RU2568421C1 - SOLAR CELL BUILT AROUND p-TYPE HETEROSTRUCTURE OF AMORPHOUS AND NANOCRYSTALLINE SILICON NITRIDE - SILICON - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: single junction solar cell comprises p-silicon substrate of p-type Si(100) pretreated HF acid. Substrate top surface is provided with the ply of 4-5 nm deep n-type film of amorphous silicon nitride mixed with silicon nitride of nanocrystalline structure applied by magnetron sputtering in argon from solid-state Si₃N₄ target. Electric contacts are produced by magnetron sputtering. Note here that contacts on element top side are made of Ag and shaped to a comb. Note also that rear electric contact on Si(100) substrate back is made of Ag or Cu.

EFFECT: higher efficiency without application of whatever extra plies and solar power concentrators.

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Description

The invention relates to semiconductor photovoltaic structures used to convert solar radiation into electrical energy, not only in spacecraft and autonomous systems, but also as a high-performance environmentally friendly means of generating electrical energy in various industries.

In the overwhelming majority of cases, the material of photovoltaic structures is silicon, for example, of 98.2% of the capacity of existing plants, 38% are based on crystalline silicon, 52% are based on polycrystalline, 5% are based on amorphous. The share of thin-film structures increases for all types of materials and for silicon modules it amounted to 25% in 2013. The share of other photovoltaic materials is significantly less (1.8%) among them are structures based on cadmium tellurium (1.6%), compounds of elements of groups III-IV ( In, Ga, As, Sb, P, etc.), polymer-based cells, liquid photovoltaic cells, etc.

Silicon carbide SiC is widely used in many branches of science and technology. For various modifications of SiC, the band gap can have a value in the range from 2.4 to 3.34 eV, which allows one to create semiconductor devices based on it that maintain operability at temperatures up to 600 ^° C.

Also promising material for photovoltaics is silicon nitride Si ₃ N ₄ with a band gap of 4.0 eV and unique chemical, mechanical, electrical and optical properties. Three modifications of Si ₃ N ₄ are known: α and β are hexagonal, for α - Si ₃ N ₄ : a = 0.7765 nm, c = 0.5622 nm, space group P31c; for β - Si ₃ N ₄ : a = 0.7606 nm, c = 0.2909 nm, space group P63 / m; α - Si ₃ N ₄ turns into β above 1400 ° С, β - Si ₃ N ₄ is stable up to ~ 1600 ° С. The cubic modification of γ is usually formed at high pressures, for γ - Si ₃ N ₄ : a = 7.7418 nm, the space group Fd-3m. Silicon nitride does not interact with nitric, sulfuric and hydrochloric acids, weakly reacts with H ₃ PO ₄ and intensely with hydrofluoric acid. The oxidation of Si ₃ N ₄ in air begins above 900 ° C.

Layers of Si ₃ N ₄ are used in photovoltaic devices to passivate the surface of the upper layers of heterostructures [JL Cruz-Campa, et al., Microsystems enabled photovoltaics: 14.9% efficient 14 μm thick crystalline silicon solar cell, Sol. Energy Mater. Sol. Cells (2010), doi: 10.1016 / j.solmat.2010.09.015.] And as antireflection layers [A. Lennie, H. Abdullah, S. Shaari and K.Sopian, Fabrication of Single Layer SiO ₂ and Si ₃ N ₄ as Antireflection Coating on Silicon Solar Cell Using Silvaco Software, American Journal of Applied Sciences 6 (12): 2043-2049, 2009]. Moreover, Si ₃ N ₄ and related silicon nitrides and solid solutions based on it Si ₃ N _{4 ± x} play the role of chemical and electrical passivation of the surface. Electrical passivation is to reduce the surface recombination of charge carriers in silicon wafers or films. The presence of a large number of traps in silicon nitride leads to a sharp decrease in the amount of one of the two types of charge carriers on the surface of the absorbing layer and a decrease in the probability of their recombination. In addition, as an antireflection layer, Si ₃ N ₄ promotes increased light absorption [Dirk-Holger Neuhaus and Adolf Münzer, Industrial SiliconWafer Solar Cells, Advances in OptoElectronics, in special issue Volume 2007, Article ID 24521, 15 pages (doi: 10.1155 / 2007/24521)].

The addition of small amounts of hydrogen, often used in passivation layers in solar cells, does not significantly affect the change in the crystal structure of the film and the electronic properties of silicon nitrides [LE Hintzsche, CM Fang, T. Watts, M. Marsman, G. Jordan, MWPE Lamers, AW Weeber, and G. Kresse, Density functional theory study of the structural and electronic properties of amorphous silicon nitrides: Si ₃ N _{4 − x} : H, Physical Review B 86, 235204 (2012) ].

Passivation effects should have a particularly large effect in thin-film solar cells due to the fact that the surface is close to the space charge region of the pn junction where charge separation occurs [J.L. Cruz-Campa, et al., Microsystems enabled photovoltaics: 14.9% efficient 14 μm thick crystalline silicon solar cell, Sol. Energy Mater. Sol. Cells (2010), doi: 10.1016 / j.solmat.2010.09.015].

In known complex solar cells, p and n conductivity layers of amorphous a-Si ₃ N ₄ layers and related silicon nitride SiN _x and solid solutions based on Si ₃ N _{4 ± x are used} , which are deposited on the surface of the absorbing layer, for example, Si emitter, then follows the region of the substrate or i-layer in the pin solar cells. A reverse electrode is applied to the back side of the substrate. The application of frontal and reverse electrodes is sometimes accompanied by additional alloying. In some cases, passivating layers are also used before applying the reverse electrode. The parameters of industrial photovoltaic cells of various designs were improved after the application of plasma-assisted vapor deposition (PECVD) methods of passivating layers of hydrogenated amorphous a-SiN _x : H silicon nitride and the mechanisms of the effect of passivating silicon nitride layers on the efficiency of photovoltaic cells were analyzed [J. Schmidt, JD Moschner, J. Henze, S. Dauwe and R. Hezel, RECENT PROGRESS IN THE SURFACE PASSIVATION OF SILICON SOLAR CELLS USING SILICON NITRIDE, 19th European Photovoltaic Solar Energy Conference, 7-11 June 2004, Paris, France, p. 391-396].

The closest technical solution is a single junction solar cell according to patent US2008241987 (published 2008-10-02), which is a p-silicon substrate having upper and reverse sides, on the upper side of the substrate there is an n-layer of silicon on which an antireflection layer is formed silicon nitride SiNx deposited on it using the screen printing technique of the upper electrical contacts in the form of a grid of Ag, and on the reverse side of the substrate an electrical contact in the form of a double layer of Al. The disadvantages of this technical solution are the complex structure and, accordingly, the complex process.

The objective of the invention is the elimination of the disadvantages of the prototype.

EFFECT: creation of a single-junction heterostructure of a solar cell with a substrate of p-type monocrystalline silicon and a n-type light-absorbing two-phase layer made in the form of mixed amorphous and microcrystalline structures of silicon nitride with an efficiency of 7.41%.

The claimed single junction solar cell comprising a p-silicon substrate having upper and reverse sides, an n-type layer on the upper side of the substrate, upper electrical contacts from Ag and contacts on the reverse side of the wafer include new features:

- a substrate with a thickness of 300 μm, with a specific resistance of 2 Ω / cm is made of p-type silicon Si (100), pre-treated with acid HF;

- an n-type light-absorbing layer deposited on the upper polished side of the substrate by magnetron sputtering in argon from a solid-state target from previously synthesized Si ₃ N ₄ is a 4-5 nm thick film of amorphous silicon nitride mixed with silicon nitride nanocrystalline structure, developed surface which, due to the presence of the nanocrystalline structure of Si ₃ N _4, has a positive effect on the absorption of light and simultaneously performs the function of passivation of the surface of the Si (100) substrate;

- the upper electrical contacts located on the n-type light-absorbing layer on the upper side of the solar cell are made in the form of a comb of Ag deposited by magnetron sputtering;

- on the reverse side of the solar cell there is an electric back contact made of Ag or Cu, formed in the form of a film by magnetron sputtering directly on the reverse unpolished surface of the Si (100) substrate.

The technical solutions are unknown in which the solar cell is a single-junction heterostructure with a p-type monocrystalline silicon substrate, on the upper side of which there is a nanoscale, nanostructured mixed amorphous and nanocrystalline light-absorbing layer (α + μs) -Si ₃ N ₄ , performing at the same time, the function of passivating the surface of the silicon substrate and creating a photovoltaic pn heterojunction (α + μs) -Si ₃ N ₄ / Si (100) with an efficiency of 7.41%.

Graphic materials.

Figure 1. Section of the claimed solar cell, side view.

Figure 2. Stated solar cell, top view.

Figure 3. Image of the Raman spectrum of a light-absorbing layer of a Si ₃ N ₄ nanofilm _.

Figure 4. Image of electron diffraction in the (α + μs) Si ₃ N ₄ / Si (100) heterostructure obtained using a JEOL Ltd. transmission electron microscope. JEM 2100:

a) the diffuse rings around the diffuse central reflection from the Si ₃ N ₄ film in the (α + μs) -Si ₃ N ₄ / Si (100) heterostructure indicate a significant amorphous nature of the film material, and the presence of weakly expressed concentric rings indicates the presence of a second phase of nanocrystalline or fine-grained in nature,

b) a picture of electron diffraction of electrons from a single-crystal Si (100) substrate in the same heterostructure.

Figure 5. Cross-sectional image of the heterostructure (α + μs) -Si ₃ N ₄ / Si (100) in a JEOL Ltd. transmission electron microscope. JEM 2100:

a) a general view of the film interface in the (α + μs) -Si ₃ N ₄ / Si (100) heterostructure;

b) the morphology of the cross section of the film Si ₃ N ₄ .

6. Image of the surface morphology of a Si ₃ N ₄ film by atomic force microscopy using an NT-MDT Ntegra Aura microscope:

a) the results of scanning the surface of the film;

b) the processed image of the scan results of the film surface.

7. Image illustrating the determination of the step height at the edge of the film obtained by atomic force microscopy using an NT-MDT Ntegra Aura microscope.

Fig. 8. The current-voltage characteristic of the structure (α + μs) Si ₃ N ₄ / Si (100), obtained according to the standard AM1.5 method on a ST-1000 solar simulator at 1000 W / m ² at 25 ° C.

FIG. 9. A table with the processing results of the surface image of the Si ₃ N _4. film shown in FIG. ₅ .

The composition of the proposed single-junction pn solar cell (α + μs) -Si ₃ N ₄ / Si (100) includes the upper electrodes 1 in the form of a comb of Ag, a layer about 4-5 nm thick film 2, consisting of amorphous silicon nitride mixed with nitride silicon nanocrystalline structure, the substrate 3, made of single-crystal silicon (Si) (100) p-type and the back contact on the back side of the substrate, made of a layer of Ag or Cu (Figure 1 and Figure 2). The advantage of the proposed structure of the solar cell is the minimum number of layers, as well as the nanoscale and nanostructured absorption layer, which ensures the effective conversion of solar energy into electricity.

An example of a method for producing a solar cell.

On a substrate 3 made of monocrystalline silicon, grade KDB 2, of orientation (100), 300 μm thick, with a specific resistance of 2 Ohm / cm p-type conductivity, pre-treated with HF acid, a two-phase layer is applied by non-reactive magnetron high-frequency sputtering from a solid state target Si ₃ N ₄ films 2 of mixed amorphous and nanocrystalline silicon nitride (α + μs) Si ₃ N ₄ . Then, by the method of magnetron sputtering, the upper electrodes 1 are applied in the form of a comb of Ag on a film layer 2, and the back contact 4 in the form of a film of Ag or Cu is applied by the method of magnetron sputtering on the back unpolished side of the substrate 3. The method is characterized by simplicity, environmental friendliness, the entire production cycle is based on the use of equipment of the same type, and also eliminates the increased requirements for cleanliness conditions, for example, the use of so-called "clean rooms" commonly used in the electronics industry.

Examples of research results obtained solar cell.

The correspondence of the film material to the Si ₃ N ₄ target material is confirmed by Raman spectra obtained from nanoscale films of the light-absorbing layer of a solar cell. The position of the maximum in the inset of Fig. 3 in the Raman scattering spectrum corresponds to Si ₃ N ₄ , and the shape of the spectrum with a maximum at a wavelength of 347 cm ^{-1 is} characteristic of the nanocrystalline state.

In addition to the lines belonging to the oscillations of Si atoms at -528 cm-1 and their harmonics near 950 cm ^-1 , the inset shows the established vibration frequencies of 288, 307 and 347 cm ^-1 that correspond to the cubic modification of silicon nitride.

The image of electron diffraction in the (α + μs) Si ₃ N ₄ / Si (100) heterostructure in Fig. 3a confirms that the layer of film 2 deposited by magnetron sputtering in argon from a solid state target Si ₃ N ₄ is predominantly amorphous, and the presence of weakly expressed concentric rings indicates the presence of a second phase of silicon nitride of a nanocrystalline or fine-grained nature. In FIG. 3b, for comparison, the electron diffraction pattern from a single-crystal Si (100) substrate in the same heterostructure is shown.

Study of the cross sectional foil of the heterostructure (α + μs) Si ₃ N ₄ / Si (100) in a JEOL Ltd. transmission electron microscope. JEM 2100 revealed the presence of a fine texture of approximately 1 nm in the form of parallel rows in a Si ₃ N ₄ film directed perpendicular to the Si (100) substrate. In addition, structural elements of the order of tens of nanometers and a clear interface between the layers (α + μs) of Si ₃ N ₄ and Si (100) are visible in this cross section (Fig. 4).

The surface morphology of the Si ₃ N ₄ film was investigated by atomic force microscopy using an NT-MDT Ntegra Aura microscope and, based on the results of the studies, an analysis of the size of the surface of the film was carried out, which is consistent with the results of transmission microscopy in terms of observing large, on the order of tens of nanometers of formations, including the texture of objects of the order of 1 nm. The results of atomic force microscopy are shown in Figure 5, and the results of image processing of the surface of the Si ₃ N ₄ film are presented _. in the table of FIG.

The film thickness of Si ₃ N ₄ , according to the results of measuring the height of the edge of the film in an atomic force microscope NT-MDT Ntegra Aura, was 4-5 nm (Fig. 6).

The claimed photovoltaic structure — a heterostructure-based solar cell mixed amorphous and nanocrystalline silicon nitride — p-type silicon — shows an efficiency of 7.41%, which is confirmed by the current – voltage characteristic of the structure (α + μs) Si ₃ N ₄ / Si (100), obtained from standard AM1.5 methodology at 1000 W / m ² , 25 ° C on a ST-1000 solar simulator (Fig. 7).

Thus, the claimed technical result without additional antireflective, protective or any other layers and without the use of solar radiation concentrators is achieved.

Claims (1)

A single junction solar cell comprising a p-silicon substrate having upper and back sides, an n-type layer on the upper side of the substrate and electrical contacts, where the electrical contact on the upper side of the cell is made of Ag, characterized in that the substrate is made of p-type silicon Si (100) and pretreated with HF acid, an n-type layer deposited on the upper side of the substrate by magnetron sputtering in argon from a solid state target Si ₃ N ₄ is a 4-5 nm thick film of amorphous silicon nitride mixed contact with silicon nitride of a nanocrystalline structure, while the electrical contacts from Ag on the upper side of the element are made in the form of a comb and the electrical back contact in the form of a film of Ag or Cu located on the back of the Si (100) substrate is also formed by magnetron sputtering.

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