Influence of the Temperature as an Environmental Factor on the Electrophysical Behavior of Flexible Polymeric Luminescent Devices

The effect of operational temperature on the electrophysical properties of polymer based electroluminescent structures is examined. For this purpose thin film of light-emitting semiconductor polyphenylenevinylene derivative is deposited between two indium-tin oxide (ITO) electrodes. DC current-voltage (I-V) characteristics of the fabricated devices ITO/polyphenylenevinylene derivative PPV-D/ITO are measured at varying ambient temperatures, ranging from room temperature (25°C) to 70°C. Several important electrical parameters like a trap factor, traps activation energy distribution, free carriers’ density, trapped carriers density, and effective mobility are estimated from measured temperature dependent I-V curves. Such analysis of the charge transport process in polymer devices may give information needed for optimization of the existing structures. 1. Introduction A great number of potential applications of organic semiconductor and polymeric materials for use in electronics and optoelectronic devices such as field-effect transistors, solar cells, and organic light-emitting diodes are found [1]. Conjugated polymers are especially modern topic in the research and developing area in this field [2]. Many soluble and semiconducting luminescent polymers with great variety of electrooptical properties were recently synthesized [3]. However, degradation of these devices with respect to temperature and the aging effect are still serious problems. To improve temperature stability and to improve device’s performance, it is very important to have a good understanding of the physical phenomena taking place in polymer layers in these devices, such as charge carriers transport, concentration and energy distribution of traps, charge carrier mobility, and other factors with respect to the temperature [4]. Various models are applied to explain different transport mechanisms in organic semiconductors at different modes of the supplied voltage. Among them, two models are used most frequently to explain the I-V characteristics: (a) the trapping model with space-charge-limited current (SCLC) [5] and (b) the field dependent mobility model [6]. The trapping model assumes that there is a certain distribution of traps, called localized states, where the free charge carriers can be trapped. The trapped carriers may be released after some period due to either temperature or other excitations to contribute to the polymer conduction. This model is applied for relatively low voltages, where only temperature dependence of the mobility is considered and electrical field activation is