Ester-substituted thiophene-fused benzothiadiazole as a strong electron acceptor to build D-A red emitters for highly efficient solution-processed OLEDs

Rational molecular design is critical to build high-efficiency red-NIR fluorescent emitters. In this work, a thiophene-fused benzothiadiazole (BTT) unit has been chosen as a strong electron acceptor to build donor-acceptor (D-A) type fluorescent emitters, in which the fusion of thiophene ring extends the conjugation system and the introduction of an ester group at alpha-position of thiophene ring strengthens the electron-withdrawing ability and simultaneously improves the solubility for solution-processed device fabrication. Herein, two new D-A luminescent molecules (BTTTPA and BTT2TPA) have been designed and synthesized by employing the triphenylamine (TPA) group as a donor. It has been found that both of them show unique characteristics of hybridized local and charge-transfer and aggregation-induced emission. Compared with BTTTPA containing only one TPA group, the introduction of two TPA groups into the BTT unit leads to BTT2TPA exhibiting much redder emission and higher photoluminescent quantum yield in the film. Solution-processed non-doped and doped organic light-emitting diodes based on BTTTPA and BTT2TPA were fabricated to evaluate the electroluminescent (EL) performances. Overall, BTT2TPA exhibited better EL performances than BTTTPA. The non-doped device based on BTT2TPA achieved deep-red emission with an EL peak at 666 nm, corresponding to the Commission International de L'Eclairage (CIE) coordinates of (0.67, 0.32), and a maximum external quantum efficiency (EQE) of 1.1%. Moreover, the doped device with BTT2TPA showed warm-white emission and a dominant peak at 632 nm and an improved EQE of 5.9% which surpassed that of the upper limit of conventional fluorescent devices.

Automated summary

In this study, thiophene-fused benzothiadiazole unit was used to build donor-acceptor type fluorescent emitters, with the introduction of triphenylamine group as donor resulting in two new luminescent molecules. It was found that BTT2TPA exhibited superior electroluminescent performances compared to BTTTPA, showing deeper red emission and higher quantum efficiency.