Energy structures of molecular semiconductors contacting metals under air studied by the diffusion potential measurements and the Kelvin probe technique

Abstract

Energy structures of molecular semiconductors are investigated under air on the basis of the Kelvin probe (KP) method; a critical examination of the applicability of this method for direct determination of work functions is performed. It is revealed that vacuum-sublimed films of three phthalocyanines ( p-type), two porphyrins ( p- and n-type) and a perylene derivative ( n-type) in contact with metals form ideal Schottky barriers whose heights are simply determined by the difference in work function between a molecular solid film and a metal. Based on this finding (achievement of a Schottky–Mott rule), 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 deduced. On the other hand, work function values measured in air directly with the KP method ( φ KP) are strongly dependent on the metal used as substrate. This metal dependence is ascribed to nonalignment of Fermi levels between a molecular solid and a contacting metal under the open-circuit situation of the KP configuration. Indeed, φ KP values of phthalocyanine films corrected for the Fermi energy difference at the metal/phthalocyanine junction become independent of the respective substrate metal, although they are still by 0.1–0.2 eV greater than the value of 4.8 eV evaluated from the Schottky–Mott rule. Possible reasons for the nonalignment of Fermi levels and the remaining discrepancies are discussed in connection with incorporation of O 2 and/or H 2O into molecular semiconductors. Influences of film thickness and illumination on φ KP are also examined in order to clarify junction properties at metal/molecular solid contacts. Based on these results, use of the Schottky–Mott rule is highly recommended for determination of work functions of molecular semiconductors exposed to air.