Study On Molecular Synthesis And Optoelectronic Properties Of Highly Efficient And Low Efficiency Roll-off Oleds With Narrowband Emission

Organic light-emitting diodes(OLEDs)have aroused tremendous attentions in display and lighting fields.Narrowband emission is of vital importance for high color purity and ultra-high-definition displays.The already commercial materials still suffer from the relatively broad emission bands due to the intrinsic vibronic coupling between the ground and excited states and structural relaxation at the excited states.In order to meet the requirements of visual verisimilitude levels,technologies such as optical filters and special optical microcavity structures have to be applied in OLEDs to narrow the electroluminescent spectra and improve color purity,which is usually accompanied by a heavy loss in energy utilization.Therefore,starting from the light-emitting mechanism of organic materials,the development of organic light-emitting materials with intrinsic narrowband emission is a highly effective strategy to enhance the color purity of the original electroluminescence spectra.In this dissertation,we focus on designing high efficiency pure organic fluorescent materials with narrowband emission and fabricating their high color purity and low efficiency roll-off devices.The main works of this dissertation are summarized as follow:1.We design three“hot exciton”materials,namely PIAn TAZ,Cz An TAZ,and TPAAn TAZ,in which phenanthroimidazole(PI)/carbazole(Cz)/triphenylamine(TPA)are selected as the electron donors;triazine(TAZ)is used as the electron acceptor;anthracene,possessing rigid planar?conjugation and large energy gap of T₁-T₂,is chosen as acceptor.The photophysical experiments combined with theoretical calculation reveal that through reducing electron-donating ability of donors,we tailor hybridization status of excited state with relatively weak CT component,which is beneficial to facilitate the electron spin flip under electrical charge injection and significantly reduce the vibrational motion to obtain blue and narrowband emission.Owing to the bipolar nature and high efficiency of PI,the nondoped device based on PIAn TAZ exhibits the best device performance,with a maximum EQE of 7.96%and a maximum luminescence of 58675 cd m^-2.It is worth noting that the nondoped device shows an ultra-low efficiency roll-off of 7.90%at a high luminance of up to 1000 cd m^-2and the EQE at a brightness of 10 000 cd m^-2remains as high as 6.49%.More importantly,the full widths at half maximum(FWHM)of non-doped PIAn TAZ device is 63 nm,which is significantly smaller than the FWHM(72 nm)of TPAAn TAZ with a strong CT state under the same device structures.These results indicate that weakening the CT component is an effective way to narrow the FWHM of the fluorescent molecules,and finally achieve high color purity nondoped blue OLEDs.2.The deep blue fluorescent materials PIAn TPh is designed and successfully synthesized by replacing of the acceptor triazine group with neutral m-terphenyl unit.Such a molecular design strategy can not only further reduce the CT component of molecule,but also inhibit the notorious bimolecular interactions of the rigid molecular structure to hinder the fluorescence quenching effect.As a comparison,we also chose a pyreneimidazole group with a more extended?-conjugated rigid planar structure to design and synthesize Py IAn TPh,which can further improve PLQY.The optimized OLEDs using PIAn TPh and Py IAn TPh as dopants display deep blue emission with the maximum EQEs of 8.09%and 8.44%,narrow FWHMs of 50 nm and 52 nm,CIE coordinates of(0.15,0.06)and(0.15,0.07),respectively,which meet well with stringent EBU CIE coordinates of(0.15,0.06).These electroluminescence performances are among the state-of-the-art deep blue OLEDs.The photophysical experiments and transient EL decay confirm that the the excellent EL performance is mainly attributed to the hot exciton mechanism during device operation.3.Four highly efficient MR-TADF emitters,namely TW-BN,TPh-BN,p Cz-BN and m Cz-BN,are obtained by introducing peripheral phenyl derivatives as steric hindrance groups on the respective para position of BCz-BN via single bond.These single bond-linked bulky groups not only inhibit the fluorescence quenching effect and reduce efficiency roll-off,but also enhance low-frequency vibronic coupling strength and substantially suppress high-frequency stretching vibrations,leading to the small reorganization energy(?)and narrow FWHM of the emission spectra.As a result,all these sterically protected B-N-based materials successfully achieve the?values below 0.12 e V,narrow FWMH values around 0.11 e V(20 nm)and fast radiative decay rates exceeding 10⁸s^-1.The OLED based on TPh-BN exhibits high EQE up to 28.9%,together with reduced efficiency roll-off and relatively long operational lifetime.In addition,the device exhibits significantly small FWHM of0.14 e V(28 nm),demonstrating that the“peripheral phenyl substituents”design strategy is an efficient approach to further decrease the FWHM on the basis of B–N skeleton and construct high-performance narrowband organic luminescent materials.4.Two novel asymmetric MR-TADF emitters(Cz-PTZ-BN and 2Cz-PTZ-BN)are designed that fully inherit the high PLQY and large rate constant of reverse intersystem crossing(k _RISC)of the properly selected parent core skeletons,namely,BCz-BN and 2PTZ-BN.BCz-BN is selected because of its rigid planar structure as well as more delocalized HOMO-LUMO distributions,benefiting a high oscillator strength and PLQY,whereas 2PTZ-BN possesses short delayed fluorescence lifetime and fast k _RISCvalue because the sulfur atom can effectively enhance spin-orbit coupling via heavy atom effect.Moreover,introduction of the peripheral electron-donating unit(carbazole)into the Cz-PTZ-BN skeleton could further improve the PLQY and facilitate the RISC process as well as suppress notorious bimolecular interactions.Importantly,the resultant OLED based on 2Cz-PTZ-BN exhibits pure green emission with a remarkable maximum EQE of 32.8%and the maximum luminance of 13518 cd m^-2without utilizing any sensitizers or exciplex cohosts.Moreover,the device sufficiently maintains high efficiency of exceeding30%under the luminance of 100 cd m^-2and 23.5%under 1000 cd m^-2,which are the highest values for green MR-TADF emitters at the same brightness.The state-of-the-art device performance imply great potential of this molecular design strategy in simultaneously achieving high efficiency and low efficiency roll-off for MR-TADF materials.