Everything You Need to Know About Photovoltaic Cells

Michael Brown

In one second, the nuclear fusion process taking place inside the sun produces enough energy to satisfy the needs of the earth’s population for nearly 500,000 years. Photovoltaic cells are capable of capturing some of that energy and converting it into usable electricity; unfortunately, today’s technology can’t do this very efficiently.

French physicist Edmond Becquerel first described the photovoltaic effect in 1839. He discovered that some materials were capable of producing small amounts of electricity when exposed to sunlight. The first photovoltaic cell, however, wasn’t created until 1883, and more than 70 years passed before the next major scientific advance took place, when researchers at Bell Labs developed the first crystalline silicon photovoltaic cell in 1954.

Most modern photovoltaic cells are still manufactured from silicon, the same semiconductor material used to produce GPUs, CPUs, and other integrated circuits. The majority of commercial photovoltaic cells are manufactured from crystalline silicon—either single- or poly-crystal silicon. The latter are less efficient than the former, but their lower manufacturing cost largely makes up for the conversion shortfall.

The bulk of the progress that’s been made since the 1950s stems from the efficiency at which absorbed light is converted into electricity. The Bell Labs product was capable of just 4 percent efficiency; today’s commercial products are approaching 20 percent efficiency.


Photons from the sun pass through the cell’s n-type layer to strike atoms in the p-type layer,
dislodging some of those atoms’ electrons in the process. The freed electrons move up toward the n-layer,
creating an electrical current that can be stored or service an electrical device.

The Photovoltaic Process

A photovoltaic cell is created by sandwiching two silicon wafers: an n-type layer and a p-type layer. The n-type layer exhibits a negative electrical charge and has an excess of electrons, while the p-type layer exhibits a positive electrical charge and has a shortage of electrons. The two layers are separated by an n-p junction. The cell is then attached to a backplane, a layer of metal used to physically reinforce the cell and provide an electrical contact on its bottom. A second electrical contact is placed on the top of the cell to create an electrical circuit. The cell is then treated with an anti-reflective coating to compensate for silicon’s otherwise shiny nature.

As photons—particles of light—hit the photovoltaic cell, they pass through the n-type layer and strike the p-type layer, where they are either absorbed by the silicon atoms, reflected, or pass straight through the material. Absorbed photons knock electrons loose from the silicon atoms, leaving empty “holes,” which are filled by electrons further back in the circuit. The loose electrons flow through the electrical contacts on the p-type layer to the contacts on the n-type layer. This flow of electrons produces an electric current that can be drawn off and stored in a battery or used to power an electrical device.

An array of cells is electrically connected and mounted into a frame to form a photovoltaic module. A narrow metal grid is applied to the top of the module to transport electrical energy, and a sheet of glass or plastic is placed on top to protect the cells from the environment (everything from bad weather to bird droppings and stray baseballs). A group of interconnected modules is known as an array.

Photons contain varying amounts of energy, depending on their wavelength. Within the visible spectrum, red light possesses the least amount of energy while violet light has the most. The same goes for the invisible spectrum: Infrared light possesses very little energy but ultraviolet light contains a great deal of it.

Most modern photovoltaic cells are capable of converting only high-energy photons into electrical current, which explains why mainstream solar panels are so inefficient. One of the most promising ideas for increasing the efficiency of solar energy is to stack cells with different properties on top of one another. This way, high-energy photons can be captured by a cell on the top of the stack, while lower-energy photons pass through to subsequent cells that are better suited to those photons’ wavelengths.

AC/DC

The electrical devices in your home (appliances, computers, air conditioners, lights, and so on) operate on alternating current (AC), but a solar array produces direct current (DC). The solution is to install an inverter that converts the solar array’s DC into AC. Inverters are designed to power off when there isn’t enough electrical current for them to operate, e.g., at night.

Solar panels produce the most power in the presence of direct sunlight, but they’ll produce some energy on cloudy or even rainy days. They can’t produce any juice at night, of course, so you’ll need some means of storing the electricity that they create when the sun is shining. Batteries can provide total independence from your local electric company, enabling you to potentially live “off the grid,” but this solution presents a host of environmental problems, and there’s no guarantee it will provide all the energy you’ll need. The more practical alternative is to tie your system into the electrical grid.

In a grid-tied system, you sell the excess energy your solar array generates to your local utility, and you buy back the electrical power you need for your home. With this method, the utility acts like an unlimited energy-storage system, giving you all the power you need whenever you need it. The inverter is connected to the meter the electric utility uses to measure your consumption, which means your meter will spin backward whenever you generate more than you consume.

For most households, the reward for going solar is more feel-good than financial: It could take a decade or longer to recoup the investment in even a moderate-size system. That situation is changing rapidly as the escalating cost of producing electricity from fossil fuels moves in inverse proportion to the cost of deriving energy from the sun.

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