Transparent Photovoltaic Cells A window to a renewable future
There is an ever-increasing demand for energy in an expanding economy with a continuously growing population. Electricity production generates the largest share of global greenhouse gas emissions, with 60% of our electricity in the U.S. coming from burning fossil fuels, mostly coal and natural gas (1),(2). To continue to meet energy consumption needs while replacing fossil fuels, a rapid expansion of clean renewable energy infrastructure is imperative. Solar energy received on Earth’s surface per year is approximately 120,000 TW, which is 6-7 thousand times more than the current global energy consumption (3). Research into solar energy has been a priority, as it is considered the most abundant source of energy that can satisfy the demands of continual economic development (4). As of 2020, only around 1% of global energy demand could be satisfied by installed solar energy plants, however (5). A large surface area is required for solar energy to be an efficient source of electricity, as photovoltaic cells rely on absorbing light (photons) to create a current. For photovoltaic cells to fulfill energy demand, they must be adopted on a wide scale. Traditional solar panels made from
BY TIA BÖTTGER
silicon can be heavy and bulky, making them difficult to widely install into existing architectural spaces (6). Emerging technologies of transparent solar cells help address the challenge of a very large amount of space required, as any already existing sheet of glass could be converted into photovoltaic cells. The prospect of converting windows in buildings, cars, agricultural sheds, and even the glass panels of electronic devices into energy harvesting devices is an exciting challenge, which would help overcome the current limitations of solar panels and address the high demand for energy (7). An understanding of a traditional photovoltaic cell is necessary before exploring emerging technology into transparent photovoltaic cells. Most basically, a photovoltaic cell functions by creating a potential difference, which will drive current to flow through the cell so that energy can be transported to electrical devices. A semiconductor material is used to achieve this, and a photovoltaic cell functions under the same working principle as a semiconducting diode. Semiconductors are crystalline solids, so they are neither metals nor insulators, and their conducting properties are determined by impurities or dopants (4). Silicon is by far the most widely used semi-conducting material, with around 80% of solar cells in the world made using silicon-based materials (4),(8). A slab or film of Si is n- and p- doped in its two surface regions (8). The p-type silicon is produced by adding atoms, such as boron or gallium, which have one less electron in their outer energy level than silicon does. Because one less electron is present than is required to form bonds with the surrounding silicon atoms, an electron “hole” is created. In contrast, n-type silicon includes atoms with one more electron in their outer energy level than silicon, such as phosphorous,
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