The average American home uses 10,649 kilowatt-hours (kWh) of electricity annually. This means that the average families or homeowners in America use 10,649 thousand, or 10.6 million, units of electricity to power their homes for one hour! The burning of fossil fuels such as coal to generate this electricity results in the release of large amounts of CO₂ (carbon dioxide), and other anthropogenic (human-generated) greenhouse gases (a-GHG) that cause the atmosphere to heat, damaging life on earth in a phenomenon known as global warming. In addition, electricity is expensive, costing 13.19¢ per kWh on average, which is a major reason why over 16% of the world does not have access to it.
In 1883, New York inventor, Charles Fritts, wanted to create a new energy generation solution and used a principle called the photovoltaic (PV) effect to develop one of the most prominent sustainable energy technologies today: the solar cell, more commonly referred to by its larger counterpart, the solar panel. The photovoltaic effect describes how two semiconductors can absorb light to produce electricity, thus photo, meaning light, and voltaic, meaning electrical force produced. A semiconductor is a type of material that conducts energy, with a force between a very powerful superconductor, and an insulator.
For the photovoltaic effect to occur, there must be variation in the semiconductors. While they can be the same element, such as silicon, one must be positive (p-type), and one must be negative (n-type). When the n-type semiconductor is placed overtop the p-type one, it forms an electrical connection called a p-n junction. As light strikes the semiconductors, the energy is absorbed by the atom’s electrons, causing them to become excited and move to a higher energy state called the conduction band. Since the valence electrons physically move, and “jump” from their shell in the n-type, it leaves behind a positive hole in the p-type. Due to the p-n junction already being an electric field, the electrons instead gravitate towards the n-side, while the holes can move towards the p-side. The flow of electrons throughout the material creates a current, which generates electricity (Figure 1).
Each semiconductor has a different bandgap, which is the measure of the minimum amount of energy needed for its valence electrons to jump from the atom. Therefore, this also determines which wavelengths, or colors of light, they absorb most strongly. This is why after the semiconductors absorb electromagnetic energy (like the sun’s rays), only a certain percentage of it can be converted into usable electricity, and this serves as the benchmark for each material’s efficiency in photovoltaics.
In solar panels, the material layers are turned into rectangular grids called cells, and multiple cells constitute a solar panel. The grids also contain lines of conductive metals to extract the current created from the PV semiconductor. Though Fritts’ initial solar panel used a selenium semiconductor and a thin gold metal sheet, the latest renditions of solar panels use various forms of silicon, which has a general efficiency of 15%, meaning 85% of the light taken in by it is not converted into electricity. Despite its low efficiency, silicon is the second most abundant element on Earth, making it a resource that is cheap and easy to experiment with.
However, the minimal-throughput performance of silicon-based solar panels is still an issue, as much of the electrical energy that could be used is lost, meaning that the technology is not suitable for large scale implementation. In addition, solar energy costs are currently not scalable, either. According to an MIT-backed study on solar energy costs, current energy output from solar panels costs around $180 per kWh, but for the world to run completely on solar panels, costs would have to drop to $10-20 per kWh, which is nearly a 90% reduction in price.
To increase the efficiency and decrease the cost of solar panels, researchers have begun to pivot away from using silicon-based solar panels, and have started using four other solar panel types: thin-film, organic, quantum-dot-based transparent, and perovskite, each of which is made up of different materials and layers to dramatically optimize photovoltaic devices’ performance, bandgap range, and operability (Figure 2).
- Thin-film solar panels are made from glass and extremely narrow layers of cadmium telluride (CdTe) or copper indium gallium diselenide (CIGS), making them a much cheaper alternative, but also less efficient.
- Organic solar panels are made from carbon-containing PV materials that are customisable and easily manipulated thanks to the versatility of carbon. They can be flexible and designed to absorb certain wavelengths of light, and designed for cheap mass manufacturing. However, they are only half as efficient as traditional crystalline silicon solar panels.
- Smaller quantities of semiconducting PV materials on the nanoscale (10-6 of a unit) called quantum dots (QDs) exhibit unique properties to absorb and emit specific colors of light (custom band gaps). Using glass layers, they can become easily manufactured transparent solar panels that can double as windows, making them portable and functional. They are currently inefficient, as their small sizes make electrical connections difficult.
- Perovskite solar panels are a sheet of PV perovskite placed on a support layer called a substrate. These solar panels are easily assembled as well as flexible and have reached efficiencies of 25%, higher than any other silicon solar panels made. Currently, much of the research and development on perovskites is to increase their operational longevity, efficiency, and develop low-cost mass manufacturing methods.
With the sun releasing 430 quintillion joules of energy each hour, which is more than enough to power the world for multiple years, efficiently harvesting solar energy gives the prospect of low-cost and near-limitless energy generation for every person on the planet. Making energy more accessible, environmentally assistive, and versatile in implementation is a very exciting future that photovoltaic technology could bring about. Ultimately, solar panels are an exciting emerging technology with a swath of environmental and economic impacts that will lead to a (literally and figuratively) brighter future for billions.
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