| Abstract: | 薄膜太陽能電池普遍具有良好的光吸收能力、短能源回收期以及高可撓性，因此可以應用在廣泛的能源領域中。近年來，無毒且地表豐富的硫銅錫鋅礦(Cu2ZnSn(S,Se)4)太陽能電池引起許多的關注。但是近十年來的研究仍舊無法克服CZTS/Se太陽能電池開路電壓缺失的問題。為了克服開路電壓缺失的瓶頸，深度探討材料本質特性是不可避免的。本研究將使用各式的材料分析技術探索CZTSe太陽能電池材料本身的極限。
嚴重影響CZTSe太陽能電池光電轉換效率的其中一項關鍵原因是CZTSe材料存在的本質缺陷(點缺陷/缺陷團簇)。但是目前的文獻中，陽離子錯位缺陷的特性探索與調控仍舊是個極大的挑戰。因此本研究藉由將銀摻雜進入CZTSe晶體中，進行陽離子替換來調控並鈍化CZTSe能隙中的缺陷能態。本實驗為了能詳盡的探討(AgxCu1-x)2ZnSnSe4(ACZTSe)的缺陷能態特性，因此合成了不同比例的ACZTSe薄膜吸收層(x=Ag/(Ag/Cu)，x = 0.00、0.05、0.10、0.15、0.20)。變強度的低溫光激發螢光光譜的量測結果顯示10%摻雜的ACZTSe具有最淺層的缺陷能態以及較少的非輻射複合。第一原理計算結果也顯示銀摻雜可抑制有害的缺陷產生，同時促進有益的缺陷形成。在太陽能電池元件表現方面，10%摻雜的ACZTSe太陽能電池在開路電壓的表現有顯著的提升，光電轉換效率可以達到10.2%。
為了理解ACZTSe材料的的陽離子錯位、缺陷濃度、整體長程晶體排列以及局部原子結構之間的關係，因此本研究結合了中子衍射以及同步輻射光源的X射線吸收技術進行材料分析。並利用Rietveld精算法量化CZTSe以及ACZTSe粉末樣品中的缺陷濃度。陽離子分布則被用來作為量化本質點缺陷的依據。精算結果發現銀摻雜可以有效的抑制CuZn錯位的深層缺陷並同時促進淺層缺陷的形成(Cu空缺)，進一步的改善太陽能電池的光電轉換效率。此外，本實驗將X射線吸收光譜作為函數進一步分析ACZTSe的原子結構，從X光近緣吸收光譜(XANES)以及延伸X光吸收精細結構(EXAFS)結果可以獲得最近殼層的結構、氧化態以及電荷轉移機制(偵測元素周圍的電子佔據狀態)
;Renewable energy sources based on thin film solar cells (TFSC) can effectively contribute in a wide variety of energy field due to their higher light absorbance, shorter energy payback time, and high flexibility. Recently, researches on earth-abundant and non-toxic kesterite based Cu2ZnSn(S,Se)4 (CZTS/Se) solar cell has sparked a lot of attention in the photovoltaic sector. However, large open-circuit voltage deficit (VOC,def) issue in CZTS/Se solar cell remains unsolved even after a decade of research and development. Therefore, in-depth research to solve fundamental bottlenecks are required to address the VOC,def issue. This thesis focuses on investigating the fundamental limitations of CZTSe solar cells, in particular the large VOC,def, by different characterization techniques.
One of the reasons that substantially impairs the photovoltaic (PV) performance of CZTSe is occurrence of intrinsic point/cluster defects. Understanding the nature of and controlling cation disorder remains a crucial challenge for improving their PV performances. Herein, a cation substitution method has been introduced to control and passivate the defect states in bandgap of CZTSe by incorporating Ag. Different x values of (AgxCu1-x)2ZnSnSe4 (ACZTSe) thin film absorbers were synthesized, where x = Ag/(Ag+Cu), i.e., with ratios of x = 0.00, 0.05, 0.10, 0.15, and 0.20 to provide a comprehensive understanding of defect states for ACZTSe solar cell. Intensity-dependent low-temperature photoluminescence measurements show that 10% Ag-alloyed CZTSe provides the shallowest defect states and less nonradiative recombination. It is also illustrated by first-principles calculations that Ag alloying enables the formation and suppresses the beneficial and detrimental defects, respectively. The best power conversion efficiency of 10.2% is achieved for the 10% Ag-alloyed CZTSe cell, along with an enhanced open-circuit voltage.
To determine the relationships among cation disorder, defect concentration, overall long-range crystallographic order, and local atomic-scale structure for (AgxCu1−x)2ZnSnSe4 (ACZTSe) material, the combination of neutron diffraction and synchrotron-based x-ray absorption techniques are implemented. The joint Rietveld refinement technique is used to quantify the concentration of defects in Ag-alloyed stoichiometric and non-stoichiometric Cu2ZnSnSe4 (CZTSe) powder samples. As main outcome, the cation distribution was determined to quantify the intrinsic point defects. This directly shows that Ag effectively inhibit the high concentration of the deep CuZn antisite and promotes shallower defects such as Cu-vacancy (VCu), which is important for improved device performance. Moreover, we studied the atomic-scale structure of ACZTSe as a function of composition using x-ray absorption spectroscopy (XAS). X-ray absorption near-edge structure (XANES) and extended X-ray fine structure (EXAFS) analyses of the nearest neighbor shell has been performed by simultaneous fitting of all K-edges to determine oxidation states, charge transfer mechanism (reflecting the occupancy of electronic states at/near the probed element) and structural parameters, respectively. |
