Development of semi-crystalline conjugated polymers for efficient organic solar cells and organic electrochemical transistors

Abstract

Organic semiconductors have attracted great attention as alternatives that can potentially replace inorganic semiconductors (e.g., silicon) owing to their advantages such as flexibility, light-weight, and solution processability for large-scale production. However, in contrast to inorganic materials that form lattice structures with strong bonding forces, organic materials rely on weaker van der Waals forces, which lead to a greater degree of conformational freedom and a looser microstructure ranging from amorphous to semi-crystalline phases. As a result, organic materials inherently possess multi-scale structural defects and resultant energetic disorders, which are undesirable for ensuring high electronic charge carrier mobility. Therefore, to develop efficient organic semiconductors for use in various electronic devices, it is crucial to achieve a high crystallinity and improve structural/energetic orderings. In this thesis, it is aimed to develop highly ordered organic semiconductors for realizing high-performance solar cells and electrochemical transistors. Organic solar cells (OSCs) are promising solar energy harvesting systems, where the active layers are composed of polymeric/small-molecular donors and acceptors. With the development of active materials, the power conversion efficiencies (PCEs) of OSCs have drastically increased to over 18%. However, in addition to photovoltaic performance, other requirements for OSC commercialization such as the feasibility for the large-scale synthesis of active materials should be simultaneously considered. In this regard, poly(3-hexylthiophene) and its derivative polythiophenes (PTs) have emerged as a promising class of polymer donors owing to their low-cost and straightforward synthesis. Despite these significant advantages, the PCEs of PT-based OSCs have generally remained low (i.e., below 10%) until the recent developments of PTs. In addition, the molecular structures of PTs have been relatively less explored than representative benzodithiophene-based donors, leaving room for further improvement in PT design. I have proposed a molecular design strategy to improve the electrical performance of PTs with simple molecular structures for the development of efficient and cost-effective OSCs. Organic electrochemical transistors (OECTs) utilizing electrolytes as gate dielectrics serve as electronic devices that not only amplify/switch signals but also transduce ionic/metabolic signals to electronic signals (and vice versa), which make themselves applicable in biosensors and neuromorphoic/synaptic devices and differentiated from other traditional transistors. To maximize these functionalities, it is essential to enhance the current flows in response to the changes in external conditions such as ion/metabolite concentration in electrolytes and voltage. Therefore, the active materials should exhibit ionic conductivity to accommodate ions from the electrolyte while possessing high electronic conductivity to enable greater current responses. I have demonstrated the molecular structural optimization of organic mixed ionic–electronic conductors to maximize polymer orderings and steady-state OECT performance.