High performance and low cost transparent electrodes based on ultrathin Cu layer

Transparent electrodes are extensively applied in a great range of optoelectronic and photovoltaic devices  where they exhibit high transmittance while offering high conductivity. Transparent conductive oxides such as aluminum-doped zinc oxide, tin-doped indium oxide, and fluorine-doped tin oxide are examples of the most common transparent electrodes materials. Among these, tin-doped indium oxide is commonly applied owing to its high optical transmittance as well as low resistivity. Unfortunately, scarcity of the rare earth element indium and its high cost inhibit the use of tin-doped indium oxide.

This has resulted in increased research interest in alternative materials, for example, graphene, carbon nanotubes, metal nanowires, metal meshes, conductive polymer films, and ultrathin metal films. Multilayered transparent electrodes based on ultrathin metals are composed of ultrathin silver, copper, gold or  aluminum embedded between two dielectric materials. Among other applications, they are  used for  low emissivity coatings. These materials show promising properties, for instance, widely tunable optical as well as electrical attributes via materials and film thickness variations, low roughness, mechanical and temperature stability, and ambient temperature deposition.

Researchers at the Austrian Institute of Technology presented a comprehensive study of dielectric-copper-dielectric electrodes, considering that copper is a low-cost substitute to the generally used  silver or gold. A wide range of dielectric materials encompassing an array of refractive indices was considered in the transfer matrix method simulations in a bid to maximize the electrode transmittance. Their research work is Optics Express.

The authors deposited by direct current magnetron sputtering on soda-lime glass substrates, without substrate heating. TiOx was first  deposited by reactive sputtering from a  Titanium target. Copper was subsequently deposited from a  copper target. The authors then sputtered aluminum-doped zinc oxide from a  doped zinc oxide target.

The proposed sputtered transparent electrode of titanium oxide-copper-aluminum doped zinc oxide combined the advantage of being made of low-cost components with high average transmittance of 0.80 and a sheet resistance of 6.5 ohms/sq when deposited on glass substrate. The electrode was therefore competitive to the typical tin-doped titanium oxide. The authors observed that high sputter power was important to curtail island-like growth of the copper film and achieved excellent optical as well as electrical attributes.

The electrode deposited on polyethylene terephthalate was far from attaining its performance on glass owing to the roughness initiated by the TiOx deposition. Eventually, the performance could be restored by introducing an additional aluminum-doped zinc oxide buffer layer. The resulting electrode had an average transmittance of 0.74 and a sheet resistance of 17 ohms/sq. The authors recommended that the optimization of the electrode design had to account for precise layer architecture of the device. For the case of inverted hybrid perovskite absorber solar cell, the authors indicated that the electrode design optimized with air as ambient medium had to be adopted.