AVS 50th International Symposium
    Contacts to Organic Materials Topical Conference Tuesday Sessions
       Session OM-TuP

Paper OM-TuP1
Study of Surface Photovoltage Effects in Doped Organic Molecular Thin Films

Tuesday, November 4, 2003, 5:30 pm, Room Hall A-C

Session: Poster Session
Presenter: C. Chan, Princeton University
Authors: C. Chan, Princeton University
W. Gao, Princeton University
A. Kahn, Princeton University
Correspondent: Click to Email
We investigate photoemission-induced surface photovoltage (SPV), i.e. non-equilibrium resulting from photoexcitation and separation of carriers, in organic films. We focus on electrically doped molecular films that exhibit interface depletion regions@footnote 1@ and can sustain charge separation and SPV. We combine ultraviolet photoemission spectroscopy (UPS) to measure the HOMO and vacuum levels under illumination, and contact potential difference (CPD) measurement with a Kelvin probe to measure the vacuum level in the dark. Zinc phthalocyanine (ZnPc) and N,N’-diphenyl-N,N’-bis(1-naphthyl)-1,1’biphenyl-4,4’’diamine (@alpha@-NPD) p-doped with the acceptor molecule, tetrafluorotetracyanoquinodimethane (F@sub 4@-TCNQ), are investigated.@footnote 1,2@ UPS and CPD measurements are performed as a function of film thickness deposited on Au. CPD measurements reproduce with excellent accuracy the UPS-observed shifts of the molecular energy levels, in particular through a depletion region ~30-50Šnear the metal-organic interface of the doped organic films. The excellent agreement shows that: (1) efficient p-doping occurs even in the dark in these two organic systems; (2) SPV is negligible in both doped and undoped organic films. The undoped films do not have depletion regions where charge separation can occur. In the doped films, electrons separated in the interface depletion region recombine in the metal before generating significant SPV, whrereas the holes readily recombine throughout the film. @FootnoteText@ @footnote 1@ W. Gao and A. Kahn, Organic Electronics 3, 53 (2002) @footnote 2@ W. Gao and A. Kahn, J. Appl. Phys. (accepted for publication; July 1, 2003).