Aza-substitution on naphthalene diimide-based conjugated polymers for n-type bottom gate/top contact polymer transistors under ambient conditions

Abstract

Four conjugated polymers based on naphthalene diimide (NDI) as the acceptor and thiophene–phenyl–thiophene with different aza-substitutions as co-units were synthesized. The aza-substitution gradually lowered the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. The polymer with four aza-substitutions exhibited the deepest LUMO/HOMO energy level (−4.10/−6.01 eV). Consequently, organic field-effect transistors (OFETs) based on the bottom-gate/top-contact configuration exhibited unipolar electron transport characteristics even if the devices were exposed directly to ambient conditions. Furthermore, the corresponding devices also maintained their n-type transport behavior during long-term storage in air (>300 days). This work demonstrated that the incorporation of nitrogen atoms into NDI-based conjugated polymers for further lowering the energy levels is a feasible strategy for the construction of air-stable n-type unencapsulated OFETs.