The Triplet-Triplet Annihilation In Organic Planar Heterojunction Devices

Organic light-emitting diodes(OLEDs)have great advantages in solid-state lighting and flat-panel displays because of their characteristics,such as,self-luminous,high color gamut,and great mechanical flexibility.Meanwhile,OLEDs have also attracted more and more interests from researchers.At present,many manufacturers are also involved in the development and application of OLED display.After a long period of development,the performance of OLEDs has been greatly improved,and some OLED productions have already emerged in the market.However,there are some challenges for large-scale practical of OLEDs,especially the short lifetime and low electroluminescence(EL)efficiency of the device.The number of singlet states in the OLEDs determines the fluorescence EL efficiency.Therefore,in order to improve EL efficiency of OLEDs,it is necessary to increase the number of singlet states.And the transition from triplet states to singlet states is a viable approach to increase singlet states.Both the reverse inter-system crossing(RISC)process and triple-triplet annihilation(TTA)process in OLEDs can achieve this transition.Adachi et al.have used the RISC process to obtain a high external quantum efficiency and have explored the mechanism of the RISC process.However,the occurrence of the TTA process usually needs a large triplet concentration,because it is an interaction between two excited states.We have observed the TTA process in exciton-based OLEDs at low temperature and large injected current.In this work,we have investigated the TTA process at room temperature in exciton-based OLEDs,and have realized the TTA process in exciplex-based OLEDs.Concurrently,we have investigated the intrinsic mechanism of the TTA process.In fact,the microscopic processes in OLEDs are very complex and they are difficult to observe.Magneto-electroluminescence(MEL),developed in recent years,is very sensitive to these spin-related microscopic processes and can be used as an effective tool to detect these processes.In this paper,two kinds of organic planar heterojunction OLEDs were prepared.By the selection of organic materials and the design of the device structure,the room TTA process in exciton-based OLEDs and the TTA process in exciplex-based devices were realized.By analyzing the MEL curve,device structure,and its EL spectra,the occurrence condition and internal mechanism of the TTA process were explored.This work is expected to understand the intrinsic mechanisms of the TTA process,which will provide some suggestions for improving the EL efficiency of exciton-or exciplex-based OLEDs.The main chapters are listed as follows:In the first chapter,the historical origins and developments of OLEDs and their applications were introduced.And also the basic EL process of OLEDs was introduced.Then the reasons why OLEDs have a low EL efficiency and the methods to solve this problem were introduced.Finally,the principle for MEL that can detect the microscopic process was introduced as well.In the second chapter,the preparation and measurement processes of OLEDs and the required instruments were introduced.In the third chapter,the room TTA and abnormal temperature dependent behaviors of MEL were observed in planar heterojunction OLEDs contained mCP and CBP layer.The structure is ITO/PEDOT:PSS/NPB(30 nm)/mCP(or CBP)(40 nm)/TPBi(50 nm)/LiF(1 nm)/Al(120 nm).The value of MEL at low fields gradually increases with decreasing temperature.Meanwhile,the high-field component of MEL shows high-field decrease(HFD)at room temperature but this HFD gradually vanishes with decreasing temperature.These phenomena indicate that the intersystem crossing(ISC)process becomes stronger with decreasing temperature,and the TTA process occurs at room temperature but ceases at low temperature.Analyses of the EL spectra of these devices and their temperature dependence indicate that both exciton and exciplex are present in the device.And as the temperature decreases,the number of exciton decreases while the number of exciplex increases.Thus,the increased population of exciplex states at low temperature may cause the abnormal behavior of intersystem crossing.Additionally,long lifetime of the exciton within mCP or CBP layer may allow TTA process occur at room temperature,while the reduced population of excitons at low temperature may account for the disappearance of TTA process.Thus,we observed the opposite temperature dependence of the MEL from these devices.In this work,the TTA process in exciton-based OLEDs was realized at room temperature,and the intrinsic mechanisms of the ISC and TTA process among the excited states(exciton and exciplex)were explored.In the fourth chapter,the TTA process in the planar heterojunction exciplex-based OLEDs was realized.The structure is ITO/TTP(80 nm)/PPT(80 nm)/LiF(1 nm)/Al(120 nm).The TTA process occurs with difficulty in exciplex-based OLEDs because it is an interaction among several neighboring donor and acceptor molecules.However,the TTA process was realized in our planar heterojunction exciplex-based OLEDs by using a thin recombination zone to enhance the interfacial density of the triplet states.The TTA process,which is characterized by a HFD in the MEL curve,occurs at approximately 150 K and becomes stronger with decreasing temperature.At a given temperature,the higher the injected current is,the stronger HFD is observed.This is consistent with the change of the TTA process at different temperatures and injection currents.The analysis of the device structure indicates that there are large LUMO-LUMO and HOMO-HOMO energy barrier(1.4 and 1.9 eV)in the double-layer organic planar heterojunction devices.The holes and electrons are confined within the donor-and acceptor-layer and they produce a large concentration of exciplex states in the vicinity of the hetero-interface.Thus the TTA process occurs in the exciplex-based OLEDs.Additionally,we found that TTA process could even happen at room temperature.And the mechanism of room TTA process in the exciplex-based OLEDs was discussed.