The fifth edition of the International Technology Roadmap for Photovoltaic (ITRPV) released today, continues to emphasise the critical need to reduce material costs from wafers to modules, but also highlights the need for increased cell efficiencies and higher throughput fabrication equipment as material cost reductions get harder to implement.

The latest ITRPV reports that the cost reduction learning curve, in which the doubling of cumulative PV module shipments results in the average selling price declining by over 20%, has continued and is expected to do so over the next few years.

However, increased global PV demand has seen polysilicon prices increase as well as solar cells, which the ITRPV noted have not been fully transferred to the price of PV modules, adding to the challenges of overall cost reductions.

Indeed, pricing of polysilicon, wafers, multicrystalline solar cells and modules has been stable throughout 2013, according to the ITRPV, which means that prices are not expected to compensate for cost increases. As a result, driving cost reductions in consumables and materials throughout the manufacturing process remains the key task for the industry during the current upturn.

Polysilicon and wafers

Although FBR technology is expected to gain further market share and provide lower cost polysilicon, the transition to FBR granular polysilicon is expected to take a further 10 years before accounting for over 40% of the market.

The latest ITRPV notes that polysilicon producers do not expect material quality reductions tp lead to any significant cost reductions and therefore PV-grade polysilicon of between 8N and 9N purity levels will remain as the standard for polysilicon supply.

Emphasis is therefore being placed on reducing wafer costs, which include a transition to larger ingots, diamond wire cutting and thinner wafers. Further improvements in recycling at the wafering stage were also said to be needed.

Silicon wafers account for approximately 57% of the current solar cell price, according to the report.

Although diamond wire reduces kerf loss, further cost reductions are required, which should see the development of using smaller diamond sizes to enable thinner wafers and further kerf loss reductions.

Yet over the last few years little has been achieved in wafer thickness reduction, with the majority of the industry sticking with wafer thicknesses of 200 microns to 180 microns.

Wafer thickness reductions are expected but pace of development for conventional modules is slow. The interesting aspect is what is characterised by ‘alternative module technology', which opens the door to a rapid wafer thickness reduction.

Although the ITRPV doesn’t explain what these alternative module technologies are, it falls into the field of wafer cleaving such as GT Advanced Technologies ‘Hyperion’ tool which is expected to enable wafer thickness in the sub 50 micron range.

The ITRPV indicates that alternative module technology could enable the use of wafers as thin as around 25 microns by 2024, with significant thickness reductions starting in the 2016 to 2018 timeframe.

Emphasis is also placed on consumables such as crucibles, graphite parts, slurry and sawing wires that have a cost/price reduction potential of between 5% and 10% per year. However, diamond wiring pricing would need to reach 25% of 2013 price levels by 2023 to achieve the expected annual savings, according to the ITRPV.

Solar cells

Not surprisingly, metallization pastes/inks containing silver (Ag) and aluminium (Al) remain the most process-critical and expensive materials used in current solar cell processing with the ITRPV continuing to push for lower paste consumption as the price of silver is expected to remain relatively high.

The good news is that metallization paste reductions have increased substantially with the ITRPV noting only 100mg of paste is actually deposited on cells today. The key reason for this has been the introduction of new pastes as fine-line printing.

However, the migration away from silver to copper has been delayed due to the lack of progress in screen printing and lack of improvements in reliability and adhesion. As previously stated in the fourth edition of the roadmap, the introduction of copper into mass production is not expected to start before 2018.

Modules

Several interesting developments are expected in relation to overall module cost reductions. Based on the recent ITRPV survey, aluminium frames are expected to remain in dominant use, however from 2018 the share of frames constructed from plastic or other alternative metals are expected to increase by up to 20% by 2024.

The cell-to-module power ratio is also expected to exceed 1.0 for both modules with acidic-textured (multicrystalline) and alkaline-textured (monocrystalline) cells in the 2016 to 2018 timeframe. This is due to light management improvements within the module in order to redirect light from active module areas on to active cell areas as well as due to new interconnection and encapsulation technologies boosting module performance.

New interconnection technologies also hold the promise of eliminating several process steps such as tabbing and stringing, further reducing silver consumption and lowering production costs.