Bringing a new generation of optoelectronic devices
Nowadays, inorganic semiconductors are the most widely used for fabrication of optoelectronic devices, yet there are increased fabrication costs due to complicated processing methods. Alternatively, organic semiconductors, such as conjugated polymers, are easier to fabricate; however, their applications are limited since the current quality of their performances is unsatisfactory. To answer this need the DISCEL project extensively explored the functional discotic liquid crystalline materials in order to replace the conjugated polymers. The liquid crystalline materials have been considered as higher performing self-assembling organic semiconductors. The project worked on not only on the synthesis and optimisation of processing conditions, but also on the determination of their key physical parameters. After performing pulse radiolysis time-resolved microwave conductivity tests in both p- and n-type materials, it was found that most of the materials used display high charge carrier mobility. Thereby, hexabenzocoronene (HBC) derivatives support rapid charge transport at room temperature. Unlike what was considered before, chirality of the branched alkyl side chains was not clearly found to affect positively the mobility, while purity does. Compared to p-type, n-type components displayed lower mobilities for the HBCs. Photo-induced charge separation was also measured on thin, spin-coated films of mixtures of a perylenediimide (PEDI) derivative and two different HBCs using flash-photolysis time-resolved microwave conductivity. At long wavelengths, where absorption occurs in composite regions in which HBC and PEDI are in close contact, direct photo-induced charge separation takes place. The higher the light intensity for direct photo-induced charge separation, the lower the efficiency of charge separation. The DISCEL project also fabricated and evaluated three different kinds of electronic devices using liquid crystalline discotic semi-conductors. The first involves photovoltaic diodes (PVD) that can be used for solar cells, including battery charger for wireless devices or flexible panels in building and greenhouse applications. The second is light emitting diodes (LED) aimed for small- and large- sized displays. The third kind is field effect transistors (FET) that can find applications in disposable microchips used in consumer markets as well as in healthcare industry.
