鎵元素在銅銦鎵硒薄膜太陽能電池吸收層之偏析行為研究

Study of segregative behavior for Gallium in Cu(In1-x,Gax)Se2 based thin film solar cells

丁于庭(Yu-Ting Ting)

指導教授 ： 蘇程裕

國立臺北科技大學/機電學院/製造科技研究所/碩士(2013年)

https://doi.org/10.6841/NTUT.2013.00621

摘要

本研究應用兩階段製程製備薄膜太陽能電池之吸收層，在此製程中，Se原子團藉由擴散的方式與CIG金屬前驅層進行硒化反應(senelization)。然而，硒化製程中容易因為化學親和力(chemical affinity)的影響，造成前驅層中In原子向Se濃度較高之薄膜表面遷移，進而導致Ga原子向薄膜底部沉積，以至於產生相分離、元素偏析等現象，進一步影響CIGSe薄膜之光電特性。有鑑於此，本研究以直流磁控濺鍍系統搭配硒化/硫化製程，利用CuGa合金靶與金屬In靶堆疊濺鍍不同Cu/(In+Ga)比例之CIG前驅層;隨後進行硒化反應，並探討硒化製程中Ga元素偏析之現象，深入研究此現象對於薄膜的表面形貌、晶體結構及光電特性的影響。
研究結果顯示，隨著Cu/(In+Ga)比例的增加，會使薄膜表層晶粒大小增加，經XRD與Raman檢測結果為單一相的CuIn0.7Ga0.3Se2黃銅礦結構，同時亦可發現硒化後的薄膜皆有嚴重的Ga元素偏析現象，利用TEM觀察後發現薄膜底部析出相為CuGaSe2。此外，EPMA及AES檢測結果也針對此薄膜元素分布不均之現象進行佐證。在薄膜光電性質部分，本研究所製備之薄膜皆呈現p-type導電形式，載子濃度則依Cu/(In+Ga) 比例不同，而在1015-1018 cm-3變化。由穿透率對波長作圖後可知薄膜能隙值隨Ga含量增加而提升，但因Ga無法有效置換黃銅礦結構中的In，因此能隙值增加效果有限。
對此，本研究提出以後續硫化處理以改善CIGSe薄膜的均勻性及Ga偏析現象，於硫化製程中將針對溫度及持溫時間做一系列的探討，觀察此參數對CIGSeS薄膜的表面形貌、晶體結構及光電特性的影響，研究結果顯示，添加S後可有效使CIGSeS均質化，SEM顯示其原有的偏析層厚度下降至300nm，薄膜表層之Ga含量亦有提升趨勢，能隙值也因此增加0.6eV，更能有效地增加元件的開路電壓，使得太陽能電池的轉換效率得以改善。

關鍵字

CIGSe ； CIGSeS ； Ga偏析 ； 兩階段製程 ； 太陽能電池

並列摘要

In this research, the tranditional two stage process was utilized to fabricate CIGS absorber.In the two stage process, CIGSe films are prepared by selenization of CuInGa (CIG) metallic percursors that CIG precursors are subjected to get diffused with Se vapor. However, In atoms might migrate toward the surface with higher Se concentration during selenization because of chemical affinity of In-Se and Ga-Se. the result would contribute to the accumulation of Ga atoms, causing detrimental influences such as Ga segregation or CuInSe2 and CuGaSe2 phase separation and further affect PV performance of CIGSe solar cells. In this study, we thus prepared the CIG precursors with various Cu/(In+Ga) ratio by sequential sputtering of In and Cu3Ga targets in DC sputtering system, followed by selenization process. In addition, we also investigated the effect of Ga segregation on the micro-structural and electro-optical characteristics on CIGS thin films.
The results indicate that the crystalline size in the morphology observation continue to extend with an increase in Cu/(In+Ga) compositional ratio. Moreover, the single phase chalcopyrite CuIn0.7Ga0.3Se2 is predominant in the XRD and Raman results. The phenomenon of apparently Ga segregation can also be observed. After TEM analysis, the phase on the bottom of films shows CuGaSe2. The observation of Ga segregation and elemental distribution is clarified using EMPA and AES as well. In the section of electro-optical characteristicsof CIGSe thin films, all the CIGSe thin film we prepared reveal P-type conductivity. The carrier contentration would alter from1015-1018 cm-3 with varied Cu/(In+Ga) composition ratio. According to result the by extrapolating the slope of the α2(hν) curve and the abscissa, Band gap (Eg) can enhance with an increase in Ga content. However, the In atoms in chalcopyrite structure can not be substituted by Ga efficiently, the trend of extending Eg would be presented apparently.
In this case, we utilized sulfurization process to improve the uniformity and Ga segregation throughout the films. In addition, we investigated the effects of temperature and duration on the characteristics like morphology, structure, and photoelectric property of CIGSeS thin films during sulfurization. The results indicate the influence of S incorporation do exert effect on the homogenization of CIGSeS thin films that can improve uniformity and segregation. The open-circuit voltage (Voc) and Band gap (Eg) would be enhanced, resulting in extending the conversional efficiency.

並列關鍵字

CIGSe ； CIGSeS ； Ga segregation ； Two Stage Process ； Solar Cells

參考文獻

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