Nature of the interfaces between stoichiometric and under-stoichiometric MoO₃ and 4,4′-N,N′-dicarbazole-biphenyl: A combined theoretical and experimental study

A combination of density functional theory and experimental measurements via ultraviolet and X-ray photoelectron spectroscopies is used to explore the nature of the interface between the stoichiometric molybdenum trioxide (MoO ₃) or its under-stoichiometric counterpart with oxygen vacancies, and an organic hole-transport layer represented by 4,4′-N,N′- dicarbazole-biphenyl (CBP). Upon adsorption of CBP, special attention is paid to i) the appearance of gap states and the reduction of the molybdenum oxide surface, and ii) the evolution of the work function. Very good agreement is found between theory and experiment. The near alignment of the CBP highest occupied molecular orbital with the Fermi level and the conduction band edge of molybdenum oxide points to facile hole collection or injection. The interface between MoO_x and an organic hole transport material is investigated in a joint theoretical and experimental study. Gap states are found to be associated with different types of oxygen vacancies and adsorption of the organic material, which induce electron donor states at different energy levels within the band gap.