Organic photovoltaics (OPVs) have garnered significant attention due to their advantageousfeatures, including tunable light absorption properties, rapid large-area fabrication, and compatibility with flexible substrates. Despite these benefits, the low long-term stability and relatively modest power conversion efficiency (PCE) continue to hinder their practical application and commercialization. This dissertation investigates the mechanisms of photoactive network formation and singlet fission in OPVs, providing comprehensive insights into these processes.

In Chapter 2, the mechanism of photoactive network formation in OPVs was studied usingreal-time analysis. It addresses the persistent issue of aggregation in non-fullerene acceptors within bulk-heterojunction. To tackle this challenge and control the network formation process, a solid additive was introduced and compared with a conventional additive. In situ UV-Vis absorption spectroscopy was employed to monitor the evaporation and solidification dynamics of the photoactive solution. The findings demonstrated that solutions containing high boiling point additives exhibited prolonged evaporation and solidification periods, resulting in densely packed molecular structures. Moreover, the attractive interaction of the additive with non-fullerene acceptor molecules induced more uniform distribution of the acceptor molecules with less aggregation in the photoactive network.

Chapter 3 focused on visualizing photoactive domains, marking a significant advancementin the field. Nanomechanical mapping using atomic force microscopy was performed to distinguish donor and acceptor domains through Derjaguin-Muller-Toporov modulus and dissipation energy measurements. This domain visualization allowed for an analysis of the final morphology of the photoactive layer, influenced by the additives discussed in Chapter 2. The results indicated that the additive induced a greater concentration of acceptors at the top surface and more uniform distribution within the bulk of the photoactive layer. This led to faster and more efficient charge extraction and transport, ultimately enhancing long-term stability and PCE.

In Chapter 4, the charge multiplication mechanism of singlet fission in a new photoactivematerial was studied to surpass the theoretical single-junction PCE limit of 34% set by the Shockley-Queisser model. The new singlet fission material overcame the limitations of existing systems, which relied on vacuum deposition and bilayer structures, by synthesizing a solution processable pentacene polymer. Material characterization revealed that increased intermolecular coupling in the pentacene polymer improved singlet fission efficiency through more orderly molecular packing and higher concentration. Subsequent device testing demonstrated that higher molecular weight pentacene polymers, combined with solvent vapor annealing treatment, significantly enhanced PCE. Additionally, the polymer acceptor photoactive material exhibited superior fill factors compared to small molecule non-fullerene acceptors with less molecular aggregation, leading to notably higher PCE.

Overall, the studies presented here encompass foundational research on the mechanisms ofphotoactive network formation and novel methodologies for photoactive domain mapping, along with an in-depth exploration of singlet fission mechanism in an innovative photoactive material. These research directions collectively contribute valuable insights for improving the long-term stability and PCE of OPVs. Based on the findings of this work, Chapter 5 presents an outlook and future directions for advancing OPV technology.