Ultrafast Photoconductivity in Organic Semiconductors
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Ultrafast Photoconductivity in Organic Semiconductors Oksana Ostroverkhova1, David G. Cooke2, Frank A. Hegmann2, John E. Anthony3, Vitaly Podzorov4, Michael E. Gershenson4, Oana D. Jurchescu5, and Thomas T.M. Palstra5 1 Physics, Oregon State University, 301 Weniger Hall, Corvallis, OR, 97331 2 Physics, University of Alberta, Edmonton, T6G 2J1, Canada 3 Chemistry, University of Kentucky, Lexington, 40506 4 Rutgers University, Piscataway, 08854 5 University of Groningen, Groningen, 9747, Netherlands
ABSTRACT We present comprehensive study of ultrafast charge carrier dynamics in a variety of organic molecular crystals. In all samples, we observed sub-picosecond charge photogeneration and band-like transport, characterized by (i) an increase in charge carrier mobility as the temperature decreases in a wide temperature range of at least 20 K– 300 K and (ii) mobility anisotropy in the a-b plane of the crystals. The temperature dependence of the decay dynamics of the transient photoconductivity reveals the presence of shallow trapping sites in herring-bone-type-structured crystals (such as pentacene (Pc), tetracene (Tc), and rubrene (Rub)), while such traps are apparently absent in “brick-wall”-π-stacked crystals (such as functionalized pentacene (FPc)). We also report on the measurements of the charge carrier mobility anisotropy in the a-b plane of two types of FPc single crystals. Anisotropies of approximately 3.5 and 11.6 were obtained in the crystals characterized by crystal structures favoring two-dimensional and one-dimensional charge transport, respectively, consistent with the degree of π-overlap along different directions in the crystals. INTRODUCTION Over the past decade, considerable efforts in organic synthesis and purification have resulted in improved organic semiconductors with room-temperature charge carrier mobilities well above 1 cm2/(Vs). A number of experimental and theoretical studies directed towards clarifying mechanisms of charge carrier generation and transport have also been carried out. However, the exact nature of charge carrier photogeneration and transport in organic semiconductors is still not completely understood due to difficulties in assessing intrinsic properties that are often masked by impurities, contact effects, or grain boundaries. Recently, ultrafast optical pump – terahertz (THz) probe techniques have been used to perform non-contact measurements of transient photoconductivity in organic single crystals and polycrystalline thin films over picosecond time scales, making it possible to probe the intrinsic nature of mobile charge carriers before they are trapped at defect sites [1]. In this paper, we summarize our studies of the transient photoconductive properties of pentacene, functionalized pentacene, tetracene and rubrene single crystals assessed using time-resolved terahertz pulse spectroscopy [2-4]. MATERIALS In our studies, we used single crystals of ultra-high-purity pentacene (Pc), pentacene derivative functionalized with triisopropylsilylethynyl (TIPS) side gr
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