Charge carrier lifetimes in a smectic liquid crystalline photoconductor of a 2-phenylnaphthalene derivative
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Charge carrier lifetimes in a smectic liquid crystalline photoconductor of a 2-phenylnaphthalene derivative Hiroaki Iino and Jun-ichi Hanna Imaging Science and Engineering Laboratory, Tokyo Institute of Technology, 4259, Nagatsuta, Midori-ku, Yokohama, 226-8503, Japan ABSTRACT We have investigated the charge carrier lifetimes for electrons and holes in smectic mesophases of a 2-phenylnaphthalene derivative, 6-(4'-octylphenyl)-2-dodecyloxynaphthalene (8-PNP-O12) by time-of-flight (TOF) measurement. For the negative charge carriers, we found two transits in different time range, which are attributed to electronic and ionic conduction. With the aid of liquid-like fluidity of the material, we could prepare very thick samples over 500µm, and it enables us to determine the carrier lifetimes, which are governed by different regimes: the hole lifetime is governed by recombination of charge carriers and estimated to be 10-2 sec; on the other hand, the electron lifetime is dominated by trapping at deep states and estimated to be on the order of 10-5 sec for both SmA and SmB phases.
INTRODUCTION Recently organic semiconductors have received much attention due to a new industrial application of organic light emitting diodes[1], which follows the photoreceptors for photocopiers and laser printers[2] and will be done by organic thin film transistors[3]. Furthermore, as for the materials, the fast electronic conduction characterized by high mobility over 10-2cm2/Vs, has been discovered not only in columnar mesophases of discotic (disk-like) liquid crystals such as triphenylenes[4], but also in smectic mesophases of calamitic (rod-like) liquid crystals such as 2-phenylaphthalenes[5] and a terthiophene[6]. This mobility is 1000 times faster than that of conventional amorphous semiconductors practically used. Thus, the liquid crystals are being recognized as a new class of quality organic semiconductors, i.e., self-organizing molecular semiconductors (SOMS). Above all, the smectic materials, which had been believed to be governed by ionic conduction due to fluidity [7], are interesting for device applications as described above, because of the bipolar high mobility independent of electric field [5,6]. Their fundamental characteristics of the charge carrier transport including the charge carrier lifetime, however, have not been clarified yet because of their relatively recent discovery. We have studied the charge carrier transport for the lifetime in the smectic mesophases of the 2-phenylnaphthalene derivative, i.e., 8-PNP-O12, by time-of-flight (TOF) measurement. The smectic liquid crystal exhibits liquid-like fluidity, which distinguishes it from the conventional organic solids including amorphous and crystalline materials. This unique feature allows us to reveal the different origins of the charge carrier lifetimes for electrons and holes. In this report, we describe the carrier lifetimes for electrons and holes elucidated by transient photocurrents measurement observed in very thick cells up to 700µm and discuss their or
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