二氧化鈦工作電極對於固態染敏化太陽能電池之元件效率的影響探討

Studies on the effect of TiO2 photoanode on the performance of solid-state Dye-Sensitized Solar Cell

李柏融(Bo-Rong Li)

中原大學/工學院/化學工程研究所/碩士(2012年)

https://doi.org/10.6840/cycu201200587

摘要

染敏化太陽能電池在近年來已成為大家爭先研究的領域，在光電極部份，目前大都採用二氧化鈦為材料。基本上，它可藉由幾種方式去改善元件的效能：(1)導入較大顆粒之二氧化鈦奈米粒子增加光散射效應；(2) 利用一維結構之光電極取代奈米粒子以提升電子傳遞速率。本研究分別探討上述兩種途徑對於固態染敏化太陽能電池之光伏特性。
首先，我們以SEM、TEM分析藉由溶膠凝膠反應所製備之二氧化鈦奈米粒子的粒徑與形態，發現此粒子係由許多15 nm 之小粒子聚集形成約100~300 nm粒徑大之顆粒。XRD繞射圖譜証實此等粒子為銳鈦礦相。若將這些粒子，再與P25混摻以提升光散射效果。然而， SEM影像中卻觀測不到這些大粒子的存在，這可能是因為這些大粒子較為鬆散，在混摻過程中已分散為小顆粒。但從UV-Vis發現混摻過後之薄膜，其染料吸附量卻有明顯提升，說明如此製備之二氧化鈦膜具有高之比表面積，此一結果對應SEM之觀測現象。我們進一步將此薄膜作為光電極，以Z907為染料， P3HT為電洞傳導材料，製備成固態染敏化太陽能電池。在元件的效能上，發現混摻多孔性二氧化鈦奈米粒子有助於效率的提升。另外，我們對商業化之較大顆粒奈米粒子與P25形成混摻薄膜。從UV-Vis發現這些顆粒可以有效提升光的吸收，但其薄膜吸附量卻無明顯提升，主要來自於光的散射效應。在元件製備上，隨著混摻比例提升，效率也慢慢提升。當混摻比例在0.5 wt%，有較佳的效率。IPCE圖譜亦顯示長波長處有較佳的光致電荷轉換效率，證明光散射增加了光的吸收量。之後，隨著混摻比例的增加，效率隨之遞減，其係因為吸附量與散射效應競爭後的結果。因此，在固態染敏化電池中，在適當之比例下導入較大顆粒之奈米粒子，有助於提升元件的效能。
由於顆粒彼此間存在著一些缺陷，不利於電子的傳遞，易提升載子的複合機率。在第二部份的實驗上，我們藉由水熱方式在FTO玻璃基材上成長出二氧化鈦奈米柱，取代藉由奈米粒子組成之光電極，以期提升元件的效能。反應時間的長短，影響著二氧化鈦在基材表面上的形態。而二氧化鈦緻密層的有無，則影響了奈米柱的的成長方向。由於此光電極表面粗糙度大，易使元件發生導通現象。為了修飾其表面粗糙程度，在本研究乃以二氧化鈦膠體塗佈於奈米柱上，構成二氧化鈦奈米柱/奈米粒子混合型光電極。並探討P3HT之分子量及濃度對於元件效能的影響。發現較大的分子量，有助於避免導通現象的產生，但在濃度的選定上，並不會影響其元件的效能。

關鍵字

二氧化鈦 ； 染敏化太陽能電池

並列摘要

Dye-sensitized solar cells (DSSC) have attracted considerable research interest because of their potential of providing cheap renewable energy. In this type of solar cell, TiO2 is the most common photoanode. To further improve cell performance, two approaches of modifying TiO2 electrode have been proposed: (1) the introduction of large-size TiO2 particle to induce light scattering effect and then enhance the light absorption of sensitizer; (2) the replacement of TiO2 nanoparticles with one-dimensional rods or tubes to shorten the paths by which electrons transport to anode. This study intensively investigates the influence of these two methods on the photovoltaics of solid-state dye-sensitized solar cells.
In the first part, porous TiO2 beads were synthesized using the sol-gel method. Both the SEM and TEM images show that the thus-prepared beads have a particle size of 100~300 nm, and are composed of numerous nanoparticles with a diameter of ~15 nm. The XRD diffraction pattern indicates they are in anatase phase. As blending with TiO2 particles (Degussa P25) to produce porous photoanode, these fragile beads hardly tolerate the shear stress and break apart into nanoparticles, thus failing to induce light scattering effect but accidently raising the total surface area of the TiO2 layer that consequently increases the dye loading and the photocurrent. Additionally, a commercial large-size TiO2 bead, P80 and P200, whose diameter is 80 nm and 200 nm, respectively, was employed as an additive of P25 for preparing the porous TiO2 layer. The UV-vis measurements indicate the presence of the large-size beads practically boosts the absorbance of the dyed TiO2 in a certain range of loading. Moreover, the power conversion efficiency of the solid-state dye-sensitized solar cell which has a device structure of FTO/dense TiO2/P25:TiO2 bead/dye/P3HT/Au increases with the loading of TiO2 beads up to 0.5 wt% and then drops slightly with further increment of the bead loading. This is probably because the large-size TiO2 has a lower specific surface area than that of P25 so that the replacement of P25 with these beads causes the decrease of the total surface are of TiO2 and then the amount of the adsorbed sensitizer.
In the second part, to increase the electron mobility of photoanode, TiO2 nanorods were grown on the surface of FTO by the hydrothermal route. Both the reaction time and the presence of a dense titania layer play important roles on the morphology of the thus-prepared TiO2 rods. For the device fabrication, these TiO2 rods were coated with a layer of TiO2 nanoparticles to enlarge the total surface area of photoanode. This porous TiO2 film has a very rough surface so a high-molecular-weight P3HT is required to fully cover the TiO2 to lower the leakage current. However, the concentration of the P3HT solution has no apparent effect on the photovoltaics of solid-state dye-sensitized solar cells.

並列關鍵字

DSSC ； TiO2

參考文獻

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