Researchers invent new triple-junction tandem solar cells with world-record efficiency

Scientists from the National University of Singapore (NUS) have developed a novel triple-junction perovskite/Si tandem solar cell that can achieve a certified world-record power conversion efficiency of 27.1 per cent across a solar energy absorption area of 1 sq cm, representing the best-performing triple-junction perovskite/Si tandem solar cell thus far. To achieve this, the team engineered a new cyanate-integrated perovskite solar cell that is stable and energy efficient.

Solar cells can be fabricated in more than two layers and assembled to form multi-junction solar cells to increase efficiency. Each layer is made of different photovoltaic materials and absorbs solar energy within a different range. However, current multi-junction solar cell technologies pose many issues, such as energy loss which leads to low voltage and instability of the device during operation.

To overcome these challenges, Assistant Professor Hou Yi led a team of scientists from NUS College of Design and Engineering (CDE) and Solar Energy Research Institute of Singapore (SERIS) to demonstrate, for the first time, the successful integration of cyanate into a perovskite solar cell to develop a cutting-edge triple-junction perovskite/Si tandem solar cell that surpasses the performance of other similar multi-junction solar cells. Asst Prof Hou is a Presidential Young Professor at the Department of Chemical and Biomolecular Engineering under CDE as well as a Group Leader at SERIS, a university-level research institute in NUS.

"Remarkably, after 15 years of ongoing research in the field of perovskite-based solar cells, this work constitutes the first experimental evidence for the inclusion of cyanate into perovskites to boost the stability of its structure and improve power conversion efficiency," said Asst Prof Hou.

The experimental process that led to this ground-breaking discovery was published in Nature on 4 March 2024.

Fabricating energy-efficient solar cell technology

The interactions between the components of the perovskite structure determine the energy range that it can reach. Adjusting the proportion of these components or finding a direct substitute can help modify the perovskite's energy range. However, prior research has yet to produce a perovskite recipe with an ultrawide energy range and high efficiency.

In this recently published work, the NUS team experimented on cyanate, a novel pseudohalide, as a substitute for bromide -- an ion from the halide group that is commonly used in perovskites. Dr Liu Shunchang, Research Fellow in Asst Prof Hou's team, employed various analytical methods to confirm the successful integration of cyanate into the perovskite structure, and fabricated a cyanate-integrated perovskite solar cell.

Further analysis of the new perovskite's atomic structure provided -- for the first time -- experimental evidence that incorporating cyanate helped to stabilise its structure and form key interactions within the perovskite, demonstrating how it is a viable substitute for halides in perovskite-based solar cells.

When assessing performance, the NUS scientists found that perovskite solar cells incorporated with cyanate can achieve a higher voltage of 1.422 volts compared to 1.357 volts for conventional perovskite solar cells, with a significant reduction in energy loss.

The researchers also tested the newly engineered perovskite solar cell by continuously operating it at maximum power for 300 hours under controlled conditions. After the test period, the solar cell remained stable and functioned above 96 per cent capacity.

Encouraged by the impressive performance of the cyanate-integrated perovskite solar cells, the NUS team took their ground-breaking discovery to the next step by using it to assemble a triple-junction perovskite/Si tandem solar cell. The researchers stacked a perovskite solar cell and a silicon solar cell to create a dual-junction half-cell, providing an ideal base for the attachment of the cyanate-integrated perovskite solar cell.

Once assembled, the researchers demonstrated that despite the complexity of the triple-junction perovskite/Si tandem solar cell structure, it remained stable and attained a certified world-record efficiency of 27.1 per cent from an accredited independent photovoltaic calibration laboratory.

"Collectively, these advancements offer ground-breaking insights into mitigating energy loss in perovskite solar cells and set a new course for the further development of perovskite-based triple junction solar technology," said Asst Prof Hou.

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Three proposals from researchers to meet EU climate goals

The EU countries have decided that the EU is to be climate neutral by 2050. By 2030, greenhouse gas emissions must have been reduced by at least 55% compared to 1990. To meet this target, continued vigorous efforts are needed to reduce emissions, but that alone will not be enough. This is the conclusion of seven researchers from Sweden and Germany in an article in the journal Communications Earth & Environment. One of them is Mathias Fridahl, associate professor at the Department of Thematic Studies -- Environmental Change at Linköping University, Sweden.

"We have painted humanity into a corner. It's no longer possible to solve the climate crisis simply by reducing emissions. We also need to clean the atmosphere of carbon dioxide," says Mathias Fridahl.

The problem is that there are currently no incentives for companies and countries to invest in new technologies to remove carbon dioxide.

That is why a change in the EU's climate policy is needed. "There are many technologies that are quite well developed, but which aren't economically viable," says Mathias Fridahl.

He and his colleagues have three proposals that they believe could soon make a difference.

Anyone contributing to the removal of carbon dioxide should be able to get paid for it under the EU emissions trading scheme.

This should only apply to methods that have a long life span, that is, capture linked to the storage of carbon dioxide for thousands of years.

To get the trading scheme up and running, the researchers propose that the EU set up a central bank for carbon dioxide.

The bank would give investors a good price for the carbon dioxide removed from the atmosphere.

In order to maintain the drive to continue reducing emissions at the same time, the proposal is that the bank strongly regulates how removal may be used to compensate for continued emissions.

The bank's financial muscle could come from revenues from carbon tariffs on goods from outside the Union.

To stimulate other measures with a shorter life span, the researchers propose an extension of the EU's land use regulation.

This sets out the measures to remove carbon dioxide that member states are allowed to be credited with when reporting their climate emissions.

Today, there is a limited amount of removal methods in forestry and agriculture.

The researchers contend that if the regulation were extended to more measures, it would encourage countries to invest resources in carbon removal.

The researchers also want the EU to identify which emissions will be very difficult or impossible to do anything about.

Greater clarity would reduce the risk of companies and member states postponing measures in the hope that their emissions will belong to the group that is difficult to tackle.

This would stimulate innovation and efforts to reduce emissions in parallel with initiatives to remove carbon dioxide.

Read more at Science Daily