beam evaporation, and the indium tin oxide (ITO) was deposited upon dielectric layer by a thermally sputtering at temperature above 250℃ in H2. Thus, the MOS-structure solar cell was formed. Transmittance of 80% (wavelength range 350~1100 nm) and conductivity of 158.73(Ω．cm)-1 were obtained from the obtained ITO film in this study.
In the selection of the material and the thickness of the space layer, the cell with a 20 nm TiO2 space layer has the best performance in the measurement of reflectance, leakage current and the increasement of effiency using the bias voltage. Under AM1.5G illumination and at temperatures of 25°C, the characteristics of the fabricated cell are characterized in different stages. (1) Bare solar cell (shadowing of 10.14%, area of 0.16cm2) : The open-circuit voltage (Voc) of 0.55V, short-circuit current density (Jsc) of 25.40 mA/cm2 and conversion efficiency (η) of 10.68% are presented. (2) Solar cell with TiO2 (20 nm) and ITO layer (50 nm)：Voc of 0.56V, Jsc of 33.31 mA/cm2 and η of 14.06% are obtained. (3) Solar cell with TiO2 (20 nm) / ITO layer (50 nm) and ITO electrode biased at + 4V：Voc is increased from 0.56V to 0.57V, Jsc is increased to 47.19mA/cm2, and η is increased to 19.68%. The capacitance-voltage (C-V) of MOS-structure silicon solar cell was measured to confirm the depletion width as a function of the biasing voltage.

並列關鍵字

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

參考文獻

[2] Ｕ.S. EIA, “Internation Energy Outlook,” 2013.

[4] H. K. Pulker, G. Paesold, and E. Ritter, “Refractive indices of TiO2 films produced by reactive evaporation of various titanium-oxygen phases,” Appl. Opt., vol. 15, 1976, pp. 2986-2991.

[5] A. Armigliato, G. Celotti, S. Guerri, P. Ostoja and R. Rosa, “Characterization Of TiOx Films And Their Application As Antireflection Coatings For Silicon Solar Cells,” Sol. Cells, vol. 3, 1981, pp. 195-208.

[6] S. Schiller, G. Beister, W. Sieber, “Influence Of Deposition Parameters On The Optical And Structurl Properties Of TiO2 Films Produced By Reactive D.C. Plasmatron Sputtering,” Thin Solid Films, vol. 83, 1981, pp. 239-245.

[7] W. D. Munz, J. Heimbach and S. R. Reineck, “Reactive High Rate Sputtering Of Oxides,” Thin Solid Films, vol. 86, 1981, pp. 175-181.