Effective Work Functions For The Evaporated Metal/Organic Semiconductor Contacts From In-Situ Diode Flatband Potential Measurements

The diode built-in potentials (V-bi) of several polymer organic semiconductor (OSC) thin films [(2,5-dialkoxy-substituted poly(p-phenylenevinylene), poly(9,9-dialkylfluorene), poly(9,9-dialkylfluorene-alt-phenylene-(N-phenyl)iminophenylene), and poly(9,9-dialkylfluorene-alt-benzothiadiazole)] sandwiched between p-doped poly(3,4-ethylenedioxythiophene) (PEDT:PSSH) and evaporated metal contacts have been measured by bias-dependent electromodulated absorption (EA) spectroscopy of the Stark-shifted pi-pi* band. From these values and the vacuum-level offsets at the PEDT: PSSH contacts evaluated by sub-gap EA spectroscopy, the following effective work functions for the buried evaporated metal contacts have been obtained: Al 3.4+/-0.1, Ag 3.7+/-0.1, Au 4.4+/-0.1, and Ca 2.4+/-0.1 eV. These work functions are smaller than those of the "clean" metal surfaces by up to 0.8 eV, and are substantially independent of the OSC in the absence of charge transfer. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4728121]