高效率濺鍍銅銦鎵硒太陽電池中鎵分佈及晶粒成長之研究

A study of Ga distribution and grain growth in a high efficient CuInGaSe2 solar cell prepared in a sputtering process using a single CuInGa precursor

許弘儒(Hsu, Hung Ru)

指導教授 ： 劉容生

國立清華大學/電機資訊學院/光電工程研究所/博士(2012年)

https://doi.org/10.6843/NTHU.2012.00111

摘要

銅銦(鎵)硒(Cu(In,Ga)Se2)薄膜是目前最具有潛力的太陽電池材料之一，能隙寬為1.02eV屬直接能隙半導體材料，光學吸收係數為 105 cm-1，所以，只需要1μm厚度便可以吸收90%以上的太陽光。經由控制材料中元素的組成比例，可使得銅銦(鎵)硒薄膜不需要經過額外摻雜便可直接形成n 或p型半導體。如果利用鎵元素來取代部分的銦的位置，可使得能隙從1.04 eV提高到1.68 eV。同理亦可利用硫元素來取代部分的硒的位置，使其形成銅銦(鎵)硒(硫) (Cu(In,Ga)(Se,S)2)薄膜來提高元件的開路電壓(Voc)。
在銅銦(鎵)硒太陽電池的製造技術中，目前以共蒸鍍製程(co-evaporation process)及濺鍍加硒化製程(sputter and selenization process)最具有發展潛力。共蒸鍍製程目前保有小面積銅銦(鎵)硒太陽電池元件最高效率(20%)的紀錄，但卻有著大面積量產不易的缺點。這時利用濺鍍製程沉積銅銦鎵(Cu-In-Ga)前驅物，再通入含硒蒸氣來成長銅銦(鎵)硒薄膜是最有機會實現大面積量產的技術，目前大面積銅銦(鎵)硒太陽電池模組的最高效率為利用濺鍍加硒化製程所保持的15.7%。
在目前濺鍍加硒化製程技術中，主要是利用濺鍍銅鎵(CuGa)及銦(In)靶在鉬玻璃上沉積銅銦鎵前驅物，再利用硒蒸氣或硒化氫(H2Se)與其反應形成銅銦(鎵)硒薄膜。在目前的製造方式中，主要存在著兩個問題需改善，第一、在濺鍍製程中所使用的銦靶屬於低熔點金屬，使其在濺鍍及成膜的製程中容易產生銦聚集的現象，進而影響銅銦(鎵)硒電池元件效率的均勻性。第二、在硒化製程中，鎵元素容易在銅銦(鎵)硒吸收層底層聚集，造成元件開路電壓的不易提升。為了改善上述的問題，本論文的主要研究可分成三個部分:
一、不同銅銦鎵(Cu-In-Ga)前驅物結構的研究及改善:在本論文中利用銅銦鎵三元合金靶來取代傳統銅鎵及銦靶做為濺鍍製程的主要靶材，由我們的實驗結果可以觀察到，傳統銅鎵及銦的疊層方式在硒化反應中，由於銦的聚集使得在硒化反應後，銅銦(鎵)硒薄膜容易產生未完全反應的銅硒(Cu2−xSe)及銦硒(InSe)二次相，這對元件的效率將造成影響，而利用銅銦鎵三元合金靶所沉積的薄膜，明顯的改善傳統沉積方式所產生銦聚集現象，進而改善元件效率的均勻性。針對硒化反應後鎵聚集的問題，在本論文中提出，利用銅銦/銅銦鎵/銦(CuGa/CuInGa/In)的三明治結構來改善銅銦鎵前驅物經硒化後，鎵的分佈及晶粒的成長。由實驗結果可以觀察到，當在銅銦鎵前驅物表面濺鍍一層厚度約125nm的銅鎵層，經硒化反應後，銅銦(鎵)硒薄膜的晶粒明顯的長大，再經由XPS的分析得知，吸收層表面Ga/(In+Ga)的比例由22%提高到32%，這使得元件的開路電壓由390 mV提升至460 mV，提升18.2%。當將膜厚約80 nm的銦層置入銅銦/銅銦鎵(CuGa/CuInGa)前驅物中時，經硒化反應後，可以觀察到銅銦(鎵)硒元件在長波長(900-1150 nm)光的吸收明顯增加，這使得元件的短路電流密度(Jsc)由29 mA/cm2 提升至 33 mA/cm2，提升13.8 %。經由銅銦/銅銦鎵/銦的三明治結構所做成的銅銦(鎵)硒太陽電池元件，可將效率由6.26 %提升至9.52 %，提升50 %。
二、硒化製程對銅銦(鎵)硒薄膜晶粒成長及鎵分佈的研究: 在本研究中針對不同的硒化方式包括1. RTP硒化、2. 硒化氫(H2Se) 硒化及3.硒化後硫化製程(sulferization after selenization process)來討論在不同硒化條件下，對於銅銦(鎵)硒薄膜中晶粒成長及鎵分佈的影響。1. 在RTP硒化製程中，我們利用快速升溫的方式來觀察升溫速率對吸收層中鎵分佈的影響，由實驗結果發現在硒化製程中利用快速升溫的方式，使得升溫過程中銅銦硒(CuInSe2)及銅鎵硒(CuGaSe2)的相幾乎同時生成，由XPS的結果發現鎵並沒有往底部聚集。這個發現提供給我們一種改善甚至有機會控制銅銦(鎵)硒薄膜中鎵分佈的可能方式。2. 在硒化氫(H2Se)硒化製程中，我們發現利用提高硒化溫度到550 oC以上時，可以使得在吸收層底層較小晶粒的銅鎵硒(CuGaSe2)晶粒成長，進而改善元件的電性及效率，使元件效率由9.5% 增加到12.8%，提高34%。3. 在硒化後硫化製程中，我們發現利用降低第一階段硒化製程的溫度，可使得後續硫化製程中的硫更容易進入銅銦(鎵)硒硫的薄膜中，提高元件的開路電壓。但過多的硫元素會讓吸收層的晶粒不易成長，而減少元件電流的收集。經由硒化後硫化製程對開路電壓的改善，可使得元件的效率達到14%。
三、改善銅銦鎵硒太陽電池近紅外光波段光的吸收：由銅銦(鎵)硒太陽電池元件外部量子效率的量測結果發現，電池元件在近紅外光波段的光吸收較差。這是由於目前用來做透明導電層(TCO)的鋁摻雜氧化鋅(ZnO: Al)在近紅外光波段中光的穿透率較差，這是由於ZnO: Al內自由載子在近紅外光波段的光吸收所造成。所以，我們利用在濺鍍純質氧化鋅(i-ZnO)時通入氫氣，與其反應形成氫摻雜氧化鋅(ZnO:H)薄膜，用來取代ZnO: Al層，由實驗結果發現ZnO:H薄膜具有較佳的電子遷移率，可改善在近紅外光波段所產生的自由載子光吸收，使其在近紅外光波段具有較佳的光穿透率，可提高元件的短路電流密度由34.5 到35.6 mA/cm2。

關鍵字

銅銦鎵硒 ； 濺鍍 ； 三元靶 ； 太陽電池 ； 鎵分佈

並列摘要

Chalcopyrite compounds of Cu(In,Ga)Se2 and related alloys are among the most promising materials for photovoltaic applications. Sputtering of Cu-In-Ga precursors followed by selenization has been a preferred industrial process for Cu(In,Ga)Se2 solar cell manufacturing. In a sputtering process, many studies using co-sputtering or sequential sputtering from CuGa and In magnetron targets for preparation of the metallic precursors. In this study, the metallic precursors were deposited by sputtering a single Cu-In-Ga ternary target and compared with the In/CuGa stocked precursors and these samples were selenized using the Se vapor. It was observed that Ga tends to segregate near the Mo electrode after selenization thus reducing the band gap of the Cu(In,Ga)Se2 absorber near the surface. Since the open circuit voltage (Voc) depends on the band gap in the space charge region (SCR) near the surface of absorber. The device fabricated using this process, however, tends to have a relatively low Voc value due to the Ga element migration to near the Mo electrode.
一、Proposed and demonstrated a novel sandwiched precursor structure
we demonstrated a novel sandwiched structure to improve the Ga distribution and grain growth in the absorption layer and thus increase the open circuit voltage Voc and Jsc. We discuss the employment of a novel precursor structure using a single CuInGa layer sandwiched between thin CuGa and In layers. This precursor structure was constructed by having a thin CuGa film on top of the CuInGa ternary layer and a thin In layer to the bottom of the CuGa/CuInGa stacked layer. It is observed that when a thin CuGa film was sputtered on top of the surface of CuInGa ternary precursor, it enhanced the grain growth of Cu(In,Ga)Se2 absorber and increased the Ga concentration in the space charge region, therefore improved the open circuit voltage (Voc). In addition, we observed when a thin In layer was added to the bottom of CuGa/CuInGa stacked layer, it reduced the minimum band gap of devices, and therefore increased the absorption of solar spectrum. By employing this novel structure, the open circuit voltage for the solar cell devices in our studies increased by 18.2% (from 390 mV to 460 mV), the short current density by 13.8% (from 29 mA/cm2 to 33 mA/cm2), and the conversion efficiency by 50 % (from 6.26 % to 9.52 %).
二、Investigation of selenization and sulferization process
we discuss three kinds of selenization methods including (a) the RTP process, (b) H2Se selenization process and (c) sulferization after selenization process which were used in studying the Ga distribution and grain growth of Cu(In,Ga)Se2 absorbers under these three selenization processes. In an experiment study of selenization using RTP, our result shows by shortening the annealing time, CuGaSe2 and CuInSe2 would produce almost within the same time and therefore could reduce the segregation of Ga into the bottom of Cu(In,Ga)Se2 absorber. These experiments directly confirmed that the segregation of Ga element due to a difference in the formation temperature of the CuGaSe2 phase higher than that of the CuInSe2 phase.
From the results of our study of the H2Se selenization process using XPS and SEM analyses, it further suggests a higher selenization temperature did not affect the Ga distribution in the absorber, however, it could enhance the grain growth near the bottom of Cu(In,Ga)Se2 absorber. As a result, it lead to an increase of the conversion efficiency of the solar cell devices from 9.5% to 12.8%; an enhancement of about 34%.
From the experiment results of sulferization after the selenization process, the sulfur element incorporate into the Cu(In,Ga)Se2 absorber would form smaller grains. By comparing the results of GIXRD and SEM, it suggests that a lower selenization temperature would increases the S content in the surface area of Cu(In,Ga)(Se,S)2 film and form smaller grains of absorber and the energy band gap of the absorber. As a result, by using sulferization after the selenization process, the open circuit voltage (Voc ) of the device was further improved by 10%, and the overall conversion efficiency of the solar cell devices increased by about 10% from 12.8% to 14%.
三、Near infrared enhancement in Cu(In,Ga)Se2-based solar
Near infrared enhancement in Cu(In,Ga)Se2-based solar cells utilizing a ZnO:H window layer were also investigated in this study. The hydrogen atoms incorporated into a ZnO film as a shallow donor could decrease the resistivity of ZnO film. The ZnO:H film has sa imilar resistivity to that of the ZnO:Al film of about 1.29×10-3 Ω-cm. The advantage of ZnO:H film is higher Hall mobility than ZnO:Al film and thus the carrier concentration of ZnO:H film is lower than that of ZnO:Al film which can decrease free carrier absorption in the NIR. It is found that the cell efficiency is enhanced by 4.8% for the ZnO:H device. This is attributed to the fact that the ZnO:H film has higher transmittance than the ZnO:Al film in the NIR which results in the improvement of short-circuit current (Jsc) from 34.5 to 35.6 mA/cm2.

並列關鍵字

CuInGaSe2 ； sputter ； CuInGa ternary target ； photovoltaic ； solar cell ； Ga distribution

參考文獻

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