Influence of vacuum chamber impurities on the lifetime of organic light-emitting diodes.

Hiroshi Fujimoto,Kaori Nagayoshi,Hong Wei,Satoshi Yukiwaki,Masayuki Yahiro,Takashi Suekane,Katsuya Imanishi,Chihaya Adachi

doi:10.1038/srep38482

Abstract

We evaluated the influence of impurities in the vacuum chamber used for the fabrication of organic light-emitting diodes on the lifetime of the fabricated devices and found a correlation between lifetime and the device fabrication time. The contact angle of the ITO substrates stored the chamber under vacuum were used to evaluate chamber cleanliness. Liquid chromatography-mass spectrometry was performed on Si wafers stored in the vacuum chamber before device fabrication to examine the impurities in the chamber. Surprisingly, despite the chamber and evaporation sources being at room temperature, a variety of materials were detected, including previously deposited materials and plasticizers from the vacuum chamber components. We show that the impurities, and not differences in water content, in the chamber were the source of lifetime variations even when the duration of exposure to impurities only varied before and after deposition of the emitter layer. These results suggest that the impurities floating in the vacuum chamber significantly impact lifetime values and reproducibility.

Highlights

The development of second-generation emitter materials based on phosphorescence[4] and, more recently, third-generation emitters based on thermally-activated delayed fluorescence (TADF), with TADF emitters eliminating the need for the rare metals used in phosphorescent materials[5,6]
We found that a major cause of batch-to-batch variation in the lifetime of Organic light-emitting diodes (OLEDs) is differences in the device fabrication times
Our results suggest that extremely small amounts of impurities at the emission layer (EML) interfaces can greatly influence lifetime

Summary

Introduction

The development of second-generation emitter materials based on phosphorescence[4] and, more recently, third-generation emitters based on thermally-activated delayed fluorescence (TADF), with TADF emitters eliminating the need for the rare metals used in phosphorescent materials[5,6]. A long lifetime is critical for commercial OLEDs, so research to develop durable device structures, reduce driving current by improving light out-coupling efficiency[7,8,9,10,11,12,13,14], and understand degradation mechanisms is in progress worldwide[15,16,17,18,19,20,21,22,23,24,25,26,27,28,29]. Residual water in the vacuum chamber during fabrication[24,25], which affects the interface between the hole transport layer and the emission layer, has been shown to reduce lifetime by participating in degradation-inducing electrochemical reactions with the organic materials. Even with the multitude of potential contamination sources in the vacuum chamber, the influence of such impurities on OLED lifetime has yet to be studied in detail

Methods

Results

Conclusion