Electric‐Field‐Induced Gap States in Pentacene

Abstract

Adv. Mater. 2009, 21, 1–5 2009 WILEY-VCH Verlag Gmb IC A T IO N Electronics based on organic materials have experienced unprecedented progress in recent years. The prospect of flexible, unbreakable, extremely low-weight electronics at relatively low cost has stimulated a lot of research and development on flexible display media, organic memories, and organic radio frequency identification (RFID) tags. Progress in this field has been sustained by the synthesis of new materials, the improvement of electronic devices, and the development of novel and improved processing techniques, such as inkjet printing, and microcontact printing. Even though significant advances have been achieved in recent years, fundamental issues surrounding the origin of gap states in small molecule materials are not resolved, and relatively little is known about the influence of these states on the electronic properties and the electrical stability of organic devices. It is important to address these questions because stable device operation is a major requirement in realizing organic displays and RFID tags. Furthermore, gap states affect the lifetime and the reliability of organic transistors and integrated circuits. Defect states might be caused by environmental conditions such as exposure to oxygen andmoisture. Therefore, it is essential to gain a fundamental understanding of such effects and their underlying microscopic processes. The formation of gap states and the influence of oxygen on the formation process were studied in polycrystalline pentacene (Pn) thin-film transistors (TFTs) using in situ electrical measurements. The experimental results are complemented by pseudopotential density functional calculations based on first principles for different types of oxygen-related defects in Pn. The results of the density functional calculations were used as input parameters in the simulation of the current/voltage characteristics of Pn TFTs. The transistor characteristics weremodeled by a density-of-states transport model. Simulations of the device characteristics will be presented and compared to experimental results. The investigation of gap states or doping effects in organic films is very often hindered by the unintentional incorporation of impurities during the purification, fabrication, and characterization of the materials and devices. Furthermore, the structural properties of the organic films have a distinct influence on the electronic properties of the materials. For example, several studies suggest that oxygen acts as a p-type dopant in a variety of semiconducting polymers and small molecules, such as Pn. Other studies show rather stable device operation under ambient conditions for weeks and months. Therefore, a systematic investigation of doping and trapping effects is essential to clarify the picture. In particular, the threshold voltage of the transistor has to be stable to realize complex organic circuitry, which is required for applications, such as organic RFID tags. The cross-section of a staggered-Pn TFTwith bottom drain and source contacts is shown in Figure 1b. The Pnmolecules (Fig. 1a) were deposited using organic molecular beam deposition (OMBD) onto a silicon oxide gate dielectric. The transfer characteristics of a polycrystalline TFT are shown in Figure 2a. The transistor had a channel length of 20mm and a width-tolength (W/L) ratio of 4000. Measurements were carried out under vacuum at room temperature. The transistor exhibited a mobility of 0.6 cmV 1 s , an on/off ratio larger than seven orders of magnitude, and a threshold voltage of 1V. Furthermore, the transistor exhibited a subthreshold slope of< 100mV/decade. The onset voltage of the drain current, defined as the gate voltage for which the drain current started to increase, was 0 V. In another experiment, the sample was exposed to dry oxygen for more than 30min. The oxygen concentration was controlled by the pressure in the high vacuum chamber (10 1 Pa). During oxygen exposure, no electrical bias was applied to the electrodes of the transistor. After exposing the device to oxygen, the transistor was pumped back to high vacuum (<10 6 Pa) and characterized. In this case, the transfer characteristics were not affected by the oxygen exposure (Fig. 2a). The experiment was repeated several times at different pressures and exposure times (up to 12 h). However, the characteristics did not change. The device behaved