High-Efficiency Organic Light-Emitting Diodes Based On Multi-Process Energy Transfer

Organic light-emitting diodes(OLEDs)have advantages of easy-adjusting photoelectric properties,light weight,thin thickness,a low driving voltage and wide viewing angle.OLEDs have great potential in solid-state lighting and display areas.Especially,OLEDs can make flexible displays and lighting sources.In recent years.the industrialization of OLEDs has been gradually realized in areas of smart phones and televisions.However,there are still several challenges existing in the development of OLEDs,mainly including efficiency enhancement of conventional fluorescent organic light-emitting diodes(FOLEDs).fabrications of high-efficiency and stable blue and deep-red OLEDs as well as reduction of fabrication costs.In this research,we fabricated high-efficiency FOLEDs and deep-red phosphorescent organic light-emitting diodes(PhOLEDs)based on multi-process energy transfer.We designed and synthesized novel organic materials as hosts for blue PhOLEDs,enabling high-performance blue PhOLEDs through multi-process energy transfer.Our research processes and results are shown as follows1.Due to spin-statistics.only 25%singlet ones of excitons are luminescent,leading to a limited external quantum efficiency(EQE)of 5%while ignoring additional light out-coupling.Thermally activated delayed fluorescence(TADF)process can help harvest triplet excitons to realize radiative decay by reverse intersystem crossing(RISC)process.Accordingly,we chose exciplex as a co-host and TADF molecules as sensitizers.The structure of the novel emitting layer proposed by us was "an exciplex-forming co-host(consisting of electron-giving and electron-drawing fluorescent materials):assisted energy transfer TADF sensitizers:conventional fluorescent molecules".While using the above emitting layer structure for the device,there are mainly three Forster resonance energy transfer(FRET)processes and two RISC processes in the emission mechanism of the excited-state process helping harvest triplet excitons.Based on the conventional fluorescent emitter DCJTB,we fabricated devices using the novel emitting layer structure.The maximum EQE of the device could be up to 12.9%,which was more than twice the efficiency of conventional FOLEDs.Besides,we also carried out corresponding theoretical analysis and calculations to understand the energy transfer processes in the device and explain the obtained experimental results.2.The conventional phosphor PtOEP is an ideal deep-red phosphorescent emitter.The peak wavelength of PtOEP's luminescent spectrum is 650 nm and the full width at half maximum of the spectrum is only 38 nm.Hence,we selected PtOEP as the emitter to carry out our research for high-efficiency deep-red PhOLEDs,The triplet exciton lifetime of PtOEP material is quite long,which can be up to 100?200 ?s.Therefore,deep-red PhOLEDs with the common host-guest emitting layer system usually behave poorly because of high triplet excitons concentration on PtOEP.We proposed a novel emitting layer structure for PtOEP-based PhOLEDs,which was an exciplex-forming co-host:TADF sensitizers:PtOEP dopant.While using the above emitting layer structure for PhOLEDs,more triplet excitons can be converted to singlet excitons in PtOEP emitter by RISC and FRET processes.As a result.the triplet excitons concentration of PtOEP emitter can be reduced,leading to efficiency enhancement of PhOLEDs.We fabricated devices using emitting layer of CBP:B4PyMPM:4CzIPN:PtOEP.The maximum EQE of the device was close to 22%.We also found the device lifetime could be extended by using the novel design emitting layer structure.3.The first step to fabricate high-performance blue PhOLEDs is synthesizing ideal organic materials as hosts.Host materials for blue phosphors should have high triplet energy level and good thermal stabilities.We designed and synthesized novel tetraarylsilane-based host materials TCzSi and DCzMzSi for blue PhOLEDs.TCzSi and DCzMzSi had good thermal stabilities and high triplet energy level(ET).ET of TCzSi and DCzMzSi were 2.86 eV and 2.97 eV,respectively.We chose traditional phosphorescent material FIrpic as the guest and TCzSi or DCzMzSi as the host material.Accordingly,we fabricated the corresponding PhOLEDs.The PhOLEDs based on TCzSi host material exhibited the highest EQE of 13.32%.