Perovskite is named after the Russian scientist Lev Perovsky, who studied these minerals. Perovskites have a crystal structure similar to a \( CaTiO_3 \). Its chemical formula is \( ABX_{3} \). Atom X (oxygen) with atom B (e.g. Ca) forms a regular octahedron \( BX_{6} \). The B atom is in the center of the octahedron and the X atom is in the middle. At the six vertex corners of the octahedron, neighbouring octahedrons are attached to form an extended three-dimensional lattice structure. Perovskite materials used in solar cells: A is generally an organic amine ion (such as \( CH_{3}NH^{3+} \), \( NH= CHNH^{3+} \)), B is generally a divalent metal ion (such as \( Pb^{2+} \), \( Sn^{2+} \) etc.), X means halogen ion ( \( Cl^{-} \), \( Br^{-} \), \( JA^{-} \)). The inorganic halide metals can form a continuous octahedral structure and a more regular cube-like crystal shape - Fig. 1 (based on [1]). In photovoltaics, new materials and new methods of PV cells production are being searched for, which will allow the production of cells with higher solar energy conversion efficiency and at the same time reduce production costs. Such potential is shown by cells obtained on the basis of materials with a perovskite crystalline structure.

This is evidenced by the fact that after only six years of research on this type of cell, cells were obtained whose solar-to-electric conversion efficiency in the laboratory is \( 21.5% \) while silicon cells, despite 60 years of research, have achieved an efficiency of only \( 25% \). The theoretical efficiency of cells based on perovskite structures is \( 31.4% \). Considering their development rate so far, it is expected that in the near future they will reach energy conversion values comparable to those obtained by silicon cells [2]. The high efficiency of solar energy conversion to electricity is due to the optical and electrical properties of perovskite materials. First of all, perovskites have a high absorption coefficient, much higher than the absorption coefficient of silicon or GaAs (the best absorber so far). This makes it necessary to use a GaAs layer of about 1 \( \mu m \) thickness to achieve a conversion efficiency of \( 21% \), while in the case of perovskites, a layer of 300 nm thickness is sufficient [3].
The use of such thin absorber layers reduces the risk of recombination of current carriers. From the application point of view, it allows the reduction of material consumption, but also makes it possible to produce transparent (possibly semitransparent) photovoltaic cells, which fits into the concept of Building Integrated Photovoltaics (BIPV).
The phenomenon of perovskite cells is that they have desirable physical properties and at the same time can be manufactured from materials that are relatively easy to obtain from commonly available raw materials using uncomplicated, well-known thin film technologies. An example of a cross-section through a perovskite thin-film cell is shown in Fig. 2 [4].

Figure 2: Scanned electron microscope image for a section through a perovskite photovoltaic cell for two different magnifications. Author photo by T. A. Nirmal Peiris et al, CC-BY 4.0 license, source: MDPI.

The high potential of perovskite cells has led to a large number of research and development centers around the world to undertake research in this area. The world's leaders at the moment are:

• Oxford University, England;
• École Polytechnique Fédérale de Lausanne EPFL, Switzerland;
• Korea Research Institute of Chemical Technology KRICT, Republic of Korea;
• The University of Valencia, Spain.

At Oxford University, Dr. H. Snaith has developed a technology for obtaining perovskite cells with an efficiency of \( 15.4 \) in which a perovskite absorber layer is obtained using a vacuum-assisted dual-source evaporation technique [5]. In 2012, a spin-off company called Oxford Photovoltaics (Oxford PV) was founded to commercialize thin film solar cell technology. In March this year, the CEO of Oxford PV declared his intention to bring perovskite solar cells to market as early as 2017. At EPFL, Prof. Michael Grätzel developed a two-step process for obtaining perovskite in a PV cell structure. Grätzel's method uses wet techniques, i.e. successive layers are deposited from a solution [6].
The perovskite cells obtained by Grätzel's team have an efficiency of nearly \( 16\% \). The technology was put into production at the Australian company Dyesol's perovskite cell factory, which was launched in Turkey in 2016. The company said it would launch pilot production of 20 000 \( m^{2} \) cells per year in 2016 and mass production with a potential of up to 600 MW in 2018. In Dyesol's favour is the fact that it is a company with a tradition in dye-sensitised cell (DSC) technology, which is the forerunner of perovskite cells. Since the invention of dye-sensitized cells, Dyesol has been developing this technology in collaboration with the Swiss polytechnic EPFL. Today, they are the world leader in DSC cells, offering both the materials and the apparatus as well as the technology needed to produce them. Recent perovskite solar cell efficiency records of ( \( 20.1\% \)) come from the Korea Research Institute of Chemical Technology (KRICT). However, it should be emphasized that the record value of efficiency was determined for a cell with an area of 0.0955 \( cm^{2} \) [7]. All the mentioned centers focus on developing the technology of obtaining high-efficiency perovskite cells on glass substrates. In comparison with other research and development centers in the world, Saule Technologies laboratory is distinguished by the fact that the whole process of cell production is carried out at relatively low temperatures, not exceeding \( 150_{}^{o}\textrm{C} \). Thanks to this, materials sensitive to high temperatures, such as polymer films, can be used as substrates, which can definitely broaden the spectrum of applications for these types of devices.
Although great progress has been made in perovskite cell research since 2012, there are still several unresolved issues. A major limitation in using perovskites for the mass production of photovoltaic cells is their high sensitivity to humidity. Upon interaction with water vapor, perovskites degrade. For this reason, the process of perovskite production must be carried out in an anhydrous atmosphere, which requires the use of special chambers providing appropriate environmental conditions. The low resistance of perovskites to moisture adversely affects the stability of perovskite cells. Moreover, the structure of perovskites used in solar cells contains lead, which is a toxic and carcinogenic element. As a result of interaction with water vapor, perovskites undergo degradation and one of the degradation products is toxic lead compounds. Laboratories are working on replacing it with other elements, e.g. Sn, but the studies carried out so far show that substitution of lead in the perovskite structure with another element significantly worsens its optical and electrical properties, and thus worsens the cell efficiency.
Currently, both the University of Oxford and the Technical University of Lausanne use encapsulation of cells in a polymer film to protect the cell from interaction with water vapor and improve its stability [8]. Hermetization is performed in such a way that the cell placed on a glass substrate is covered with another glass sheet and the space between them is filled with appropriate polymer solution. This solution on one hand prevents water vapor penetration and the release of lead compounds, on the other hand however, it raises the production costs of the cell. The possibility of using a flexible substrate, on the one hand, greatly expands the range of applications of perovskite photovoltaic cells, on the other hand, strongly raises the level of difficulty in developing an appropriate deposition technology. An example of a perovskite cell on a flexible substrate is shown on fig. 5 in article D. Kim i C. Kim "A Ladder-Type Organosilicate Copolymer Gate Dielectric Materials for Organic Thin-Film Transistors" [9]. Very high repeatability must be maintained between the successive layers of the perovskite solar cell (electrodes, blocking layer, absorber layer), the layers must lie perfectly (down to the nanometer) on top of each other. Such a technological operation is difficult to realize even when the substrate is a glass sheet. When the substrate is a polymer film that is much more susceptible to deformation than glass, the execution of the process, especially in the "roll to roll" technology, is a huge technological challenge [10].