Single-Carrier Devices for the Understanding of the Voltage Drift in Organic Light Emitting Diodes
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Single-Carrier Devices for the Understanding of the Voltage Drift in Organic Light Emitting Diodes J. Boizot1, V. Gohri1 and H. Doyeux1 Laboratory of Technology and Components for the Visualization, Optronic Department, CEALETI Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France 1
ABSTRACT The aim of this study is to analyze and mitigate the voltage drift phenomenon observed in top-emitting organic light emitting diodes (OLED) when driven at constant current. An operating device may experience voltage increase over time due to factors such as interface or bulk material degradation, charge accumulation and formation of trap states. Single-carrier devices were fabricated to understand the contribution to voltage drift from each of these causes. Doping in electron injection layer (4, 7-diphenyl-1,10-phenanthroline or Bphen) and hole injection layer (2,2’,7,7’-tetra(N,N-di-tolyl)amino-spiro-bifluorene or Spiro-TTB) were optimized to obtain ohmic injection contacts. Devices with tris(8-hydroxy-quinoline) aluminium (Alq3) degrade significantly with holes injection and undergo high voltage increase in lifetime test measurements. On the contrary, devices with N,N’-di(naphtalen-1-y1)-N,N’-diphenyl-benzidine (NPB) exhibit an ambipolar charge transport behavior and low voltage drift under both hole and electron injection. INTRODUCTION Although the lifetime and reliability of organic light-emitting diodes (OLED) have been improved significantly since the first reported OLEDs by Tang and Van Slyke [1,2], a complete understanding of the device degradation mechanisms remains elusive. When driven at constant current, electroluminescence efficiency tends to decrease and driving voltage tends to increase. This study is meant to target the origin of voltage drift observed in a typical top-emitting stack during operation. The charge balance is crucial parameter for device stability [3]. An excess of one kind of charge carrier would lead to low device efficiencies. This excess does not contribute to radiative recombinations, rather it can just pass through the thickness of the device. This can lead to deleterious effects such as degradation of organic materials, interfaces, charge accumulation and trap formation. Thus, the choice of charge transport, injection and blocking materials is very important for maximizing radiative recombinations and achieving highlyefficient and long-lifetime devices. In the present work, we studied charge transport in some common small molecule materials by fabricating simple devices. The doping for hole and electron transport layers was optimized for achieving high charge mobilities and ohmic injection contacts. Further, the charge blockers NPB and Alq3 have also been studied to ascertain their behavior to both electrons and holes. EXPERIMENT Electron and hole only devices are done using doped charge injection layers. For the electron and hole transport layers, Bphen is doped with an alkaline earth metal M and STTB is doped with F4-TCNQ (2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane) respec
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