May 31, 2021
R&D into Thin-film Solar Cells That Can Be Fabricated to Fit Any Material
- Achieving World-leading Power Conversion Efficiency of 18.6 Percent for a Flexible CIS Solar Cell Mini-module -
The Toyota Motor Corporation Frontier Research Center (Toyota) is engaged in solar cell research, as part of its efforts to achieve carbon neutrality by 2050. Solar cells have already been equipped to some Toyota models, with the electricity generated used to power driving and other operations. Improving the amount of electricity generated by solar cells—in other words, increasing the amount of high-efficiency solar cells equipped to vehicles—is one way of extending the electric-powered cruising range of electrified vehicles. To this end, the development of lighter, thinner, more shape-following, and higher-efficiency solar cells is essential. For this report, we spoke to Taizo Masuda, who is engaged in joint research into thin-film solar cells being carried out by Toyota and the National Institute of Advanced Industrial Science and Technology (AIST).
Is this a solar cell?
— Taizo Masuda, I have come here today to find out more about your research into thin-film solar cells. To begin with, there is a black film stuck to a sheet (the photo above)—what is it?
Masuda:
Well, this is a solar cell.
— Really? But I can’t see any electrodes, and it looks just like a pure-black film—is this a solar cell?
Masuda:
There are many types of solar cell, as shown in the diagram below. The market is currently dominated by crystalline silicon solar cells. These solar cells are cheap and extremely reliable, and they were equipped to previous generations of the Prius and to the Prius PHV. However, when we install them in a limited space like a vehicle, we want solar cells to have high power conversion efficiency, outstanding durability, and be thin enough to follow the shape—just like a plastic wrap. When it comes to future products, the focus is on thin-film type solar cells such as CIS compounds and Group III-V compounds.
The above photo shows a CIS compound solar cell (CIS solar cell). CIS solar cells are compound semiconductor-type solar cells primarily made from copper, indium, and selenium; the name “CIS” is an acronym of its constituent materials.
Taizo Masuda is group manager of the R-Frontier Department’s Advanced research group No.6. He is tasked with leading creative projects for the creation of new mobility societies that take advantage of solar energy. He is also visiting professor at the University of Electro-Communications. He is involved in R&D for the realization of a carbon-neutral world—which has been an ambition of his since he was a student.
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Types of solar cell and their respective highest power conversion efficiencies
If it doesn’t exist, let’s make it! Beginning research into onboard solar cells
— Why is Toyota engaged in solar cell research?
Masuda:
It’s quite a long story, so let me begin by describing the background to onboard solar cells.
As global trends toward decarbonization accelerate, the automotive industry as a whole is engaged in implementing measures. Toyota equipped the third-generation Prius, which went on sale in 2009, with solar cells, and in this way sought to use renewable energies to supply at least some of the energy needs of its vehicles.
In 2017, Toyota launched the Prius PHV, and one of the options available was to equip it with the world’s first 180 W crystalline silicon solar cells; the solar cells was capable of generating enough energy to drive a maximum of 6.1 km per day under the irradiance condition in Japan. In 2019, Toyota worked together with the New Energy and Industrial Technology Development Organization (NEDO) and Sharp Corporation (Sharp) to develop a test car for joint demonstration tests*1; it was equipped with solar cells with a total output of approximately 860 W, and made use of high-efficiency solar cells used on satellites. The solar cells were capable of generating enough energy to drive a maximum of 44.5 km per day.
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Performance comparison of the Prius PHV production model and the demonstration car*2
— If you wish to contribute to carbon neutrality, costs must be controlled and cars must be affordable to buy. What solar cell performance are you targeting to popularize the use of onboard solar cells?
Masuda:
In Japan, passenger cars are driven for an average of 24 km per day. Accordingly, if they were equipped with solar cells capable of generating between 800 W and 1000 W, approximately 70 percent of passenger cars would be able to run off renewable energy alone.
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Average distance driven per day by passenger cars in Japan*3
Masuda:
If we could develop affordable, thin-film solar cells with a power conversion efficiency of around 20 percent, and if they could be equipped to the roof, hood, back door, and other parts of a car, then our preliminary calculations indicate that approximately 70 percent of passenger cars could run off solar cell-generated electricity alone. For this reason, we have focused our attention on thin-film solar cells, such as CIS, since they are made from comparatively affordable materials.
— Can you tell us more about CIS solar cells?
Masuda:
Generally speaking, the power conversion efficiency of a solar cell decreases when the solar cells become hot; in addition, when the solar cells are even partially shaded, their power generation output is reduced. CIS solar cells are not only thin films with a high power conversion efficiency; their efficiency does not significantly decrease when hot, nor when partially shaded. For this reason, they are well-suited for use on cars, which at times can be exposed to the burning sun and at others may be partially shaded. In addition, compared to their crystalline silicon counterparts, CIS solar cells are more resistant to breaking, it has potential to enabling their protective covers to be made thinner. And, since they are jet-black and have inconspicuous electrodes, the solar cell modules could be used as they are as outer panel coloring.
— The CIS solar cell module that you developed is the pure-black films you showed me, is that correct?
Masuda:
CIS solar cells currently in mass production are sandwiched between glass, and so their potential as thin-film solar cells is not being fully realized. For this reason, in our research we chose to use flexible ceramic sheets as a substrate, since they are highly flexible and resistant to breaking when bent. We succeeded in developing thin-film CIS solar cell mini modules with a power conversion efficiency of 18.6 percent. Of existing thin-film solar cell mini-modules that have been registered with certification organizations, our power conversion efficiency is among the best in the world.
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The CIS solar cell mini-module achieved world-leading power conversion efficiency; it can be fabricated and attached to a variety of substrates
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The cross-section of CIS solar cell mini-modules manufactured using conventional production methods (glass plate substrate; protective glass required) (left)
The cross-section of CIS solar cell mini-modules manufactured using new production methods (flexible ceramic sheet substrate; protective glass not required) (right)
— Wow, it’s incredible that you achieved world-leading power conversion efficiency! In your research, you made use of flexible ceramic sheets as substrates—would it be possible to use car exteriors, such as curved sheet metal, resin, and other materials, instead?
Masuda:
Yes. One of the results of our research was that we were able to fabricate our high-performance CIS solar cells onto a variety of substrates made from different materials, in other words, it’s possible we could attach our solar cell modules directly onto car bodies.
The mechanism is a little difficult to explain, but when manufacturing the CIS solar cells, we can improve the performance of the solar cell by adding sodium or other alkali metal elements to the CIS light-absorption layer. For this reason, on our substrate we first deposited a precursor layer containing a high concentration of alkali metal (using the ASTL method, Patent Number 5366154); we then deposited molybdenum, the metal that would act as an electrode; on top of this, we grew the CIS light-absorption layer. By adding alkali metal from both the upper and the bottom surface of the CIS light-absorption layer, we improved the CIS thin-film photoelectric conversion performance. For further details, please see the AIST news release (Japanese only).
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CIS solar cell cross-section
— And the jet-black color of the CIS solar cell modules could be utilized as the color of vehicle exterior itself, is that correct?
Masuda:
The possibility exists. If we want solar cell modules of a different color, then we can add color and patterns using decorative films for photovoltaic modules, as we outlined on this website in March, 2021.
Seeking R&D partners for practical implementation
— What an exciting story! What are your future research goals?
Masuda:
With these mini-modules, we managed to achieve a power conversion efficiency of 18.6 percent; however, our aim is to further increase power conversion efficiency by making quality improvements to both the CIS light-absorption layer and its interfaces. In addition, since our ultimate goal is to use these solar cells on cars, we intend to continue R&D so that we can increase the size of the modules while still achieving higher power conversion efficiency. To this end, rather than AIST and Toyota working in isolation, I hope that we can engage in R&D together with solar cell manufacturers and materials manufacturers, and aim for the practical implementation of these technologies together. If we succeed in realizing these technologies, I have no doubt we can accelerate the popularization of solar cells.
References
- 1
- NEDO, Sharp, and Toyota to Begin Public Road Trials of Electrified Vehicles Equipped with High-efficiency Solar Batteries (Toyota Motor Corporation Official Global Website)
- 2
- Maximum amount of charge generated by the solar charging system while the vehicle is parked or being driven, converted into travelling distance according to the JC08 Japan test cycle. Calculated based on the Japan Photovoltaic Energy Association Labeling Guidelines (FY2016), taking into account the various losses incurred in onboard systems. Solar radiation amount calculated based on the daily data of the average year between 1990 and 2009 in the Nagoya district (source: NEDO).
- 3
- Japanese Ministry of Land, Infrastructure, Transport and Tourism, “Road Traffic Census” (in Japanese, Sep. 2015)