Universal energy-level alignment of molecules on metal oxides

Abstract

Transition-metal oxides improve power conversion efficiencies in organic photovoltaics and are used as low-resistance contacts in organic light-emitting diodes and organic thin-film transistors. What makes metal oxides useful in these technologies is the fact that their chemical and electronic properties can be tuned to enable charge exchange with a wide variety of organic molecules. Although it is known that charge exchange relies on the alignment of donor and acceptor energy levels, the mechanism for level alignment remains under debate. Here, we conclusively establish the principle of energy alignment between oxides and molecules. We observe a universal energy-alignment trend for a set of transition-metal oxides--representing a broad diversity in electronic properties--with several organic semiconductors. The trend demonstrates that, despite the variance in their electronic properties, oxide energy alignment is governed by one driving force: electron-chemical-potential equilibration. Using a combination of simple thermodynamics, electrostatics and Fermi statistics we derive a mathematical relation that describes the alignment.

Efficient and bright organic light-emitting diodes on single-layer graphene electrodes

“…Although OA-doped graphene has an enhanced work function of B5.1 eV, it is not sufficient for direct hole injection into host materials such as 4,4 0 -bis(carbazol-9-yl)biphenyl (CBP), which has a HOMO level of 6.1 eV. Here we use anode interface layers consisting of the transition metal oxide to further enhance the work function of the graphene anode, similar to their roles of modifying the work function of ITO electrodes in organic electronics [38][39][40][41][42][43] . Note that metal oxides do not grow as uniform films on graphene due to the lack of nucleation to initialize the film growth 44,45 .…”

MoOx and V2Ox as hole and electron transport layers through functionalized intercalation in normal and inverted organic optoelectronic devices

“…43,44 Here, we use a Kelvin Probe system to investigate the work function of various Csintercalation mole ratios of MoO x and V 2 O x . The MoO x and V 2 O x films formed on ITO/glass substrates exhibit work functions of 5.30 eV and 5.42 eV, respectively.…”

Undoped and Ni-Doped CoOx Surface Modification of Porous BiVO4 Photoelectrodes for Water Oxidation

“…10), it is suggested that Ni incorporates into the CoOx lattice as opposed to forming a composite structure. Considering the work functions of monoclinic BiVO4 (5.27 eV) 54 and Co3O4 (6.1 eV) 63 , the interface band diagram for the n-type BiVO4/CoOx heterojunction is shown schematically in However, under the conditions used to evaluate ηcs and ηci (presence of a hole scavenger SO3 2-), the photo generated surface holes are consumed very rapidly. In this case, the enhanced charge transfer enabled by the larger band bending has only a small effect, as can be seen in Figures 9a and S5.…”