D. Yermukhamed

al-Farabi Kazakh National University, Almaty, Kazakhstan

Formation of the thin layers of organometal perovskites by vacuum evaporation method for application in solar cells

Nowadays the subject of attention from specialists in the field of solar energy become materials, which called perovskites. During the short time we obtained solar cells with coefficient of performance (COP) of 3.8% in 2009 to 20.29% in 2015. World began to talk about the remarkable success of CH₃NH₃PbX₃ organometallic threehalogen perovskites (where X - I, Br, Cl halogens, or their mixtures), and about perovskite photovoltaics era. With this material began to have great expectations for the development of highly efficient solar cells with cost much lower (4-6 times) than cost of elements based on crystalline silicon, which is basic material of modern photovoltaics.

It should be noted that the creation of perovskite solar cells was observed by «Science» journal, as one of the technical achievements of 2013. Perovskite solar cells manufacturing do not require expensive clean rooms and technological equipment, and materials. Formation of perovskite material required temperature that do not exceeding 150 ° C.

In this article represented results on the formation of perovskite layers by different vapor phase methods. In vapor phase method organic part is treated by vapour of inorganic part; it is a method of perovskite layers formation with wide band gap, which provides method of producing CH₃NH₃PbBr₃ wide-perovskite. This perovskite may be used to create cascade solar cells, as frontal heterojunction element.

In this experimental work used vacuum evaporation methods such as vapor deposition with dual source and deposition by vapour assisted solution process to produce organometallic perovskites.

During vapor deposition with double source, we put in separate crucibles 500 mg CH₃NH₃I and 100 mg PbCl₂. Substrate fixed on substrate holder, which disposed above sources with FTO coated TiO₂ converted down to the sources. After evacuating of pressure in chamber to 10^-5 mbar, two sources were heated slightly above the desired deposition temperature for 5 minutes (CH₃NH₃I heated to 120 ° C, and PbCl₂ to 325 ° C) to remove volatile impurities before material deposition on substrate. The substrate holder rotated for uniform deposition of coating, because the right source predominantly covers the right side of substrate, which is similar to the left source. The substrate holder cooled by water at about 21 ° C, although the precise measurement of substrate temperature during the deposition process was not carried out.

In Figure 1 schematically illustrated system of deposition with dual source.

Figure 1 - Schematic image of deposition system of materials. Thermal evaporation system with dual source for deposition of perovskite absorber; organic source - methylammonium iodide and inorganic source - lead chloride

Perovskite films have been optimized for improved device performance [1] by applying the changes in key parameters such as speed and deposition time for two sources. In particular, the performance of device [2] is very dependent on the ratio of CH₃NH₃I to PbCl₂ content, and the total deposition thickness.

Perovskite films obtained by vapor deposition with dual source were homogeneous [3]. Vapor deposition can lead to fully optimize the electronic contact on the borders through plurality of layers with controlled doping levels.

The following method which was used for manufacturing the organic / inorganic hybrid perovskite films – is a deposition by vapour assisted solution process. In this method, inorganic film structures is formed by depositing precursor solution onto substrate, followed by treatment with vapors of necessary organic compounds [4]. PbI₂ and CH₃NH₃I - corresponding vapours of precursors that form CH₃NH₃PbI₃. PbI₂ films deposited on the FTO glass coated by compact and mesoporous TiO₂ layer, followed by annealing in CH₃NH₃ vapour at 150 ° C under nitrogen atmosphere for 2 hours to form perovskite films (CH₃NH₃PbI₃).

In Figure 2 represented schematic image of perovskite film forming by vapour assisted solution process.

Figure 2 - Schematic image of the formation of perovskite film with vapour assisted solution process

Vapour assisted solution process method is a low-temperature method for production of perovskite films based on kinetically favorable reactions between the newly deposited PbI₂ film and CH₃NH₃I vapour [5].

A key step in this method is the growth of film during the direct reaction of just deposited PbI₂ film with CH₃NH₃I vapor phase. It uses kinetic reactivity of CH₃NH₃I and thermodynamic stability of perovskite during the immediate growth of film; in the result produced films with well defined grain structure with grain size of several microns, with complete coverage of the surface and small surface roughness of surface suitable for photovoltaic applications.

In Figure 3 shown SEM images of films surface prepared from the solution and by vapor-phase method. The films deposited from vapor is extremely homogeneous. In contrast, the films treated in the solution, covered partially the substrate with crystalline islands on tens of micrometers of scale length. Voids between crystals in the film treated in solution apparently reach directly glass coated by compact TiO₂ and FTO. Perovskite film, deposited from vapour, is uniform and similar to the FTO layer with a little more crystallinity in appearance. Perovskite film obtained from solution, has an extremely smooth surface, in accordance with the larger size of crystalline particles.

Figure 3 - Characteristics of the topology of thin films. SEM images of the surface (a) perovskite films deposited from vapour and (b) perovskite film treated in the solution

Perovskite film obtained by deposition by vapour assisted solution process method have full coverage of the surface, uniform grain structure with a grain size of more than micron and 100% conversion of the precursor. Rebuilding of PbI₂ film during the implementation of CH₃NH₃I conditioned by decreasing energy of grain boundaries. Vapour assisted solution process method is simple, manageable and universal method in an effort to achieve high quality of perovskite films, and therefore, photovoltaic devices with high efficiency. The inclusion of organic compounds in the deposited inorganic structures by vapour effectively prevents from a high rate of reaction (formation) of perovskite during coprecipitate precursor, and also eliminates the possibility of film deterioration which sometimes observed during the immersion of inorganic structure to solution with organic compounds.

The roughness of film produced by vapour assisted solution process method is small. The resulting film has thickness of about 350 nm, with distinct grains across the whole thickness of film. 100% surface coverage, microscale grain size and uniform grain structure make this prepared film promising to use in photovoltaic devices. These characteristics may be associated with relative smoothness of PbI₂ films.

1. Were carried out research works on the formation of perovskite films by vapor deposition methods with a dual source and deposition by vapour assisted solution process.

2. We used three-layer version of the front contact consisting of FTO layers, blocking and mesoporous TiO₂ layers. Blocking TiO₂ layer prevents entering of holes to the FTO. Photoconverters based on mesoporous have several advantages over the planar devices such as higher efficiency, less hysteresis effect.

3. Using the methods of scanning microscopy were studied structural properties and determined the thickness of perovskite layer. In work was found optimal deposition regimes, under which formed perovskite thin layers.

4. Considering methods show that can get organometallic perovskites films with high-quality for further practical application in solar cells and lasers. Another advantage of metal halide perovskite lasers is their frequency tuning in the visible range. This can be easily achieved through the replacement of iodide ions by bromide.

5. Based on the experimental data one can conclude that the obtained films is not inferior to known literature methods and are suitable for use in photovoltaic devices.

Literature:

1. Lee, M. M., Teuscher, J., Miyasaka, T., Murakami, T. N. & Snaith, H. J. Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites. Science 338,643–647 (2012).

2. Ball, J. M., Lee, M. M., Hey, A. & Snaith, H. J. Low-temperature processed meso-superstructured to thin-film perovskite solar cells. Energy Environ. Sci. 6, 1739–1743 (2013).

3. Liu M., Johnston M. B., Snaith H. J. Efficient planar heterojunction perovskite solar cells by vapour deposition //Nature. – 2013. – Vol. 501. – №. 7467. – P. 395-398.

4. Chen Q. et al. Planar heterojunction perovskite solar cells via vapor-assisted solution process //Journal of the American Chemical Society. – 2013. – Vol. 136. – №. 2. – P. 622-625.

5. K. Liang, D. B. Mitzi and M. T. Prikas, Synthesis and Characterization of Organic-Inorganic Perovskite Thin Films Prepared Using a Versatile Two-Step Dipping Technique, Chem. Mater., 1998, 10, 403–411.

Резюме

В этой работе рассматривается получение пленок перовскита с помощью методов вакуумного испарения. А именно методами, парофазного осаждения с двойным источником и осаждения обработкой из раствора индуцированное паром.

Были определены преимущества и недостатки данных экспериментальных методов. Были исследованы структурные свойства пленок с помощью сканирующего электронного микроскопа. Были найдены оптимальные режимы осаждения тонких слоев перовскита.

Ключевые слова: органометаллический перовскит, парофазное осаждения с двойным источником, осаждение обработкой из раствора индуцированное паром, СЭМ изображение.