Doping is poised to become an important tool to increase film conductivity, control Fermi level position and facilitate carrier injection in molecular semiconductors. P-type doping of common hole-transport materials has been achieved with molecules like the electronegative fluorinated F4-TCNQ over the past few years. In this talk, we describe work on new p-type and n-type dopants aimed at enhancing film conductivity and charge carrier injection via tunneling through narrow depletion regions. Energetics of doping are determined by ultra-violet and inverse photoemission spectroscopies (UPS, IPES). We report on very recent work on p-doping with a fluorinated molybdenum compound that exhibits a larger electron affinity (EA) than F4-TCNQ. N-doping is challenging because of the low electron affinity (EA~1.5-2.5 eV) of most electron-transport molecular materials of interest. N-dopants must have low ionization energy, which makes them inherently unstable against oxidation. We demonstrate n-doping of a variety of small molecule materials, such as THAP with cobaltocene (CoCp, IE = 4.07 eV), and CuPc, pentacene and C60 with decamethylcobaltocene (CoCp*2; IE=3.30 eV). Orders of magnitude increase in current density in doped devices demonstrate enhanced electron injection and materials conductivity. Applications to the formation of organic p-i-n homojunctions and to the improvement of CuPc/C60 photovoltaic cells are demonstrated.
Host: Eric Garfunkel