Understanding Structure‒Property Relationships of Photoactive Materials for Efficient and Stable Organic Photovoltaics

Abstract

Organic photovoltaics (OPVs) have gained great attention as next-generation photovoltaic technologies due to their lightweight nature, mechanical flexibility, tunable optical properties, and compatibility with low-cost, solution-based fabrication. These advantages enable their use for both outdoor and indoor energy harvesting, such as powering low-power Internet of Things (IoT) devices. Over the past few decades, the remarkable efficiency improvements in OPVs, now exceeding 20%, have been driven primarily by advances in molecular design of photoactive materials. While the development of advanced p-type polymer donors and n-type non-fullerene acceptors (NFAs) has driven notable efficiency gains, key challenges remain in terms of device stability, green-solvent processability, and large-area scalability for practical applications. Achieving these advances demands a molecular- level understanding of how structural variations influence optoelectronic properties, blend morphology, and device performance. Therefore, rational design of both n-type and p-type materials is essential. A deep understanding of their structure‒property‒performance relationships is critical for realizing highly efficient and stable OPVs. In this dissertation, I aim to develop both n-type and p-type materials for efficient and stable OPVs, and to elucidate the structure‒property relationships that guide the rational design of next-generation organic semiconductors. Firstly, an asymmetric NFA named IPC-BEH-IC2F, incorporating a tricyclic pyrazine-based IPC unit, was designed to strengthen π-π intermolecular interactions and stabilize blend morphology. The resulting IPC-BEH-IC2F devices exhibited higher power conversion efficiency (PCE) and excellent long-term stability compared to the symmetric IC2F-BEH-IC2F-based devices. This demonstrates the effectiveness of asymmetric structural modification and the tricyclic IPC unit in enhancing crystallinity and suppressing Ag electrode-induced degradation pathways. Secondly, long alkyl chains and halogen-functionalized IPC-based end groups were introduced to finely tune light absorption, energy levels, and miscibility with polymer donors under non-halogenated solvent processing. Eight asymmetric NFAs—IPCnF-BBO-IC2X and IPCnCl-BBO-IC2X (where n = 1 and 2, X = F and Cl)—enabled efficient and additive-free devices processed from o-xylene. They achieved over 15% PCEs and maintained more than 94% of the initial performance over 2000 hours without encapsulation. These results demonstrate that incorporating halogenated IPC units into asymmetric NFAs provides an effective route to efficient, stable OPVs that are compatible with eco-friendly fabrication. Finally, PB2FQxn (n = 5, 10, and 15) terpolymers were developed via random terpolymerization of PM6 with a bulky quinoxaline-based B2FQx unit to achieve high-performance indoor OPVs (IOPVs). Incorporation of the B2FQx unit weakened pre-aggregation and improved miscibility with NFA L8-BO. This resulted in deeper highest occupied molecular orbital (HOMO) levels, reduced non-radiative energy losses, and enhanced VOC under indoor light conditions. As a result, PB2FQx15-based IOPVs achieve a PCE of up to 31% under LED illumination (1000 lx), outperforming the reference PM6-based IOPVs. Collectively, this dissertation highlights the importance of rational molecular design in developing efficient, stable, and environmentally sustainable OPVs. By systematically establishing correlations between molecular structure, morphology, and device performance, this work provides valuable design guidelines for next-generation photoactive materials that can achieve both high efficiency and long- term stability for practical outdoor and indoor applications.