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A series of pentacene derivatives with unsaturated hydrocarbon substituents at the 6,13-positions as side groups have been synthesized. The effects of conjugated π-bonding throughout the side groups on the solubility, oxidative stability, crystallinity, and electrical properties of the compounds were investigated. Here we report that the solubility of pentacene can be greatly improved by the insertion of linear long side groups at the 6,13-positions: the obtained pentacene derivatives were soluble in common organic solvents. Time-dependent UV-visible spectra in chloroform solutions as well as thin films were recorded to investigate the oxidative stability of each pentacene compound under ambient conditions. The photooxidative stability of the pentacene derivatives was remarkably enhanced by increasing the electron conjugation length through the installation of unsaturated hydrocarbon substituents at the 6,13-positions. The solution-processed TFT devices based on 6,13-bis(4-pentylphenyl-ethynyl)pentacene (P5) and 6,13-bis(4-hexylphenylethynyl)pentacene (P6) showed charge transport mobilities of 2.14 × 10−2 cm2 V−1 s−1 and 3.94 × 10−2 cm2 V−1 s−1, respectively, and exhibited an on/off ratio of 105. Here we demonstrate that P5 and P6 revealed much more stable electrical performances than the parent pentacene under the oxidative environment.

Introducing conjugated side chains onto the backbone of two-dimensionally (2D) conjugated polymers has been utilized for tuning the optoelectronic characteristics of the polymer and the morphological properties of organic photovoltaics. To investigate the effects of conformational symmetry of conjugated side chains, two benzo[1,2-b:3,4-b′]dithiophene (BDT)-based derivatives, one with the asymmetric alkoxythienyl (Th) side chain and the other with the symmetric alkoxyphenyl (Ph) side chain, were synthesized as donor units and copolymerized with fluorinated benzothiadiazole (2FBT). These two side chains were selected for the distinct differences between their structures, and were found to affect the intrinsic characteristics of these BDT polymers. The introduction of the symmetric conjugated side chain to the conjugated backbone of the polymer was observed to improve both light harvesting and the charge carrier mobility, apparently by increasing the extent of packing between the polymer chains. Power conversion efficiency (PCE) values of photovoltaic devices fabricated using these conjugated polymers were strongly related to the light absorbance and crystallinity in a film of the blend of polymer and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM). PBDT2FBT-Ph showed effective light absorption, an optimum morphology that we argue is due to the symmetry of the conjugated Ph side chain, and a maximum PCE of 6.23%, with an open-circuit voltage of 0.83 V, a short-circuit current density of 11.33 mA cm−2, and a fill factor of 66.3%. These results demonstrate that symmetric conjugated side chains are promising groups to produce 2D-conjugated polymers for high-performance photovoltaics. This systematic study of side chain engineering provides a valuable strategy to synthesize 2D conjugated polymers and to achieve high PCE values in organic photovoltaics.

We report the preparation of a new acceptor unit, 3-alkoxy-4-cyanothiophene, and a 3-alkoxy-4-cyanothiophene-based low band-gap polymer (PBDT–CT) with a deep HOMO energy level (−5.5 eV). A bulk heterojunction solar cell containing PBDT–CT and PC61BM was found to exhibit a high open circuit voltage (Voc) of 0.90 V and an efficiency of 3.36%.