LONDON -- Long-term forecasts on the availability of silver, the most widely used electrode material in solar photovoltaic technologies, suggest that the price of this already valuable material is likely to rise as demand from the solar industry soars.
The photovoltaic market is currently still dominated by crystalline silicon (c-Si) solar cells – which make up 85% of roof-top installations – and production costs of silicon-based PV are continuously being lowered by measures such as improved manufacturing practices, up-scaling of fabrication installations and vertical integration within the value chain as well as efforts to reduce the volume of silicon required per unit of rated output – with the cost of Si representing about a third of the module cost.
However, while silicon is the earth’s second most common element in a solid state and is therefore available in considerable abundance, taking into account the increasing size of the PV market, some experts predict a shortage of silver – which also accounts for a large part of the solar cell’s manufacturing cost – within the next decade. If nothing is done to address this trend these materials costs will be expected to increase dramatically.
One way to tackle this issue is through recycling of existing materials which reduces consumption. PV Cycle and member companies like SolarWorld are already actively recycling solar panels for re-use of materials like silicon and silver, and recycling in general contributes to both more sustainable production of solar cells and reduced energy payback time. However, while this process is desirable, it will probably not be sufficient to prevent the depletion of silver supplies, particularly as production ramps up.
Indeed, a major avenue of solar PV research is dedicated to discovering and developing alternatives which offer similar conductivity and mechanical qualities but at considerably lower costs. One promising substitute for this material is copper, and results emerging from the industry indicate that copper contacted silicon solar cells are emerging as an important alternative.
Sourcing Alternative Materials
Using copper as an electrode material for solar PV cells holds great potential in terms of sustainability and cost effectiveness, but, according to imec scientists Dr Jef Poortmans and Dr Joachim John, such a move also presents a number of challenges for the solar cell manufacturing engineer. First of all, copper is a lifetime killer for silicon-based solar cells. This is as a result of copper diffusing into the silicon where it forms a trap for the charge carriers in the semiconducting material. Consequently, a diffusion barrier is required that prevents this process from occurring. Secondly, copper, unlike silver, oxidises into a porous compound when exposed to air. Addressing this issue requires extra protection of the electrode contact, for example by capping.
Thirdly, the use of copper as an electrode material increases the complexity of the solar cell manufacturing process. Current ‘standard’ crystalline silicon solar cell production involves the process of silver metallisation and ‘co-firing’ which consists of depositing silver paste on the surface of the cell via a screen-printing technique. In this way, a silver-based conductive grid is formed that compromises between high electrical conductivity and light absorption by the underlying silicon nitride layer which is used for passivation and antireflection purposes. Subsequent co-firing comprises a simple high-temperature step that allows the silver paste to pass through the 80 nm thick silicon nitride layer and make contact with the silicon. Key to this process step is the silver paste, which typically consists of a suspension of fine particles of silver and glass. For copper, no similar paste is available and consequently extra process steps are needed to establish electrical contact.
For example, in order to make contact with silicon semiconductor material, the silicon nitride passivation layer must be opened by either etching or laser ablation techniques. Subsequently, a diffusion barrier – a series of materials such as titanium nitride, tantalum nitride and nickel have been investigated for use as an effective barrier layer against copper diffusion – should be deposited followed by copper deposition. The latter can be done by electroplating, a technique that is well known in the metal industry.
Introducing copper into silicon solar cell production will thus increase the complexity of the process and will inevitably drive up the costs of the manufacturing process. However, since a significant part of the manufacturing cost is related to the purchase of raw materials, this extra processing cost is expected to be largely compensated for by the use of more readily available and cheaper materials.
Another key issue for sustained production growth for solar power is potential impact on conversion efficiency. However, research work at imec apparently indicates that introducing copper has a positive impact on cell efficiencies. The research agency claims that higher efficiencies can be obtained with copper-plated solar cells compared with cells based on screen-printed silver contacts – stating that efficiencies with silver contacts were up to 19.5% whereas 20.0% was obtained with copper contacts. These results were achieved on large-area cells of 148 mm2 with 160 µm thickness, proving the industrial viability of the process. Poortmans and John suggest they expect that further improvements will enable efficiencies of up to 21%, a target that they argue would be hard to reach with screen-printed silver contacted cells.
Certainly imec is not alone in making claims for high efficiency from copper metallisation technologies. For example, late in 2011 Schott AG stated that it had achieved cell efficiencies of 19.7% with a silver-free solar cell by combining several of its most recent development achievements.
The company says it was able to use standard production processes to manufacture a high-efficiency solar cell that contains no silver, using frontside electroplated copper contacts. The backside is passivated using Passivated Emitter and Rear Contact (PERC) technology and aluminium screen printing. This new type of cell is based on an industrial wafer size of 156 mm2 and was confirmed by the Fraunhofer ISE. If long-term stability can be demonstrated successfully, Schott says, it will no longer be necessary to use expensive silver in cell manufacturing.
‘This new cell represents a real milestone on its way to achieving higher output at lower production costs,’ explains Dr Axel Metz, director of solar cell development at Schott Solar. ‘We have been exploring every possible aspect of this technology over the last few years and have made important progress these last few months. Now, we were successful in combining these excellent results in a new cell. The next step will be to provide actual proof of its long-term stability,’ he adds.
This development followed on the heels of another copper-based technology in which Schott Solar says it was also able to achieve efficiency of 18% in the research project Las VeGaS while using nickel-copper plating on the front side of a multicrystalline cell. The company says the manufacturing technology used can reduce the costs of front side metallisation by over 50%.
Launched six months previously, the Las VeGaS project – in which Schott is working together with RENA GmbH and CiS Forschungsinstitut für Mikrosensorik und Photovoltaik GmbH – used a multicrystalline wafer from Schott Solar AG that features standard screen-printed backside metallisation. The goal of the Las VeGaS project is to largely replace the silver contacts with less expensive nickel-copper plating.
In order to overcome the copper diffusion issue the project team has developed an electroplated nickel layer that serves as a diffusion barrier as well as the appropriate manufacturing techniques for applying both the nickel barrier and the copper contacts to the cell.
The project team says it has used RENA’s new ‘InCellPlate’ technology to manufacture rather promising prototypes on standard industrial tools. These solar cells will now be used to fabricate test modules so that they can demonstrate their long-term stability in reliability tests, they add.
Imec's cu contact cells (Source: Imec)
The company further claims that the Las VeGaS method offers another advantage: the electroplated layers are environmentally friendly because they are free from both lead and solvents and thus meet the requirements of the EU RoHS Directive which places restrictions on the use of hazardous substances in electrical and electronic devices. Now, only a very thin electroplated silver layer is needed to solder the cells to the copper tabs to make a module. This, in turn, lowers the consumption of silver by at least 95%, they claim.
RENA GmbH is one of the largest suppliers of process technology for wet chemical applications, mainly for use in the PV industry. CiS Forschungsinstitut für Mikrosensorik und Photovoltaik GmbH is an institution dedicated to application-oriented research and development. The Las VeGaS project receives funding support from the German Federal Ministry of Education and Research as part of the ‘Innovation Alliance on Photovoltaics’.
Similarly, in the ETAlab at the Fraunhofer ISE – a laboratory for new solar cell structures and processing steps – the technology of producing solar cell contacts using industrially feasible galvanic processes to replace silver, mostly by copper, has been demonstrated. Fraunhofer researchers claim to have achieved a solar cell efficiency of 21.4% using this approach, comparable with values from solar cells using a highly efficient titanium/palladium/silver contact system, which must be created in comparatively expensive vacuum laboratory processes.
With the enormous cost difference between silver and copper, simply by changing the material and keeping the efficiency the same, it is possible to reduce the production costs by about 8 cents/Wp, or in other words, by up to 10%, Fraunhofer ISE says.
Again, in order to ensure the loss-free operation of the solar cell, nickel was used as a diffusion barrier. Nickel can, in addition to the required barrier function, also create an electrical contact to the silicon. Furthermore, it offers the advantage that, like copper, it can be deposited onto the solar cell with low-cost galvanic processes. A galvanic nickel-copper system on printed silver contact layers, the current standard process of the industry, is a first possible use for this reason. With only minor adjustments to industrial production lines, the costs here can be dramatically reduced.
Even greater efficiency potential is provided by the galvanic nickel-copper system with direct deposition to silicon, without a printed silver contact layer. Using an industrially feasible process, such as laser ablation, the anti-reflection coating (ARC) is removed locally. Structural widths in the range of 20 µm are achieved, which significantly reduces shading in comparison to screen printing. In the affected areas of the ARC, nickel will be selectively deposited, which is then reinforced and made solderable by the addition of copper and zinc or silver. On solar cells with front and rear side passivation in a 2x2 cm² format, this technology reached an efficiency of 21.4% in ETAlab, as confirmed by CalLab PV Cells at Fraunhofer ISE.
‘The metallisation based on copper and nickel offers a significant potential for cost savings for the next generation of silicon solar cells, and with it for power from sunlight,’ says Dr. Markus Glatthaar, head of the Advanced Processes Group.
Most recently, Korea’s Hyundai Heavy Industries (HHI) announced that it has also achieved a 19.7% conversion efficiency in copper-contact solar cells, in this case using standard 156 mm commercial-grade p-type silicon wafers. The efficiency was independently confirmed by the Fraunhofer ISE.
Researchers and engineers at HHI – currently the largest cell and module producer in Korea – say they have improved UNSW’s original LDSE (laser-doped selective emitter) technology to create their new solar cell by copper plating. The company claims the world efficiency record with its copper-front-contact selective-emitter cell with a full-area aluminium-alloyed back electrode. The previous record for cells created with the LDSE process was 19.6%, achieved by a Chinese company on smaller 125 mm wafers.
As in the original LDSE technology, a laser-based selective doping process was combined with a plating technology to form copper contacts. They claim costs are 10%-30% lower when compared with regular six-inch cells.
‘The new cell is a critical milestone in our ongoing effort to improve the performance of our photovoltaic products while reducing the production costs,’ said Dr. Choong-dong Lee, COO of HHI’s Green Energy Division. ‘The record-setting new solar cell technology is a testament that we now compete head-to-head with the leading solar institutes and companies in terms of R&D,’ he added. HHI has recently restructured its R&D workforce, and the researchers in the renewable energy sector have been brought together as the Hyundai Green Energy Research Institute (HGERI). A new solar R&D centre is being prepared in Eumseong and existing research tools are being relocated into the centre, along with newly purchased, state-of-the-art research equipment.
Copper's Cost Advantage
Reductions in the electricity generation costs for PV can be reached through two mechanisms: improving the efficiency of solar cells and reducing their production costs. With advanced processes for metallisation of solar cells, both effects can be reached at the same time. Copper can be deposited from chemical solutions in galvanic processes, which are economical and have high rates of deposition. If the solar cell efficiency is further improved with such industrially feasible processes, the advantage of the specific costs will be even higher.
The challenge of the solar cell metallisation with copper lies in the creation of a homogenous and qualitatively high-value layer between silicon and copper. This serves as a barrier against diffusion of copper into the semiconductor. Copper-based front side metallisation in Si solar cells is a significant step towards lower cost.
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