Application of Anion-Physisorbed Multi-walled Carbon Nanotubes on Photoelectrode for Dye-sensitized Solar Cells

本論文以物理吸附的方法來對多層奈米碳管進行表面改質，並將其分散在二氧化鈦薄膜中，作為染料敏化太陽能電池的光電極。在表面改質劑的選擇上，分別為陰離子型的十二烷基苯磺酸鈉(SDBS)及分子量為9350、20800、70000的聚苯乙烯磺酸鈉(NaPSS)，而非離子型表面改質劑則使用兩性三嵌段共聚物(PEG-PPG-PEG)，分別為P123、F68及F108。碳管的表面吸附製程如下，首先於水溶液中將表面改質劑與多層奈米碳管以超音波震盪法進行分散及表面吸附，且將多餘的改質劑利用過濾的方法去除，乾燥後取得表面處理過後的奈米碳管。由UV-vis光譜測定、表面電位分析及粒徑的量測發現，陰離子型表面吸附的碳管在水中之再分散能力比非離子型表面吸附碳管來得好。原因在於陰離子型改質劑具有苯環結構，可與碳管表面進行π-π堆疊。此外，磺酸根離子(SO3-)提供的靜電排斥分散能力比PEG-PPG-PEG的分散力來得大。之後將陰離子型表面吸附碳管分散在二氧化鈦薄膜中，由XRD的結果證實碳管的存在可幫助二氧化鈦結晶形成更多銳態礦相(anatase)，並且使穿透度上升。在染料敏化太陽能電池的元件方面，加入0.03wt%陰離子型表面吸附碳管於二氧化鈦能使短路電流值(Jsc)提升約10~12%。光電流的提升使元件具有較高的光電轉換效率、較佳的電子收集效率及較多的電量，原因來自於碳管能提供良好的導電性、二氧化鈦有較多的anatase相及光穿透度的上升，效率最佳的表現為NaPSS(70000)-MWCNT於添加量為0.03wt%時，短路電流及效率分別為20.68mA/cm2及8.42%。另外，為了進一步提升效率，光電極的外層採用大粒徑的二氧化鈦光散射薄膜，得到最佳短路電流及效率分別為21.44mA/cm2及8.84%。In this thesis, multi-walled carbon nanotubes(MWCNTs) physisorbing anionic and nonionic dispersing agent for surface modification were dispersed in TiO2 thin film utilizing as a photoelectrode for dye-sensitized solar cells (DSSCs). Anionic dispersing agents such as sodium dodecylbenzenesulfonate(SDBS) and poly (sodium4-styrenesulfonate) (NaPSS) with molecular weight of 9350, 20800 and 70000 were used to disperse MWCNTs. We also chose amphiphilic triblocks of PEG-PPG-PEG as nonionic surface modifying agents, such as P123、F68、and F108. The procedure for surface treatment MWCNTs is as following. Surface modifying agents and MWCNTs were dispersed and adsorbed in water by sonication. After fully adsorption, the solution was filtrated to remove the extra dispersing agents. After drying, the surface-modified MWCNTs were obtained. From the results of UV-vis spectroscopy, zeta potential and dynamic light scattering measurements, anionic surface modifying agents have better dispersing power than nonionic surface modifying agents because anionic agents have benzene ring to physisorb onto CNTs via π-π stacking. Furthermore, the electrostatic repulsion from the SO3- groups leads to the charge stabilization of MWCNTs compared to the nonionic agents. Then we dispersed anion-physisorbed MWCNTs onto the TiO2 system. From the XRD results, we found that TiO2 can crystallize into more anatase structures in the presence of MWCNTs. The resulting materials also had higher light transmittance. For the performance of DSSCs with MWCNT-TiO2 photoelectrode, the addition of 0.03wt% CNT to the TiO2 film could enhance the Jsc of DSSC from 18.65mA/cm2 to 19.26 mA/cm2. And the increase of Jsc resulted in higher power conversion efficiency and better charge collection efficiency because of good conductivity of MWCNTs, more anatase TiO2 crystal structures and higher light transmittance of the MWCNT-TiO2 films. The best Jsc and power conversion efficiency of 20.68mA/cm2 and 8.42% were achieved with MWCNT(70000) dispersing in TiO2. In addition, in order to enhance the light harvesting efficiency, the larger size of TiO2 scattering layer was deposited on the outer layer of MWCNT-TiO2 thin films. After addition of the scattering layer, the Jsc and power conversion efficiency were both increased to 21.44mA/cm2 and 8.84%