Exciton-polaron interaction in blue fluorescent organic light-emitting diodes

Abstract

OLEDs are popular as display technology nowadays, which have been widely used in commercial application. However, there are still some problems that blue light devices are not as efficient or stable as red and green light devices. Although the use of phosphorescent dyes can significantly improve the internal quantum efficiency, the high production cost and unstable performance limit the industrialization of phosphorescent OLEDs. In the development of OLEDs, the researchers found that OLEDs suffered from a decline in their efficiency at high brightness levels, a behavior known as “efficiency roll-off”. The efficiency roll-off is more pronounced in phosphorescent devices due to the longer lifetime of triplet exciton than singlet exciton, so that it has been widely investigated in recent years. Little is known, still, about fluorescent devices. Accordingly, unraveling the exciton loss mechanism in blue fluorescent OLEDs is particularly important, as it is a limiting factor for the improvement of efficiency. In this work, the efficiency roll-off in blue fluorescent OLEDs is investigated by observing the quenching of DPAVBI excitons. Firstly, the effects of electron current and hole current on photoluminescence(PL) behavior of unipolar devices are studied by steady-state and transient-state measurements, and we analyze PL spectrum and calculate the exciton quenching rate constant according to the transient PL decay curves to clarify the exciton quenching dynamics. The results show that the holes are much more efficient in quenching the excitons when the host is a hole transport material. This is different from the general understanding that exciton-polaron quenching effect with higher carrier mobility is weaker. Because the existence of bound charges produces additional charge density, and it is inferred that the exciton is mainly quenched by trapped charge rather than moving charge. We also exclude the effect of exciton–exciton annihilation and electric-field-induced dissociation on the efficiency degradation of the OLEDs. It is confirmed experimentally that exciton-polaron interaction is the dominant mechanism of the efficiency roll-off in fluorescent OLEDs. We then fabricate organic light-emitting diode devices with different doping concentrations to figure out the effect of doping concentration on exciton-polaron interaction, and obtain a blue fluorescence device with good comprehensive performance. We also summarize some feasible methods to optimize the efficiency of the OLEDs. In this paper, our findings about exciton-polaron interaction might provide a viable source for efficiency improvement by regulating charge trapping in light emitting layer.