The ability of light to pass through an electrode (optical transparency) and the ability of an electrode to carry a current (electrical conductivity) are necessary for technology like solar powered generators, lights, and touch screens. Usually, the electrodes have to be unfavorably less transparent than the structures they are built on, so the relative transmittance is less than 100%. This study used a dielectric-metal-dielectric-based electrode with a higher transparency than what is previously used, which allows the relative transmittance to equal about 100%.
This study proposes designs for a dielectric-metal-dielectric (DMD) transparent electrode in place of traditional methods like indium tin oxide (ITO) and PET.
Indium tin oxide (ITO) is usually used as the go to transparent electrode because it has a high transmittance and electrical conductivity. But ITO has plenty of disadvantages like the low level of indium on Earth and the low flexibility of the compound. To find a better alternative to ITO, scientists are considering clear electrodes with metal networks instead. These clear electrodes have the benefits of ITO, but have downsides like shorting out and looking hazy. The most recent solution uses dielectric-metal-dielectric (DMD) transparent electrodes. In this case, thin metal film is placed between two layers of antireflection devices, called dielectrics. Dielectric 1 simply refers to the dielectric on the top of the structure, and dielectric 2 refers to the dielectric under the metal film. Together, dielectric 1 and 2 sandwich the metal, and the substrate (usually PET) is on the bottom of the whole structure. This version has all of the benefits of the other materials previously used. To get an ideal conductivity, the entire electrode system has to be more transparent than the structure it is built on.
This study aims to see how close they could get the transmittance while maintaining a functioning conductivity. They used a DMD-based transparent electrode on polyethylene terephthalate (PET) as the substrate to achieve a relative transmittance of greater than 100%, which could potentially replace ITO.
Results and Methods
Strategy to achieve relative transmittance surpassing 100%
The relative transmittance was optimized by using seven different factors. The high number of factors can make it difficult to develop the best electrode system design. This study aims to maximize two of the factors: optical transmittance and electrical conductivity. Silver, copper, and gold are all considered as metallic layers, but silver has the lowest absorption so it is the material used. Complex vector addition and algebraic manipulation of variables are used to determine the maximum thickness and maximum speed that light travels through the material (called the refractive index).
Proof of Concept
Though most other studies have used the same material for both of their dielectric components in the DMD, the math done in this study suggests that two different dielectric materials may actually be the most successful design. PET is always used as the optical substrate. Titanium dioxide (TiO2) is used as the dielectrics. The ideal structure to obtain a relative transmittance of over 100% as described in the study is: PET substrate/24nm TiO2/9nm Ag/63nm. Later, we will see that different materials are used for dielectric 1 and dielectric 2 with favorable results.
Using thin silver films can lead to high transparency, but this can cause the issue of large surface roughness. Large surface roughness can decrease the electrical and optical function of the DMD electrode. This study demonstrates that the roughness can be reduced by using impure silver, that means adding copper (Cu) or another metal in small amounts to make the silver sample impure. This is called Cu-doped silver. Cu-doped silver was used with PET as the substrate, zinc oxide (ZnO) as dielectric 1, and aluminium oxide (Al2O3) as dielectric 2. This highly optimized model was the first study to obtain over 100% transmittance for a DMD electrode. With more optimization, the transmittance could reach even higher levels, and the DMD electrode could be used in a variety of settings, like high temperature and humidity conditions.
Using a Cu-doped silver film in a DMD electrode can provide a thick, smooth, low optical loss, and highly electrically conductive system. The DMD electrode provides 88.4% absolute transmittance compared to the PET substrate, which has an absolute transmittance of 88.1%. The DMD electrode offers an alternative to the ITO electrodes, and offers a relative transmittance of over 100%.