  Organic semiconductors based on small conjugated molecules generally behave
as insulators when undoped, but the hetero-interfaces of two such materials can
show electrical conductivity as large as in a metal. Although charge transfer
is commonly invoked to explain the phenomenon, the details of the process and
the nature of the interfacial charge carriers remain largely unexplored. Here
we use Schottky-gated heterostructures to probe the conducting layer at the
interface between rubrene and PDIF-CN2 single crystals. Gate-modulated
conductivity measurements demonstrate that interfacial transport is due to
electrons, whose mobility exhibits band-like behavior from room temperature to
~ 150 K, and remains as high as ~ 1 cm2V-1s-1 at 30 K for the best devices. The
electron density decreases linearly with decreasing temperature, an observation
that can be explained quantitatively based on the heterostructure band diagram.
These results elucidate the electronic structure of rubrene-PDIF-CN2 interfaces
and show the potential of Schottky-gated organic heterostructures for the
investigation of transport in molecular semiconductors.
