Intro to Solar Photovoltaics (PV)

So Hello! My name is Jeffrey Brownson, I’m an associate professor in the department of energy and mineral engineering and material science and engineering. I am also a program lead for an online master’s program in renewable energy and sustainability systems where I specialize in solar. And my career is in the researching and the teaching and the outreach tied to solar energy systems. And what we’ll be talking about in our future lectures is tied to solar energy, lifecycle assessment and thinking about the specific technologies of photovoltaics, the materials tied to them and what are their impacts on the environment and society.

Solar energy is this wonderful topic. It’s a discipline that is very broad reaching. It impacts so many other fields and it’s really something that even kids appreciate. Kids love sunshine, right? They can go out and they can play in it, and the interesting thing here is that we can use that sunshine, harvest that sunshine, for electricity generation, for the generation of hot air, hot water that we use in society, and we already use sunshine regularly for photosynthesis. For turning plants into fuels and plants into foods. And that’s all being delivered by solar energy. And so, specifically, solar energy in the case that we use it in my field is how do we use solar energy for useful purposes in society and how can we think about using solar energy to help in the process of reducing greenhouse gas emissions for providing health and safety to society and to do that in a way that is equitable, that is environmentally sound, and that will ultimately be sustainable as a source of energy. However, solar energy, as we know, has an intermittency to it. And there’s the act of every cycle the sun rises and sets, and it has a peak at noon, and that’s a variable intensity of light across the day. But the sunlight is also interfered with by clouds, and clouds will change the intensity of sunlight. And if we are using a technology that provides instantaneous conversion of sunlight to electricity, the technology called photovoltaics, then we need to be aware of that intermittency. And we need to think about how we design our technological systems to adapt to that intermittency and to adapt as it is tied together with the system of our electrical grid across the nation.

The technology that we are really focusing on for solar energy conversion in this discussion and subsequent discussions is photovoltaics. And photovoltaics is really an action; it’s a process that involves a couple of steps, but we’re really taking the sun's light and turning it into electricity. And so, let’s discuss what those steps are. So, the sun's light comes in; it has to be absorbed. It has to be absorbed by an opaque material. That material typically is something called silicon. Silicon absorbs the sun's light; then electrons are generated; they’re excited; they have a potential energy to them that we can use to do work. The electrons are generated. They are separated from their constituent parts. So, we have electrons within the crystalline lattice. They’re excited; they’re separated from the lattice; they travel through an external circuit, the copper wires and the grid, and they return back depleted of their potential after they’ve done some work, maybe to spin a fan, to power our lights, to power air conditioning systems. But they use that energy to do work, they return back to the photovoltaic cell and the loop is closed. So, this process, altogether, is called photovoltaic action (PVA). PVA is the absorption of light, the generation of charge carriers, those excited electrons, and the separation of charge carriers. And then, finally, once those charged carriers are separated, they go through an external circuit and they do work. And, for us, work just means energy being used for something we find useful.

So, photovoltaics are technologies and they can be purchased in several different flavors, we’ll say. The main technology for photovoltaics is based upon the material silicon. Silicon is a semiconductor. It’s readily available in the environment; it's actually the second most abundant element in the earth’s crust. And the silicon is converted into a technical material, into a cell, that is then arranged into multiple cells into a module which was then sold onto the market. The other technology that can be purchased is a smaller percentage of the industry but is growing fast, is called thin film photovoltaics; the main material being used to absorb light for photovoltaic action is called cadmium telluride. So, thin films of cadmium telluride are deposited in a module and then sold into industry to be installed. So, we have silicon technologies and cadmium telluride technologies and in both cases, we’re describing the absorber material that is really doing the photovoltaic action, the actual work of absorbing light, generating charge carriers and separating charge carriers. There are other materials in these modules but those are the main ones that we really want to focus on when we’re specifically thinking of different photovoltaic technologies that we can buy and use in society.

The sun is very hot. It is effectively a black body that is emitting light because it’s approximately 6,000 degrees Kelvin at its surface. That light is going to be emitted across a broad spectrum of wavelengths, so - many different wavelengths of light. If you think about it, the light on a red laser might be around the order of 650 nanometers (nm), and a green laser might be on the order of 530 nanometers. Think about the sun’s spectrum being many, many, many more wavelengths around that. The light from the sun, by the time it reaches us, will be called the shortwave band, so that group of wavelengths that are coming from the sun that reach the earth’s surface is called the shortwave band, and it will go from the ultraviolet around the order of 250 nanometers all the way out into the infrared to about the range of 2500 nanometers at the far end. So, 250 to 2500 - that's a rough range for the full spectrum of the sun's light and there are a whole number of photons that are in that curve, in that whole spectrum, of wavelengths of light. That is all called the shortwave band. And the shortwave band has all these high energy photons, many of which can be used by a photovoltaic device to absorb and generate charge carriers and then be separated to do work in the process of photovoltaic action.

We’ve talked about how the sun has many different wavelengths, and that these wavelengths that come and are received at the earth’s surface underneath the atmosphere is called the shortwave band. From 250 nanometers to about 2500 nanometers, UV to infrared. The range of that spectrum is very broad, but a portion of that spectrum can be absorbed by our photovoltaic technologies and deliver excited electrons. That portion of the spectrum that can absorb light and generate excited electrons that can be separated and do work in the process of photovoltaic action, that range is above a threshold that we will call a band gap. And the band gap is an energy level above which, so higher energies, are absorbed and generate excited electrons. Lower energies are effectively filtered out. They don’t generate excited electrons. They might warm up the module, they might heat it up because the module is still opaque, but they’re not going to generate the electrons that we want to make electricity for work. So, we want energies higher than the band gap, and the interesting thing about energies is that the energy of the band gap is the reciprocal or the inverse of the wavelength. So, if I want higher energies, I, actually, in terms of wavelengths, am thinking about shorter wavelengths. In the case of silicon, the band gap is about 1.1 electron volts. Now, in terms of that energy unit, that’s a little bit obscure for the general audience, so what we think of it in wavelengths is about 1100 nanometer wavelengths of light. And so, I want 1100 nanometers and smaller, which is equivalent to 1.1 electron volts and higher. Again, because of that inverse relationship of energy and nanometers. Cadmium telluride is the other material that we think of as a main material for photovoltaic cells. Now, cadmium telluride absorbs light at a slightly higher band gap than silicon, and so, you’re going to have a higher wavelength, excuse me, a smaller wavelength of light absorbed to generate excited carriers in cadmium telluride. Now, the electrons that are generated from this higher energy are going to have a higher potential, so it’s kind of like thinking of more photons at a lower potential versus less photons at a higher potential will contribute different levels of power from the photovoltaic modules. For our purposes, for the general audience, these really work in very similar ways, and will both generate clean power for the grid. So, one is not necessarily better than the other, just because of the band gap, they’re just going to absorb lights at different thresholds and that’s something to consider in terms of the intrinsic material properties of silicon and cadmium telluride.