前驅物堆疊及硒硫化製程對CIGS太陽電池之微觀組織與電性改善之研究

Study on the Microstructure and Performance of CIGS Solar Cell by Stacked Precursors and Selenization/Sulfurization Process

吳政翰(Wu, Cheng Han)

指導教授 ： 朝春光 ； 吳樸偉

國立交通大學/工學院/材料科學與工程系所/博士(2019年)

摘要

本研究係以二階段前驅物溅鍍及無毒性硒化方式製備 CIGS太陽能電池。前驅物金屬層分別是以 Cu0.7Ga0.3 和 In 靶交替堆疊，其中 In (220 nm)/CuGa (150~300 nm)/In (450 nm)金屬層是以直流溅鍍方式鍍製於含 Mo 的鈉玻璃基板上，隨後並以蒸鍍方式製備 Se 層完成前驅物。完成的前驅物經過三階段硒化熱處理並在退火最高溫持溫 20 分鐘後，將 CIGS 吸收層化學計量比控制在 Cu/Im+Ga 與 Ga/In+Ga 分別為 0.93 及 0.34。從 XRD 分析中發現，在 280℃左右 CIGS 三支獨立的繞射峰(112),(220/204),(312/116)開始出現並且強度隨著溫度增加而增強直到 550℃，而硒化的持溫退火時間從 10 分鐘增長至 20 分鐘，會讓 CIGS 吸收層表面更平坦、緻密，晶粒也變更大。拉曼光譜分析中顯示主峰174cm-1與兩支較弱的分峰(212 與 231cm-1
)，這樣的 CIGS 太陽電池電池元件經過適當的硒化處理後，元件最高效率達12.58%。
而經 500℃低溫硫化後，CIGSS 表面結構不僅更平整也更緻密，XRD 分析發現到會多一支 CIGSS 繞射峰並往右邊偏移，主要是因為 S 原子半徑較 Se 原子半徑小，使得 S 原子更容易產生取代的效果，當 d 變小θ值就會變大，繞射峰就會往右邊偏移，同時能隙從未硫化的 1.19eV 增加至 1.34eV(500℃,10 分鐘)。SISM 分析也顯示 S 原子擴散在 CIGS 表層深度約有 250nm。經過 500℃且 5 分鐘持溫的 CIGSS 元件樣品效率可增加至13.30%。
為了獲得更高效率的元件，透過後退火的方式調整 CIGS 吸收層與 CdS 表面的結構關係，CdS 能隙不僅會隨退火溫度增加而增加，經退火後的 CdS 表層結晶會比未退火的 CdS 來得更小且平整，另外發現到退火後的 CdS 膜厚會有變薄的現象，這現象與 XPS 分析結果一致，推測 CdS 會透過擴散方式進入到CIGS 形成更好的 P-N 介面特性，將 CdS/CIGS 空乏區介面擴大並更靠近 CIGS 以減少複合載子缺陷產生，而這樣的結果也可以看到在 300℃的退火溫度，元件效率約可增加 3.12 到 4.71%。

關鍵字

CIGS 太陽能電池 ； CIGSS 太陽能電池 ； 前驅物溅鍍 ； 硒化 ； 硫化 ； CdS 緩衝層後退火

並列摘要

In this research, Cu (In, Ga)Se2 (CIGS) films were fabricated using a two-step precursors sputtering and selenization process. Precursors stacked with In (220 nm)/CuGa (150~300 nm)/In (450 nm) layers are deposited onto Mo bilayer soda-lime glass by sputtering, using Cu0.7Ga0.3 and In targets, followed by vapor stacking of the elemental Se layers.Using three sequential stages for the selenization process, with an annealing time of 20 min, the stoichiometry of the CIGS absorbers with
the Cu/(In+Ga) and Ga/(In+Ga) controlled at atomic ratios of 0.93 and 0.34, respectively. From XRD analysis, three independent CIGS (112),CIGS (220/204) and CIGS (312/116) occurred crystallization at ~280oC and phase peaks intensity reinforced until 550oC. With the increased
selenization annealing time from 10 min to 20 min, the CIGS absorbers were evidently dense and relatively smooth, and the CIGS grains size became larger. Raman spectra of CIGS absorbers revealed a main peak (174 cm-1
) and two weak signals (212 and 231 cm-1). The performance of CIGS solar cell device prepared with proper selenization, a maximum efficiency of 12.45% was obtained.Through sulfurization, CIGSS surface structure become smooth and densely. From XRD analysis, the peak (112) of CIGSS films shifts after sulfurization. Because S has a smaller atomic radius, so it is easily substituted for Se. The band gap energy was increased from 1.19 eV (for the as-grown) to 1.34 eV (for the absorber layer that is sulfurized at 500oC for 10 min). SISM analysis also showed that the S atoms diffuse into the CIGS surface region to a depth of approximately 250nm. At 500℃,5 min holding time, the best efficiency of CIGSS solar cell device increased to 13.30%.
In order to get higher efficiency of solar cell, through annealing process to modify the interface of CIGS absorber layer with CdS buffer layer. The band gap of CdS thin film was increased from 2.51eV (unannealing) to 2.35eV (350℃ annealing) through by the annealing
temperature 150、200、250、300、350℃. The surface grains of CdS layer for annealing process were smaller and smoother than un-annealing process. Besides, the thickness of CdS buffer layer was decreasing after annealing process, we inferenced that the CdS diffuse into CIGS layer correspond to XPS analysis. The benefit was the depletion zone of the interface of CdS/CIGS layer move into CIGS layer to avoid trapping at the more defect interface. Hence, the carriers were collected more easier
to gain efficiency more than 3.12~4.71% at 300℃ annealing temperature.

並列關鍵字

CIGS solar cell ； CIGSS solar cell ； precursor sputtering ； selenization ； sulfurization ； annealing CdS buffer layer

參考文獻

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Chapter 1

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