低維度氧化鋅奈米結構：製程、光學性質與染料敏化太陽電池應用之研究

Abstract

利用氣相沉積法搭配預鍍氧化鋅緩衝膜，氧化鋅奈米柱可成功的垂直成長於玻璃、矽、碳化矽以及藍寶石基板上。氧化鋅奈米柱在與基板水平方向的磊晶性質，與預鍍的氧化鋅膜的磊晶性息息相關，均受基板影響。同樣地，在氧化鋅奈米柱的光激發光方面，亦受到選擇不同的基板而有所不同。此外，高密度、垂直成長的氧化鋅奈米陣列亦可成功的磊晶於預鍍氮化鎵緩衝膜的藍寶石基板上。我們發現氧化鋅奈米線成長於氮化鎵緩衝膜上會受到基板影響，而有水平方向 121.9 MPa 的雙軸壓縮應力。在共振拉曼頻譜上，我們發現 n 階 A1 與 E1 縱模光學聲子強度比例（A1(nLO)/E1(nLO)）隨階數趨於增加，原因歸咎於聲子在氧化鋅奈米線空間中受到空間侷限所造成的結果。氧化鋅奈米線於常溫及低溫下的光激發光均與能帶附近激子的複合有相關連性。 我們利用溶膠-凝膠法，成功地合成具自組裝的奈米等級氧化鋅二次粒子。藉由穿透式電子顯微鏡的分析，氧化鋅奈米二次粒子是由具相同結晶方向的微小一次粒子所凝結而成。在共振拉曼分析上，我們發現聲子的頻譜有紅移現象，電子與聲子的耦合強度比起經攝氏350以及500度熱處理過後樣品來得小。這種隨著尺寸而變化的電子與聲子耦合，主要是受到 Fröhlich 交互作用所影響；這現象在氧化鋅量子點系統也觀察到。另外，在氧化鋅量子點的吸收光譜與光激發光光譜研究中，發現譜峰有明顯的藍位移現象，再利用有效質量模型可粗略估計其量子侷限效應在不同氧化鋅奈米晶粒尺寸下的結果。 在染料敏化太陽電池應用上，我們利用水熱法成長氧化鋅奈米線與分歧狀氧化鋅奈米線於導電玻璃上，做為光電極。比起奈米粒子，一維奈米線結構更能減少載子於傳輸過程中的所造成的躍遷損耗。而具分歧狀的氧化鋅奈米線於染料敏化太陽電池的光電流以及效益表現上分別為4.27 mA/cm2 以及 1.51 %，均為單純奈米線的兩倍。表面積增加提高了染料吸附是效益提升的主要原因。 我們亦嘗試利用具自組裝性的氧化鋅奈米二次粒子做為染料敏化太陽電池之光電極試驗。我們發現這種具多層級的結構體，除了利用一次粒子維持了染料的吸附效率外，二次粒子的結構並增加了光散射效益，間接提供了更多的光獲取能力。我們也嘗試使用兩種吲哚啉染料，D149、D205，與氧化鋅光電極搭配，分別達到具高效益的 4.95 % 以及 5.34 % 染料敏化太陽電池。在使用 D205染料，我們發現開路電壓以及短路電流提升的原因主要是增加了長鏈的疏水官能基，有效的抑制了電子與電解中液碘離子的再複合。對氧化鋅奈米粒子而言，D205同時也比D149染料具有更優越的電子傳輸率。我們亦利用交流阻抗分析法，比較兩種染料所製備的染料敏化太陽電池，提供電子生命期長短的更直接證據。

Vertically well-aligned ZnO nanorods were synthesized without employing any metal catalysts on various substrates including glass, Si (111), 6H-SiC (0001) and sapphire (0001), which were pre-coated with c-oriented ZnO buffer layers, by simple vapor phase deposition. The in-plane alignments of ZnO nanorods depend on the crystallographic alignment of pre-coated ZnO buffer layer. The photoluminescence of ZnO nanorods are basically related to the type of the substrates. In addition, high-density, vertically oriented arrays of ZnO nanowires were also successfully epitaxial grown on the GaN (0001)-buffered sapphire substrate. We demonstrated that the arrays of ZnO nanowires are well aligned along the c-axis and suffer a small biaxial compressive stress of 121.9 MPa. The increasing intensity ratio of nth-order longitudinal optical (LO) phonon (A1(nLO)/E1(nLO)) with increasing scattering order in resonant Raman spectra (RRS) reveals the spatial phonon-confinement as shrinking the diameter of ZnO nanowires. The exciton-related recombinations near the band edge dominate the UV emissions at room temperature as well as at low temperature. Self-assembled secondary ZnO nanoparticles (NPs), recognized with the agglomeration of crystalline subcrystals, are successfully synthesized by a simple sol-gel method. TEM images display that one artificial cluster behaves in a single crystal like wurtzite structure owing to the fact that subcrystals coagulate at the same crystal orientation. Moreover, from the RRS measurement, the as-grown sample exhibits phonon redshift; meanwhile, the coupling strength between electron and longitudinal optical phonon, determined by the ratio of the second- to the first-order Raman scattering cross sections, diminishes compared with the samples after post-annealing at 350˚C and 500˚C. The size dependence of electron-phonon coupling is principally as a result of the Fröhlich interaction. ZnO quantum dots (QDs) of controlled sizes have been fabricated by a simple sol-gel method. The blueshift of room-temperature photoluminescence (PL) measurement from free exciton transition are observed decreasing with the QD size that is ascribed to the quantum-confinement effect. From the RRS, the coupling strength between electron and longitudinal optical phonon, deduced from the ratio of the second- to the first-order Raman scattering intensity, diminishes with reducing the ZnO QD diameter. The size dependence of electron-phonon coupling is principally a result of the Fröhlich interaction. For further dye-sensitized solar cell (DSC) applications, the solvothermal method was utilized to fabricate the ZnO nanowires and branched nanowires on FTO substrates. The one-dimensional branched nanostructures can afford a direct conduction pathway instead of interparticle hopping while using nanoparticles. Furthermore, the short-circuit current density and the energy conversion efficiency of the branched ZnO nanowire DSCs are 4.27 mA/cm2 and 1.51 %, which are twice higher than the bare ZnO nanowire ones. The improvement was consequent on the enlargement of internal surface area within the photoelectrode and achieving higher dye adsorption to significantly enhance the performance of the DSCs. Moreover, self-assembled ZnO secondary NPs have been fabricated as an effective photoelectrode for DSCs. The hierarchical architecture, which manifested the significant light-scattering, can provide more photon harvesting. In addition, dye-molecule adsorption retained sufficient due to enough internal surface area provided by the primary single nanocrystallites. Two indoline dyes, coded D149 and D205, were used as the sensitizers of ZnO DSCs with the optimal energy conversion efficiencies of 4.95% and 5.34%, respectively. The enhancement of Voc and Jsc for D205-sensertized ZnO DSCs was ascribed to the effective suppression of electron recombination by extending the alkyl chain on the terminal rhodanine moiety from ethyl to octyl. The higher charge-transfer rate and retardant fluorescence decay reveal that D205 has better electron injection dynamics for ZnO NPs as compared to D149. The further evidence is performed by the electrochemical impedance spectroscopy (EIS) which exhibits the longer electron lifetime for D205-sensitized ZnO DSC in comparison with D149-sensitized one.