Contact characterization by photoemission and device performance in P3HT based organic transistors

Abstract

The operation of organic devices as organic field-effect transistors (OFET) depends critically on the contact between the organic layer and the material for source/drain electrodes. Small barriers for carrier injection are required for efficient operation. In order to support the understanding of organic devices, photoemission spectroscopy has been used to determine the properties of metal/organic interfaces. Values for the hole injection barrier determined in the last decade by different groups are frequently of the order of 0.5–1 eV. It is not clear whether barrier lowering due to the image charge is sufficient to make contacts with such barriers efficient for carrier injection. Indeed, no results have been reported where the preparation of the samples for the photoemission study and for the devices are the same. Here we present results of such an investigation for OFETs with gold source/drain contacts. The measured hole barrier at the gold contact of 0.6 eV results from the Au work function of 4.6 eV. Taking into account the dependencies of the mobility on the carrier concentration and on the field for the Gaussian density of states (DOS) of disordered organics, measured OFET current characteristics cannot be described well with such contacts but rather for work functions of 4.7 eV or larger. Considering the method in determining the barrier from photoemission data and the Gaussian DOS of the hopping transport states, we present a quantitative connection between the barrier as determined from photoemission and the barrier as used in the device simulation.