黃銅礦奈米晶體的合成與光學特性研究

Abstract

銅銦二硫半導體近來被認為是最佳的太陽能材料，因為其具有適合的能帶寬(1.5eV) 和高吸收係數，與理想的太陽能光譜幾乎完全匹配，若作為太陽能電池的吸收層可將入射光子完全吸收。過去十年，銅銦二硫薄膜被廣泛研究，其太陽能最佳轉換效率達到12.7%。此外，銅銦二硫奈米晶體仍被認為具有相當大的潛力可提高太陽能轉換效率，這是因為奈米晶體產生的量子侷限效應具有多重激子(multiple excitons)與內部能帶(intra band-gap)的優點。因此本論文將探討銅銦琉與類銅銦二硫奈米晶體的合成，結構鑑定與在太陽能電池元件的相關應用。本論文首先利用膠體方法合成銅銦琉的奈米晶體與不同厚度包覆的核-殼結構的銅銦二硫-硫化鋅奈米晶體，並以X 光繞射儀(XRD)、穿透式電子顯微鏡(TEM)、X 光光電子能譜儀（XPS）、動態雷射散射分析儀(DLS)鑑定合成奈米晶體的結構與粒徑大小。本論文合成的均勻銅銦二硫奈米晶體的粒子半徑為2.4 奈米，核-殼結構耐米晶體粒子半徑大小則為4.6 奈米。由於量子侷限效應，所得銅 銦二硫奈米晶體的放射光波長在450 奈米，明顯高於塊材的銅銦二硫半導體能帶。硫化鋅表面改質消除了銅銦二硫奈米晶體表面的缺陷，因此可明顯提高其發光強度。合成銅銦二硫奈米晶體後，我們將三種不同比例的鋅導入銅銦二硫奈米晶體中，分別為n=1，n=2和n=3(n=Cu/Zn)。並將這些奈米晶體嫁接到5μm長的氧化鋅奈米線上，探討其太陽能轉換效率。合成後的鋅參雜銅銦二硫奈米晶體的吸收 波長分別為530(n=1)，570(n=2)和650(n=3)奈米。當n=2的時候，可得到最佳的轉換效率，其值為0.28%，短路電流(Jsc)為1.71mA/cm2，開路電壓(Voc)為0.34V，而填充係數(FF)則為0.48。此類型量子點太陽能電池的限制來自於激子快速的再結合與電解液，當沉積硫化鋅保護層於鋅參雜銅銦二硫奈米晶體表面後，主要太陽能電池參數都獲得提升(Jsc = 3.21 ma/cm2, Voc = 0.45 V, FF = 0.49)，因此得到 轉換效率為0.71%。本論文提出的第二個方法是在高頻磁場(High frequency magnetic field)下合成高品質的鋅參雜的銅銦二硫奈米晶體，所有實驗過程在正常大氣環境與室溫下即可進行。磁性的參雜提供了主體(host)在外加磁場下產生超順磁性加熱，使得晶體以高於傳統高溫爐加熱速度的2-3個級數快速成長。隨磁場時間的作用，鋅 參雜的銅銦二硫奈米晶體形貌隨之變化，從球體，錐體，立方體，最後形成長方體。本論文最後將磁場下合成的，含有不同形狀銅銦二硫奈米晶體的膠體溶液製成＂奈米墨水＂並應用於太陽能電池元件中，將其塗佈成銅銦二硫薄膜吸收層，最佳轉換效率為1.01%，吸收層膜厚為1.012μm(長方體樣品)。吸收層膜厚提高到2.132μm後效率可達到1.44%。

Copper indium disulfide (CuInS2) semiconductor has been considered as the best materials for solar-cells because it has a superior band-gap of 1.5 eV and high absorption coefficient, which matches almost ideally to the solar spectrum and assures a complete absorption of the incident photon flux in an absorber layer. In the past decade, CuInS2 film has been widely studied and the best conversion efficiency for solar cells is about 12.7 %. On the other hand, CuInS2 nanocrystals still have great potential to promote power conversion efficiency due to quantum confinement effect, such as multiple excitons and intra band-gap. Therefore, in this thesis, the synthesis, characterization and application of CuInS2 and derivative nanocrystals, will be systematically studied. The bare CuInS2 with different shell thickness, CuInS2@ZnS core-shell nanocrystals, are first produced via a colloidal method and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and dynamic light scattering (DLS). The obtained nanocrystals are quasimonodisperse with an average particle size of 2.4 (bare) and 4.6 (core-shell) nm, respectively. The emission peak of obtained CuInS2 is around 450 nm, which is higher than bulk CuInS2 due to the size-dependent quantum confinement effect. ZnS modification eliminates defects on the surface of CuInS2 and significantly enhances photoluminescence consequently. After the CuInS2 nanocrystals were developed, the Zn was introduced into CuInS2 nanocrystals with three different Cu/Zn ratios (n value) which were and then deposited onto 5 μm long ZnO nanowire substrate as a quantum dot (QD)-based solar cell. The absorbance peak of obtained Zn doped CuInS2 nanocrystals shift from 530, 570, to 650 nm correspond to n = 1, 2, and 3, respectively. The best conversion efficiency of bare Zn doped CuInS2 is 0.28 % for n =2 sample, short-circuit current density (Jsc) is 1.71 mA/cm2, open-circuit voltage (Voc) is 0.34 V, and fill factor (FF) is 0.48. It was fond that the restriction of such quantum dot-based solar cells could be due to the fast recombination time and corrosion from electrolyte. But, after depositing ZnS layer on the surface of Zn-CuInS2 nanocrystals, an optimal efficiency at 0.71 % could be obtained, resulting from the promotion of all solar cell parameters. (Jsc = 3.21 ma/cm2, Voc = 0.45 V, FF = 0.49) The second novel method we present is using magnetic Zn doping to synthesize high quality CuInS2 nanocrystals under high frequency magnetic field at ambient conditions. The magnetic doping gives superparamagnetic heating of the resulting nanocrystals via magnetic induction, causing an accelerating growth rate of the doped CuInS2 under ambient conditions of 2-3 orders of magnitude faster than conventional autoclave synthesis. Shape evolution of the Zn doped CuInS2 nanocrystals from initially spherical, to pyramidal, cubic, and finally to a bar geometry, were detected as a function of time of exposure to magnetic induction. Finally, these colloidal solvent with different shape nanocrystals were further used as “nanoink” to fabricate a simple thin film solar device, the best efficiency of these crystals we obtained is 1.01 % with a 1.012 um thick absorber layer (bar geometry). The efficiency can be promoted to 1.44 % after thickening absorber to 2.132 um.