Perovskite nano-domains can affect solar-cell efficiency
A perovskite crystalline rock © iStock photos
EPFL scientists have studied, for the first time, the nanoscale elemental distribution of mixed perovskites, which is particularly relevant for the photovoltaic efficiency and reproducibility.
Perovskites are a group of materials that present a cost-effective and improved alternative to Silicon solar panels They consist of hybrid organic and inorganic units, made out of Carbon, Nitrogen, Hydrogen, a metal (e.g. lead) and Halogens (e.g. Iodide and/or Bromide), forming a 3D crystal-lattice structure. Perovskites are usually deposited as thin films on a surface and self-organize into crystals that can be used for efficient solar cells. However, very little is known about the self-organization of the material, or how the different elements distribute — all of which is paramount for optimizing perovskite photovoltaics. Now, EPFL scientists, working in collaboration with researchers from LIST (Luxembourg Institute of Science and Technology) have partly solved the mystery by combining state-of-the-art microscopy with mass spectrometry. The work is published in the Journal of the American Chemical Society.
The scientists, led by Mohammed Nazeeruddin at EPFL, mainly used the information from three different methods to reveal important micro- and nanoscale structural and elemental properties in self-organizing mixed perovskite films.
First, they used a helium-ion microscope (HIM), which uses a helium ion beam to scan the samples, coupled to a secondary ion mass spectrometer (SIMS). Whereas the helium ion microscope provides high-resolution morphological information, SIMS analyzes the elemental composition of solid surfaces and thin films. By combining both techniques, this unique HIM-SIMS instrument provides chemical information about the surface of the perovskite film with a lateral resolution down to 10-20 nm.
The second technique is referred to as “Kelvin probe force microscopy”, which is a type of atomic force microscopy. Here, an ultrasensitive cantilever scans over a surface to map local surface potential variations.
Lastly, combined photoluminescence and Raman spectroscopy mapping revealed optical and structural non-homogeneities on the pervoskite.
By interpreting the results, the scientists were able to conceptualize the nanoscale distribution of charge carriers and elements in mixed perovskite solar cells with 20% efficiency. Specifically, the study showed that part of the mixed perovskite film intrinsically segregates into pure iodide perovskite nano-domains up to a few hundred nanometers in size. This means that the homogeneity of the film is disrupted, which also leads to severe non-homogeneities in the optical properties of the film.
The findings can help optimize the manufacturing of perovskite solar cells in the future, prompting the authors to state: "Our results provide unprecedented understanding of the nanoscale perovskite composition.”
This work included involved a collaboration between EPFL’s Institute of Chemical Sciences and Engineering with the Luxembourg Institute of Science and Technology, and included contributions from EPFL’s Molecular Engineering of Optoelectronic Nanomaterials Lab (LIMNO) and Laboratory for Photonics and Interfaces (LPI), ISTM-CNR (Perugia), and Laboratory for Photonics and Interfaces (LPI). It was funded by the Qatar Environment and Energy Research Institute (QEERI), the Hamad Bin Khalifa University (HBKU), the Qatar Foundation the Marie Curie Institute and the National Research Fund of Luxembourg.
Reference
Paul Gratia, Giulia Grancini, Jean-Nicolas Audinot, Xavier Jeanbourquin, Edoardo Mosconi, Iwan Zimmermann, David Dowsett, Yonghui Lee, Michael Grätzel, Filippo De Angelis, Kevin Sivula, Tom Wirtz, Mohammad Khaja Nazeeruddin. Intrinsic Halide Segregation at Nanometer Scale Determines the High Efficiency of Mixed Cation/Mixed Halide Perovskite Solar Cells. JACS 29 November 2016. DOI: 10.1021/jacs.6b10049