Transparent Conductive Anodes for Solid-State Lighting

The U.S. Energy Information Administration estimates that in 2017, the U.S. residential sector and the commercial sector used about 273 billion kilowatthours of electricity for lighting. This was about 10% of the total electricity consumed by both of these sectors and about 7% of total U.S. electricity consumption. Residential lighting consumption was about 129 billion kilowatthours or about 9% of total residential sector electricity consumption in 2017. Increasing the efficiency of current lighting technologies can be a substantial energy and cost saver. Currently, inorganic based light emitting diodes are the most efficient lighting present on the market. Even though traditional light emitting diodes lighting is currently the clear leader in efficiency, lifespan and cost savings, this might not be the case in the near future. When the basic semiconductor light emitting diode first hit the market, it was more expensive and less effective, but technological advances have reduced cost, while increasing efficiency. This is expected to be the same case as advancements are made in organic light emitting diodes technology. In fact, experts expect organic light emitting diodes lighting to be a $1.3 billion market by 2023. The transparent conducting electrode is one of the major constructive elements of the organic light emitting diodes. It provides the means to supply current to the light emitting material and allow generated light to escape from the diode, so it can be used for lighting purposes. Indium–tin oxide is currently the most widely used as transparent conductive electrode. However, it has several drawbacks: (I) its cost has increased gradually owing to a scarcity of indium; (II) indium and tin ions that diffuse from the electrode to overlying organic layers can act as charge- trapping centers during the charge injection process, degrading the luminous efficiency and operational stability of organic light emitting diodes; (III) majority of the emitted light is trapped in the glass or plastic substrates (substrate mode) and the indium-tin oxide/organic medium (waveguide mode) due to the refractive-index mismatch between these layers lowering efficiency of the organic light emitting diodes; (IV) indium tin oxide films are brittle, which makes them unsuitable for practical flexible electronics. In this SBIR program, scalable production methods for alternative materials for transparent conductive electrodes will be developed. We will improve the reproducibility of fabrication of reduced graphene oxide films on non-conductive substrates with control over the number of graphene layers in the films. Specific attention will be paid to develop low temperature graphene oxide reduction methods and optimization of the films for applications as transparent conductive electrodes for solid state lighting. Work function of the graphene films and sheet resistance will be optimized by chemical doping of the films. Electrically conductive, transparent graphene films can find potential applications in touchscreens, liquid crystal displays, organic photovoltaic cells, and organic light-emitting diodes. Large area, flexible graphene films are particularly suitable as indium tin oxide substitution in flexible electronics.