Role of Singlet and Triplet Excitons on the Electrical Stability of Polymer Light‐Emitting Diodes

Bas Vander Zee,Paul W M Blom,Lay‐Lay Chua,Peter K H Ho,Sarina Paulus,Gert‐Jan A H Wetzelaer,Rui‐Qi Png

doi:10.1002/aelm.202000367

Abstract

AbstractStability under electrical stress is an important aspect for the function of organic light‐emitting diodes (OLEDs). Degradation is currently one of the key topics in this field, concerning all types of OLEDs, including fluorescent‐, phosphorescent‐, and thermally activated delayed fluorescence‐based OLEDs. For single‐layer polymer light‐emitting diodes (PLEDs) it has recently been found that degradation is the result of hole trap formation due to exciton–polaron interactions. However, whether singlet or triplet excitons are responsible for degradation is an open question. Here, their contributions are disentangled by systematically manipulating the singlet and triplet exciton populations and their effect on PLED degradation is studied. To control singlet excitons the emission of a blue‐emitting PLED is modified to green by adding a small amount of a perylene‐monoimide based green‐emitting dye. The triplet population is manipulated by blending the light‐emitting polymer with a dye that has either a longer or shorter triplet lifetime as compared to the polymer host. The results reveal that the degradation in fluorescent PLEDs is governed by the interaction between polarons and triplet excitons.

Highlights

Stability under electrical stress is an important aspect for the function of organic light-emitting diodes (OLEDs)
The results reveal that the degradation in fluorescent polymer light-emitting diodes (PLEDs) is governed by the interaction between polarons and triplet excitons
In this work we experimentally show, with the support of numerical modelling, that triplet excitons rather than singlet excitons are responsible for the intrinsic degradation of PLEDs

Summary

Introduction

Stability under electrical stress is an important aspect for the function of organic light-emitting diodes (OLEDs). Degradation is currently one of the key topics in this field, concerning all types of OLEDs, including fluorescent-, phosphorescent-, and thermally activated delayed fluorescence-based OLEDs. For single-layer polymer light-emitting diodes (PLEDs) it has recently been. Small-molecule organic LEDs exploiting triplet excitons using phosphorescent molecules (ph-OLEDs) or thermally activated delayed fluorescence (TADF) clearly outperform fluorescent PLEDs in terms of efficiency. To reach high efficiencies, both ph-OLEDs and TADF based OLEDs generally have complex singlet excitons the emission of a blue-emitting PLED is modified to green by device architectures comprising different adding a small amount of a perylene-monoimide based green-emitting dye. The multilayer TADF and phosphorescent OLEDs as well as single-layer PLEDs all show typical degradation features of a voltage increase and a luminance decrease under continuous electrical operation.[4] For multilayer OLEDs the physical processes. Introduction behind degradation are hard to elucidate due to the presence of many materials and interfaces.[5,6] The standard PLED device

Results

Discussion

Conclusion