Electronic structures of CuI interlayers in organic electronic devices: An interfacial studies of N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine/CuI and tris-(8-hydroxyquinolinato)aluminum/CuI

Electronic structures with the copper iodide (CuI) interlayer in organic electronic devices were measured and its strong electron-withdrawing properties were revealed. In situ ultraviolet and X-ray photoelectron spectroscopy showed the interfacial electronic structures of N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB)/CuI/indium-tin-oxide (ITO) and tris-(8-hydroxyquinolinato)aluminum (Alq₃)/CuI/ITO as a representative hole- and electron-transport material. The large work function of the CuI interlayer induces electron transfer from both molecules and ITO to CuI. As a result, CuI dramatically reduces the hole injection barrier (HIB) from ITO to NPB and Alq₃ layers. Notably, CuI assists molecular ordering of the NPB layer, which would increase the intermolecular interaction, so would enhance the charge transport properties. Simultaneous enhancement in HIB and molecular ordering with the CuI interlayer would improve the device performance.