KR101035979B1 - Method for growth of zinc oxide thin films and thin film solar cell using the same and fabricating method for the thin film solar cell - Google Patents

Abstract

PURPOSE: A method for growing a ZnO thin film, a thin film solar cell using the same, and a method for manufacturing the thin film solar cell are provided to prevent a void and a crack in a silicon thin film deposition process by making the surface of the ZnO thin film with a U-shape. CONSTITUTION: Textures in a (0002) direction and a (1120) direction are simultaneously grown by controlling a deposition temperature, a deposition pressure, or the injection ratio of precursors. The deposition temperature is controlled between 90 and 150 degrees centigrade. The deposition pressure is controlled between 0.1 and 10 torr. The precursor is made of oxygen materials and zinc materials.

Description

Method for growth of zinc oxide thin films and thin film solar cell using the same and fabricating method for the thin film solar cell}

The present invention relates to a ZnO thin film growth method and a thin film solar cell using the same, and a method for manufacturing the same, and more particularly, the surface uneven state of the ZnO thin film by inducing the growth in the <0002> direction and the <1120> direction when the ZnO thin film is laminated The present invention relates to a ZnO thin film growth method that does not require a separate post-treatment process such as texturing by implementing a U-shape, and a thin film solar cell using the same and a method of manufacturing the same.

The thin film solar cell has a structure in which a first electrode, a semiconductor layer, and a second electrode are sequentially stacked on a transparent substrate. In this structure, the first electrode is made of a transparent material such as indium tin oxide (ITO) and ZnO having excellent light transmission properties, and the semiconductor layer is made of crystalline (microcrystalline) or amorphous (amorphous) silicon.

Thin-film solar cells with micro-crystalline Si as a semiconductor layer have an energy band gap of 1.1 eV, which is lower than that of amorphous silicon, resulting in a wider light absorption range from visible to near-infrared. In addition, deterioration due to light exposure has a small advantage. However, low light absorption requires a film thickness of about 50 mu m or more for sufficient light absorption. Therefore, there is a need for a technique capable of minimizing the thickness of the thin film layer while maximizing light absorption in the semiconductor layer. This effect is made possible by the so-called light trapping, which causes light scattering at the surface of the film and increases the distance that light travels within the film. Light trapping may be achieved by increasing the surface roughness of transparent conductive films such as ITO, SnO ₂ , and ZnO, which are generally used as transparent electrodes of thin film solar cells.

Meanwhile, ZnO thin films have many advantages over ITO due to their low light transmittance and electrical conductivity and low price. In addition, it is also highly usable compared to SnO _{2 because} it has excellent resistance to hydrogen plasma used in the deposition of silicon thin films. ZnO is generally deposited by sputtering or metal-organic chemical vapor deposition (MOCVD). In the case of a thin film deposited by the sputtering method, the crystal growth plane has a direction of <0002> and the surface of the thin film is Very flat as shown in FIG. Therefore, diffuse reflection of light cannot be expected. On the other hand, the thin film deposited by the MOCVD method forms a <0002> direction or <1120> direction according to the deposition conditions, the thin film in the <1120> direction is a pyramidal particles as shown in Figure 2 on the substrate It is embedded in shape and forms an uneven structure as a whole. Such surface irregularities allow light to be diffusely reflected and to capture light. For reference, in the present patent, the <1120> direction means a direction in which particles having a hexagonal system grow perpendicular to the substrate, and the same also applies to the following.

Meanwhile, as the surface of the thin film grown in the <1120> direction has a shape in which pyramidal particles 201 are disposed adjacent to each other, as shown in FIG. 3, a V-shaped valley is formed between the particles 201. ) Is formed. In this state, when the silicon thin film is deposited on the ZnO thin film as a semiconductor layer, the silicon thin film 202 does not completely fill the V-shaped valleys, causing voids 203 or cracks due to the cracks. There is a problem that (crack) occurs. These voids and cracks are generated due to the shadowing effect when the silicon thin film is deposited on the V-shaped bone, which is an important factor in reducing the efficiency of the solar cell ('M. Python et al., Solar energy materials and solar cells, 93, 1714 (2009) ',' HBT Li et al., Thin solid films, 517, 3476 (2009) ')

In order to solve such a problem, a method of forming Z-shaped bones in a U shape by depositing ZnO using MOCVD and performing plasma treatment ('J. Bailat et al., Proce. Of the 4th WCPEC.Hawaii, 1533 (2006) ') has been proposed a method of making a U-shaped by wet etching (US Patent Application 2009-239326). These methods are commonly referred to as texturing and are effective in removing voids and cracks between the silicon thin film and ZnO and thus increase the efficiency of the solar cell.

However, the method of making U-shaped bone by plasma treatment or wet etching requires an additional process after ZnO deposition, and it is possible to cause contamination of the ZnO thin film by the gas used in the plasma treatment or the etchant used in the wet etching process. There is a need for a separate process for surface cleaning, and may also damage the surface of the ZnO thin film during treatment, which may cause a decrease in physical properties of the ZnO thin film.

The present invention has been made to solve the above problems, by inducing growth in the <0002> direction and <1120> direction at the time of the ZnO thin film stacking to implement the surface irregularities of the ZnO thin film in the U-shaped to separate the texturing, etc. It is an object of the present invention to provide a ZnO thin film growth method and a thin film solar cell using the same and a method for manufacturing the same, which do not require post-processing.

ZnO thin film growth method according to the present invention for achieving the above object in the growth of ZnO thin film using a metal-organic chemical vapor deposition (MOCVD) method, 1) deposition temperature control, 2) deposition pressure during the deposition of ZnO thin film Control, 3) through the one or more of adjusting the injection rate ratio of the precursor, characterized in that to grow the texture in the <0002> direction and the texture in the <1120> direction at the same time.

1) In the deposition temperature control, the deposition temperature may be adjusted at 90 ~ 150 ℃. 2) In the deposition pressure control, the deposition pressure may be adjusted from 0.1 to 10 torr. 3) In controlling the injection rate ratio of the precursors, the precursor is composed of <material including oxygen> and <material containing zinc>, the amount of the injection amount of the <material containing oxygen> and <material containing zinc> The ratio may be <material comprising oxygen> / <material comprising zinc> = 0.1 to 4. In addition, the <material containing oxygen> is H ₂ O, the <material containing zinc> may be DEZ (diethylzinc) or DMZ (dimethylzinc).

The method for manufacturing a thin film solar cell using the ZnO thin film growth method according to the present invention includes preparing a substrate and depositing a ZnO thin film having a surface uneven structure on the substrate. Through one or more of temperature control, 2) deposition pressure control, 3) injection rate ratio adjustment of precursors, it is characterized in that the growth of the texture in the <0002> direction and the texture in the <1120> direction at the same time.

The thin film solar cell using the ZnO thin film growth method according to the present invention is laminated on the substrate and the substrate, and comprises a ZnO thin film having a surface irregularities structure, 1) deposition temperature control, 2) deposition pressure during the deposition of ZnO thin film 3) A thin film solar cell using the ZnO thin film growth method, characterized by simultaneously growing an aggregate structure in the <0002> direction and an aggregate structure in the <1120> direction through at least one of adjusting the injection rate ratio of the precursors.

ZnO thin film growth method and a thin film solar cell using the same and a manufacturing method according to the present invention has the following effects.

As the ZnO thin film is formed such that the texture in the <0002> direction and the texture in the <1120> direction are mixed, the surface structure of the ZnO thin film is U-shaped. Accordingly, the light scattering effect can be ensured and the generation of voids and cracks during subsequent silicon thin film deposition can be suppressed.

Therefore, since an additional process such as plasma etching is not required, the thin film solar cell process can be simplified, and the photoelectric conversion efficiency of the thin film solar cell can be improved by suppressing voids and cracks.

In addition, by controlling the deposition temperature, the deposition pressure, and the injection ratio of precursors, it is possible to mix the texture in the <0002> direction and the texture in the <1120> direction, thereby making it easier to control the process conditions. Do.

1 is a SEM image of the surface of the ZnO thin film grown in the <0002> direction.
2 is a SEM image of the surface of a ZnO thin film grown in a <1120> direction.
3 is a reference diagram illustrating voids generated when a silicon thin film is laminated on a ZnO thin film grown in a <1120> direction.
4 is a schematic cross-sectional view of a ZnO thin film in which an aggregate structure in a <0002> direction and an aggregate structure in a <1120> direction are mixed.
FIG. 5 is a surface structure photograph of a ZnO thin film deposited for 90 minutes under a deposition temperature of 120 ° C. and a deposition pressure of 0.67 torr, with a bubble bath having a temperature of 80 ° C. and a DEZ of 36 ° C. adjusted to H ₂ O. FIG.
Figure 6 shows the XRD spectrum results for the ZnO thin film of FIG.
FIG. 7 illustrates the total transmittance (TT) and transmittance (DT) due to diffuse reflection (TT) for the ZnO thin film of FIG. 5.

The present invention is to control any one or more of the deposition temperature, the deposition pressure, the injection ratio of the precursors (precursors) in the deposition of ZnO thin film using a metal-organic chemical vapor deposition (MOCVD) method and <1120 Its characteristic is to induce growth in the> direction simultaneously.

Through this, the ZnO thin film in which the aggregated structure in the <0002> direction and the aggregated structure in the <1120> direction can be realized can be realized, and the surface of the ZnO thin film is caused by the mixed structure in the <0002> direction and the aggregated structure in the <1120> direction. Form a U shape rather than a V shape (see FIG. 4). Accordingly, the surface of the ZnO thin film forms a concave-convex structure capable of light scattering, and has a structure capable of suppressing voids and cracks during subsequent deposition of the silicon thin film layer.

In addition, the ZnO thin film having a U-shaped concave-convex structure in which the aggregated structure in the <0002> direction and the aggregated structure in the <1120> direction are mixed may be applied as a transparent conductive film of a thin film solar cell, in which case, Further processing such as texturing (texturing), that is, plasma etching process or the like is not required.

Meanwhile, when crystal growth of a ZnO thin film, there are 1) controlling the injection ratio of precursors, 2) controlling the deposition temperature, and 3) controlling the deposition pressure as factors for inducing growth in the <0002> and <1120> directions simultaneously. 1) Controlling the injection rate ratio of precursors is as follows.

Precursors used for ZnO thin film deposition are classified into <materials containing oxygen> and <materials containing zinc (Zn)>, H ₂ O may be used as <materials containing oxygen> and <zinc As a material containing>, DEZ (diethylzinc) or DMZ (dimethylzinc) may be used.

According to the experiment, the ratio of <material containing oxygen> / <material containing zinc>, the crystal growth direction of the ZnO thin film is <1120 in the <0002> direction with the oxygen / zinc ratio being changed from 0.1 to 4 Direction is changed. That is, as the ratio of <material containing oxygen> / <material containing zinc> approaches 0.1, crystal growth occurs in the <0002> direction, and <material containing oxygen> / <material containing zinc> is As close to 4, crystal growth occurs in the <1120> direction. At this time, when the ratio of <material including oxygen> / <material containing zinc> is 0.1 or less, the metal film is deposited in the form of a Zn metal thin film. Does not look

Therefore, by adjusting the ratio of <material including oxygen> / <material containing zinc> to 0.1 to 4, it is possible to simultaneously grow the texture in the <0002> direction and the texture in the <1120> direction.

Next, 2) the deposition temperature control will be described. During deposition of the ZnO thin film, as the deposition temperature is increased, the ZnO thin film shows a growth pattern in the <1120> direction. In contrast, as the deposition temperature decreases, the ZnO thin film shows a growth pattern in the <0002> direction. Specifically, when the deposition temperature is 150 ° C. or more, the ZnO thin film grows only in the <1120> direction, and when the deposition temperature reaches 90 ° C. or less, the ZnO thin film grows only in the <0002> direction. On the other hand, when the deposition temperature is 90 to 150 ° C., the ZnO thin film grows in the <1120> direction as well as the <0002> direction. Therefore, by controlling the deposition temperature to 90 to 150 ℃ during the deposition of the ZnO thin film it is possible to grow the texture in the <0002> direction and the texture in the <1120> direction at the same time.

Finally, 3) the deposition pressure control will be described. In the pressure range of 0.1 to 10 torr, the texture in the <0002> direction and the texture in the <1120> direction are simultaneously grown. In addition, the lower the pressure, the higher the growth in the <1120> direction, and the higher the pressure, the <0002. The aggregate of directions grows. The pressure range in which two aggregates grow simultaneously depends on the temperature, but in the example of 120 ℃, it is possible to grow the aggregate in the <0002> direction and the aggregate in the <1120> direction by adjusting from 0.3torr to 6torr. have.

As described above, in the present invention, when the ZnO thin film is grown, the aggregated structure in the <0002> direction and the aggregated structure in the <1120> direction are mixed through 1) controlling the injection ratio of precursors, 2) controlling the deposition temperature, and 3) controlling the deposition pressure. It is characterized by forming a ZnO thin film having a U-shaped uneven structure. Among these, 1) the injection rate ratio of the precursors and 3) the deposition pressure control do not change the crystal growth direction by changing the deposition temperature, there is no effect of the deposition temperature on the crystal growth direction, accordingly It is possible to form a ZnO thin film under temperature conditions.

As such a low temperature ZnO thin film can be deposited, the width of the material for the substrate on which the ZnO thin film is deposited becomes wider, and the application of the polymer substrate and the like is sufficiently possible. Therefore, the ZnO thin film growth method according to the present invention can be applied to a transparent conductive film of a thin film solar cell requiring a surface irregularities structure, and the substrate selection of the thin film solar cell is possible as the ZnO thin film can be deposited at low temperature. Wider).

In addition, when the ZnO thin film of the present invention is applied to a thin film solar cell, as the ZnO thin film surface structure is U-shaped, it is possible to suppress voids (see FIG. 3) and crack generation in depositing subsequent silicon thin films. So that no additional aftertreatment process is required.

Hereinafter, the crystal structure and the light transmission characteristics of the ZnO thin film grown with the growth of the ZnO thin film according to an embodiment of the present invention will be described.

Example 1

Using a bubbler, precursors of H ₂ O and DEZ (diethylzinc) were supplied together with a carrier gas into the MOCVD chamber to deposit a ZnO thin film on a glass substrate. Argon gas was used as the carrier gas, and the pressure in the MOCVD chamber was 0.67torr and the temperature was 120 ° C. In addition, the injection amount of H ₂ O was 100 sccm, and the injection amount of DEZ was 200 sccm. The MOCVD chamber was in the form of a hot wall with a 5.5 cm diameter quartz tube in the heating element, evacuating the quartz tube to 10 ^-3 torr and feeding the substrate to the desired temperature before supplying the precursor and carrier gas. In order to simultaneously achieve crystal growth in the <0002> direction and the <1120> direction, the injection amount of the precursor supplied into the MOCVD chamber was controlled by independently controlling the temperatures of the Bubbler baths of H ₂ O and DEZ.

Figure 5 is a photograph of the surface structure of ZnO thin film was deposited 90 minutes while the temperature of the H ₂ O bath Bubbler 80 is ℃, DEZ is in a controlled state in 36 ℃, deposition temperature 120 ℃, deposition pressure 0.67torr. Referring to FIG. 5, it can be seen that large particles having clear crystal planes are mixed with very small particles. Particles with a clear crystal plane are particles in the <1120> direction, and very small particles are the particles in the <0002> direction. The surface shape of the ZnO thin film is such that <1120> particles exist as islands on a matrix of <0002> particles. Assuming that only <1120> particles grew without such particles in the <0002> direction in these tissues, the boundary between these particles is estimated to form a very sharp deep valley (ie, V-shaped). In the case of FIG. 5, it can be seen that not only <1120> particles but also <0002> particles grow simultaneously to fill the valleys between the <1120> particles.

Meanwhile, the XRD spectrum of the ZnO thin film shown in FIG. 5 is shown in FIG. 6. Referring to FIG. 6, diffraction peaks of (0002) and (1120) are shown together, and through this, particles in the <0002> direction and particles in the <1120> direction may be simultaneously grown.

Example 2

Example 2 is a result of measuring the light transmission characteristics of the ZnO thin film prepared in Example 1. FIG. 7 shows the total transmittance (TT) and transmittance (DT) due to diffuse reflection (TT) for the ZnO thin film of FIG. 5. Referring to Figure 7, it can be seen that there is a diffuse reflection effect of a predetermined level or more for almost all wavelength bands.

Claims (16)

In growing a ZnO thin film using a metal-organic chemical vapor deposition (MOCVD) method,
When depositing a ZnO thin film, 1) controlling the deposition temperature, 2) controlling the deposition pressure, and 3) adjusting the injection rate ratio of precursors, simultaneously growing the texture in the <0002> direction and the texture in the <1120> direction.
1) In the deposition temperature control, the deposition temperature is controlled at 90 ~ 150 ℃,
2) in the deposition pressure control, the deposition pressure is controlled at 0.1 ~ 10torr,
3) In controlling the injection rate ratio of the precursors, the precursor is composed of <material including oxygen> and <material containing zinc>, the amount of the injection amount of the <material containing oxygen> and <material containing zinc> The ratio is <material containing oxygen> / <material containing zinc> = 0.1 to 4, wherein the ZnO thin film growth method.
delete delete delete The ZnO thin film growth method of claim 1, wherein the <material including oxygen> is H ₂ O, and the <material containing zinc> is DEZ (diethylzinc) or DMZ (dimethylzinc).
According to claim 1, 3) In adjusting the injection ratio of the precursors,
The precursor is composed of <material containing oxygen> and <material containing zinc>, the <material containing oxygen> is H ₂ O, <material containing zinc> is DEZ (diethylzinc), ZnO thin film growth method, characterized in that the ratio of H ₂ O / DEZ is 0.1 to 4.
Preparing a substrate; And
And depositing a ZnO thin film having a surface uneven structure on the substrate,
When depositing a ZnO thin film, 1) controlling the deposition temperature, 2) controlling the deposition pressure, and 3) adjusting the injection rate ratio of precursors, simultaneously growing the texture in the <0002> direction and the texture in the <1120> direction.
1) In the deposition temperature control, the deposition temperature is controlled at 90 ~ 150 ℃,
2) in the deposition pressure control, the deposition pressure is controlled at 0.1 ~ 10torr,
3) In controlling the injection rate ratio of the precursors, the precursor is composed of <material including oxygen> and <material containing zinc>, the amount of the injection amount of the <material containing oxygen> and <material containing zinc> The ratio is <material containing oxygen> / <material containing zinc> = 0.1 to 4,
The <material containing oxygen> is H ₂ O, and the <material containing zinc> is DEZ (diethylzinc) or DMZ (dimethylzinc) characterized in that the thin film solar cell manufacturing method using a ZnO thin film growth method.
delete delete delete According to claim 7, 3) In controlling the injection rate ratio of the precursors,
The precursor is composed of <material containing oxygen> and <material containing zinc>, the <material containing oxygen> is H ₂ O, <material containing zinc> is DEZ (diethylzinc), A method of manufacturing a thin film solar cell using the ZnO thin film growth method, characterized in that the ratio of H ₂ O / DEZ is 0.1 to 4.
Board; And
Stacked on the substrate, including a ZnO thin film having a surface irregularities structure,
When depositing a ZnO thin film, 1) controlling the deposition temperature, 2) controlling the deposition pressure, and 3) adjusting the injection rate ratio of precursors, simultaneously growing the texture in the <0002> direction and the texture in the <1120> direction.
1) In the deposition temperature control, the deposition temperature is controlled at 90 ~ 150 ℃,
2) in the deposition pressure control, the deposition pressure is controlled at 0.1 ~ 10torr,
3) In controlling the injection ratio of the precursors, the precursor is composed of <material including oxygen> and <material containing zinc>, wherein <material containing oxygen> is H ₂ O, <material containing zinc > Is DEZ (diethylzinc), the ratio of the H ₂ O / DEZ is a thin film solar cell using a ZnO thin film growth method, characterized in that 0.1 to 4.
delete delete delete The method according to claim 12, 3) in controlling the injection ratio of precursors,
The precursor is composed of <material containing oxygen> and <material containing zinc>, the <material containing oxygen> is H ₂ O, <material containing zinc> is DEZ (diethylzinc), The thin film solar cell using the ZnO thin film growth method, characterized in that the ratio of H ₂ O / DEZ is 0.1 to 4.

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