Series of Quinoidal Methyl-Dioxocyano-Pyridine Based π-Extended Narrow-Bandgap Oligomers for Solution-Processed Small-Molecule Organic Solar Cells
作者:Ailing Tang、Chuanlang Zhan、Jiannian Yao
DOI:10.1021/acs.chemmater.5b01350
日期:2015.7.14
Small molecules with narrow bandgap of <1.6 eV can harvest the visible and near-infrared solar photons. In this Article, we report a new method to achieve narrow bandgap small molecule donors by using electron-deficient quinoidal methyl-dioxocyano-pyridine (MDP) to induce possible quinoidal resonance structure along the conjugated A−π–D−π–A backbone. Practically, two MDP moieties are covalently linked onto an electron-rich benzodithiophene (BDT) through the oligothiophene (0T–5T) π-bridge. The affording small molecules, namely, nTBM, exhibit broad and strong absorption bands covering the visible and near-infrared region from 400 to 870 nm. The estimated optical bandgap is down to 1.4 eV. The narrow bandgap is associated with the low-lying lowest unoccupied molecular orbital (LUMO) energy level (about −3.7 eV) and the high-lying highest occupied molecular orbital (HOMO) energy level (around −5.1 eV). Density-functional theory calculations reveal that the HOMO and LUMO energy levels, with the increase of the size of the oligothiophene bridge, become localizations in different moieties, i.e., the central electron-donating and the terminal electron-withdrawing units, respectively, which provides necessary driving force for the delocalization of the excited electrons and formation of the quinoidal resonance structure. The quinoidal structure enhances the photoinduced intramolecular charge-transfer, leading to the absorbance enhancement of the low-energy absorption band. With the increase of the size of the oligothiophene from 0 to 5 thienyl units and the change of the direction of the alkyl chains on the bridged thiophene from “outward” to “inward”, the crystalline nature, fibril length, and phase size of the blend films as well as the cell performance are all fine-tuned, also. With the “inward” alkyl chains, the terthiophene bridged molecule is amorphous, while the pentathiophene bridged one is relatively crystalline. Both molecules form nanoscale interpenetrating networks with a phase size of 15–20 nm when blended with PC71BM, showing the higher hole mobility and promising electric performance.
窄带隙<1.6 eV的小分子可以收获可见光和近红外太阳光子。在本文中,我们报告了一种获得窄带隙小分子供体的新方法,通过使用缺电子的醌型甲基二氧氰基吡啶(MDP)沿着共轭的A−π–D−π–A主链诱导可能的醌型共振结构。实际上,两个 MDP 部分通过低聚噻吩 (0T–5T) π 桥共价连接到富电子苯并二噻吩 (BDT) 上。这种小分子,即 nTBM,具有覆盖 400 至 870 nm 可见光和近红外区域的宽而强的吸收带。估计光学带隙低至 1.4 eV。窄带隙与低位最低未占分子轨道(LUMO)能级(约-3.7 eV)和高位最高占据分子轨道(HOMO)能级(约-5.1 eV)相关。密度泛函理论计算表明,随着低聚噻吩桥尺寸的增加,HOMO和LUMO能级分别定位在不同的部分,即中心给电子单元和末端吸电子单元,这提供了激发电子离域和形成醌型共振结构的必要驱动力。醌型结构增强了光诱导分子内电荷转移,导致低能吸收带的吸光度增强。随着低聚噻吩尺寸从0个增加到5个噻吩基单元以及桥联噻吩上烷基链方向从“向外”到“向内”的变化,低聚噻吩的结晶性质、原纤长度和相尺寸都发生了变化。混合薄膜以及电池性能也都经过微调。由于烷基链“向内”,三噻吩桥联分子是无定形的,而五噻吩桥联分子则相对结晶。当与 PC71BM 混合时,两种分子形成相尺寸为 15-20 nm 的纳米级互穿网络,表现出更高的空穴迁移率和良好的电性能。