Power and progress in photovoltaics

Published 12:00 pm Thursday, July 21, 2016

We’ve all seen a working solar cell. It may have been powering the lights on that blinking stop sign near the Whirlpool Trails, or at the top of your TI-30XS scientific calculator, or perhaps in one of the solar farms in nearby Chickasaw County.

Solar cells are incredible devices that absorb sunlight and generate electricity that can be used to do work, like powering a microwave or an electric car. Solar cells have no moving mechanical parts. They are silent, durable, and clean. They offer additional independence. Want to live off the grid? They are good for national security by relieving our dependence on foreign oil and decentralizing our energy infrastructure.

On-site electrical power generation is also more efficient as significant resistance losses in the long-distance transmission and distribution of electricity are avoided. It’s also good stewardship of our natural resources; fossil fuels (coal, oil, and natural gas) are nonrenewable sources of energy. Why not make them last for as many future generations as possible?

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In essence, a solar cell uses light energy to move electrons around in a circuit. Solar cells, or photovoltaics, are solid-state materials that contain two different semiconducting layers. One layer starts off positively charged (p-type) or electron deficient, while the other layer is negatively charged (n-type) having extra electrons. When sandwiched together, a so-called p-n junction is established that causes electrons to migrate between the two layers leaving the p-type layer negatively charged and the n-type layer positively charged. That’s the tricky part.

Briefly, a single unit of light, indeed the smallest quantity of light called a photon, is absorbed by the cell, meaning light energy promotes (or knocks free) an electron from one of the atoms in the material to a higher energy state, still in the material. The position in the semiconductor that was just vacated by the excited electron is called a “hole.”

This gives us a higher energy “electron-hole pair” as a consequence of light absorption. Notably, electron-hole pairs don’t form on their own; their formation is due to the transfer of solar energy! The free, negatively charged electron moves toward the now positively-charged n-type layer while the positively charged “hole” moves through the now negatively-charged p-type layer.

Opposites always attract (in electrostatic interactions anyway)! When an external load is applied across the two layers to collect the charges, an electrical current is allowed to flow that can power your appliance or charge a battery. This electron dance repeats itself over and over again while the sun is present.

So, where’s the catch? Conventional solar cells have been around for decades, but they are still quite expensive. It’s best to think about residential solar cells (or panels of cells) as an investment. There is a large investment cost up front that can often be financed, and a federal energy tax credit of 30% of the total expense is also available to further offset their initial cost. Both the City of Oxford Electric Department and North East Mississippi Electric Power Association participate in a dollar-for-dollar buy back of solar-generated electricity from participating customers that reduces the customer’s power bill. You can read more about this program online at https://www.tva.gov/Energy/Renewable-Energy-Solutions/Green-Power-Providers. The average payback on investment is around 7 to 10 years, but solar panels typically operate for at least 25 years and the sun’s energy will always be free. It’s an economic investment, but also an investment in a cleaner future.

As the volume of solar cell manufacturing has increased, their price tag has continued to drop. The majority of commercial solar cells are single crystalline silicon wafers that require high temperatures and high purities in the manufacturing process to achieve optimal cell performance.

This is an expensive process. Materials such as polycrystalline silicon, cadmium telluride, and dye-sensitized titanium dioxide-based solar cells are available or under development as potentially cheaper alternatives. However, these currently have lower efficiencies and cadmium is a heavy metal with toxicity concerns. Research challenges remain in lowering the cost while maintaining safety and matching, or even exceeding, the efficiency of conventional solar cells. However, the economic barriers to this technology are becoming smaller.

Did you know that the Center for Manufacturing Excellence at the University of Mississippi has the largest roof-mounted solar panel array in the state? This solar farm is capable of generating roughly 109 kilowatts of electrical power at any given time. Over the course of a year, this system will produce around 136,000 kilowatt-hours (kWh) of clean energy. For comparison, the average American household consumes about 11,000 kWh of electricity each year. Solar energy is an investment worth looking into.

Jonah W. JURSS, Ph.D. is a chemistry professor at the University of Mississippi who does research in the field of renewable energy.