Researchers from Japan pave the way for a clean energy economy with low-cost, cadmium-free, thin-film solar cells
Ritsumeikan
University
While solar cells are a great alternative to fossil fuels, the environmental impact of the processes involved in manufacturing solar cells has been a concern. Solar panel fabrication often involves toxic materials such as cadmium and industrial waste.
Climate change, one of the major global concerns today, has made it clear that fossil fuels are detrimental to our environment and are not sustainable. It is imperative to adopt clean sources of energy, and solar cells are a popular candidate on this front.
While efficiency is a primary concern for solar cells,
researchers have also focused on developing solar cells that are lightweight,
low-cost, and flexible. However, the fabrication process itself has posed a
serious environmental concern: the use of toxic materials and generation of
industrial waste.
For instance, copper indium gallium selenide (CIGSe) is a thin-film solar cell
that offers several advantages over traditional silicon solar cells. Thin-film
solar cells are about 100 times thinner, cheaper to make, and are easier to
install on rooftops and vehicles. Moreover, compared to other photovoltaic
materials used in thin-film solar cells like amorphous silicon,
cadmium-telluride, and organic materials, CIGSe absorbs light more strongly and
can be prepared into thinner films.
However, they contain a buffer layer of
cadmium sulfide, which is highly toxic and carcinogenic. This makes finding alternative,
non-toxic materials essential for large-scale production and installation of
CIGSe panels.
For Professors Jakapan Chantana and Takashi Minemoto at Ritsumeikan University,
Japan, removing cadmium from solar cells was as important as developing an eco-friendly
manufacturing process that is both efficient and affordable.
Addressing these
issues in a new study, a research team led by them developed a strategy in
which the traditional cadmium sulfide buffer layer was replaced with a native
buffer layer formed by oxidizing the surface of the Cu(In,Ga)(S,Se)2 CIGSSe
layer with an air-annealing process. The study was published on March 26, 2022, in the journal Solar RRL.
While attempts to oxidize the CIGSSe layer have been made before, the surface
usually takes months to oxidize. With the new method, however, the team reduced
the oxidation time to a few hours, allowing for a faster manufacturing by a
“roll-to-roll” process. In this process, a CIGSSe layer is initially deposited
on a flexible stainless-steel substrate.
The deposition is then followed by an
air-annealing process where the surface of the CIGSSe layer is oxidized to form
native buffer layers of Inx(O,S)y. By experimenting with different oxidation
conditions, the researchers fabricated a CIGSSe solar cell with a maximum
energy conversion efficiency of 16.7% after 6 hours of oxidation at 130°C.
“We have disclosed for the first time that the CIGSSe surface
oxidized through an optimized air-annealing process leads to a strong
enhancement in energy conversion efficiency,” says Prof.
Minemoto.
Although the reported efficiency is lower than that of conventional solar cells
(which typically exceed 20%), the developed method manages to do away with
cadmium, making the solar cells eco-friendly. “In the conventional process,
cadmium is deposited on the CIGSSe layer via a chemical bath deposition
process. By eliminating this step, we have created a completely dry
manufacturing process that generates less waste,” explains
Prof. Chantana. Moreover, the process is also cost-effective.
In order to make solar energy a viable source of clean energy, solar panels
must become more efficient, economical, and eco-friendly. “The method developed in our study can be scaled to large-scale
manufacturing applications, which is what we need to make solar cells a clean
energy resource not only in Japan but all over the world,” concludes
Prof. Minemoto.
We certainly hope the vision shared by the researchers become a reality soon!
Reference
Title
of original paper: Formation of Native Inx(O,S)y Buffer through Surface
Oxidation of Cu(In,Ga)(S,Se)2 Absorber for Significantly Enhanced Conversion
Efficiency of Flexible and Cd-Free Solar Cell by All-Dry Process
Journal: Solar RRL
DOI: 10.1002/solr.202200250