Parallel Donor and Acceptor Semiconductor Crystals for Organic Field Effect Transistors

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.

Two systems of p-type & n-type organic semiconductors suitable for directional growth have been identified. Thin films composed of parallel stripes of p-type & n-type crystals have been successfully prepared by a temperature gradient approach, on glass substrates, and on polymer dielectric layers. The comprehensive characterization of the structure and morphology of the most promising films has been carried out by the combined use of the polarized optical microscope, optical profilometer, in-house X-ray diffraction, and synchrotron X-ray diffraction. The conductivity of the thin films has been measured at INP, University of Bordeaux. However, the fabrication and characterization of OFETs based on the prepared films could not be completed because of the lockdown due to the global pandemic of the COVID-19.
The research results of PARADA have been disseminated in five international scientific conferences, including 32nd European Crystallographic Meeting (Austria), Crystal Growth and Assembly (GRC) Gordon Research Conference (the USA), the 24th International Conference on the Chemistry of the Organic Solid State (the USA), 10th Belgian crystallography symposium (Belgium), and Crystal Growth and Assembly (GRS) Gordon Research Seminar (the USA). Additionally, the outreach activity relevant to PARADA was delivered at European Researchers’ Night 2019, (“Dynamic Crystals”, Sci4all, 27. September, University of Applied Arts, Vordere Zollamtsstraße 7, 1030 Wien).

Although the directional crystallization of thin films of organic molecules from melt has commonly been used for the study of common organic compounds, the directional crystal growth of mixtures of molecular semiconductors have never been studied, despite it offers an opportunity to fabricate parallel single-crystal stripes of p-type & n-type organic semiconductors. The PARADA project has demonstrated that the directional crystal growth of mixtures can be expanded to organic semiconductors. Our experimental results indicate that not only crystal growing mode (digitated, dendritic, or branched sea-weed type) but also the length-scale of phase separation between the two organic semiconductor crystals, are linked to the growth rate, that is externally imposed by the pulling rate, and the magnitude of the thermal gradient. Moreover, the comprehensive characterization of the structure and morphology of the thin films demonstrates the high preferred orientation and the excellent in-plane crystalline order of the two organic semiconductors crystals. PARADA showed a new method of growth of organic semiconductor thin film single crystals. Moreover, the results of PARADA would deliver a fresh look to the research field of molecular semiconductors for logic operations.

Periodic Reporting for period 1 - PARADA (Parallel Donor and Acceptor Semiconductor Crystals for Organic Field Effect Transistors)

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