Traditional photovoltaic cells are typically expensive to produce and hard to incorporate into organic building forms. They may also utilize rare earth materials, which require energy-intensive extraction. However, new research is suggesting we should head in a different direction. A class of materials called perovskites is being explored to create solar panels within the next few years. They seem useful for creating a photovoltaic system that is lightweight, cheap and can be placed onto various surfaces, regardless of texture or curvature.

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What are perovskites?

Perovskites are a family of materials with a special crystal structure, similar to the mineral perovskite. The crystalline structure is made of three parts and uses a lattice to interlace them. These perovskites can contain various combinations of elements for each of the three parts. These specific elements can be chosen to best suit the needs of the final product they are required for.

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How perovskite photovoltaics work

Perovskites have metals like tin or lead in them which are useful for the photovoltaic effect. This is the process of using light to produce energy. It occurs when light strikes two different atoms in close proximity, generating an electrical current and voltage. This is because light provides energy to free the outermost electron(s) in one of the atoms from their structure. It then pushes the electrons to join the structure of another molecule.

The minimum amount of energy required to shift electrons is called the bandgap. The electron that has moved from one molecule to another becomes a charge carrier that can move through this new material, carrying electrical energy. These electrons are directed along a wire to produce electricity and can continue to generate a current and voltage while the light continues to shine.

In traditional solar cells, silicon is used to carry out the photovoltaic effect. It has to be meticulously heated to extreme temperatures for perfect purity and crystal structure to be able to produce electricity. On the other hand, perovskites are relatively easier to make and do not require extreme conditions. They also can tolerate defects in their molecular structure, unlike silicon which requires high purity for functional solar cells. 

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How are perovskite solar cells made?

The molecular structure of the perovskite is first tuned to maximize the electricity that can be generated. This is then made into a solution using additives like methyl-ammonium lead iodide and methyl-ammonium halide. The solution is used to coat a surface, thus creating a perovskite solar cell.

Substrates can include sheets of glass, flexible polymers, or even transparent wood. Using the solution as a surface coating makes the process cheaper and easier to carry out than making silicon-based photovoltaic panels.

To evenly spread the coating onto a surface, scientists use spin-coating techniques. The solution is poured onto the surface and evenly spreads over the top in a thin layer by spinning the surface at high speeds. Once the solvents in the mixture evaporate, films of perovskite are left behind. These form the layers of perovskite crystals and can be wired to create a solar cell. 

Perovskite solar panel efficiency

Power Conversion Efficiency (PCE) is the amount of light energy a solar cell can convert into electricity. Silicon solar cells have a maximum PCE of 32%. Meanwhile, the most efficient perovskite materials observed so far can reach an efficiency of 31%.

Current research is exploring the effects of manipulating the chemical composition of perovskite crystals. This way, scientists can produce perovskite materials that have an ideal bandgap for converting light to electricity.

Researchers are also experimenting with multi-layered perovskite solar cells. In these, each crystalline layer has a different bandgap. Multiple layers ensure that higher energy light particles (photons) can penetrate through to electrons in layers that require more energy to excite (wide bandgap), and vice-versa. This way, more of the solar energy can be converted into electricity. Within research in the past decade, this method has attained 26% conversion efficiency. 

Finally, there is a third type of cell that is being explored. This is a hybrid or tandem system that incorporates a perovskite layer with a traditional silicon photovoltaic panel to utilize the energy from photons that the silicon cannot convert. This increases overall efficiency. Currently, the highest PCE of this system is 22.8%. However, with further exploration, researchers are aiming for a 50% PCE rate, which would revolutionize solar energy.

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The advantages of perovskite solar cells

Perovskite solar cells have quite a few perks. Since they adopt a coating-based solution, they are easy to produce and cost-efficient. Though experimentation is still underway, they also can potentially have high PCE rates.

Because of their design, perovskite systems only require 5% of the material needed for traditional silicon solar panels. They also do not use rare earth metals that need to be mined, which can disrupt local ecosystems. Their design also requires significantly fewer resources and energy to manufacture, making them both cost- and energy-efficient.

The drawbacks of perovskite systems

Unfortunately, since the perovskite cells utilize a thin-film coating, this layer can break down over time. Exposure to moisture, heat, light and oxygen is a given as they are used outdoors, but further experimentation is needed to prevent excessive and rapid weathering.

Currently, some of the most efficient perovskite systems use lead to generate electrical energy. Nevertheless, studies are being carried out to reduce the potential perovskite toxicity without compromising efficiency.

Looking to the future

In 2020, the Office of Energy Efficiency and Renewable Energy in the US dedicated $20 million in funding for perovskite research. This was spread across advancements in cell technology, manufacturing and testing. If we can develop systems that are energy-efficient and potentially less expensive and energy-intensive to produce, harnessing the sun’s (almost) infinite energy will become even more sustainable to power the planet.

Via SolarReviews and MIT News

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