Harmonizing Triplet Level and Ambipolar Characteristics of Wide-Gap Phosphine Oxide Hosts toward Highly Efficient and Low Driving Voltage Blue and Green PHOLEDs: An Effective Strategy Based on Spiro-Systems

Abstract

A series of phosphine oxide (PO) hosts based on diphenylphosphine oxide and spiro[fluorene-9,9′-xanthene] (SFX) moieties, SFX2PO, SFX27PO, SFX2′PO, and SFX2′7′PO, were designed and synthesized. On the basis of the different electrical properties of xanthene and fluorene in SFX, the influence of substitution position on the chemical and optophysical properties of the ambipolar-core based systems were investigated in detail. The effective strategy of introducing electron-withdrawing PO moieties in electron-deficient moieties in the molecules accompanied with suitable linkages insulating the electron-rich and -deficient moieties was convincingly demonstrated, which can endow the hosts with much better carrier injecting and transporting ability and high enough T1 for blue and green phosphors. As the results, the operating voltages of the devices based on SFX2PO and SFX27PO were much lower than those of the devices based on SFX2′PO and SFX2′7′PO. Simultaneously, the efficiencies of the SFX2PO based devices wer...