Luminance degradation and recovery studies in Alq 3 based Organic light emitting diodes
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Luminance degradation and recovery studies in Alq3 based Organic light emitting diodes K. Sudheendra Rao1, 2, Girija S. Samal 2, Y. N. Mohapatra 1, 2, 3 1
Department of Physics, Indian Institute of Technology, Kanpur-208016, India. Samtel Centre for Display Technologies, Indian Institute of Technology, Kanpur-208016, India 3 Material Science Program, Indian Institute of Technology, Kanpur-208016, India. 2
Abstract Though organic light emitting diodes are being commercialized in many applications, issues relating to lifetime and degradation remain as fundamental concerns limiting performance. A coherent understanding of degradation mechanisms is yet to emerge. We focus on intrinsic degradation of high quality Alq3 based diodes due to electrical stressing. We monitor progressive luminance degradation and recovery by introducing well defined relaxation time windows in the current stress cycles. The method helps to clearly distinguish between recoverable and permanent degradation systematically. The voltage shift due to degradation and recovery is also monitored as a function of time. Further, we introduce a method of reconstructing the transients of the recoverable part using progressive isolated current pulses as a probe. The recovery of degradation is related to the charging and discharging of the traps in the device and our method provides a technique of measuring significant parameters of trapping through luminance transients. The origin and distinguishing features of the two types of degradation are discussed. INTRODUCTION Organic electronics has emerged as a promising technology in many areas of applications involving large area, flexible substrates and inexpensive processing techniques. However, lifetime and degradation are of key concerns which limit widespread use of such devices [1]. There have been several studies on degradation of molecular semiconductors, of which Alq3 based devices are the most common as prototypes [2-9]. Many studies on degradation due to electrical stress have reported both permanent and recoverable part to degradation in performance [2, 3, 5-9]. The origins of the two types of degradation are unclear [9]. The permanent degradation is often attributed to mechanisms such as cationic instability of Alq3 or chemical reaction of molecules in presence of oxygen, moisture in presence of light or indium ion migration into active material [1]. The other type of degradation, which is recoverable, is attributed to dipole formation [2, 10] or internal field due to traps. However, there is a lack of systematic study to separate these contributions which can aid better understanding of their origin. In this paper, we focus on intrinsic degradation in the material due to electrical stress in Alq3 based bilayer OLED structures. We propose a variety of systematic stress scheduling in order to distinguish between permanent degradation and recoverable degradation of luminance. We also demonstrate a method by which we can get the time evolution of the recoverable part of degradation and attribute it to
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