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Diffusion mechanisms vary in charge transport materials during the solidification from liquid to solid but mechanisms have only been deduced intuitively, without experimental evidence. However, these hypotheses have limited precision when performing mechanistic analysis of new emerging materials emerge. For example, transition metal complexes (TMCs) have shown a remarkable improvement in their properties when converted from liquid to solid-hole transport materials (HTMs) in mesoporous solar cells but it has been difficult to reveal what change in mechanism led to the advancement in HTMs. Herein we have successfully observed the change of diffusion mechanism for the TMCs, Cu(dmby)2 2+/+ and Co(bpy)3 3+/2+, during the solidification by the in-situ solidification analysis. The modified Dahms-Luff equation, which was partially substituted by the viscosity term, was used to evaluate the overall mechanism, and it was confirmed that the calculated self-exchange rate constant (k ex) varies with phase and the viscosity-dependent value. Eventually, as the solidification progressed, the transport mechanism dominating Cu(dmby)2 2+/+ was changed from the ionic to the electrical diffusion, which led to the increase in the apparent diffusion coefficient (D app), indicating a high conductivity and a high k ex (8.7 x 108 M-1s-1) in the HTM. However, in the case of Co(bpy)3 3+/2+, the transport mechanism continued to be dominated by the ionic diffusion during the solidification progresses and the D app was reduced due to the viscosity increase. This change in the diffusion mechanism, due to the solidification of the electrolyte, led to the reduction in the reorganization energy of TMCs calculated by Marcus Theory. This in turn could also affect the heterogeneous charge transfer reaction with the electrode, which may indicate the fundamental cause of the change in the performance in the devices. The results of this study provide an important milestone in establishing the charge transport mechanism for solid-state TMCs. |
