以電泳沈積法製作可撓曲染料敏化太陽能電池

Fabricating Flexible Dye-sensitized Solar Cells by the Electrophoretic Deposition Method

張原睿(Yuan-Ruei Chang)

指導教授 ： 何國川

國立臺灣大學/工學院/化學工程學研究所/碩士(2008年)

https://doi.org/10.6342/NTU.2008.10563

摘要

近年來，染料敏化太陽能電池已被組裝在塑膠基板上，例如ITO-PEN (indium-tin-oxide coated polyethylene naphthalate)為市面上可獲得的透明導電塑膠基材。ITO-PEN無法承受500 oC的高溫處理，但此高溫燒結程序卻為製程中，欲使染敏電池達到高效能表現所必須的。因此，開發低溫製程製備高效率的可撓式染敏電池，便成為一個重要的議題。
本研究使用電泳沈積法製備介孔洞的二氧化鈦薄膜於導電塑膠基材上作為正電極，而白金對電極則以濺鍍法製備在FTO玻璃上。異丙醇中分散良好的奈米二氧化鈦(Degussa, P25)，在電泳沈積中以定電壓程序沈積到導電塑膠基材，經過低溫(150 oC)燒結可獲得沒有龜裂的二氧化鈦薄膜。
實驗發現，經由化學後處理將Ti(ΙV) tetraisopropoxide (TTIP)溶液滴覆在二氧化鈦薄膜上，能夠提升元件效率表現。TTIP濃度對元件效率表現所造成的影響，經由電化學阻抗頻譜法及染料吸附量分析，發現在二氧化鈦薄膜上的TTIP化學後處理最佳濃度為1.2x10-3 (mole/cm3)。為了增加膜內染料的吸附量，藉由電泳沈積中不同鍍膜時間的選擇獲致不同膜厚的二氧化鈦薄膜，結果顯示薄膜厚度在11 micrometer時具有最佳的元件效率。膜厚超過此值會促進注入膜內的電子與電解液中的氧化還原對發生再結合反應，並讓電解液較難進入薄膜內層，造成元件效率下降。此外，由BET量測發現，低溫燒結程序也可促使二氧化鈦粒子間相互連結，減少粒子間的晶界阻力。在最佳化實驗的燒結程序後，元件效率可達到3.4%。
因為預期電泳沈積中，擁有較佳分散性質的粉體應擁有較高導電性質的正電極，而Q25二氧化鈦粒子含有比P25粒子更高的界面電位，故嘗試以Q25取代P25粒子製備正電極。然而，實驗發現以Q25組裝的電極效率較P25所組裝的電極為低，可能是因為Q25粒子表面有較多的雜質而佔據了染料的吸附位置。此結果說明電泳沈積中，P25的分散性質並未使製備的電極效率降低。而Q25粉體也以不同的電壓，如200、300、400及500 V電泳沈積於正電極上。發現電壓越高，元件效率就越差。由於粒子在沈積過程中產生聚集的速率正比於電場強度，因此較高的電場強度會形成鬆散的薄膜，而呈現較差的元件表現。
由於在正電極中含有大顆粒的二氧化鈦可收集入射光，增加元件的效能表現，因此本研究中，200 nm的二氧化鈦(U200)被加入電泳槽中使之與Q25粒子共電泳沈積，而得到一具有光捕獲效率較高的二氧化鈦薄膜。藉由交流阻抗分析發現，此型態的正電極由於其鬆散的結構，其膜內電阻相較於完全不含大顆粒子的薄膜高，最佳化的效率也僅有2.4%。另一方面，組裝雙層結構(內層由Q25粒子所製備，外層則包含Q25粒子與光散射粒子)的正電極則成功地將元件效率由3.3%提升到3.9%。這是因為雙層結構的正電極不僅能維持奈米結構薄膜的低內電阻，也能藉由外層以200 nm與Q25的二氧化鈦粒子所組成的光散射層捕捉入射光。
雙層結構的二氧化鈦薄膜經由紫外線/臭氧的處理去除二氧化鈦表面殘留的有機分子，能成功的提升元件效率。其在AM 1.5 (100 mWcm-2)一個模擬太陽光照射下達到的光電壓、短路電流、填充因子及光電轉換效率分別為830 (mV),.2 (mA/cm2),0.71及4.2%。而當白金對電極由FTO-glass換成ITO-PEN基材時，也有同樣4.2%的光電轉換效率。
最後，塑膠染料敏化太陽能電池以UV膠封裝，探索元件的穩定性。結果顯示，元件在室溫下的環境中，經過靜置三個小時仍能維持相同的效能表現。然而，由於UV膠易被電解液所剝蝕，讓液態電解液揮發到外界，使的效率逐漸產生衰減的現象，元件效率在第七個小時相較於原始效率衰減了3.8%。

關鍵字

染料敏化太陽能電池 ； 塑膠基材 ； 電泳沈積 ； 交流阻抗 ； 穩定性

並列摘要

Recently, dye-sensitized solar cells (DSSCs) have been fabricated on the plastic substrates, including the indium-tin-oxide coated polyethylene naphthalate (ITO-PEN), which is a commercially available transparent conductive plastic substrate. However, ITO-PEN could not endure the high temperature (500 oC) treatment, which is a necessary step for sintering TiO2 in DSSCs to achieve high cell efficiency. Fabrication of flexible DSSCs at low temperature with good cell performance thus becomes an important issue.
In this study, the electrophoretic deposition (EPD) method was used to deposit mesoporous TiO2 film onto the plastic substrate as the photo-anode, and platinum was sputtered on the FTO-glass as the counter electrode. Nanocrystalline titanium dioxide (Degussa, P-25) was well dispersed in isopropanol (IPA) and deposited potentio statically onto the plastic substrate by the EPD followed by sintering at low temperature (150 oC), then the crack-free mesoporous TiO2 film was obtained.
The cell performance was improved by chemical post-treatment through the drop coating of Ti(ΙV) tetraisopropoxide (TTIP) solution on the TiO2 films. The effect of TTIP concentration on the cell performance was investigated by EIS analysis and dye loading analysis, and the optimum TTIP concentration was found to be 1.2x10-3 mole/cm3. To enhance the amount of dye loading within the film, different thicknesses of the TiO2 layers as photo-anodes were obtained by changing the deposition time in the EPD. It was found that the optimum cell performance can be achieved at the TiO2 film thickness of 11 micrometer. Greater film thickness would promote the recombination reaction between the injected electrons within the film and the redox couple in the electrolyte. This also would prevent the electrolyte’s penetration into the film, thus decrease the cell performance. In addition, it was found from the BET measurement that low -temperature sintering process could also enhance the connection among TiO2 particles, and decrease the grain boundary resistance among particles. After optimizing the sintering process, the cell efficiency of 3.4% was achieved.
Q25 TiO2 particles possessing higher zeta potential in the solution were used to replace P25 TiO2 particles. It is expected that Q25 powders suspend better in the EPD and may produce more conductive photo-anode. However, the result showed that the photo-anode preparing from Q25 particles performed slightly lower efficiency, as compared to that prepared from P25 particles. This might be that Q25 particle’s surface contained more impurities and occupied the dye adsorbing sites. The results also showed that P25 powders’ suspension property dosen’t harm the photo-anode performance. The photo-anodes prepared with Q25 powder were carried out at different applied electric fields in the EPD, including 200, 300, 400 and 500 V. The higher the voltage was used, the lower the cell efficiency was obtained. Because the aggregation rate of the particle was proportional to the electric field during the deposition, higher electric field was found to produce looser TiO2 film thus exhibited poor cell performance.
If large TiO2 particles were incorporated in the photo-anode, it could harvest incident light and increase cell efficiency. In this study, 200 nm TiO2 particles (U200) were added to the EPD cell to co-deposit with Q25 particles and got a TiO2 film with a higher light harvesting efficiency. However, from the AC impedance analysis, it was found that this photo-anode had high interanl electric resistance, as compared to the film made without any large particles. Due to its looser structure, the optimal cell efficiency was only achieved at 2.4%. On the other hand, preparation of the photo-anodes with bilayer structure (inner layer was prepared by Q25 particles, and outter layer was composed of Q25 particles and light scattering particles) was improved the cell efficiency from 3.3% to 3.9%. This is because bilayer structure not only maintain a low internal electric resistance in the nanocrystalline film, but also trap the incident light by the outer scattering layer, which was prepared by mixing 200 nm and 25 nm TiO2 particle.
TiO2 film with bilayer structure was subjected to UV/Ozone treatment to remove the residual organic molecular on the TiO2 surface, and the cell efficiency was improved. The values of the open–circuit photovoltage, short-circuit photocurrent density, fill factor and sunlight -to-electric energy conversion efficiency achieved were 830 mV, 7.2 mA/cm2, 0.71 and 4.2%, respectively, under illumination with AM 1.5 (100 mWcm-2) simulated sunlight. When the platinum counter electrode was changed from FTO-glass to ITO-PEN, it performed the same cell efficiency of 4.2%.
Finally, plastic DSSC was sealed by UV glue to study the stability of the cell. The result showed that the cell stored at room temperature for three hours performed the same as its original states. However, because the UV glue was degraded by the electrolyte and evaporated, the cell efficiency decayed 3.8% at seven hours compared with the initial performance.

並列關鍵字

DSSC ； Plastic substrate ； Electrophoretic deposition ； AC impedance ； Stability

參考文獻

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