Anisotropic transport properties of organic single crystals

Organic thin film devices based on conjugated small molecules and polymers are receiving quite a large attention due to their potential in optoelectronic, photovoltaic and sensor applications, given the interesting possibility to fabricate cheaply and quickly devices able to be applied in many fields of electronics [1]. In spite of the rapid progress that has taken place on the applied side, our basic understanding of the electronic properties of organic materials is still limited the rather low chemical and structural purity of the thin films used in device fabrication that has so far prevented a systematic study of the intrinsic electronic properties of organic semiconductors. In organic thin films, the presence of grain boundaries and interfacial disorder limits the charge carrier mobility and results in the broadening of the device on/off transition. Grain boundaries can be eliminated in organic semiconductors by growing single crystals of small conjugated organic molecules. We have investigated the electronic transport properties of single crystals based on a thiophene derivative recently synthesized [2], by Space Charge Limited Current (SCLC) analyses and photocurrent (PC) spectroscopy. The structural properties of the crystals have been investigated by X-ray diffraction and Atomic force microscopy. SCLC analyses allowed to obtain information on the charge carrier mobility and on the presence of electrically active deep levels associated to structural defects that affect the carrier transport. PC spectra provided information on the distribution of the electronic excited states and on the major energy gap transition energies We identified a strong anisotropic behaviour of the crystals that we discuss by correlating the electrical and structural results.