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Post-Treatment-Free Solution-Processed Reduced Phosphomolybdic Acid Containing Molybdenum Oxide Units for Efficient Hole-Injection Layers in Organic Light-Emitting Devices.

We report the development of solution-processed reduced phosphomolybdic acid (rPMA) containing molybdenum oxide units for post-treatment-free hole-injection layers (HILs) in organic light-emitting devices (OLEDs). The physical and chemical properties of rPMA, including its structure, solubility in several solvents, film surface roughness, work function, and valence states, were investigated. The formation of gap states just below the Fermi level of rPMA was observed. Without any post-treatment after the formation of rPMA films, OLEDs employing rPMA as an HIL exhibited a very low driving voltage and a high luminous efficiency. The low driving voltage was attributed to the energy level alignment between the gap states formed by reduction and the HOMO level of the hole-transport layer material N,N'-bis(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine.