Evaluation of work functions of organic semiconductors and electronic structures of organic/metal interfaces

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Energy structures of molecular semiconductors are investigated in air on the basis of the Kelvin probe (KP) method. Vacuum sublimed films of phthalocyanines (p-type), porphyrins (p- and n-type) and perylene derivative (n-type) contacting metals form ideal Schottky-barriers. A common work function of 4.8±0.05 eV for p-type molecular semiconductors and 4.1-4.3 eV for n-type ones are evaluated from the Schottky-Mott rule. Work function values measured directly with the KP method strongly dependent on the metal used as substrate. This is ascribed to nonalignment of Fermi levels between a molecular solid and metal under the open-circuit situation of the KP configuration. Correction for the Fermi energy differences reduces the metal dependencies. Possible reasons for the nonalignment of Fermi levels are discussed in connection with incorporation of O_2 and/or H_2O into molecular semiconductors. Similarly, the Schottky-Mott rule has been successfully applied to evaluating work functions of va … More rious poly (3-arylthiophenes) (PATs). Edges of the valence and conduction bands of the PAT films are estimated from the onset potentials for anodic p-doping and cathodic n-doping of the polymer films. On these bases electronic structures of the PAT films and their interfaces with metals in air are depicted.Apparent mobilities of positive charge carriers in a poly [(ethoxypropylsilanylene) quinqe-(2,5-thienylene)] (MS5T-OEt) film are evaluated. The mobility decreases monotonously with the increase in doping level up to a few percents, in contrast to four order-of-magnitude increase of mobilities reported earlier for the poly (3-methylthiophene) (PMT) film. This indicates that the two kinds of diamagnetic species, cation radical π-dimers in the MS5T-OEt film and bipolarons in the PMT, are related to the electrical conduction in completely different manners. An in situ ESR study performed to characterize charge-carrying species in the MS5T-OEt film demonstrates that there are oligothienylene segments where polarons are isolated so as not to form π-dimers. Less