Design/Synthesis And Optoelectronic Properties Of Deep Blue Semiconductors Based On Phenanthro [9,10-d] Imidazole

In recent years, along with the great potential in full-color, organic light-emittingdevices (OLEDs) received more and more attention from both scientific and practicalpoints due to their applications in large-area flat-panel displays and solid-state lighting.Compared with green-emitting and red-emitting materials, blue-emitting materials arestill inferior such as low efficiency and especially poor color purity for the intrinsicwide band gap. According to the European Broadcasting Union (EBU) standard blueCIE coordinate of (0.15,0.06), there have been only a few reports on deep blueOLEDs that can match the emission with y coordinate lower than0.06with highefficiency. One reason for the scarcity of reports is that the wide energy bandgap ofdeep blue material would confine the molecular size to render a limited π-conjugationlength, which leads to the difficulty in choosing efficient deep blue emitting buildingblock. In addition, fluorescence can be easily quenched in the solid state due toaggregation, which usually results in the decreased efficiency and emission out of thedeep blue spectral region in OLEDs. More importantly, it is difficult to simultaneouslyinject electrons and holes into such wide energy bandgap emitters since the restrictionin π-conjugation length would result in the reduction of carrier injection and transportproperties and hamper the devices performance. Overall, high performance deep bluelight emitting materials are still rare, and the design for the blue emitters with good thermal stability, high efficiency, and reliable color purity is quite challenging.The development of high efficiency saturated deep blue emitters is especiallyimportant for OLEDs. First of all, deep blue emission can efficiently reduce the powerconsumption in full-color displays, i.e. the lower the y coordinate, the less the powerconsumption. Secondly, deep blue emitters can be used to generate light of all colorsby energy transfer to emissive dopants. The preparation process of full color displaycan be simplified with more excellent stability. Once again, highly efficient blueOLEDs are necessary for high-resolution full-color displays. As one of the threeprimary colors, the higher saturation blue can reproduce more color saturated images,improving the human visual perception. So, the develement of high efficiencysaturated blue emitting materials is an ever-increasing issue to promote thecommercial application of OLEDs.Phenanthro[9,10-d]imidazole (PI) unit has attracted extensive attentions inorganic optoelectronic field recent years due to its intrinsic wide band gap, highluminescence efficiency, good thermal stability, balanced charge carrier injection andtransport property as well as low fabrication cost. In this paper, PI as a building blockfor blue light emitting materials are modified at the C2position of imidazole, derivingfrom the analysis the electronic structure of PI. A series of blue light emittingmaterials are tried to design and synthesize. Mainly from the following three aspectsof this work:1. Different donors are tried to attach to PI, including carbazole, indole, anilineand phenothiazine for bipolar molecular structures. The final results showthat, while the hole and electronic separate obviously, the full width at halfmaximum of the devices emission increases and the y coordinate value isimproved obviously. The deep blue emitting materials M2consisting ofcarbazole and PI moieties exhibit excellent performances. The non-dopeddevice based M2with the CIE of (0.166,0.056) and an EQE of3.02%wasobtained.2. A series of PI-aromatic type derivatives are designed and synthesized. Theemission of materials based on anthracene and pyrene are out of the deep blue range. Different acceptor are tried to attach to PI, includingdiphenylphosphrine oxide and dimesityboryl. The fluorescence quantumefficiency is increased obviously. However, the emission of materials is outof the deep blue range, too. The device based on B-PIM exhibit the currentefficiency of8.80cd A-1, the external quantum efficiency of4.87%. It isproved that the introduction of electron withdrawing group would make thephenanthroimidazole derivatives to have intramolecular charge transferphenomenon and reduce the saturation of electroluminescence spectra.3. SiPIM using PI and triphenylsilane was obtained. The non-dopedvacuum-deposited device based on SiPIM achieves extremely highefficiency of6.29%(1.94cd A1) with true deep blue CIE coordinates of(0.163,0.040). It can be distinctly ascribed to the introduction oftriphenyl-substituted silane, which entitles SiPIM with promising physicalproperties, including high quantum yield of deep blue emission, appropriateflexibility, and high thermal and morphological stabilities. All of theseresults gives us a new foreground for the design of deep blue material andinspire its application in the future.