Studies On The Magnetic Field Effects In Exciplex-based Organic Light Emitting Diodes

Organic light emitting diodes(OLEDs)have shown great application prospect in solid-state lighting and display recently.More and more organic materials are designed for higher efficient and performance OLEDs.People's research on OLED has also converted from improving the efficiency of the devices into studying the internal mechanism of them.More experimental tools and measurement methods are applied to investigate the luminescence mechanism of OLEDs.Organic magnetic field effects(OMFEs)are related to the spin dynamic process of electrons and have attracted more interests as a good tool to study the internal physic mechanism of OLEDs.The working OLEDs are placed into a small external magnetic field(about a few hundred mT)and the following current and luminescence intensity will change with the strength of external magnetic field revealing different lineshapes.According to these lineshapes,the correlated physical models are proposed to analyze the working mechanism of devices.After a couple of years development,OMFEs have not only grown into a good tool to detect the luminescence mechanism of OLEDs,but also plays an important role in guiding the improvement of OLED performances and the preparation of new spin-related devices.Although some theory models of OMFEs have been raised for the observed experimental results,such as electron-hole pair model,bipolaron model and exciton-exciton or exciton-carrier interaction models,these models still can't explain all the experimental phenomena.There exist competitions and interrelationships among them,bringing the opportunities and challenges in studying the OMFEs.Based on the above results,we choose the popular exciplex-based systems as the research object.Firstly,we systematically analyze the origin of OMFEs and the relations among them,confirming the suitable models for exciplex-based OLEDs.Then,the dependence of device performances on OMFEs are investigated in detail and the correlation between them are got.Finally,the enhanced performances of exciplex-based OLEDs are fabricated by simple and reasonable method on the basis of previous experimental results.Detailed works of this thesis are showing bellow:1.We study the OMFEs of the m-MTDATA:3TPYMB exciplex-based systems.The device shows large OMFEs with MCmax=16%and MELmax=32%under the constant voltage test,and MELmax=12%under constant current.Furthermore,the single-carrier devices of this system are fabricated to investigate the origin of OMFEs and the relations between the MC and MEL responses.It is found that the MC of single-carrier devices mostly come from the dissociation process under the photo excitation and obvious MC can only exist in hole-only device.By adjusting the device structures,it is found that the origin of MC in exciplex-based single carrier device is closely relate to the generation of charge transfer states and the choices of hole or electron transporting materials.Comparing the MC lineshapes of single-carrier device with bipolar device,the results show that there exist different contributions of excitons on current under the photo and electrical excitations.The initial states are excited donor D*under photo excitations,which is important for the photo-induced MC.However,the first formed polaron pair(PP)states under the electrical excitation will also influenced by external magnetic field,bringing the broaden lineshapes of MC and MEL.By modifying the injected current or illumination intensity,the concentrations of carrier and excitons in single-carrier devices can be controlled well.It is noted that there exist serious triplet-polaron quenching(TPA)in exciplex-based OLEDs according to the OMFEs results.And the polaron plays an important role in the TPA process.2.We use the OMFEs as the tool to study the influences of exciplex-based OLEDs.The distinction between the triplet states of donor or acceptor materials and their formed exciplex states will influence the performances of devices.The exciplex excitons will transfer their energy to the triplet states of the donors or acceptors when their triplet energy levels are lower,resulting in the energy loss.This energy loss will accelerate the exciton quenching through triplet-triplet quenching(TTA).The transient MEL method confirms that this energy loss will affect the rising and turning off periods of the working device,which is different with the devices of well restrained excitons.And then,the MEL are used as the tool to study the relationships between external quantum efficiency(EQE)and the amplitude of MEL.It shows that there exist positive correlations between them,which is helpful for us to obtain enhanced performance of exciplex-based OLEDs.During the researches,we also find a special "W" lineshape of MEL in the devices.After excluding the influences of device structures,it is demonstrated that the decreasing trend of low-field region is induced by the hole-transporting material of TAPC.We study the pure TAPC films and devices by OMFEs and observe that the formation of electromer in TAPC device decides the negative MC and MEL responses,affecting the MEL of exciplex-based OLEDs.3.We propose an efficient method to enhance the performances of exciplex-based OLEDs according to the experimental results in section 2.A higher energy material defined as spacer is co-doped into the donor:acceptor blending layers to enlarge the distance between electrons and holes in spatial,decreasing the singlet-triplet energy gap(?EST)and promoting the reverse intersystem crossing(RISC)processes.As a result,more triplet excitons will be utilized effectively,improving the performances of exciplex-based OLEDs.The EQE of the prepared TCTA:PO-T2T exciplx-based OLEDs is enhanced by approximately 105%,from 3.9%to 8.0%,and maintains 7.5%at a luminance of 1000 cd/m2.MEL is used to verify the intrinsic luminescence process of this co-doped system and it is noted that the long-range coupled electron-hole pair mainly contribute to the luminescence.At last,based on this co-doped system,high performances of phosphorescence OLEDs are achieved,indicating the advantage of this method in exciton regulation and utilization.