以奈米TiO2製作色素增感型太陽電池工作電極之初步探討

Nanocrystalline TiO2 for Porphyrin-Sensitized Solar Cells: A Preliminary Study

姚品全(Pin-Chuan Yao) ； 王立民(Li-Ming Wang) ； 姜穎宏(Eric Chiang) ； 何浩千(Kevin Ho) ； 陳禹誠(Yu-Cheng Chen)

《科學與工程技術期刊》 4卷1期 (2008/03) Pp. 35-42

https://doi.org/10.7117/JSET.200803.0035

摘要

本研究以奈米TiO2粉體製作色素增感型太陽電池之工作電極，研究製程參數對太陽電池光電轉換效率之影響，主要製程變因包括：寬能隙半導體工作電極的製備、增感染料系統篩選、對電極製備與太陽電池封裝等。研究結果發現：以Porphyrin爲敏化染料的色素增感型太陽電池，經由適當製程調控，整體的最佳值V(下标 OC)=0.53V，J(下标 SC)=5.81mA/平方公分，FF=0.34，光電轉換效率爲0.39%（實驗室自製光源，未經濾光與強度調整）。Co-adsorbent(deoxycholic acid, DCA)的添加，會影響太陽光的吸收。由Ⅰ-Ⅴ量測的暗電流分析(dark current analysis)中發現：本電池結構之串聯電阻大小與太陽電池之填充因子(fill factor, FF)高度相關，本系統的低光電轉換效率，主要來自低填充因子。由TiO2工作電極表面結構研究發現：旋塗法較刮刀法更容易控制膜厚與表面孔隙度等特性。以簡單的光生載子擴散模式，推估載子可能穿越的TiO2膜厚爲10μm，與膜厚對光電流之影響實驗所得結果相符。此外，由添加PEG以調整TiO2工作電極孔隙度之研究發現：添加PEG可增加TiO2工作電極表面積，當PEG添加量爲20%時（相對於TiO2重量），TiO2工作電極有最大BET表面積48.2平方公尺/g。PEG的添加對DSSC光電效率影響最大的是短路電流。當PEG添加量爲30%時，得到最大J(下标 SC)值3.91mA/平方公分。此一結果與BET表面積增加趨勢大致吻合。此外PEG添加量與開路電壓，V(下标 OC)的關聯較不明顯，可能原因是添加PEG後，略微改變TiO2的表面狀態，但是受到各種製程中不理想因素之掩蓋，在高PEG添加量時，與V(下标 OC)的關聯，呈現不規則的變化趨勢。填充因子與PEG添加量之關聯，隨PEG添加量增加而呈現逐漸下降之趨勢。綜合本研究結果可知，以呲喀紫質(Porphyrin)系列花青素染料取代目前效率最佳但是價格昂貴的Ru-complex dye製作色素增感型太陽電池之工作電極，經由適當製程調控，所製作的電化學太陽電池，仍然可以得到不錯的V(下标 OC)與J(下标 SC)。如何進一步提昇此系列電池的FF值，以達到合理的效率，仍然有許多改善空間。

關鍵字

奈米TiO2 ； 色素增感型太陽電池 ； 呲喀紫質 ； 孔隙度

並列摘要

In this study, porphyrin-sensitized nanocrystalline TiO2 was used as the working electrode (photo-anode) for dye-sensitized solar cells (DSSC). The key process parameters in fabricating high-efficiency DSSC, including the deposition of wide band-gap thin semiconductor films, were investigated as preliminary tests for further detailed study later. The results indicate that the best cell performances were V(subscript OC)=0.53V, J(subscript SC)=5.81mA/cm^2, and FF=0.34 with a solar energy conversion efficiency of 0.39% (in laboratory conditions). The addition of a co-adsorbent shifted the absorption spectrum of the sensitizer. The dark-current diode analysis revealed that the series resistance (R(subscript o)) for the equivalent circuit of solar cells was closely related to the filled factor (FF). In this case, the low conversion efficiency was a result of the very low FF value. Moreover, the morphology of TiO2 films revealed that spin-coated films have better quality with uniform thickness compared to those coated by the doctor-blade process. When analyzing the transport behavior of photo-generated carriers by a simple diffusion model, the anticipated diffusion length of the photo-injected electrons lies within 10 μm, being in accordance with our experimental results. To control the porosity of a TiO2 working electrode, poly (ethylene glycol) was introduced. As the amount of PEG was increased, the BET surface area of the thin TiO2 films also increased. The TiO2 working electrode reached a maximum BET surface area of 48.2 m^2/g as the added PEG content reached 20%. After the amount of PEG was increased to 30%, the BET surface area declined to 44.6 m^2/g while the maximum J(subscript SC) (3.91 mA/cm^2) was being obtained. The normalized surface-area-to-photocurrent ratio indicated that the result was approximately consistent with the increase in the trend of the BET surface area. Consequently, it was revealed that one of the governing factors for photo-injected electron transport is the surface area of thin TiO2 films. In contrast to the relationship between BET and J(subscript SC), the relationship between BET and V(subscript OC) was oblique because the change in surface states for nanocrystalline TiO2 was compensated by other non-ideal factors such as impurities in the entire cell and hydrocarbon residues after calcinations. Furthermore, the DSSC fill factor in this study was quite low compared to other published results of studies using Ru-complex dyes. Therefore, working porphyrin-sensitized nanocrystalline TiO2 electrodes show a satisfactory high J(subscript SC) of 5.81 mA/cm^2 at V(subscript OC)=0.53 volt, and FF=0.34. The low FF in this study still needs research toward further improvement.

並列關鍵字

TiO2 ； dye-sensitized solar cells ； porphyrin