Periodic Reporting for period 1 - TALNET (Transparent Aluminium Networks)
Période du rapport: 2020-03-01 au 2022-02-28
PROJECT OBJECTIVE AND BENEFITS
The objective is to determine the economic and technical feasibility of using electrospun polymer (poly-methyl-methacrylate, PMMA) templates to fabricate mechanically robust, corrosion resistant transparent conductors (TCs) based on a network of aluminium nanowires on flexible substrates for display, lighting, solar cells and other applications where light transmission is critical to device operation. TALENT exploits the use of inexpensive aluminium networks that surpass the performance of indium tin oxide (ITO) - the industry standard. The brittle nature of ITO and the scarcity of indium worldwide means that alterative solutions are required to meet the enormous growth in display production and the increasing demand for light-weight, mechanically robust and flexible form factors. TALNET involves the use of a sacrificial polystyrene (PS) layer which facilitates processing and ensures quantitative lift-off and high fidelity pattern transfer of the PMMA network into a network of Al nanowires even in areas of dense coverage where multiple wires intersect on the substrate to form seamless high conducting junctions. Mechanical robustness requires a shift from glass to flexible substrates, e.g. polyethylenephthalate (PET) or polyethylenenaphthalate (PEN), and also provides a reduction in device weight. Numerous replacement candidates for ITO have been proposed that provide mechanical flexibility, while potentially future proofing for emerging form factors. Of these, metal networks comprised of silver and copper that allow light to pass through them have demonstrated the highest figure-of-merit (FOM) performance but suffer from corrosion under ambient conditions. TALNET exploits an electrospun polymer template to fabricate aluminium metal networks with excellent sheet resistance and transmission, which are mechanically robust and corrosion resistant.
RESULTS AND CONCLUSIONS
As an initial step it was demonstrated that high quality Al films with conductivities of up to 94% bulk Al metal can be deposited onto flexible PET or PEN substrates with low surface roughness. Controlled PMMA fibre formation was enabled through the addition of surfactants to the PMMA prior to electrospinning. The diameter of the electrospun fibre was controlled by controlling the concentration of the PMMA solution and the level of surfactant used. In this manner, it was possible to achieve high fidelity transfer of PMMA nanofibers into Al nanowires, with tuneable thicknesses of between 25 nm and 300 nm. Nanowire widths were tuneable between 200 nm and 1μm. This aspect of the process is substrate independent, allowing for the same degree of control over wire width on both glass and polymeric substrates.
The need to demonstrate patterning necessary for incorporation of the TALNET technology into devices was accomplished by masking regions of the template during metal deposition using either a metal shadow mask or using UV lithography. S1813 resist processing was found to be chemically compatible with the template materials allowing minimum feature resolution of 1μm.
A systematic study of sheet resistance and optical transmission as a function of nanowire dimension (thickness and width) demonstrated performance levels that are comparable to ITO but with the added ability to independently tune sheet resistance and transmission. Flexibility was tested using a 5mm bending radius and no degradation in sheet resistance was observed during mechanical life testing for PEN based networks. The networks also exhibited a superior and featureless optical transmission (200nm - 2000 nm) compared to ITO. Excellent corrosion resistance in ambient was observed with no degradation in conductivity after 1 year. No devices have been fabricated at this stage, but the conformal deposition of Al networks on roughened silicon device layers in solar cells was demonstrated.
POTENTIAL SOCIOECONOMIC IMPACTS
Today’s display and touch-screen technologies rely on high performing TCs. Smart phones and handheld interactive displays represent a rapidly increasing share of this market and the biggest challenges facing the sector is screen failure and limited battery life. TALNET offers the potential to replace glass with PET/PEN and the development of shatterproof screens. The reduced density of PET (1.3 g cm-3) compared to gorilla glass (2.4 g cm-3) used in conventional displays reduces the weight of these handheld devices, facilitates larger batteries or additional functionality or simply lowers weight of the devices. TALNET may also help realise the integration of flexible solar cell technologies for energy harvesting into buildings and homes, and into the design of future vehicular transportation.
These impacts are consistent with the outcome of a preliminary market assessment which pointed to (i) manufacturers of flexible solar films, and (ii) manufacturers of flexible displays as the likely customers for this technology. Direct engagement with industry revealed interest but some aspects of the process as currently envisioned are challenging to industry (e.g. lift-off process). This slow step such as lift-off would seem to preclude a high throughput roll-to-roll manufacturing process, which is important for many technologies. Also the vacuum condition associated with the metal deposition and plasma etch steps are not compatible with many current device technologies.
No cost point analysis has been undertaken at this juncture. However, it is clear the margins in the manufacture of display and solar cell technologies are very small and ease of integration into the existing manufacturing process is essential.
The objective is to determine the economic and technical feasibility of using electrospun polymer (poly-methyl-methacrylate, PMMA) templates to fabricate mechanically robust, corrosion resistant transparent conductors (TCs) based on a network of aluminium nanowires on flexible substrates for display, lighting, solar cells and other applications where light transmission is critical to device operation. TALENT exploits the use of inexpensive aluminium networks that surpass the performance of indium tin oxide (ITO) - the industry standard. The brittle nature of ITO and the scarcity of indium worldwide means that alterative solutions are required to meet the enormous growth in display production and the increasing demand for light-weight, mechanically robust and flexible form factors. TALNET involves the use of a sacrificial polystyrene (PS) layer which facilitates processing and ensures quantitative lift-off and high fidelity pattern transfer of the PMMA network into a network of Al nanowires even in areas of dense coverage where multiple wires intersect on the substrate to form seamless high conducting junctions. Mechanical robustness requires a shift from glass to flexible substrates, e.g. polyethylenephthalate (PET) or polyethylenenaphthalate (PEN), and also provides a reduction in device weight. Numerous replacement candidates for ITO have been proposed that provide mechanical flexibility, while potentially future proofing for emerging form factors. Of these, metal networks comprised of silver and copper that allow light to pass through them have demonstrated the highest figure-of-merit (FOM) performance but suffer from corrosion under ambient conditions. TALNET exploits an electrospun polymer template to fabricate aluminium metal networks with excellent sheet resistance and transmission, which are mechanically robust and corrosion resistant.
RESULTS AND CONCLUSIONS
As an initial step it was demonstrated that high quality Al films with conductivities of up to 94% bulk Al metal can be deposited onto flexible PET or PEN substrates with low surface roughness. Controlled PMMA fibre formation was enabled through the addition of surfactants to the PMMA prior to electrospinning. The diameter of the electrospun fibre was controlled by controlling the concentration of the PMMA solution and the level of surfactant used. In this manner, it was possible to achieve high fidelity transfer of PMMA nanofibers into Al nanowires, with tuneable thicknesses of between 25 nm and 300 nm. Nanowire widths were tuneable between 200 nm and 1μm. This aspect of the process is substrate independent, allowing for the same degree of control over wire width on both glass and polymeric substrates.
The need to demonstrate patterning necessary for incorporation of the TALNET technology into devices was accomplished by masking regions of the template during metal deposition using either a metal shadow mask or using UV lithography. S1813 resist processing was found to be chemically compatible with the template materials allowing minimum feature resolution of 1μm.
A systematic study of sheet resistance and optical transmission as a function of nanowire dimension (thickness and width) demonstrated performance levels that are comparable to ITO but with the added ability to independently tune sheet resistance and transmission. Flexibility was tested using a 5mm bending radius and no degradation in sheet resistance was observed during mechanical life testing for PEN based networks. The networks also exhibited a superior and featureless optical transmission (200nm - 2000 nm) compared to ITO. Excellent corrosion resistance in ambient was observed with no degradation in conductivity after 1 year. No devices have been fabricated at this stage, but the conformal deposition of Al networks on roughened silicon device layers in solar cells was demonstrated.
POTENTIAL SOCIOECONOMIC IMPACTS
Today’s display and touch-screen technologies rely on high performing TCs. Smart phones and handheld interactive displays represent a rapidly increasing share of this market and the biggest challenges facing the sector is screen failure and limited battery life. TALNET offers the potential to replace glass with PET/PEN and the development of shatterproof screens. The reduced density of PET (1.3 g cm-3) compared to gorilla glass (2.4 g cm-3) used in conventional displays reduces the weight of these handheld devices, facilitates larger batteries or additional functionality or simply lowers weight of the devices. TALNET may also help realise the integration of flexible solar cell technologies for energy harvesting into buildings and homes, and into the design of future vehicular transportation.
These impacts are consistent with the outcome of a preliminary market assessment which pointed to (i) manufacturers of flexible solar films, and (ii) manufacturers of flexible displays as the likely customers for this technology. Direct engagement with industry revealed interest but some aspects of the process as currently envisioned are challenging to industry (e.g. lift-off process). This slow step such as lift-off would seem to preclude a high throughput roll-to-roll manufacturing process, which is important for many technologies. Also the vacuum condition associated with the metal deposition and plasma etch steps are not compatible with many current device technologies.
No cost point analysis has been undertaken at this juncture. However, it is clear the margins in the manufacture of display and solar cell technologies are very small and ease of integration into the existing manufacturing process is essential.