氧化鋅薄膜成長技術及其在光電相關元件的應用

A study of ZnO thin film deposition and its optoelectronic applications

楊宏仁(H.J. Yang)

指導教授 ： 黃振昌

國立清華大學/工學院/材料科學工程學系/博士(2015年)

https://doi.org/10.6843/NTHU.2015.00593

摘要

本論文研究氧化鋅薄膜成長並以創新方式應用於矽薄膜太陽電池與異質接面矽基太陽電池。與工研院太電中心技術合作，使用國內廠商自行研發組立之磁控濺鍍系統以及工研院機械所開發之低壓化學氣相沉積系統，研究各沉積參數對氧化鋅薄膜光電性質的影響，找出一最適當的製備參數與薄膜性質，供給太電中心作為矽薄膜太陽電池之基板使用。另外也研究低壓化學氣相沉積法製備BZO薄膜並嘗試創新之太陽電池領域應用。
由研究結果知，使用靶材為氧化鋅摻雜氧化鋁（2 wt%），功率密度為2.26 W/cm2，氧氣流量為O2/Ar = 0.1 %，沈積壓力控制在2 mTorr，電極距離7 mm，脈衝頻率30 kHz，的沉積參數，製備1 μm 厚之AZO薄膜做為基準片，其片電阻為6~8 Ω/□，可見光的穿透率可達80 %以上。再以0.5 %HCl蝕刻AZO表面，以獲得具有texture之良好光電性質的透明導電膜，供給工研院太電中心使用。
另外本研究與工研院共同合作開發多彩的非晶矽薄膜太陽能電池，藉由在非晶矽薄膜太陽電池上基板玻璃外層導入了ACR多層膜結構，使得整體太陽電池元件的顏色可以自由控制，最終提出了五種顏色分別是暗藍色、天空藍、淺黃、暗橘以及藍紅色，且成功地將此五種顏色客製化成NTHU的圖案應用於非晶矽薄膜太陽電池元件上。
另外本研究也嘗試改善以低壓化學氣象沉積技術所製備的BZO薄膜的光電性質，經由導入ITO作為一緩衝層，其中緩衝層厚度介於0~100 nm之間，研究其不同緩衝層厚度對於整體BZO/ITO堆疊薄膜結構的光電性質的影響。最終得到ITO緩衝層可使得整體BZO/ITO堆疊薄膜有較大的晶粒大小以及粗糙的表面，因此更適合應用於太陽電池。最後經由模擬驗證，矽薄膜太陽電池在使用ITO緩衝層厚度為100 nm的BZO/ITO堆疊薄膜，作為前向接觸層的條件下，可使得元件的短路電流提升1.06倍。
接著我們嘗試了另一個創新的想法，將異質接面矽基太陽電池上的前向接觸層ITO薄膜以LPCVD-BZO薄膜取代。由於LPCVD-BZO的光電特性可藉由基板溫度和B2H6流量來控制，且相對於ITO平坦的表面，LPCVD-BZO的薄膜為粗糙的表面，更利於元件光的吸收，更有其優勢，在本研究中，得出將異質接面矽基太陽電池上的前向接觸層ITO薄膜以LPCVD-BZO薄膜取代的情況下，元件效率由15.64 ％提高到16.30 ％，且元件填充因子由72.17 ％提高到78.05 ％。
最後由於非晶矽薄膜太陽電池對於TCO基板的表面形貌有特殊的要求。本研究提出了經由改變沉積溫度調整LPCVD-BZO薄膜的表面形貌，並研究彼此的關係。粗糙的LPCVD-BZO薄膜表面可以增加入射光散射的比例，進而提升非晶矽薄膜太陽電池元件的短路電流。然而太粗糙的基板表面使得元件有孔洞產生，降低了光電流與填充因子，本研究最終提出了最適合應用於非晶矽薄膜太陽電池的LPCVD-BZO製程溫度為160 oC。

關鍵字

氧化鋅 ； HASH(0x5e03820) ； 太陽能電池 ； HASH(0x5e03960) ； 低壓化學氣相沉積

並列摘要

Abstract
In this thesis, we report a study of ZnO thin film deposition method cooperating with Industrial Technology Research Institute (ITRI). Our target is trying to find the most proper deposition parameter and ZnO thin film optoelectronic properties for so-lar cell applications. In our research, the proper optoelectronic properties could be ap-proached by using the deposition parameters, ZnO:Al2O3 target, power density 2.26 W/cm2, O2/Ar = 0.1 %, deposition pressure 2 mTorr, the target distance 7 mm and pulse frequency 30 kHz. After 1 μm AZO thin film was fabricated, the sheet re-sistance 6~8 Ω/□ and transmittance above 80 % for visible light could be achieved under the above parameters. Then etching 1 μm AZO thin film by 0.5 % HCl for tex-ture, the roughness of AZO thin film surface is easily controlled by etching time which could be satisfied by ITRI for silicon thin film solar cell research.
An adjusted color of amorphous silicon thin film solar cells is illustrated by us-ing a multilayer film design. The multilayer film consists of silver (Ag) and gallium doped Zinc oxide (GZO), which serves as an adjusted color reflection (ACR) multi-layer film. The color can be adjusted with wide range from dark blue to blue red col-ors when the GZO thickness increases from 50 to 150 nm. The color adjusted by the ACR multilayer film is in good agreement with the reflectance response of the multi-layer. The influence of the ACR multilayer film on the solar cell performance is dis-cussed.
Boron-doped ZnO (BZO) films were deposited by low pressure chemical vapor deposition (LPCVD) on glass substrates with tin-doped indium oxide (ITO) buffer layers varying from 0 to 100 nm in thickness. The effects of ITO thickness on the structural, optical and electrical properties of BZO/ITO stacks were investigated. The sheet resistances of BZO/ITO stacks were decreased with increasing of ITO thickness while haze factors for BZO/ITO stacks were saturated at an ITO thickness of 50 nm. X-ray diffraction spectra indicate that ITO (222) can promote the preferred orientation of BZO films changing from <110> to <100>. The grain size of the BZO films with ITO buffer layers was larger than that of those without ITO buffer layers. The simu-lated short circuit current of the silicon thin film solar cell when using BZO/ITO stacks as the TCO layers can be increased by a factor of 1.06 at an ITO thickness of 100 nm.
This paper demonstrates the growth of highly-textured boron-doped ZnO (ZnO:B) film by using low-pressure chemical-vapor-deposition (LPCVD) for effi-cient light harvesting and carrier collection in heterojunction silicon-based (HJS) solar cells. The optical and electrical characteristics have been optimized versus the sub-strate temperature and B2H6 flow rate for tradeoffs among the sheet resistance, free-carrier absorption, and optical transmission of blue/green wavelengths. A HJS solar cell with a 1.6 um thick ZnO:B film achieves a high power conversion efficien-cy of 16.30 % and fill factor of 78.05 %, compared to 15.64 % and 72.17 % , respec-tively, from a counterpart with a conventional 80-nm-thick indium tin oxide layer.
Boron doped ZnO (ZnO:B) films were textured simply by changing the deposi-tion temperature used in low pressure chemical vapor deposition (LP-CVD). Surface morphologies of textured ZnO:B films are very sensitive to deposition temperature, which were characterized by using atomic force microscope and scanning electron microscope. The textured ZnO:B films were applied to fabricate amorphous hydro-genated silicon (a-Si:H) thin film solar cells. The a-Si:H thin film was prepared by plasma enhanced chemical vapor deposition (PECVD) operated at 40 MHz. The best device performance of a-Si:H thin film solar cells occurs at 160 oC, which is attribut-ed to the optimization of the surface morphology of ZnO:B. A textured ZnO:B film with a higher root-mean-square (RMS) value can enhance diffusion transmittance and contribute short-circuit current (Jsc). However, critical surface roughness exists, be-yond which long stripe nano-cracks are generated near the surface valleys of ZnO:B. This accompanies with the reduction of Jsc and fill factor (F.F.).

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