A novel technique for fabricating inexpensive, transparent electrodes from common metals has been developed by engineers and scientists at Iowa State University and Ames Laboratory. They exhibit very high transparency (see Figure 1) and are very good electrical conductors. This is a combination of properties that is difficult to achieve with common materials. The most frequently used transparent electrode in today's high-technology devices (such as LCD screens) is indium tin oxide (ITO). While ITO performs well in these applications, the supply of indium is very limited. In addition, it is rapidly decreasing as consumer demand for flat-panel electronics is skyrocketing. According to a 2004 US Geological Survey report, as little as 14 years' exploitation of known indium reserves remains.1 In addition to increasing prices, the dwindling supply of indium suggests its use is not sustainable for future generations of electronics enthusiasts.
Figure 1.Sample of transparent electrode over text.
Solar cells represent another application where transparent electrodes are used. To make solar-energy collection economically feasible, all parts of solar photovoltaics must be made more efficient and cost-effective. Our novel transparent electrodes have the potential to do both. In addition, there is much interest in developing more efficient, cost-effective, and environmentally friendly lighting. Incandescent light bulbs are very inefficient, because most of their energy consumption is wasted as heat. Fluorescent lighting is much more efficient but still uses mercury, an environmental toxin. An attractive alternative is offered by LEDs, which have very high efficiencies and long lifetimes, and do not contain mercury. If made bright enough, LED use for general lighting could provide a viable alternative.
We have fabricated electrodes from more commonly available materials, using a technique that is cost effective and environmentally friendly. Most of today's electronic devices are made in specialized facilities equipped with low-particle-count clean-room facilities and multimillion-dollar equipment. On the other hand, the novel process we developed uses a method that makes use of polymer molds and standard deposition techniques in an ambient laboratory environment.2 The final structure consists of tall ribbons of metal (standing on edge) that are so thin that they do not block light but are very good conductors: see Figure 2. The advantage of this design is that it avoids the competition between conductivity and transparency inherent in transparent oxide electrodes. By making the structure taller, conductivity can be increased without impacting transparency.
Figure 2.(left) Polymer-bar structure on a glass substrate. (right) Structure with just sidewalls coated in gold.
We have measured both electrical conductivity and transparency for these structures. We performed two-wire electrical measurements to quantify the structures' resistance using metal contacts deposited on each end. The total sample area was 4×4mm2. We measured a resistance of structures with 40nm gold sidewalls of 7.3Ω, which is lower than that of ITO glass (which has a sheet resistance around 10Ω/square).
We investigated the structures' optical properties based on both specular- and total-transmission measurements. Specular transmission is measured by collecting the transmitted light at normal incidence, while total transmission is obtained by collecting transmitted light at normal incidence and diffracted light using an integrating sphere. Figure 3 shows the total transmission of a grating with 40nm gold or silver sidewalls on a glass substrate compared to that of ITO. Additionally, the transparency changes very little within 30° off normal incidence. This high visible-light transmission of our metal-patterned structures is very promising for their application as transparent electrodes, because most visible light was allowed to propagate through the patterned metallic/polymeric structures.
Figure 3.Transmission of metallic structures compared to commercial indium tin oxide (ITO).
Researchers in our group continue to refine the fabrication methods and are investigating methods to make large-scale structures for use in a variety of applications that require both transparency and high electrical conductivity. We are also applying these fabrication techniques to build other structures that can manipulate light in novel ways.
Work at the Ames Laboratory was supported by the Department of Energy (Basic Energy Sciences) under contract DE-AC02-07CH11358.
Iowa State University
Kristen Constant is an associate professor of materials science and engineering. She is also an associate at Ames Laboratory. She works in photonic bandgap materials and other structures, specifically toward designing and fabricating structures that enhance energy efficiency.