Orientation of pentacene films using surface alignment layers and its influence on thin film transistor characteristics

2 There has been great interest in organic semiconductor materials composed of molecules such as sexithiophene derivatives and pentacene for organic thin film transistors (TFTs), due to their high field effect mobilities, which approach that of amorphous silicon. It is thought that the high mobilities observed in these materials are due to intermolecular interactions which lead to larger grain sizes and fewer grain boundaries, facilitating charge transport [1-4]. Liquid crystals(LC) are another class of organic materials in which increased mesoscopic order is technologically desirable. High contrast in liquid crystal displays is achieved through the use of underlying alignment layers such as rubbed or irradiated polymers [5-7] and self-assembled monolayers [8]. A liquid crystal film without an alignment layer will show locally ordered domains with frequent grain boundaries and discontinuities while an LC film over an alignment layer exhibits enhanced long range order. Some of the more promising organic semiconductor materials, such as dihexylsexithiophene, have striking similarities to liquid crystal materials. Thus it appears promising to investigate the affect of liquid crystal alignment layers on organic semiconductor materials. In this paper we investigate the use of liquid crystal alignment layers to produce order in thin pentacene films. Both rubbed and photoirradiated layers of polymers and self-assembled monolayers were investigated. Their ability to alter the structure of pentacene films was screened by linear dichroism measurements and grazing-incidence x-ray diffraction. Finally, organic transistors were constructed employing an alignment layer and its influence on the device performance was measured. Self-assembled monolayer (SAM) photoalignment layers were synthesized by conversion of cimmamic, 4-fluorocinnamic and 4-nitrocinnamic acids to the corresponding acid chlorides and subsequent reaction with aminopropyltriethoxysilane. Films of these materials were prepared 3 by immersion of the substrates (fused silica) in a 1% solution of xylene for 1 hour [9]. To generate alignment, these layers were irradiated with polarized light at 280 nm with an irradiation dose of ~ 300 mJ/cm 2. The photoalignment polyimide (PI) Nissan RN1332 was also used, prepared and irradiated following manufacturer specifications. For rubbed alignment layers, polyvinylalcohol (PVA, Aldrich) was spun from a 2% solution and baked for two hours at 130 C. Film thickness was found to be ~ 500 Å by ellipsometry. The commercial alignment polyimides Nissan SE150 and SE12710 were prepared following manufacturer specifications. PVA and PI film surfaces were rubbed on a LCBM buffing machine (Optron Systems, Inc., Bedford MA) having a roller radius of 3 cm. …