Styrene and quinoline groups are commonly incorporated in the organic fluorescent materials for organic light-emitting diodes (OLEDs). In this paper, a small molecule derived from styrene and quinoline, with the chemical structure of 2,2`-(2,5-dimethoxy-1,4-phylenedivinylene)bis-8- acetoxyquinoline (MPV-AQ), were employed as the emitter and electron transporting material in the OLEDs, and its optoelectronic characteristics such as charge-carrier injection, transporting and recombination were investigated by the steady-state and transient technologies. It was found that the electron injection from the cathode to the MPV-AQ layer showed the Fowler-Nordheim (FN) tunneling characteristic in the N,N`-di(naphthalene-1-yl)-N,N`-diphenyl-benzidine (NPB)/MPV-AQ bilayer OLED, which is different to the Richardson-Schottky (RS) thermionic emission in the electron-only device based on the MPV-AQ single-layer. The difference of electron injection was attributed to the bend of energy bands of MPV-AQ in the NPB/MPV-AQ device, which might be caused by the charge accumulation at the NPB/MPV-AQ interface. The accumulated charges should mainly be the holes at the side of NPB layer because the electron mobility of MPV-AQ is much lower than the hole mobility of NPB. Due to the bend of lowest unoccupied molecular orbital (LUMO) of MPV-AQ, the tunneling distance for electrons is significantly reduced, which is favorable for the FN tunneling. The barrier height for electron injection was calculated as 0.23 eV by fitting the current-voltage curve of the NPB/MPV-AQ bilayer OLED. And the electron mobility of MPV-AQ was determined by the delay time of transient electroluminescence (EL) and showed field-dependence with the value of the order of 10-6 cm2/Vs. In addition, the electron-hole recombination coefficient was obtained from the long time component of the temporal decay of the EL intensity, and the coefficient was found to reduce with increasing the applied voltage, which is consistent with the efficiency roll-off in this bilayer OLED. This study may provide a foundation for understanding the electronic processes of carrier injection, transport and recombination in the OLEDs, which is helpful to improve the device performance.
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