Luminescent 2D single crystals of thiophene–phenylene co-oligomers for field-effect devices

Abstract

Two-dimensional (2D) organic semiconductor macroscopic single crystals have recently demonstrated their excellent charge-transport properties competing with their 3D counterparts. However, the combination of efficient charge transport and prominent luminescent properties in an organic 2D material is still a challenge. Here, we report on 2D emissive semiconductor single crystals based on four and five-ring thiophene–phenylene co-oligomers with didecyl terminal substituents—DD-PTTP and DD-PTPTP, where P and T stand for phenylene and thiophene, respectively—and investigate how the insertion of a P-ring in the PTTP conjugated core impacts the molecular and various solid-state properties of the material. DD-PTTP and DD-PTPTP were synthesized and characterized, and then macroscopic 2D single crystals of both oligomers in the form of monolayer and few-layer films were grown from solution and studied by optical interference, photoluminescence (PL), and atomic-force microscopies. Polarization-sensitive PL microscopy revealed the orientation of the conjugated cores in 2D crystals relative to their facets and indicated that the molecules are H-aggregated. 2D organic field-effect transistors (OFETs) based on both oligomers showed good stability, and their performance nicely followed the Shockley model with a charge-carrier mobility of up to 0.2 cm² V⁻¹ s⁻¹, which is among the best values for various materials based on linearly conjugated oligomers. The highest charge-carrier mobility was demonstrated for the monolayer devices indicating high molecular order in the monolayer single crystals. 2D organic light-emitting transistors (OLETs) with DD-PTTP and DD-PTPTP active layers were demonstrated, and the DD-PTPTP-based 2D OFETs and OLETs showed higher performance. The latter was assigned to the stronger intermolecular interactions in DD-PTPTP stemming from its longer conjugation length despite the almost equal optical gaps of DD-PTTP and DD-PTPTP. The results obtained show that macroscopic 2D single crystals of thiophene–phenylene co-oligomers are a promising platform for 2D organic optoelectronics.