Renewable energy will play a leading role in the energy industry of the future
An automaker in Korea, alongside US-tech giants Apple, were recently reported to be discussing a collaboration plan to produce electric self-driving vehicles. Apple’s possible business expansion into the electric vehicle (EV) market demonstrates the level of interest that global companies have in the EV boom, triggered by the success of companies like Tesla. According to a market outlook released by Bloomberg, the number of global EV sales, currently around several million units worldwide, will grow significantly to account for at least 30 percent new vehicle sales by 2040. For EVs to become a truly sustainable means of transportation, they have to use electricity generated from renewable energy sources, and therefore, the demand for renewable energies, including PV, is forecast to rise.
The PV industry has been driven by silicon solar cell modules, which account for 95% of the global module market. Silicon solar cells are produced using first-generation technology, ensuring high reliability and maturity with excellent price competitiveness.
Since the barrier for new entrants into the silicon PV cell sector is quite low, Chinese companies are dominating the global market by leveraging their economies of scale. For example, the combined production capacity of three major Chinese PV cell producers’ 183 mm wafers is expected to reach over 50 GW (pv-magazine.com, November 2020). The figure suggests an aggressive investment strategy in China, because global installed PV capacity is around 120 GW per year.
Securing competitiveness is the key to promoting the PV industry in Korea
For the Korean PV industry to secure competitiveness against Chinese PV producers, it must develop so-called ‘super-gap’ technologies. However, this is not a straightforward task because silicon module-related technologies are relatively standardized. Perovskite solar cells, first introduced in 2012, began to receive much more attention in 2020 for achieving over 25% efficiency. Of note, in Korea, scientists continue to set world records for the technology, and this will likely improve the competitiveness of the PV industry in Korea. (Figure 1. Perovskite solar cell efficiency trends)
(KRICT: Korea Research Institute of Chemical Technology / UNIST: Ulsan National Institute of Science and Technology)
Perovskite is a material that meets almost all the requirements of a solar cell. Because of its high light absorption rate, it absorbs most incident light even with a thickness of less than 1 micrometer. This makes it possible to produce large-area solar cells by applying a thin film of perovskite on to glass or plastic. This cell is called a thin-film solar cell, and it forms the basis of second-generation solar cell technology.
A low temperature process is essential to producing cells using a plastic substrate. With perovskite, high-efficiency solar cells can be produced at low temperatures (<150℃). They are typically fabricated using non-vacuum spin coating with a perovskite solution, which is advantageous in that it lowers the cost. Spin coating is used to apply a certain amount of viscous solution to a rotating substrate and distribute it uniformly via centrifugal force. It is common to apply it in post-processing, including heat treatment, after dehydrating the sample.
Using flexible substrates like plastics or stainless steel plates offers advantages in terms of utility and the possibility of reducing manufacturing costs significantly, as it allows manufacturers to apply the roll-to-roll (R2R) method. R2R processing is a technique of producing electronic devices on a roll of flexible material, such as aluminum foil, and that material is fed continuously from one roller to another. The technique is popular because it can reduce the size of the footprint required for cell and module manufacturing.
Second-generation cells: thin-film technology
There also are some thin-film solar cell materials that had been considered for commercialization prior to the development of perovskite. CIGS (CuInGaSe2 with 23.4% efficiency), CdTe (22.1% efficiency), amorphous silicon (a-Si:H with 14.0% efficiency), dye-sensitized cell (13.0% efficiency) are major thin-film solar cell types. (https://www.nrel.gov/pv/cell-efficiency.html
). CIGS and CdTe have been commercialized already, but their market share is low. There are several reasons why thin-film solar cells, whose market share reached as much as 30% as of the early 2000s, failed to keep up with the rapid pace of market growth.
The first reason is efficiency. A solar cell’s efficiency is a factor that determines the price of the entire system, and eventually the price of electricity produced in the system. Previously, price (price per watt) was more important than efficiency, but as solar cell prices fall, other factors, including the cost of balance of systems (BOS) such as inverters and frames, are becoming more important. Since thin-film solar cells are less efficient than silicon, they are disadvantageous in terms of the overall system installation cost. Secondly, the supply of module mass production equipment was not always so smooth because the technological maturity was lacking, and it was therefore difficult to quickly make large-scale investments.
(The module consists of a light absorption layer sandwiched between two electrodes.
The structure consists of a series of narrow and long cells divided by laser scribing.)
The manufacturing technology for thin-film solar cells appears to be relatively simple compared to the 8.5th generation TFT LCD panels for TVs or laptops, which are produced in 2.5x2.2m2 sizes with a pixel that is less than a millimeter. Thin-film solar panels are fabricated with connecting long ribbon-shaped cells being scribed in a width of about 5–8 mm in series, which are also connected in series, having opposed front and rear electrode surfaces (Figure 2. Thin-film solar module structure).
A thin-film solar panel, since all its cells are connected in series, is a single device. Therefore, a panel’s entire efficiency can be significantly decreased if there is even a pin hole or tiny defect present. It requires advanced processing control capability to apply 5-6 thin layers uniformly to a large-area substrate using low-cost technologies. The production of this cell type becomes difficult to maintain in the market if the cost of equipment and process increases, as its competitiveness compared to silicon will fall.
In order to commercialize perovskite thin-film solar cells, much research has been conducted actively to improve the thermal and environmental stability of the cells, and address issues with efficiency reduction due to increasing product size. Some promising achievements are being reported. For the commercialization of perovskite solar cells, mass production-related issues should be reviewed seriously. If solar cell theories accumulated since the 1970s are well integrated with technologies and production know-how, perovskite thin-film solar cells can be commercialized sooner than expected.
(Tandem solar cells are fabricated by pairing silicon with perovskite. Some of the incident light is absorbed in the perovskite layer and then light with other wavelengths is absorbed in the bottom silicon solar cell. (Refer to Figure 4.))
Perovskite overcomes the efficiency limit of silicon solar cells
The application of combining perovskite with silicon solar cells to improve efficiency is gaining strength as a fabrication method that increases the possibility of its commercialization. The principle is to produce electricity by absorbing light at different wavelengths by placing perovskite solar cells on top of silicon cells (called tandem cell structure: Figure 3). Perovskite is a compound that consists of three or more elements, and its photo detection bandwidth varies according to the composition ratio of each element. As such, perovskite sensitivity can be adjusted to the bandwidth where silicon cannot respond to efficiently. That is, perovskite absorbs light at shorter wavelengths and silicon absorbs light at longer wavelengths that pass through the perovskite (Figure 4. Principle of a tandem solar cell operation). Through this structure, silicon solar cells can achieve 44% efficiency, which is higher than its theoretical limit of 30%.
(The top sell absorb shorter wavelength light and longer wavelength light that pass the cell will be absorbed in the bottom cell.)
This fabrication method has a number of technical issues to overcome since the perovskite solar cell should be added on to the silicon solar cell. The two different solar cells must be connected to each other electronically, and the development of a transparent medium layer is required. In addition, since perovskite and module productions are processed at low temperatures, the development of new materials and process is necessary. A high-level silicon manufacturing technology is also essential as the silicon solar cell must be optimized for the tandem structure.
Q CELLS becomes a global top-tier company with proprietary technologies
I believe that Korea has secured the most favorable position for the commercialization of perovskite-silicon tandem technology, as it is adept at developing both high-efficiency silicon technology and perovskite technology. I am also confident that this will set the direction to develop the super-gap technology to overcome China's economies of scale.
Q CELLS was selected as a national research project organization for the next-generation solar cell technology “perovskite/crystalline silicon solar cell (tandem cell)” at the “2020 Renewable Energy R&D Project Evaluation” held by Korean Energy Technology Evaluation and Planning (KETEP) at the end of 2019. I hope that Q CELLS will dominate the rapidly growing PV market in the near future with the successful commercialization of perovskite-based solar cells by securing advanced technologies.