Mechanism And Design Of Highly Efficient Thermally Activated Delayed Fluorescent Device

Since organic light emitting diodes(OLEDs) were applied in panel displays and solid state lightings, various organic electroluminescence(EL) devices and materials have been investigated, especially for industrial development of fluorescent and phosphorescent materials based OLEDs. It is well known that fluorescence emissions are generated from singlet exciton while phosphorescence generated from triplet exciton recombination that provide a theoretical maximal internal quantum efficiency(IQE) of 100%. For example, the devices with iridium(Ir)-complex, tris(2-phenylpyridinato)iridium(III)(Ir(ppy)3), exhibits very high efficiency due to its heavy atom effect. Despite they offer high IQE, phosphorescent OLEDs still present shortages like serious efficiency roll-off at high current density owing to a long lifetime of triplet excited states and high cost due to adoption of noble metal-complexes. Although fluorescent OLEDs have high reliability only a limited IQE of 25% from singlet exciton can be reached.In view of respective disadvantages in both fluorescence and phosphorescence OLEDs, great efforts have been devoted to harvest triplet excited states via reverse intersystem crossing(RISC) from triplet to singlet excited states, where relatively small energy gap(△E(S-T)) between the singlet and triplet excited states is required. Usually three kinds of typical excited states involve small △E(S-T): D:A exciplex, intramolecular charge transfer(ICT) states and dimer. The reported exciplex can exhibit either positive or negative △E(S-T) value, the ICT states usually behave negative and zero △E(S-T) value, while the calculated dimers mostly possess smaller positive △E(S-T) value. Adachi group has firstly demonstrated a maximal external quantum efficiency(EQE) of larger than 5% in exciplex OLED based on thermally activated delayed fluorescence(TADF) mechanism. Thus selection and design of materials with smaller △E(S-T) becomes a general route for achieving high efficient delayed fluorescence EL devices based on RISC process. RISC mechanism based on TADF is considered that triplet energy must be up converted to singlet level by rising temperature because the triplet excited state is generally lies below the singlet one. Up till now, the EQE of devices based on red, blue and green ICT materials have been reached 15.8%, 21.2% and 29.6%, respectively. the EQE of devices based on blue and green exciplex have been reached 8.0% and 15.4%, respectively. We aimed to reveal the micro mechanism of TADF system and promote the development of TADF based OLED. During the doctoral study, the followings products have been harvested:Firstly, We demonstrated RISC mechanism based high efficient exciplex delayed fluorescence OLEDs in which 4,4’,4’’-tris [3-methylphenyl(phenyl) amino] triphenylamine(m-MTDATA) and 4,7-diphenyl-1, 10-phenanthroline(Bphen) were chosen as donor(D) and acceptor(A), respectively. Our m-MTDATA:Bphen exciplex EL offers an EQE of 7.79% which increases by 3.2 and 1.5 times in comparing that reported by references at 10 m A/cm2, respectively. The excellent results would be attributed to very easily RISC process because the energy difference between the singlet and triplet excited states is almost around zero, which is based on the rate analysis at different temperatures and also on time-resolved-spectra analysis data, respectively. Besides, the study on transient EL indicates that unbalanced charge in the exciplex devices is responsible for lower EQE and its effciency roll-off. When placed in 100 m T magnetic field, permanent positive magneto-EL and magneto-conductivity of our devices exclude triplet-triplet annihilation process in the exciplex EL which only arise from delayed fluorescence with RISC mechanism.secondly, EL and PL characters of a series of blue exciplexes from combinations between three electron acceptors and one electron donorwere studied. The emissions of three exciplexes, ranging from sky blue to purple blue, though with quiet different acceptor triplet levels, possess the same thermally activated delayed fluorescence character. Temperature dependence of the PL decay measurement suggests that the long-lived exciplex would be quenched heavily by phonon and the monomer triplet quenching effect was not the origin of low efficiency. Detailed analysis indicates that the RISC rate could be more affected by the spatial structure of D/A materials and substituent group in the D or A molecules. Trap effect and energy transfer from exciplex to the yellow dopant were also studied.Thirdly, By changing the coordinating metal in the accepting materials, we systematically studied the heavy metal effect and paramagnetic effect on the performance of the exciplex with m-MTDATA as donor. The environmental heavy metal of Lu could enhance the photoluminescence and electroluminescence intensity of exciplex two times by decreasing its decay time. This shortened decay time could enhance the exciplex intensity by preventing it from phonon quenching effect under high temperature(300 K). As a result, high external efficiency of 3% of pure red exciplex based OLED was achieved. While the paramagnetic metal Gd might decrease the quantum yields of both singlet and triplet exciplexes.Forthly, a new mechanism is developed to realize efficient white OLED with extremely simple structure by redistributing the energy of triplet exciplex to both singlet exciplex and the orange dopant. The micro processes of energy transfer could be directly examined by detailed photoluminescence decay measurement and time resolved photoluminescence analysis. This strategy overcomes the low RISC efficiency of blue exciplex and complicated device structure of traditional WOLED, enables us to achieve efficient hybrid WOLEDs. Based on this mechanism, we successfully constructed both exciplex-fluorescence and exciplex-phosphorescence hybrid white OLEDs with remarkable efficiencies.Finally, New exciplexes formed between a typical intramolecular charge transfer(ICT) material(bis[4-(9,9-dimethyl-9, 10-dihydroacridine)phenyl]sulfone(DMAC-DPS)) and a series of electron donor and acceptors in donor:acceptor(D:A) system have been systematically demonstrated. It was found that such ICT materials could form exciplex with both standalone electron donor and acceptor materials with itself as acceptor and donor components, which is based on the presence of both donor and acceptor species in the ICT material. The emission spectra of exciplex OLEDs based on ICT materials could be regularly tuned ranging from blue to yellow color by changing energy level alignment between ICT and standalone donor/acceptor materials. Among these exciplexes, DMAC-DPS:PO-T2 T combination offered the highest exciplex EL performance, with its peak external quantum efficiency, luminance and current efficiency of 9.07%, 35000 cd/m2 and 30 cd/A, respectively. On the other hand, we also found that the exciplex efficiency was insensitive with the weight ratio between ICT material and acceptor, which means doping of ICT material into the acceptor. Our finding extend the usage and selection scope of the TADF material.