Molecular level alignment at metal-organic (MO) and organic-organic (OO) semiconductor heterojunctions defines charge injection into and transport through multi-layer organic devices. Breakdown of vacuum level alignment is generally observed at MO interfaces, whereas vacuum level alignment prevails at a majority of OO interfaces, consistent with weaker intermolecular interactions and absence of free charges at interfaces between wide gap materials [1]. We focus here on the impact of electrical doping on the energetics of these interfaces. Using direct and inverse photoemission spectroscopy (UPS, XPS, IPES), we demonstrate that doping at MO interfaces between Au and ZnPc or a-NPD p-doped with the strong electron acceptor F₄-TCNQ induces a depletion region in the organic film, but does not alter the molecular level alignment at the interface, consistent with strong metal-organic molecular level anchoring [2]. We investigate a number of OO interfaces between electron transport layers (ETL) (CBP, BCP, Alq₃) and hole transport layers (HTL) (ZnPc, a-NPD) as a function of p-doping in the HTL [3]. The first important result is that, unlike MO interfaces, OO interfaces exhibit a systematic shift in molecular level alignment upon doping the HTL. The shift is accompanied by the formation of an interface dipole, the size of which depends on the constituents of the heterojunction. The second key result is that the position of the HOMO of the undoped ETL remains fixed with respect to the Fermi level (E_F) and independent of the HTL when the HTL is doped. This observation suggests that the introduction via doping of charges and/or electronic states at the interface in the gap of the HTL and ETL results in pinning of E_F at or near a specific energy level, tentatively related to the charge neutrality level of the organic material. The notion of charge neutrality level and its relevance and applicability to MO and OO interface level alignment are discussed.

 
1. A. Kahn, N. Koch and W. Gao, Journal of Polymer Science, Polymer Physics 41, 2529-2548 (2003)
2. W. Gao and A. Kahn, Organic Electronics 3, 53 (2002) and J. Appl. Phys. 94, 359 (2003)
3. W. Gao and A. Kahn,, Appl. Phys. Lett. 82, 4815 (2003)