Think back to your favorite family recipe — the food that only your mom or grandpop can make just the way you like it. Is it pasta? Tacos? Cake? Many electronics companies and research organizations have their own “family” recipes for special cakes called photovoltaic or solar cells.
The name “photovoltaic” comes from the words “photon” and “voltage” (National Renewable Energy Laboratory, n.d.-b). This is because photovoltaic cells convert photons, or particles of light, that can carry energy from the sun, into voltage, or electricity (NREL, n.d.-b). But what do these cells have to do with cakes and recipes?
Like many cakes, a solar cell is made of layers, and the process of developing a new cell can be as laborious as the process your great-great-great-grandparents might have gone through to craft a new family recipe. We start with the base of our solar cell cake: the semiconductor layer.
Each atom in this layer is juggling several electrons. However, as NASA scientist Gil Knier explains, a traveling photon of sunlight can hit an electron and steal it away from its atom (Knier, 2008). Once the electron has been freed from its atom, the semiconductor’s properties pull the electron towards a wire, which acts as a path for the electron to move through (Knier, 2008). This flow of electrons through the wire is our newly generated electricity (Knier, 2008)! A good semiconductor layer will facilitate the movement of its electrons towards the wire in order to maximize the amount of electricity that gets generated.
For our semiconductor layer to do this job properly, we need to optimize its main ingredient: the semiconductor itself. Semiconductors can be made of many different combinations of elements, but currently, the Institute of Electrical and Electronics Engineers lists silicon as the primary element in most commonly used semiconductors (IEEE, n.d.). Take a look at the electronic devices around you. Do you see a computer? Phone? Microwave? 95% of these electronics use the unique properties of silicon as a semiconductor (Zulehner et al., 2000), as do 90% of the world’s solar cells (Chandler, 2020).
However, when it comes to solar cells, silicon is becoming too inefficient. Solar cell silicon has to be very pure, so the high temperatures and elaborate facilities required to produce that level of purity (Kumar & Sethiya, 2014) have caused researchers in materials science, chemical engineering, electrical engineering, and energy research (Jeong et al., 2021) to seek cheaper, easier-to-produce, and more eco-friendly alternatives (Kumar & Sethiya, 2014).
A team of these renewable energy researchers from Korea’s Ulsan National Institute of Science and Technology and Switzerland’s École Polytechnique Fédérale de Lausanne (UNIST/EPFL) is working with the newest ingredient: perovskite (Jeong et al., 2021). Perovskites are a class of materials composed of three types of particles (Yi et al., 2019). As seen in Figure 1, one particle is surrounded by the other two types, which form the corners of a diamond and a cube (Yi et al., 2019). Different combinations of particles result in different properties, opening the door for many possible replacements for silicon. The perovskite designed by the UNIST/EPFL team, for example, uses formamidinium, lead, and iodine (Jeong et al., 2021).
Figure 1: A diagram of the perovskite structure, with the three types of particles, denoted as A, B, and X. Image credit: Yi et al., 2019.
With the main layer developed, the UNIST/EPFL team finishes its cake. The team adds an anti-reflective frosting that can absorb more photons than the perovskite alone, thereby allowing the cell to generate more electricity (Sainthiya & Beniwal, 2017). Like the decorations on a cake, solar cells also have grid-like patterns on their anti-reflective frosting. These patterns are made of slim strips of metal, or the wires through which the electricity generated by the cell travels until it reaches your house and powers your devices (U.S. Department of Energy, n.d.-b).
After adding these final touches, the cake and its recipe are ready to be shared with the world, both for people to eat and continue improving upon it to create more delicious cakes. Similarly, in April 2021, the UNIST/EPFL team shared its perovskite solar cell recipe with the world through Nature, a scientific journal (Jeong et al., 2021), and the National Renewable Energy Laboratory (NREL), the Michelin critics of the solar cell world (NREL, n.d.-a).
During its review, NREL both measures and ranks each solar cell’s efficiency, which describes how much solar energy from the photons gets converted into electricity by the cell (NREL, n.d.-a). Despite the relative youth of the perovskite solar cells — the first of this type was evaluated eight years ago — UNIST/EPFL’s solar cell already matches the records of many existing silicon solar cells with an efficiency rating of 25.5% (NREL, 2021)!
For now, due to safety and durability concerns, we will not be seeing perovskite solar cells on people’s roofs or in large-scale power generation plants just yet (U.S. Department of Energy, n.d.-a). However, various companies and laboratories continue to build on and support the advancements made by the UNIST/EPFL team (U.S. Department of Energy, n.d.-a). For example, engineers at the Massachusetts Institute of Technology (MIT) have discovered that perovskite can be manufactured at a mere 200 degrees Celsius, whereas silicon requires more than 1,000 degrees Celsius (Chandler, 2021). That’s the difference between the heat of a household oven and a volcano (U.S. Geological Survey, 2017).
Like any good family recipe, the first iteration of the perovskite solar cell will not end up as either the best or the most iconic.
Like any good family recipe, the first iteration of the perovskite solar cell will not end up as either the best or the most iconic. As the UNIST/EPFL team’s recipe gets passed down to each new generation of solar cell engineers, countless researchers, like the team at MIT, are making discoveries and improving the perovskite cell design. With materials scientists and engineers around the world referring to the perovskite solar cell as “a leading candidate for eventually replacing silicon as the material of choice” and “an astonishing achievement for solar cells” (Chandler, 2021), you can hope to see perovskite as the semiconductor of the future.
November 2021
References
Chandler, D. (2020, January 26). For cheaper solar cells, thinner really is better. MIT News | Massachusetts Institute of Technology. https://news.mit.edu/2020/cheaper-solar-cells-thinner-0127
Chandler, D. (2021, February 24). Researchers improve efficiency of next-generation solar cell material. MIT News | Massachusetts Institute of Technology. https://news.mit.edu/2021/photovoltaic-efficiency-solar-0224
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