Transparent conducting films (TCFs) are optically transparent and electrically conductive in thin layers. They are an important component of a number of electronic devices including liquid-crystal displays, OLEDs, touchscreens and photovoltaics. While indium tin oxide (ITO) is the most widely used, alternatives including other transparent conductive oxides (TCOs), conductive polymers, metal grids, and carbon nanotube(CNT), graphene, nanowire and ultra thin metal films all show promise in some applications.
Transparent conducting films act as a window for light to pass through to the active material beneath (where carrier generation occurs), as an ohmic contact for carrier transport out of the photovoltaic, and can also act as transparent carrier for surface mount devices used between laminated glass or light transmissive composites. Transparent materials possess bandgaps with energies corresponding to wavelengths which are shorter than the visible range of 380 nm to 750 nm. As such, photons with energies below the bandgap are not collected by these materials and thus visible light passes through. However, applications such as photovoltaics may require an even broader bandgap to avoid unwanted absorption of the solar spectra.
They have a multitude of applications for solar energy utilization and for energy savings, especially in buildings. The largest of these applications, in terms of area, make use of the fact that the TCs have low infrared emittance and hence can be used to improve the thermal properties of modern fenestration. Depending on whether the TCs are reflecting or not in the near infrared pertinent to solar irradiation, the TCs can serve in “solar control” or “low-emittance” windows. Other applications rely on the electrical conductivity of the TCs, which make them useful as current collectors in solar cells and for inserting and extracting electrical charge in electrochromic “smart windows” capable of combining energy efficiency and indoor comfort in buildings.
Transparent conducting oxides are integral components in flat panel displays, solar cells, and smart windows. The role of the transparent conductor in these devices is to deliver or collect electrons from the active part of the device while at the same time allowing visible photons to pass through relatively unimpeded. The best combination of transparency and conductivity is in In2-xSnxO3, or ITO. Efforts to produce a transparent conductor, with conductivity superior to ITO yet retaining high transparency through the visible region, has so far proved to be a challenging task.
In addition, the processing and economic factors of such material must be taken into account. Transparent conductors which are to be produced commercially need to be processed easily on a large scale and be cost effective. With a handful of transparent conductors to currently choose from, new technological devices may not be plausible for large scale consumer use due to processing and cost control, even if the material exhibits exceptional properties.