利用瞬態光電流與光電壓衰減技術研究染料敏化太陽能電池之電荷轉移動力學Ⅰ.不同TiO2奈米管成長方法(cv, cc and cv/cc Hybrid)Ⅱ.不同紫質染料(YD11, YD12 and YD13)

Abstract

本論文是藉由瞬態光電流/光電壓衰減技術，研究不同陽極處理方法製備之一維二氧化鈦奈米管(TiO2 nanotube, TNT)光陽極微結構以及YD11-YD13系列紫質染料分子結構對於染料敏化太陽能電池(DSSC)元件之電子傳遞及電荷重組過程所造成的影響。 在第一部分，我們採用傳統定電壓、定電流以及本實驗室開發的定電壓-定電流混合陽極處理方法，製備出不同微結構的一維二氧化鈦奈米管，分別標示為cv-TNT、cc- TNT及Hybrid-TNT，將其作為DSSC中的光陽極材料。在其吸附N719光敏染料後，封裝成背照式的NT-DSSC元件。藉由瞬態光電流/光電壓衰減技術的量測分析，我們可以得知電子擴散係數(Dn)之趨勢為：Hybrid-TNT≧cv-TNT＞cc-TNT；而電子生命期(□R)之趨勢為：cc-TNT＞cv-TNT＞Hybrid-TNT。與傳統定電壓陽極處理法相比，本實驗室開發的定電壓-定電流陽極處理法不僅可以縮短陽極處理的成長時間，且Hybrid-TNT與cv-TNT的電子傳遞速率相近，表示兩者的內部缺陷結構密度及trap states分佈情形相近，使得兩者的JSC也很相近；反之，cc-TNT的缺陷結構較多及trap states分佈較寬廣，造成其電子傳遞速率降低，但電荷重組速率受限於電子傳遞速率，因此cc-TNT的電荷重組速率最慢，且由於cc-TNT之導帶band edge最高，故其元件之VOC最高，但三者差異不大。而較快的電子傳遞速率，使得JSC呈現Hybrid-TNT≧cv-TNT＞cc-TNT之趨勢，因而導致整體元件效能呈現相同的趨勢。 在第二部分，我們採用Hybrid-TNT作為光陽極，搭配YD11-YD13紫質染料封裝成Porphyrin-based NT-DSSC元件，藉由瞬態光電流/光電壓衰減技術的量測分析，得知電子擴散係數(Dn)之趨勢為：YD12≧YD11＞YD13；而電子生命期(□R)之趨勢為：YD12＞YD11＞YD13，顯示YD13的電荷收集效率最差。經由估算可知，YD13的電子注入效率最低，推測是由於YD13染料分子間容易因自身的□-□作用力而堆疊聚集，發生能量轉移，導致其電子注入效率大幅降低，其不僅使JSC降低，也會使TiO2導帶的band edge降低；此外，YD13的電荷重組速率最快，使TNT內電子密度降低，TiO2費米能階因而往正電位移動，因此YD13的VOC最低，整體元件效率最差。

In this work we studied electron transport and charge recombination kinetics of one-dimensional TO2 nanotubes (TNT) photoanodes and YD11-YD13 porphyrin sensitizers in dye-sensitized solar cells using transient photocurrent and photovoltage decay techniques. In first part, 1D TNT arrays were fabricated by conventional potentiostatic anodic method (cv-TNT), galvanostatic anodic method (cc-TNT) and a novel hybrid anodic method (Hybrid-TNT) developed in the laboratory. We fabricated back-illuminated NT-DSSC after the TNT films adsorbed with N719 sensitizer. By means of the transient photocurrent and photovoltage decay techniques, we found that the electron diffusion coefficients show the order of Hybrid-TNT≧cv-TNT＞cc-TNT and the electron lifetimes show the order of cc-TNT＞cv-TNT＞Hybrid-TNT. Compared to the conventional potentiostatic anodic method , the hybrid anodic method has the advantage to produce TNT with high charge collection efficiency in a short anodization period. In addition, the electron diffusion coefficient of Hybrid-TNT is comparable to that of cv-TNT, implying that the state density and the potential distribution of the trap states are similar for both TNT. Therefore, the resulting JSC values are similar too. In contrast, the electron diffusion coefficient of cc-TNT is slower than the other two TNT due to the existence of more defect states for cc-TNT. Furthermore, charge recombination of cc-TNT is the slowest due to its lower electron transport rate. The high VOC of cc-TNT is attributed to the upward shift of the conduction band edge and slower charge recombination, but the discrepancy is small. However, the faster electron transport leads to the variation of JSC showing the order of Hybrid-TNT≧cv-TNT＞cc-TNT, eventually resulting in the overall cell performance to have the same order. In second part, we fabricated back-illuminated porphyrin-based NT-DSSC after the TNT films adsorbed with YD11-YD13 sensitizers. Investigating using the transient photocurrent and photovoltage decay techniques, we found the electron diffusion coefficients show the order of YD12≧YD11＞YD13 and electron lifetimes show the order of YD12＞YD11＞YD13. Charge collection efficiency of YD13 is the lowest. Under estimations, we found electron injection effi- ciency of YD13 is the lowest due to dye aggregation resulting in energy transfer. The down- ward electron injection efficiency of YD13 would lower JSC and induce downward shift of TiO2 conduction band edge. In addition, the low VOC of YD13 is attributed to the faster charge recombination and the downward shift of the TiO2 conduction band edge. This would reduce efficiency of YD13 dramatically.