UC Santa Barbara

The development of near-infrared (NIR) non-fullerene acceptors (NFAs) has led to a surge in research interest in semitransparent organic optoelectronics, such as solar cells and photodetectors. These technologies offer numerous advantages, including ultra-thin film processibility, synthetic flexibility, and low-cost solution-based techniques for potential industrial applications. However, the interaction between solvents and bulk-heterojunction (BHJ) components can significantly impact the film morphology and blends self-assembly. To achieve high efficiency in fullerene-free optoelectronics using halogenated solvents, there is a strong focus on processing the active layer from low-cost and eco-friendly solvents derived from renewable resources. Chlorine-free solvents such as toluene, xylene, tetrahydrofuran (THF), and 2-methyl THF are preferentially chosen for green solvent processed organic optoelectronics. The first two chapters of this thesis introduce the molecular engineering of narrow bandgap NFAs based on the original structure of published near-infrared absorber COTIC-4F molecule. The impact of side chain modification on the optoelectronic properties of NFAs built on the COTIC-4F conjugated framework has been examined and tested within the applications of corresponding organic solar cells and NIR-OPDs. Via modification on the center donor core, a set of C-O bridged NFAs CO6ICs have been successfully synthesized and applied into NIR-OPDs achieving high responsivity at NIR-region. Moreover, those set of small molecular NFAs have presented promising processibility in “green solvent” of 2-MeTHF. This leads to the following chapter of this thesis presenting a project investigating molecular engineering of green solvent processable A-D’-D-D’-A structured non-fullerene acceptors (NFAs). These NFAs exhibit high solubilities and potential processability in 2-MeTHF. By using DFT and HSP simulation of the small molecules, and various thin-film characterization techniques (UV-vis, AFM, GIWAXS) on the 2-MeTHF solvent-cast thin-films, a better understanding of the materials' chemical structural influences on NFAs' green solvent processibilities and their corresponding solid-state properties is achieved. The last chapter of this contribution focus on the molecular design and synthesis of a series of ultra-narrow bandgap NFAs and their efficient NIR-OPDs applications processed from non-halogenated solvent of toluene. Systematic modifications on the end-dye acceptor halogen substituents within the framework were made to investigate the influence of the electron negativity of terminated groups on the materials' optical properties and the corresponding device performance. Semitransparent organic optoelectronic devices were fabricated with the originally designed NFAs and processed using chlorobenzene and toluene as non-halogenated solvents for comparison. Toluene-based devices achieved higher specific detectivity due to the better quality of blend thin-film morphologies and more compacted crystallinity in the photoactive layer. This confirms that this novel series of DaTICs NFAs have satisfying chlorine-free solvent processibilities without sacrificing device performance.