Intermolecular Interaction And Exciton Regulation For Efficient Thermally Activated Delayed Fluorescent Organic Light-Emitting Diodes

In recent years,organic light-emitting diodes（OLEDs）are continuously flourishing the fields of lighting and small and medium-sized display thanks to the unremitting efforts by scientists and related industry engineers.Currently,the commercialized conventional fluorescence（first-generation）materials and heavy-metal-containing phosphorescence（second-generation）materials face low-efficiency and high-cost issues,respectively.By comparison,the third-generation material thermally activated delayed fluorescence（TADF）has the advantage of high efficiency and low cost,presenting great potential for further commercialization.Although it has developed rapidly under the extensive attentions and in-depth explorations of scientists,different radiation mechanism of the new generation emitting materials means new challenges and tremendous research space in molecular design and device optimization.In addition,the progressions of computational simulation technologies have played an important auxiliary role in the research from molecular scale to device scale.Herein,molecular dynamics,3D kinetic Monte Carlo（3D KMC）electrical simulation,optical simulation and other simulation methods are sufficiently and systematically employed to explore the material properties,carrier transport,exciton dynamics and light extraction of TADF based OLED devices.We provide novel and effective strategies for high-efficiency and low-cost purely organic monochromatic and white doped/doping-free OLEDs.Three research works were carried out as follows:1.We proposed a novel strategy to improve light extraction of devices by synergetic horizontal dipole orientation induction of TADF host-guest system,achieving highly efficient white OLEDs.A TADF material Dspiro S-TRZ with high horizontal dipole ratio(HDR,Θ_//)was utilized as the blue emitter as well as the host for an orange-red TADF emitter TPA-AQ. Molecular dynamics simulations indicatesthe parallel alignment between host and guest molecules,which shows the orientation induction by host-guest interactions.When doping TPA-AQ into the horizontally orientated Dspiro S-TRZ host,a synergetic horizontal dipole orientation induction on the transition dipole moment of TPA-AQ was achieved,resulting in high HDR(Θ_//=96%)of the orange-red emitter.Owing to the simultaneous efficient triplet exciton utilization of TADF mechanism and highly horizontal orientaion in the binary system with white light emission,a maximum external quantum efficiency(EQE_max)of 29.3% was achieved for single emission layer（EML）white OLED,with an excellent white light out-coupling efficiency of 34%.In addition,the carrier recombination in the EML was further regulated by inserting an exciton regulation emission layer,which can relieve the trapping effect of the orange-red emitter and guarantee stable Langevin recombination and energy transfer processes for improved electroluminescence spectral stability.Finally,an EQE_max of 31.2%was ultimately achieved for the double-EML white OLED,which is one of the highest efficiency of all TADF white OLEDs.2.Based on the effect of intermolecular interaction on TADF molecules arrangement,a new device optimization strategy of ultrathin quasi-host-guest structure composing of separated host layer and ultrathin emission layer is proposed,achieving highly efficient doping-free OLEDs equivalent to the traditional doped ones.Firstly,we selected representative green TADF molecules p ACRS and p ACRSO with the difference of the valence states of sulfur atoms（-S-and-SO2-）.The sp³ hybridized sulfur atom in p ACRS can weaken the intermolecular interactions and suppressed aggregation-caused quenching（ACQ）,thus showing better device performance in non-doped devices.Subsequently,the two molecules were then used to explore a UEML structure,where the varied intermolecular interactions of emitters result in different extend of aggregations on the interface during the deposition process.A quasi-host-guest system with suppressed ACQ effect formed by simple UEML thickness regulation,resulting in comparable ultrathin OLEDs efficiency to that of the doped ones.Noting that the suppressed trap effect is the key factor to achieve high efficiency of ultrathin OLEDs via 3D KMC simulation.Moreover,weak intermolecular interactions are more conducive to achieving excellent efficiency roll-off suppression.We further prepared high-efficiency red ultrathin devices by the introduction of interfacial exciplex, which verify the feasibility of the above strategy and meanwhile propose a feasible design direction for doping-free OLEDs with high efficiency and low turn-on voltage in the next step.3.Based on the understanding of previous work,a novel pn heterojunction strategy was proposed to achieve color-tunable OLEDs with low efficiency roll-off.In addition to conventional TADF molecules,intermolecular charge transfer between donor and acceptor（D and A）materials can also present TADF characteristic.Traditional p-type hole transport materials 1,1-Bis[4-[N,N-di（p-tolyl）amino]phenyl]-cyclohexane（TAPC）and 4,4′,4′′-tris[phenyl（m-tolyl）amino]triphenylamine（m-MTDATA）act as donors,while 9-（5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-yl）-9H-carbazole（BOCz）and 3-（5,9-dioxa-13b-boranaphtho[3,2,1-d,e]anthracen-7-yl）-9-phenyl-9H-carbazole（BOCzph）is synthesized to serve as acceptor material.Firstly,molecular dynamics simulation and photophysical characterizations were conducted to investigate the intermolecular distances（intermolecular interactions）for different D-A systems and radiation transition process for the formed exciplex systems.Owning to the minimum energy effect of exciplex formation and interfacial dilution effect,ultrathin p-type interlayer is then introduced to prepare the red and white pn heterojunction devices with EQE_max of 15.5%and 16.1%,respectively,which is the highest efficiency among the OLEDs based on interfacial exciplex at present.In addition,the negligible trap effect and efficient exciton utilization of exciplex lead to low efficiency roll-off.This simple ultrathin heterojunction strategy provides a new way to meet the efficiency and cost requirements for further commercialization of OLEDs.