Molecular Design And Optoelectronic Properties Of Novel Organoboron Emitters

Since the birth of organic light-emitting diodes（OLEDs）,both materials and devices have been developed rapidly.However,achieving efficient and stable deep blue OLEDs remains a challenge.To meet high-quality display requirements,both materials and devices must be continuously improved to enhance the color purity and stability of OLED.In recent years,the development of new synthesis methods has led to emerging a large number of organoboron emitters with excellent properties.Among them,boron-oxygen emitters have achieved highly efficient deep blue OLEDs.Through the combination of thermally activated delayed fluorescence（TADF）and short-range separation of frontier molecular orbital（FMO）,boron-nitrogen emitters have exhibited highly efficient and narrow full-width at half-maximum（FWHM）emission.In addition,boron-fluorine emitters have achieved high-efficiency red OLEDs with solution processing.Through the theoretical calculation of organoboron emitters,researchers have revealed that the short-range charge transfer（SRCT）character is the key to achieving high oscillator strength and small structural relaxation.These studies suggest that boron-based materials have broad application prospects for achieving high-performance OLEDs.In this thesis,the main line is the study of boron-oxygen materials.By designing multiple donor and rigid structures to regulate the excited states,high-efficiency narrowband blue OLEDs with low efficiency roll-off were successfully realized.Furthermore,the relationship between the structure and properties of organoboron emitters was analyzed from the aspects of chemical synthesis,theoretical calculation,photophysics measurement and exciton dynamic analysis.Furthermore,the design of narrowband emitters was also explored in depth.The research work is mainly carried out from the following aspects:Firstly,to address the slow radiative transition and low efficiency issues of donor-acceptor（D-A）type blue emitters,multiple charge transfer channels and local triplet states were introduced to achieve fast radiative transitions and effective reverse intersystem crossing（RISC）process.In addition,the rigid disk-shaped structure is conducive to high orientation and small FWHM.Due to the efficient up-conversion process and high dipole orientation,the radiative exciton utilization and light-out-coupling factor were both successfully enhanced for the deep blue OLED device based BO3N emitter,giving a maximum EQE of more than 10%and CIE coordinates of（0.14,0.08）.This study provides a multi-donor rigid disk-shaped strategy for the development of deep blue emitters.Subsequently,developing efficient blue exciplex OLEDs remains challenging.Though regulating intra-and inter-molecular charge transfer,bipolar boron-based materials with shallow lowest unoccupied molecular orbital（LUMO）were used to construct blue exciplex.Due to the rigid boron-oxygen fused ring unit participating in the intermolecular charge transfer,the boron-containing exciplex exhibited a high radiative transition rate.Not only high-efficiency blue PN heterojunction OLEDs were constructed,but also sufficient energy transfer,small FWHMs and low turn-on voltage(V_on)were achieved in a series of OLEDs based on the boron-nitrogen emitter and phosphorescent emitter.Owing to the small energy barrier,good carrier transport ability and efficient upconversion process,blue PN heterojunction OLED based on BOCz:TAPC exhibited high power efficiency of 36.8 lm W^-1 and low V_on of 2.6 V.In addition,the blue ultrathin layer OLED based boron-nitrogen emitter showed a small FWHM of 35 nm and a low V_on of 2.6 V.Compared to OLEDs with multi-host emitting layers,simple ultrathin layer OLEDs achieved narrower emission,sufficient energy transfer and smaller FWHMs broadened by environmental polarity.These results show great potential for novel bipolar boron-based materials in constructing highly efficient,structurally simple and cost-effective blue exciplex OLEDs.In addition,although blue guest and exciplex were successfully constructed by boron-oxygen units,the organoboron emitters with conjugated plane structures still suffer from low triplet states and low luminance in the devices.Therefore,inheriting the advantages of the multi-donors strategy,organoboron emitters with a non-planar structure and multi-carbazole donors are designed to avoid low triplet states and suppress vibration,while achieving efficient narrowband blue OLEDs with low efficiency roll-off.By manipulating the peripheral substitution sites,the o22Cz BO emitter achieves narrowband blue emission with a peak of 478nm and a FWHM of 17 nm/111 em V.Furthermore,the charge transfer characteristics,intramolecular interactions,vibration frequencies,configuration changes and energy distribution of molecules were analyzed.Besides,the relationship between narrowband emission and molecular structure was studied.Ultimately,the blue OLED device achieved a high EQE of over 27%,low efficiency roll-off and excellent color purity.To our knowledge,these results represent the narrowest bandwidth emission and the highest device brightness among the reported boron-oxygen emitter OLEDs,while providing a design approach for non-planar emitters with narrowband emission.Lastly,three organoboron emitters,BOFCz,BO2FCz and BO3FCz,were designed to further investigate the relationship between narrowband emission and molecular structure.On the one hand,the planar fused ring structure is designed to obtain a narrowband emission by reducing structural relaxation changes and suppressing non-radiative transitions such as molecular rotation.On the other hand,the combination of weak electron donor carbazole and weak electron acceptor boron-oxygen unit formed weak charge transfer,which is beneficial for deep blue emission.By increasing the number of carbazoles,the conjugated plane was expanded.Meanwhile,with the decreased Huang-Rhys（HR）factors,reduced Root Mean Squared Errors（RMSDs）and suppressed high-frequency vibrations,the redshifted spectra and smaller FWHMs were successfully achieved.Using computational simulation,the relationship between vibration modes,reorganization energy,structural changes and emission spectra are deeply analyzed,while revealing the causes of spectra narrowing.This study provides a new design concept for narrowband blue emitters.At the same time,the FWHM of BO3FCz is only8 nm in solution.To our knowledge,this is the narrowest emission among the organic emitter.