Effective work functions for the evaporated metal/organic semiconductor contacts from in-situ diode flatband potential measurements

The diode built-in potentials (Vbi) 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 π–π* 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.