  The surface molecular doping of organic semiconductors can play an important
role in the development of organic electronic or optoelectronic devices.
Single-crystal rubrene remains a leading molecular candidate for applications
in electronics due to its high hole mobility. In parallel, intensive research
into the fabrication of flexible organic electronics requires the careful
design of functional interfaces to enable optimal device characteristics. To
this end, the present work seeks to understand the effect of surface molecular
doping on the electronic band structure of rubrene single crystals. Our
angle-resolved photoemission measurements reveal that the Fermi level moves in
the band gap of rubrene depending on the direction of surface electron-transfer
reactions with the molecular dopants, yet the valence band dispersion remains
essentially unperturbed. This indicates that surface electron-transfer doping
of a molecular single crystal can effectively modify the near-surface charge
density, while retaining good charge-carrier mobility.
