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

Two carboline derivatives 5-(30-(9H-carbazol-9-yl)-[1,10-biphenyl]-3-yl)-5H-pyrido[3,2-b] indole (CzBPDCb) and 9-(30-(9H-carbazol-9-yl)-[1,10-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 (iota, 1.05 ms) and a prompt decay component (iota, 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 d-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.