The secrets of the perovskite

The largest source of clean and inexhaustible energy available to us is the Sun. To capture it, we use photovoltaic panels capable of transforming it into electricity. But the true secret of this extraordinary transformation is in the materials that make up the panels.

Among them, the most popular is silicon. It is a semiconductor material that absorbs photons –particles of light– from solar radiation. Once absorbed, they produce electrical charges (holes or absence of electrons and electrons) that are extracted in the solar cells, obtaining electricity with very high efficiency.

However, silicon is not the only material capable of carrying out this process. Currently we have several alternatives, already established or emerging, that have nothing to envy.

third generation materials

These technologies are divided into three generations.

The first generation focuses on crystalline silicon, the second is divided into multicrystalline silicon, chalcogenides –such as cadmium telluride (CdTe) or copper indium gallium selenium (CIGS)– and composite materials made up of elements from groups III-V, as gallium arsenide (GaAs).

The first and second technologies are already implemented in buildings, photovoltaic farms or even in aerospace technology.

The third generation –and the least known– is made up of emerging materials such as dye-sensitized solar cells, organic solar cells and, lastly, perovskite cells, the use of which is not yet widespread.

More efficient than silicon

To compare photovoltaic technologies we must look at the conversion efficiency. This term refers to the relationship between the power output of the device and the incoming solar radiation energy. Their values ​​are used to compare the performance between solar cells and between photovoltaic technologies.

The US National Renewable Energy Laboratory graphically compares the highest certified efficiencies in each type of solar cell.

If we carefully analyze the graph above, we see that perovskite solar cells have achieved 25.7% efficiency, surpassing the performance of commercial multicrystalline silicon solar cells (23.3% efficiency).

Perovskites, after only 15 years of their first use in solar cells in 2009, when it obtained only 3.8% efficiency, have managed to be the most promising photovoltaic technology among all emerging and commercial technologies.

What is special about perovskite?

We are going to see what the perovskite material is, its characteristics and its particularities.

The term perovskite is linked to the discovery in the 19th century of the mineral calcium and titanium trioxide (CaTiO₃), whose crystalline structure is a three-dimensional network formed by the combination of elements of the form ABX₃. This structure was called perovskite in homage to the mineralogist Lev Alekseyevich von Perovski and from then on, all materials with this crystalline structure are called by that name.

There are several families with this crystalline structure, for example based on oxides or halides. The materials of this last family (halides) are the most promising for the efficient absorption of solar energy. Specifically, they are organic-inorganic hybrid materials, formed by the combination of:

• A: methylammonium, formamidinium or cesium.

• B: a metal like lead, tin.

• X: a halide such as chlorine, bromine, or iodine.

These hybrid materials present some fascinating optical and electrical properties, such as a highly efficient absorption of ultraviolet and visible light, rapid dissociation of excitons –hole-electron pair created in a semiconductor after the absorption of photons of light– in free charges due to to its low exciton binding energy, and finally, a high diffusion length of these free charges to be able to be extracted in the photovoltaic device.

In addition, by modifying the composition in A, B and/or X with pure chemical compounds or combinations, we can modulate at will the electrical properties of this material (the energy bands and their corresponding band gap) closely related to the optical properties of the material modulating the fraction of the incident radiation that it will absorb.

How are these materials made?

There are many ways to manufacture perovskites by dissolving the precursors or by thermal evaporation of the same in vacuum, and with laboratory or industrial techniques.

He spin-coating is a laboratory technique that consists of adding a small amount of liquid to a substrate. The thin layer is created by spinning removing excess liquid by centrifugation.

He roll to roll (R2R) is an industrial process in which the substrates are continuous rolls where the thin layer is printed using a rotary press, a technique widely used in the printing of newspapers or brochures.

This material does not require high-temperature heat treatments, so flexible solar cells or fabrics can be manufactured. It also has a high performance while maintaining a certain transparency, ideal for its possible installation in windows. In addition, the low cost of the precursors necessary to produce it is another incentive for the development of this new photovoltaic technology.

So, when will we be able to enjoy the wonderful properties of perovskite in our daily lives? There are still some aspects to improve, such as the presence of lead and its stability.

Lead is essential to achieve the desired high efficiency and its amount is minimal in the device. Other alternative perovskite materials are currently being developed with elements such as tin substituting for lead.

Finally, the stability problem can be solved by using encapsulation techniques in which the use of an encapsulating agent (for example, the combination of an epoxy resin and glass) will prevent the entry of humidity and therefore its degradation.

María Cristina Momblona Rincón, Materials for energy, Zaragoza’s University

This article was originally published on The Conversation. Read the original.