Pentacene Schottky diodes studied by impedance spectroscopy: Doping properties and trap response

Abstract

We study doping properties and charge carrier trap distributions in pentacene Schottky diodes doped by the fluorinated fullerene derivate ${\text{C}}_{60}{\text{F}}_{36}$ and 2,2'$\ensuremath{-}$(perdiylidene)dimalononitrile (${\text{F}}_{6}\text{-TCNNQ}$) upon small signal excitation. We show that the charge carrier depletion zones present in these Schottky diodes are tunable by the applied bias and temperature. Mott-Schottky evaluations yield reduced doping efficiencies and dopant activation energies between 19 and 54 meV. In the low-frequency regime, we resolve additional capacitive contributions from inherent charge carrier traps. A Gaussian distributed trap center 0.6 eV above the hole transport level with a density in the range of ${10}^{16}$ cm${}^{\ensuremath{-}3}$ depending on the material purity is found to be an intrinsic feature of the pentacene matrix. Upon doping, the deep Gaussian trap center saturates in density and broad exponentially tailing trap distributions arise. Subsequent ultraviolet photoelectron spectroscopy measurements are conducted to inspect for energetic broadening due to doping.