Modulating charge transport and optoelectronic properties through N-substitution in annulated thiophene–phenylene oligomers

Abstract

The connection between the molecular structure and the (opto)electronic properties of organic semiconductors is crucial for their smart design, but it is still poorly understood. In this study, we computationally and experimentally address the impact of the substitution of S atoms by N–CH₃ groups in a partially annulated thiophene–phenylene co-oligomer, 2,7-bis(4-decylphenyl)[1]benzothieno[3,2-b][1]benzothiophene (DPBTBT), on the optical and electronic properties of isolated molecules and the charge transport and optoelectronic properties in thin films. The crystal structure of the N–CH₃ substituted oligomer 2,7-bis(4-decylphenyl)-5,10-dimethyl-5,10-dihydroindolo[3,2-b]indole (DPIDID) was resolved so that charge transport properties were calculated. The charge transport, electroluminescence (EL), and photoelectric properties were compared in thin-film organic field-effect transistors based on DPBTBT and DPIDID. Unexpectedly, DPIDID films showed electron transport despite the N–CH₃ substitution raising the LUMO energy of DPIDID. We assign this unexpected feature to an electrode–film interface dipole facilitating electron injection. Thus, in contrast to the hole-transporting DPBTBT films, the DPIDID ones demonstrated bipolar charge transport. Moreover, the DPIDID-based field-effect devices demonstrated a substantially higher EL efficiency assigned to the higher photoluminescence efficiency, bipolar charge transport and more favorable orientation of light-emitting dipoles. Finally, the photoresponsivity values of DPIDID and DPBTBT devices were compared. The obtained results are anticipated to facilitate the rational design of organic semiconductors for (opto)electronic applications.