Suppressing efficiency roll-off of orange-red thermally activated delayed fluorescence polymer-based OLEDs via copolymerizing co-hosts with cascade energy levels

Abstract

Thermally activated delayed fluorescence (TADF) polymers are key to advancing organic light-emitting diodes (OLEDs) by utilizing 100% of excitons. This study focused on designing high-performance orange-red TADF polymers by incorporating carbazole with a dibenzothiophene (DBT) or dibenzofuran (DBF) co-host as the polymeric backbone with cascade energy levels. By manipulating the attachment sites of these units within the polymer backbones, four series of polymers were synthesized, and their photophysical and electrochemical properties were thoroughly investigated. The influence of sulfur in DBT and oxygen in DBF on exciton dynamics was found to be critical, in which the heavier mass sulfur atom enhanced spin–orbit coupling, contributing to a higher reverse intersystem-crossing (k_RISC) rate of 5.13 × 10⁵ s⁻¹ for the polymer pDBT3705. The prepared polymers pDBT3705 and pDBF3705 displayed relatively high photoluminescence quantum efficiencies of 63% and 53% and achieved maximum external quantum efficiencies (EQEs) of 17.1% and 14.2%, respectively. Moreover, pDBT3705 maintained an EQE of 13.0% at 500 cd m⁻², which is one of the highest reported EQEs at 500 cd m⁻² for orange-red TADF polymers. These findings underscore the role of co-hosts in attaining high-efficiency orange-red TADF polymers and demonstrate the importance of backbone engineering for achieving OLED advancements.