RU2567191C1 - Method of making photosensitive chalcopyrite films - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: method of producing photosensitive chalcopyrite films includes two steps, where a precursor film is produced at the first step and the precursor film is fired at the second step. The precursors used are intermetallic compounds Cu₂In, CuGa₂ and indium metal.

EFFECT: invention enables to obtain homogeneous films with good adhesion.

3 cl, 9 dwg

Description

The invention relates to a technology for creating thin-film environmentally friendly solar cells. The invention may find application in the creation of solar cells with heterojunction Cu _1-δ In _1-x Ga _x Se ₂ / CdS. More specifically, the invention relates to the creation of thin films of Cu _1-δ In _x Ga _x Se ₂ with a gallium gradient, used as absorbing layers in such devices.

Solar cells based on the use of thin-film (4-5 microns) materials provide an opportunity to significantly increase the ratio of specific power to mass while reducing their cost, because due to the small thickness of the components of solar cells, it is possible to create them on light and flexible substrates, which greatly simplifies the deployment process, reduces the weight of structures, reduces the cost of both the solar panels themselves and associated systems. At the same time, the modern most promising for space types of thin-film CIGS solar cells are still far from perfect, although they are comparable to traditional silicon in energy efficiency. The main candidates satisfying the above conditions are such compounds based on CIGS chalcopyrites (Cu-In-Ga-S, Cu-In-Ga-Se). The history of Cu-In-Ga-Se technology began with the involvement of EPV Corporation (Energy Photovoltaics, Inc.) in research in 1991. These were thin-film elements CuInSe ₂ (CIS) and Cu (In, Ga) Se ₂ (CIGS). In April 1998, the EPV demonstrated the unencapsulated CIGS module (9.7%, 3100 cm ² ), which produced 24 watts of power. A prototype a-Si / CIGS tandem module was also created. EPV has consistently adhered to CIGS vacuum technology. It was believed that such layers would be more homogeneous, less defective, and their production safer. These properties were supposed to reduce the cost of the product. In the same 1998, the NREL subcontracting company showed a record conversion factor of 18.8% (for 0.44 cm ² ).

The work was carried out with the following objectives:

1.) the study of various methods for producing CIGS,

2.) obtaining a linear characteristic,

3.) obtaining an efficiency of at least 15%,

4.) increase in the working surface to 4300 cm ² ,

5.) control of the ratio Cu / (In + Ga) and Ga / (In + Ga),

6.) optimization of the rear Mo electrode,

7.) using a Cd-free buffer layer,

8.) ensuring a high deposition rate of ZnO,

9.) low losses during template operations.

Currently, three methods are used to obtain such layers of the CIGS type: a) co-evaporation [Moharram AN, Hafiz MM, Salem A. // 2001 Applied Surface Science. 172. p. 61], b) spraying and selenization [Muler J, Nowoczin J, Schmitt H. 2006 // Thin Solid Films. 496. p. 364], c) electrodeposition [Calixto M E, Sebastian P J, Bhattacharya R N, Noufi R. // 1999 Solar Energy Materials and Solar Cells. 59. p. 75.]. So far, the co-evaporation technique gives the best results in terms of solar energy conversion efficiency. However, this is a very expensive technique and, in addition, when using it, it is difficult to scale the production of thin-film solar cells (TSE) on large panels.

The main difficulty in creating solar cells using this method is the difficulty of creating layers of photosensitive CIGS with a gallium gradient.

The lack of convenient and economical methods for the synthesis of such films is a significant obstacle to further increase the efficiency of solar panels based on it. In addition, there is practically no information in the world literature on the effect of annealing conditions on the properties of such films.

The claimed invention discloses a method for the synthesis of thin (~ 1-2 μm) films of Cu _1-δ In _x Ga _1-x Se ₂ (0 <x <1) with a gallium gradient. To illustrate, a general synthesis scheme is given, as well as the synthesis of images of the composition Cu _0.7 In _0.7 Ga _0.3 Se ₂ and CuIn _0.33 Ga _0.67 Se ₂ .

Closest to the proposed are the methodology described in [Huan-Hsin Sunga, Du-Cheng Tsaia, Zue-Chin Chang et al. // Surface and Coating Technology, 2014, in press], where Cu-Ga alloys and metallic indium are used as CIGS precursors, as well as [Ingrid Repins, Miguel A. Contreras, Brian Egaas et all. // Progress in Photovoltaics: Research and Applications. 2008. V. 16, pp. 235-239], where films of the composition Cu _0.7 Ino _0.7 Ga _0.3 Se ₂ are obtained by evaporation from four sources. These techniques have a number of significant drawbacks. The first of them uses a non-equilibrium system of Cu-In-Ga metals as precursors, which does not allow producing CIGS films with a holistic structure and acceptable adhesion [Gapanovich MV, Odin I.N., Novikov G.F., // Inorganic materials. 2014, in print]. And the second technique is quite expensive, in addition, they will not allow to obtain films of a large area.

A distinctive feature of our proposed technique is the use of equilibrium equilibrium CIGS systems as precursors at a selenization temperature, for example CuGa ₂ -Cu ₂ In-In. This allows you to create uniform films with acceptable adhesion. In addition, the use of a strictly fixed mass of selenium in the reactor will allow reproducibility of the electrophysical properties of the samples. The proposed technology is scalable and potentially cheaper than that proposed in [Ingrid Repins, Miguel A. Contreras, Brian Egaas et al. // Progress in Photovoltaics: Research and Applications. 2008. V. 16, pp. 235-239].

The general synthesis scheme is shown in FIG. 1. The values of x and δ lie in the range 0 <x <1, 0 <δ <1.

The synthesis consists of several stages. At the first stage, intermetallic compounds Cu ₂ In and CuGa ₂ are obtained by direct synthesis in evacuated quartz ampoules.

At the second stage, the precursors are sputtered onto a substrate by vacuum thermal or magnetron sputtering. The required number of precursors are calculated based on the circuit shown in FIG. 1. To create a gallium gradient in a CIGS film, precursors are sprayed in the following sequence: substrate / CuGa ₂ / Cu ₂ In / In.

At the third stage, precursors are annealed in a three-zone furnace (Fig. 2).

The reactor is a quartz tube sealed at one end and having a lid with a vacuum tap. Annealing is carried out in vacuum.

The temperatures of zones I and III are T ₁ = 700 ° C; in zone II, the temperature T ₂ = 550 ° C was maintained. The first is the heating of zones I and III. Heating time t ₁ = 10 min. Then, zone II t ₂ = 5 min is heated.

The precursor film obtained in the previous step is placed in zone II simultaneously with a fixed amount of selenium and small (~ 5 mg) amounts of metallic gallium and indium. The required amount of selenium is determined by the stoichiometry of the final film and the characteristics of the reactor. The presence of metallic gallium and indium in the reaction zone is necessary to prevent possible loss of these elements in the precursor film due to the formation of volatile compounds [Wei Liu, Jian-Guo Tian, Zu-Bin Li et al. // Semicond. Sci. Technol .. 2009. V. 24. 035019 (4 pp.)]. For the reactor used by us with a length L = 45 cm and a volume V = 150 cm ³ during the synthesis of a film of the composition Cu _0.7 In _0.7 Ga _0.3 Se _{2, the} required mass of selenium is m (Se) = 30 mg.

To create CIGS photosensitive films, the most optimal annealing time is 25-30 minutes. At long annealing times, the film is homogenized and the gallium gradient disappears.

The invention is illustrated, but not limited to the following examples.

Example 1. Synthesis of a CIGS film of composition Cu _0.7 In _0.7 Ga _0.3 Se ₂ with a thickness of d = 1 μm

On samples of this particular composition in the world, the maximum efficiency of a solar battery of this type was achieved [Ingrid Repins, Miguel A. Contreras, Brian Egaas et all. // Progress in Photovoltaics: Research and Applications. 2008. V. 16, pp. 235-239].

The general synthesis scheme is shown in FIG. 3.

Calculation of the mass of precursors:

At a CIGS density of ρ = 5.6 g / cm ³ , thermal spraying from a tape-shaped evaporator from a distance of R = 7 cm would require a film with a thickness of d = 1 μm

m = 2 · π · ρ · R ² d = 2 · 1416 · 5.6 · 49 · 10 ^-4 = 0.1724 g CIGS. Molar mass Cu _0.7 In _0.7 Ga _0.3 Se ₂ Mr (CIGS) = 303.6 g / mol, Mr (Cu ₂ In) = 241.8 g / mol, Mr (CuGa ₂ ) = 202.9, Mr (In) = 114.8

The required number of precursors is calculated based on the circuit of FIG. 3. With mass m (CIGS) = 0.1724 g

Accordingly, the thickness of the layers

The deposition was carried out sequentially on glass and glass / molybdenum substrates (5 × 5 cm) from tungsten crucibles at T ~ 1500 ° C in vacuum at a residual pressure of 8 · 10 ^-7 mm Hg. For uniform deposition, rotation of the substrate was used.

The structure of the obtained precursor films was as follows: glass / Mo / CuGa ₂ / Cu ₂ In / In.

Pieces 2 × 2 cm in size were cut from the obtained samples, then annealed in a three-zone furnace. Annealing time ranged from 15 to 90 minutes. The mass of selenium in the reactor is from 10 to 100 mg.

CIGS film formation was confirmed by XRD. Photosensitivity of samples was studied by the method of photoelectrochemical cells (PEC) [JJ Scragg, PJ Dale, L.M. Peter et al. // Phys. Stat. Sol. (b). 2008. V. 245. No 9. P. 1772-1778.]. The samples were illuminated with a DRSh-250 mercury lamp at P = 100 mW / cm ² . To reduce the glow in the IR and UV regions, light filters were used. A 0.1 M aqueous solution of Eu (No ₃ ) _{3 was} used as an active electrolyte.

From FIG. 4 (1-15 minutes, 2-30 minutes, 3-60 minutes, 4-90 minutes, 5-120 minutes) and FIG. 5 (1-15 min, 2-30 min, 3-90 min) it can be seen that with increasing annealing times the phase composition of the films does not change (tetragonal syngony (I-42d), lattice parameters. A = 5.760 (4), c = 11.59 (3)), but the amplitude of the photocurrent under illumination varies nonlinearly. The maximum is observed at annealing times of 15-30 min. With longer annealing, a decrease in photosensitivity is observed. The observed phenomenon can be associated with a uniform distribution of gallium over the film thickness.

In FIG. 6 shows the RES data depending on the mass of selenium in the reactor. Annealing time 30 min.

From FIG. Figure 6 shows that the amplitude of the photocurrent is maximum for samples obtained with a selenium mass in the reactor of m (Se) = 30 mg. In FIG. Figure 7 shows the RES results for samples obtained under similar conditions (1 and 2), as well as data for a CIGS sample obtained at Chung Gang University, Taiwan, by evaporation from four sources (solar cell efficiency based on this sample η ~ 10%). From FIG. Figure 7 shows that the increase in the amplitude of the photocurrent for these samples is comparable.

This example illustrates the possibility of synthesizing high-quality CIGS films of the composition Cu _0.7 In _0.7 Ga _0.3 Se _{2 by} a simpler method than the evaporation method from four sources described in the literature.

Example 2. Synthesis of a CIGS film of composition CuIn _0.33 Ga _0.67 Se ₂ with a thickness of d = 1 μm

The general synthesis scheme is shown in FIG. 8.

Calculation of the mass of precursors:

At a CIGS density of ρ = 5.6 g / cm ³ , thermal spraying from a tape-shaped evaporator from a distance of R = 7 cm would require a film with a thickness of d = 1 μm

m = 2 · π · ρ · R ² d = 2 · 3.1416 · 5.6 · 49 · 10 ^-4 = 0.1724 g of CIGS. Molar mass CuIn _0.33 Ga _0.67 Se ₂ Mr (CIGS) = 306.3 g / mol, Mr (Cu ₂ In) = 241.8 g / mol, Mr (CuGa ₂ ) = 202.9, Mr (In) = 114.8

The required number of precursors is calculated based on the circuit of FIG. 8. With mass m (CIGS) = 0.1724 g

Accordingly, the thickness of the layers

The deposition was carried out sequentially on glass and glass / molybdenum substrates (5 × 5 cm) from tungsten crucibles at T ~ 1500 ° C in vacuum at a residual pressure of 8 · 10 ^-7 mm Hg. For uniform deposition, rotation of the substrate was used.

The structure of the obtained precursor films was as follows: glass / Mo / CuGa ₂ / Cu ₂ In.

Pieces 2 × 2 cm in size were cut from the obtained samples, then annealed in a three-zone furnace. Annealing time 30 min. The mass of selenium in the reactor is 30 mg.

CIGS film formation was confirmed by XRD (Fig. 9).

Tetragonal syngony (I-42d), crystal lattice parameters a = 5.659 (5), c = 11.19 (3), a / c = 1.98.

This example illustrates the possibility of synthesizing single-phase films of other compositions.

Thus, the claimed invention discloses a method for synthesizing high-quality gallium gradient CIGS photosensitive films. This technique can be useful in creating high-performance, environmentally friendly thin-film solar panels of a new generation.

Claims (3)

1. A method for producing photosensitive chalcopyrite films, which includes two stages, the first stage receiving a precursor film, and the second stage annealing it, characterized in that the first stage uses intermetallic compounds CuGa ₂ , Cu ₂ In and metal indium. 2. The production method according to p. 1, characterized in that the process is carried out in selenium vapor. 3. The production method according to claim 1, characterized in that the annealing is carried out in a three-zone furnace.

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