Characteristic Control of n-Channel Organic Thin-Film Transistors Using a Dimethyl-Substituted Benzimidazole Dopant

4-(N,N-Dimethylamino)phenyl-substituted 1,3-dimethyl-2,3-dihydro-1H-benzimidazole (N-DMBI-H) has been utilized as a solution-processable n-type dopant in organic electronics. In this study, a dimethyl-substituted N-DMBI-H derivative (DMe-N-DMBI-H), in which two methyl groups are attached at the terminal 5- and 6-positions of the benzimidazole moiety of N-DMBI-H molecule, has been examined to control its electron-donating ability. The effectiveness of DMe-N-DMBI-H as a solution-processable donor dopant has been clarified by evaluating electrical characteristics of DMe-N-DMBI-H-doped PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) thin-film transistors, such as field-effect mobility, gate threshold voltage, and contact resistance. Our electrochemical and electrical characterizations as well as quantum chemical calculations have suggested that DMe-N-DMBI-H works as a donor dopant somewhat stronger than N-DMBI-H.