The greenhouse effect
from the fossil fuels has played havoc with our environment and solar is the only renewable energy source that
in principle could meet all the world's energy requirements. Solar energy means
the direct conversion of sunlight into electricity, heat, or hydrogen.
A path-breaking new
research from Brown University, USA, has revealed a faster and more economical
straightforward method to produce flexible, high-efficiency, thinner solar
panel cells. Currently silicon solar cells
are used and these are expensive to
manufacture, however the research from Brown University may change all this and
lead to a paradigm shift in how green energy is produced.
The technique involves
a room-temperature solvent bath to create a class of crystalline materials
known as perovskites, which is different from the blast of heat used in current
crystallization methods to produce the solar cells.
The new method can
produce superior-quality perovskite films with precise control over thickness across
large areas. It also points the way toward mass production methods for
The study was
published in the Royal Society of Chemistry's Journal of Materials
Efficiency of Perovskite Cells
Perovskite films in
solar cells are excellent light absorbers. The films are much cheaper to make
than the silicon wafers used in standard solar cells.
The first perovskite
cell was introduced in 2009. It had shown an efficiency of only about 4 percent
(the percentage of sunlight converted to electricity), a far cry from the 25%
efficiency flaunted by standard silicon cells.
However, the efficiency
of perovskite cells has increased at a staggering pace in just a few years.
Last year, perovskite cells got certified as having more than 20-percent
efficiency. That rapid advancement is promising and scientists are racing to
start using perovskite cells in commercial products.
Using High Temperature
to Produce Perovskite Films Causes a Number of Problems
There are many
different ways to generate Perovskite films, and most of them so far have
required but all of them require heat.
"People have made good
films over relatively small areas - a fraction of a centimeter or so square.
But they have had to go to temperatures from 100 to 150 degrees Celsius, and
that heating process causes a number of problems," said Nitin Padture,
professor of engineering and director of the Institute for Molecular and
heat-treatment, the crystals often form unevenly leaving tiny pinholes in the
film which in turn reduces the efficiency of a solar cell. Also, flexible
plastic substrates cannot be used as they are damaged by high temperatures.
Crystal Thin Films at Room Temperature in Solvent Baths
Yuanyuan Zhou, a graduate
student in Padture lab at Brown University, USA, was searching for a way to
make perovskite crystal thin films without having to apply heat. He could
create the films using a solvent-solvent extraction (SSE) method.
In this approach,
perovskite precursors are dissolved in a solvent called N-Methyl-2-pyrrolidone
(NMP) and coated onto a substrate. Then, the method totally avoids heating
process. Instead, the substrate is bathed in diethyl ether (DEE), a second
solvent that selectively captures the NMP solvent and whisks it away. What's
left is a thin film of perovskite crystals.
According to Zhou, the
crystals can be formed on virtually any substrate and the entire SSE
crystallization process takes less than two minutes, compared to hours long
heating method. Another important thing is that the process is more amenable to
The approach also
ensures very thin films maintaining high quality. Standard perovskite films are
generally on the order of 300 nanometers thick, but the SSE method promises
high quality films as thin as 20 nanometers.
Zhou says that the SSE
films could also be made larger several centimeters square without
generating pinholes. "Using the other methods, when the thickness gets below
100 nanometers you can hardly make full coverage of film. You can make a film,
but you get lots of pinholes. In our process, you can form the film evenly down
to 20 nanometers because the crystallization at room temperature is much more
balanced and occurs immediately over the whole film upon bathing," Zhou said
Researchers said that
method could be used to make the cells in different colors by tweaking the
perovskite precursor solution composition. "The method could be used for
decorative, building-integrated windows that can make power," they said.
Initial Testing Shows
The research team plans
to do more work to refine the process. Researchers conducted initial testing on
the method with the help of the National Renewable Energy Laboratory in
The results of the initial
testing of cells made with SSE films showed conversion efficiency of over 15%.
Also, solar cells based on semi-transparent 80-nanometer films have shown very
Researchers believe that the results could be an
important step toward a variety of commercially available perovskite cell
products and will bring down not only the cost of the panels but also the cost
of producing dollar energy.