利用同步輻射光源技術研究薄膜太陽能與鋰電池材料的晶體結構與電子特性

Abstract

本論文主要目的在於利用同步輻射相關技術研究再生能源材料以及儲能材料在電性與材料特性之間的關聯。本文分為兩個部份：第一個部份是研究磷酸鋰鐵儲能材料添加釩元素後影響材料結構與電化學特性間的關聯；第二部份是研究銅銦鎵硒硫五元太陽能電池材料中，元素分佈造成吸收層中的導電帶與價帶形成雙重梯度變化後與載子傳輸之間的相互影響。 在第一個部份中，我們利用X光繞射以及吸收光譜研究釩元素在磷酸鋰鐵材料中造成的長程與短程結構改變，以及此結構變化對於電化學特性的影響，另外利用中子繞射技術研究超價元素添加在磷酸鋰鐵材料中所誘發的鋰離子空缺效應與其電化學特性的改變。藉由延伸X光吸收近邊緣細微結構(EXAFS)分析法，可以清楚的顯示釩元素添加在磷酸鋰鐵材料結構中，取代了鋰離子在結構中的位置(M1 site)。由於添加了的超價元素提供系統多餘的自由電子，使磷酸鋰鐵材料導電度由4.75 × 10-4 S/cm增加至1.9 × 10-2 S/cm。另外，整個材料系統電荷需要平衡，所以摻雜的釩離子在磷酸鋰鐵結構週圍產生三至四個鋰離子的空缺，此空缺效應增加了鋰離子在材料中的遷移速度，使得磷酸鋰鐵材料在添加微量釩元素後的電容量由138 mAh/g增加至155 mAh/g。此外，藉由X光繞射以及吸收光譜在非臨場(ex-situ)電池充放電實驗中，可以再次證明釩離子在磷酸鋰鐵結構中取代鋰離子位置，並且產生額外的離子通道，使鋰離子更容易遷出/遷入磷酸鋰鐵結構中，進而減少未反應之鋰離子在磷酸鋰鐵結構中的殘留量。 在第二部份中，我們將銅銦鎵硒硫五元化合物薄膜太陽能電池裝置研磨出一微小角度的斜面，藉以製作一厚度梯度的樣品並增加可分析之橫截面面積，配合使用掠角X光繞射(GIXRD)以及掃描式光電子能譜(SPEM)研究銅銦鎵硒硫薄膜太陽能電池元件中各層材料縱深元素的分佈以及結構的變化，利用此分析技術在銅-銦-鎵三元合金薄膜的硒化與硫化製程(two-stage process)中，可以觀察到薄膜太陽能電池的主要吸收層在縱深分佈上會形成四個獨特的化學計量比結晶結構，而藉由熱力學反應能量可以解釋此獨特的化學計量比結晶結構的反應成長過程。另外，可以利用此縱深結構分佈模擬出元件實際的能帶結構，並瞭解此能帶結構中，價帶與導電帶雙重梯度效應造成電子與電洞之間傳輸與複合的影響。進一步，利用掃描式聚焦光電子能譜測量材料中價帶的最大值，確認此模擬能帶結構方法的準確性，而此測量結果除了符合模擬的價帶結構外，還觀察到在吸收層表面上形成了有序缺陷化合物(ODC)。此外，在太陽能電池電壓-電流曲線(IV curve)中，可以觀察到薄膜樣品的吸收層與背電極之間有剝離(peeling-off)的現象，利用二維面積偵測器研究材料各層之間的殘留應力(residual stress)變化以及結構優選取向(preferred orientation)的程度可以瞭解此剝離現象發生的原因以及可能的解決方式。在比較陰離子硫與硒之間在厚度縱深上元素濃度的分佈以及在結晶結構中的比例後，可以推測在吸收層底層會形成硒的間隙型缺陷，以及在吸收層表層會形成硫的間隙型缺陷。 最後，利用X光吸收近邊緣結構(NEXAFS)以及延伸X光吸收細微結構(EXAFS)分析技術，研究銅銦鎵硒硫五元化合物薄膜太陽能電池材料中原子尺度的結構資訊以及電子狀態。在氧化鋅透光層中，可以觀察到其與吸收層表層中擴散上來的硫原子在界面形成硫酸鋅化合物，而在原子尺度結構中，銦與鎵原子周圍環境與元素分佈較無關，而是依據熱力學中的反應能量，銦與鎵原子周圍分別偏好與硒原子、硫原子鍵結形成四面體結構，而兩個四面體中間由硒硫化銅四面體隔開。另外，在吸收層表層中，硒與銅原子在結構中之空缺，可以推測有序缺陷化合物的形成與硒周圍銅原子之空缺有關。硒原子周圍環境還可以佐證硒的間隙型缺陷在吸收層底層；硫的間隙型缺陷在吸收層上層出現。

In this doctoral dissertation, we investigated the correlations between electrical performances and material properties of clean energy materials, energy renewable or energy storage materials. It describes two parts: one is the study of mechanism of the electrochemical properties affected by vanadium substitution effect in the LiFePO4-based lithium-ion batteries; the other is the study of carrier transport mechanism induced by double-grading effect in the CIGSSe-based thin film solar cells. In the first part, by using X-ray powder diffraction and X-ray absorption spectroscopy, we investigated the mechanism of improving the electrochemical properties of a pristine LiFePO4 via changing the long-range and short-range ordering of the structure with the vanadium (V) incorporation in the LiFePO4 crystal structure. The effect of the Li vacancies induced by the doped supervalent V on affecting the electrochemical properties was studied by neutron powder scattering as well. Our results present that the doped V is preferentially substituted at the M1 site and induced 3~4 lithium vacancies in the LiFePO4 crystal structure and the increased lithium vacancies would enhance the Li-ion diffusion rate in the LiFePO4 material. As a result the capacity increases from 138 mAh/g for the pristine LiFePO4 to 155 mAh/g for the V-added compound and the corresponding conductivity increases from 4.75 × 10-4 S/cm to 1.9 × 10-2 S/cm. The ex-situ investigations of lithiation and delithiation states also provide an evidence of the substituted V at the M1 site, which can reduce an inactive lithium-ion in the Li1-x-vVxFePO4 cathode materials. In the second part, a novel approach is proposed to study the depth-dependent information of the thin-film solar cell, providing a gradient thickness and a wide cross section of the sample by polishing process. Utilizing a scanning photoelectron microscope (SPEM) and grazing-incidence X-ray powder diffraction (GIXRD), we studied the electronic band structure and the crystalline properties of the pentanary Cu(In,Ga)(S,Se)2 (CIGSSe) thin-film solar cell as a function of sample depth on measuring the thickness-gradient sample. The results exhibit that the CIGSSe absorber layer possesses four distinct stoichiometries. The growth mechanism of this distinctive compositional distribution formed by a two-stage process is described according to the thermodynamic reaction and the manufacturing process. On the basis of the depth-profiling results, the gradient profiles of the conduction and valence bands were constructed to elucidate the performance of the electrical properties (in this case, Voc = 620 mV, Jsc = 34.6 mA/cm2 , and η = 14.04%); the valence-band maxima (VBM) measured with a SPEM in the spectroscopic mode coincide with this band-structure model, except for a lowering of the VBM observed in the surface region of the absorber layer due to the ordered defect compound (ODC). In addition, the depth-dependent texturing X-ray diffraction pattern presents the crystalline quality and the residual stress for each depth of a thin-film device. We find that the randomly oriented grains in the bottom region of the absorber layer and the different residual stress between the underlying Mo and the absorber interface, which can deteriorate the electrical performance due to peeling-off effect. An anion interstitial defect can be observed on comparing the anion concentration of the elemental distribution with crystalline composition; a few excess sulfur atoms insert at the interstitial sites in the front side of the absorber layer whereas the interstitial selenium atoms are distributed in the back side. In the end of work, the local environments and electronic structure of atoms in the depth-dependent CIGSSe-based solar cell devices are studied by near edge X-ray absorption fine structure (NEXAFS) and extended X-Ray absorption fine structure (EXAFS) techniques. The results show the sulfate compound forming in the glass and ZnO window layer, as SiSO4 and ZnSO4 respectively. Moreover, the corner-shared Cu(S,Se)4 tetrahedra structure separates the GaS4 and InSe4 tetrahedra structure in the practical CIGSSe local environments. The deficiencies of Cu and Se are found at the top of the absorber layer and the interstitial defects of S and Se are found at the top and bottom of the absorber layer, respectively.