Radical Sensitized Fluorescent Organic Electroluminescent Devices

In the field of contemporary display and lighting,organic light-emitting device（OLED）has become a new generation focus with many advantages such as self-luminous,flexible,low energy consumption,full perspective,high brightness and simple production.With OLED technology becomes more mature,the technology begins to realize industrial production.New products with OLED technology are emerging,ranging from 70-inch TVS to smart wristband displays that are less than an inch in size.The OLED industry is still in its infancy,although the flexible nature of OLED makes more conceptual solutions and products emerge in the display field,however,there are still some problems to be solved in the research and development process,which make the product price remain high.Therefore,how to improve the efficiency and reduce the cost of OLED materials has become a breakthrough goal of relevant researchers.Conventional fluorescent molecules generate 25%singlet excited states and 75%triplet excited states when excited into excited states,a large number of excited excitons are wasted due to the transition confinement between the triplet excited state and the ground state.Therefore,the maximum internal quantum efficiency（IQE）of OLED devices made by conventional fluorescent molecules is only 25%.To solve the problem of low molecular luminous efficiency,researchers in related fields began to develop new molecules to improve the luminous efficiency.For closed shell molecules,the excited single and triple states are still 25%and 75%,Therefore,the main ways to improve the luminous efficiency of closed shell molecules are as follows:1).materials that use triplet exciton luminescence directly to improve efficiency,such as phosphorescent materials;2).materials systems that use the conversion of triplet states to single states to improve luminous efficiency,such as thermally activated delayed fluorescence（TADF）,triplet-triplet quenching（TTA）,and hybrid localized charge transfer excited state（HLCT）;3).materials system in which a triplet exciton interacts with a polaron to produce a single-state exciton（TPI）.All these systems can improve the molecular luminous efficiency.Due to the need to use expensive heavy metals as raw materials for the preparation of phosphor molecules and the scarcity of heavy metals in the earth,the phosphor material system still has the disadvantage of being expensive.However,TADF,TTA,HLCT and TPI materials can reduce manufacturing costs through relatively low prices.In addition to using traditional closed shell molecules for OLED research and development.Our group proposed a new OLED light-emitting material system that generates doublet and radiative transitions by stimulating neutral stable radicals in 2015.No matter for traditional fluorescent molecular materials or for TADF,TTA,HLCT materials,their luminescence mechanism is studied around the radiation transition generated by the return of the excited state related to the single and triple states to the ground state,The core of the research is around the use of triplet excitons,but the transition from triplet state to ground state is usually forbidden transition.The organic neutral radical molecule is an open-shell molecule,which has only one unpaired electron in its outermost shell,the ground state is doublet D₀.When the molecule is excited,the paired electrons in the inner layer of this single-electron orbital are excited to jump into the single-electron orbital,where they form a pair.The original orbital becomes a single electron orbital after the transition,and the excited state formed is called doublet D₁.Since the transition orbital corresponding to the excited electrons in the doublet radiation transition when it returns to the ground state is the orbital formed by single electrons and holes,there is no transition prohibition in the transition from the excited state to the ground state,and the upper limit of quantum efficiency in its theory can reach 100%.However,most organic radical molecules are difficult to exist stably at room temperature,while those that can emit light stably at room temperature are even rarer.So far,has been published in the preparation of OLED can be neutral stable free radical which revolve around the trityl radicals（TTM）as the core of the stability of neutral radicals,in addition to our group in recent two benzyl radical（BTM）as the core of free radical molecules to modify and adjust the production of neutral stable radical molecules,The emission range of the neutral stable radical molecules in these two systems is mainly from red light to near infrared.As the emission wavelength in red light to near infrared increases,the external quantum efficiency（EQE）of OLED is low,for conventional fluorescent molecules,the fluorescence quantum efficiency is relatively difficult to achieve high efficiency.Since the excited radiation transition of neutral stable radical molecules is not restricted by the transition confinement,it is possible to have the upper limit of the theoretical quantum efficiency of 100%.So we can take advantage of that,we use stable neutral radical molecules as sensitizers to sensitize traditional fluorescent molecules with low efficiency,so as to improve the radiation transition ratio of traditional fluorescent molecules,to obtain fluorescent OLED with high efficiency.Therefore,the following work was carried out in order to improve the efficiency of traditional fluorescent OLED molecules by studying the energy transfer between the doublet characteristic with the upper limit of100%internal quantum efficiency and the traditional fluorescent molecules:The neutral radical molecule TTM-1Cz,which is stable and luminescent at room temperature,was selected as the energy donor in the doublet sensitization system,The EL emission peak is 668nm and EQE is as high as10.5%inOLEDoftheluminescentstructurewithαHTTM-1Cz:2.5wt%TTM-1Cz.According to the position of its emission peak,we selected traditional fluorescence molecules whose absorption peak overlapped TTM-1Cz emission peak,2,4-bis[4-（N,n-diisobutylamino）-2,6-dihydroxyphenyl]square cyanine（SQ）or 2-(4-（N-butyl-N-phenyl amine）-2,6-dihydroxy phenyl]-4-[（4-（N-butyl-N-phenyl amine）-2,6-dihydroxy phenyl）-2,5-diene-1-sub]-ring-1-3-oxygen generation olefin-1-alcohol sodium（SQ-BP）as an energy acceptor,allowing Forster energy transfer to occur between these molecules.In OLED with SQ as the receptor,when the SQ doping concentration is 1%,EL emission peak is 672nm and EQE is 6.93%;in OLED with SQ-BP as the receptor,when the SQ-BP doping concentration is 2%,EL emission peak is 680nm and EQE is 8.12%.This sensitization method greatly improves the performance of OLED in the traditional fluorescent material system,and both of the two sensitized structures have achieved high EQE in the deep red region non-phosphor material system.The method of neutral and stable radical material with the characteristic of doublet radiation transition doped in the traditional fluorescent molecularmaterial becomes a new method which can help the traditional fluorescent molecular material OLED to improve the device performance.