Dicyano-substituted Aromatic Conjugated Groups As Electron Acceptors For The Construction Of Thermally Activated Delayed Fluorescence Material Systems

Organic light emitting diodes?OLEDs?based on noble metal-free thermally activated delayed fluorescence?TADF?materials are raising increasing interest for their potential to achieve 100% internal quantum efficiencies through efficient up-conversion of non-radiative triplets to radiative singlets.After a rapid development of TADF materials and devices in recent years,the external quantum efficiency?EQE?of TADF OLEDs especially for blue or green device can reach a value of over 20% and in some cases over 30%,which is comparable to the best values reported of phosphorescent OLEDs.Without using expensive noble metals,such as iridium?Ir?and platinum?Pt?,pure organic TADF materials are the cheaper candidates for OLED applications,and now are regarded as the next generation OLED materials.But several challenges remain to be overcome for practical applications.One is the dearth of high performance red TADF emitters.The performance of red TADF devices is still far from satisfactory in terms of the EQE values.The other is that most TADF devices suffer from serious efficiency roll-off at high brightness or current density due to the triplet-triplet or singlet-triplet exciton annihilation.Therefore,new molecular design strategies and detailed theory to construct materials with TADF feature should be established.These will contribute to achieving red TADF materials with high performance and stability.Also,the performance of TADF OLEDs needs to be improved for practical display applications.New device configurations and fabrication technologies developed with a profound understanding of the physical process will help in promoting the overall device performance for advanced optoelectronic applications.In this thesis,we carried out the following research using dicyano substituted aromatic conjugated units as electron acceptors to build novel TADF material systems:1.In chapter II,we designed a near-infrared TADF molecule,featuring a D-?-A-?-D configuration with 2,3-dicyanopyrazino phenanthrene?DCPP?as the electron acceptor,diphenylamine?DPA?as the electron donor,and phenyl?Ph?rings as ?-bridges.The theoretical calculations demonstrated that the molecular orbital distribution possessed an effective HOMO/LUMO separation which was beneficial to obtain a small singlet-triplet splitting??EST?0.20 e V?.And partial HOMO/LUMO overlaps on the DCPP acceptor could be found,which was advantageous for obtaining a large oscillator strength?f?0.1508?to enhance the radiative transition rate.Thus,a small ?EST and a high radiative transition rate can be simultaneously obtained in this molecular design.The nondoped film showed high fluorescence quantum efficiency??PL?of 14% with an emission peak at 708 nm.The first near-infrared TADF OLED device employing this molecule as the non-doped emitter exhibited a maximum EQE of 2.1% with a CIE coordinate of?0.70,0.29?.Notably,the doped device achieved an extremely high EQE of 9.6% with an emission peak at 668 nm,which was comparable to the values for some efficient near-infrared phosphorescent OLEDs with similar electroluminescent spectra.This research work provides some design ideas for the development of highly efficient near-infrared TADF materials.2.In chapter III,we further investigated the effect of different donor on the properties of TADF molecules.A series of long-wavelength TADF emitters were developed using DCPP as the electron acceptor?A?,carbazole?Cz?,diphenylamine?DPA?or 9,9-dimethyl-9,10-dihydroacridine?DMAC?as the electron donor?D?.With or without the phenyl ring as ?-bridge,we synthesized the other five compounds with D-A-D or D-?-A-?-D configuration.TADF molecules with emitting colors ranging from yellow to deep-red were realized with different electron-donating groups and ?-conjugation length.Compared with the Cz and DPA donor based molecules,the DMAC donor based molecules showed extremely smaller ?EST values due to the large D-A twisting angles.Using a new bipolar host,the device efficiency of the yellow,orange and deep-red device can reach 47.6 cd A^-1?14.8%?,34.5 cd A^-1?16.9%?,and 13.2 cd A^-1?15.1%?,respectively.However,the devices of Cz and DPA donors based compounds suffered from more severe efficiency roll-offs than that of DMAC donor based compounds,which was due to the fact that larger ?EST values and longer delayed fluorescence lifetimes induced serious exciton annihilation.Therefore,in addition to high ?PL,the ?EST and delayed fluorescence lifetime should be further reduced to suppress exciton quenching for fabricating low efficiency roll-off TADF OLEDs.3.In chapter IV,in order to tune the emission wavelength of the TADF molecule to longer wavelength,we expanded the ? conjugation of the DCPP acceptor,which enhanced electron-withdrawing ability and reduced its LUMO level.Two novel red and near infrared TADF compounds were obtained by combining the acceptor with diphenylamine?DPA?or dimethylacridine?DMAC?donor.The theoretical calculation showed that the LUMO levels of these new compunds were reduced by 0.25⁰.28 e V compared to their counterpart and the HOMO levels were basically unchanged,so the band gaps were reduced and the emission spectra could be red shifted.Their nondoped thin films showed near infrared emission with the identical peaks at 765 nm,which were red shifted by nearly 60 nm compared to the compound in chapter II.A record high EQE of 5.4% was achieved in the near infrared fluorescent doped OLED device based on the DPA donor compound with the emission peak at 708 nm,which was among the highest values reported in near infrared fluorescent OLEDs with similar EL spectra.The results indicate that extending the ?-conjugated length as well as increasing the electron-withdrawing ability of the acceptor was a promising method to realize efficent red to NIR TADF emitters.4.In chapter V,we developed high performance fluorescent OLEDs using the dicyano-substituted green TADF material 4Cz IPN as the sensitized host material and quinacridone derivatives as guest dopants.Triplet excitons generated on the TADF host can be harvested by conventional fluorescent dopants via reverse intersysterm crossing?RISC?and F?rster energy transfer process.The TADF host based flurescent OLED device exhibited excellent performance with a maximum EQE of 14.6% and power efficiency of 53.4 lm W-1,which were nearly three times higher than the best values of conventional fluorescent OLEDs.Moreover,the device maintained a high EQE of 13.7% at a practical brightness of 1000 cd m-2,indicative of a low efficiency roll-off.This approach not only improved the efficiencies of conventional fluorescent OLEDs,but also helped to achieve low efficiency roll-off and high color purity TADF OLEDs.In summary,we designed and synthesized a series of yellow to near infrared TADF molecules based on the cyano substituted aromatic conjugated acceptors.The relationship between molecular structure and physical properties was thoroughly investigated.This study contributes to expanding and enriching the red TADF material system.Meanwhile,we achieved high power efficiency and low roll-off TADF OLEDs via efficient energy transfer from dicyano substituted TADF host to the traditional fluorescent dopants.This device fabrication approach provides some guidance for further development of high performance TADF materials and devices.