Photosensitization of smectic photoconductive liquid crystal by C70-doping

The spectral sensitization and photosensitization efficiency of a liquid crystalline photoconductor, 2-(4'-octylphenyl)-6-dodecyloxy-naphthalene (8-PNP-O12) with C(70) were investigated by steady-state and transient photocurrent measurements in terms of temperature, electric field, and doping concentration of C(70). The C(70)-doped liquid crystal cell exhibited a photoresponse in visible region of 400-700nm corresponding to the optical absorption of C(70). In the time-of-night measurement, the fast transient photosignals with fast rise and decay on the order of microseconds were obtained even in a bulk excitation condition, which is governed by the ambipolar carrier transit. In visible region, the same photogeneration efficiency for hole and electron indicated that C70 can inject both electron and hole into 8-PNP-O12 when photoexcited. The phase transition temperature did not changed by doping C(70) but the phase transition was found to have a great influence to the photogeneration yield. In the high ordering SmB phase, the photosensitization yield was found to be about two orders larger than that in the low ordering SmA phase and isotropic phase, where different interaction of C(70) was obvious in optical absorption and texture s under polarized microscope. These results indicate a possible application of liquid crystalline photoconductors to high-speed photosensors with a fast response in microseconds.