金屬氧化物薄膜與奈米微結構之多功能電漿處理與光電應用

Abstract

為了滿足全世界提升太陽能轉換效率的目標,對於發展與部署大規模、省錢、可重複利用的能源有相當的需求。近年來,在透明導電基板上,有機或是無機材料製成的太陽能電池 ,都具有良好的能量轉換效率。透明導電基板或是奈米微結構可讓光通過以被底下的活性 材料層吸收,更可將電子從此光伏器裡傳導出。這些將是此論文探討的主題。 第一部份,噴霧沉積的方式用來探討載氣中氧成份對 FTO 薄膜造成的影響。此載體 含有不同濃度的氧和氮 (0, 20, 50, 80 and 100%) ,導致對薄膜厚度、晶格長成的大小 和形狀有顯著不同。此種方式沉積而得的薄膜具有很低的電阻率約為 10-4Ω-cm, 對 550nm 光波約有 76%~96%的透光率。最後,此種光電效應製成的染料敏化太陽能電池以及高分子 太陽能電池皆顯示特別的性質,例如電荷傳導,重組以及針對表面和介面效應而成的均化 特性。 然而對於非晶質太陽能電池,作為透明基材的 FTO 薄膜,經氫化處理後會變質。因 此,此研究係針對ㄧ種雙層結構的透明導電薄膜(AZO/FTO),結果發現 AZO 的存在可保護底下的 FTO 薄膜表面不受氫離子或是氫的自由基破壞,提供良好的磊晶介面。更近一步, 在經過 400 度的再退火,藉由 Sn 和氧鍵結的再氧化,可復原已被氫離子破壞的 FTO 薄膜 ,此氫離子擴散和 Sn-O 氧化還原的過程皆,經由 XPS 和 SIMS 做詳盡分析。 第二部份主要著重在選擇性的電漿處裡,針對表面的官能化,保護層的形成,以及 蝕刻,來製備奈米柱、奈米線和奈米管。因此,我們製造 P 型 SnO2 的透明導電電極,是 藉由氮氣電漿將 In 和 N 共同參雜到已參雜不同濃度 In 的 SnO2 薄膜。根據 In 參雜濃度, In 和 N 共同參雜的 SnO2 薄膜可製備成 p 型或是 n 型,其中氮原子藉由取代 SnO2 中氧的位 置,可提升其導電性、霍爾遷移率、以及 In 參雜原子的溶解度。此特性也顯著的呈現在 pn 包覆核層的異質介面,顯示 IV 曲線的對稱性以及精準二極體特型。 同樣的方法也更近一步的用來比較室溫下經由氮氣電漿處理的 ZnO 的奈米線和奈米管。經 過 900 秒處裡,ZnO 的奈米管展現高穩定性的光學反應特性,優於奈米線約 20 倍,且結 構維持完整。這代表奈米管其極高的表體比,對於表面修飾、藉由氮離子修補氧缺陷、及 ZnO 中的雜質,為指標性關鍵。 另一方面,我們提出一種選擇性的氧電漿蝕刻技術,藉由已合成的 ZnO奈米柱為犧牲基材 ,再將 FTO 奈米粒子藉由簡單的噴霧熱解法沉積其在 ZnO 奈米柱表面,形成 ZnO-FTO 的異 質結構奈米管。XPS 分析顯示氧電漿處裡可降低 O2-/OH-濃度比,而引起 Zn-O 鍵結的解離, 以及 Zn 離子的向外擴散,形成內部中空的結構,其可由 FTO 表面羥基官能團的形成來解 釋。藉由反覆 UV 光照射,隨時間變化的光電流測量,驗證此結構具有光感應性,且其暗 電流三倍優於未經電漿處裡的試片。

To satisfy the universal goal of improving solar-energy conversion efficiency, the need to develop and deploy large-scale, cost-effective, renewable energy is becoming increasingly important. In recent years, a solar cell consisting of organic or inorganic materials along with good transparent conducting oxide (TCO) films has achieved good power conversion efficiencies (PCE). Transparent conducting oxide (TCO) films or nanostructures serve as a window for light to pass through to the active material beneath and as an ohmic contact (electrode) for carrier transport out of photovoltaic. All detailed studies will become the focus in this thesis. In the first part, a spray deposition process was used to investigate the effect of oxygen content in the carrier gas on FTO film morphology and properties. The carrier gas containing various O2/N2 concentrations (0, 20, 50, 80 and 100%) led to significant change in thickness, size and shape of grain growth. The deposited films reach a low resistivity of ~10-4Ω-cm and a transmittance of 76%~96% at 550 nm. Finally, photovoltaic implement of dye-sensitized solar cells (DSSCs) and polymer-based solar cells reveals the particular behaviors such as charge transport, recombination, and collection properties respected to the surface and interfacial effects. However, in case of amorphous silicon solar cells (a-Si:H), it was found that a hydrogenated effect results in the deterioration of FTO film as TCO electrode. Therefore, a double-layered transparent conducting AZO/FTO thin film was investigated and the results suggest that the AZO film acts as a protecting layer for the beneath FTO film surface, providing an excellent epitaxial interface, from the direct bombardment of H ions and radicals. Moreover, following by a post-annealing treatment at 400oC, the degraded properties of FTO can be recovered via the reoxidization of Sn-O bounds, in which H-ions diffusion and Sn-O redox process was interpreted by XPS and SIMS analysis. In the second part, the investigation is mainly focused on a selective plasma-treatment technique for introducing surface functionalization, passivation, and etching to form nanorods (NRs), nanowires (NWs) and nanotubes (NTs). Herein, we fabricate the p-type SnO2-based transparent conducting electrode which was constructed by a novel approach of In and N co- doping by nitrogen plasma (5-40 min), to In-doped SnO2 films with several In contents (0, 3, 7, 15 and 30%). Depending on In doping concentration, (N, In)-codoped SnO2 can be modified to either p-type or n-type where N atoms primarily substitute in O sites in SnO2 with enhanced conductivity, Hall mobility and solubility of the In dopant. Significantly, such behavior is also exhibited in term of pn core-shell heterojunction, showing the symmetrical I-V curve with rectified diode characteristics. Furthermore, similar method was applied to investigate the key aspects in comparison between ZnO NWs and NTs under nitrogen plasma treatment at room temperature condition. Upon an extended treatment of 900 s, the ZnO NTs exhibit higher reliability of photoresponse, 20 times of NWs without deteriorated structure. This indicates that higher surface-to-volume ratio of NTs is critically important factor for inducing the surface modification, occupying the oxygen defects and impurities in the ZnO matrix by the presence of N ions. On the other hand, we proposed a selective oxygen-plasma-etching technique for the formation of ZnO-FTO heterostructure nanotubes using presynthesized ZnO nanorods (NRs) as sacrificial templates, and FTO nanoparticles are deposited onto the ZnO nanorods by a simple spray pyrolysis method. XPS analysis demonstrated that the oxygen-plasma treatment decreased the O2-/OH- concentration ratio, resulting in dissociation of the Zn-O bonds and the outward diffusion of Zn cations to form an interior hollow, which is related to the formation of the hydroxyl functional group, Sn-OH, at the FTO surface. Time-dependent photocurrent (I-T) measurements under ON-OFF cycles of UV illumination confirms a rectified photoresponse characteristic and a dark current increased by about 3 orders of magnitude over that of the unetched sample.