分枝側鏈對聚噻酚物性與太陽能電池效能之影響

Abstract

以共軛高分子作為電子予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；除了發展新材料或添加添加劑來提高效率，其穩定性提升也是另一個很重要的議題。 在本研究中，我們合成了聚（3-2-甲基戊基噻吩）（P3MPT），一種具有分枝(branch)側鏈的聚噻吩，並研究了P3MPT的形態，熱性能和光伏性能以及搭配不同電子受體(PC61BM、PC71BM、IC60BA)製作反式太陽能電池，觀察到P3MPT/PC61BM系統擁有最高的效率，可能原因為此系統兩相的混合程度較其他兩者來得佳，且較不容易產生相分離。將P3MPT物性結果與具線性(linear)側鏈的常用聚（3-己基噻吩）（P3HT）進行比較，研究側鏈構型的影響。我們發現P3MPT / PC61BM太陽能電池不僅有較高的效率，而且可以比P3HT / PC61BM系統的太陽能電池維持更長時間的高效率。除此之外，通過小角X光散射（SAXS）得知P3MPT / PC61BM系統具備更穩定的結構。由循環伏安法，得知P3MPT的HOMO較P3HT的高，此為P3MPT/PC61BM系統之Voc較高的原因。 另外，我們認為具分枝側鏈比線性側鏈更有立體障礙，這更剛性的側鍊可以減緩P3MPT和PC61BM之間的相分離速率。通過差示掃描量熱法（DSC）了解到P3MPT的玻璃轉移溫度(Tg)高於P3HT，且P3MPT/PC61BM系統的高分子與PC61BM相容度較好，較不易產生大量的PC61BM聚集。此外，DSC和廣角X射線散射表明，由於較大的側鏈，P3MPT的結晶能力低於P3HT的結晶能力。P3MPT較高的玻璃轉移溫度和較低的結晶能力有助於P3MPT / PC61BM系統在效率方面具有優異的長期穩定性。

Over the past decade, the polymer/fullerene-based organic solar cell has rapidly developed. In addition to the increase of power conversion efficiency (PCE) by adding additives or synthesizing new structure, the improvement of stability has also been a key issue for the organic solar cell. In this study, we synthesized poly(3-2-methylpentylthiophene) (P3MPT), a polythiophene with branched side chain and studied the morphology, thermal properties, and photovoltaic performance of P3MPT blended with different fullerene derivatives. The results are compared to those of the commonly used poly(3-hexylthiopene) (P3HT) with linear side chains to investigate the effects of side chain configuration. We found that P3MPT/PC61BM solar cells not only show a higher efficiency but can maintain the high efficiency much longer than the P3HT/PC61BM solar cells. Compared to P3HT/PC61BM systems, the size of domains extracted by the small-angle X-ray scattering (SAXS) profiles implies a more stable phase separated structure in P3MPT/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 P3MPT and PC61BM. The higher glass transition temperature of P3MPT than that of P3HT was confirmed by differential scanning calorimetry (DSC). In addition, DSC and Wide-angle X-ray scattering show that the crystallization ability of P3MPT is lower than that of P3HT also due to the bulkier side chains. The higher glass transition temperature and the lower crystallization ability of P3MPT contribute to the superior long-term stability of P3MPT/PC61BM system.