Hole Transport in Self-Organized Oligosilane Thin Films with Highly Ordered Hopping Sites
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Hole Transport In Self-Organized Oligosilane Thin Films With Highly Ordered Hopping Sites H. Okumoto, T. Yatabe, A. Richter, M. Shimomura, A. Kaito, N. Minami Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan ABSTRACT Self-organized oligosilane thin films possess molecular orientation normal to substrates with multilayered structure. This unique order of -conjugated molecules results in good hole transport properties. In the present work, carrier transport properties at low temperature are studied for 1,10-diethyldecamethylsilane polycrystalline films. Even at a temperature as low as 173 K, a time-of-flight transient photocurrent waveform showed a clear plateau and a sharp decay, whose shape is similar to that at room temperature. Their hole mobility followed Arrhenius type temperature dependence with a small activation energy of 0.09 eV. The hole mobility of /Vs at 193 K was more than 2 orders of magnitude higher than that of typical polysilanes, which inevitably contain disordered structures hindering smooth carrier transport. INTRODUCTION The self-organization of molecules is one of key factors in developing functional molecular materials for device fabrications. For example, -conjugated discotic [1] and smectic [2] liquid crystalline materials attract attention as carrier transport materials with mobilities exceeding /Vs. On the other hand, -conjugation [3] is another key factor in improving carrier transport materials. Polysilanes, possessing delocalized electrons along silicon main chains, have been extensively studied because they show high hole mobilities among conducting polymers on the order of /Vs at room temperature [4, 5]. Oligosilanes can combine the above two key factors. In our previous papers, we proposed self-organized oligosilanes as a new class of hole transport materials [6, 7]. Self-organization of oligosilanes occurs as a smectic B (S ) phase [8, 9, 10], but its high molecular order is well- preserved even in a polycrystalline phase. In these ordered states, molecules are oriented almost normal to the substrate surface, forming multi-layer structure with exceptionally large domain sizes ( m). Such a structural peculiarity results in good carrier transport properties in crystalline phase as well as in S phase. In permethyldecasilane (CH [Si(CH ) ]CH , MS10) polycrystalline films, hole mobility exceeded /Vs at room temperature. Moreover, the time-of-flight photocurrent showed nondispersive waveform having a clear plateau and a sharp decay, reflecting very low density of carrier traps that would hinder smooth carrier transport. This is an advantage over the -conjugated liquid crystalline materials. While these materials show high carrier mobilities in the liquid crystalline phase, their carrier transport properties deteriorate accompanying crystallization due to many carrier traps created at grain boundaries.
BB10.46.1
(a) Si
EMS10 (b)
C2H5
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