How tandem solar cells can speed the energy transition

Solar energy is an abundant and renewable energy resource. In fact, the amount of sunlight that hits the Earth in a single hour contains more energy than the world needs in a year. Sunlight can be converted directly into electricity with a device called a photovoltaic cell (more commonly known as a solar cell). Traditional silicon-based solar cells are in use worldwide. However, further advancements are already in the pipeline.

Now, tandem solar cell technologies – specifically, stacking an ultrathin perovskite solar cell on top of a standard silicon solar cell – are breaking records in converting sunlight into electricity. With tandem solar cells now achieving power-conversion efficiencies of over 30%, experts say these high-tech photovoltaics will play a major role in the much-needed rapid transition to renewable energy.

“We are very excited about these tremendous advances,” says Steve Albrecht, Head of the Young Investigator Group of Perovskite Tandem Solar Cells at Helmholtz-Zentrum Berlin (HZB), Germany. “They give us hope that this technology can make an important contribution to a sustainable and affordable energy supply not only for a large part of the population but also for the fight against climate change in the coming years, because the upscaling and industrial production of perovskite-silicon tandem solar cells is also feasible.”

Today’s silicon-based photovoltaics convert only a small range of longer wavelengths of sunlight into electricity. Perovskite, a lightweight, low-cost semiconductor compound, can be “tuned” to absorb the shorter wavelengths of light that silicon solar cells miss. When combined, an ultrathin layer of perovskite on top of a silicon solar cell can convert more sunlight into usable electric energy than either cell alone.

The trick is that the perovskite layer uses the entire wavelength range of visible light [380 – 700 nm] and converts it into electric current. Near-infrared light [800 – 2,500 nm], on the other hand, penetrates the perovskite layer, hits the silicon cell underneath and is converted into electrical energy. By working in tandem, the solar cell duo increases its power-conversion efficiency to over 30% (the amount of energy converted from the sunlight is 30% of its total incoming energy). Theoretically, 40% is possible.

In addition to HZB, teams from several other research institutes and industries have been making progress in increasing the efficiency of tandem solar cells:

Unlike silicon, perovskite properties can be customized (tuned) by the manufacturer to best fit their needs. By using different ratios of materials to build the perovskite, the manufacturer can create a perovskite with a specific colour that absorbs a specific wavelength range of light. Perovskite cells are also much thinner, lighter and less costly to manufacture than their silicon counterparts.

A report from the UN climate science panel states that renewable energy “can and should” supply 90% of electricity by 2050 to give the world a good chance of limiting warming to 1.5°C per the Paris Agreement and avoiding the worst impacts of climate change.

By 2050, the global demand for electricity is predicted to reach 75 terawatts (TW). To meet this demand with renewable energy sources, it is crucial for the photovoltaics industry to grow at an annual rate of 25% over the next seven years. This will, unsurprisingly, place huge demands on solar cell R&D and manufacturing industries.

While 75TW by 2050 is a lofty goal, it is doable. The solar cell industry has doubled its annual production every three years and the costs for building new production lines have fallen by 50% worldwide. Last year, the first TW of solar cell-derived electricity was generated. The next TW is expected to be reached in 2024, and a production rate of one TW per year is predicted by 2028.

The cost of solar technologies has also reduced significantly over the past 30 years, with technological advances – including tandem perovskite and silicon solar cells – and public policies offering additional cost reductions. However, achieving 75 TW by 2050 will depend on continued innovation, cost reductions and supportive public policies.

The solar cell efficiency of silicon-perovskite tandem cells is now in the range previously only achieved by expensive III/V semiconductors, used in the power supply of satellites and in concentrator power plants.

With the technology and manufacturing of tandem solar cells tested in principle, the focus is now on testing tandem solar cell performance in real-world settings and pushing forward policies that support photovoltaic industry expansion.

The Intergovernmental Panel on Climate Change describes photovoltaic technology as the renewable energy solution that can make the greatest and fastest contribution to the global energy transition. International Energy Agency executive director Fatih Birol has also described solar as the new king of the world’s energy markets.

There may indeed be a bright future ahead for solar energy. However, it is necessary to maintain a sense of urgency in bringing tandem solar cells and other photovoltaic technologies to market because, as the HZB team urges, “waiting is not an option.”