Surface modification for more effective textured perovskite/silicon tandem solar cells
A coating of solar cells with special organic molecules could pave the way
for a new generation of solar panels. As a research team reports in the
journal Angewandte Chemie, this coating can increase the
efficiency of monolithic tandem cells made of silicon and perovskite
while lowering their cost—because they are produced from industrial,
microstructured, standard silicon wafers.
© Wiley-VCH, re-use with credit to 'Angewandte Chemie' and a link to the original article.
In solar cells, light “knocks” electrons out of a
semiconductor, leaving behind positively charged “holes”. These two
charge carriers are separated from each other and can be collected as
current. Tandem cells were developed to better exploit the entire
spectrum of sunlight and increase solar cell efficiency. Tandem cells
are made of two different semiconductors that absorb different
wavelengths of light. Primary contenders for use in this technology are
a combination of silicon, which absorbs mostly red and near-infrared
light, and perovskite, which very efficiently uses visible light.
Monolithic tandem cells are made by coating a support with the two types
of semiconductor, one on top of the other. For a perovskite/silicon
system, this is usually achieved by using silicon wafers that are
produced by the zone melting process and have a polished or
nanostructured surface. However, these are very expensive. Silicon
wafers produced by the Czochralski process with micrometer-scale
pyramidal structural elements on their surfaces are significantly
cheaper. These microtextures result in better light capture because they
are less reflective than a smooth surface. However, the process of
coating these wafers with perovskite results in many defects in the
crystal lattice, which affect the electronic properties. Transfer of the
released electrons is impeded, and electron-hole recombination
increasingly occurs through processes that do not emit light. Both the
efficiency and the stability of the perovskite layer are decreased.
Headed by Prof. Kai Yao, a Chinese team at Nanchang
University, Suzhou Maxwell Technologies, the CNPC Tubular Goods
Research Institute (Shaanxi), the Hong Kong Polytechnic University, the
Wuhan University of Technology, and Fudan University (Shanghai) has now
developed a strategy for surface passivation that allows the surface
defects of the perovskite layer to be smoothed out. A
thiophenethylammonium compound with a trifluoromethyl group (CF3-TEA) is
applied by a dynamic spray coating process. This forms a very uniform
coat—even on microtextured surfaces.
Due to its high polarity and binding energy, the CF3-TEA
coating very effectively weakens the effects of the surface defects. Nonradiative
recombination is suppressed, and the electronic levels are adjusted so
that the electrons at the interface can be more easily transferred to
the electron-capturing layer of the solar cell. Surface modification
with CF3-TEA allows perovskite/silicon tandem solar cells based on
common textured wafers made of Czochralski silicon to attain a very high
efficiency of nearly 31% and maintain long-term stability.
(3148 characters)
About the Author
Dr Kai Yao is a Professor at Nanchang University (China) with
appointments in Institute of Photovoltaics. His research interest is in
the field of organic–inorganic hybrid materials for optoelectronic
applications. He is also Director of emerging photovoltaic technologies
research group at the Nanchang University.
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