A Mechanistic Investigation Of P3HT Film Morphology Modulated By Additive And Corresponding Photoelectric Properties

Controling active layer morphology is an important means to improve the efficiency of polymer solar cells. The additive is a common and effective method of optimizing morphology of the active layer. It can not only induce the crystallization and improve the crystallinity of the polymer, but also can control the phase separation between donor and acceptor.In this paper, polystyrene of different tacticities and two kinds of liquid crystals with different structures have been served as additives to study their effects on the morphology of poly(3-hexylthiophene)/fullerene(P3HT/PCBM) system and the crystallization of P3 HT.First of all, the atactic polystyrene(aPS) or syndiotactic polystyrene(sPS), P3 HT and PCBM blended with different weight ratio, and the morphology and device of the blending films were characterized and analyzed to study the effects of different tacticities on the morphology of P3HT/PCBM system and its device performance. The results show that it is not immiscible between aPS or sPS and P3 HT. When the content of aPS is lower than 50 wt%, the crystallinity of P3 HT improve, the lamellar spacing of the molecular chain increase from 1.63 nm to 1.76 nm and the crystalline size of P3 HT decrease with the increasing of aPS. In the aPS/P3HT/PCBM blend films, the morphology of the film changes with increasing aPS concentration. The columnar structure and lamellar structure are constructed and PCBM tends to enrich in aPS and P3 HT interface. When the doping content of sPS is 3 wt%, sPS can induce the crystallization of P3 HT. However, with the doping amount of sPS increasing, the crystallization of P3 HT is inhibited. When the aPS content is 50 wt%, the aPS/P3HT/PCBM system still owns relatively good efficiency(1.2 %). However, when the doping amount of sPS is only 3 wt%, the device efficiency of sPS/P3HT/PCBM system decays rapidly to 0.6 %. Therefore, different tacticities have important effect on the morphology and device performance of P3HT/PCBM system.Secondly, the 4-cyano-4’’-pentylterphenyl(5CT) and 2,2’’-dioctylterthiophene(8-TTP-8) are two kinds of liquid crystals(LCs) with different structures, which were selected as LCs additives to blend with P3 HT with different weight ratio. The LCs/P3 HT solution was spin coated on quartz plate, and applied a magnetic field of 1 T in the process of solvent-evaporation. Under the treatment of magnetic field, the liquid crystal molecules firstly self-assemble into ordered structures and then as a template to induce the formation of P3 HT nanofibers. The results show that under the magnetic field both 5CT and 8-TTP-8 can assemble into well-ordered structure, which is served as the template to induce the crystallization of P3 HT, strengthen the π-π stacking of P3 HT molecular chain and induce P3 HT to form nanofibers. The P3 HT nanofibers can enhance the carrier mobility of P3 HT films. After the magnetic field treatment, the carrier mobilities of 8-TTP-8/P3 HT specimen increase from pure P3 HT film of 1.092×10-4 cm2/Vs to 3.346×10-4 cm2/Vs. And the carrier mobilities of 5CT/P3 HT specimen increase to 3.114×10-4 cm2/Vs. Compared with the pure P3 HT specimen, the carrier mobilities increase about 3 times. Therefore, under the external treatment of magnetic field, the liquid crystal can induce the formation of P3 HT nanofibers and improve carrier mobility of film.