Growth and physical properties of organic semiconducting single crystals
Tan, Ke Jie
Date of Issue2012
School of Materials Science and Engineering
Electronic instruments that operate using organic semiconductors are already available on the market. Organic light emitting diodes are in numerous displays of mobile phones or some TV sets. Organic solar cells with efficiency larger than 5% are produced by small start-up companies. Organic field effect transistors have achieved charge mobilities larger than that of amorphous silicon. Therefore it is justified to ask for the intrinsic limit of organic semiconductors and how to improve the physical properties of organic semiconductors. This study focuses on the most perfect form of solid state made from an organic semiconductor, on organic single crystals. High defect concentrations and grain boundaries cause hindrance to the charge carrier‘s transport. Relatively long radiative decay of excitons can be achieved in high quality Light Emitting Organic Field Effect Transistors (LEOFET) built from high charge carrier mobility materials. Therefore, using high quality molecular single crystals for fundamental research in organic electronics will be valuable in pushing forward the cutting edge of these technologies. To study organic semiconductors, single crystals of numerous organic semiconductors have been produced. Furnaces with high precision temperature control were built for growing of high quality single crystals. The high quality of a single crystal was defined in terms of high crystallinity, high purity and molecularly smooth crystal surface. Numerous high quality mono-component and charge-transfer (CT) binary-component molecular single crystals have been grown using solution phase and gas phase methods. Powder X-ray diffraction (PXRD), single crystal X-ray diffraction (SCXRD) and cross-polarized light microscopy (CPLM) were done to characterize the crystallinity and the crystal phase of the single crystals. High performance liquid chromatography (HPLC), liquid chromatography mass spectrometry (LCMS), Matrix-assisted laser absorption/desorption mass spectrometry (MALDI-ToF-MS), photoluminescence (PL) and attenuated reflection fourier transform infrared spectroscopy (ATR-FTIR) were used to study the purity of the single crystals. Atomic force microscopy (AFM) was used to investigate the surface roughness of the single crystals. The bandgap of the molecular single crystals could be reduced by growing CT binary-component molecular single crystals. High conductivity molecular single crystals could be attained with peri-substituted dichalcogenide polycene, such as tetrathiotetracene (TTT). The highest charge carrier mobilities of holes and electrons were measured for rubrene, tetracene, F16CuPc and few more single crystal FETs. Yet, the limit of the mobility of organic materials is still a mystery. It was found that the measured mobilities from the single crystal FETs do not represent the intrinsic mobility of the materials. The measured mobility highly depends on the surface states of the single crystals which are largely affected by the fabrication process of the single crystal FETs. The physical properties of these molecular single crystals are highly sensitive to impurities and defects in the single crystal whose impurities level is still far higher than its inorganic counterpart. For instance, oxides of rubrene and pentacene were detected in high mobility rubrene and pentacene single crystals. Current theoretical knowledge doesn't allow prediction and modeling of the structure-properties relation with precision sufficient for material selection. Therefore, experimental studies aids in finding parameters responsible for the physical properties of materials. This study describes numerous efficient organic semiconductors, structure of these materials and their physical properties useful for device preparation.