使用交流阻抗法中傳輸線模組分析氧化鈮阻隔層對染料敏化太陽電池陽極界面逆電流抑制之研究

Investigation of Niobium Oxide Blocking Layer on the Suppression of Recombination at Anodic Interface in Dye Sensitized Solar Cells by Transmission Line Model in Impedance Analysis

張育銘(Yu-Ming Chang)

指導教授 ： 衛子健

國立清華大學/工學院/化學工程學系/碩士(2014年)

摘要

如何使染料敏化太陽能電池(Dye-sensitized solar cells,DSSC)中的電子傳輸單向化是達到高光電轉換效率的關鍵之一，但是事實上卻會有相反方向的電子再結合損失，主要的損失位置在TiO2/電解質界面和FTO/電解質界面，本研究致力於探討後者，以Nb2O5阻隔層來抑制該界面的再結合反應，利用Nb2O5的導帶位置具有些微高於TiO2的特性，可形成能階障礙抑制電子的再結合反應。
本研究中，阻隔層的製備方式為使用5mM的鈮前驅物(Nb ethoxide)以旋轉塗佈法沉積Nb2O5於FTO導電基材上，經過XRD、XPS和TEM鑑定為Nb2O5，並呈現少部分的TT相結晶態，以光學顯微鏡和SEM初步判別阻隔層較佳的均勻性為使用旋轉塗佈使用轉速3000rpm和塗佈5層，經由紫外/可見光譜儀檢測其不影響光線的穿透度，接著透過紫外/可見光譜儀量測半導體能隙和XPS量測半導體價帶，得到Nb2O5的導帶位置高於TiO2，能形成能階障礙於FTO/電解質界面；經由暗電流檢測，阻隔層具有抑制暗電流的功用；阻隔層的厚度有其最佳值，過厚反而會使得電子擴散係數下降，本研究的最佳厚度約在68~80nm左右。
交流阻抗法中傳輸線模組分析可量測DSSC內部的再結合電阻變化，使用揮發性電解質時，於低施加偏壓時(等同照射弱光)，再結合反應主要發生在FTO/電解質界面，阻隔層使得再結合電阻約增加3倍，於弱光(0.05sun)下，光電轉換效率由11.1%上升至11.8%；使用非揮發性電解質時，阻隔層使得再結合電阻約增加14倍於低施加偏壓下，增加範圍延伸至中高偏壓，隨著施加偏壓上升，增加幅度會下降，於弱光(0.05sun)下，使得光電轉換效率由7.87%上升至8.53%。
結果可知，Nb2O5阻隔層能夠抑制FTO/電解質界面的再結合反應，於弱光下顯示出阻隔層的重要性，而使用非揮發性電解質之光電轉換效率增加的比例為8.4%多於揮發性電解質的6.3%於照度0.05sun，因此於更弱光的室內環境下，配合使用低揮發性的電解質，氧化鈮阻隔層能夠抑制再結合反應使得DSSC的光電換效率提升。

關鍵字

染料敏化太陽能電池 ； 再結合反應 ； 氧化鈮 ； 阻隔層 ； 傳輸線模組

並列摘要

Creating a unidirectional electron transport is one of the key factors of reaching high power energy conversion efficiency in dye-sensitized solar cell. Unfortunately, undesired electron pathway called recombination exists in real devices. The recombination causes photocurrent loss and mainly occurs at TiO2/electrolyte as well as FTO/electrolyte interface. Herein we focus on retarding the recombination at latter interface by using Nb2O5 blocking layer. Owing to slightly higher conduction band position of Nb2O5 comparing to TiO2, which can suppress charge recombination and enhance power conversion efficiency.
In this study, the Nb2O5 blocking layer was prepared by the spin coating 5mM niobium ethoxide ethanol solution on FTO substrate. The film was then characterized by X-ray diffraction (XRD), X-ray photoelectron spectrometer (XPS) and transmission electron microscopy (TEM), which indicated a small part of TT-phase crystalline was formed. The uniformity of such blocking layer was scrutinized by optical microscope (OM) and scanning electron microscope (SEM). UV/Vis spectroscope was used to determine the transmittance of so-prepared Nb2O5 thin film on FTO. Band structure was positioned by XPS and UV/Vis spectroscope.
In order to evaluate the recombination extent, the electrochemical impedance spectroscopy with transmission line model was employed and the recombination resistance (Rct) can be obtained. At low forward bias, considering the same condition as low-sun irradiation, recombination takes place primarily at FTO/electrolyte interface. In the case of volatile electrolyte based devices, the Rct with the Nb2O5 blocking layer increases 3 times higher than the blank one. As non-volatile electrolyte was applied, more significant effect could be observed for 14 times improvement in Rct value in the device fabricated with this Nb2O5 blocking layer.
As the result, the Nb2O5 blocking layer could suppress the recombination at FTO/electrolyte interface, especially under low-sun condition. An increase in power conversion efficiency from 11.1% to 11.8% and 7.87% to 8.53% were obtained in volatile and non-volatile electrolyte based devices, respectively. This research reveals the importance of blocking effect under low illumination, which is the crucial issue of indoor application for DSSC and this niobium oxide thin film acts as a remarkable blocking layer.

並列關鍵字

dye-sensitized solar cell ； recombination ； niobium oxide ； blocking layer ； transmission line model

參考文獻

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