Thermally activated delayed fluorescence of co-deposited copper(i) complexes: cost-effective emitters for highly efficient organic light-emitting diodes

Abstract

Two carboline derivatives 5-(3′-(9H-carbazol-9-yl)-[1,1′-biphenyl]-3-yl)-5H-pyrido[3,2-b]indole (CzBPDCb) and 9-(3′-(9H-carbazol-9-yl)-[1,1′-biphenyl]-3-yl)-9H-pyrido[2,3-b]indole (CzBPCb) were employed as both a ligand and a host and co-deposited with CuI to form emissive Cu(I) complexes. Their structure–property relationships were studied to demonstrate the strategy of achieving highly efficient organic light emitting diodes (OLEDs) with thermally activated delayed fluorescence (TADF) characteristics. Co-deposited CzBPDCb:CuI films showed photoluminescence at 520 nm with TADF features of a delayed decay component (τ, 1.05 μs) and a prompt decay component (τ, 5.11 ns). Their OLEDs exhibited a maximum external quantum efficiency (EQE) of 17.5%, a current efficiency (CE) of 53.84 cd A⁻¹, and a power efficiency (PE) of 48.31 lm W⁻¹. The device also showed a low efficiency roll-off of 48.1 cd A⁻¹ at 100 cd m⁻², 41.8 cd A⁻¹ at 1000 cd m⁻², and 38.5 cd A⁻¹ at 2000 cd m⁻², respectively. While as a comparison, the CzBPCb:CuI-based OLED only showed a maximum EQE of 3.24%, a CE of 8.58 cd A⁻¹, and a PE of 6.24 lm W⁻¹. CzBPDCb has a δ-carboline unit with the benefit of the nitrogen atom locating outside of the molecule, which could efficiently coordinate with CuI resulting in higher efficiencies. This promising result gives us a unique strategy to develop a series of TADF emitters by co-depositing a normal organic ligand and a copper source, resulting in cost-effective OLED fabrication.