Not quite sure how solar cells work? Think of them as pizza (really!)

Yes, there are more similarities between solar cells and pizza than you might think. 
I'm a solar energy researcher working towards eliminating the defects in and improving the performance of industrial solar cells. A PhD is a long journey full of untimely experiments and countless sleepless nights, so I often find myself eating while working (definitely not in the labs!) and working while eating. One day, intrigued by how delicious a cheese pizza is, I realized how alike the pizza and the cell samples I work with are.

Build your own Solar Cell

A solar cell is a device that generates electricity when the sun shines over it. A combination of these cells linked in sequence makes a solar panel that can generate significant power, and that is what you see on people's rooftops, in solar-powered streetlights, and calculators. As a good pizza starts with a perfect dough base, solar cells begin with a very pure form of silicon wafer (which is also the second most abundant element in the Earth's crust), scientifically called a 'base.' Some extra elements like boron or phosphorus are then added to this silicon base to make it more conductive. 
Then comes the toppings. Yes, both for the pizza and the cells! Pizzas are loaded with a bunch of toppings for various flavors; solar cells are also coated with some very thin layers that help enhance their performance. These layers are called 'dielectrics.' They help reduce the reflection off the surface to increase the light absorption, passivate some surface defects, and possess some hidden benefits for the base. The most common dielectric, silicon nitride, used in the industry is also responsible for the blue color you see on most solar panels (a silicon wafer is otherwise grey!). 
We all know that pizza does not taste great after sitting in the fridge for a week. A similar degradation occurs in most solar cells. Once the panels are installed and are out in the sun, their performance degrades after the first few years (anywhere between 2-10% relatively). And losing a slice or two of a pizza might not make a dent in your pocket, but this degradation is responsible for a loss of billions of dollars every year. We call it 'light-induced degradation' (or LID). 

Light-Induced degradation (Staleness)

LID is a family of defects that occur in the presence of light; however, technically speaking, the resulting charge carriers are responsible, not the light. This degradation is not a new phenomenon, and researchers have been working on understanding and solving it for years. The good news is one of the most common defects responsible for LID has now been nearly solved in most panels worldwide. Unfortunately, we now have a new variant of LID in all kinds of panels. However, it only occurs at high-temperatures under light: "light- and elevated temperature-induced degradation," or LeTID. More sunlight is essential for higher electricity generation from the solar panels, but higher temperatures are detrimental (we only need the light, not the heat, for solar electricity generation). 

Limiting Staleness

This new kind of degradation is a focus of numerous researchers globally, including me. In my research, I work on mitigating this degradation by simply playing with the dielectrics (after all, it is all about the toppings, right?). 

Firstly, we have found that reducing the thickness of the dielectrics can significantly mitigate this degradation (1). You can imagine how applying less tomato sauce can prevent the pizza from going soggy. Reducing the thickness means using less material and thus lower costs. However, there is a threshold beyond which reducing the thickness might lead to other kinds of losses. 

Secondly, we also devised that the placement of the dielectrics plays a vital role in the extent of potential degradation (2). By studying multiple industrial cells, it was observed that adding a very thin layer of a second dielectric can strongly modulate the degradation. This reduced the degradation by creating a barrier layer between the first dielectric (Silicon nitride) and the silicon base. Another solution we found is the dependence of degradation on the silicon wafer thickness (3). By thinning the wafers, severely low degradation was observed. Similar to how a thin-crust pizza can help prevent you from gaining extra calories if you are on a diet!
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These three solutions effectively alleviate the degradation in current solar cells without increasing their manufacturing cost. With solar installations progressing at record levels each year, the mitigation of these defects will accelerate the transition to a cleaner world. So, we can leave the next generations with tastier pizzas and a healthier planet! 

Author

Utkarshaa Varshney is a solar energy researcher at UNSW Sydney, Australia with a master’s in
Material Science from Stony Brook University, NY, USA. Born in India, she has always been
deeply passionate about sustainability and global warming. She is also a science communicator
and frequently speak on renewable energy at academic conferences and science events to
spread the awareness and urgency of it.
Outside of work, she loves to bake, travel, and read books. You can follow her on Twitter
(@utkarshaavarshn) and LinkedIn. 

And you can read her fabulous publications about solar cells listed below!

1. U. Varshney, M. Abbott, A. Ciesla, D. Chen, S. Liu, C. Sen, M. Kim, S. Wenham, B. Hoex, and C. Chan, "Evaluating the Impact of SiNx Thickness on Lifetime Degradation in Silicon," IEEE J. Photovoltaics, vol. 9, no. 3, pp. 601–607, 2019.

2 . U. Varshney, C. Chan, B. Hoex, B. Hallam, P. Hamer, A. Ciesla, D. Chen, S. Liu, C. Sen, A. Samadi, and M. Abbott, "Controlling Light- And Elevated-Temperature-Induced Degradation with Thin Film Barrier Layers," IEEE J. Photovoltaics, vol. 10, no. 1, pp. 19–27, 2020.
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3. U. Varshney, M. Kim, M. U. Khan, P. Hamer, C. Chan, M. Abbott, and B. Hoex, "Impact of Substrate Thickness on the Degradation in Multicrystalline Silicon," IEEE J. Photovoltaics, vol. 11, no. 1, pp. 65–72, 2020.