| Organic light-emitting diode(OLED)is a new type of lighting and display device,which has the advantages of self-illumination,low driving voltage,fast response,rich color variations,low power consumption and flexibility.After more than 30 years of efforts made by many scientific researchers and the investment of major manufacturers,OLED has been widely used in the field of display and lighting.Unlike traditional fluorescent and phosphorescent materials,thermally activated delayed fluorescence(TADF)materials can capture singlet(S1)and triplet(T1)excitons via an efficient reverse intersystem crossing(RISC)process to achieve theoretical 100%exciton utilization.With ingenious molecular design and device preparation,the external quantum efficiency(EQE)of blue and green TADF-OLEDs has increased to approximately 40%,while there are merely a few reports of red and deep red TADF with EQEs exceeding 30%,and research in the near-infrared field is even less.In recent years,red and near-infrared OLEDs have shown their potentials in photoelectric sensor,night vision display,biomedical imaging,optical communication and many other fields,and the development of high-performance red TADF is also imminent.In this thesis,a series of high-performance red TADF organic light-emitting devices were constructed by modulating the molecular length-diameter ratio,using multiple acceptors,and using sensitized and non-doped structures in the device.Increasing the aspect ratio of the molecule can enhance the horizontal orientation of the emitting dipole,thus increasing the light intensity of the device.In the first work,we designed the TADF molecule PH-TPA with an aspect ratio of 1.93 by introducing mxylene on the molecular skeleton with high luminous efficiency,and its horizontal dipole orientation in a 3 wt%doped film exceeded 80%.In the electroluminescent device,it exhibited an external quantum efficiency of 33.0%under the red emission peak of 624 nm,which realized the efficiency breakthrough of red TADF molecules,and proved that the regulation of molecular aspect ratio is a simple and effective strategy in the design of TADF molecules.Subsequently,by introducing a TADF sensitizer into the device,the EQE of the red device at high luminance was more than doubled.To narrow the band gap and obtain efficient red molecules,people tend to use strong donors and strong acceptor units,which is contrary to the design requirements of TADF rapid RISC process,so in the second work we designed an efficient red TADF molecule DT-TPA by using multiple acceptors.The simultaneous addition of cyanogen and dipyridinylphenazine units as acceptors not only enhances the overall acceptor strength,but also makes DT-TPA featured with rigid skeleton,so that DT-TPA has a horizontal dipole orientation of up to 78.0%.Based on the DT-TPA,a red-light device with the highest external quantum efficiency of 34.3%was fabricated,which represents one of the best efficiencies of reported red-TADFs.By increasing the doping concentration,the emission wavelength can be redshifted to the deep red region.In addition,the efficiency roll-off of the device was improved by adding a TADF sensitizer with high flip Forster energy transfer rate.This work provides an effective method for designing red TADF molecules,explores the feasibility of multi-donor molecular design strategy,and points out a device fabrication idea to improve the stability of red-light devices.To achieve TADF emission in the near-infrared region,we attempted to increase the doping concentration of guests in the EML to 100 wt%,as described in Chapter 3.The NIR TADF emitter AQTC-DTPA exhibited its electroluminescent properties of emission above 800 nm and achieved an emission wavelength of 910 nm and an external quantum efficiency of 0.22%in non-doped device with the thickness of 40 nm,which was almost the maximum emission peak of a TADF molecule.The preparation of DT-TPA-based nondoped devices with a similar device structure also redshifted the luminescence peak to more than 800 nm with a maximum external quantum efficiency of more than 1%.As the thickness of the light-emitting layer increased,the emission spectrum of the non-doped device also slightly redshifted.This work fills the gap of TADF in the near-infrared field and provides an idea for the preparation of non-doped devices. |
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