Electronic Properties of Furan-Quinone Pigments. II. Photoinjection and Photoemission Studies

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Abstract
The positions of the valence, conduction, and vacuum levels of the vacuum‐deposited yellow furan‐quinone pigments 8, 13‐dioxodinaphtho (2, 1−b:2′, 3′−d)‐furan‐6‐(2‐pyridyl)‐carboxamide (DFC) and 8, 13‐dioxodinaphtho (2, 1−b:2′, 3′−d)‐furan (DF) with respect to the Fermi level of the substrate Au, Al, or Cu were determined from measurements of the internal and external photoeffect. The structure of the conduction and vacuum levels for the two pigments is the same within experimental error. A weak photoresponse due to bulk states was observed below 2 eV photon energy. These states position the Fermi level about 2.7 eV above the valence band edge. An experimental value of 3.55 ± 0.13 eV for the separation of the valence and conduction bands was obtained. An interfacial dipole layer was found for all substrates investigated. The resulting drop of the vacuum level of the pigment with respect to the vacuum level of the substrate increases with increasing substrate work function. From the voltage dependence of the injection current, a mean free path for the scattering of the photoinjected carriers, in the image‐force potential well between the emitter and the potential maximum in the pigment, was calculated. Qualitative agreement was obtained with the screening length derived from the bending of the band‐type energy states close to the pigment/substrate interface. Depending upon bias polarity, strong polarization effects were observed for DFC deposited on Au and DF deposited on Al substrates. The experimental results are compared with the theoretical predictions made from molecular energy level calculations.