Molecular design strategy for high EQE and low efficiency roll-off TADF organic light emitting diodes

Abstract

Purely organic thermally activated delayed fluorescence (TADF) emitters are cost-effective alternatives to the rare-earth-metal-based phosphorescent emitters, offering high exciton utilization for advanced optoelectronic applications. However, organic light-emitting diodes (OLEDs) using TADF emitters often experience significant efficiency roll-off at higher brightness levels, thereby reducing their energy efficiency in practical applications. My research focuses on developing new TADF emitters for high external quantum efficiency (EQE) OLED devices and reducing efficiency roll-off through suitable molecular design modifications.

In the first chapter, I investigated the role of molecular flexibility on OLED device performance by synthesizing three organic emitters: 4BPy-PTA, 4BPy-PBPA, and 4BPy-PMPA. Despite having a large ΔEST in the rigid host matrix, all three emitters showed enhancement in PLQY under vacuum, suggesting the triplet contribution in the radiative decay process. However, no delayed fluorescence emissions were observed in the transient PL measurements. The fabricated OLED devices of 4BPy-PTA, 4BPy-PBPA, and 4BPy-PMPA exhibited EQEmax of 6.5, 17.3, and 16.2%, respectively, which exceeded the theoretical limit of fluorescent emitters. After a careful transient lifetime measurement without using a neutral density filter, weak delayed emissions were detected, demonstrating examples of hidden TADF emitters.

In the second chapter, I have designed a series of TADF emitters, namely 3BPy-mDPXZ, 3BPy-mPXZ, 3BPy-pPXZ, and 4BPy-pPXZ, by modifying the number and position of a strong donor 10H-phenoxazine unit, which helps in reducing ΔEST. The para derivatives exhibit higher PLQY (74 and 64%) compared to the meta ones (9 and 18%), due to their better electronic conjugation. OLED devices of 3BPy-pPXZ (17%) and 4BPy-pPXZ (24%) showed higher EQEmax than 3BPy-mDPXZ (1.5%) and 3BPy-mPXZ (2.7%). A close analysis of para donor derivatives revealed that 3BPy-pPXZ is ideal for low efficiency roll-off, while 4BPy-pPXZ provides high EQE. The strong efficiency roll-off in the para derivative is likely due to its higher delayed fluorescence component.

In the third chapter, a rigid planar group “triazatruxene/ TAT,” was utilized as the donor unit for the design of TADF emitters TAT-(4BPy), TAT-2(4BPy), and TAT-3(4BPy). The synthesized emitters exhibited a systematic increase in the locally excited character of their phosphorescence spectra as the number of acceptors increased. All three emitters demonstrated sub-microsecond delayed lifetimes (0.85-0.26 μs) and high PLQY (100-83%). TAT-(4BPy), TAT-2(4BPy), and TAT-3(4BPy) showed EQEmax of 29, 25, and 20%, respectively. Among the three emitters, TAT-2(4BPy) exhibited the least efficient roll-off (6.5% @ 1000 cd/m2), simultaneously maintaining a high EQEmax. This is due to its faster reverse intersystem crossing rate and lower delayed fluorescence contribution to radiative decay.

In the fourth chapter, I investigated the moisture sensitivity of 4BPy-PTA and 2BPy-PTA, which are isomers differing in the position of the pyridine nitrogen. A controlled study revealed that 4BPy-PTA, with the pyridine “N” at the 4th position exhibits greater moisture sensitivity than 2BPy-PTA having pyridine “N” at 2nd position. The former emitter responds more rapidly to moisture and shows a larger shift in PLmax. Both DFT calculations and crystal structure analysis suggest that the strong intramolecular N-H interaction in 2BPy-PTA contributes to its weaker moisture sensitivity.