Real-Time Observation of the Diffusion Mechanism Progression from Liquid to Solid-State of Transition Metal Complexes

Copper complexes have recently shown remarkable performance upon conversion from liquid-based to solid-state hole transport materials (HTMs) in mesoscopic solar cells, however the diffusion mechanism is not clear. In this work we apply an in-situ solidification analysis of the charge diffusion and find that the dominant mechanism of [Cu(dmbpy)2]2+/+ (dmbpy = 6,6′-dimethyl-2,2′-bipyridine) changes from ionic to electronic diffusion. Through use of the modified Dahms-Luff equation, a fast self-exchange rate constant of hole-hopping in the HTM of (8.3 ± 0.5) × 108 M-1s-1 is calculated, which indicates a small reorganization energy of approximately 0.47 eV. These results introduce a new methodology to analyze the transport mechanism of solids, reveal the mechanism of charge transport in molecular-based HTMs, and offer insight into ways to control the flow of charge in optoelectronic systems.