A Molybdenum Dithiolene Complex as<i>p</i>-Dopant for Hole-Transport Materials: A Multitechnique Experimental and Theoretical Investigation

Abstract

Molybdenum tris-[1,2-bis(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd)3) is investigated as a p-dopant for organic semiconductors. With an electron affinity of 5.6 eV, Mo(tfd)3 is a strong oxidizing agent suitable for the oxidation of several hole transport materials (HTMs). Ultraviolet photoemission spectroscopy confirms p-doping of the standard HTM N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl-4,4′-diamine (α-NPD). Strong enhancement of hole injection at α-NPD/Au interfaces is achieved via doping-induced formation of a narrow depletion region in the organic semiconductor. Variable-temperature current−voltage measurements on α-NPD: Mo(tfd)3 (0−3.8 mol %) yield an activation energy for polaron transport that decreases with increasing doping concentration, which is consistent with the effect of the doping-induced filling of traps on hopping transport. Good stability of Mo(tfd)3 versus diffusion in the α-NPD host matrix is demonstrated by Rutherford backscattering for temperatures up to 110 °C. Density functional theory (DFT) calculations are performed to obtain geometries and electronic structures of isolated neutral and anionic Mo(tfd)3 molecules.