A molybdenum dithiolene complex as p-dopant for hole-transport materials: A multitechnique experimental and theoretical investigation

Molybdenum tris-[l ,2-bis(trifluoromethyl)ethane-l,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 -doping of the standard HTM N,N-di-[(l-naphthyl)-N,N-diphenyl]-l, l'biphenyl-4,4'-diamine (a-NPD). Strong enhancement of hole injection at a-NPD/Au interfaces is achieved via doping-induced formation of a narrow depletion region in the organic semiconductor. Variable-temperature current-voltage measurements on a-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 a-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.