Solution-processed organic materials are promising alternative to traditional solar cells based on silicon. Organic solar cells (OSCs) are attractive due to their low cost, light weight, non-toxicity, flexibility, and roll-to-roll large area fabrication. Recently, the performance of bulk-heterojunction (BHJ) OSCs based on non-fullerene acceptor molecules have significantly improved over their fullerene-based counterparts. Crucially, these non-fullerene OSCs show high charge photogeneration yields without requiring a large energy offset at the donor/acceptor (D/A) interface, which reduces the open-circuit voltage loss and enables higher power conversion efficiency.

This thesis focuses on understanding the underlying charge generation mechanism in high-performance non-fullerene OSC...[ Read more ]

Solution-processed organic materials are promising alternative to traditional solar cells based on silicon. Organic solar cells (OSCs) are attractive due to their low cost, light weight, non-toxicity, flexibility, and roll-to-roll large area fabrication. Recently, the performance of bulk-heterojunction (BHJ) OSCs based on non-fullerene acceptor molecules have significantly improved over their fullerene-based counterparts. Crucially, these non-fullerene OSCs show high charge photogeneration yields without requiring a large energy offset at the donor/acceptor (D/A) interface, which reduces the open-circuit voltage loss and enables higher power conversion efficiency.

This thesis focuses on understanding the underlying charge generation mechanism in high-performance non-fullerene OSCs. Working closely with my collaborators, I use transient optical spectroscopy and other supplementary techniques to comprehensively study the charge dynamics in non-fullerene OSCs, as well as dependence on temperature and applied electric field. We find that in contrast to most fullerene-based OSCs, the charge transfer excitons (CTEs, or called charge transfer states) in high-performance non-fullerene OSCs have similar energy as the excitons, and are long-lived at the D/A interface. The free charge generation process from CTEs in non-fullerene OSCs is strongly dependent on thermal energy, and can be assisted by applied electric field. A quasi-equilibrium of excitons, CTEs, and free charges can form. We also characterize the influence of exciton diffusion length in a state-of-the-art non-fullerene acceptor material, as well as the role of electron/hole transport interlayers in reducing charge recombination and improve charge extraction.

This thesis provides a fundamental physical model of thermally-activated and field-assisted charge photogeneration in non-fullerene OSCs, and shows the importance of long exciton lifetime in active layer and the utilization of interlayers for achieving high-performance solar cells. Our work helps to design next-generation highly-efficient non-fullerene OSCs with very low voltage losses. The experimental and analysis methods can also be applied to research in other novel optoelectronic material/device fields.