Organic optoelectronic devices: modelling of charge transfer in metal/organic and metal-oxide/organic interfaces

Organic photovoltaic technologies require semiconductors to have a long enough exciton diffusion length in order to reach the donor/acceptor interface; excitons would then separate into an electron and a hole and be collected at the device electrodes. At the same time, the metal electrode needs to have a work function that is sufficiently low to facilitate efficient transfer of electrons to the cathode, while the transparent metal-oxide electrode needs to have a work function that is sufficiently high for efficient transfer of holes to the anode.

In this talk, I will focus on organic/inorganic interfaces of optoelectronics devices. I will firstly show, via density functional theory (DFT) calculations, that surface modifiers based on molecules containing simple amine groups can significantly modify the surface properties of the metal electrode in terms of charge injection/collection across the interface as well as work function modification. Our results are in very good agreement with ultraviolet photoemission spectroscopy (UPS) measurements performed on the Au(111) surface. In addition, a bilayer cathode, consisting of a thin film of high work function metal, such as Al and Au, and a layer of amine-functionalized polystyrene, was also fabricated and tested as an organic light-emitting diode exhibiting substantially enhanced efficiency. Our combined theoretical and experimental investigation gives insight into how a thin layer of a commodity polymer can be used to transform rather high-work-function metals into high-performance cathodes to provide efficient electron injection [1, 2, 3].

Secondly, a combination of DFT and experimental measurements via ultraviolet and X-ray photoelectron spectroscopies is used to explore the nature of the interface between the stoichiometric molybdenum trioxide MoO₃ and its under-stoichiometric counterpart with oxygen vacancies MoO_x, 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 processes, which are vital for the fabrication of efficient optoelectronics devices [4, 5].

Bibliography

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