Electrical Transient Based Defect Spectroscopy in Polymeric and Organic Semiconductors
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Electrical transient based defect spectroscopy in polymeric and organic semiconductors Y.N. Mohapatra, V. Varshney, V. Rao, Samarendra P. Singh and G.S. Samal Samtel Centre for Display Technology, IIT Kanpur India Materials Science Programme, IIT Kanpur India Department of Physics, IIT Kanpur India ABSTRACT The relation between electrically active defects in organic and polymeric semiconductor materials and degradation of devices such as loss of luminance is currently not understood. In this paper, we study defect related charge processes using electrical transients in polymeric and organic diodes. We monitor slow charging and discharging currents as a function of time for both virgin and electrically aged devices. The current transients are analyzed spectroscopically in time domain using a technique called Time Analyzed Transient Spectroscopy, which is similar to DLTS in its implementation. We observe highly stretched exponentials over 3-4 orders of magnitude in time up to thousands of seconds indicating existence of large distribution in time constants in aged PLED devices. We demonstrate ability to distinguish between different mechanisms of charge storage related to defect related processes in typical OLED and PLED. INTRODUCTION Organic semiconductors are emerging as attractive material base for many display and electronic applications, especially as light emitting diodes and thin film transistors. Though there has been considerable progress in device structures and materials, there is a lack of studies on electrically active defects in these materials and their relation to degradation. This necessitates developing research programs to understand trapping and other charge processes in these devices at a level similar to defect studies in inorganic semiconductors. The electrically active defects may have origin from breaks in conjugation (intrinsic traps) or processes during the synthesis (extrinsic traps). These deeper traps play major role in controlling the electrical behavior of devices and they may have significant correlation with loss of performance as has been shown by some recent studies [1-6] through charge, capacitance and luminescence based techniques. There is currently a lack of methods of analysis in defect recognition to facilitate their study and underlying mechanisms. In this paper, we study defect related charge processes using electrical transients in polymeric and organic diodes. We use popular light emitting diode structures consisting of ITO/PEDOT:PSS/PPV/CN-PPV/Al for PLED, and ITO/PEDOT:PSS/PPV/MEH-PPV/Al and similar structures wherein the active layers are based on small molecules. We monitor slow charging and discharging currents as a function of time for both virgin and electrically aged devices. The current and charge transients are analyzed spectroscopically in time domain using a technique called Time Analyzed Transient Spectroscopy, which is similar to DLTS in its implementation. We observe highly stretched exponentials over 3-4 orders of magnitude in time up to thousands o
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