AVS 49th International Symposium
    Organic Films and Devices Tuesday Sessions
       Session OF+SS+EL+SC-TuA

Paper OF+SS+EL+SC-TuA8
Controlled p-Doping of Organic Molecular Films

Tuesday, November 5, 2002, 4:20 pm, Room C-102

Session: Organic Molecular Films
Presenter: W. Gao, Princeton University
Authors: W. Gao, Princeton University
A. Kahn, Princeton University
Correspondent: Click to Email
We investigate the controlled electrical p-doping of the hole-transport organic molecular material @alpha@-NPD with the strong electron acceptor tetrafluoro-tetracyano-quinodimethane (F@sub 4@-TCNQ) using ultraviolet and inverse photoelectron spectroscopies (UPS/IPES), and in-situ I-V measurements. We previously examined p-doping of ZnPc co-evaporated with F@sub 4@-TCNQ,@footnote 1@ and found an excellent energy match between the ionization energy (IE) of ZnPc (5.28eV) and the electron affinity (EA) of F@sub 4@-TCNQ (5.24eV), demonstrating host HOMO-to-guest LUMO charge transfer. The ZnPc thickness dependence of I-V data further demonstrated a 7 orders of magnitude increase in hole current injected from Au due to tunneling through the doping-induced narrow depletion region at the metal/organic interface.@footnote 2@ In the present study, we show that @alpha@-NPD (IE=5.52eV) is also efficiently doped with F@sub 4@-TCNQ. The hole injection current increases by almost 5 orders of magnitude when only the first 80Å of @alpha@-NPD away from the metal/organic interface is doped, and increases by another factor of 10 when the entire organic film is uniformly doped. However, the doping-induced movement of E@sub F@ toward the HOMO appears to be more restricted than in ZnPc. Having excluded extrinsic effects like surface photovoltage, we propose that the substantial (~0.2 eV) ionization-induced relaxation of molecular energy levels moves the "doped" @alpha@-NPD HOMO upwards, and thus pins E@sub F@ deeper into the gap than for ZnPc, which is a planar molecule with a negligible relaxation energy (< 0.05eV). This interpretation is further confirmed with UPS study of the host material growth on a film of the dopant molecules. We also show that molecular level alignment at organic/organic interfaces is controllable by doping-induced dipole. Work supported by the NSF (DMR-0097133). @FootnoteText@@footnote 1@ W. Gao and A. Kahn, Appl. Phys. Lett., 79, 4040 (2001) @footnote 2@ W. Gao and A. Kahn, Organic Electronics (in press).