Research On The Effect Mechanism Of Interface Charge Storage On The OLED Efficiency Roll-off

Organic light-emitting diodes?OLEDs?are the next generation of display and lighting technology,facing the bottleneck of OLED efficiency roll-off.OLED's light emission and efficiency roll-off are electronic and energy processes,in which charge injection,storage,and dissipation play an important role.In order to study the mechanism of OLED efficiency roll-off during interface charge storage,a series of organic light-emitting devices with Ir?ppy?₃doped CBP as the light-emitting layer were designed.The thickness of PMMA layer is used to control the injection and storage of carriers.The effects of interface charge accumulation on carrier injection and transport and the roll-off of the luminous efficiency of the device are studied.The details are as follows:Firstly,the 5nm and 6nm charge storage layer devices have a higher hole current density than the control device,while the 8nm and 9nm charge storage layer devices'hole injection is suppressed.By comparing the luminescence characteristics of devices with different thicknesses of charge storage layers and the control devices,it was found that the more effective and unbalanced the carrier injection is,the more severe the interface charge storage and the more serious the device efficiency roll-off.Next,the capacitance-voltage test can obtain the relationship between carrier injection and recombination.From this,it is found that carrier injection is more effective in the control devices and devices with a 5nm and 6nm charge storage layer,and it will cause charge accumulation and storage.In the devices of this thesis,the interface charge storage will occur at the interface between the charge storage layer and the light-emitting layer.The interface charge storage region and excitons formation region inside the device will overlap.Accumulating charge in the overlapping region will quench the excitons.The effect reduces the number of excitons and affects the light-emitting performance of the device.In the comparison of PL and EL life tests,it is found that the PL life of different devices is not much different,but the EL life is different,indicating that the change in exciton life is affected by the electric field.Time-resolved emission spectroscopy also proves the relationship between interface charge and exciton quenching.OLED efficiency roll-off is positively correlated with interface charge storage,especially under OLED unbalanced charge injection.Finally,the discharge of the internal charge of the device was tested by the method of transient electroluminescence under the off state.After the forward and reverse turn-off voltages,luminescence spikes appeared,corresponding to the electron diffusion and the release of trap carriers,respectively.The luminescence spikes under different reverse voltages were compared separately.The luminescence spikes increased with the increase of the reverse voltage.The reason for this change is that there are three types of stored charges at different positions inside the insulating layer:The interface storage charges,charges at the shallow trap,and charge at the deep trap.The charge at the shallow trap are easily released at a relatively small reverse voltage,while the charges at the deep trap can only be released at a relatively high reverse voltage.