Organic electronics is an active research field aiming at the use of organic semiconductors in various optoelectronic applications, such as organic light-emitting diodes or organic field-effect transistors (OFETs). The essential characteristic of organic semiconductors is their ability to transport electrical charges that are either holes, h+, or electrons, e-. Organic semiconductors are of two types: conjugated polymers, which exhibit various degrees of order, and molecular crystals. Performances are, mainly, assessed by the charge carrier mobility µ (cm2/V.s) i.e. the higher, the better. Molecular semiconductors outperform conjugated polymers because charge transport is faster in highly ordered media. Not surprisingly, the highest µ values have been measured for OFETs fabricated with single crystals of molecular semiconductors, because of the absence of grain boundaries. As a general matter of fact, conjugated compounds can transport both h+ and e-. It is, however, observed that semiconductors with electron-donating (withdrawing) groups form more stable radical cations (anions). Few semiconductors exhibit ambipolar charge transport, but more interesting in view of industrial applications is the complementary logic that is possible with simple circuits composed of both p-type & n-type OFETs. Single crystal OFETs have been fabricated with both p-type & n-type molecular semiconductors. However, the fabrication is tedious and requires growing single crystals, to select the best ones in terms of size and shape, and to delicately connect them between source & drain electrodes. Crystals are grown by the physical transport method that is a vapor deposition technique. Overall, such a tedious fabrication method resembles more to craftwork than to technology.
Research on OFETs is of great significance because they can be used as control elements in flat panel displays, as parts of radio frequency identification card (RFID), electronic skin (E-skin), and other flexible electronic materials. The PARADA project was mainly devoted to studying crystallographic problems in the field of OFETs and developing organic single-crystal thin films with the coexistence of p-type & n-type organic semiconductors, using directional crystallization as a tool. These single-crystal stripes were designed to fabricate single-crystal transistors able to operate in complementary logic mode. Therefore, the knowledge and results produced from the PARADA project will contribute to both fundamental and applied sciences.
By the PARADA project, we successfully demonstrate that the parallel p-type & n-type OSCs thin-film crystals can be prepared by a temperature gradient approach. And the uniaxial in-plane alignment of crystallites along the temperature gradient direction was observed in the blended thin films.
