Assistance of partially reduced MoO3 interlayer to hole-injection at iron phthalocyanine/ITO interface evidenced by photoemission study

• Published in: Applied surface science Vol. 271; pp. 352 - 356
• Main Authors: Liu, L.Y., Wan, L., Cao, L., Han, Y.Y., Zhang, W.H., Chen, T.X., Guo, P.P., Wang, K., Xu, F.Q.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 15-04-2013; Elsevier
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Hole injection; Interface; Iron phthalocyanine; Molybdenum oxide; Photoemission; Physics; Molybdenum oxide; Hole injection; Iron phthalocyanine; Photoemission; Interface; Inorganic compounds; Transition elements; Transition element compounds; Iron; Phthalocyanines
• ISSN: 0169-4332; 1873-5584
• DOI: 10.1016/j.apsusc.2013.01.200

► Pronounced gap states were yielded in partially reduced MoO3 (MoOx) interlayer. ► A barrier of 0.45eV was derived for electron extraction from FePc to MoOx smaller than to MoO3. ► The energy level alignment diagrams were obtained for the FePc/MoO3 and FePc/MoOx interfaces. ► The nominal thickness was optimized to be 3nm for MoO3 interlayer.
Molybdenum trioxide (MoO3) has been widely used as an interlayer between electrode and functional organic layer inducing significant improvement in the performance of organic electronic and optoelectronic devices such as organic light-emitting diodes (OLEDs) and organic photovoltaics (OPVs). In this work, the electronic structures and energy level alignment at FePc/MoO3 (3nm)/ITO and FePc/MoOx (3nm)/ITO (x<3) interfaces have been comparatively investigated in situ by means of photoemission spectroscopy (PES) in an attempt to understand the influence of MoOx layer on the interfacial hole injection property. It is found that the electron injection barrier is only 0.45eV for electron extraction from the highest occupied molecular orbital (HOMO) of FePc to the conduction band of MoOx. Simultaneously, two gap states at binding energy of 1.15eV and 2.30eV present in 3nm MoOx, which extend from the valence band maximum of MoOx to the Fermi level. These gap states are suggested to originate from the partial occupation of Mo 4d states due to the reduction of MoO3 during the annealing process. Both lowered electron extraction barrier and presence of interfacial gap states appear to assist the hole injection from ITO anode into FePc by the insertion of MoOx layer.