(Invited) Electro-Optical Techniques to Measure Traps in Organic-Based Devices: Why the Methods Originally Developed for Silicon-Based Devices Must be Modified

Abstract

Electrical techniques to detect traps are a crucial part of the development of a new electronic materials and devices. Since most of our electronic technology relies on silicon, the well-established methods to measure traps were originally developed for silicon-based devices. The intense work developed in the 60’s to map all electronic defects in silicon brough the technology to today’s standards. Among the techniques developed are the deep level transient spectroscopy, photo-induced current spectroscopy, and admittance spectroscopy. Blood and Orton wrote and excellent textbook on these methods [1]. The methodology behind these techniques is the following: Traps are first filled using a voltage pulse or a light pulse, then the kinetics of the charge carrier release is measured usually as function of temperature. This allow us to determine the trap signature (energetic depth and cross-capture section). In organic semiconductors this methodology often does not apply, this is because the traps are extremely deep. A trap with an energetic depth of 0.1 eV is considered a very deep trap in silicon, but in organic electronics, is just a shallow trap. Traps in an organic semiconductor may have an energetic depth higher than 0.5 eV they can be so deep that a charge carrier may take minutes, hours or even days to be released. This makes unpractically the use of the popular methods to study traps in silicon. To detect the presence of the traps and their effects on the device performance the standard methods developed for silicon must be modified.In this contribution we present a methodology to detect the presence of deep traps in organic based devices. This methodology relies on the measuring the trap filing process and not the trap emptying process. To perform this analysis, the traps must be first emptied, so the device is first brough into a state called “trap emptied state”. This state is then frozen to a specific low temperature and the trap filling recorded at that temperature. To reach a “trap empty state” several methods can be used, namely high temperature while the device is kept under short circuited conditions and/or light exposure for certain types of traps. This methodology was used to detect the effect of traps in organic- based transistors. Transistor devices produced by spin coating and by ink-jet printing methods were studied and presented in this work. The use of this methodology revels the presence of metastable traps that only become active above certain temperatures (200 K, 240 K, and 310 K). This type of metastable traps is only observed organic thin film transistors.[1] J. W. Orton and P. Blood “Electrical Characterization of Semiconductors: Measurement of Minority Carrier Properties (Techniques of Physics)”, Academic Press (November 11, 1990).