奈米碳材料應用於染料敏化太陽能電池對電極之研究

Abstract

本論文是應用奈米碳材料於染料敏化太陽能電池對電極，來降低白金於染料敏化太陽能電池中的使用量。傳統染料敏化太陽能電池需使用白金作為對電極，在大面積製作及普及化需求下將會大幅提高染料敏化太陽能電池製作成本。然而，碳含量於地表上十分豐富，加上奈米碳材料具有許多良好的電學、光學及電化學特性，因此本研究將奈米碳材料（包含石墨烯、氧化石墨烯及巴克紙）對電極中，試圖降低白金於染料敏化太陽能電池中的消耗量。此外，本研究中同時探討含有奈米碳材料對電極的染料敏化太陽能電池的光伏特性及載子傳輸行為，從中了解這些奈米碳材料對染料敏化太陽能電池的影響。 研究發現白金-碳複合材料能降低白金與導電基板的接觸電阻，有效降低染料敏化太陽能電池串聯電阻。因此，染料敏化太陽能電池的短路電流及能量轉換效率分別改善達8%及13%。氧化石墨烯具水溶液步驟製備而成的優點，能大量提供高品質的奈米碳材料。因此，白金-氧化石墨烯複合材料可被作為高穿透及高效率的對電極供雙面染料敏化太陽能電池使用。白金-氧化石墨烯複合材料具有較佳的電化學催化能力，能加速碘離子於對電極上的還原速率，使得雙面染料敏化太陽能電池具有較佳的光伏特性。巴克紙是由多重交錯的奈米碳管組成的多孔性材料，具有極高的電化學作用表面積提供碘離子還原，可在沒有電催化物（白金）複合情況也可加速染料敏化太陽能電池中碘離子還原速率，並改善了電池中漏電問題。透過這些奈米碳材料的應用，白金於染料敏化太陽能電池中的使用量分別被減少75%（石墨烯）、80%（氧化石墨烯）及100%（巴克紙）。 奈米碳材料的電化學催化及維度對染料敏化太陽能電池的表現影響極大。因材料先天特性，奈米碳材料有用較差的電化學催化特性。因此，奈米碳材料的維度極具重要性。實驗中驗證，三維尺度的巴克紙因具有極大的電化學作用面積，因而抵銷電化學催化的缺點。然而，相較於高溫合成的石墨烯，可由水溶液步驟合成的氧化石墨烯與巴克紙能提供低成本且大量製作的優點，因此可應用性較高。雖然現今白金對染料敏化太陽能電池成本影響不高，但隨著白金存量不斷消耗，在大面積製作及提高染料敏化太陽能電池普及性的需求下，減少白金用量或完全取代白金是必要的課題；屆時，奈米碳材料將成為極具潛力的材料。

In the thesis, carbon-based nanomaterials are applied to the counter electrodes (CEs) for dye-sensitized solar cells (DSCs) to reduce the consumption of Pt. The applied carbon-based nanomaterials to CEs include graphene, graphene oxide (GO), and buckypaper (BP). Because of storage limitation of Pt, the cost of DSC manufacturing is increasing. Carbon is an abundant element in the Earth’s crust, and carbon-based nanomaterials have lots of excellent electrical, optical, and electrochemical properties. They exhibit great potential to reduce the Pt consumption in DSCs. In addition to developments of low Pt-loading CE with carbon-based material, the photovoltaic performance and charge dynamics of DSCs with graphene, GO and BP-incorporated CEs are investigate to understand the influences of these carbon-based nanomaterials. The Pt-C composite can reduce the contact resistance of Pt/fluorine-doped tin oxide (FTO) interface, resulting in the enhancements of short-circuit current density and power conversion efficiency of DSC with Pt/few-layer graphene CE (DSCPt/FLG). Solution-processable GO provides a low-cost method to prepare large-quantity and high-transparent carbon source. Pt/GO composites are developed as the high-transparent and high-efficient CEs for bifacial DSCs. DSCPt/GO exhibits the better bifacial photovoltaic behaviors because of the outperformed PCE under rear illumination, attributing to the efficient I3- reduction ability of Pt/GO composite. The solution-processable BP is fabricated with entangled multi-walled carbon nanotubes, providing extreme electroactive surface area for I3- reduction. BPs can increase the I3- reduction rate at CE and suppress the charge recombination at photo-anodes. By applying BP, the DSCBP present a comparable performance to DSCPt, and the Pt-free DSCs are achieved. Accordingly, by applications of graphene, GO, and BP, the Pt consumption in DSCs can be reduced by 75%, 80%, and 100%, respectively. According the experimental results, it is found that the electrocatalytic ability and dimension of carbon-based nanomaterials are important. The naturals of carbon-based nanomaterials lead to the worse electrocatalytic ability to I3- reduction. Therefore, the dimension of CEs becomes critical. The 3-dimensional BP demonstrates the large electroactive surface area can compensate the degradation of electrocatalytic ability, which results in the comparable performance to DSCPt. On the other hand, the low-cost CE manufacturing with inexpensive material can be realized by the solution-processable GO and BP, whereas the complex manufacturing is required to synthesize graphene. Pt consumption is a foreseeable cost issue in DSC deployments. This research demonstrates that carbon-based nanomaterials are the potential materials to reduce Pt consumption in DSCs.