Organic radicals as innovative multifunctional materials in field-effect transistors with light-emission, light-detection and neuromorphic characteristics

Open-shell molecules such as organic radicals are attracting attention thanks to their optical, magnetic and electrical properties that make them appealing as active materials in optoelectronic devices. However, the use of open-shell molecules as active species into transistor-based optoelectronic devices has still to be fully exploited. In this thesis, both light-emitting and light-sensing organic field-effect transistors based on organic radicals are engineered and investigated. In organic light-emitting transistors (OLETs) luminescent mono-radicals and biradicals are successfully used as emitters in ad-hoc developed unipolar n-type transistor architecture. Indeed, electroluminescence from doublet excitons unaffected by spin limitation (in the case of mono-radical) and from zwitterionic excited states with high values of photoluminescence quantum yield in solid state (in the case of biradical) are reported in OLETs. The mechanisms at the basis of excitons formation under electrical excitation are analyzed by means of time-resolved electroluminescence measurements, which demonstrated an efficient electron trapping in radical-based emissive layer in OLETs. At this regard, the tendency of the adopted radical emitters to trap electrons improves the charge recombination efficiency, thus resulting to be beneficial for the OLET’s operation. In the second part of the thesis, open-shell molecules are used as acceptors in photoactive bulk heterojunctions of all solution-processed organic phototransistors (OPTs) sensitive to near-infrared (NIR) light. In OPTs, the well-assessed tendency to trap electrons of radicals is exploited to boost the photo-gain effect. As a result, radical-OPTs exhibit state-of-the-art photosensitivity values in the NIR region. Moreover, the long-living trapped electrons in radical sites enables neuromorphic characteristics in the OPT upon patterned light stimulation, which resulted in synapse-like neuroplasticity. Finally, dual mode operation is obtained by the integration of a ferroelectric polymer as the dielectric layer, so that neuromorphic modulation of the channel current in the device can be obtained also through electrical inputs at the gate electrode.