Greenhouse gases, the major cause of global warming, are at all-time highs according to the World Meteorological Organization. Increased CO2 concentrations are triggering these record-breaking levels, and are mainly caused by the burning of fossil fuels for electricity and transportation.
One of the most promising alternatives to fossil fuels for electricity is solar power, yet it only accounts for about one percent of the total energy production in the United States. Crystalline silicon is a primary component in the most common type of solar cells, because it absorbs a large portion of the light from the sun and is resistant to water damage. Nexight’s Jared Kosters previously discussed the SunShot Initiative’s successes at decreasing the cost of solar cells. However, some properties of silicon are limiting the amount of cost reductions possible and preventing the widespread adoption of solar cells:
Crystals are expensive. Large silicon crystals are created by inserting a small seed crystal in a large tank of high purity molten silicon. It requires a tremendous amount of energy to keep the silicon melted, because the melting point of silicon is 2577 degrees Fahrenheit. The seed crystal is then slowly pulled out of the tank and rotated precisely to control the cooling rate of the material. Once the crystal has reached the desired size, it can be cut into individual wafers. These can be used for solar cell and other electronic applications. The energy, purity requirements, and precision needed for this processes drives up manufacturing costs.
Large amount of energy lost. All colors of light contain specific amounts of energy. Materials can absorb the light energy as long as it is above a specific threshold, which is called the material’s bandgap. As long as the energy is greater than or equal to the bandgap, it will be absorbed. In solar cells, this absorption is what creates electricity. Most of the light silicon absorbs from the sun has energy greater than the bandgap. This excess energy is not used to create more electricity; instead it is lost as heat.
Crystals are rigid. The highly ordered structure of silicon crystals creates an “express lane” for electricity to make it out of the solar cell. However, it also makes crystalline silicon rigid and brittle. Because of this, silicon solar cells have to be flat. Yet, this can lead to inefficiencies and increased cost in solar systems. For example, if the flat solar cells are installed on a roof, the amount of energy they produce will change depending on the angle of the sun relative to the solar cell. Expensive tracking systems can be installed to keep the efficiency consistent throughout the day.
Researchers are designing alternatives to silicon solar cells that eliminate many of these problems. The most common approaches are to create materials and cells that absorb and convert light more efficiently and reduce material processing costs. None of these new designs have been able to compete with silicon for a number of reasons: they are made from materials that are not as abundant as silicon, use toxic materials, and stop working when exposed to water and oxygen. If any emerging solar materials could decrease manufacturing costs significantly and remain working when exposed to water and oxygen, they could spur a more widespread use of solar cells.