Europe’s Plan B: thin-film tandem solar cells
To meet the 2050 energy transition goals, the world will need more than half a million square kilometers of additional solar area—roughly the size of Spain. However, current photovoltaic applications are nearing their theoretical limits. The PERCISTAND project, led by imec, has brought together experts from around the globe to push the boundaries of efficiency, achieving world-record levels of up to 30% in thin-film tandem solar cells. These advanced solar cells not only offer the potential for low-cost mass production but are also more sustainable than current silicon panels, particularly when produced in Europe. With the growing demand for solar energy, this technology presents an exciting complement to traditional silicon panels.
This article was also published in Eos Wetenschap (Dutch only).
A new dawn for solar power
Solar panels are a well-known and promising source of renewable energy, crucial to making the energy transition successful. Current solar panels on the market, which are mainly made of silicon, have an efficiency of around 25%. This means that a quarter of the available sunlight is actually converted into electricity.
In addition, their mass production, mainly in low-wage countries in Asia, makes them economically attractive. As a result, they are being used on a massive scale to generate renewable energy generators. Because current solar cells are mostly made on glass substrates – and are therefore rigid – they can only be placed on flat surfaces. This makes them ideal for roofs, facades or unused land. However, the latter is becoming increasingly controversial, with the Netherlands even banning the installation of solar power on agricultural and natural land, due to its scarcity.
According to recent estimates, if we continue to use current technologies, we will need up to 600,000 square kilometres of additional land for solar energy production by 2050 if we are to meet our climate targets – the equivalent of the entire area of Spain. The quest for efficient solar cells therefore continues, to minimise the need for additional, often costly, land, while at the same time making the technology sufficiently sustainable.
Material combinations for maximum efficiency
What if we could make solar cells even more efficient? In other words, what if we could harness more sunlight and therefore use less space to generate the same amount of energy? The theoretical upper limit for the optimal performance of a solar cell consisting of a single material is 30%. But by combining materials, we can achieve higher efficiencies.
The solution lies in tandem solar cells. These consist of two or more layers of different materials, each designed to absorb a different part of the solar spectrum. By combining a top and bottom solar cell, we can achieve the theoretical 60% energy conversion efficiency. Initial commercial options are likely to remain around a realistic 30-35% efficiency.
A promising material combination for tandem cells is that of perovskite and silicon. Perovskite, a promising material due to its attractive chemical properties, has made impressive progress in stability and scale-up in recent years. However, this combination still suffers from the limitations of silicon: only flat surfaces are suitable for placing this type of tandem solar cell.
Invisible solar power production
More efficient would be to add solar cells to existing building materials. This would provide almost invisible solar power generation without taking up valuable space. However, these solar cells require a completely new approach to their manufacture, as they are no longer placed on existing flat surfaces, but integrated into existing materials and therefore much more flexible in nature.
Solar cells based on thin-film material are ideally suited for this application. In thin-film solar cells, a wafer-thin layer of photovoltaic material is applied to various substrates, such as flexible plastics or light metals. However, the technology is still in its infancy. Enfoil and Soltech, both spin-offs from imec’s EnergyVille research, have already shown that it is possible to integrate thin-film solar cells into vehicles or buildings.
For example, the integration of a solar roof into an electric vehicle (EV) only becomes interesting when the efficiency is high enough to relieve the car’s battery. The same applies to building integration: the higher the efficiency, the less dependent on the grid, the better. So why not combine two thin-film materials on the same surface to increase energy conversion? Such combinations not only offer higher efficiency, but also retain their flexibility and lightness, making them suitable for various applications.
PERCISTAND: a promising combination
An interesting thin-film combination is that of perovskite and Cu(In,Ga)Se2 (CI(G)S, a combination of copper, indium or gallium and selenide). This combination has been investigated in the PERCISTAND-project, a Horizon 2020-funded initiative led by imec and involving both academic and industrial partners. Coordinator Professor Bart Vermang (EnergyVille/imec/UHasselt): “Theoretically, the combination of a perovskite and CIS solar cell is very interesting, but it requires two different fields, that of organic and inorganic thin films, to work together. This is also the strength of our tandem technology within the PERCISTAND consortium; for the first time, we have brought together experts from Europe, Australia and the US.”
While the properties are fixed in traditional solar cells by the silicon layer, CIS offers the possibility of playing with the exact proportions (e.g. adding more gallium to the mixture) and thus also adjusting the physical properties. This allows the CIS solar cell to be perfectly matched to its corresponding top or bottom cell (here: perovskite) to capture photons as efficiently as possible.
“With this combination of materials, we have succeeded in producing stable solar cells and have broken world records for their efficiency: up to 30% for cells smaller than 1 cm2, and 21% for 5 x 5 cm2 modules. The consortium has produced more than 50 peer-reviewed publications, including in the prestigious Nature Energy en Joule journals.”
Europe's strategic issue: (in)dependence?
Efficient thin-film tandem solar cells therefore offer a Plan B as Europe faces an important decision: do we continue to rely on cheap PV imports from China, or do we strategically rethink our own production capacity? The question is all the more urgent as China has gained a dominant position in the global solar and electric vehicle markets, with 80% of the world’s solar PV production capacity and a growing market for EVs.
In February 2023, the Commission has therefore presented the Green Deal Industrial Plan to strengthen the competitiveness of Europe’s net-zero industry and accelerate the transition to climate neutrality. This plan, together with the Net-Zero Industry Act of March 2023, aims to produce at least 40% of the net-zero technologies needed in the EU by 2030. These measures should reduce dependence on imported technology and create a more sustainable energy system.
From laboratory to marketplace
But it is a long way from laboratory tests to market launch. Professor Sebastien Lizin (EnergyVille/UHasselt) and his team have therefore calculated how feasible and sustainable the PERCISTAND technology is in the long term. They calculated the levelised cost of energy (LCOE, a measure of the total cost of an energy project versus the energy yield) and the GHG emission factor (GEF, a measure of kg CO2 equivalent emitted per kWh of energy produced) per technology, for its production and deployment in either the EU, the US or China. This took into account projections for the composition of the energy mix in each region and differences in labour and material costs.
This analysis showed that thin-film tandem solar cells, when produced at the megawatt scale, could potentially be competitive with silicon in terms of price (provided a competitive lifetime can be achieved and scale-up losses are limited). In addition, they were also found to offer a more sustainable option for the future of solar power production. If produced in Europe, they would result in almost 60% less CO2 emissions than traditional silicon PV production in China.
Professor Lizin explains, “Compared to traditional silicon panels, the production of thin-film tandem solar cells requires less energy and raw materials, resulting in a similar LCOE and lower carbon footprint. In addition, the sustainable electrification of our energy system is already much further advanced than in the US or China, and will remain so even as their energy transition plans come into effect.”
The road to renewable energy
It is clear that additional production capacity will be needed in the coming years to meet the growing demand for solar energy. In addition to today’s silicon solar cells, thin film tandems now present an opportunity to diversify our European supply chain. The goal is clear, the knowledge is there and the technology is in full development. The question now is: which path do we take to a more sustainable electricity system by 2050?
