Yash
The Tandem solar cell technology has the potential to improve the efficiency of traditional silicon-based solar panels by over 50%.
Sunlight can be converted directly into electricity with a photovoltaic cell (known as a solar cell). Traditional silicon-based solar cells are in use worldwide.
However, further advancements are already in the pipeline.
The Shockley-Queisser limit
Over 60 years ago, in 1961, scientists William Shockley and Hans-Joachim Queisser made an interesting discovery known as the Shockley-Queisser limit.
They determined that solar cells with a single layer (one p-n junction) can achieve a maximum efficiency of only 33.7%. It means that a single-layer solar cell can convert a maximum of 33.7% of incident light into electric current.
Tandem Cells breaking the limit
With tandem solar cells now achieving power-conversion efficiencies surpassing 30%, experts believe that advanced photovoltaics will play a crucial role in the transition to renewable energy sources.
One method to increase the efficiency of a solar cell is to split the spectrum and use a solar cell of different bandgap optimized for each section of the spectrum.
When the 2 solar cells of different bandgaps are paired, it increases the light absorption and the cell efficiency.
Usually, the Perovskite solar cells are paired with the c-silicon solar cells.
Perovskites made the top layer while c-Si is at the bottom of the stack.
The Working of Tandem Solar Cell
The top cell layer is semi-transparent, efficiently converting large-energy photons into electricity, while the bottom cell converts the remaining or transmitted low-energy photons into electricity.
This allows a larger portion of the light energy to be converted to electricity. Hence improves the Tandem solar cell’s efficiency.
This cell converts part of the solar spectrum into electricity and transmits the infrared light to the bottom silicon solar cell.
The structure of Tandem Solar Cell
The solar energy spectrum falls on the front transparent electrode.
The Perovskite solar cells absorb the high energy photons and by-passes the low energy photons to bottom layer of c-Si through a tunnel junction.
c-Si absorbs the low energy photons and convert them into electricity.
The current passes through the back contact and runs the load.
Why do we pair with Perovskites to make Tandem solar cell?
Current silicon-based photovoltaics mainly convert longer wavelengths of sunlight, limiting their efficiency. Perovskite (ABX₃), a lightweight and cost-effective semiconductor, can absorb shorter wavelengths, including high-energy photons.
By layering perovskite on silicon solar cells, overall efficiency improves, enhancing sunlight conversion into usable electricity.
In contrast to silicon, the properties of perovskite can be tailored by manufacturers to suit their specific requirements.
By adjusting the ratios of materials used to create perovskite, they can produce a variant that has a particular color and absorbs specific wavelengths of light.
Unlike silicon, perovskite properties can be customized (tuned) by the manufacturer to best fit their needs.
By using different ratios of materials to build the perovskite, the manufacturer can create a perovskite with a specific colour that absorbs a specific wavelength range of light.
Perovskite cells are also much thinner, lighter and less costly to manufacture than their silicon counterparts.
The Problems with Tandem Solar Cells
Today, the Tandem cells are on the radar by many scientists around the world. However, there are a bunch of problems associated with them that need to be resolved to utilize their full potential.
Let us cover a few of them:
i) Tandem solar cells are costly
Although, Tandem solar cells have the potential to improve the existing efficiency of traditional solar cells by another 50% or more.
But they are costly.
The stacking of solar cells of different materials and bandwidth requires a complex production line which requires a huge investment.
Hence increases the production cost of the Tandem cell.
However, the team of scientists is working on making the process more cost-effective.
ii) Compatibility Issues
The compatibility problems are natural when two different solar cells of different materials are stacked together.
a) Material compatibility is another major issue with the Tandem solar cells.
The two stacked solar cells have different mechanical characteristics. They contract and expand at different rates when exposed to temperature variations. This makes the tandem cell prone to cracks at the interface of two cells.
b) Another is electrical compatibility.
Tuning and making the top layer of Perovskite transparent enough to pass through the low-energy photons for the lower layer of c-Si to make the photovoltaic effect is difficult.
Also, these two cells work at different voltages and currents. Therefore, it is very difficult to extract the maximum power output.
iii) Durability
It makes little sense to develop a tandem solar cell where one cell ceases functioning after a short duration of the lifespan of the other cell.
The Perovskite solar cell used as the top layer in the Tandem cell makes oxide when it comes in contact with the surroundings. And it starts deteriorating very fast.
The useful life of Perovskite is less than 1 year where as silicon solar cells produce current for over 25 years.
In other words, the silicon cell is 25 times more durable than the Perovskite solar cell.
Future of Tandem Solar cells
Despite these challenges, the South Korean company Q Cells has launched the first production line for perovskite-silicon tandem solar cells, which could enhance efficiency by 50-75% compared to standard panels.
Traditional silicon panels peak at 22% efficiency, while tandem technology could raise this to 33%. Q cells is investing $100 million to transition this technology from research to production, indicating that tandem solar cells may become widely used in the coming years.
