Perovskites
are a class of materials made up of organic materials bound to a metal. Their
fascinating structure and properties have propelled perovskites into the
forefront of materials' research, where they are studied for use in a wide
range of applications.
Metal-halide
perovskites are especially popular, and are being considered for use in solar
cells, LED lights, lasers, and photodetectors.
For
example, the power-conversion efficiency of perovskite solar cells (PSCs) have
increased from 3.8% to 25.5% in only ten years, surpassing other thin-film
solar cells - including the market-leading, polycrystalline silicon.
Perovskites are usually made by mixing and layering various materials together on a transparent conducting substrate., which produces thin, lightweight films. The process, known as "chemical deposition", is sustainable and relatively cost-effective.
A
Large Perovskite Solar Cell. Courtesy: Ecole Polytechnique Fédérale de Lausanne
But there
is a problem. Since 2014, metal halide perovskites have been made by mixing
cations or halides with formamidinium (FAPbI3). The reason is that this recipe
results in high power-conversion efficiency in perovskite solar cells.
But at the
same time, the most stable phase of FAPbI3 is photoinactive, meaning that it
does not react to light - the opposite of what a solar power harvester ought to
do. In addition, solar cells made with FAPbI3 show long-term stability issues.
Now,
researchers led by Michael Grätzel and Anders Hafgeldt at EPFL, have developed
a deposition method that overcomes the formamidinium issues while maintaining
the high conversion of perovskite solar cells.
The work
has been published in Science ("Vapor-assisted deposition of highly
efficient, stable black-phase FAPbI3 perovskite solar cells").
In the new
method, the materials are first treated with a vapor of methylammonium
thiocyanate (MASCN) or formamidinium thiocyanate FASCN. This innovative tweak
turns the photoinactive FAPbI3 perovskite films to the desired photosensitive
ones.
The
scientists used the new FAPbI3 films to make perovskite solar cells. The cells
showed more than 23% power-conversion efficiency and long-term operational and
thermal stability. They also featured low (330 mV) open-circuit voltage loss
and a low (0.75 V) turn-on voltage of electroluminescence.