Film morphology and molecular orientation in oligothiophene-fluorene films

The effects of processing conditions on film morphology and molecular orientation were studied for a novel conjugated fluorene–bithiophene oligomer, oligo(9,9-dioctylfluorene-alt-bithiophene) (OF8T2). Depending on the method of film preparation, OF8T2 molecules adopt different orientations in the films. X-ray diffraction peak at 4.9° of the OF8T2 film deposited from petroleum ether/dichloromethane mixture is attributed to a layering distance between sheets of OF8T2 chains, which are separated by the octyl side chains. Preferred orientation is clearly inferred through the absence of peaks corresponding to π–π stacking. For the spin-coated film after annealing, X-ray diffraction patterns indicate the presence of lamellar structure with the plane of the conjugated backbone normal to the substrate. The molecules were aligned with long axes along the rubbing direction when the spin-coated film was rubbed and then annealed. These results suggest a convenient approach for preparing active layers for organic optoelectronic devices by simple solution methods.