Design And Synthesis Of High-Performance Thermally Activated Delayed Fluorescence Emitters And Their Applications In Organic Light-Emitting Diodes

This thesis focused on the current frontier of organic light-emitting diodes(OLED)based on the third-generation thermal activated delayed fluorescence(TADF)mechanism.Aiming at the respective critical scientific issues of TADF emitters which are concentration sensitivity and color purity,the main achievements are summarized as follows:(1)TADF emitters generally show high concentration sensitivity because their long-lived triplet excitons easily suffer serious quenching.Therefore,precisely controlling doping weights is required to achieve optimized device performances.However,the most common method is introducing extra host matrix which not only complicates the fabrication of OLEDs,but also significantly increases the manufacturing cost.To address this issue,a facile molecular design strategy of introducing additional hindrance groups was proposed to develop TADF emitters with low concentration sensitivity.Based on a previously reported TADF emitter DPS-PXZ,a novel TADF emitter SPFS-PXZ was designed and synthesized.Introduction of the fluorene group can not only enhance the molecular rigidity and effectively suppress the non-radiative molecular relaxations,but also significantly inhibit the potential intermolecular π-π interactions to suppress concentration quenching.As a result,comparing the DPS-PXZ-based device,the SPFS-PXZ-based OLED realized a superior device performance with a maximum external quantum efficiency(EQE)of 22.9%.Moreover,by gradually increasing the doping concentration(10 wt%to 50 wt%),the devices exhibit similar performances,indicating that SPFS-PXZ has low concentration sensitivity.All these results suggest that introducing rigid fluorene groups is a wise method to exploit novel high-performance TADF emitters with low concentration sensitivity.(2)The quenching of long-lived triplet excitons greatly delays the development of TADF emitters for efficient non-doped OLEDs.To address this issue,a systematic molecular design strategy that merges several reported molecular design strategies is proposed to exploit novel TADF emitters for efficient non-doped OLEDs.Two novel TADF emitters,Trz-Py-NCS and Trz-Py-SAC,were thus designed and synthesized.Trz-Py-NCS and Trz-Py-SAC process the following advantages:(1)their large steric hindrance groups can effectively isolate the adjacent triplet active cores and reduce the concentration sensitivity;(2)both emitters show dual stable conformations in ground states,which are the quasi-axial(QA)forms with wide bandgaps and the quasiequatorial(QE)forms with TADF characteristic.QA forms with large populations can effectively disperse the minority QE forms,thus resulting in a "self-doping" system which can further reduce the concentration sensitivity of the TADF emitters;(3)long linear molecular structures favor the horizontal orientation and benefit the out-coupling efficiency of OLEDs.Therefore,Trz-Py-NCS and Trz-Py-SAC amorphous neat films both realize nearly 100%PLQYs and high horizontal dipole orientations.Trz-Py-NCS and Trz-Py-SAC-based non-doped OLEDs achieve maximum EQEs of 30.8%and 30.3%,respectively,which set new records for the non-doped OLEDs.More importantly,both devices exhibit extremely small efficiency roll-offs at ultra-high luminance of 10000 cd m-2.All the results further confirm that such systematic molecular design strategy is a feasible method to exploit high-performance non-doped TADF emitters.(3)TADF emitters based on the multiple resonance(MR)effect have become a hot research topic since they can help to realize OLEDs with high efficiency and high color purity simultaneously.However,most of the currently reported MR-TADF emitters are in the blue region and show inefficient RISC process.To address this issue,we have designed and synthesized an organoboron-based emitter DBTN-2 with a highly distorted fused π-conjugated molecular structure.Such a highly distorted conjugated extended molecular structure can not only reduce the recombination energy between the excited states and the ground state,and achieve a full width at half-maximum(FWHM)of only ca.20 nm,but also induce different excitation characters of its singlet and triplet states to enhance the spin-orbit couplings.Furthermore,introducing multiple carbazole moieties favor the charge-resonance-type excitation feature of the T1 and T2 states,reducing the T1-T2 energy gap and thus opening the T2→S1 up-conversion channel.Eventually,it realizes a fast reverse intersystem crossing rate of 1.7×105 s-1.As a result,the DBTN-2-based OLED device achieves an ultra-pure green emission with high efficiency(maximum EQE of 35.2%)and high color purity(FWHM of 29 nm).All the results indicate that DBTN-2 has the potential to improve the performance of currently commercial OLED displays and provide theoretical guidance for the further development of MR-TADF emitters with high efficiencies and excellent color purity.(4)Currently,majority of the ever-reported MR-TADF emitters are in the blue region and few attempts in the longer wavelength region.On the other hand,although commonly used molecular design strategies by extending the molecular conjugation can successfully red-shift the emission wavelengths of the MR-TADF materials,but in some cases the color purity would decrease simultaneously.Therefore,it is urgent to exploit novel efficient molecular design strategies.To address this issue,three novel TADF emitters,mBDPA-TOAT,pBDPA-TOAT and DMAC-TOAT with D3-A-typed molecular structures were designed and synthesized by employing a MR-typed TOAT as the A fragment.Compared to the parent TOAT,emissions of all three emitters are successfully red-shifted to the red region.Nevertheless,derived from different D fragments,they exhibit different MR and intermolecular charge transfer(ICT)contributions in their excited states.Although the near-orthogonal molecular structure of DMAC-TOAT favors the smallest ΔEST and most efficient RISC process,the DMAC-TOAT-based device displays low efficiency and unsatisfied color purity due to the dominated ICT nature of the lowest excited states.While the mBDPA-TOAT and pBDPA-TOAT-based OLEDs achieve high efficiencies(maximum EQEs of 17.3%and 11.3%,respectively)and satisfied color purities(FWHM of 45 and 62 nm,respectively),derived from the dominated MR characteristics of their lowest excited states.Moreover,CIE coordinates of the pBDPA-TOAT-based device are(0.66,0.34),very close to the NTSC red standard CIE coordinates(0.67,0.33).All these results demonstrate that the characteristics of the excited states determine the device performance when using MR backbones to construct D-A-typed TADF emitters.This work provides theoretical guidelines for the development of novel TADF emitters with high efficiency and high color purity in the long wavelength region.