RU2667689C2 - Method for producing nanocrystalline silicon/amorphous hydrogenated silicon heterojunction for solar elements and solar element with such heterojunction - Google Patents

Abstract

FIELD: electronic equipment; optics.SUBSTANCE: invention relates to the field of optoelectronic technology and can be used to create cheap and effective solar cells based on layers of amorphous hydrogenated silicon. Method for producing a nanocrystalline silicon/amorphous hydrogenated silicon heterojunction in a sample consisting of a film of amorphous hydrogenated silicon, deposited on a quartz substrate, involves irradiating the sample with femtosecond laser pulses in a vacuum at a pressure of not more than 2⋅10–2 mbar, with a central emission wavelength of 257–680 nm, a pulse repetition rate of 20–500 kHz, a pulse duration of 30–500 fs, and an energy density of laser pulses of 4–500 mJ/cm. To obtain a solar cell, the nanocrystalline silicon/amorphous hydrogenated silicon heterojunction obtained by the method described above is made on a quartz substrate with a transparent conductive sublayer, and a metal electrode (aluminum, gold or magnesium) is thermally deposited on top. Achieved properties of solar cells will eventually result in higher values (in comparison with solar cells based on a-Si:H) of the photovoltaic parameters (the following values can be achieved: photoconversion efficiency – 14 %, the open circuit voltage – 0.68 V, short-circuit current – 36.5 mA/cm).EFFECT: technical result achieved by using the claimed group of inventions is to provide increased stability of electrical and photoelectric properties under illumination, increased mobility of charge carriers, increased efficiency and relative cheapness in its manufacture (in comparison with solar cells made of crystalline silicon).9 cl, 3 tbl, 4 ex, 9 dwg

Description

Technical field

The group of inventions relates to the field of photovoltaics and can be used to create effective thin-film solar cells based on a nanocrystalline silicon / amorphous hydrogenated silicon heterojunction. In particular, the group of inventions relates to a method for producing a heterojunction of nanocrystalline silicon / amorphous hydrogenated silicon and to a solar cell containing a heterojunction layer made by the above method.

State of the art

Recently, the attention of researchers has been attracted to micro- and nanocrystalline silicon - promising materials from the point of view of their use in thin-film electronic devices. These materials have a structure similar to that of amorphous hydrogenated silicon (a-Si: H), however, unlike the latter, they contain crystalline silicon granules surrounded by an amorphous silicon matrix. The size and concentration of these granules determines the type of material: with a large fraction of the crystalline phase of Si in amorphous silicon, one speaks of nanocrystalline (nc-Si) or microcrystalline silicon (μc-Si), otherwise the material is called polymorphic or protocrystalline silicon. The advantages of these materials include increased stability of their electrical and photoelectric properties compared to a-Si: H under illumination (suppression of the Stebler-Wronsky effect), as well as an increase in the mobility of charge carriers due to the presence of crystalline inclusions. Therefore, the use of micro- and nanocrystalline silicon instead of a-Si: H in thin-film devices can significantly improve their characteristics, in particular, increase the efficiency of solar cells. Compared to crystalline silicon, the advantage of μc-Si and nc-Si materials is their relatively low cost of manufacture, since the same traditional methods of low-temperature deposition can be used for this as in the case of a-Si: H.

The use of pulsed laser radiation with a pulse duration in the femtosecond range to modify the structure of materials has certain advantages over nano- and picosecond modes. In particular, ultrashort (femtosecond) laser pulses lead to a purely local modification of the area of influence, while the surrounding areas are not affected [A.S. Elshin, N.Yu. Firsova, M.A. Marchenkova, V.I. Emelyanov, I.P. Pronin, S.V. Senkevich, E.D. Mishina, A.S. Sigov // Letters in ZhTF, 2015, Volume 41, no. 9, p. 16-23]. In addition, if it is necessary to crystallize thin films deposited on a substrate, heating of the substrate is minimized due to the almost complete absorption of radiation by the film and its short cooling time, which significantly reduces heat diffusion from the film [T.T. Korchagina, V.A. Volodin, B.N. Chichkov // Physics and Technology of Semiconductors, 2010, Volume 44, Issue. 12, p. 1660-1665].

The femtosecond laser annealing method allows locally modifying the structure of amorphous silicon due to the complete or partial crystallization of the irradiated region. In the first case, under the action of laser radiation, a phase transition occurs with the formation of a layer of polycrystalline silicon. This method is described, in particular, in patent RU 2431215 C1 (published on 10/10/2011, CL H01L 21/268, B82B 3/00). According to the invention, an amorphous silicon layer located on a glass or silicon substrate with a silicon dioxide layer is subjected to crystallization by laser treatment using pulsed femtosecond radiation from 30 to 120 fs. The wavelength and energy density are selected so as to induce a crystallization phase transition in amorphous silicon, stimulated by an electron-hole plasma and characterized by the absence of energy transfer to the substrate. An energy density of 20 to 150 mJ / cm ² and a radiation wavelength selected from 390 to 810 nm determine the formation of a continuous nanocrystalline layer in the radiation exposure section for a given amorphous silicon layer thickness of 20 to 130 nm.

With the appropriate choice of the parameters of femtosecond laser radiation, partial crystallization of amorphous silicon is possible with the formation of crystalline silicon granules in the irradiated region [Choi T.Y., Hwang D.J., Griporopoulos S.R. // Optical Engineering. 2003. V. 42. P. 3383; Lee G.Y., Park J., Kim E.K., Lee Y.P., Kim K.M., Cheong H., Yoon C.S., Son Y.-D., Jang J. // Optics Express. 2005. V. 13. P. 6445; Shien J.-M., Chen Z.-H., Dai B.-T., Wang Y.-C, Zaitsev A., Pan C.-L. // Appl. Phys. Letters. 2004. V. 85. P. 1232]. Depending on the exposure conditions, the size of these granules can vary over a wide range from ~ 50 to ~ 800 nm. Note that in almost all works, a titanium-sapphire laser with a wavelength of 800 nm was used. There is also known a method of forming silicon quantum dots in amorphous silicon, described in patent CN 101866836 B (published on October 20, 2010, CL H01L 21/205; H01L 21/268; H01L 31/20). The authors propose using for the crystallization of amorphous silicon films the interference effect of two beams of a femtosecond laser with a wavelength in the range from 400 to 800 nm, a pulse duration of 5-150 fs, an pulse energy of 0.1 to 20 μJ and a pulse repetition rate of 2 to 100 Hz The method is based on sequential layer-by-layer scanning of a film of amorphous silicon prepared by the method of plasma-chemical deposition from the gas phase, as a result of which an array of ordered crystalline granules with a controlled size of 30 to 50 nm is formed. The resulting material is proposed to be used as an element of thin-film solar cells (as one of the layers in a multilayer structure) to increase their efficiency. The disadvantages of the technical solution include the complication of the crystallization method of amorphous silicon due to the use of the interference effect. In addition, to create a solar cell based on it, it is necessary to study the question of what other active layers will be combined with the proposed materials and how they will be obtained.

A necessary element of a solar battery is a photoconverter, based on one or more p-n junctions. The latter can be both homogeneous, i.e. which are parts of the same semiconductor with different levels of donor and acceptor impurities, and heterogeneous, i.e. consisting of two different materials. The use of heterogeneous transitions (heterojunctions) is now of greatest interest in micro- and optoelectronics, as it allows you to create unique in their properties elements of promising devices.

The use of a composite structure of amorphous and nanocrystalline phases of various materials has been repeatedly proposed previously to create heterojunctions of solar cells. In particular, in patent RU 2568421 C1 (published on November 20, 2015, CL H01L 31/06, B82B 1/00,) a solar cell based on a heterostructure describes mixed amorphous and nanocrystalline silicon nitride / p-type silicon nitride. The single junction solar cell includes a p-type silicon substrate Si (100) pretreated with hydrofluoric acid HF. On the upper side of the substrate, there is a layer of an n-type film 4–5 nm thick made of amorphous silicon nitride mixed with silicon nitride of a nanocrystalline structure deposited by magnetron sputtering in argon from a solid state target Si ₃ N ₄ . Ag or Cu electrical contacts are formed by magnetron sputtering. The invention provides an efficiency of 7.41% without additional antireflective, protective or any other layers and without the use of solar radiation concentrators.

As analogues to the claimed technical solution, solar cells have been identified whose technical essence is disclosed in publications US 20090071539 A1 (published March 19, 2009, class H01L 31/00) and US 20070272297 A1 (published on 11/29/2007, class H01L 31/00) .

US20090071539 A1 describes a solar cell made using a composite thin film of amorphous and nanocrystalline silicon and a method for its manufacture. Composite thin films of amorphous and nanocrystalline silicon are obtained by dispersing silicon nanoparticles in a liquid silicon precursor. The resulting dispersion is used to coat the substrate by standard methods, or transferred to the substrate by printing. Subsequent heating of the substrate with a deposited film of amorphous and nanocrystalline silicon is used to convert the precursor from liquid silicon to amorphous silicon. A distinctive feature of this invention is the lack of the need for expensive equipment to obtain composite thin films of amorphous and nanocrystalline silicon. In addition, the described method allows one to obtain composite films consisting of multiple materials with different bandgap energies, which can significantly improve the energy conversion efficiency of a solar cell obtained from a solution of a liquid precursor and nanocrystalline particles, characterized by a low manufacturing cost. The disadvantages of the above technical solution include the complexity of the process of transferring a high-temperature dispersion of liquid silicon with silicon nanoparticles (silicon melting point above 1400 ° C) onto the substrate, as well as the need for subsequent heating of the substrate.

US 20070272297 A1 describes a similar technical solution dedicated to disordered silicon-based nanocomposite structures for photovoltaics, solar cells, and light-emitting devices. Nanocomposite structures are described, including layered pn and pin homo- and heterojunctions consisting of semiconductor nanoparticles, such as colloidal semiconductor nanocrystals, nanorods, nanowires, nanotubes, etc., in which at least one of the layers is made of hydrogenated amorphous or microcrystalline / nanocrystalline silicon or their alloys obtained by low-temperature deposition methods such as plasma-chemical vapor deposition to prevent degradation of physical properties in nanoparticles. An intrinsic, as well as p- or n-type nanocomposite silicon layer containing disordered embedded nanoparticles in direct electrical contact with the silicon matrix can be made on the basis of hydrogenated amorphous silicon, microcrystalline silicon, or their alloys, including hydrogenated silicon-germanium and carbide silicon. The proposed technology, which includes a low-temperature (75-250 ° С) process of plasma-chemical deposition from the gas phase, allows nanoparticles to be embedded in a semiconductor matrix without degradation of their physical properties, which is inevitable with high-temperature methods. This solves the problem of direct electrical contact between the matrix and nanoparticles. Moreover, the energy structure, including the width and position of the forbidden zone of the melt of amorphous and microcrystalline silicon, can be controlled over a wide range from about 1.2 to more than 3 eV due to changes in the composition of the melt. This, according to the authors of the patent, allows to optimize the photovoltaic properties of the material, as well as to vary the parameters of the corresponding light-emitting devices in a wide spectral range from ultraviolet to near infrared. This technical solution is taken as a prototype.

However, in this technical solution there is no direct indication of a specific method for incorporating semiconductor nanoparticles into a silicon matrix, since the mentioned method of plasma-chemical deposition from the gas phase is characterized by many parameters and allows to obtain a wide range of substances, however, the formation of nanoparticles is possible only if a strictly defined procedure is observed, not described nevertheless in the patent itself.

The technical problem is the creation of a solar cell made on the basis of a heterojunction nanocrystalline silicon / amorphous hydrogenated silicon during the formation of this heterojunction through irradiation with laser pulses.

Disclosure of invention

The technical result achieved by using the claimed group of inventions is to provide increased stability of electrical and photoelectric properties under lighting, increased mobility of charge carriers, increased efficiency and relative cheapness in its manufacture (in comparison with solar cells made of crystalline silicon). These properties of solar cells will ultimately produce higher (compared to a-Si: H based solar cells) photovoltaic parameters (the following values can be achieved: photoconversion efficiency - 14%, open circuit voltage - 0.68 V, short-circuit current short circuit - 36.5 mA / cm ² ).

The technical problem is solved by the method of producing a heterojunction nanocrystalline silicon / amorphous hydrogenated silicon in a sample, which is a film of amorphous hydrogenated silicon, deposited on a quartz substrate, including irradiation of the sample with femtosecond laser pulses in vacuum at a pressure of no more than 2-10 ^-2 mbar, with a central a radiation wavelength of 257-680 nm, a pulse repetition rate of 20-500 kHz, a pulse duration of 30-500 fs, and a laser pulse energy density of 4-500 mJ / cm ² .

The film deposition on a quartz substrate can be performed by plasma-chemical deposition of a layer of amorphous hydrogenated silicon on a quartz substrate.

As films, it is possible to use films obtained by ion sputtering of silicon.

As films, it is possible to use films obtained by the thermal decomposition of monosilane.

It is possible to irradiate a sample with laser pulses by scanning with a focused beam at a scanning speed of 1 to 10 mm / s.

The distance between the centers of the scanned bands is 40 μm, and the scanning step is chosen so that the overlap of the laser beams from adjacent bands is at least 80%.

The diameter of the beam varies from 20 to 100 microns.

The technical problem is also solved by a solar cell containing:

- heterojunction layer nanocrystalline silicon / amorphous hydrogenated silicon, obtained in accordance with the above method, made on a film of amorphous hydrogenated silicon and deposited on a quartz substrate;

- a metal electrode deposited on this layer;

- and a conductive transparent sublayer deposited on a quartz substrate.

The metal electrode may be made of aluminum or gold or magnesium.

Brief Description of the Drawings

FIG. 1 is a schematic representation of a solar cell containing a nanocrystalline silicon / amorphous hydrogenated silicon heterojunction layer obtained on the silica substrate with a conductive transparent sublayer (ITO) obtained in accordance with the claimed method and a metal electrode deposited on this layer (A1).

FIG. 2 is a photograph of an amorphous hydrogenated silicon film from Example 1 on a quartz substrate (1) before (2) and after (3) laser crystallization.

FIG. 3 - spectral dependences of the absorption coefficient for amorphous, nanocrystalline and single crystal silicon [A.V. Shah, N. Schade, M. Vanecek, J. Meier, E. Vallat-Sauvain, N. Wyrsch, U. Kroll, C. Droz and J. Bailat, Prog. Photovolt: Res. Appl., 2004, v. 12, p. 113].

FIG. 4 - Raman spectra (Raman), taken from the side of the film surface and from the side of the substrate, for a-Si: H film irradiated with laser pulses with a wavelength of 515 nm.

FIG. 5 - Raman spectra taken from the side of the film surface and from the substrate side for a-Si: H film modified (irradiated) with laser pulses with a length of 1030 nm.

FIG. 6 shows the distribution of the fraction of the nanocrystalline phase over the thickness of a silicon film modified by femtosecond laser radiation with a wavelength of 515 nm and 1030 nm.

FIG. 7 - load current-voltage characteristics for solar cells obtained by laser irradiation with wavelengths: 300 nm (sample 1) and 600 nm (sample 2).

FIG. 8. - load current-voltage characteristics for solar cells obtained by laser irradiation with a pulse duration of 30 fs (sample 3) and 500 fs (sample 4).

FIG. 9. - load current-voltage characteristics for solar cells obtained by laser irradiation with different energy densities: 5 mJ / cm ² (sample 5) and 100 mJ / cm ² (sample 6).

The implementation of the invention

In order to show how the wavelength of crystallizing laser radiation affects the a-Si: H structure, it is sufficient to analyze Raman spectra for two wavelengths: 515 and 1030 nm - see FIG. 3, which shows the spectral dependences of the absorption coefficient of hydrogenated amorphous, microcrystalline and single crystalline silicon. FIG. 3 allows you to understand the features of the used wavelengths. The quantum energy corresponding to the minimum wavelength (2.4 eV) exceeds the width of the mobility gap of amorphous hydrogenated silicon (1.8 eV). At the same time, the quantum energy corresponding to the maximum wavelength (1.2 eV) practically coincides with the band gap of crystalline silicon (1.15 eV). Accordingly, the dynamics of the crystallization process and the structure formed as a result of laser processing depend not only on the used laser energy density, but also on the wavelength of the laser pulses.

Note that the wavelength of the laser used to measure Raman spectra, which in turn were used to estimate the fraction of the crystalline phase, was 488 nm. According to the spectral dependences of the absorption coefficient in amorphous hydrogenated silicon, shown in FIG. 3, such radiation is well absorbed in amorphous silicon. Using the absorption coefficient (a) for a given wavelength allows us to estimate the penetration depth (d) of the probe radiation into the silicon film [I. de Wolf, Semicond. Sci. Technol., 1996, v. 11, p. 139]:

d = (2,3 / 2α),

which is 50 nm. It is worth noting that according to the data for the absorption coefficient of nanocrystalline silicon, presented in FIG. 1, crystallization of the film surface should lead to an increase in the depth of sounding of the structure by the Raman method to 200 nm. Nevertheless, the use of the Raman spectra of a laser with a wavelength of 488 nm allows us to study the distribution of crystallites over the film thickness for modified films obtained as a result of their irradiation with femtosecond laser radiation with different wavelengths. For this, we measured Raman spectra recorded at a wavelength of 488 nm both from the side of the substrate and from the side of the film surface. The measured spectra are shown in FIG. 4 and FIG. 5 for films modified by femtosecond laser radiation with a wavelength of λ = 515 (Fig. 4) and 1030 nm (Fig. 5). As noted above, the analysis of the obtained spectra allows one to evaluate the structure of the material in a layer ~ 50 nm thick located near the substrate or near the surface, respectively. As can be seen from FIG. 4, for a film modified by femtosecond laser radiation with a wavelength of 515 nm, the Raman spectra taken from the side of the film surface and from the side of the substrate are significantly different. In the Raman spectrum taken from the side of the film surface, an intense peak of about 520 cm ⁻¹ is observed, which indicates the presence of a significant fraction of the crystalline phase in the surface structure of the film. At the same time, the Raman spectrum taken from the side of the quartz substrate corresponds to the spectrum of amorphous silicon. Thus, in the case of laser modification of the structure by radiation with a wavelength of 515 nm, a change in its structure nonuniform in film thickness occurs. The film crystallizes near the surface, while the film structure near the quartz substrate remains amorphous.

Other results are obtained when films are modified by femtosecond laser radiation with a wavelength of 1030 nm. As can be seen from FIG. 5, Raman spectra taken both from the surface side and from the side of the quartz substrate coincide with each other. This indicates that the crystallites formed are distributed uniformly throughout the film. The different distribution of the crystalline phase over the film thickness when it is modified by radiation with a wavelength of 515 and 1030 nm is associated with a difference in the absorption coefficient of radiation with a wavelength of 515 and 1030 nm. In the latter case, the absorption leading to crystallization of the films is determined by two-photon transitions, the probability of which is relatively small. Weakly absorbed IR radiation penetrates the entire depth of the amorphous film, causing a material modification uniform in film thickness.

To obtain information on the distribution over the thickness of modified films of phase composition and the effect of the used wavelength of femtosecond laser radiation on this distribution, Raman spectra were measured by layer-by-layer etching of a layer of modified silicon. Liquid chemical etching was carried out in a mixture of HNO ₃ , H ₃ PO ₄ , HF and H ₂ O in the ratio (9: 3: 0.3: 4.5). The film thickness after layer-by-layer etching was determined using a profilometer. From an analysis of the Raman spectra, the volume fraction of the crystalline phase in the ƒ _c films was estimated. Note that the obtained value of ƒ _c reflects the fraction of the crystalline phase in the sensing region of the laser beam used in Raman scattering. In this case, the sounding depth may decrease as the film is etched and the transition from the region with a large fraction of nanocrystals to the region with a small fraction of the crystalline phase. This is due, as mentioned above, to a larger absorption coefficient in amorphous silicon for the laser radiation that we used in Raman scattering.

The results obtained for films modified by femtosecond laser radiation with a wavelength of 515 and 1030 nm are presented in FIG. 6. In the case of using a wavelength of 515 nm, measurements were carried out for a film irradiated with radiation with an energy density of 8 mJ / cm ² . In the case of using a wavelength of 1030 nm, the measurements were carried out for a film with irradiated radiation with an energy density of 100 mJ / cm ² . In FIG. 6, the Δd value corresponds to the thickness of the “etched” part of the film, which initially had a thickness d = 300 nm.

As can be seen from FIG. 6, the obtained results confirm the data obtained by measuring Raman spectra from the side of the film surface and from the side of the quartz substrate. Namely, in the case of using femtosecond laser radiation with a quantum energy exceeding the width of the mobility gap of amorphous hydrogenated silicon, crystallization occurs near the surface of the film at a depth of tens (or less) nanometers. In the case of using femtosecond laser radiation with quantum energies substantially smaller than the width of the mobility gap, crystallization more uniform across the film thickness occurs. In the latter case, the creation of a heterojunction is not possible.

Thus, the technical problem is solved by irradiating the initial film of amorphous hydrogenated silicon (a-Si: H) with femtosecond laser pulses with quantum energies exceeding the width of the mobility gap of amorphous hydrogenated silicon (~ 1.8 eV). As a result, the upper part of the film partially crystallizes to form a thin (less than 1 μm) layer of nanocrystalline silicon (nc-Si), while the lower part of the film remains in its original state. As a result of this, an nc-Si / a-Si: H heterojunction forms, which can serve as the basis of the solar cell. To exclude oxidation of a-Si: H films during their processing, irradiation is carried out in vacuum at a pressure of P≤2⋅10 ^-2 mbar by femtosecond laser pulses with a central radiation wavelength of 257-680 nm, a pulse repetition rate of 20-500 kHz, and pulse duration 30-500 fs and a laser pulse energy density of 4-500 mJ / cm ² . The size of the obtained silicon nanocrystals can vary from 4 to 10 nm.

Examples of carrying out the invention

Example 1. The implementation of solar cells based on a heterojunction nanocrystalline silicon / amorphous hydrogenated silicon using femtosecond laser pulses with different wavelengths

The initial a-Si: H film doped with boron, 0.5 μm thick, is deposited on a quartz substrate with a conducting layer of a mixture of indium and zinc oxides (ITO) by plasma-chemical deposition from a gas mixture of monosilane SiH ₄ , argon Ar and diborane В ₂ Н ₆ when the ratio of flows [In ₂ H ₆ ] / SiH ₄ ] equal to 10 ^-5 . The film is scanned by a focused beam of a pulsed laser on a chromium forsterite crystal, while the laser beam diameter is 20 μm, the pulse duration is 300 fs, the pulse repetition rate is 100 kHz, and the laser pulse energy density is 85 mJ / cm ² . To process amorphous hydrogenated silicon films, the scanning method is used, i.e. the laser beam moves along the surface of the sample. Scanning speed is 5 mm / s. The distance between the centers of the scanned bands is 40 microns. The scanning step (the distance between the scanning “bands”) is selected so that the overlap of the laser beams from adjacent bands is at least 80%.

To realize a solar cell, a nanocrystalline silicon / amorphous hydrogenated silicon heterojunction, for example, by vacuum deposition paths, is applied to a layer (which is contained on a quartz substrate with a conductive transparent sublayer - an ITO layer that plays the role of a lower electrical contact), for example, by vacuum deposition paths, and a metal electrode. The metal electrode can be made in particular of aluminum (see Fig. 1), but options are possible in gold or magnesium. The role of the lower electrical contact is played by the part of the ITO film that is not occupied by the a-Si: H film, for which a quartz substrate with an ITO conductive layer is not completely dusted by the a-Si: H film (see Fig. 2).

In this way, two solar cells were obtained. One solar cell (sample 1) at a laser wavelength of 300 nm, a second solar cell (sample 2) at a wavelength of 600 nm. The current-voltage characteristics for these two modes under illumination under AM1.5 are shown in FIG. 7. The resulting photovoltaic parameters are shown in table 1.

Example 2. The implementation of solar cells based on a heterojunction nanocrystalline silicon / amorphous hydrogenated silicon using femtosecond laser pulses with different durations

Solar cells were obtained in a manner similar to that described in example 1. The only one, in this case, the wavelength of the laser pulses was fixed and amounted to 515 nm, and the duration varied and was equal to 30 fs (sample 3) and 500 fs (sample 4).

The current-voltage characteristics for these two modes under illumination under AM1.5 are shown in FIG. 8. The resulting photovoltaic parameters are shown in table 2.

Example 3. The implementation of solar cells based on a heterojunction nanocrystalline silicon / amorphous hydrogenated silicon using femtosecond laser pulses with different energy densities

Solar cells were obtained completely similar to the method described in example 1, except that the wavelength was fixed and amounted to 515 nm, and the laser pulse energy density varied and was 5 mJ / cm ² (sample 5) and 100 mJ / cm ² (sample 6).

The current-voltage characteristics for these two modes under illumination under AM1.5 are shown in FIG. 9. The resulting photovoltaic parameters are shown in table 3.

Example 4. The implementation of solar cells based on a heterojunction nanocrystalline silicon / amorphous hydrogenated silicon using femtosecond laser pulses with different repetition rates

Solar cells were obtained by a method completely similar to that described in example 1, except that the wavelength was fixed and amounted to 515 nm, and the repetition rate of laser pulses varied and was either 20 or 500 kHz.

The obtained load characteristics, and, accordingly, the photovoltaic parameters did not differ within the measurement error from those obtained for sample 4.

As can be seen from the above examples, by means of the proposed method, it is possible to form a nanocrystalline silicon / amorphous hydrogenated silicon heterojunction and, based on the proposed method, a solar cell having increased photovoltaic parameters (in particular, photoconversion efficiency) compared to solar cells based on amorphous hydrogenated silicon - 14%, open circuit voltage - 0.68 V and short circuit current - 36.5 mA / cm ² ).

Claims (9)

1. A method of producing a heterojunction nanocrystalline silicon / amorphous hydrogenated silicon in a sample representing an amorphous hydrogenated silicon film deposited on a quartz substrate, comprising irradiating the sample with femtosecond laser pulses in vacuum at a pressure of not more than 2⋅10 ^-2 mbar, with a central radiation wavelength 257-680 nm, a pulse repetition rate of 20-500 kHz, a pulse duration of 30-500 fs and a laser pulse energy density of 4-500 mJ / cm ² . 2. The method according to p. 1, characterized in that the deposition of the film on a quartz substrate is performed by plasma-chemical deposition of a layer of amorphous hydrogenated silicon on a quartz substrate. 3. The method according to p. 1, characterized in that as the films use films obtained by ion sputtering of silicon. 4. The method according to p. 1, characterized in that the films used are films obtained by the thermal decomposition of monosilane. 5. The method according to p. 1, characterized in that the irradiation of the sample with laser pulses is carried out by scanning with a focused beam at a scanning speed of from 1 to 10 mm / s. 6. The method according to p. 5, characterized in that the distance between the centers of the scanned strips is 40 microns, and the scanning step is chosen so that the overlap of the laser beams from adjacent strips is at least 80%. 7. The method according to p. 5, characterized in that the diameter of the beam varies from 20 to 100 microns. 8. A solar cell containing a layer with a heterojunction nanocrystalline silicon / amorphous hydrogenated silicon, obtained in accordance with paragraphs. 1-7, made on a film of amorphous hydrogenated silicon and deposited on a quartz substrate, a metal electrode deposited on this layer and a conductive transparent sublayer deposited on a quartz substrate. 9. The solar cell according to claim 8, characterized in that the metal electrode is made of aluminum, or gold, or magnesium.

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