Researchers from the School of Photovoltaic Renewable Energy and Engineering (SPREE), University of New South Wales, estimate that the annual photovoltaic (PV) module production rate must increase from ~135 GW in 2020 to ~3 TW (terawatt) in 2030 to make any significant impact on climate change. However, a substantial obstacle on the pathway towards sustainable manufacturing at a multi-TW scale is the reliance of commercial PV production on scarce metals like silver, indium, and bismuth.

The primary concern in heading towards sustainable PV manufacturing at the TW scale comes from silver due to its widespread use in all major industrial solar cell technologies. It contributes a significant fraction of the non-wafer fabrication cost of the solar cell. Silver consumption level must be reduced to less than 2mg/W to enable a 3 TW market.

The current consumption of silver for industrial PERC solar cells is approximately 15.4 mg/W. In the tunnel oxide passivated contact (TOPCon) and Silicon heterojunction solar (SHJ) cells, silver is used on both the front and rear contacts. So, consumption is roughly double at 25.6 mg/W (TOPCon) and 33.9 mg/W (SHJ). This would result in respective silver limited sustainable manufacturing capacities of 380 GW, 230 GW, and 170 GW, given 20% of the 2019 global silver supply.

Although International Technology Roadmap for Photovoltaic (ITRPV) projections expect a 50-60% reduction in silver usage for each of these mainstream technologies over the coming decade, the expected values in 2031 are 8.5 mg/W, 13.8 mg/W, and 14.3 mg/W, respectively, still well above the 2mg/W targets.

Silver’s offtake by the PV industry reached 101 million ounces (Moz) or 3,142 tons in 2020, according to the ‘World Silver Survey 2021’ by the Silver Institute. The survey said that the demand for the precious metal surged despite an 80% drop in the average silver loading per cell over the last decade.

The Silver Institute had also forecast a global demand of over 1 billion ounces of silver in 2021. Spot silver prices stood at $26.11/ounce in March 2021. In February, the Institute had reported that silver prices hit $31.10/ounce, an eight-year high.

SPREE researchers also suggest solar cell technologies incorporating copper plating as a pathway to reduce silver consumption. They maintain that copper plating technology compatible with sustainable TW-scale manufacturing is already available and has been successfully deployed for large-scale production by numerous companies.

Indium does not pose a challenge for the mainstream passivated emitter and rear cell (PERC) or the emerging TOPCon solar cell technologies. Indium only poses a potential challenge for the PV industry if it deploys technologies requiring transparent conductive oxide (TCO) layers, such as SHJ solar cells and futuristic tandem devices. Current SHJ solar cells with 200 nm of indium tin oxide (ITO) (100 nm on both surfaces) consume approximately 10.7 mg/W of indium. This provides a minimal sustainable manufacturing capacity of less than 40 GW. Consumption must be reduced to 0.38mg/W to enable a 3 TW market of solar cells using indium.

This would equate to no more than 3 nm-thick of ITO layers that can be tolerated for a 30% efficient tandem solar cell. As such, the accelerated development and deployment of indium-free TCO layers is critical for current SHJ solar cells and to replace interlayers in future tandem devices. Essentially no solar PV technology requiring indium can be manufactured at scale sustainably.

The researchers argue that even at an upper material consumption limit of 20% of the 2019 global supply, the present industrial implementations of PERC have a sustainable manufacturing capacity of 377 GW, limited by silver supply. Tunnel oxide passivated contact has a capacity of 227 GW, also determined by silver supply. At the same time, SHJ cells have a capacity of 37 GW, limited by indium supply.

The researchers propose material consumption targets of 2 mg/W, 0.38 mg/W, and 1.8 mg/W for silver, indium, and bismuth, respectively, indicating that significant material consumption reductions are required to meet the target production rate sustainable multi-TW scale manufacturing in about ten years from now.