Fermi level equilibrium at donor–acceptor interfaces in multi-layered thin film stack of TTF and TCNQ

Organic hetero-junctions in multi-layered thin film stacks comprising alternate layers of the molecular donor – tetrathiafulvalene (TTF) and the acceptor – tetracyanoquinodimethane (TCNQ), have been studied by ultraviolet photoelectron spectroscopy (UPS). We show that the energy level alignment at the organic–organic interfaces in the stacks depends only upon the relative energy structure of the donor and acceptor molecules, in particular, the molecular integer charge transfer (ICT) states. The observed interfacial dipoles, across the multi-layered organic stacks, correspond to the difference in energy between the positive and the negative charge transfer states of the molecules constituting the interface. Consequently, Fermi level across the multi-layer system is pinned to those states, since the energetic conditions for the charge transfer across the interface are fulfilled. Hence the energy level alignment at donor–acceptor interfaces studied can be rationalized on the basis of integer charge transfer model (ICT-model). Moreover, we present the photoelectron spectra where 0.85 eV shift of the highest occupied molecular orbital (HOMO) of TTF during formation of TCNQ over-layer is directly observed. These studies contribute to the understanding of the nature of the offset between the frontier electronic levels of the donor and acceptor components which is of high importance in the engineering of efficient organic solar cells.