Perovskite (or calcium titanate) is one of the most abundant minerals on earth. It was discovered by German geologists more than 170 years ago, and until recently it was widely used mainly as a dielectric. When the first solar cells were born, perovskite cells were not considered for their creation. The reason was the inability to achieve its long-term stability during power generation, which is a difficult engineering task even today.
Electrical properties of perovskite
Interest in the mineral, as a potential semiconductor for photovoltaic systems, arose only in the 21st century, with the advent of thin-film technologies. The very first experiments confirmed that the transfer of electric charge by perovskite solar cells is no worse than the “classics” made of silicon. But in this case, the specific absorption of the same amount of radiation was achieved at a thickness:
- silicon wafer – 180 microns;
- perovskite film – 1 micron.
The reason turned out to be about the same number of times the large effective width of the absorbing spectrum of an inconspicuous mineral. Moreover, commercial scale production of calcium titanate is cheaper and easier to produce.
Why haven’t perovskite solar panels already ousted silicon from the market today? The reason is in some of the physical and technical features of this natural material.
Disadvantages of calcium titanate and the search for their elimination
Structurally, the CaTiO3 mineral molecule includes three chemical elements:
- calcium – Ca;
- titanium – Ti;
- oxygen – O.
In the crystal lattice, they are arranged in a special way, which dictates the physicochemical properties of the behavior of the material during various electrochemical reactions.
The main problem of obtaining energy from this fantastically promising semiconductor is the rapid loss of stability of its structure under the influence of high temperature.
This drawback could be eliminated in only one way – to rearrange the arrangement of molecules inside the layer of the substance. This process is still going on, and each new modification of the structure becomes more and more stable, and solar perovskite elements – more productive.
The approximate dynamics of the improvement of photovoltaic cells based on CaTiO3 is as follows:
In the process of searching for optimal solutions, engineers found another, compromise solution to use the unique properties of calcium titanate. This is how tandem photovoltaic batteries were born, in which a layer of perovskite worked in parallel with a layer of electroactive silicone.
In prototypes of 2021, prototypes of such panels showed an efficiency of 26.3%, industrial ones – 23-25%.
At the same time, the structural integrity without loss of operational characteristics, according to calculations, should be maintained from 3 to 5 years. That, with a multiple lower production price, will already make perovskite solar panels a very serious competitor to the classics.
Benefits of innovative 3rd generation panels
Along with photovoltaics based on polymers and organics, thin-film perovskites belong to the third generation of solar panels. They have a number of important advantages.
- Inorganic structure. The absence of organic matter in the composition dramatically improves the thermal stability of the cells, and, as a consequence, reduces the rate of their degradation.
- Wide range of absorption range. To further increase generation, the latest panel models contain manganese.
- The ability to use carbon instead of gold as electrodes. This did not reduce the electrical conductivity during the transfer of energy from perovskite cells to conductors, but the cost of the films was significantly reduced.
- High speed and availability of manufacturing equipment. Perovskite solar panels can be printed today with a mid-range 3D printer. Moreover, at the exit it is not difficult to obtain whole rolls of film with the possibility of their further cutting while maintaining the performance of each piece.
- Ecological cleanliness. Films based on CaTiO3, incl. heterostructural, do not contain environmentally hazardous chemical elements. This greatly simplifies the process of their subsequent disposal and does not require the installation of expensive carcinogen traps at the production stage.
Solving the problem of durability through encapsulation
For a long time, experts have fiercely debated about the problem of the fragility of perovskite elements. Until 2020, it was believed that in the near future it would not be possible to increase their effective service life to more than 1.5-2 years. However, it was in this year that the technology of encapsulation was discovered – “continuous” sealing of calcium titanate cells into a special capsule.
To maintain the efficiency of the panels, it was necessary not only to create a single whole from perovskite “filling” and an external continuous coating. The latter was supposed to represent the outer layer of the “capsule” with the following set of properties:
- absolute moisture and especially heat resistance;
- practically infinite period of preservation of integrity, as well as mechanical and chemical strength;
- extremely light weight;
- maximum transparency.
A complex glass-polymer compound that satisfies all the specified properties has become such a material. The perovskite solar cells themselves have undergone changes. Now they consisted of not one, but two varieties of the modified mineral, with complex formulas Cs0.05FA0.8MA0.15Pb (I0.85Br0.15) 3 and FA0.85MA0.15Pb (I0.85Br0.15) 3.
The creators of the tandem were a group of scientists from New South Wales (Australia), and the samples on tests showed the following results inherent in the mandatory standard IEC61215: 2016:
- 1000 hours of continuous operation at t = + 85 ° C (new panels lasted 1800 hours);
- 40 cycles of daily temperature differences from – 40 ° C to + 85 ° C (imitation of extreme conditions) – the new product has withstood 75 cycles;
- preservation of at least 90% efficiency at each of the tests.
In addition, perovskite solar panels in glass polyisobutylene (PIB) capsules have demonstrated efficiency in excess of 25%.