TIME-RESOLVED PHOTOCURRENTS IN LANGMUIR-BLODGETT-FILMS OF SUBSTITUTEDHEXAALKOXYTRIPHENYLENE

By successive deposition of monolayers of the hexaalkoxytriphenylene d erivative Te(OH)(2), organized multilayers with a thickness between 18 and 360 Angstrom can be obtained. Photoexcitation of the Langmuir-Blo dgett films, deposited on interdigitating electrodes with a spacing of 100 mu m, by a laser pulse of 4 ns full width at half maximum at a wa velength of 320 nm and with an energy between 5 and 200 mu J, induces transient photocurrents. At large applied fields or upon increasing th e energy of the laser pulse and at-reduced pressure, besides a transie nt current pulse of which the features are determined by the bandwidth of the experimental setup, a second current pulse grows in and shows a maximum at 1.5 mu s after excitation. At low fields, charge generati on occurs mainly in the bulk of the sample. In this field range, most charge carriers disappear by trapping or recombination before they can reach the opposite electrode, making it difficult to get information about the transit time. In the high field range, the charge carriers a re still due to light absorbed by the Te(OH)(2) layers. In this field range, the pressure of air dr nitrogen strongly influences the feature s of the second maximum. The observation of the second maximum above a critical voltage and under optimal pressure conditions, suggests that corona discharging is also an important process. According to this mo del, the second maximum is related with the transit time of the charge carriers which migrate perpendicular to the substrate. (C) 1998 Ameri can Institute of Physics.