A three-stage engineered deposition rate profile in evaporation processes for ultra-thin metallic transparent conductors

Abstract

Transparent conductors are employed in many optoelectronic components such as liquid crystal displays, light-emitting diodes, and solar cells. Indium-tin-oxide (ITO) is the most common transparent conductor, which suffers from difficulties such as high annealing temperature and brittleness. One of the attractive alternative solutions is to use ultra-thin metal films (UTMFs). In this study, the electrical and optical properties of ultra-thin composite metal films have been studied in terms of the effect of simultaneous controlling of the deposition rate and the doping. A three-stage deposition process using electron-beam and thermal evaporation was purposed for the fabrication of UTMFs. The three steps are: (I) deposition of an ultra-thin seed layer, (II) a co-deposition process with a precisely engineered deposition rate during the transition of the two layers, and (III) an accelerated deposition of the metallic layer at the end. A chromium ultra-thin layer was used as a seed layer, and gold or silver metal was studied in the subsequent deposition stages. The results showed that the three-deposition process could effectively improve the percolation threshold, leading to high average transparency of ≥71% over 400–700 nm, the sheet resistance of ≤22Ω/□, and roughness of ≤0.6 nm root mean square in UTMFs samples employing gold as the metal layer, where the UTMF total thickness was ≈8.3 nm. As a proof of concept, organic light-emitting diodes (OLEDs) were fabricated employing an optimized chromium-gold UTMF and also with a witness ITO. According to the results, UTMFs based on the three-stage process, provide acceptable performance in OLEDs compared to the ones exploiting the ITO anode, indicating that such UTMFs are applicable in various optoelectronic devices.