UC Santa Barbara

Organic semiconductors have been widely applied to electronic devices including organic solar cells (OSCs), and organic photodetectors (OPDs) in the last decades for their lightweight, flexibility, and great industry potential for high throughput solution processing and roll-to-roll printing technologies. However, traditional solvents used for these organic photovoltaics processing conditions are commonly highly toxic halogenated organic solvents like chlorobenzene (CB), o-dichlorobenzene (o-DCB), and chloroform (CF) which are not only harmful to human health upon exposure but can also cause long-term effects on the ecosystem. Consequently, research on halogen-free/ “green solvents” (such as xylene, 2-methyltetrahydrofuran (2-MeTHF), methanol, water, etc.) processable organic semiconductors are essential to moving organic photovoltaics from bench chemistry towards real-life applications. In this thesis, three studies are carried out to enhance the sensing performance, as well as to better understand the device physics of bulk heterojunction (BHJ) OPVs and OPDs. The design, fabrication, optimization, as well as characterization of green-solvent processing organic semiconductors, have been discussed in detail including OPV and OPD systems. The characterization of the solid-state properties of the optimized active layers in these organic electronic devices is further included. The bulk-heterojunction (BHJ) thin films processed from green solvents were characterized with different techniques at different length scales, including atomic force microscopy (AFM), transmission electron microscopy (TEM), grazing-incidence wide-angle X-ray scattering (GIWAXS) and solid-state nuclear magnetic resonance (ssNMR) spectroscopy. The first study gains insights into the structure–processing–property correlation of a PPDT2FBT: PC61BM bulk-heterojunction (BHJ) system processed from a green solvent, ortho-xylene (o-XY). This system is investigated in comparison with the same blend processed from a traditional halogenated solvent, chlorobenzene (CB). The optimized PPDT2FBT:PC61BM devices processed from o-XY can achieve a noteworthy higher power conversion efficiency (PCE) owing to a higher short-circuit current density and fill factor. The second study develop a series of 2-MeTHF processing OPV systems based on newly designed bulk heterojunction (BHJ) consisting of PM7-D3/D5 donors and Y-series non-fullerene acceptors with systematically changing in their side chain length and positions. The optimized 2-MeTHF casting device based on PM7-D3:PTI04 exhibits the best PCE of approaching 15%, which is comparable with the well-known PM6:Y6 system processing from traditional halogenated solvents. The third study reports a designed bulk heterojunction (BHJ) consisting of PM7-D5 donor and Y12 non-fullerene acceptor processed from 2-methyltetrahydrofuran (2-MeTHF) sets a record in light detectivity, which is also comparable with commercially available silicon-based photodiodes. Newly designed PM7-D5:Y12 OPD can be employed in wearable self-powered devices to monitor heart rate and blood oxygen saturation. This development of a framework for a detailed understanding of the structure-processing-property relationship provides insight into the mechanisms that lead to the improved performance of green-solvent-processed organic electronics.