相分離結構與高分子太陽能電池效率及穩定度之相關性

Abstract

以共軛高分子作電子施體並以富勒烯衍生物作電子受體的異質接面太陽能電池近年來被廣泛研究；除了發展新材料來提高效率，其穩定性提升也是另一個很重要的課題。 本研究中研究了一具分枝側鏈的聚噻吩(polythiophene)－聚3-(2-甲基丁基)噻吩(poly(3-2-methylbutylthiophene), P3MBT)的形態、熱性質與以其為主動層材料之太陽能電池效率並與具線形側鏈的聚3-己基噻吩(poly(3-hexylthiophene), P3HT)比較，以探討側鏈結構差異造成的影響。由製作反式結構太陽能電池之效率與熱穩定性量測結果中，發現P3MBT/PC61BM系統的效率及熱穩定度都比P3HT/PC61BM系統佳。使用光學顯微鏡與原子力顯微鏡觀察熱退火時間不同膜形態的變化，發現到P3MBT/PC61BM系統的形態較不易產生大量PC61BM聚集，使得其穩定度較佳，而P3MBT/PC61BM系統效率佳的原因可能與P3HT傾向形成一束束、而P3MBT傾向形成網狀的纖維形態有關；此外，由示差掃描量熱儀知道P3MBT具較高之玻璃轉移溫度，也顯示出P3MBT/PC61BM系統有較佳之熱穩定性。 此外，我們使用X光繞射技術發現到P3HT與P3MBT同樣傾向以層狀(Lamella)的方式堆疊；而由小角X光散射剖線得到的晶域尺寸也顯示出P3MBT/PC61BM系統具較穩定相分離結構；此分枝側鏈之較大的立體障礙，導致其主鏈較為剛硬，減緩了P3MBT與PC61BM間的相分離。

In the past decade, the polymer/fullerene-based organic solar cell has rapidly developed. In addition to the increase of power conversion efficiency (PCE), the improvement of stability has also been a key issue for the organic solar cell. In this study, we studied the morphology, thermal properties, and photovoltaic performance of poly(3-2-methylbutylthiophene) (P3MBT), which is a poly(thiophene) bearing branched side chains. The results are compared to those of the commonly used poly(3-hexylthiophene) (P3HT) with linear side chains to investigate the effects of side chain configuration. We fabricated the solar cells in an inverted structure and found that P3MBT/PC61BM not only show a higher PCE but can last much longer than P3HT/PC61BM does. P3HT and P3MBT tend to form fibrous bundles and networks, respectively, which may be one of the reasons that cause a higher PCE for P3MBT-based solar cells. Moreover, we used X-ray diffraction techniques to show that both P3MBT and P3HT tend to form lamellas commonly seen in conjugated polymers. We utilized optical microscope and AFM to study the dependence of the morphology on the annealing times and found that the morphology of P3MBT/PC61BM is more stable than that of P3HT/PC61BM. The size of domains extracted by the small-angle X-ray scattering (SAXS) profiles also reveal a more stable phase separated structure in P3MBT/PC61BM blends. We suggest that the branched side chains are bulkier than the linear side chains and thus can provide a higher steric hindrance, which causes more rigid side chains and backbones to slow down the phase separation rate between P3MBT and PC61BM. The higher glass transition temperature of P3MBT than that of P3HT is confirmed by differential scanning calorimetry (DSC).