Triplet-Polaron Interaction Induced Upconversion from Triplet to Singlet: a New Way to Obtain Highly Efficient OLEDs

TL;DR

This paper introduces a novel triplet-polaron interaction mechanism that enables efficient triplet-to-singlet upconversion in OLEDs, surpassing traditional efficiency limits and achieving record-high EQE in deep-blue devices.

Contribution

It proposes and experimentally validates a new triplet-polaron interaction mechanism for triplet-to-singlet upconversion in OLEDs, breaking the 25% efficiency limit.

Findings

Achieved a maximum EQE of 6.8% in deep-blue OLEDs.

Eliminated thermally activated delayed fluorescence and triplet-triplet annihilation as dominant processes.

Proposed one-electron transfer mechanism for triplet-to-singlet upconversion, supported by experiments and computations.

Abstract

The triplet harvesting is a main challenge in organic light-emitting devices (OLEDs), due to the radiative decay of triplet is spin-forbidden. Here, we designed and synthesized two D-A type molecules, TPA-TAZ and TCP. The OLEDs based on them exhibit deep-blue emission and the singlet formation ratios are higher than the simple spin-statistics of 25 %. Specially, a TPA-TAZ-based OLED achieves a maximum EQE of 6.8 %, which is the largest value of the undoped OLEDs with CIE(y)< 0.06 (the EBU blue standard) up to date. Comprehensive experiments eliminate the triplet-harvesting processes of thermally activated delayed fluorescence and triplet-triplet annihilation. Instead, the triplet-polaron interaction induced upconversion from triplet to singlet through one-electron transfer mechanism is proposed, and proven by the magneto-current measurement and quantum chemistry computation. Our results may offer a new route to break through the 25 % upper limit of IQE of fluorescent OLEDs, especially, the deep-blue fluorescent OLEDs.