How to obtain highly crystalline organic-inorganic perovskite films for solar cells

Members of the Laboratory of New Materials for Solar Energetics, working at the Faculty of Material Sciences, in cooperation with their colleagues from the Faculty of Chemistry of the Lomonosov Moscow State University have obtained highly crystalline organic-inorganic perovskite films for solar cells. Their results are published in the journal Materials Horizons.

The researchers previously worked on nanowires of hybrid organic-inorganic perovskites, which are promising for the creation of light emitting diodes, lasers and photodetectors. However, the most promising application for these substances is the elaboration of perovskitesolar cells - namely, next-generation photovoltaic devices. Efficiency of these devices has risen by several times over the last five years, and now comprises even more than 22 percent. This is significantly higher than the maximum efficiency of polycrystalline silicon solar cells. Efficiency of the most popular industrially produced solar cells is 12 to 15 percent.

There are two main approaches for obtaining this material. The first involves a coating of vaporous chemical agents, and the second is solution crystallization. Projects aimed at improving these methods have been intensively developed in recent years. However, further perspectives of these approaches are almost exhausted.

Alexey Tarasov, Doctor of Chemistry, the Head of the Laboratory and the Study Lead says, "As part of the study, we've found several new compounds - polyiodides, which are liquid at room temperature, and have unique properties. They look like viscous liquids of dark brown color with a metal gleam, obtained from two solid powders, which simply melt while blending. Their liquid state makes them a good substitute for hazardous solvents and, their chemical composition contributes to the formation of a necessary perovskite upon contact with a metallic lead film or other lead compounds. As a result of the chemical interaction between lead film and polyiodide melts, a perovskite film composed of large interpenetrating crystals is formed."

Polyiodide melts are deposited on lead using a so-called spin coating technique. For this purpose, a glass substrate with lead layer is fixed on a whirling rod and rotates. Polyiodide is poured on the whirling glass substrate and the residue is flushed using isopropanol. This produces stable perovskite films from 200 to 700 nm in thickness.

The lab currently continues studying properties of discovered polyiodides and elaborating technologies to obtain solar cells with high efficiency.

Discovery of new transparent thin film material could improve electronics and solar cells

A team of researchers, led by the University of Minnesota, have discovered a new nano-scale thin film material with the highest-ever conductivity in its class. The new material could lead to smaller, faster, and more powerful electronics, as well as more efficient solar cells.

The discovery is being published today in Nature Communications, an open access journal that publishes high-quality research from all areas of the natural sciences.

Researchers say that what makes this new material so unique is that it has a high conductivity, which helps electronics conduct more electricity and become more powerful. But the material also has a wide bandgap, which means light can easily pass through the material making it optically transparent. In most cases, materials with wide bandgap, usually have either low conductivity or poor transparency.

"The high conductivity and wide bandgap make this an ideal material for making optically transparent conducting films which could be used in a wide variety of electronic devices, including high power electronics, electronic displays, touchscreens and even solar cells in which light needs to pass through the device," said Bharat Jalan, a University of Minnesota chemical engineering and materials science professor and the lead researcher on the study.

Currently, most of the transparent conductors in our electronics use a chemical element called indium. The price of indium has gone up tremendously in the past few years significantly adding to the cost of current display technology. As a result, there has been tremendous effort to find alternative materials that work as well, or even better, than indium-based transparent conductors.

In this study, researchers found a solution. They developed a new transparent conducting thin film using a novel synthesis method, in which they grew a BaSnO3 thin film (a combination of barium, tin and oxygen, called barium stannate), but replaced elemental tin source with a chemical precursor of tin. The chemical precursor of tin has unique, radical properties that enhanced the chemical reactivity and greatly improved the metal oxide formation process. Both barium and tin are significantly cheaper than indium and are abundantly available.

"We were quite surprised at how well this unconventional approach worked the very first time we used the tin chemical precursor," said University of Minnesota chemical engineering and materials science graduate student Abhinav Prakash, the first author of the paper. "It was a big risk, but it was quite a big breakthrough for us."

Jalan and Prakash said this new process allowed them to create this material with unprecedented control over thickness, composition, and defect concentration and that this process should be highly suitable for a number of other material systems where the element is hard to oxidize. The new process is also reproducible and scalable.

They further added that it was the structurally superior quality with improved defect concentration that allowed them to discover high conductivity in the material. They said the next step is to continue to reduce the defects at the atomic scale.

The research was funded by the National Science Foundation (NSF), Air Force Office of Scientific Research (AFOSR), and U.S. Department of Energy.

In addition to Jalan and Prakash, the research team included Peng Xu, University of Minnesota chemical engineering and materials science graduate student; Cynthia S. Lo, Washington University assistant professor; Alireza Faghaninia, former graduate student at Washington University; Sudhanshu Shukla, researcher at Lawrence Berkeley National Laboratory and Nanyang Technological University; and Joel W. Ager III, Lawrence Berkeley National Laboratory and University of California Berkeley adjunct professor.

Researchers at the Technical University of Eindhoven in the Netherlands have staked claim to the highest conversion efficiency yet achieved with a nanowire-based solar cell: 17.8 percent.

Researchers at the Technical University of Eindhoven in the Netherlands have staked claim to the highest conversion efficiency yet achieved with a nanowire-based solar cell: 17.8 percent.  While this new mark edges out the previous record of 15.3 percent, it still falls well short of the 46-percent theoretical limit for these cells.

The record-breaking achievement was actually reported in the doctoral dissertation of Dick van Dam. As a result, the work has not yet been published. When we contacted Van Dam to get more details on the device he fabricated, he explained he was limited with regard to what he could say about it until publication.

However, in the press release, Van Dam did express hope that the record he has achieved will fall quickly. He expects that to be the case within the next couple of years. With the fairly recent development of the first nanowire-based solar cells and the high ceiling for their performance, that's a reasonable expectation.

Three years ago, this blog reported on the work of a joint Danish-Swiss research team that proposed a way to surpass the Shockley-Queisser limit for the maximum efficiency of a solar cell by employing nanowires. The Shockley-Queisser limit posits that only 33.7 percent of all the solar energy hitting a solar cell with a single p-n junction can be converted into electricity.

The nanowire-based solar cell proposed by the Danish-Swiss team was going to be able to surpass this limit by exploiting the nanowires' unique ability to concentrate light to intensities up to 15 times greater than normal. Because the diameter of a nanowire is smaller than the incoming wavelength of light, resonances occur in the intensity of the light surrounding the nanowires. These resonances concentrate the light right at the spot where it is converted into electricity.

Since that research there’s been a steady stream of new uses of nanowires in solar cells that promise higher and higher energy conversion efficiency.

Van Dam did say that the nanowire solar cells he fabricated operate in the same way as regular solar cells, but that the solid layer that normally absorbs the light and converts it into electricity is simply replaced by a layer of vertical nanowires.

In general, says Van Dam, improving the performance of these cells mainly involves increasing the internal radiative efficiency of the cells, which correlates to decreasing the number of defects. “This is basically processing optimization,” he added in an email interview with IEEE Spectrum.

“For commercial availability, the process needs to be optimized as well, in order to lower the production costs,” Van Dam explained. “Fabrication of nanowire solar cells without using the thick substrate (which is probably possible) would be an important step forward in this direction.”