Manufacturers of PV devices are under constant pressure to improve the cost per watt in order to drive market growth and compete with other forms of energy. Reducing manufacturing costs is one approach that has enabled significant market growth, but continuing with this strategy alone will meet with diminishing returns and profitability for the manufacturers themselves.
Clearly, increasing the number of watts from the same device will also help reduce the cost per watt and thus there is a lot of recent attention on ways to increase the efficiency of solar cells. New materials and new applications will be required to achieve some of these gains and below we’ll look at some of the approaches being taken and the implications for material supply for crystalline PV devices.
The photovoltaic effect relies on light generating free electrons from the photovoltaic material, which are then effectively collected thus generating a current. Any loss of these electrons on their path to the contacts will reduce the efficiency of the device.
Surface recombination is one mechanism that can reduce the efficiency and this becomes more important to reduce as devices get thinner and increase the surface-to-volume ratio. One approach to minimise this effect is to “passivate” the surfaces — this helps to tie up any potential chemical recombination sites and provides a more neutral electrical surface to minimise electrical recombination.
The approach currently undergoing the most study is the deposition of a thin layer of Al2O3 typically by Chemical Vapour Deposition using tri-methyl aluminium. This material is routinely used for LED manufacturing and so the handling challenges are known but will still be new to PV manufacturers. It is a liquid at room temperature but will spontaneously ignite in air or with moisture and so it needs to be kept in a closed, dry system. Vapor is typically delivered to the process tool via a stainless steel bubbler.
Potential impact: Results show that this process can lead to a 1 percent absolute improvement in cell efficiency (roughly a 5 percent effective improvement in cell performance).
Traditionally, p-type wafers have been used for the manufacture of crystalline Si solar cells. P-type wafers were what was required by the semiconductor industry and thus there was an established manufacturing and supply capability in place with the numbers of wafers required by the semiconductor industry far outnumbering those required for PV.
That position has now changed with the solar wafer market becoming important in its own right, and thus they are now looking at what sort of wafers would be best for PV devices. P type wafers are bulk doped with Boron, but Boron can form defects with oxygen present in the bulk material when exposed to light (a process known as light induced degradation), which impacts the efficiency of the cell causing it to drop over time. Boron-doped devices are also more sensitive to metal impurities that can also cause recombination. It is possible to make wafers to minimise these effects but at great cost.
N-type wafers, which are bulk-doped with phosphorus, do not exhibit the same light-induced degradation and due to the difference in carrier type (holes rather than electrons) there is much less recombination due to metal impurities. The use of n-type wafers requires the deposition of a p-type emitter, which will require a boron source. Boron tribromide is the material currently being investigated. This has similar properties to the phosphorus oxychloride (POCl3) traditionally used for the n-type emitter used with p-type wafers and so it is only a material change rather than a material and process type change. BBr3 is also a liquid at room temperature and is highly corrosive and it traditionally delivered via quartz bubblers.
Potential impact: The use of n-type wafers can lead to an increase in efficiency of up to 1 percent absolute (approximately 5 percent effective improvement in cell performance).
Large tanks of wet chemicals have traditionally been used for batch processing of wafers for cleaning, etching, texturing, and saw damage removal. The semiconductor industry has shown that dry processing, particularly as device dimensions reduce in scale, offers more control and use significantly less chemical materials, which makes safe treatment and disposal less of an environmental concern.
As wafer thickness continues to decrease, wafer handling in chemical baths becomes more problematic and solutions already exist for single wafer mechanical handling from the experience of chip packaging. Wet processes also affect both sides of the wafer — as the PV devices become more complex, single sided processing may be required. Thus companies are beginning to explore replacing wet processing with dry and some have already implemented for texturizing using fluorinated gases. Clearly this is a significant change in both process equipment and the materials required.
Potential impact: While this may not lead to a reduction in either wafer cost or efficiency in the current generation of devices, it is seen as an enabling techniques for future high performance devices.
Traditional devices contain a uniform emitter layer onto which are deposited contacts. A balance needs to be reached between high conductivity required at the contacts and the low conductivity that is ideal elsewhere. By selectively heavily doping only the area where the contacts will go, the cell efficiency can be increased. This does require a more complex process and there is a range of techniques under investigation — printing, laser treatment or ion implantation.
Possible impact: Efficiency could be increased up to 1 percent absolute.
While reducing manufacturing costs has enabled PV devices to reach a wider market and get closer to grid parity with non-renewable sources, for applications where space is limited, then a higher cell efficiency is desirable and will also help lower the cost per watt. A wide range of techniques is under investigation, all of which can make significant improvements in efficiency.
Material changes and handling challenges compared to the existing processes are required for some, while for others a complete change in process technique is needed and not all of the approaches described will necessarily be adopted into mass production. However, the challenges of developing and delivering the required materials and the application know-how to better support the implementation of these processes, provide many opportunities for material suppliers.
By Andreas Weisheit, Head of Global FPD/Solar & Asia Market Development, Linde Electronics, Jean-Charles Cigal, OEM Program Manager, Linde Electronics, Greg Shuttleworth, Equipment Product Manager, Linde Electronics