Electrical doping of poly(9,9-dioctylfluorenyl-2,7-diyl) with tetrafluorotetracyanoquinodimethane by solution method

We investigate p-type doping of poly (9,9-dioctylfluorenyl-2,7-diyl) (PFO) films with tetrafluorotetracyanoquinodimethane (F-4-TCNQ) introduced via cosolution. Doped and undoped films are compared using ultraviolet photoelectron spectroscopy (UPS) and current-voltage (I-V) measurement. In spite of the difference between the ionization energy of PFO (5.8 eV) and the electron affinity of F-4-TCNQ (5.24 eV), p doping occurs, as seen from the movement of the Fermi level (E-F) toward the polymer highest occupied molecular orbital (HOMO). Interface hole barriers are measured for undoped and doped PFO deposited on three substrates with different work functions, indium-tin-oxide (ITO), gold (Au), and poly-3,4ethylenedioxythiophene-polystyrenesulfonate (PEDOT-PSS). Doping leads to the formation of a depletion region at the PFO/ITO and PFO/Au interfaces. The depletion region is believed to be at the origin of the (hole) current enhancement observed on simple metal/PFO/substrate devices. (c) 2005 American Institute of Physics.