Effect of External Electric Field on Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(bithiophene)] Chain Orderness, Morphology, and Carrier Mobility in Different Condensation Processes

Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(bithiophene)] (F8T2) is a kind of crystal and liquid-crystal conjugated polymer (CP). In this research, the effect of external electric field (EEF) on F8T2 chain orderness, morphology, and carrier mobility in different condensation processes of annealing at 130 °C (above glass-transition temperature), 200 °C (crystallization temperature), and 280 °C (liquid-crystal temperature) was investigated by UV–vis absorption spectroscopy, photoluminescence spectroscopy, X-ray diffraction, atomic force microscopy, polarizing optical microscopy, transmission electron microscopy (TEM), and current density–voltage (C–V) curves. It was found that EEF could enhance F8T2 chain orderness and manipulate F8T2 chain self-assembly in all the above three types of annealed films. However, the responses of F8T2 chains to EEF were different. After applying EEF, a preferred arrangement along the EEF direction with needle-like crystals was presented in 130 °C annealed films, and ordered nanorod structures were observed in 280 °C annealed films. Meanwhile, subcrystallization with an increased crystallite size of 14.6% was also discovered in the annealed film at 200 °C. As a result, the hole mobilities of these films were improved 127% (130 °C), 367% (200 °C), and 320% (280 °C). The mechanism for both microstructure changes and hole mobility enhancement was revealed. This research sheds a new light to improve carrier mobility by utilizing EEF to control the chain-condensed state structure and film orderness. This also enlightens us to consider the relationship between external forces and chain self-assembly of CPs based on intrinsic properties of CPs and basic theory of condensed matter physics.