Probe exciplex structure of highly efficient thermally activated delayed fluorescence organic light emitting diodes

Tzu-Chieh Lin,Shih-Hung Liu,Yi-Chen Liu,Ken-Tsung Wong,Pi-Tai Chou,Monima Sarma,Wei-Chieh Tang,Hsiu-Fu Hsu,Yi-Ting Chen,Pin-Yi Chiang,Ke-Ting Lin,Wei-Tsung Chuang,Wen-Yi Hung

doi:10.1038/s41467-018-05527-4

Abstract

The lack of structural information impeded the access of efficient luminescence for the exciplex type thermally activated delayed fluorescence (TADF). We report here the pump-probe Step-Scan Fourier transform infrared spectra of exciplex composed of a carbazole-based electron donor (CN-Cz2) and 1,3,5-triazine-based electron acceptor (PO-T2T) codeposited as the solid film that gives intermolecular charge transfer (CT), TADF, and record-high exciplex type cyan organic light emitting diodes (external quantum efficiency: 16%). The transient infrared spectral assignment to the CT state is unambiguous due to its distinction from the local excited state of either the donor or the acceptor chromophore. Importantly, a broad absorption band centered at ~2060 cm−1 was observed and assigned to a polaron-pair absorption. Time-resolved kinetics lead us to conclude that CT excited states relax to a ground-state intermediate with a time constant of ~3 µs, followed by a structural relaxation to the original CN-Cz2:PO-T2T configuration within ~14 µs.

Highlights

The lack of structural information impeded the access of efficient luminescence for the exciplex type thermally activated delayed fluorescence (TADF)
organic light emitting diodes (OLEDs) based on intramolecular TADF emitters with >30% external quantum efficiencies (EQE) have been reported[10,11]
The bipolar feature of CN-Cz2 is supported by the density function theory (DFT) calculation, revealing that highest occupied molecular orbital (HOMO) resides on the peripheral carbazole, while lowest unoccupied molecular orbital (LUMO) is located on the CNsubstituted carbazole core (Fig. 1b)

Summary

Introduction

The lack of structural information impeded the access of efficient luminescence for the exciplex type thermally activated delayed fluorescence (TADF). The characteristic transient IR spectra were acquired for CN-Cz2, PO-T2T, and CN-Cz2:PO-T2T exciplex, and they are shown in Fig. 2a (1200–1700 cm−1) and b (1700–3000 cm−1) at a delay time of 1.6 μs.

Results

Conclusion