Transparent conducting film

They are an important component in a number of electronic devices including liquid-crystal displays, OLEDs, touchscreens and photovoltaics.

Transparent conducting films are typically used as electrodes when a situation calls for low resistance electrical contacts without blocking light (e.g. LEDs, photovoltaics).

Typically, these applications use electrode materials that have greater than 80% transmittance of incident light as well as electrical conductivities higher than 103 S/cm for efficient carrier transport.

Suitable p-type transparent conducting oxides are still being researched, though the best of them are still orders of magnitude behind n-type TCOs.

The lower carriers' concentration of TCOs with respect to metals shift their plasmonic resonance into the NIR and SWIR range.

Indium, the film's primary metal, is rare (6000 metric tons worldwide in 2006), and its price fluctuates due to market demand (over $800 per kg in 2006).

AZO is composed of aluminum and zinc, two common and inexpensive materials, while indium-doped cadmium oxide only uses indium in low concentrations.

These systems are typically n-type with a carrier concentration on the order of 1020 cm−3, provided by interstitial metal ions and oxygen vacancies which both act as donors.

However, these simple TCOs have not found practical use due to the high dependence of their electrical properties on temperature and oxygen partial pressure.

Researchers have varied parameters enough and found combinations that will optimize the short circuit current as well as the fill factor for TCOs such as indium tin oxide.

[citation needed] Doped metal oxides for use as transparent conducting layers in photovoltaic devices are typically grown on a glass substrate.

This in turn helps maintain a low temperature of the active region of the solar cell, which degrades in performance as it heats up.

[20] Growth typically is performed in a reducing environment to compensating acceptor defects within the film (e.g. metal vacancies), which degrade the carrier concentration (if n-type).

[13] For AZO thin film deposition, the coating method of reactive magnetron sputtering is very economical and practical way of mass production.

In this method, a zinc-aluminum target is sputtered in an oxygen atmosphere such that metal ions oxidize when they reach the substrates surface.

This stabilization of the 5s orbitals causes a formation of a donor level for the oxygen ion, determined to be 0.03 eV below the conduction band.

Thus to enhance their electrical properties, ITO films and other transparent conducting oxides are grown in reducing environments, which encourage oxygen vacancy formation.

Increasing the scattering decreases the mean-free path of the carriers in the oxide, which leads to low electron mobility and a high resistivity.

By manipulating the band structure, polythiophenes have been modified to achieve a HOMO-LUMO separation (bandgap) that is large enough to make them transparent to visible light.

However, because transparent conductive polymers do absorb some of the visible spectrum and significant amounts of the mid to near IR, they lower the efficiency of photovoltaic devices.

FTO-coated glass provides thermal insulation in buildings by reflecting infrared radiation while allowing visible light, reducing heat loss and improving energy efficiency.

However, these thin films are usually fragile and such problems as lattice mismatch and stress-strain constraints lead to restrictions in possible uses for TCFs.

Nanotubes can be grown using laser ablation, electric-arc discharge, or different forms of chemical vapor deposition (such as PECVD).

One method of spray deposition used for CNT film creation is an ultrasonic nozzle to atomize CNTs in solution to form PEDOT layers.

[38][39] By optimizing spray parameters, including surfactant, drop size (dictated by the ultrasonic nozzle frequency) and solution flow rate, sheet resistance characteristics can be tuned.

Due to the ultrasonic vibration of the nozzle itself, this method also provides an additional level of sonification during the spray process for added separation of agglomerated CNTs.

These photovoltaic devices had much higher efficiencies compared to the devices made with CNT thin films: Britz et al. report an efficiency of 8%, with an open circuit voltage (Voc) of 0.676 V, a short circuit flux (Jsc) of 23.9 mA/cm2, and a fill factor of 45.48%.

This paves the way for new applications, indicating that CNT thin films can be used as heat dissipaters in solar cells because of this high transmittance.

Before mass production can occur, more research is needed in exploring the significance of tube diameter and chirality for transparent conducting films in photovoltaic applications.

Randomly conducting networks of wires or metal meshes obtained from templates are new generation transparent electrodes.

Figure 1. Cross-section of thin film polycrystalline solar cell. The transparent conducting coating contacts the n-type semiconductor to draw current.
This solar cell, made of monocrystalline silicon , has no transparent conducting film. Instead, it uses a "grid contact": a network of very thin metal wires.
Figure 2. Polymer photovoltaic cell using transparent conducting polymers.
Figure 3. CNTs of various diameters separated within a centrifuge tube. Each distinct diameter results in a different color.
Figure 4. Schematic of metal network based Transparent Conducting Electrodes. Electrical transport is through the percolating metal network, while optical transmittance is through the voids. Source: Ankush Kumar (JNCASR) Thesis.