Sequential Codoping Making Nonconjugated Organic Radicals Conduct Ionically Electronically

Mixed conduction through both ionic and electronic pathways in an organic radical has received enormous attention recently, owing to its high conductivity and exceptional processibility amenable to future organic electronics. While the majority of previous works have centered on polymeric systems, the study on the mixed conduction in a small molecular radical has gained less attention despite its enormous potential. Herein, a study on the mixed conduction behavior of such system, 4-substituted 2,2,6,6-tetramethylpiperidyl-1-oxy (4-hydroxy TEMPO, HT), via sequential codoping with an ionic dopant, lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI, LT), and an electronic dopant, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ, FT), is presented. It is found that the coupling between the components plays an important role in determining the total conductivity, in which a maximum conductivity of ≈10^-4 S cm^-1 was obtained for a HT/LT/FT mixture. A systematic study to connect the physical changes associated with doping and the observed mixed conductivity is provided. It is believed that these findings establish a starting point to study mixed conduction behaviors in small molecular organic radical systems in general, ultimately targeting next-generation organic electronic devices and batteries.