EPFL

Researchers from Switzerland's Ecole Polytechnique Fédérale de Lausanne (EPFL), Chinese Academy of Sciences (CAS) and Peking University have developed a perovskite solar cell with a 2D/3D heterojunction architecture.

The cell uses a 2D perovskite layer at the interface between the perovskite and the hole transport layer, which the researchers said can improve charge-carrier transport/extraction while suppressing ion migration. Cells with this architecture usually exhibit large exciton binding energies and are generally more stable than conventional 3D devices due to the protection provided by the organic ligands.

Researchers from the University of Toronto, the University of Kentucky, EPFL, North Carolina State University and Northwestern University have designed a perovskite solar cell that can stand up to high temperatures for more than 1,500 hours — an important achievement on the to commercialization. 

“Perovskite solar cells offer new pathways to overcome some of the efficiency limitations of silicon-based technology, which is the industrial standard today,” said Ted Sargent, professor of electrical and computer engineering at the McCormick School of Engineering, professor of chemistry in the Weinberg College of Arts and Sciences, and a former professor at the University of Toronto. “But due to its multi-decade head start, silicon still has an advantage in some areas, including stability. This study shows how we can close that gap.”

In two separate studies, researchers report novel methods that enable the fabrication of high-performance perovskite-silicon tandem solar cells with power conversion efficiencies exceeding 30%. 

Combining perovskite and silicon solar cells into a tandem device could provide a promising path toward high-performance PVs. Here, in the two separate studies, researchers present different strategies for developing perovskite-silicon tandem solar cells with a PCE exceeding 30%. In one study, Xin Yu Chin from Ecole Polytechnique Fédérale de Lausanne (EPFL) and colleagues showed that the uniform deposition of the perovskite top cell on a silicon bottom cell featuring micrometric pyramids – the industry standard configuration – can facilitate high photocurrents in tandem solar cells.

An international team of scientists from Fritz Haber Institute of the Max Planck Society, École Polytechnique in Paris, Columbia University in New York, and the Free University in Berlin have demonstrated laser-driven control of fundamental motions of the lead halide perovskite (LHP) atomic lattice.

Sketch of the experimental pump-probe configuration. Image from Science Advances

By applying a sudden electric field spike faster than a trillionth of a second (picosecond) in the form of a single light cycle of far-infrared Terahertz radiation, the team unveiled the ultrafast lattice response, which might contribute to a dynamic protection mechanism for electric charges. This precise control over the atomic twist motions could allow to create novel non-equilibrium material properties, potentially providing hints for designing the solar cell material of the future.

An international research team that includes scientists from EPFL in Switzerland, Middle East Technical University (METU) in Turkey, Lomonosov Moscow State University in Russia and The University of Tokyo has fabricated a quasi-2D perovskite solar cell with a unique type of salt to enhance hole extraction. 

The triple-cation perovskite absorber was treated with phenethylammonium iodide (PEAI), a modulator that alters the perovskite film's surface energy and forms a quasi-2D structure without further annealing. The result is a 23.08%-efficient device that is also able to retain 95% of its initial efficiency after 900 hours.

Researchers from the University of Toronto in Canada, Northwestern University, The University of Toledo and University of North Carolina at Chapel Hill in the United States, King Abdullah University of Science and Technology (KAUST) in Saudi Arabia, Yunnan University in China, Ecole Polytechnique Fedérale de Lausanne (EPFL) in Switzerland and University of Warwick in the UK have developed a triple-junction perovskite solar cell with a record efficiency of 24.3% with an open-circuity voltage of 3.21 V. 

The NREL has certified the cell’s quasi-steady-state efficiency as 23.3%, which the team stated is the first reported certified efficiency for perovskite-based triple-junction solar cells. They added that triple-junction perovskite solar cells have so far demonstrated a maximum efficiency of around 20%.

A team of researchers, led by Professor Michael Grätzel at EPFL and Xiong Li at the Michael Grätzel Center for Mesoscopic Solar Cells in Wuhan (China), have developed a technique that addresses stability concerns of perovskite solar cells (PSCs) and increases their efficiency.

The researchers introduced a phosphonic acid-functionalized fullerene derivative into the charge-transporting layer of the PSC as a “grain boundary modulator”, which helps strengthen the perovskite crystal structure and increases the PSC’s resistance to environmental stressors like heat and moisture.

Researchers at Switzerland's EPFL and Sungkyunkwan University in Korea have addressed the issue of perovskite solar cells' stability. They focused on the degradation of perovskite thin films, which can be damaged by exposure to moisture, heat, and light. The team looked at two specific crystal facets (the crystal's flat surface), characterized by a particular arrangement of atoms. The arrangement of atoms on these facets can affect the properties and behavior of the crystal, such as its stability and its response to external stimuli like moisture or heat.

The researchers looked at the (100) and (111) facets of perovskite crystals. The (100) facet is a plane that is perpendicular to a crystal's c-axis with its atoms arranged in a repeating pattern in the form of a square grid. In the (111) facet the atoms are arranged in a triangular grid. The study found that the (100) facet, which is most commonly found in perovskite thin films, is particularly prone to degradation as it can quickly transition to an unstable, inactive phase when exposed to moisture. In contrast, the (111) facet was found to much more stable and resistant to degradation.

A team of researchers at EPFL have developed a method that improves both power conversion efficiency and stability of solar cells based on pure iodide as well as mixed-halide perovskites. The new method aslo suppresses halide phase segregation in the perovskite material. The research was carried out by the groups of Professors Michael Grätzel and Ursula Rothlisberger at EPFL and led by Dr Essa A. Alharbi and Dr Lukas Pfeifer.

The method treats perovskite solar cells with two alkylammonium halide modulators that work synergistically to improve solar cell performance. The modulators were used as passivators, compounds used to mitigate defects in perovskites, which are otherwise promoting the aforementioned degradation pathways.

Researchers from Empa, EPFL, Sichuan University, Jiaxing University, Soochow University, University of Cologne, University of Potsdam, HZB and the University of Oxford have developed a flexible all-perovskite tandem solar cell with a mitigated open-circuit voltage deficit and reduced voltage loss. The team reported flexible tandem efficiency of almost 24% on small area cells using the spin coating method.

Flexible all-perovskite tandem cells are currently less developed than rigid cells, due to a difficult deposition process for the cell's functional layers and a lower open-circuit voltage. This is due to high defect densities within the perovskite absorber layer and at the perovskite/charge selective layer interface.