Charge Transport in Ternary Charge‐Transfer Solid Solution Single Crystals

Abstract The electronic effects of structural defects introduced through chemical doping are challenging to characterize in organic semiconductors, especially when measured in thin film devices where the performance is sensitive to structural heterogeneity. Here, a simple approach is presented to probe the roles of indirect and direct electronic coupling on charge transport in a series of compositionally varied charge‐transfer single crystals. In this system, carbazole (CBZ) is controllably substituted with N ‐methylcarbazole (NMCBZ) in the cocrystal formed between CBZ and 1,3,4,5,7,8‐hexafluoro‐11,11,12,12‐tetracyano‐2,6‐naphthoquinodimethane (F 6 TNAP), producing a series of single crystals with compositions that range between 0 – 50% CBZ replacement and preserve the structure type of the parent cocrystal. Gas‐phase electronic structure calculations predict that substitutional replacement of CBZ with NMCBZ introduces two competing effects: ( i ) strengthening of indirect coupling by increasing the average degree of charge transfer and ( ii ) weakening of direct exchange by increasing the distance between adjacent charge‐transfer π‐stacks. Charge transport measurements reveal an initial decrease in the mobility upon substitution of CBZ with NMCBZ, rationalized by a combination of hole‐trapping and weakened direct coupling with increasing NMCBZ content. Critically, these results demonstrate the potential for solid solutions to offer insight into charge transport mechanisms and their chemical tunability in molecular electronic materials.