AVS 51st International Symposium
    Organic Films and Devices Thursday Sessions
       Session OF+EM-ThA

Paper OF+EM-ThA6
Electrical Doping of Poly(9,9-dioctylfluorenyl-2,7-diyl) with Tetrafluoro-Tetracyanoquinodimethane by Solution Method

Thursday, November 18, 2004, 3:40 pm, Room 304C

Session: Molecular and Organic Films and Devices - Optoelectronic
Presenter: J.H. Hwang, Princeton University
Authors: J.H. Hwang, Princeton University
A. Kahn, Princeton University
Correspondent: Click to Email
Electrical doping of organic materials has received attention for enhancing carrier injection and lowering drive voltages. We investigate here p-type doping of poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) with tetrafluorotetracyanoquinodimethane (F4-TCNQ) using a solution method. Doped and undoped films were compared using ultraviolet photoelectron spectroscopy (UPS) and current-voltage (I-V) measurement. Undoped PFO was prepared from 0.1wt% of tetrahydrofuran (THF) and p-xylene. For doped PFO, 5% of F4-TCNQ relative to each repeat unit of PFO was added to each solution. Undoped and doped films (~100Å thick) were spun on ITO or Au substrates in nitrogen, annealed in nitrogen at 50°C to remove residual solvent, and loaded without ambient exposure in an ultra-high vacuum chamber. UPS spectra were recorded for each film. The energy of the highest occupied molecular orbital (HOMO) was measured with respect to the Fermi level (E@sub F@). The ionization energy of PFO, determined as the difference between vacuum level (measured from the onset of photoemission) and HOMO, was found to be 5.75 eV, which is ~0.5 eV larger than the electron affinity of F4-TCNQ (5.24eV)@footnote 1@.In spite of the fact that this difference is significantly larger than for ZnPc (0.04 eV)@footnote 1@ and a-NPD (0.28 eV)@footnote 2@, F4-TCNQ p-dopes the polymer. EF-HOMO is 1.1 eV and 1.4 eV for undoped PFO on Au and ITO, respectively, and drops by 0.2 eV for PFO:F4-TCNQ, showing a shift in the expected direction for p-doping. We also performed I-V measurements on Au/1200Å PFO/ITO, which show an order of magnitude increase in current in doped PFO, consistent with higher conductivity and/or lowering of the hole-injection barrier. These measurements show that electron transfer from host to dopant occurs and produces p-type doping. @FootnoteText@ @footnote 1@ W. Gao, A. Kahn, Appl. Phy. Lett., 79, 4040 (2001)@footnote 2@ W. Gao and A. Kahn, J. Appl. Phys. 94, 359 (2003).