中孔性二氧化鈦光電極對染料敏化太陽能電池效能的影響

Abstract

本論文利用添加聚苯乙烯球的二氧化鈦薄膜經過燒結所產生的微米孔洞所製作之中孔性二氧化鈦光電極改善染料敏化太陽能電池的短路電流與效率，本實驗所研究的二氧化鈦光電極主要包含了三種結構。第一種結構採用均為相同成分的二氧化鈦膠體溶液，網印共三層所製作出之 12 um 勻相薄膜結構光電極，定義為 Monolayer，在此結構下，除了本身的奈米多孔隙之外又增加了微米的大孔洞，因此增加了光的散射與光路徑(optical path)，使染料對於光的吸收機會增加，進而提升了元件的效能。在此結構下，使用摻雜濃度 2 wt.%、平均直徑 2um的聚苯乙烯球有最佳化元件效能，其元件光電流為14.50 mA/cm2、光電轉換效率為6.70 %，比起傳統奈米多孔隙薄膜染料敏化太陽能電池光電流提升約11.2 %，而光電轉換效率也提高11.16 %。第二種結構則是含有兩種不同濃度的聚苯乙烯球之二氧化鈦膠體溶液製作出的非勻相薄膜結構光電極，第一層膜厚為8um，第二層膜厚為4 um；利用添加不同濃度的聚苯乙烯球於不同膜層中，此結構不 但能利用微米的孔洞增加光的散射與光路徑，而膜層之間濃度的改變亦能使光侷限於薄膜之中，提升了光在二氧化鈦光電極行走的路徑，進而增加染料對光的吸收，再次提升了元件的光電轉換效率。在摻雜濃度2 wt.% 為第一層膜、10 wt.%為第二層膜、平均直徑2um 聚苯乙烯球所製備的雙層中孔性二氧化鈦薄膜之染料敏化太陽能電池有最佳化條件，元件的光電流為15.3 mA/cm2、光電轉換效率為7.00 %，其光電流大於傳統奈米多孔隙薄膜染料敏化太陽能電池約11.90%， 而元件效率也提升了11.67 %。為了進一步利用光侷限效應提升染料敏化太陽能電池的光電轉換效率而採用第三種Trilayer 結構，其為含有三種不同濃度的聚苯乙烯球之二氧化鈦膠體溶液製作出的非勻相薄膜結構光電極，每一層不同濃度的膜厚均為4um。在聚苯乙烯球平均直徑2um、濃度2 wt.% 為第一層膜、5 wt.%為第二層膜、10 wt.%為第三層膜之聚苯乙烯球所製備三層中孔性二氧化鈦薄膜之染料敏化太陽能電池有本實驗最佳化條件，光電流密度和轉換效率分別提高 了22％和20％（光電流密度從12.9 mA/cm2 提升15.7 mA/cm2 和光電轉換效率從6％至7.2％）。在長波長區域可觀察到光吸收率和入射光子-電子轉換效率（IPCE）有顯著的提升，在加入聚苯乙烯小球的中孔性二氧化鈦薄光電極提升了光在長波長的吸收，也彌補了N719 染料於長波長吸收率不佳的情形。

This paper reports the enhanced performance of dye-sensitized solar cells (DSSCs) with microcavity-embedded nanoporous TiO2 photoanodes. The microcavities were formed by sintering TiO2 pastes with the addition of polystyrene (PS) microspheres. In this study, the study of TiO2 photoanode consists mainly three types of structure. The monolayer structure (M) was composed of three sublayers with PS microspheres of identical size and concentration, the composite pastes were screen printed layer-by-layer to achieve a total thickness of 12 um (three 4-um-thick sublayers).The present of microcavities in the TiO 2 photoanode increase the light scattering and light path (optical path), so there was more opportunity for dye to absorb the light, thereby enhancing the device performance. For DSSCs made with TiO 2 paste mixed with 2 wt.%, 2 um PS microspheres, the short-circuit current density was 14.5 mA/cm 2 and the conversion efficiency was 16.3 mA/cm 2 . Compared to conventional dye sensitized solar cell, it was improved by 11.2% and 11.16% respectively. The bilayer structure (B) comprises two sublayers with PS microspheres of identical size and concentration, denoted as the first layer, and one sublayer with PS microspheres of the same size but different concentration, denoted as the second layer. The first layer is 8 um and second layer is 4 um in thickness. In this structure , the further enhancement of cell performance was observed when the incident light passes from the first layer with high effective index (low concentration) of refraction to the second layer with low effective index(low concentration ).It can be attributed to light confinement effect between different concentration layer. For DSSCs made with TiO 2 paste mixed with 2 wt.% for the first layer and 10 wt.% for the second layer with 2 um PS microspheres, theshort-circuit current density was 15.3 mA/cm2 and the conversionefficiency was 7.00 % . Compared to conventional dye sensitized solar cell, it was improved by 11.9% and 11.67% respectively. To furtherimprove the cell performance by utilizing the light confinement, atrilayer structure was proposed and investigated. The trilayer structure(T) contains three sublayers with PS microspheres of the same size but in different concentrations and the 4 um in thickness each. The I-Vcharacteristics of the best cell with photoanode made using PSmicrosphere concentrations of 2 wt.%, 5 wt.% and 10 wt.% in the first,second, and third sublayers are shown in Figure 8. A short-circuit currentdensity of 16.30 mA/cm2 and a conversion efficiency of 7.2% wereobtained, which were improved by 26% and 20%, respectively, compared to those of the DSSC without microcavities. Pronouncedincrease in both optical absorbance and incident monochromaticphoton-to-current conversion efficiency (IPCE) in the long wavelengthregion was observed, implying that the enhancement of cell performance was due to the multiple scattering of light by the microcavities and also to the light confinement by the stack of TiO2 sublayers with high-to-low graded effective index of refraction.