Controllable beta-phase formation in poly(9,9-dioctylfluorene) by dip-coating for blue polymer light-emitting diodes

The performance of polymer light-emitting diodes based on poly(9,9-dioctylfluorene) (PFO) is strongly affected by the presence of beta-phase in the polymer films. In this work, beta-phase PFO films were developed by a facile method dip-coating in one step. Absorption spectra with an additional peak at 435 nm and red-shifted emission spectra demonstrate the formation of beta-phase in the dip-coated films. Notably, beta-phase content in the PFO films can be tailored by controlling the withdrawal speed. The experimental results show that the content of beta-phase in PFO films increases from 1.4 % to 23.2 % with the decreasing of the withdrawal speed from 60 mm/min to 5 mm/min, and the device efficiency and color purity for the blue emission get improved due to the formation of beta-phase. When the withdrawal speed is 5 mm/min, the devices possess the optimal performance with a current efficiency of 2.47 cd/A, which is 3.6 times as high as that of the device with the spin-coated PFO film as emitting layer. These results demonstrate that as a simple methodology, the dip-coating technology can effectively induce the molecular chain orientation in the film to regulate the optoelectronic properties for applications in efficient optoelectronic devices.