AVS 52nd International Symposium
    Electronic Materials and Processing Thursday Sessions
       Session EM1-ThA

Paper EM1-ThA6
Molecular Level Alignment and the Role of the Charge Neutrality Level at Organic-Organic Heterojunctions

Thursday, November 3, 2005, 3:40 pm, Room 309

Session: Organic and Molecular Optoelectronics
Presenter: W. Zhao, Princeton University
Authors: W. Zhao, Princeton University
H. Vazquez, Universidad Autonoma de Madrid, Spain
F. Flores, Universidad Autonoma de Madrid, Spain
A. Kahn, Princeton University
Correspondent: Click to Email
The electronic structure of organic-organic (OO) interfaces is a key aspect of organic devices such as OLEDs and PV cells. Molecular level offsets at OO interfaces determine transport across devices, and directly affect their performance. This talk reports a recent investigation of the electronic structure of several OO interfaces between films of molecules such as tris(8-hydroxy-quinoline)aluminum (Alq@sub 3@), 1,4,5,8-naphthalenetetracarboxylicdianhydride (NTCDA), iridium-bis(4,6,-di-fluorophenyl-pyridinato-N,C@super 2@)-picolinate (FIrpic) or copper phthalocyanine (CuPc). The filled and empty states of these materials, their ionization energy (IE) and electron affinity (EA), and the interface molecular level alignment are determined via ultra-violet and inverse photoemission spectroscopy (UPS, IPES). Unlike many previously investigated OO heterojunctions,@footnote 1@ these are found not to follow vacuum level alignment and exhibit significant interface dipoles ranging from 0.2 to 0.5 eV. The analysis of these and previous results on OO heterojunctions is performed using the extension of the concept of charge neutrality level (CNL), developed for metal/organic interfaces,@footnote 2@ to the OO heterojunction.@footnote 3@ The energy-level alignment is driven by the alignment of the CNLs of the two organic semiconductors. The initial offset between the CNLs gives rise to a charge transfer across the interface, which induces an interface dipole and tends to align the CNLs. The initial CNL difference is reduced according to the screening factor S, a quantity related to the dielectric functions of the organic materials. Good quantitative agreement with experiment is found. This allows, for the first time, a semi-quantitative prediction of the electronic structure of OO heterojunctions. @FootnoteText@ @footnote 1@ I.G. Hill et al., Appl. Surf. Sci. 166, 354 (2000); @footnote 2@ H. Vazquez et al. EuroPhysics Lett. 65, 802 (2004); @footnote 3@ H. Vazquez et al. Phys. Rev B Rapid Comm. 71, 041306 (2005).