Solar power set to become cheaper with laboratory-synthesized material

Perovskite, a class of crystalline material with potential applications in photovoltaic technology, is being studied by researchers affiliated with the Center for Innovation in New Energies (CINE), an Engineering Research Center supported by FAPESP.

By José Tadeu Arantes  |  Agência FAPESP – Perovskite is currently one of the most studied functional materials thanks to its use in photovoltaic solar power cells. The light-to-electricity conversion efficiency of perovskite solar cells has reached 25%, surpassing that of polycrystalline silicon cells, the type of solar cell most widely sold worldwide. The main advantages of the new technology are ease of manufacturing, low cost, and low impact on the environment.

“Silicon cells can only be produced in cleanrooms with very tight particle control and at temperatures exceeding 1,500 °C. That makes silicon-based solar panels expensive, although the price has fallen quite a lot in recent years. In our lab, we’re producing perovskite film using solutions, also called dyes, at room temperature,” said Ana Flávia Nogueira, a professor at the University of Campinas’s Chemistry Institute (IQ-UNICAMP), São Paulo State, Brazil, and principal investigator for the Dense Energy Carrier Division of the Center for Innovation in New Energies (CINE).

CINE is an Engineering Research Center (ERC) established by FAPESP and Shell.

Nogueira led a project designed to characterize hybrid perovskite films in collaboration with Hélio Tolentino and Raul de Oliveira Freitas, researchers at Brazil’s National Synchrotron Light Laboratory (LNLS). The results of the study are reported in the article “Nanoscale mapping of chemical composition in organic-inorganic hybrid perovskite films”, published in Science Advances, a journal of the American Association for the Advancement of Science (AAAS).

The study was supported by FAPESP and conducted during the PhD research of Rodrigo Szostak.

“In the past five years, all research groups have been racing to see who could achieve the best conversion efficiency rate,” Nogueira said. “We’re close to the theoretical limit, which is around 30% efficiency. However, most researchers now tend to want to take a step back in order to understand these materials more deeply. Szostak’s work is aligned with this trend. His techniques involve synchrotron light and infrared nanospectroscopy. This is the first time they’ve been used to characterize perovskite materials.”

Szostak used synchrotron infrared nanospectroscopy at the LNLS to map individual nanometric grains in the films. This is important because the film fabrication method, which consists of depositing a solution of the material’s precursors on a substrate in layers only a few nanometers thick, can give rise both to the structural phase of interest and to undesirable phases. Circumstantial factors, such as humidity or temperature, influence the organization of the atoms, which may form an inactive structure instead of a structure with photovoltaic activity. The aim of the study was to investigate how these different phases are distributed in the film and hence how they influence cell performance.

Diverse class

Perovskite proper is a calcium titanium oxide mineral with the molecular formula CaTiO₃. It was discovered in the Russian Urals in 1839 and named in honor of Russian mineralogist Lev Perovski (1792-1856), Tsar Nicholas I’s interior minister. What CINE researchers and others call perovskite is in fact a diverse class of materials synthesized in the laboratory and with the same crystal structure as the mineral. The general chemical formula for perovskite compounds is ABX₃, where A and B are cations (positive ions) of different sizes, and X represents halogens. 

The research conducted at CINE to investigate the photovoltaic use of these materials focuses on hybrid perovskites with an inorganic (carbonless) cation and an organic cation (with carbon).

“Szostak worked with three-dimensional perovskites. Another project of our group, led by Raphael Fernando Moral, resulted in the synthesis of a new material, a two-dimensional perovskite. Moral also used synchrotron light to characterize the material but in this case with X-ray scattering,” Nogueira said.

The research conducted by Moral also received funding from FAPESP via a master’s scholarship and a scholarship for a research internship abroad. Moral and his group reported the findings in an article entitled “Synthesis of polycrystalline Ruddlesden-Popper organic lead halides and their growth dynamics,” which was featured on the cover of Chemistry of Materials, a journal of the American Chemical Society (ACS). 

Moral served his research internship abroad at the SLAC National Accelerator Laboratory in the United States, where he used the Stanford Synchrotron Radiation Lightsource (SSRL) to observe the material’s growth as the chemical reaction occurred by small-angle X-ray scattering (SAXS). After returning to Brazil, he resumed work with the CINE group, using the LNLS to analyze the material’s stability under different operating conditions.

“Moral was even able to determine the average speed at which the 2D plates overlapped during formation of the material. When traversed by an electric current, this perovskite emits very strong light. It can be excellent material from which to make LEDs [light-emitting diodes],” Nogueira said.

The article “Nanoscale mapping of chemical composition in organic-inorganic hybrid perovskite films” can be read at: advances.sciencemag.org/content/5/10/eaaw6619. The article “Synthesis of polycrystalline Ruddlesden-Popper organic lead halides and their growth dynamics” can be retrieved from: pubs.acs.org/doi/pdf/10.1021/acs.chemmater.9b03439.