利用原子層化學氣相法在TiO2電極上沉積Al2O3覆層以改進染料敏化太陽能電池之光電效能

Abstract

本論文研究原子層沉積法在染料敏化太陽電池之二氧化鈦電極鍍上超薄之氧化鋁膜層。此太陽電池之效率(PCE)因二氧化鈦電極鍍上一層約0.2 nm厚之氧化鋁而從5.75%增加到6.5%，比未鍍氧化鋁時之效率增進了13%。在本研究中，我們研究了此太陽電池之氧化鋁(殼)/二氧化鈦(核)電極之能階、覆蓋率與成長模式三大題目。主要成果依序敘述如下： 我們發現介面Ti-O-Al(OH)2及極性層對氧化鋁/二氧化鈦電極之功函數有極大之影響，且氧化鋁厚度愈厚則氧化鋁/二氧化鈦電極之價電子能帶最大值及能隙就愈接近氧化鋁體材之值。在效率最大時，其介面功函數差距為0.4 eV，介面復合能障為0.1 eV。當氧化鋁厚度愈厚時因為穿隧效應則效率大幅下降，此太陽電池之效率增加時，因超薄氧化鋁之適宜能階而使得電子傳輸過程中並沒有穿隧效應之發生，因此，我們提出了介面功函數差距、介面復合能障及介面能障等參數與此材料之效率提升之新看法。此外，也確認介面Ti-O-Al(OH)2及極性層對氧化鋁/二氧化鈦電極之影響。 此外，我們建立了一覆蓋率之計算模型以測量染料敏化太陽電池之殼/核電極之覆蓋率，我們利用光電子能譜儀分析此殼/核電極之奈米二氧化鈦顆粒，確認此模型可應用在染料敏化太陽電池之氧化鋁/二氧化鈦電極之覆蓋率計算。結果顯示氧化鋁之覆蓋率隨其厚度及反應循環次數增加而增加，且以原子沉積法鍍上之氧化鋁膜層呈現島狀成長模式。而此太電最高之轉換效率出現在氧化鋁覆蓋率值為0.25時，顯示此太電電極若鍍上單原子層之氧化鋁且其覆蓋率值為1時,可能有機會增加其原太電轉換效率之52%。 最後，我們發現此研究中之氧化鋁膜層之成長模式在ALD 反應次數為5 次以上時，特別是在銳鈦礦之二氧化鈦電極上會開始從島狀成長轉換成層狀成長。我們推論這是因為受到奈米二氧化鈦顆粒之壓應力減少之影響，而我們也證實了壓應力隨著ALD 反應次數增加而減少，也導致了島狀成長轉換成層狀成長之狀況發生。

Ultra-thin Al2O3 films on nanoporous TiO2 electrodes of dye-sensitized solar cells (DSSCs) were successfully fabricated by atomic-layer-deposition (ALD). The power conversion efficiency (PCE) of the DSSCs increases from 5.75% to 6.5%, an improvement of 13%, when the Al2O3 overlayer reaches an average thickness of ~0.2 nm. The thesis focused three interfacial issues regarding the Al2O3/ TiO2 electrode; they are (a) the determination of energy levels of the Al2O3 overlayer, (b) analysis of the surface coverage of the as a function of the ALD conditions, and (c) study of the growth mode of the core/shell electrode. The formation of Ti-O-Al(OH)2 and interfacial dipole layers exhibits a strong influence on the work function of the Al2O3 overlayers. As the Al2O3 overlayer becomes thicker and thicker, the valence band maximum and the band gap gradually approach the values characteristic of pure Al2O3. The one-monolayer thick Al2O3 overlayer has a maximum PCE, and a work function difference of 0.4 eV and a recombination barrier height of 0.1 eV were found. However, as the Al2O3 overlayer is thicker than one monolayer, the PCE decrease significantly and the interfacial energy barrier height between the N719 dyes and TiO2 electrode increases. We found that the electron transfer from the dye to the TiO2 required no energy barrier as a result of the modification of interfacial energy levels due to the ultra-thin Al2O3 overlayer and, therefore, improved the PCE of the cell. The interfacial energy levels depends on the work function difference, the recombination barrier height and the interfacial barrier height, which are a function of the thicknesse of the Al2O3 overlayer and interfacial properties, such as the formation of the Ti-O-Al(OH)2 moiety and the dipole layer. Proper modifications of the interfacial energy levels may result in an optimal performance of the dye-sensitized TiO2 solar cell. We proposed a core/shell (C/S) model to determine the surface coverage of an Al2O3 overlayer deposited on TiO2 nanoparticles by XPS. We used the model to estimate the coverage of the Al2O3 shell layer on the nanoporous TiO2 electrode of the DSSCs as a function of the number of ALD reaction cycles. By analyzing the XPS signals of the TiO2 electrode with the Al2O3 coverage increasing from 0.25 to 1.0, we found that the ALD-Al2O3 deposition on the nanoporous TiO2 electrode was via the island growth mode. On the basis of the coverage analysis, we predict that improvement in the PCE of ~52% is obtainable when a uniform monolayer of ALD-Al2O3 (i.e. at the coverage of 1.0) is deposited on the nanoporous TiO2 electrode. To understand why the ALD-Al2O3 deposition on TiO2 nanoparticles is via the island growth mode, we used XPS and x-ray diffractometry (XRD) to study the chemical and microstructure properties of the interface between the ALD-Al2O3 overlayer and the TiO2 nanoparticle as a function of the thickness of the ALD-Al2O3 overlayer, which was used to derive the growth per cycle of the ALD reaction. The growth mode of the ALD-Al2O3 overlayers changes from the island growth to the layer-by-layer growth after the first 5 ALD reaction cycles, and the growth mode transition is much more pronounced for the anatase electrode layer. We suggest that the growth transition of the ALD-Al2O3 overlayer is correlated with the reduction in the lattice strain of the TiO2 nanoparticle. The contractive lattice strain in the hydroxylated TiO2 nanoparticle progressively decreases during the ALD Al2O3 deposition, resulting in the growth mode transition.