High Throughput Electrochemical Method for Contact Optimization in Printed Rectifying Diodes
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High Throughput Electrochemical Method for Contact Optimization in Printed Rectifying Diodes Petri S. Heljo1, Himadri S. Majumdar2, Donald Lupo1 1 Tampere University of Technology, Department of Electronics and Communications Engineering, Printed and Organic Electronics Group, P.O. Box 692, 33101 Tampere, Finland 2 VTT Technical Research Centre of Finland, Tietotie 3, 02150 Espoo, Finland ABSTRACT We report a low cost and high throughput electrochemical anodic oxidation method to enhance the metal-semiconductor contact between a silver electrode and an organic semiconductor in a rectifying diode application. The oxidized layer enhances the contact properties, leading to better device performance. Three different anodic oxide thicknesses were used in the study. Current-voltage and AC rectification measurements were used to characterize the printed devices. The DC output voltage of the half-wave rectifier increased consistently as a function of the oxide thickness. This procedure points toward a cost-effective way to optimize printed organic devices. INTRODUCTION The performance of the printed semiconductor devices depends heavily on the bulk semiconductor and charge injection properties. For example, in vertical organic diodes the bulk semiconductor properties dominate the device characteristics with thick semiconductor layers. However, with thin semiconductor layers the charge injection and collection properties of the electrodes become more important. Especially in such cases, formation of a good ohmic contact between the metal electrode and the semiconductor is essential for efficient device operation. Ptype semiconductor materials are widely used in printed electronics due to their good stability compared to most n-type materials. The HOMO levels of the p-type semiconductors are often close to -5 eV. Thus, formation of a good ohmic contact to these materials may be problematic. In the literature, use of interlayers of PEDOT:PSS [1], molybdenum oxide [2] and vanadium pentoxide [3] has been reported to improve the ohmic contact for the p-type organic semiconductors. However, deposition of ultra-thin additional layer makes the fabrication process more complex and limits the production volume. Due to the relatively low band gap in some metal oxides, including Ag2O and TiO2, they have rather semiconducting than insulating properties [4, 5]. In 2003, Chen et al. showed enhanced hole injection properties in organic light emitting diodes using a thin oxide layer on an Ag electrode [6]. In 2010, Lilja et al. proposed that a thin oxide layer on top of the printed Ag anode electrode enhances the ohmic contact with p-type poly(triarylamine) (PTAA) semiconductor in printed rectifying diodes [7]. Therefore, optimization of the oxide layer thickness could enhance the ohmic contact and device operation. We report a rectifying diode contact optimization method using a simple yet reproducible anodic oxidation of silver. The method is suitable for high volume production and enables subnm modifications to oxide layer thickness. The
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