Molecular Orientation and Femtosecond Electron Transfer Dynamics in Halogenated and Nonhalogenated, Eco-Friendly Processed PTB7-Th, ITIC, PTB7-Th:ITIC, and PTB7-Th:PCBM Films

Abstract

Synergistic effects of the electronic structure and morphology in the PTB7-Th polymer and the nonfullerene acceptor ITIC molecule resulted in state-of-art systems for organic solar cells, achieving over 12% power conversion efficiency (PCE) values. Furthermore, the substitution of harmful halogenated solvents to environmentally friendly solvents that do not compromise PCE values has become a subject of great interest in recent literature. In this work, we employed sulfur K-edge Angle-Resolved Near-Edge X-ray Absorption Fine Structure, Resonant Auger Spectroscopy, and the core-hole clock spectroscopy in an attempt to gain information on the effect of different processing methods on the electronic and morphological properties of these systems. Comparison with the PTB7-Th:PCBM blend, which uses a conventional acceptor molecule, was also performed. Steady-state and time-resolved photoluminescence results were obtained for neat PTB7-Th and blends with ITIC and PCBM. Theoretical calculations were performed utilizing restricted-excitation-window time-dependent density functional theory. Our results shows that ortho-methylanisole is a potential replacement to the conventional halogenated solvent ortho-dichlorobenzene, as they show that in terms of morphology and charge transfer dynamics, no expressive changes occurred by varying the solvent and by thermal annealing.