高分子/奈米粒子之混摻太陽能電池研究

Research on Polymer-Nanoparticle Solar Cells

廖佑加(Yu-Chia Liao)

指導教授 ： 林唯芳

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2009年)

https://doi.org/10.6342/NTU.2009.00512

摘要

聚三己基噻吩(poly(3-hexylthiophene), P3HT)為擁有高電洞遷移率且匹配太陽光光譜能隙的導電高分子，近年來在有機太陽能電池裡常用來和碳60的衍生物混摻製作成元件，目前最高的光電轉換效率已可達到約6%。聚三丁基噻吩(poly(3-butylthiophene), P3BT)為流動性差的導電高分子，但其對退火的反應很小，故假使能有效提升元件效率則可省略退火的步驟而降低製作成本。二氧化鈦為目前地球上含量豐富的無機金屬氧化物之一，成本便宜、化學和熱穩定性佳、對人體無毒以及機械強度高的優點，使得許多企業跟進發展有關鈦的相關產品。於本研究裡，主要是將P3HT以及P3BT分別和二氧化鈦奈米桿(TiO2 nanorod)混合製作成有機/無機混摻薄膜太陽能電池，並進行一系列的研究。首先以不同表面改質物改質二氧化鈦奈米桿的表面，發現末端為羧基的寡聚物3-己基噻吩 (oligomer 3HT-COOH)和兩種導電高分子混摻有較好的載子遷移率、激子分離效率以及膜的表面型態，進而有最高的效率。

我們也針對P3BT以及P3HT和二氧化鈦奈米桿混摻並製作元件，發現P3BT在退火前後的效率無明顯的差異，且都比P3HT低，顯示由於P3BT的流動性和混摻性差的原因，使得膜的表面型態有太多過度聚集，除了降低結晶度，也減少載子遷移率和連續傳導路徑，因而有較差的元件效率。

我們亦探討溶劑和分子量對P3HT混摻二氧化鈦奈米桿的元件影響，發現以氯苯溶解P3HT則分子鏈較延展，使得分子內的結構較規則，會有較高的電洞遷移率，但也使得電子和電洞的遷移率不平衡而降低光電流；另一方面，以氯仿溶解P3HT則有較均勻且分散性好的膜表面型態，且電子電洞的遷移率更加平衡，進而提升光電流和增進元件效率。而低分子量15k的P3HT具有分散性和混摻性良好的優點，可避免過度聚集的情形發生，高分子量60k的P3HT則有較長共軛長度，可提升載子遷移率和電荷連續傳導路徑；中分子量30k的P3HT同時擁有低分子量和高分子量P3HT的優點，故能收集到的載子最多，光電流越大，而有最高的元件效率。

由上述實驗結果的描述可知，在我們實驗裡的P3HT混摻二氧化鈦奈米桿的系統裡，其最佳化的參數為以末端為羧基的寡聚物3-己基噻吩作為二氧化鈦的表面改質物，並和中分子量且溶解於氯仿的P3HT混摻，並作退火的處理，即可得到最高光電轉換效率的元件。於本實驗裡，太陽能電池元件的最高效率為1.34%。

關鍵字

聚三己基噻吩 ； 聚三丁基噻吩 ； 二氧化鈦奈米桿 ； 表面改質物 ； 溶劑 ； 高分子的分子量 ； 退火

並列摘要

Poly(3-hexylthiophene) (P3HT) possesses some unique properties such as high hole mobility and extended absorption near infrared region in solar spectrum so that it is often blended with derivatives of C60 to fabricate organic solar cell. Up to now, the highest PCE (photon-to-electron conversion effieciency) in P3HT/PCBM system is about 6%. Different from P3HT, poly(3-butylthiophene) (P3BT) is not mobile enough to form high degree of crystalline region. Futhermore, anneal treatment has little influence on P3BT as it is mixed with other electron acceptor to fabricate solar cell. If a high PCE could be achieved as the same as P3HT, it would not need for P3BT with anneal treatment so as to decrease the cost of fabrication process. On the other hand, TiO2 is one of the most abundant metal oxides on the earth. Because of its low cost, nontoxic property, thermal and chemical stability and high mechanical strength, it is widely used in many commercial applications. In this work, we will present a series of study about organic/inorganic hybrid solar cell composed of either P3HT or P3BT and TiO2 nanorod.

Surface modification is a topic of this work. We find that when the surface modifier is oligomer 3HT-COOH, the performance of device can be improved through exciton dissociation efficiency, charge recombination, carrier mobility, and nano-scale morphology. Thus, a high PCE could be achieved by attacking oligomer 3HT-COOH to TiO2 surface.

We also present that P3BT is not mobile enough to develop ordered structure with and without anneal. Futhermore, large aggregates observed in the nano-scale morphology of P3BT is due to the low solubility that reduces carrier mobility and continuous transport path. Therefore, whether the device of P3BT/TiO2 system is under annealing or not, the PCE is lower than that of P3HT/ TiO2 system.

The effects of P3HT molecular weight and solvent type on the performance of the device are also investigated. The low molecular weight P3HT has good compatibility and prone to form local highly ordered crystaline domain. On the other hand, the high molecular weight P3HT have longer conjugation length and better interconnected network to balance hole and electron mobility. The medium molecular weight P3HT exhibits more interface for exciton dissociation than that of high molecular weight P3HT and longer carrier transport path than that of low molecular weight P3HT. We have observed the highest PCE is from medium molecular weight P3HT. With regard to solvent effect, we find that P3HT which is dissolved in chloroform has better device performance than that is dissolved in chlorobenzene. This result is due to the more balanced charge carrier mobility and good morphology so as to achieve higher PCE.

The best device is fabricated by medium MW P3HT which is dissolved in chloroform and blended with TiO2-(oligomer 3HT-COOH) with solvent anneal treatment. Under A.M. 1.5 at 100mW/cm2 illumination, the efficiency of 1.34% is obtained.

並列關鍵字

poly(3-hexylthiophene) ； poly(3-butylthiophene) ； TiO2 nanorod ； surface modifier ； solvent ； molecuar weight ； anneal treatment

參考文獻

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