Energy level alignment and morphology of interfaces between molecular and polymeric organic semiconductors

Ultraviolet photoelectron spectroscopy (UPS) was used to determine the energy level alignment at organic-organic conductor-semiconductor and semiconductor-semiconductor hetero-interfaces that are relevant for organic optoelectronic devices. Such interfaces were formed by in situ vacuum sublimation of small molecular materials [C-60 and pentacene (PEN)] and ex situ spin-coating of poly(3-hexylthiophene) (P3HT), all on the common substrate poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS). We found that the deposition sequence had a significant impact on the interface energetics. The hole injection barrier (HIB) of C-60 on PEDOT:PSS could be changed from 1.0 eV (moderate hole injection) to 1.7 eV (good electron injection) by introducing a layer of P3HT. The HIB of P3HT/PEDOT:PSS was increased by 0.35 eV due to an interfacial PEN layer. However, PEN deposited on PEDOT:PSS and P3HT/PEDOT:PSS exhibited the same value. These observations are explained by material-dependent dipoles at the interfaces towards PEDOT:PSS and substrate dependent inter-molecular conformation. (C) 2007 Elsevier B.V. All rights reserved.