A team of researchers from the University of Toronto’s Faculty of Applied Science
& Engineering has leveraged quantum mechanics to optimize the active layer within a device
known as an inverted perovskite solar cell – a technology that could one day result
in mass-market solar cells that a fraction of those currently on the market.
At present, virtually all commercial solar cells are made from high-purity silicon, which
takes significant energy to produce.
But researchers around the world are experimenting with alternative solar technologies that could
be manufactured and installed with less energy and at lower cost.
One of these alternatives is known as perovskite.
The power of perovskite materials comes from their unique crystal structure, which enables
them to absorb light in a very thin layer and convert it into electricity efficiently.
A new research paper related to perovskite solar cells is published in Nature Photonics.
One of the authros of the paper says that Perovskite crystals are made from a liquid
ink and coated onto surfaces using technology that is already well-established in industry
such as roll-to-roll printing.
Because of this, perovskite solar cells have the potential to be mass produced at much
lower energy cost than silicon.
The challenge is that right now perovskite solar cells lag traditional silicon cells
in stability.
In this study, the researchers aimed to close that gap.
The researchers are using a strategy based on an inverted solar cell structure.
In most prototype perovskite solar cells, electrons exit through a negative electrode
at the bottom layer of the cell, with the “holes” they leave behind exiting through
a positive electrode at the top.
Reversing this arrangement enables the use of alternate manufacturing techniques and
past research has shown that these can improve the stability of the perovskite layer.
But the change comes at a cost in terms of performance.
It’s hard to get good contact between the perovskite layer and the top electrode.
To solve this, researchers typically insert a passivation layer made of organic molecules.
That works really well in the traditional orientation, because ‘holes’ can go right
through this passivation layer.
But electrons are blocked by this layer, so when you invert the cell it becomes a big
problem.
The team overcame this limitation by taking advantage of quantum mechanics – the physical
principle that states the behaviour of materials at very small length scales is different from
what is observed at larger ones.
The researcher says that in their prototype solar cells, the perovskites are confined
to an extremely thin layer – only one to three crystals in height.
This two-dimensional shape enables the researchers to access properties associated with quantum
mechanics.
They can control, for example, what wavelengths of light the perovskites absorb, or how electrons
move within the layer.
The team first used a chemical technique established by other groups to produce a two-dimensional
perovskite surface atop their solar cell.
This enabled the perovskite layer to achieve passivation on its own, eliminating the need
for the organic layer altogether.
To overcome the electron blocking effect, the team increased the thickness of the perovskite
layer from one crystal in height to three.
Computer simulations had shown that this change would alter the energy landscape sufficiently
to enable electrons to escape into an external circuit, a prediction that was borne out in
the lab.
The power conversion efficiency of the team’s cells was measured at 23.9 per cent, a level
that did not fade after 1,000 hours of operation at room temperature.
Even when subjected to an industry-standard accelerated ageing process at temperatures
up to 65 C, the performance only decreased by eight per cent after more than 500 hours
of use.
Future work will focus on further increasing the stability of the cells, including under
even higher temperatures.
The team would also like to build cells with a larger surface area, as the current cells
are only about five square millimetres in size.
Still, the current results bode well for the future of this alternative solar technology.
In their paper, the researchers compare their prototypes to both traditional and inverted
perovskite solar cells that have been recently published in the scientific literature.
The researcher says that the combination of high stability and high efficiency they achieved
really stands out.
Perovskite technology is only a couple of decades old, whereas silicon has been worked
on for 70 years.
The researcher says that there are a lot of improvements still to come.