To investigate the impact of impurities on properties of single crystals of rubrene (C48H24), we consider structural,
charge transport and optical properties. We show that transport properties are strongly influenced by the presence of a
bandgap acceptor state, commonly occurring in crystals of rubrene. The bandgap state is likely caused by incorporation
of an oxygen-related impurity. We demonstrate that the impurity presence can be detected using optical spectroscopy.
The impurity model explains commonly-observed variations of rubrene properties. The acceptor-like characteristics of
the impurity suggests a possible approach for modification of the charge transport in molecular crystals.
Organic semiconductors attract considerable attention due to promising applications in organic light emitting diodes,
field effect transistors, and organic solar cells. Moreover, solubility of some organic semiconductors in organic solvents
favors them for printed large area OLED displays and inexpensive printed microelectronics. However, low mobility of
carriers in organic semiconductors limits usability of organic semiconductors in integrated circuits and need to be
overcome. For this reason, the knowledge of intrinsic properties achievable in very pure and perfect crystals is important.
Therefore, we have carried out a program to grow high quality single crystals of organics. Solution growth, melt growth,
solvothermal method and vapor transport crystal growth have been applied and will be reported. For research purpose,
using a gas phase transport method, we have produced millimeter - sized crystals of numerous organic semiconductors
with higher quality and purity. Structure quality has been evaluated by x-ray topography methods. Field effect transistors
have been prepared on surfaces of single crystals. Some of organic semiconductors like rubrene, pentacene, copper
phthalocyanine exhibit carrier mobilities comparable or even higher than amorphous silicon. However, characterization
of starting materials, crystals, thin films and resulting devices remains the crucial issue. The relation between organic
semiconductor properties, used device fabrication technologies and resulting device characteristics is the object of
presented here studies.
Recent research on organic and polymeric semiconductors is directed towards highly ordered molecular structures in solid states. Through molecular design and engineering, it has been shown possible to control the molecular orientation and processing conditions of these materials as well as fine tuning their energy levels and color emissions. Thin film field-effect transistors (FETs) have been used as testing structures for evaluating the semiconducting properties of new organic semiconducting materials. Performance similar to amorphous-Si can now be realized with some organic materials. Large-scale integration of organic transistors has been demonstrated. In addition, several low cost novel non-lithographic patterning methods have been developed, which resulted in the first flexible electronic paper. The field-effect transistor device structure can also be utilized as a means to induce a great amount of charge carriers in organic thin films through the gate field. Using this type of structure, superconductivity was observed in a highly ordered conjugated regioregular poly(3-hexylthiophene).
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