  We investigated transport properties of organic heterointerfaces formed by
single-crystals of two organic donor-acceptor molecules,
tetramethyltetraselenafulvalene (TMTSF) and 7,7,8,8-tetracyanoquinodimethane
(TCNQ). Whereas the individual crystals have un-measurably high resistance, the
interface exhibits a resistivity of few tens of MegaOhm with a temperature
dependence characteristic of a small gap semiconductor. We analyze the
transport properties based on a simple band-diagram that naturally accounts for
our observations in terms of charge transfer between two crystals. Together
with the recently discovered tetrathiafulvalene (TTF)-TCNQ interfaces, these
results indicate that single-crystal organic heterostructures create new
electronic systems with properties relevant to both fundamental and applied
fields.
