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Solar Energy 3.0 — The Perovskite Solar Cell

Renewable energy is very much welcomed, in order to bring self sufficiency to the means of producing electricity. We have popular sources that include solar, but it has some shortcomings. They require investment not just in the technology, but also in land to develop solar farms that produce electricity for consumption. The other issue is that they lack the energy density to produce electricity to meet high demands from industry.

The materials used in building solar cells are also another issue. They cannot provide maximum efficiency to produce power. These issues can be addressed, through breakthrough technologies that increase the efficiency of the materials. One solution is to use a hybrid organic-inorganic material called perovskites.

There are 2 types of solar cells. These cells collect photons from sunlight causing electro-chemical reactions that produce electricity. We have the crystalline silicon-based wafer cells that many solar panels use. The other type are thin film cells, which can use perovskites. The thin film cells are made of layers of photovoltaic materials that are embedded on a substrate made of glass, metal or plastic. These substrates tend to be more flexible and versatile materials.

Thin film solar cells are more flexible and versatile (Source PowerFilm)

Perovskite is made from material like methylammonium lead halides and all-inorganic cesium lead halide. The advantage they have is that they are cheaper to produce and simpler to manufacture, compared to silicon-based solar cells. These cells also increase the conversion of sunlight to power by up to 25% (in 2021). They are more efficient at producing electricity rather than wasted energy in the form of heat.

A perovskite material has an ABX3 configuration. This consists of of two positively charged ions (A and B) called cations and a negatively charged ion (X) or anion. In general, materials that have the same crystal structure of CaTiO3 are referred to as perovskite materials.

Crystal structure of CH3NH3PbX3 perovskites (X=I, Br and/or Cl).(Source: Nature, Christopher Eames et al.)

(Note: Perovskite was named after scientist Lev Perovski, who discovered the minerals that have these properties.)

Following the same arrangement, scientists discovered they can synthesize their own material that has the same or similar properties. Materials that can be used for perovskites include lead iodide and organic salts, which are examples of metal halide salts. They are then rolled into a thin film to be used as semiconductors.

The solar cells in a typical panel are ~30% efficient, according to the Shockley-Queisser Limit. This is the theoretical limit of a solar cell using a single P-N junction. In reality, solar panels that use silicon semiconductor solar cells only reach between 20 to 25% efficiency. Thin film cells using perovskites can increase the efficiency to provide more power production for electricity.

This graph shows the Shockley-Queisser Limit, which is the maximum efficiency of solar cells ~30% at 1.1 eV (Source: Sbyrnes321, Public domain, via Wikimedia Commons)

Perovskites can use a wider bandgap to support multiple P-N junctions (also called tandems) that can absorb different bands from visible light. These are the different wavelengths that come from sunlight. These tandems can also be used to work with silicon cells to produce more power. As a result, perovskites can increase efficiency by up to ~45% (in theory).

The rise in efficiency can be attributed to the following:

  • Continued improvement in device layers and architecture
  • Perovskite growth techniques
  • Use of mixed halide perovskite with enhanced electronic properties

Perovksites are also more abundant than silicon, so the supply can bring the cost of materials down as it is not rare. These are naturally occurring and found throughout the world. It is also easier to produce, requiring a lesser amount of energy, compared to silicon semiconductors to process. This is during the manufacturing of the solar cells. Silicon requires temperatures at or above 1400+ ºC (2500+ ºF) to crystallize, while perovskites can crystallize at < 100 ºC (212 ºF).

In terms of its form factor, since it can be produced into thin film cells, it provides more versatility. This makes it ideal for building products with higher flexibility. It makes it possible to fit solar arrays on objects that may not normally handle rigid silicon type cells. An example would be for use in windows and even on cars.

The question is why are perovskites not so prevalent in the market, after all the benefits they can provide. This is due to the challenges in producing them. Perovskite solar cells face the following issues:

  • Toxicity of substances like lead
  • Material instability once released to production
  • Longevity, how to make them last longer
  • Still in its early stages with no proven track record

The use of lead for perovskite solar cells is of concern to the environment. Lead is a toxic substance, and leaks from the cells can be hazardous to human health. It can also pollute environments where the lead can contaminate nearby foliage and could makes its way to water supplies and soil. The task for engineers is to find other types of perovskites to replace lead or use a process to effectively prevent any lead contaminants from leaking out.

The materials are also not as stable once the cell has been created. They are subject to rigorous tests before they can be considered production level quality. The problem is that even if these cells pass the test initially, they did not behave as expected in the real world. They can decompose at high temperatures (>90 ºC). This can lead to hysteresis in materials, altering their electrical properties.

The longevity of perovskites is also in question. The lifetime of silicon cell solar panels last about 20–25 years (data from the US EPA). Thus, manufacturers give consumers a 25 year warranty on solar panels. The problem with perovskites is that they are sensitive to moisture, oxygen and heat. Thus, they need more protection from the elements in order to last longer. Unfortunately, that could increase production costs.

Over time, it will reveal whether it can be a viable product. We have had decades of use from silicon solar cells. It is tried and tested, but does not produce as much power due to inefficiencies. However, they are quite stable and can be produced for mass consumption.

Once higher energy density and more efficient solar cells can be developed, it opens the world to less carbon dependency. The next decades could see further drop in carbon as solar becomes more utilized. Solar cells that use perovskite will allow more electricity to be produced. Since it can use more flexible substrates, it can be applied to new types of surfaces. Think of its applications in providing power not just for homes, but buildings, cars, trains, trucks and even aircraft.

This is an optimistic outlook for the potential use of solar energy (now dubbed version 3). The use of multiple junctions and new materials for creating solar cells is expected to improve. Once perovskite solar panels are produced in mass quantities, they can bring the average price of solar panels down to the most affordable level for consumers. The task for engineers in building perovskite solar cell panels is to increase efficiency, use less material, reduce manufacturing complexity and lower production costs. Easy to say but not as easy to execute.



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Vincent Tabora

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