Novel metallic transparent conductive layers for optoelectronics
Date of Issue2018-02-09
School of Electrical and Electronic Engineering
The increasing demand for optoelectronic applications coupled with novel areas of their application drive the research in the area of transparent conductive layers (TCLs) towards more convenient fabrication methods, lower costs and flexibility of materials, while not compromising on solid performance. The current industry benchmark TCL - the indium tin oxide (ITO) - benefits from an excellent trade-off between optical transparency and electrical sheet resistance, yet suffers from some well-known drawbacks, including but not limited to: high materials and fabrication cost, scarcity of indium, toxicity of ITO fabrication and inadequate flexibility. This thesis focuses on a tuning of the optoelectronic performance of two types of metallic nanostructured TCLs: 1) aluminum nanoporous (AlNP) layers with random and uniform distribution of NPs and 2) silver nanowire (AgNW) layers with random and uniform distribution of AgNWs. The first half of the thesis investigates the trade-off between transmittance and sheet resistance of random and uniform AlNP layers with various geometrical configurations of nanopores (NPs). A non-lithographic approach to fabrication of the transparent conductive AINP mesh was presented based on electrochemical anodizing of AI bulk layer. Computational models for estimating the optical and electronic properties of nanostructured metallic TCLs was proposed founded on the finite-difference time-domain (FDTD) method and the percolation theory, respectively,· and brought to good agreement with experimental data. The optimal geometrical configuration of AINPs was specified and compared with ITO and other NP layers based on Ag and gold (Au). We found that AINP TCLs possess the optoelectronic performance which is comparable to ITO, but do not outperform it. Obtained results demonstrated porous AI mesh as a strong candidate for low-cost non-lithographic low-temperature TCL, which is especially attractive for flexible electronics. The second half of the research focuses on a tuning of the optoelectronic performance of AgNW layers- promising candidates to outperform the benchmark ITO TCLs. Theoretical model for estimating the optical and electronic properties of AgNW layers was proposed based on FDTD method and an electrical approach considering the volume of NW crossings. The model was brought to good agreement with experimental data. The trade-off between the transmittance and the sheet resistance of AgNW electrodes was investigated through adjusting the diameter, length and surface coverage (SC) of AgNWs. The influence of the angle deviation of the nanowire crossings on the transmittance and the sheet resistance was demonstrated and estimated. According to the results, AgNW TCLs possess strong optoelectronic performance which is not only comparable with ITO but may also exceed it, making them promising candidates for many optoelectronic applications such as thin displays, touch screens, solar cells, light-emitting diodes, smart windows, transparent heaters, electroluminescent panels and other devices.
DRNTU::Engineering::Electrical and electronic engineering