合成有機可溶噻吩小分子於太陽能電池的應用

Abstract

本研究合成含噻吩-噻吩共軛臂鏈之小分子並應用於溶液製程有機太陽能電池。利用雙噻吩(bi-thiophene, TT)為S之核心，逐步反應形成含有四個噻吩-噻吩共軛臂鏈之星狀S-(T-TT)。進一步比較以TT為中心含兩個噻吩-噻吩共軛臂鏈之線性(linear, L) 小分子材料L-(T-TT)，探討多維共軛臂鏈對小分子光電物理性質之影響。並進一步與學長的小分子材料S-(F-TT)、L-(F-TT) 比較導入不同的共軛單體對於小分子的光電性質之影響。S-(F-TT) 是以TT為核心，逐步反應形成含有四個呋喃-噻吩共軛臂鏈的星狀小分子；L-(F-TT)則是以TT為核心，逐步反應形成含有兩個呋喃-噻吩共軛臂鏈的線型小分子。 四個材料從溶液至薄膜狀態，最大吸收峰的表現上有紅位移現象。而在 onset上，可由兩個方面來比較。第一部分觀察線型(L)和星狀小分子(S)的比較，可以看出L比S的 onset更加紅位移，這是因為L能在薄膜下得到較好的堆疊。第二部分觀察側鏈為噻吩-噻吩(T-TT)及呋喃-噻吩 (F-TT)在薄膜上的情形，可以看出 onset是 (F-TT)系列比 (T-TT)更加的紅位移，推測因為呋喃-噻吩側鏈的平面性來的更好，因此在薄膜時有更好的分子堆疊效果。 由低能光電子光譜( AC-2 )實驗顯示，芳香環共軛臂鏈可有效改變TT核心的旋轉角度，側鏈導入噻吩-噻吩能使平面性變差，並有效降低 HOMO 能階。由理論計算顯示，導入共軛臂鏈於中心雙噻酚間，可有效改變其雙噻酚間之夾角，進而達到星狀3D結構之效果，且多維共軛臂鏈能有效調整小分子之能階與多維方向的電子分布。另外也可得知，側鏈導入呋喃單體可以降低側鏈的平面性。 元件採用正式結構( ITO/ PEDOT：PSS/ Active Layer/ Ca (30 nm)/ Al (100 nm))，將小分子材料與PC61BM混摻 (1:1.2, w/w) 製備成太陽能電池之主動層。在100 mW AM 1.5照射下，L-(F-TT) 之轉換效率為0.84% ，開路電壓為 0.59 V，短路電流為3.1 mA/cm2，填充因子為0.46；S-(F-TT) 之轉換效率為0.93%，開路電壓為 0.52 V，短路電流為4.2 mA/cm2，填充因子為0.46；L-(T-TT) 之轉換效率為0.46 %，開路電壓為 0.61 V，短路電流為2.0 mA/cm2，填充因子為0.37；S-(T-TT) 之轉換效率為0.59 %，開路電壓為 0.54 V，短路電流為2.8 mAcm-2，填充因子為0.39。 在此研究中，可以觀察到導入共軛臂鏈可以利用其較大的立體障礙效果，有效的降低小分子結晶情形，改善元件效率。在此研究中，另一個重點，因側鏈為噻吩-噻吩時比側鏈為呋喃-噻吩時平面性較差，導致小分子無法得到較好的堆疊，因而降低短路電流和填充因子使元件效率變差。

Two novel small molecules with oligothienyls as conjugated arms and bithiophene (TT) as central core was synthesized for solution-processable organic solar cells (OSCs). To systematically investigate the characteristics of multi-diemensional conjugated systems, a star-shaped small molecule with four conjugated arms S-(T-TT) and a linear-shaped (L) small molecule with two conjugated arms L-(T-TT) were synthesized. To systemically investigate how inserting different conjugated bridges would affect optical and electrical properties, a system from previous work, which consists of a linear-shaped small molecule with two oligo(thienylfuran)s as conjugated arms L-(F-TT) and a star-shaped with four oligo(thienylfuran)s as conjugated arms S-(F-TT) are compared with the current work. According to the UV-vis measurements, two phenomena could be observed in the onset wavelength. The onset wavelengths of linear-shaped small molecules displayed a red-shift from solution state to film state. This red-shift is larger than that of star-shaped small molecules. This implies that linear-shaped small molecules exhibited much better molecular packing in film state. What’s more, the onset wavelengths of the molecules comprising furan-thiophene side-chains displayed more red-shift than did the molecules comprising thiophene–thiophene side-chains. This implies that furan-thiophene side-chains exhibited more co-planarity than did the thiophene–thiophene side-chains, resulting in better molecular packing in film state. According to the AC-2 measurements, the aromatic conjugated chains could effectively twist TT core. By introducing thiophene –thiophene side-chains, HOMO energy levels were decreased to a certain degree. Computational analysis provided further evidences that the additional conjugated arms of multi-dimensional conjugated systems were capable of adjusting energy levels and electronic distribution. The analysis also provided evidences that the incorporation of furan-thiophene side-chains would decrease the dihedral angle between conjugated side-chain, and make molecules more planar. Converted OSCs were respectively fabricated by spin-coating the blends of L-(F-TT), S-(F-TT), L-(T-TT), or S-(T-TT) as a donor and the fullerene derivative (PC61BM) as an acceptor. The conventional BHJ solar cell devices of L-(F-TT) achieved a PCE of 0.84% with a Voc of 0.59 V, Jsc of 3.1 mA/cm2, and FF of 0.46；the devices of S-(F-TT) achieved a PCE of 0.93% with a Voc of 0.52 V, Jsc of 4.2 mA/cm2, and FF of 0.46；the devices of L-(T-TT) achieved a PCE of 0.46% with a Voc of 0.61 V, Jsc of 2.0 mA/cm2, and FF of 0.37；the devices of S-(T-TT) achieved a PCE of 0.59% with a Voc of 0.54 V, Jsc of 2.8 mA/cm2, and FF of 0.39. The preliminary photovoltaic performances indicate that the photocurrent and power conversion efficiency were benefited greatly by inserting multi-dimensional conjugated arms and side-chains of furan bridges in small molecules.