Surface doping in T6/ PDI-8CN2 Heterostructures investigated by   transport and photoemission measurements
  In this paper, we discuss the surface doping in sexithiophene (T6) organic field-effect transistors by PDI-8CN2. We show that an accumulation heterojunction is formed at the interface between the organic semiconductors and that the consequent band bending in T6 caused by PDI-8CN2 deposition can be addressed as the cause of the surface doping in T6 transistors. Several evidences of this phenomenon have been furnished both by electrical transport and photoemission measurements, namely the increase in the conductivity, the shift of the threshold voltage and the shift of the T6 HOMO peak towards higher binding energies. 
L. Aversa, R.Verucchi, R. Tatti, F. V. Di Girolamo, M. Barra,   F.Ciccullo, A. Cassinese and S.Iannotta
cond-mat.mtrl-sci