Molecular Design, Synthesis and Characterization of Organic Semiconductors for Applications in Photonics and Electronics

Doctoral thesis, 2025

The study of organic optoelectronic lies at the intersection between the fields of physics, chemistry, and materials science. Organic semiconductors based on benzodiquinoline (BDQ), benzothiadiazoledicarboximide (BTDI), diketopyrrolopyrrole (DPP), indacenodithiophene (IDT), naphthalenediimide (NDI), and quinoxaline (Qx) moieties have been designed for different devices, including organic photovoltaics (OPVs), light emitting electrochemical cells (LECs), organic electrochemical transistors (OECTs) and organic supercapacitors. Special efforts are devoted to developing materials that can be processed from aqueous-based solvents. Over sixty polymers and two small molecular electroactive materials are synthesized. The organic semiconductor characterizations are conducted using gel permeation chromatography, thermal gravimetric analysis, ultraviolet-visible spectroscopy (UV-vis), photoluminescence spectroscopy (PL), and cyclic voltammetry (CV) techniques. Nine BDQ-based polymers are synthesized and characterized. Two BDQ monomers are synthesized in simple and cost-efficient four steps. Five of the BDQ polymers from the dibrominated monomer are wide bandgap, suggesting a potential for applications in ternary OPVs. The tetra-brominated BDQ monomer is used to synthesize two-dimensionally networked thiophene- and BDT-based polymers. Two-dimensional networking results in bandgap lowering and decreased thermal stability. Three BTDI-based terpolymers are synthesized by incorporating small amounts of BTDI monomer as a second acceptor in the PM6 system. The resulting polymers demonstrate excellent thermal stabilities and suitable optical and electronic properties for applications as donors in OPVs. Fourteen DPP-based polymers are synthesized and characterized. Long alkyl spacers generally enhance the thermal stabilities of these polymers. OECT devices from the DPP polymers demonstrate typical OECT characteristics, with peak transconductance up to 2.34 S cm⁻¹ and 99.5% current retention after 315 s operation. Eight IDT-based polymeric and two aqueous processable small molecular organic semiconductors are synthesized and characterized. Two thiophene-based and one benzothiadiazole-based NDI polymers are synthesized. The thiophene-based polymers are processable from aqueous-based solvents. A fully stretchable supercapacitor device prepared from hydrogel using the NDI electroactive polymer-based composite achieves an areal capacitance as high as 29.84 mF cm-2 at a scan rate of 5 mV s-1. Twenty-four Qx-based polymers are synthesized and characterized. Seven of these are based on thiophene-quinoxaline (TQ) moieties, while two are derived from simple selenophene-quinoxaline (SeQ) moieties. Material synthesis cost analysis of both TQ and SeQ polymers reveals their promise as cost-efficient materials. An investigation on the Hansen solubility parameters is conducted. The OEG-decorated TQ and SeQ polymers were found to be soluble in aqueous solvents. Notably, the aqueous-processed all-polymer photovoltaic cells (aq-APPVs) based on the TQ polymer deliver high power conversion efficiency up to 2.27% and open-circuit voltage approaching 0.8 V, which are among the highest values reported for aq-APPVs to date. Additionally, LEC devices are fabricated from TQ polymers. The best device exhibits an emission peak at 670 nm with a peak radiance of 185 µW/m² at a low drive voltage of 2.3 V. Other LEC devices are fabricated using TQ as a host system, exhibiting near-infrared emissions with electroluminescence peaks beyond 900 nm. Overall, this work demonstrates the logical design, synthesis, and application of organic semiconductors for optoelectronic applications. Importantly, it contributes valuable insights into how functional and sustainable organic semiconductors can be designed.