EV-expert talk: material sustainability aspects for PV technology
Solar energy is one of the most important sources of energy for decarbonising our economy. Since strong growth is still expected in this sector, it is important to consider not only efficiency, but also the sustainability of PV- production. In this Expert Talk we take a closer look at the sustainability aspects of various PV-technologies.
Written by Jonathan Parion, PhD student at IMO-IMOMEC, and Bart Vermang, professor at UHasselt and imec.
The multi-TW era
Solar photovoltaic (PV) electricity generation is currently foreseen as key energy for decarbonizing our economy. It offers many advantages such as low cost, good reliability, low maintenance and very high integration capabilities. Several scenarios are currently considered for the predicted installed PV capacity in the coming years, as also presented in figure 1 from the ITRPV roadmap (1).
Each of these scenarios considers a different target of electrification by 2050, which is a very important key to decarbonize our society. Except for the most conservative scenario, there is a consensus that the annual PV market will need to grow beyond the TWp before 2050. In this so-called “TW market”, 1TWp of additional PV capacity is installed each year, which is approximatively equivalent to 100 modern nuclear power plants (i.e. with an individual capacity of 1.6GW). For comparison, the total installed capacity in 2022 was about 268GWp (2) or 27 nuclear power plants. In the most optimistic scenario, which is also the one most likely to help us meet the 2015 Paris agreement, it is estimated that this annual market should even grow to 1 TWp before 2030, and to more than 3 TWp after 2040. As a result of this increase in production, there will be an equivalent increase in material needs. Therefore, it is of the utmost importance to investigate whether the current technological requirement in materials is compatible with the TW market.
Material considerations in a multi-TW solar PV market
In a recent study by P. Verlinden (3), the material challenges for PV manufacturing at the TW level were investigated. From this study, several materials such as copper, aluminium, steel, glass and polysilicon are identified as non-critical towards the TW era. The term non-critical mainly implies that their production will remain high enough so that they do not pose scarcity issues for long-term PV manufacturing. However, it is still possible that due to high pressure on supply and demand, these materials cause significant PV cost fluctuation. This was recently observed in the case of Polysilicon, which constituted 12% of the total cost of a PV module in 2019 and had increased to 44% of the total cost in 2021 (4) due to production price increase. Despite this, because of the abundance of the raw material (quartz), it is expected that production capacity will strongly ramp up in the following years, therefore pulling the prices down. Overall, because of the non-criticality of these materials at PV module level, they are not the focus of this study. Nevertheless, it is necessary to remember that they might become an issue when considering implementation in a large system, and that solutions such as recycling of these materials should be implemented as soon as possible.
Putting aside these earth abundant materials, the real challenge towards the multi-TW scale of PV production actually lies in the scarcity of some other materials (3, 5). This scarcity issue is nowadays acknowledged by an increasing number of scientists but also industry leaders such as Trina Solar (second largest producer of PV in 2022). Materials such as silver, indium and bismuth, are more than a million time less abundant than Silicon in the Earth’s crust (Figure 2). These materials are used in the three main PV technologies in diverse quantities, mainly depending on the type of electrical contact that is used, as presented in Figure 3.
To quantify the material needs for a certain technology, the material consumption in tons is generally considered for each GW of PV installed (t/GW, also equivalent to mg/W). In order to remain sustainable, the annual consumption of each material for PV cannot exceed 20% of its annual production. This threshold accounts for all other usages of these materials (5). In function of this, a maximal consumption of a material is established for targets of respectively 1 TWp or 3 TWp of production a year.
In the current PV technology (PERC), silver is the main limiting factor for sustainable manufacturing (Figure 4). For a targeted production of 1 TWp, the silver consumption in each module should be at least two times lower, and for a production of 3 TWp it should be at least six times lower. This is even higher for other technologies that use silver for both the front and back contact (TOPCON & SHJ). In Figure 4, the expected reduction forecasted by ITRPV due to technology improvement is also represented. It appears that this is not ambitious enough to enter the TW market, and therefore other silver-free contacts should be considered. Among these, the use of copper and aluminium are currently under investigation.
Indium is also a very important limiting factor when considering the SHJ technology (Figure 4). It is used in transparent conductive layers (TCOs), under the form of indium-tin oxide (ITO), that helps to increase the lateral conductivity of the electrical contacts. In order to achieve a potential production of 1 TW/y, the indium consumption should be reduced by at least a factor 4. This could be a solid argument against a market transition towards this technology, in favour of PERC (and/or TOPCON, but that consumes more silver). To reduce the indium consumption in SHJ cells, other indium-free TCO layers should be investigated.
Finally, bismuth is another material that could cause scarcity issues in the near future. It is currently used in low-temperature soldering paste, the so-called “SmartWire technology”. It is mostly interesting for manufacturing of SHJ solar cells, whose reduced thickness imposes lower processing temperatures, but could be used in the future in other technologies as well. The low-temperature soldering constitutes a significant advantage, since it enables the transition to thinner and larger wafers that increase the cost-effectiveness of the manufacturing process. Viable options of low-temperature, earth abundant, solder must thus be investigated to tackle this challenge
The case of tandem solar cells
As single-junction solar cells are reaching their practical efficiency limit, more and more research focus is put on tandem solar cells (6). These combine two different absorption layers to reduce the so-called “thermalization losses” which are responsible for less absorbed sunlight and therefore a lower overall efficiency. The ITRPV roadmap predicts this technology to enter the market around the year 2026. It is therefore of particular interest to also raise the question whether these tandem cells are sustainable on a material standpoint. In the case of silver and indium, this is presented in Figure 5.
In the case of silver, it appears that the transition to tandem solar cells is very beneficial, with PERC tandem technology closely approaching the target for the 1 TWp annual production. This is mainly due to the increased efficiency of the cell, for an equal amount of silver used. In the case of indium, the picture is mitigated. At the moment, there is a large variation in the reported indium consumption depending on the manufacturer as well as on the exact technology used. Four terminal (4T) tandems are not sustainable in terms of indium consumption, due to the higher quantity of TCO required. Two terminal tandems have the potential to require less indium if PERC is used instead of SHJ as a bottom cell. Moreover, there is a dependence on the amount of indium required for the perovskite top cell, which can vary largely. Overall, the key message is that tandems have the potential to be more sustainable than single junction cells, but this sustainability aspect must be considered early in their development process.
Key takeaways
- The main takeaway message is that there is an urgent need on focusing not only on improving efficiency in PV, but on guaranteeing a sustainable PV manufacturing even in a multi-TW annual market. At the moment, there is no technology that is able to guarantee this, even at 1 TW level (Figure 6).
- We need to be much more ambitious in our targets than what is suggested by the ITRPV report, in which material scarcity is to this day not even considered.
- Reduction of silver in contacts and indium-free transparent conducting layers are the main technological keys for sustainable PV production.
- The PERC technology that is currently on the market is the most sustainable single-junction technology, and it could eventually be joined by 2T-tandems. By reducing silver consumption and using indium-free TCOs, these should together enable the transition towards the TW market.
References
1. International Technology Roadmap for Photovoltaic (ITRPV) (2022).
2. E. F. Dijeau, Global solar capacity additions hit 268 GW in 2022: BNEF. Pv Mag. USA (2022), (available at https://pv-magazine-usa.com/2022/12/23/global-solar-capacity-additions-…).
3. P. J. Verlinden, J. Renew. Sustain. Energy. 12, 053505 (2020).
4. What’s next for polysilicon? – pv magazine International, (available at https://www.pv-magazine.com/magazine-archive/whats-next-for-polysilicon/).
5. Y. Zhang, M. Kim, L. Wang, P. Verlinden, B. Hallam, Energy Environ. Sci. 14, 5587–5610 (2021).
6. The tandem module, a unique opportunity to reboost EU PV industry, (available at https://www.imec-int.com/en/imec-magazine/imec-magazine-june-2020/the-t…).