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 (F4-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 F4-TCNQ (5.24 eV), p doping occurs, as seen from the movement of the Fermi level (EF) 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-3,4-ethylenedioxythiophene∙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.