Device Metallisation (PP4.1)

PP4.1: Device Metallisation

 

This work package will focus on the development and characterisation for device metallisation, with a focus on industrially relevant processes which can enable electrical connection between cells in a module. As such, this stream will complement research being conducted in PP1, PP2 and PP3. Some initial device metallisation activities are described below, however it is expected that new activities will emerge during the lifetime of the ACAP program as metallisation challenges presented by new emerging devices become evident.

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PP4.1.1 Advanced Screen Printing

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Investigator: Brett Hallam

 

Due to its simplicity and constant improvements, the International Technology Roadmap for Photovoltaic (ITRPV) projects that screen-printing will maintain its position as the mainstream metallisation technology. A key challenge for screen-printed solar cells is bringing down silver consumption to values below ~5 mg/W (20 mg/cell), well below the ITRPV target for PERC at ~8 mg/W in 2030 (50 mg/cell).

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Silver is the highest non-silicon cost in the cell fabrication step, and hence work is required to reduce silver consumption not only from a sustainability perspective, but also for the levelized cost of electricity. With conventional metallisation designs using homogenous screen-printed fingers, resistive losses will become prohibitively high as we target silver reductions needed for terawatt scale manufacturing. This is particularly the case for solar cells like TOPCon and SHJ that currently use silver metallisation on both the front and rear of the device.

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Another key challenge is reducing metal/silicon interface area below 2%, even with futuristic narrow 20-micron finger widths. Work in this package will develop novel approaches to reduce metal/silicon interface areas to 1-2% independent of print-widths using both metal and insulating pastes, and modified metallisation designs to increase efficiency for a given consumption of silver. Activities will also focus on specific challenges for tandems and other devices requiring low-temperature metallisation and interconnection schemes.

 

In particular, it will draw on the unique opportunity for tandems to maintain using screen-printed contacts with <5 mg/W silver or alternative conductors due to the low inherent resistive losses. The project will also investigate opportunities for new screen-printing approaches at the cell level with electrically conductive adhesives and alternative materials to integrate with novel module fabrication processes including low-temperature interconnection technology for SHJ and tandems, using both screen-printed contacts and plated contacts.

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Objectives

• Investigate impact of using alternative (non-silver) screen-printed conductors on cell performance and reliability for at least three non-silver conductors

• Demonstrate finished solderable screen-printed solar cells with <5 mg/W silver consumption on single junction and tandem devices, well below the 2021 ITRPV target for 2030

• Demonstrate a modified interconnection method drawing on screen-printing suitable for solar cells with screen-printed and plated contacts

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PP4.1.2 Metallisation of Passivated Contact Cells

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Investigators:  Brett Hallam

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Passivated contact higher efficiency structures are increasingly being seen as a logical next step to PERC. Cell efficiencies exceeding 25% on large-size industrial Cz wafers have been demonstrated for both SHJ and TOPCon cells, however, for both technologies the cost of silver metallisation is currently a barrier to wide scale deployment.

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This task will focus on low silver and silver-free metallisation solutions for passivated contact cells. It will include fundamental studies into copper penetration through various passivating contact layers (e.g., polysilicon and a range of transparent conducting oxides), as well as the development of patterning processes in collaboration with activities in PP1.

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Objectives

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• Develop metallisation processes for passivated contact cells with < 50 mg silver per M6 cell (or equivalent) by 2030.

• Investigate the possibility of copper diffusion through passivating contact layers.

• Demonstrate silver-free or low silver passivated contact metallised cells with an efficiency exceeding 25% by 2030.

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PP4.1.3 Plated Metallisation for Silicon Tandem Cells

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Investigators: Anita Ho-Baillie, Ned Ekins-Daukes, Dr Heping Shen 

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Although a large effort has been directed at perovskite-silicon device fabrication and efficiency, very few studies have investigated the metallisation of these devices in commercial tandem devices on industrial-sized silicon wafers.

 

This activity will be conducted as a collaboration between UNSW, the University of Sydney and SunDrive Solar using high efficiency SHJ cells as the bottom cell of the tandem device. Of particular interest, will the adaptation of SunDrive’s proprietary patterning and copper plating process for TCO-coated SHJ cells to the metallisation of the top perovskite cell. In the later phases of this activity, cell interconnection processes developed for SHJ cells will also be adapted for the perovskite-silicon tandem devices, thereby providing a direct commercialisation route than can result in higher efficiency silicon tandem modules in the field.

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A key challenge that will need to be addressed in this activity is the development of adaptions to both the cell metallisation and interconnection methods that can avoid moisture degradation of the perovskite material and the carrier transport layers. New light trapping strategies will also need to be devised to maximise light trapping in the module. These strategies will be modelled and simulated in partnership with PP4.2 activities.

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This activity will also include the development of cell metallisation methods for silicon tandem devices comprising GaAs top cells.  A key challenge for this process is the use of optimal interface contact layers.The advanced metallisation work can also benefit non-silicon multi-junction solar cells, such as those made from III-V-Ge semiconductors that are used in high efficiency solar concentrator systems. 

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Objectives

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• Investigate processes and optimal interface layers required for copper-plated GaAs top cells

• Develop new processes to replace photolithography for the fabrication of metal contacts on GaAs top cells of silicon tandem devices.

• Demonstrate 45% CPV efficiency on commercial epitaxial wafers, 50% CPV solar cell efficiency on research lab scale devices.