ITO-Free OLED and OPV Devices with a Transparent Silver Nanowire Electrode
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ITO-Free OLED and OPV Devices with a Transparent Silver Nanowire Electrode Florian Pschenitzka Cambrios Technologies Corporation, Sunnyvale, CA 94085, U.S.A. ABSTRACT Cambrios has developed a transparent conductor material based on silver nanowires which can be used to replace the ITO layer in organic photovoltaics (OPV) device and in organic light-emitting devices (OLED). After being deposited from a liquid suspension by conventional coating or printing methods onto a transparent substrate, these nanowires form a transparent conducting network. The sheet resistance of the resulting film is determined by the density of the nanostructures and can thus be easily controlled during the coating process. The elimination of the ITO layer also results in a reduced microcavity effect and thus has a positive impact on the optical performance for OLED lighting devices. This paper will focus on the use of this material as an ITO replacement for OLED devices for lighting applications and for OPV devices. The performance of ITO-free OPV and OLED devices with a nanowire anode will be discussed. We also will present the optical performance data of OLED lighting devices which show the implications of a reduced microcavity effect. In addition, we will show lifetime data for these devices which demonstrate the viability of this technology. INTRODUCTION The high cost of ITO substrates for OLED Solid-State-Lighting (SSL) and OPV panels is a major barrier for commercialization. In addition, fabricating flexible devices with an ITO electrode with a sheet resistance of less than 50 ohms/sq is very challenging. To address both of these crucial issues, we propose to utilize a transparent conductive film formed by a random network for silver nanowires (AgNW) as device electrodes. This material has the potential of lowering the material and substrate cost as well as enable flexible devices. Cambrios Technologies has developed a dispersion of silver nanowires (ClearOhm® ink) which, when deposited onto a transparent substrate, forms a random and highly conductive network while preserving the transparency of the stack. The ClearOhm® nanowire suspension can be coated by conventional coating techniques, such as spin-coating and slot-die coating. After the deposition and a drying / anneal cycle, the nanowire forms a conductive transparent network. The temperatures necessary to obtain optimal conduction between the nanowires is compatible with plastic substrates, such PET and PEN. An SEM image of such a network is shown in Figure 1. [1, 2] The resulting sheet resistance of the film is determined by the density of nanowires deposited on the substrate surface. As such, it can be easily controlled by either ink dilution or wet film thickness. The nanowire film can be patterned with conventional techniques, including photolithography and wet etching as well as laser patterning.
Figure 1: SEM of random network of silver nanowire (AgNW)
The light transmission (measured with integrating sphere and photopically weighted) as a function of sheet resistance
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