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 × 108 (±5 × 107) M–1 s–1 is calculated, which indicates a small reorganization energy of 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.