JP7169508B2 - Manufacturing method of n-type Br-doped SnS semiconductor - Google Patents

Description

The present invention relates to a SnS (tin sulfide) semiconductor, and more particularly to a semiconductor exhibiting n-type electrical characteristics and a solar cell using the same.

Photovoltaic power generation using solar energy is clean and promising for prevention of global warming, so it is highly expected as a future energy source, and its research and development is proceeding.

Semiconductor materials used in solar cells include silicon-based semiconductor materials and compound semiconductor materials.

As semiconductor materials for these solar cells, silicon-based semiconductor materials have conversion efficiencies of around 20%, but their high material costs pose a problem.

Compound semiconductor-based materials include III-V group semiconductors represented by GaAs and InP, CIGS-based materials, and CdTe-based materials. Among these, there are high-performance materials with a high conversion efficiency of nearly 40% by combining technologies such as multi-junction and light condensing, but they are generally expensive and are mainly used for space applications. there is In addition, among compound semiconductors such as CIGS, CdTe, and GaAs, those containing rare elements such as In and Ga and toxic elements such as Cd and As are the mainstream.

A solar cell is basically composed of a pn junction in which a p-type semiconductor and an n-type semiconductor are joined. The pn junction includes a homojunction in which the same material is used for both the p-type semiconductor and the n-type semiconductor, and a heterojunction in which the p-type semiconductor and the n-type semiconductor are made of different materials. In the case of a heterojunction, there is a problem that defects occur at the junction interface due to differences in crystallinity and lattice constant, electrons and holes generated by light absorption are trapped by the interface defects, and recombination is likely to occur. Therefore, it is difficult to take advantage of the properties of the original material.

SnS (tin sulfide), which is a compound semiconductor, consists of only non-toxic elements/non-rare elements, and its calculated electron mobility is 73,500 cm ² /Vs, and its hole mobility is 23,700 cm ² /Vs, compared to other semiconductors. has a very large value. It also has a high light absorption coefficient of about 10 ⁴ cm ⁻¹ and a bandgap of about 1.35 eV, which is close to the optimum 1.4 eV for sunlight absorption. Therefore, if a SnS homojunction solar cell is realized, a conversion efficiency of 20% or more is theoretically expected.

However, only non-patent document 1 and non-patent document 2 report examples of making SnS n-type. Non-Patent Document 1 is a research report on n _- type _SnS by adding Pb to SnS. is used. On the other hand, Non-Patent Document 2 is an example of Cl-doped SnS showing n-type conduction by the present inventors. This reported example is the only demonstration of an n-type SnS semiconductor produced by doping a halogen element. The n-type SnS semiconductor materials are currently in such a situation, and SnS homojunction solar cells have not been made. Moreover, in a solar cell partially using SnS, the conversion efficiency remains at about 5%. Under these circumstances, a homojunction SnS solar cell has not yet been produced.

F.-Y. Ran, Z. Xiao, Y. Toda, H. Hiramatsu, H. Hosono, and T. Kamita, Sci. Rep. 5, 10428 (2015). Hiroshi Yanagi, Yuki Iguchi, Taiki Sugiyama, Toshio Kamiya and HideoHosono “N-type conduction in SnS by anion substitution with Cl”, AppliedPhysics Express, 9, 051201 (2016).

A new n-type SnS semiconductor material prepared by doping with a halogen element without using toxic elements/rare elements is provided.

The method for producing an n-type Br-doped SnS semiconductor according to the present invention is characterized in that a sample of SnS (tin sulfide) to which Br is added is heated to 600° C. while being pressurized.

By doping (adding) Br (bromine), it is possible to provide an n-type SnS semiconductor material that does not use toxic/rare elements.

It is a figure which shows the production flow of the n-type SnS semiconductor of this invention. It is a figure which shows the outline of a spark plasma sintering apparatus. It is a figure which shows the Seebeck coefficient of the n-type SnS semiconductor of this invention. FIG. 3 shows XRD patterns of Br-doped SnS samples; FIG. 4 shows XRD patterns of fluorine-doped SnS samples; FIG. 4 shows XRD patterns of iodine-doped SnS samples;

The formation of n-type SnS by doping SnS with a halogen element is as follows. (2) When Sn defects are formed, holes are generated, which are thought to work to cancel the electrons generated by the halogen element. has not been much researched. In addition, it was not known whether the material would be n-type unless the material was actually doped with a halogen element.

As mentioned above, only the case where Cl-doped SnS shows n-type conduction according to Non-Patent Document 2 is a demonstration example of an n-type SnS semiconductor created by doping a halogen element, which requires non-toxic elements and rare elements. It was a case of not doing so.

As will be described later, F (fluorine) and I (iodine) cannot form SnS crystals well, and n-type SnS semiconductors cannot be produced.
(Example)
FIG. 1 shows a method for producing an n-type SnS semiconductor according to the present invention.

In order to minimize the effects of moisture and oxygen in the air during weighing and mixing, the work was carried out in a nitrogen-purged glove box (UNICO UN-650L).

First, as shown in FIG. 1, S and Sn are each weighed. Sn was obtained by cutting a Sn ingot (Step 1).

Here, in order to reduce weighing errors due to substances adhering to the boat-shaped weighing dish due to static electricity, etc., the boat-shaped weighing dish was previously stained with S and Sn for weighing and used.

The weighed S and Sn were transferred to another board-type weighing dish and mixed. (Step 2)

The dopant material was then weighed. Here, since Br (bromine) is used as a dopant, SnBr ₂ is used in order to stably add a desired amount. (Step 3)

S and Sn mixed in step 2 and SnBr ₂ weighed in step 3 are mixed in a quartz tube, sealed with a balloon, and taken out of the nitrogen-purging glove box (step 4). Here, the reason why the quartz tube containing these materials is sealed with a balloon when it is taken out from the nitrogen-purging glove box is to prevent it from coming into contact with the atmosphere as much as possible. Therefore, other methods may be used to seal the tubes as long as they can achieve this effect.

The quartz tube taken out is sealed in a vacuum of 10 Pa or less. (Step 5)

The vacuum-sealed quartz tube was placed in an electric furnace, and a mixture of S, Sn, and _SnBr2 was heated to bake the Br-added SnS. At this time, the temperature was raised from room temperature to 520° C. over 12 hours (12 hours), and then, while the temperature was kept at 520° C., firing was continued for another 12 hours (12 hours). After that, it was further cooled from 520°C to 20°C over 3 hours (3H). (Step 6)

The Br-added SnS obtained in step 6 was taken out and pulverized into powder (step 7).

The Br-added SnS powder obtained in step 7 was subjected to spark plasma sintering (SPS) to produce a high-density sintered body (step 8).

In discharge plasma sintering, in addition to heating the die by mechanical pressure and pulse current, the self-heating of the sample and heating by discharge plasma energy between particles produce a uniform sintered body with no bias in density in a short time. is a possible sintering method.

FIG. 2 is a diagram showing an outline of a discharge plasma apparatus. The inside of the die of the discharge plasma sintering apparatus in FIG. A high-density sintered body can be produced by sandwiching the sample with punches from above and below, applying pressure to the sample, and heating the sample.

In this example, the Br-added SnS powder prepared in step 7 was heated in a vacuum of 10 to 12 Pa at a temperature of 600 ° C. for a heating time of 12 minutes (temperature rise in 6 minutes) and a pressure of about 3.5 kN. was sintered under the conditions of As a result, a high-density sintered body sample having a relative density of 80% or more could be produced.

The Seebeck coefficient was measured for the Br-added SnS high-density sintered body sample obtained in Step 8. FIG. 3 shows the measurement results of the Seebeck coefficient of these Br-added SnS high-density sintered samples with different Br addition amounts. Triangular marks in FIG. 3 indicate the measurement results of Br-added SnS. In addition, FIG. 3 also shows the results of Cl-added SnS prepared and measured in the same manner for comparison (in FIG. 3, circles indicate the results of Cl-added SnS).

As shown in FIG. 3, Br is 0.4 at. % or more of Br-added SnS, the Seebeck coefficient is a negative value, indicating that the n-type has been achieved. In addition, it was found that the Seebeck coefficient was negative at approximately the same amount of addition as in the case of Cl. Incidentally, as is clear from FIG. 3, the Br-added SnS is at least 0.4 at. % to 1.0 at. % or less, Br-added SnS exhibits n-type characteristics. On the other hand, 0.4 at. %, the Seebeck coefficient is positive and does not exhibit n-type characteristics.

Next, an XRD measurement was performed on a sample of Br-added SnS. FIG. 4 shows the XRD measurement results thereof. The upper part of FIG. 4 shows a single-phase non-doped SnS (undoped SnS) sample, and the lower part shows Br at 0.95 at. % added samples are shown respectively. The XRD measurement of the Br-added SnS was performed using a powder obtained by pulverizing the high-density sintered body obtained in step 8. In addition, single-phase non-doped SnS (no additive SnS) was also prepared by preparing a high-density sintered body of single-phase non-doped SnS in the same manner as for Br-added SnS, and performing XRD measurement on a powder obtained by pulverizing it.

Here, the three-digit number described in the XRD pattern of undoped SnS (pure SnS) in the upper part of FIG. 4 indicates the surface index of SnS. Since all the peaks in the XRD pattern in the upper part of FIG. 4 can be attributed to respective planes of SnS, it is clear that a single phase is formed. Comparing the upper and lower parts of FIG. 4, there are peaks only at the same positions, and there are no unnecessary peaks, so Br 0.95 at. % added (bottom of FIG. 4) is also a single-phase sample. Note that the vertical axis of this XRD pattern is a logarithmic scale (a.u.).

FIG. 5 shows the XRD pattern of a sample when the same experiment as in this example was performed using fluorine instead of Br. When fluorine was added, peaks of SnS ₂ and Sn ₂ Sn ₃ were also observed in addition to the peak of SnS. This indicates that it is difficult to produce polycrystals of SnS in the first place. The XRD measurement in FIG. 5 was performed on a powder obtained by pulverizing a high-density sintered compact of fluorine-added SnS in the same manner as the Br-added SnS.

Also, FIG. 6 shows the XRD pattern of a sample when the same experiment as in this example was performed using iodine instead of Br. In the case of adding 2 to 3% of iodine, the peak of SnSl ₂ is visible, and it is understood that it is difficult to prepare even the polycrystal of SnS itself. In addition, although no heterogeneous phase was observed in the XRD pattern with 1% iodine added, in the actual experiment, after heating in the quartz tube, precipitates were observed in the tube, so it could not be determined that a single phase was obtained. The XRD measurement in FIG. 6 was performed on a powder obtained by pulverizing a high-density sintered body of iodine-added SnS similarly to the Br-added SnS sample.

From the above, it can be seen that the Br-added SnS semiconductor of the present invention is one of the few semiconductor materials that can be used as an n-type SnS semiconductor.

Since non-doped SnS semiconductors generally exhibit p-type, if the Br-added SnS of the present invention exhibiting n-type is used, a homojunction SnS solar cell can be realized. A p-type non-doped SnS semiconductor and a Br-added n-type SnS semiconductor shown in this embodiment are configured to form a pn junction, and a homojunction SnS solar cell is obtained by constructing a solar cell having this pn junction. It becomes possible to

Claims (4)

A method for producing an n-type Br-doped SnS semiconductor, which is a high-density sintered body, wherein a sample of SnS (tin sulfide) to which Br is added is pressurized and heated to 600° C. n-type Br A method for producing a doped SnS semiconductor. 2. The method for manufacturing an n-type Br-doped SnS semiconductor according to claim 1, wherein said pressure is 3.5 kN. The Br content is 0.4 at. % or more . The Br content is 0.4 at. % or more and 1.0 at. % or less , the method for producing an n-type Br-doped SnS semiconductor according to claim 1.

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