Xiaolin Zheng: Solar Stickers to Power the World

Good morning. It has been a truly exciting week for me. It's a very empowering experience for me to get to know my fellow emerging explorers. For me, I'm passionate about energy problems. I hope we can have a sustainable energy future.

Growing up in China, in the period that the cultural revolution was in the past, China has opened up its doors to look for better economy and better lives. Our lives are getting better and better, but energy is always scarce. I remember I used to watch my Mom working in the kitchen under very dim light trying to save electricity. About 10 years ago, my parents bought a solar thermal system. Like many city people, they live in apartments, those high-rise buildings. They installed a solar system on the rooftop of their building. Such a system will heat up their domestic water so they can have access to hot water, and that makes a huge difference for their life quality. Like many families, they want to install such systems on the rooftop and it becomes a competition. Everyone wants a space there. One day my father said, “It will be really nice, we can hang solar thermal panels outside our window, the piping will be simpler, and everyone will have access to the renewable energy.”

That makes me think about there really is a strong need for this kind of flexible and lightweight solar systems. That motivates me to develop later on the solar cell stickers, and this work is done by my students and me at Stanford University. The solar stickers are like downside just like the stickers your kids put on your clothes to say, 'Thank you, Mom,' 'Good job, Mom.' You peel them off and then stick it to anywhere you want, so that we can increase the places we can use solar energy. The reason I'm interested in solar energy is very simple. There is abundant resources of solar energy. Here we show the world's energy resources. The big one in the center is the annual energy from the sun. The ones on the right-hand side are from the finite resources. For example, the petroleum, coal, natural gas, etc. The small circles in the center represent those renewable sources, their annual energy output, for example, wind, geothermal, etc. The two blue dots on the very left-hand side represent our annual consumption of energy. The smaller one is about energy consumption in 2010. The bigger one is the estimated energy consumption in 2015. You can see our energy consumption is increasing as we want to preserve our lifestyle and people from developing countries are looking for better life quality. If we continue to rely on fossil fuels, we are going to deplete them one day, sooner or later, because we are using them simply at a rate much faster than can be regenerated. We have to rely more and more on renewable energy and solar is a wonderful resource.

We are using solar energy now. We are using it in the form of solar panels. You probably all have seen this kind of flat panel looking thing, dark color, facing the sun. Those solar panels essentially is a sun-powered battery. When the sun illuminates those solar panels, they will generate electron holes. The flow of electrons in the holes will generate current and that current powers the electrical load. For example, your phones, a lamp, etc. You can think about the solar cell as a sun-powered battery constantly charged by sunlight. If we look a little deeper into the solar cells, a solar cell just like a battery, as I mentioned, as all batteries you will have a positive and a negative side. The greyish color on the top and the bottom are the positive and negative part. They are metallic layers. What's in between is semiconductor, frequently silicon, that can absorb sunlight and generate electron hole that drives the current. You can think about from the structure perspective, this solar cell looks like a sandwich. It's the semiconductor sandwiched between two electrodes. If this whole thing is as thin as one type of solar cell called a thin film solar cell, then it has the potential to become flexible and lightweight.

If we look at the key parameters we characterize solar cell, the one parameter is called efficiency. How much power can be generated comparing to how much power is incident by sunlight? In other words, what percentage of sun power you can convert to electricity? For most solar cells in the market, it's about 10 to 20 percent. For example, if we have 15 percent solar panels, if you have one square centimeter area, you can generate about 150 watts. That's enough to power two to three lamps. If I have a rock the size of this rug, it will probably roughly generate about 500 watts. That's the magnitude. To increase the energy production by solar cells, one is to increase efficiency; the other to increase the area you want to use it. We want to make them to be able to apply it not only on rooftops. For example, these are the rooftop solar panels at Stanford, that's over the Business Center, and those are great for rooftop applications. But as I mentioned, we want to increase the places. There's only limited space on rooftops. It will be nice if we can put on cars, or maybe on the side walls of buildings, then those solar cells have to be flexible.

What's the challenge? This is how solar cell looks like, called the thin-film solar cells. The bluish color is the sandwich structure, that's the solar cell that's the actual part that generates electricity. Those things have to be manufactured on the substrate. The substrate is the holder for them. The substrate typically is glass or silicon wafer or stainless steel. The solar cell part is only a couple microns, about one-tenth of the thickness of the plastic food wrap you use. That's very thin, very flexible. But the substrate is about a millimeter scale. It's about a thousand times thicker. That's the part that causes the solar cells to be heavy, to be rigid. But we need those substrates as a holder to fabricate the material. During the fabrication, they put the substrate there and they deposit material on top. It's similar as the process as ladies doing make-up, we have the face as the substrate, we put layers and layers of materials and they are very thin, so they don't show up very much. The idea here is if we can replace the rigid substrate by something flexible, for example, paper or plastic, then essentially we can have a flexible substrate. But we cannot directly just stick the paper into the solar cell manufacturers. They cannot sustain the environment, the temperature, the chemicals. We are required to manufacture solar cells.

So what do we do? It's the inspiration from food. It's the pizza approach. When we bake the pizza, we put this pizza in a ceramic pan. The ceramic pan can sustain the high temperature in your oven to cook the pizza. But when we deliver the pizza, we transfer that pizza out, put it in the paper box. That's much cheaper so we can have pizza at a very economical price. When we look at the pizza, essentially we have two carriers for two different functions. The pan is for the high temperature manufacturing process, and the paper is for the delivery and transportation purpose. We can do the same thing for solar cells.

This is how we do it. This is again the two-layer structure I showed you. The blue is the solar cell and the bottom is the thick glass or silicon wafers. What we do differently is we insert a metallic layer in between, and that layer serves as a sacrificial layer later on. The solar cells will be manufactured as identical procedures and conditions as your regular thin-film solar cells. What we do next is we soak it in water. Room temperature water, just the tap water, and inside water we just peel it. During this peeling process, the metallic layer separates from the substrate, the glass. Now we have this thin solar cell coming out, kind of like when ladies apply the face mask. Sometimes you paint it and when you remove it, it comes off as entire layer. It's the same thing but we have water there. Now the thin solar cells, as I mentioned, is only a couple microns. It's very thin and you can stick it like a sticker to the back of your cell phones. That realizes the transfer process. Now we have two substrates. One for fabrication, one as the carrier when you use it.

The magical part to me is really what happens in the water, and that is where we have worked on. This shows the process in water. What you have is a four-inch wafer with the metallic layer on silicon. My students are just working through the edges, trying to make sure the edges are clean. You can see in the initial one-half minute, he's just working through the edges, and then only within a few seconds, he peeled off the entire thing. What you see in the front is the silicon wafer that you can use for the fabrication of solar cells. You can see, after this process, it's very clean so you can use it again, just like the pans for pizza, for solar cell fabrications. The one on the back, the shiny part, is the metallic layer and the solar cells come in behind that. Now you have a solar cell sticker. You can stick it to anywhere. This process only requires water and is really fast. You can see this takes less than two minutes.

We collaborated with National Renewable Energy Lab. They fabricated this amorphous silicon solar cell on our modified silicon wafers. Those circles are the solar cells. After this peel-off process, we attached it to my business card. It's very flexible. This is the initial silicon wafer and it's very clean. You can use it again. The key parameter for solar cell is efficiency. After this transfer process, the efficiency remains the same. That means the process is so gentle, it doesn't damage the solar cell. That's the key.

This peel technology, we think can really go beyond solar cell. It really doesn't depend on the solar cell. We are just controlling the interface. If you replace the solar cells with thin-film electronics, for example, display. We can have flexible displays, or batteries. We can have thin-film batteries, or sensors for human health monitoring or environmental monitoring, or any other thin-film electronic devices you can imagine. It is a very powerful technique for flexible electronics. With this technique, I dream one day we can stick those solar cells to the side wall of the buildings, to the shade of your windows, to your helmet, to the back of your phones, your iPad, to your backpack. With that, we can power the world, and with that, we can contribute to a sustainable energy future. Thank you very much.