Metal-Enhanced Adsorption of High-Density Polyelectrolyte Nucleation-Inducing Seed Layer for Highly Conductive Transparent Ultrathin Metal Films

Abstract

In recent years, ultrathin Ag films (UTAFs), which are attractive owing to its extremely low resistance, relatively high transparency, excellent mechanical flexibility, and mature mass production, have been reported as potential candidates to replace traditional indium tin oxide (ITO). To achieve a high-quality UTAF, a nucleation-inducing seed layer (NISL) is required to address the issue of irregular Ag islands growth. However, the structures of films so deposited are still far from being ideal and consist of rough surfaces with high densities of voids and grain boundaries when the film thickness is < ∼6 nm. Here, a hybrid structure composed of a gold (Au)/polyethyleneimine (PEI) bilayer is employed as a high-density NISL for the fabrication of an UTAF. Compared to the conventional single-layered PEI NISL that physisorbed on the substrate via the weak electrostatic attraction between the negatively charged substrate and the positively charged amine groups in PEI, our novel bilayered Au/PEI NISL exhibits a much higher density of nucleation sites due to the formation of strong coordinate covalent bonds between the Au atoms and amine groups. As a result, the percolation threshold thickness of the UTAF based on the Au/PEI bilayer can be reduced to as low as 3 nm. After capping with a high-refractive-index tantalum pentoxide (Ta 2 O 5 ) anti-reflection layer, the resultant Au/PEI/8 nm Ag/30 nm Ta 2 O 5 (APAT) electrode exhibits an excellent optoelectrical performance with a sheet resistance of 9.07 /sq and transmittance of 92.9% in the spectral range of 400-800 nm as well as outstanding long-term environmental and mechanical stabilities. The findings demonstrate a novel strategy for the development of high-performance UTAF-based transparent electrodes.

Ultrathin Gold Microelectrode Array using Polyelectrolyte Multilayers for Flexible and Transparent Electro‐Optical Neural Interfaces

“…It has been reported that the intensity of the chemical reaction can be quantified by the amount of shift of the binding energy peak of a gold atom (e.g., in the Au-4f region) revealed by X-ray photoelectron spectroscopy (XPS) for different PEM conditions. [25,26] From the XPS analysis of the 8 nm-thick gold deposited on various combinations of PEMs (see the details in Experimental Section), all of the combinations of PEMs led to a negative shift of the binding energy of Au-4f from the gold on bare substrate condition. Among all the combinations of PEMs, the Au/(PEI/ PSS) 5 condition resulted in the largest negative shift of the Au-4f binding energy (the XPS spectra in Figure 1c, the values of the negative shift of energy in Figure 1d).…”

“…Recently, for optoelectronic devices such as photovoltaic cells and light-emitting diodes, it has been reported that vacuumdeposited ultrathin metal film (<10 nm), which is transparent but originally non-conductive, can become highly conductive when a certain type of polymer layer is used as the NISL. [25][26][27][28] The NISL prevents random migration and aggregation of evaporated metallic elements during vacuum deposition, and therefore improves electrical conductivity by reducing the grain boundaries. In this work, we adopted this approach to create transparent and flexible microelectrodes for efficient electro-optical neural interfaces.…”

Metal‐Based Flexible Transparent Electrodes: Challenges and Recent Advances

“…These seeds provide dense nucleation centers for metal deposition, and suppress the growth of large metal islands during the PVD process. Such functional seeding materials include metals (Al, [40][41][42] Au, [43] Ag, [44] and Cu [45] ), dielectrics (Ta 2 O 5 , [46] ZnO, [47][48][49] NiO, [50] TeO 2 , [51] Nb 2 O 5 , [52] MoO 3 , [53] and Cs 2 CO 3 [54] ), polymers (polyethyleneimine [PEI], [19,[55][56][57][58] Ormoclear, [59] and photoresist SU-8 [60] ), and organic monolayers (11-mercaptoundecanoic acid [MUA], [48] [3-aminopropyl]-trimethoxysilane: [3mercaptopropyl]-trimethoxysilane [APTMS:MPTMS], [42,61,62] methyl-terminated alucone, [63] and 1,4-bis[2-phenyl-1,10-phenanthrolin-4-yl]benzene [p-bPPhenB] [64] ). For example, Schubert et al enhanced the wetting of deposited Ag films by using high surface energy metal seed materials such as Ca, Al, and Au (Figure 2a).…”

High‐Performance Semi‐Transparent Organic Photovoltaic Devices via Improving Absorbing Selectivity

“…[19] A branched polyethylenimine (PEI) layer is deposited on top of ZnO nanoparticles (NP) layer to reduce the granule size of ultra-thin Ag transparent electrode. [47][48][49] The resultant smooth ultra-thin Ag film effectively alleviates light scattering and improves the absorbing selectivity of ST-OPV. Further, with the guidance of optical simulation, an optical interference layer, that is, TeO 2 , is deposited on top of the ultra-thin Ag layer, and this dramatically improves absorbing selectivity via increasing the transmittance in visible region without significantly sacrificing absorption in invisible region.…”