Today, one of the most important and prevalent issues in the media regards the politics of climate change and renewable energy. Surely you have seen it in political discourse—the conflicting notions of support for free market economics and the current status quo of energy moguls, or the triumph of more innovative approaches to our ever-expanding energy crisis. It would seem that with the complete politicization of energy, such an issue may be hopelessly lost in the chasms of mindless disagreement. Nevertheless, a proposed solution to the issue of pollutants like CO2 is to rely on the promise of alternative methods of consumption and production. Among this group of energy proposals lie wind and solar, the two most widely known and notable in terms of both investment in money and research. Since the sun and its energy is readily available for people throughout the world for numerous hours in a day, for this and upcoming blog posts, we will focus on the benefits of one of these solar alternative energy approaches and its technological applications.
Quick question: Have you ever seen one of these before?
My bet is that you have. This is a solar panel, which is the base of it all, the foundation of solar energy, and the mechanism that harvests the sun’s power. You might ask yourself, however, if you know quite how it works—in which case the answer to that question would probably be “no.” What exactly goes on in the inside of the solar panel is of chief concern for not only the scientists who make them but the energy proponents who tout their benefit. To start off, let’s get into the photovoltaic process.
The Photovoltaic and Photoelectric Process
The photovoltaic effect is complementary to the photoelectric effect, well-understood since the early twentieth century from the experiments of physicists Heinrich Hertz and Max Planck, and contributions to light and color theory by Albert Einstein. In its simplicity, the interactions between photons and electrons represent a transfer of kinetic energy of one kind to kinetic and potential energy of another, such that a photon instancing on a valence electron causes it to jump to a higher energy level. Simply put another why, try to imagine an electron buzzing around in the center of an atom. Along comes a photon, emitted by the sun’s rays, which crashes into that electron. When that electron is hit with this photon, it now has more energy, so it begins to whirl faster around the center of the atom in what we call an excited state. Within that atom, the electron now has more kinetic and potential energy, since it is both faster moving and farther away from the nucleus. If the energy from the photon is great enough (its frequency is greater than the cutoff, or minimum frequency required to excite electrons in a material beyond a particular state), the electron is ejected and flows. A brief animation illustrating this process can be found here.
Moving away from our atomic scale now, let’s assume that with enough photons hitting enough electrons causing them to leave their atoms, they flow through the material as negative charges at a certain rate in coulombs per second, or amperes. This is our current, then. Therefore, our induced current multiplied by our potential difference, or voltage, represents our power output (P = IV), which is what we care about in the end when considering how much sun exposure a solar panel will need to power a home. In order to understand how our voltage contributes to this power output, then, we must look to the role that the material itself—our conductor or semiconductor—plays in influencing the voltage of the moving electrons. This requires a brief consideration of solid-state physics.
Band Gap Energy
For various semiconductor and regular conductor materials there exists a property called the band gap, the minimum amount of energy required to elevate an electron in the material from its outer band to the conduction band (this energy is written as Eg in standard equations). The band gap varies among different materials. For example, insulators tend to have a very high band gap (greater than 5.0 eV) while semi-conductors are usually around 3.0 eV and below. This means that insulators have a greater threshold of energy and thus are much last conductive than semi-conductors. On the other hand, conductors, metals such as copper and gold, have overlapping valence and conductor bands, therefore making them the most ideal materials for maximum electrical output since they have no band gap at all. (Keep an eye out for future posts that delve deeper into the quantum chemistry involved with how and why these band gaps differ for various materials.) It is also known that, for all conductors and semiconductors, the width of their band gap decreases with a decrease in temperature, therefore meaning that a cooler material conducts electricity more efficiently (a notion useful in the creation of superconductors). This also plays a role in the efficiency of solar panels as well:
Note that the maximum photovoltage was recorded at a temperature closest to absolute zero. It would seem to make sense then, that if a superconductor conducts so well due to its low temperature, a solar panel that contains conductors would produce more photovoltage on a colder day rather than on a warmer one.
Thus, we have arrived to our proper understanding of how a solar panel works. Here is a quick and comprehensive review of the basics of a solar panel.
This will be really important when we talk about what type of materials influence the output from solar energy. The future of solar panels, bright as it is, will only go as far as the ingenious minds that contribute to it. For now, that challenge to ingenuity is finding the proper material. Innovations in material selection to optimize solar cell function are on the forefront of cutting edge research in the photovoltaic field. (A sneak preview for the extra eager can be found here.) In the next blog post, we’ll explore just how important this is, and why every engineer in the solar innovation fast lane must give it proper and measured consideration.
About the authors: BCAB5 is a subgroup within the BCA Advanced Chemistry student community devoted to promoting awareness about renewable energy from a chemical perspective. Its contributing editors are Princeton Ferro, Sejal Jain, and Sylvia Sawires.