The Emerging Perovskite Photovoltaics | ScienceMonk

The term “perovskite” was first used by Gustav Rose in 1839 when he discovered a new material based on Calcium Titanate (CaTiO3). He named this material “perovskite” after the Russian mineralogist Lev Von Perovski. The concept of perovskites was then changed to any material having a composition ABX3, and, since then, there have been a large number of discoveries of materials with the same composition. One of these has been halide perovskites based on caesium and lead.

 

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Perovskite

When the new century started, Albert Einstein published his paper on the photoelectric effect (for which he would later go on to win a Nobel prize).

In his paper, he described how electrons interact with light. The principle states that, when an electron is hit with a beam of light, it will gain energy and therefore get “excited” and get promoted to higher energy levels in the atomic orbitals.

This new orbit is unstable, though, and the electron will quickly lose energy and therefore drop back down to the lower, more stable, energy level. Due to the first law of thermodynamics, which states that energy cannot be created or destroyed, the recombination of the electron to the lower energy level will release energy in the form of a photon i.e., a particle of light.

This opened the doors for photovoltaic technologies. Approximately 50 years later, in 1953, a research group at Bell Labs developed the first silicon-based solar cell. This was the first-ever solar cell capable of converting solar energy into electrical energy to power electrical objects. Since then, there has been a very large improvement in photovoltaic technologies, and the field has been growing ever since.

This has also produced a very large number of branches in the technology. The main one still being silicon-based solar cells. Other technologies include but are not limited to Cadmium Telluride (CdTe) solar cells, organic solar cells, dye synthesized solar cells, and perovskite solar cells. Although these are all great emerging and rapidly growing fields, in this blog post, we will explore perovskite photovoltaics.

The power conversion of perovskite photovoltaics has grown rapidly in the past decade and has now exceeded 23%. The National Renewable Energy Laboratory (NREL) best research cell efficiencies graph is a clear indication of how rapidly this technology has improved.

In just a few years, efficiency has improved and has exceeded the efficiency of well-established technologies such as CdTe, which has been developing for over forty years.

The constant optimization of the laboratory techniques and the improvement in the fabrication mechanisms has significantly helped in obtaining perovskite materials with improved crystallinity and therefore increased the efficiency of the solar cells.

Perovskite also falls in the category of thin-film solar cells. This means that one of their main advantages is that it can be both flexible and transparent, meaning that it might be possible to position them on any surface.

A lot of work is currently being done to make the cells fully transparent, which would then allow them to be built inside windows, therefore, maximizing the surface area which is covered by solar panels, therefore, making the buildings self-sufficient. The flexible and printable aspects of this technology also mean that it could potentially be placed anywhere.

However, these lead halide perovskite materials are incredibly unstable. If not protected, they will easily degrade when exposed to oxygen, moisture, and UV radiation. This is a major issue which scientists are currently addressing, and it is probably the biggest challenge to overcome before the material can be upscaled and compete with the well-established silicon solar cell technology.

The most studied perovskite solution is the CH3NH3PbI3, also known as MAPI and/or MAPbI3 (methylammonium lead iodide). As stated previously, this perovskite material has an ABX3 structure. “A” stands for the methylammonium, “B” stands for the lead, and “X3” stands for the halide (usually iodine or chlorine).

The MAPI solar cells are usually built with the following structure:

This is the first layer of the solar cell, and it is of extreme importance that it is as clean as possible. This is because all of the other layers will depend on this, and any imperfection will affect the final efficiency of the cell.

  • Titanium Dioxide (TiO2)

This layer serves as the electron transport material (ETM). As the name suggests, the ETM is mainly used to transport the electrons in the cell.

  • Perovskite

This is the photoactive layer of the solar cell.

  • Spiro-OMeTAD

This is the opposite of the ETM. It acts as the hole transport material. When an electron moves, it will leave behind a hole, which is exactly what the same suggests. It is the area of space which was previously occupied by the electron. Since electrons have a negative charge, the hole will have a positive charge.

  • Top Gold Contacts

This could also be silver, and it is used to conduct the electricity which is produced by the solar cell.

Spin coating is the main technique that is used to deposit most of these layers onto the FTO glass. This technique consists of placing a small amount of solvent onto the substrate (in this case, FTO glass). The volume of the deposited solvent will vary depending on the desired thickness as varying; this will change the cell properties.

The glass is then rotated at high speeds to fling most of the solvent onto the sides, leaving behind a very thin layer of the solvent. The glass is then placed onto a hotplate to dry. This is then done for all the layers up to the hole transport layer. Thermal evaporation is then used to deposit the top contacts.
In conclusion, perovskite technologies have continuously been rapidly growing and have great potential. A lot of money and research is being put into them to overcome many problems that they are facing, and it will take some time before we see them on the market competing with silicon-based solar cells but, once this is done, their impact on our life will be massive.

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