Contact potential difference measurements of doped organic molecular thin films

Calvin Chan, Weiying Gao, Antoine Kahn
2004 Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films  
The possibility of nonequilibrium conditions in doped organic molecular thin films is investigated using a combination of ultraviolet photoemission spectroscopy ͑UPS͒ and contact potential difference measurements. Surface or interface photovoltage is of particular concern in materials with large band gap and appreciable band ͑or energy level͒ bending at interfaces. We investigate here zinc phthalocyanine ͑ZnPc͒ and N,NЈ-diphenyl-N,NЈ-bis͑1-naphthyl͒-1,1Јbiphenyl-4,4Љ diamine ͑␣-NPD͒ p-doped
more » ... ͑␣-NPD͒ p-doped with the acceptor molecule, tetrafluorotetracyanoquinodimethane (F 4 -TCNQ). In both cases, we observe an upward movement of the vacuum level away from the metal interface with respect to the Fermi level, consistent with the formation of a depletion region. We show that photovoltage is not a significant factor in these doped films, under ultraviolet illumination during UPS. We suggest that the carrier recombination rate in organic films is sufficiently fast to exclude any photovoltage effects at room temperature.
doi:10.1116/1.1688363 fatcat:gagmyghsuja2rkgwtctmzrsh5q