利用噴射式大氣電漿系統製備高霧度鎵與鋯共摻雜氧化鋅透明導電薄膜

High Haze Ga and Zr Co-doped Zinc Oxide Transparent Electrodes Prepared by Atmospheric Pressure Plasma Jet

吳承洋(Cheng-Yang Wu)

指導教授 ： 莊嘉揚

國立臺灣大學/工學院/機械工程學系/碩士(2022年)

https://doi.org/10.6342/NTU202203026

摘要

具有紋理表面的透明導電氧化物(Transparent conductive oxide, TCO)薄膜通常具有高霧度的特性，能使光產生散射與漫射，增加光的路徑長，有望提高太陽能電池的光電轉換效率。而沉積具有紋理表面的TCO通常需要額外的製程步驟，例如蝕刻和塗佈奈米顆粒，這些步驟會使製程複雜化，成本也隨之增加。本研究使用噴射式大氣電漿系統(Atmospheric pressure plasma jet, APPJ)搭配共摻雜法(Co-doping)一步驟鍍製出高霧度鎵和鋯共摻雜氧化鋅(GZO:Zr)透明電極於5 × 5 cm^2之玻璃基板上，相較於文獻中使用的複雜製程，APPJ系統不需在真空環境下操作，過程中也不需要更換機台以及製程參數，基板溫度也較低(180℃)，既省時又省成本。最佳參數之GZO:Zr (2 at%)薄膜具有高霧度(34.8%)、低電阻率(7.88 × 10^-4 Ω-cm) 和出色的品質參數(Figure fo merit, F.O.M, 8.22 × 10^3 Ω^-1)。本研究發現當2 at% Zr摻雜到 GZO 薄膜中時，霧度從7.19%提升到34.8% (+ 384%)，此霧度的提升可歸因於薄膜表面粗糙度的增加。本研究進行了電子顯微鏡（Scanning electron microscopy, SEM）和原子力顯微鏡（Atomic force microscopy, AFM）來探討薄膜的表面形態。SEM圖像顯示，將Zr摻雜到GZO薄膜中會使薄膜表面產生球狀顆粒。AFM結果表明，這些顆粒會使薄膜之均方根粗糙度顯著增加，從而增加了霧度。此外，鮮少研究針對共摻雜薄膜之化學組成進行探討，因此本研究也透過能量散佈X光光譜儀(Energy-dispersive X-ray spectroscopy, EDS)及X光光電子能譜儀（X-ray photoelectron spectroscopy, XPS）分析共摻雜對薄膜化學組成的影響，EDS結果顯示，粒徑大小較小的球狀顆粒之鋯含量較基材的含量高；而XPS及XRD皆有量測到ZrO2的訊號，再搭配霍爾量測的結果可推測，2 at% Zr之摻雜濃度可能就已超過氧化鋅薄膜之摻雜極限，多數的鋯都是以氧化鋯團簇之形式出現在薄膜中，只有少數的鋯有參與摻雜。綜合上述量測分析，我們推論共摻雜薄膜表面出現球狀顆粒的原因為：反應物過多時會因為解離不完全而預先在空中成核，並被薄膜表面所吸附。
為了驗證高霧度薄膜是否適用於提高鈣鈦礦太陽能電池之光電轉換效率，本研究進一步將GZO及GZO:Zr (2 at%)薄膜組成鈣鈦礦太陽能電池，結果顯示雖然GZO:Zr (2 at%)有良好的光電性質及接近35%的高霧度，但組成之太陽能電池光電轉換效率卻較低，這可能是因為旋轉塗佈法(Spin coating)容易受到底層材料的表面形貌影響，GZO:Zr (2 at%)之表面粗糙度太大將影響旋轉塗佈之均勻性，導致鈣鈦礦層長晶時內部產生缺陷造成元件開路電壓下降，也會使各層間的接觸電阻提升而提高串聯電阻，此外，因為鈣鈦礦太陽能電池之總厚度(不包含透明電極)不超過200 nm，過高的表面粗糙度會使電極刺穿電池造成短路，導致並聯電阻過低使光電轉換效率變低。因此GZO:Zr (2 at%)並不適用於厚度較薄，且製程以旋轉塗佈法為主的鈣鈦礦太陽能電池，更適合應用於製程不受表面粗糙度影響，或總厚度較厚之電池元件，例如矽基太陽能電池。

關鍵字

大氣電漿 ； 共摻雜 ； 鎵摻雜氧化鋅 ； 鎵與鋯共摻雜氧化鋅 ； 高霧度透明電極 ； 光散射 ； 鈣鈦礦 ； 太陽能電池

並列摘要

Transparent conductive oxides (TCOs) with textured surfaces often have high haze factors. High haze factors enable light to be scattered, leading to longer optical path lengths, promising to enhance solar cells' power conversion efficiency. Deposition of TCOs with textured surfaces often requires additional process steps, such as etching and coating nanoparticles. However, those steps take substantial effort and time, making the process inefficient and complicated to handle. Here, we demonstrate a one-step fabrication process to deposit high haze gallium and zirconium co-doped zinc oxides (GZO:Zr) prepared by atmospheric pressure plasma jet (APPJ) on 5 × 5 cm^2 glasses. GZO:Zr (2 at%) films achieve a high haze (34.8%), a low resistivity (7.88 × 10^-4 Ω cm), and a great Figure of merit (F.O.M, 8.22 × 10^−3 Ω^−1). When 2 at% Zr is doped into GZO films, the haze factor increases from 7.19% to 34.8% (+ 384%). Such an increase in haze may be attributed to the increased surface roughness. We conducted SEM and AFM to investigate the surface morphology of films. SEM images show that doping Zr into GZO thin films creates spherical particles on the film surface. AFM results show that these particles significantly increase the root-mean-square roughness from 18.4 to 122 nm after 2% Zr is doped, thus increasing the haze factor. This study proposes a novel and convenient way to enhance the haze factor of the TCO layer and discover some new phenomena of co-doping TCO thin films. Our vacuum-free method does not need a change of materials or machines/tooling during the process and is suitable for industrial-scale mass production.
In addition, to know the chemical composition of the spherical particles on the surface of GZO:Zr thin film, we conducted EDS and XPS to analyze it. EDS results show that Zr mainly gathers on the smaller spherical particles. XPS and XRD results show the existence of ZrO2 in co-doped films. Combined with the results of Hall measurement, we conclude that 2 at% Zr doping concentration is already too high in our experiment. The excessive reactant would nucleate in the gas phase and then be absorbed on the film surface, hence the spherical particles.
In order to verify whether high haze GZO:Zr thin film would enhance the power conversion efficiency of solar cells, we use GZO and GZO:Zr (2 at%) as the electrodes to fabricate perovskite solar cells. The results show that using GZO:Zr (2 at%) as a front electrode would have lower PCE. This is because the large surface roughness affects the spin coating process significantly. The spherical particles might affect the contact of layers and hinder the crystallization of the perovskite layer. Besides, the spherical particles might contact the perovskite layer to cause shunting. Therefore, GZO:Zr (2 at%) might be more suitable for thicker solar cells or processes that would not be affected by the roughness of electrodes, such as silicon solar cells.

並列關鍵字

Atmosphere pressure plasma jet (APPJ) ； co-doping ； Gallium doped Zinc Oxide (GZO) ； Gallium and Zirconium co-doped Zinc Oxide (GZO) ； High haze electrode ； Light scattering ； Perovskite solar cell

參考文獻

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