以準分子雷射處理奈米碳管薄膜提升染料敏化太陽能電池之光電特性

Abstract

近年來，染料敏化太陽能電池逐漸受到重視，並被認為可取代傳統矽基太陽能電池。由於鉑金屬具有高的電解液催化能力與化學穩定性，所以現今染敏電池仍以鉑金屬作為對電極之材料。因此，本論文提出利用準分子雷射處理鑲嵌有鉑奈米粒子之奈米碳管的薄膜作為染敏電池之對電極，此結構不僅可以降低鉑金屬之使用量，同時可提高電池之效率。 首先，我們利用純奈米碳管薄膜作為染敏電池之對電極，碳管薄膜因具有三維結構，能使電解液能均勻分布於碳管間隙中，大幅增加接觸面積，改善電子由對電極導入電解液的能力，但實驗結果發現，利用純碳管薄膜作為對電極之染敏電池量測光電轉換效率只能到達5.168 %，相較於傳統鉑電極元件之效率7.12 %，仍具有改善的空間。 因此，為了進一步增加碳管與電解液的反應面積，本論文使用KrF準分子雷射（λ= 248 nm）對碳管薄膜做熱退火處理，發現隨著雷射能量增加，多數的碳管會因受到高能量退火由管狀結構剝開形成石墨烯的片狀結構，此奈米碳管與石墨烯的混成薄膜可大幅增加與電解液的反應面積。此外，雷射退火可使碳管再結晶化，增加其導電性，並改善碳管與基板之間的附著力，進而增加載子的傳遞能力。在雷射能量為600 mJ/cm2時，其元件光電轉換效率由原本的5.168 %提升至6.352 %，增加了23 %，足見雷射熱退火對碳管薄膜特性有顯著的提升。 由於鉑金屬對電解液催化的效果較碳管優異，為了進一步增加對電極的載子交換能力，本論文利用雷射退火製備鑲嵌有鉑奈米粒子之奈米碳管/石墨烯片混成薄膜作為染敏電池對電極，也因使用鉑奈米粒子做沉積，相較於傳統鉑電極，其鉑金屬用量大幅減少，降低製程成本。實驗結果發現，由於雷射處理過的碳管薄膜提供較大的比表面積，沉積在碳管上的鉑奈米粒子與電解液接觸面積也因此增加，載子傳遞更加容易，因此光電轉換效率由6.352 %提高到8.791 %，增加了約40 %。相較傳統鉑電極之效率7.12 %，元件效率提高約 23%。 本研究利用雷射退火製備鑲嵌有鉑奈米粒子之奈米碳管/石墨烯片混成薄膜可提供高比表面積與電解液進行載子交換外，特性穩定且導電性良好也為其優勢，未來在可撓性及半穿透性的光電元件中深具潛力。

In recent years, the dye-sensitized solar cells (DSSCs) have become an interesting and promising alternative to the traditional silicon-based solar cells. The Platinum (Pt) film was the most common counter electrode (CE) material of DSSCs because of its chemical stability, and high electrocatalytic properties. However, the high cost will increase the difficulty of commodification. Therefore, in this letter, the novel CE film composed of Pt nanoparticles supported by graphene sheet (GS) and carbon nanotube (CNT) hybrid structure was proposed to replace the traditional Pt CE. First, the pure CNT was used to the CE of DSSC. The CNT thin film kept three-dimensional structure to increase the contact area with electrolyte. It would increase the charge-transfer ability between CE and electrolyte. However, to compare with the performance of traditional Pt CE which was 7.12 %, the performance of DSSC with pure CNT was just 5.168 %. It was not consistent with the expectation. Therefore, for increasing more surface area, the CNTs thin film for CE would be irradiated by KrF excimer laser ( λ=248 nm ) for instantaneous annealing. The surface morphology variations of CNT thin film were systematically compared under different excimer laser energy irradiation. We could find that the surface morphology would be changed when the laser energy increased. Some of the CNTs would be unzipped into GSs resulting in the CNT and GS hybrid structures. Moreover, the laser energy also assisted the CNT recrystallization for improving the charge transfer ability. Finally, the adhesion between FTO and CNT film would also be enhanced. The efficiency of CNT CE under 600 mJ/cm2 laser irradiation was promoted to 6.352 %, which shown a considerable 23 % improvement in performance. Because Pt has high electrocatalytic properties, we would use the Pt particles which were decorated on CNTs and GSs to increase the ability of charge transfer. The structure with Pt particles would also decrease the quantity of metal. The hybrid structures not only reacted with electrolyte itself but supported Pt particles with larger contact area than traditional Pt CE for charge-transferring. The current density would be increased. After decorated the circular Pt nanoparticles on CNT and GS structures which annealed by KrF excimer laser, the CE could react with electrolyte rapidly. The conversion efficiency was improved from 6.352 % to 8.791 % with 40 % enhancement. To compare with Pt CE which was 7.12 %, the efficiency of CNT which decorated with Pt nanoparticles under 600 mJ/cm2 laser irradiation was shown a considerable 23 % improvement in performance. In this work, the excimer laser treatment on Pt-nanoparticles decorated CNTs had been investigated to exhibit the superior conversion efficiency of 8.791 %. The promoted conversion efficiency was attributed to the improved conductivity and the elevated reactive surface area with the electrolyte. Therefore, the laser-irradiated PtCNT network demonstrates excellent chemical stability, elevated conductivity, and exfoliated surface morphology, making it promising for the future applications in the flexible and semi-transparent solar cells.