OIST research was
featured on the frontispiece of the Journal “Advanced Materials Interfaces”.
Yuichi Kato, Luis
K. Ono, Michael V. Lee, Shenghao Wang, Sonia R. Raga and Yabing Qi.
Silver Iodide
Formation in Methyl Ammonium Lead Iodide Perovskite Solar Cells with
Silver Top
Electrodes.
Advanced Materials
Interfaces 2015, Volume 2, Issue 13, September 7, 2015.
Copyright
Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.
(October 15, 2015) Perovskite
solar cells are the rising star in photovoltaics. They absorb light across
almost all visible wavelengths, they have exceptional power conversion
efficiencies exceeding 20% in the lab, and they are relatively easy to
fabricate. So, why are perovskite solar cells yet to be found on the top of our
roofs? One problem is their overall cost, and another is that cheaper
perovskite solar cells have a short lifespan. A study published in Advanced
Materials Interfaces by the Energy
Materials and Surface Sciences Unit at the Okinawa Institute of Science and
Technology Graduate University (OIST), reveals a cause for the short lifetime
of perovskite solar cells with silver electrodes.
Currently, the most common electrode material in perovskite
solar cells is gold, which is extremely expensive. A low-cost alternative to
gold is silver, around 65 times cheaper. To keep the cost even lower, the team
wants to use solution-processed method to fabricate the layers of the solar
cell, instead of expensive vacuum-based techniques. The problem of using silver
electrodes and the solution-based method is that silver gets corroded within
days of the solar cell fabrication. The corrosion makes the electrode turn
yellow, and reduces the efficiency of the cell. The OIST team, headed by Prof.
Yabing Qi, has demonstrated the cause of this degradation and proposed an
explanation.
Flexible
perovskite solar cell device before (top) and after (bottom) corrosion
of the silver
electrode (Energy Materials and Surface Sciences Unit, OIST).
The device
prepared by Dr. Mikas Remeika.
Perovskite solar cells are composed of a sandwich of layers
that work together to transform light into electricity. Light is absorbed by
the perovskite material and stimulates electron excitations, generating the
so-called electron-hole pairs. In simple terms: when electrons are excited,
they “jump and leave holes behind.” Excited electrons and holes are transported
in opposite directions by the adjacent layers of the solar cells, comprising of
an electron-transport titanium dioxide layer, a spiro-MeOTAD hole-transport
layer (HTL), a glass layer coated with a transparent conductive material, and a
silver top electrode. The whole mechanism generates current, but it needs the
correct functioning of each layer of the solar cell in order to work
efficiently. “If one layer fails, the whole solar cell will suffer,” explains
Luis Ono, a staff scientist and group leader in Prof. Qi’s unit.
In this study, the team analysed the composition of the
corroded silver electrode and identified the formation of silver iodide as the
reason for the electrode corrosion. The color change was due to the oxidation
from silver to silver iodide. They also found that exposure to air accelerates
the corrosion, when compared to dry nitrogen gas exposure.
Some of the
researchers of the Energy Materials and Surface Sciences Unit
From left:
Shenghao Wang, Sonia R. Raga, Yabing Qi and Luis K. Ono
The team proposed a mechanism for this damage: silver iodide
forms because gas molecules from ambient air reach the perovskite material and
degrade it forming iodine containing compounds. These iodine-containing
compounds diffuse to the silver electrode and corrode it. The migration of both
air molecules and iodine-containing compounds could happen through small
pinholes present in the spiro-MeOTAD HTL layer (see animation). The pinholes
present in the spiro-MeOTAD HTL layer produced with the solution-processed
method were identified some months ago by Zafer Hawash, a PhD student in the
same laboratory.