Molecular packing-dependent photoconductivity in functionalized anthradithiophene crystals

We relate features observed in time-resolved photocurrent with crystal structure and disorder in crystals of two anthradithiophene (ADT) derivatives with different packing motifs, diF TES-ADT and diF TSBS-ADT. A factor of ∼10–12 higher peak photocurrent was obtained in the diF TES-ADT crystals characterized by a 2D “brickwork” packing as compared to diF TSBS-ADT with a 1D “sandwich-herringbone” packing, primarily due to differences in intrinsic charge carrier mobility. A change from shallow trap-mediated thermally activated regime at 230–300 K (200–300 K), with a zero-electric field activation energy of ∼40–50 meV, in the TES (TSBS) derivative to the temperature independent behavior at lower temperatures in both derivatives was observed. The shallow traps were attributed to structural defects associated, for example, with a solid-solid phase transition in the TES derivative. In diF TES-ADT, most charge trapping in shallow traps occurred on <0.4 ns time scales at all temperatures (80–300 K) and the carriers that were mobile at 0.4–10 ns time scales at 300 K remained mobile at 80 K. In contrast, shallow trapping in the diF TSBS-ADT crystals proceeded on the nanosecond time scales, with a pronounced loss of mobile carriers at low temperatures. This was followed by deep trapping at longer time scales, which led to a factor of ∼250 lower steady-state photocurrent obtained under continuous wave excitation in the diF TSBS-ADT crystals as compared to diF TES-ADT.