Solar technology has had a huge impact on our consumption of energy. As a renewable resource that produces no direct greenhouse gas emissions, it contributes to a reduction in the carbon footprint of energy production. The fact it can be harnessed locally not only minimizes dependency on centralized power plants and costly transmission infrastructures, but also makes it a viable source for remote or off-grid areas that are in need of practical and affordable alternative energy solutions. Because there is an estimated 1,000 times more solar energy reaching the earth’s surface than is needed at any given time, it is even more imperative that we take advantage of this potential.
For some time, however, the majority of solar cells, such as those based on silicon, have been manufactured using processes that are very energy-intense, requiring large amounts of the active material, which is difficult to recycle, and makes the solar cell heavy. Halide perovskite solar cells (PSCs) have emerged in the last decade as a promising alternative which is not only lightweight but can be manufactured under remarkably mild conditions. While the solar cells based on this technology enable competitive power conversion efficiencies, they degrade rapidly, offering only a year or so of optimal use compared to the 25 years guaranteed by the already established silicon technology. Scientists believe that achieving the Net Zero commitments and stopping our reliance on fossil fuels can only be achieved if we diversity the range of solar cell technologies available today – and the advantages offered by PSCs are unprecedented.
There is, therefore, a pressing need to extend the lifespan of PSCs. A range of spectroscopic techniques, particularly solid-state nuclear magnetic resonance (NMR), is being used to understand the degradation process in PSCs at the atomic level, and to facilitate the development of improved solar cells for a lower-carbon future.
PSCs: a challenge of stability
Halide PSCs are a promising technology for the development of a more ecologically sound carbon economy. While silicon solar cell technology is well established, silicon has to be processed at very high temperatures (~1400 °C) and comparatively large amounts of it are needed to make an efficient solar cell. On the other hand, PSCs not only can be fabricated close to room temperature, but they absorb sunlight more effectively than silicon, have comparable or higher efficiencies, and are lightweight. Both technologies have their advantages – PSC research is not about replacing silicon but about making the two technologies coexist and complement each other. For example, some of the most efficient solar cells today are so-called multijunction devices, where silicon is combined with halide perovskites to harvest energy in different regions of the solar spectrum.
However, PSCs are subject to quite rapid degradation, their soft ionic nature makes them more susceptible to air, light, temperature, and moisture. They have also posed a challenge from an analytical perspective; their chemical composition is complex, and commonly used analytical techniques have been insufficient for isolating the atomic-level impact of external stimuli on the stability of devices. To allow the manufacture of more stable and longer lasting solar cells, understanding the nature and prevention of degradation is essential – and solid-state NMR is emerging as a technology that could drive this understanding forward.
Halide perovskite solar cells are a complex mixture of chemical ions and molecular additives, making their analysis a challenging task. Past research has seen the combination of a variety of spectroscopic techniques – for example, Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy – with X-ray diffraction and microscopy, but these methods have rarely offered quantitative information on specific isotopes, essential for understanding component behavior within a functioning cell.
Solid-state NMR, by comparison, can provide unique element-specific and quantitative insight into the local structure, structural dynamics, and chemical transformations in materials. Combining the insights of diffraction techniques with structural information from NMR analysis, which allows access to atomic-level information, leads to structure-property relationships that provide a more comprehensive picture of the material.
Using in situ NMR, the team is able to study the mobility and dynamics of ions with the perovskite structure, allowing real-time monitoring of solar cell degradation caused by ambient air, light, and humidity. When making new halide PSCs, for example, the team introduces very dilute additives, on the order of one percent relative to the perovskite, which are impossible to identify using diffraction techniques alone. The examination of specific nuclei within the additive using solid-state NMR enables the researchers to determine its structure, interactions and role within the solar cell.
The second element of the research involves the development of the materials needed to manufacture more environmentally sustainable solar cells. They accomplish this using a variety of synthetic strategies, X-ray diffraction, and optical spectroscopies in combination with solid-state NMR. An important strategy underpinning this research is mechanosynthesis, a green synthetic protocol which enables the researchers to rapidly make large libraries of materials with diverse properties and structures.
Previously published research into halide perovskites has, for example, shown that the stability of certain promising compositions is enhanced when lead is partially replaced by europium or cadmium. The research also found that efficiency can be improved by incorporation of barium which passivated grain boundaries, and that the power conversion efficiency of hybrid organic-inorganic halide perovskites is almost as high as that of silicon solar cells.xii
Pushing the limits of solid-state NMR
The University of Birmingham team’s work has already led to promising developments in the area of triple-junction solar cells (TSCs), in research led by Prof. Ted Sargent at Northwestern University. In multi-junction SCs, solar cell absorbers, each made from a different semi-conductor material, are stacked on top of each other. The differing bandgap energy of each stack leads to more efficient harvesting of different section of the solar spectrum, creating a more effective solar cell. The use of solid-state NMR has allowed the team to understand and overcome the challenges limiting the effectiveness of such stacked assemblies, the main one being light-induced degradation of one of the perovskite layers.
Recent work, led by Prof. Lyndon Emsley at the Swiss Federal Institute of Technology Lausanne (EPFL), combined magic-angle spinning (MAS) and dynamic nuclear polarization (DNP) to deliver a major breakthrough in how quickly and selectively NMR analysis can be done on halide perovskites, especially on mass-limited samples. MAS DNP had not previously been used to analyze halide perovskites, but the team has proved that this technique can reduce experiment time from several months to only a few hours, as well as leading to significant improvements in the information content.
More specifically, they have studied cesium lead chloride, an emerging light-emitting material, using two DNP methods; incipient wetness impregnation (IWI) DNP, which involves using an organic biradical solution to coat the particles of the solid, and metal-ion DNP, which incorporates paramagnetic metal ions (Mn2+) into the perovskite structure. By looking at how the material’s surface interacts with liquids, and considering factors such as relaxation times, particle size and dopant concentration, they have found that IWI DNP is highly effective for studying the material’s surface, whereas metal-ion DNP can provide detailed information on the bulk material’s structure.
In the future, DNP NMR will continue to be invaluable for determining structure in mass-limited perovskite samples at the atomic level. This includes photovoltaic and other optoelectronic devices based on halide perovskites, which in their most industrially relevant form are extremely thin (less than 500 nanometers).
A cleaner alternative
The pressing climate crisis requires the development of advanced solar cell technologies that are stable, efficient, and less damaging to the environment. Halide-based PSCs are a promising and cost-effective alternative to traditional silicon-based solar cells; their ability to be tuned to absorb a wide range of wavelengths of light makes them not only suitable for traditional rooftop solar panels, but also for transparent or flexible solar cells in other applications such as wearable electronics, windows, and aircraft. This could allow greater harnessing of the sun’s energy.
However, the tendency of halide PSCs to degrade rapidly – lasting approximately 25 times less than their silicon-based counterparts – has, until now, posed a serious challenge. Solid-state NMR is driving forward the understanding of this degradation process and allowing the development of more stable perovskite solar cells while maintaining their high efficiency. The team’s developmental research is poised to bring about enhanced solar cell efficiency and an improvement in the stability and lifespan of PSCs, as well as the use of fewer toxic materials and reduced manufacturing costs.
As an abundant and renewable energy source, solar power is one of the most significant alternatives to fossil fuels, both to regions that currently use electricity, but also the estimated 800 million people for whom location or cost prevents ease of electrical access. Solid-state NMR and other experimental techniques such as MAS DNP are helping to transform halide-based PSCs into a truly viable clean energy technology.
About the author
Dominik J. Kubicki is an Assistant Professor in the School of Chemistry at the University of Birmingham, UK, and a Visiting Professor in the Department of Physics at the University of Warwick. He graduated from the Warsaw University of Technology, Poland in 2013 and completed his PhD at EPFL, Switzerland in 2018. He held a Marie Curie-Skłodowska Fellowship at the University of Cambridge, UK from 2018 to 2021. In 2022, he received an ERC Starting Grant underwritten by UK Research and Innovation to develop new atomic-level strategies to study perovskite solar cells. His research focuses on new materials for sustainable optoelectronic technologies and benefits from the unique capabilities of the UK High-Field Solid-State NMR Facility at Warwick.