The Synthesis And Application Of Hole Transport Materials Based On Aromatic Heterocyclic Substituted 9,9'-spirofluorene Xanthene Derivatives

In the recent years,organic semiconductors have shown great interests in scientific research and commercial markets.Organic hole transport material(HTM)as one of important organic semiconductors has been widely used in organic light-emitting diodes(OLEDs),organic solar cells,organic field-effect transistors et al.Among the building blocks for constructing organic HTMs,9,9'-spirofluorene xanthene(SFX)as core unit has been widely studied because of its simple synthesis and easy optimization.Motivated to obtain high performance HTMs,based on their molecular design principle,we synthesized two kinds of derivatives with SFX core,carbazole and phenothiazine as the substituent group,respectively.The molecular structures of the compounds were characterized by 1H NMR,13 C NMR and elemental analysis.Their geometries and substituents effect are analyzed through theoretical calculation and experiments.Finally,their applications as hole transport layer in OLEDs are explored.The results are obtained as follows:1.Two spiro [ fluorene-9,9'-xanthene]-centered(SFX),tetra-carbazole substituted molecules 2,2',7,7'-tetra-kis(N,N-carbazole)-spiro(fluorene-9,9'-xanthene)(SFX-Cr1)and 2,3',6,7'-tetrakis(N,N-carbazole)-spiro(fluorene-9,9'-xanthene)(SFX-Cr2)were synthesized.It demonstrated that both of the two materials possess high thermal stability: SFX-Cr1 with Tg=106.7 ? and Td=495.0 ?,SFX-Cr2 with Tg=130.0 ? and Td=480.0 ? were observed from DSC analyses and TGA analyses,respectively.Single charge carrier devices were fabricated to calculate the hole mobilities of the two materials using the space charge limited current method.It was found that the hole mobility of SFX-Cr1 and SFX-Cr2 is 9.63×10-4 cm2V-1s-1 and 1.57×10-3 cm2V-1s-1,respectively.SFX-Cr2 based OLED device showing a maximum EQE of 16.3%,a maximum current efficiency of 55.8 cd A-1,and a maximum power efficiency of 46.6 lm W-1.2.By introduced rich electron phenothiazine,two spiro(fluorene-9,9'-xanthene)-centered(SFX),tetra-phenothiazine substituted molecules 2,2',7,7'-tetra-kis(N,N-phenothiazine)-spiro(fluorene-9,9'-xanthene)(SFX-PT1)and 2,3',6,7'-tetrakis(N,Nphenothiazine)-spiro(fluorene-9,9'-xanthene)(SFX-PT2)were synthesized.As reference,9,9?–spirobifluorene as core unit,phenothiazine substituted 2,2',7,7'-tetra-kis(N,N-phenothiazine)-spiro(9,9?–spirobifluorene)(Spiro-PT)was synthesized.The three kinds of compounds have high thermal stability: SFX-PT1 with Tg=153.0 ? and Td =435.0 ?,SFX-PT2 with Tg=148.0 ? and Td=421.0 ?,Spiro-PT with Tg=110.0 ? and Td=324.0 ? were observed from DSC and TGA analyses,respectively.Single charge carrier devices were fabricated to calculate the hole mobilities of the three materials using the space charge limited current method.It was found that the hole mobility of SFX-PT1,SFX-PT2 and Spiro-PT is 2.08×10-3 cm2V-1s-1,2.76×10-4 cm2V-1s-1 and 1.29×10-4 cm2V-1s-1,respectively.The HOMO energy level of the three materials is-5.20 eV,-5.08 eV and-5.33 eV,respectively.Compare with the commercial hole transport material Spiro-OMeTAD,SFX-PT1 is more reasonable,which has potential value as the hole transport material for high efficient perovskite solar cells.