JP4289768B2 - Photovoltaic device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a photovoltaic device using microcrystalline silicon germanium (μc-SiGe) as a photoactive layer.
[0002]
[Prior art]
Conventionally, photovoltaic devices mainly made of amorphous silicon (hereinafter referred to as a-Si) formed by glow discharge decomposition of a source gas or a photo-CVD method have the advantage of being thin and easy to increase in area. And is expected as a low-cost photovoltaic device.
[0003]
As a structure of this type of photovoltaic device, a pin type a-Si photovoltaic device having a pin junction is generally used. FIG. 7 shows the structure of such a photovoltaic device. On the glass substrate 21, a transparent electrode 22, a p-type a-Si layer 23, an intrinsic (i) -type a-Si layer 24, an n-type a-Si layer. 25 and the metal electrode 26 are sequentially laminated. In this photovoltaic device, photovoltaic power is generated by light incident through the glass substrate 21.
[0004]
The a-Si photovoltaic device described above is known to undergo photodegradation after light irradiation. Therefore, there is microcrystalline silicon as a material that is a thin film and highly stable to light irradiation, and a photovoltaic device using this microcrystalline silicon for a photoactive layer has been proposed . This microcrystalline silicon is a thin film in which a microcrystalline Si phase and an a-Si phase are mixed.
[0005]
[Problems to be solved by the invention]
As described above, microcrystalline silicon (Si) has attracted attention as a technique for overcoming photodegradation, which is a drawback of amorphous silicon (Si) -based semiconductor films, but microcrystalline silicon is amorphous silicon. The absorption coefficient is smaller than For this reason, if it is used for the photoactive layer, a film thickness of 2 μm or more is required. Therefore, when considering the productivity of the solar cell, a very high film formation rate is required. However, at present, such a deposition rate cannot be achieved while maintaining good quality characteristics.
[0006]
Therefore, the present inventors solved the conventional problems by using microcrystalline silicon germanium (SiGe), which has a light absorption coefficient larger than that of microcrystalline silicon, for the photoactive layer and reducing the required photoactive layer thickness. We studied earnestly. The following points must be satisfied to solve the problem.
[0007]
In order to make the thickness of the active layer 1 μm or less, an absorption coefficient at least about three times that of microcrystalline silicon is required. For this purpose, the composition ratio of germanium (Ge) in microcrystalline silicon germanium (SiGe) needs to be 20 atomic% or more.
[0008]
The present invention has been made in view of the above circumstances, and an object thereof is to provide a photovoltaic device using a thin microcrystalline silicon-based semiconductor thin film as a photoactive layer.
[0009]
[Means for Solving the Problems]
In the present invention, the composition ratio of germanium is 20 atom% or more and 40 atom% or less, and the signal intensity from the bond between germanium and germanium is 35% with respect to the signal intensity from the bond between silicon and silicon observed by Raman spectroscopy. More than 55% , microcrystalline silicon germanium whose signal intensity from the bond of silicon and germanium is between the two signal intensities is used as the photoactive layer, and the film thickness is 1 μm or less.
[0010]
The signal intensity from the germanium-germanium bond is preferably 35% to 55% with respect to the signal intensity from the silicon-silicon bond observed by Raman spectroscopy.
[0011]
According to said structure, a photovoltaic device with favorable conversion efficiency is obtained using a microcrystalline silicon germanium with a thin film thickness for a photoactive layer.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a cross-sectional view showing a photovoltaic device according to an embodiment of the present invention using a microcrystalline silicon germanium (SiGe) film as a photoactive layer.
[0013]
As shown in FIG. 1, in the photovoltaic device according to the present invention, a highly reflective metal film 2 such as silver (Ag) is formed on a support substrate 1 made of glass, metal or the like. In addition, in order to provide a light confinement effect on the surface of the substrate 1, minute irregularities are formed by etching or the like. The unevenness may be provided on the surface of the highly reflective metal film 2. A transparent conductive film 3 made of ZnO having a thickness of 500 mm is provided on the highly reflective metal film 2. The transparent conductive film 3 prevents an alloying reaction between the n-type microcrystalline silicon (Si) layer 4 and the highly reflective metal film 2 to be formed next.
[0014]
On this transparent conductive film 3, an n-type microcrystalline Si film 4 having a thickness of 300 mm, an i-type microcrystalline SiGe film 5 according to the present invention having a thickness of 5000 mm, and a p-type microcrystalline Si film 6 having a thickness of 300 mm are sequentially laminated. Is formed. A surface transparent conductive film 7 made of ZnO with a thickness of 500 mm is provided on the p-type microcrystalline Si film 6. Further, a comb electrode 8 made of silver or the like is provided on the transparent conductive film 7. Light enters from the transparent conductive film 7 side.
[0015]
The above ZnO film is formed by sputtering, and the n-type microcrystalline Si film 4 and the p-type microcrystalline Si film 6 are formed by parallel plate RF plasma CVD at 13.56 MHz. It should be noted that there is no particular designation of the production method for portions other than the microcrystalline SiGe film 5, and any material can be used as long as the effect of the present invention can be obtained. The transparent conductive films 3 and 7 may be SnO ₂ films other than ZnO films and ITO.
[0016]
By the way, a photovoltaic element using microcrystalline silicon as a photoactive layer usually requires a film thickness of 2 μm or more. However, considering the amount of material used, throughput, element stability, etc., the film thickness of the photoactive layer is 0.1-1.0 micrometer is suitable. Therefore, the i-type microcrystalline SiGe film 5 which is a feature of the present invention is formed as follows.
[0017]
The microcrystalline SiGe film 5 is formed by parallel plate RF plasma CVD at 13.56 MHz at an input power of 200 mW / cm ² , a pressure of 39.9 Pa, and a substrate temperature of 250 ° C. The hydrogen dilution ratio (H ₂ / SiH ₄ + GeH ₄ ) was 30 and the germane flow rate ratio (GeH ₄ / SiH ₄ + GeH ₄ ) was 10%. The power supply frequency of plasma CVD is not particularly specified, and may be a higher frequency or a direct current.
[0018]
When prepared under the above conditions, the Ge composition ratio of the microcrystalline SiGe film 5 is 30 atomic%, and the deposition rate is about 2 Å / sec. The microcrystalline SiGe film 5 is composed of Si, Ge, SiGe crystal grains having a particle diameter of 20 to 300 mm and an amorphous part, and the ratio of the amorphous part is less than 10%. Further, 5000 cm ^-1 respectively the light absorption coefficient 800 nm, it is at 1500 cm ^-1, 1000 nm at 800 cm ^-1 or more at 900 nm, which is about 4 times the value of the microcrystalline silicon. For this reason, the film thickness was set to 5000 mm, which is ¼ that of microcrystalline silicon.
[0019]
FIG. 2 is a Raman spectroscopic spectrum diagram of the microcrystalline SiGe film formed by the above-described method and having a Ge composition ratio of 30 atomic% measured by Raman spectroscopy. Note that when a monochromatic light having a frequency υ ₀ is applied to the material and scattered, Raman lines of Stokes lines υ ₀ -υ _mn and anti-Stokes lines υ ₀ + υ _mn appear. A method for identifying and quantifying a substance by measuring the wavelength and scattering intensity of the Raman line is called Raman spectroscopy.
[0020]
As shown in FIG. 2, in the microcrystalline SiGe film formed by the above-described method and having a Ge composition ratio of 30 atomic%, a peak near 500 cm ⁻¹ is a signal from a silicon-silicon bond (Si—Si), The peak near 400 cm ⁻¹ is the signal from the bond between silicon and germanium (Si—Ge), and the peak near 280 cm ⁻¹ is the signal from the bond between germanium and germanium (Ge—Ge). The signal from Si-Ge is 70% compared to that of Si-Si when compared with peak height, and the signal from Ge-Ge is 50% compared to that of Si-Si. The signal from the Si-Ge bond is between the signals from Si-Si and Ge-Ge.
[0021]
In addition, when the photovoltaic device using this microcrystalline silicon germanium (SiGe) film as the photoactive layer was measured for conversion efficiency under AM-1.5, 100 mW / cm ² light irradiation, the conversion efficiency was 8%. Indicated. This is the same value as the photovoltaic element formed under the same conditions except that microcrystalline Si having a film thickness of 2 μm was used as the active layer, and the same characteristics were obtained with a film thickness of ¼ of the photoactive layer. It will be.
[0022]
Next, for comparison, microcrystalline silicon germanium (SiGe) having a composition ratio of Ge of 30 atomic% is different from the above-described embodiment in the signal intensity measured by Raman spectroscopy. Formed. FIG. 3 is a Raman spectrum obtained by measuring a microcrystalline SiGe film having a Ge composition ratio of 30 atomic% formed for this comparison by Raman spectroscopy. The formation conditions are the same as the above conditions except that the input power is 1000 mW / cm ² and the pressure is 399 Pa.
[0023]
When formed under these conditions, the composition tends to be biased due to the occurrence of polymerization in the gas phase and insufficient surface reaction time. For this reason, the signal from Si—Ge was 45% compared to that of Si—Si, and the signal from Ge—Ge was 70% when compared in terms of peak height.
[0024]
When the conversion efficiency of the photovoltaic device using this microcrystalline silicon germanium (SiGe) film as a photoactive layer was measured under AM-1.5, 100 mW / cm ² light irradiation, the conversion efficiency was 3%. .
[0025]
Next, by changing the input power and pressure, microcrystalline SiGe was formed in which the signal from the Ge-Ge bond changed with respect to the signal intensity from the Si-Si bond, and this film was used as a photoactive layer. A power device was created. FIG. 4 shows the change in conversion efficiency measured for these photovoltaic devices under AM-1.5, 100 mW / cm ² light irradiation.
[0026]
As can be seen from FIG. 4, the signal intensity from the Ge—Ge bond is less than 30% and more than 60% compared to the peak height with respect to the signal intensity from the Si—Si bond observed by Raman spectroscopy. Even a slight change greatly reduces the conversion efficiency. On the other hand, when the signal intensity from the Ge—Ge bond is 30% or more and 60% or less compared with the peak height, the signal intensity from the Si—Si bond observed by Raman spectroscopy. Even if changes slightly, the conversion efficiency changes only slightly. Considering mass production efficiency and the like, it is not preferable that the conversion efficiency changes greatly due to a slight change in composition. For this reason, if the signal intensity from the Ge—Ge bond is 30% or more and 60% or less compared with the peak height with respect to the signal intensity from the Si—Si bond observed by Raman spectroscopy, Even if the composition changes, the conversion efficiency does not change significantly and good characteristics can be obtained. Furthermore, when the signal intensity from the Ge—Ge bond is 35% or more and 55% or less as compared with the peak height, the signal intensity from the Si—Si bond observed by Raman spectroscopy is better. Results are obtained.
[0027]
Next, by 13.56 MHz parallel plate RF plasma CVD, the input power is set to 200 mW / cm ² , the pressure is set to 39.9 Pa, the substrate temperature is 250 ° C., and the hydrogen dilution rate (H ₂ / SiH ₄ + GeH ₄ ) is set to 30. The microcrystalline SiGe film was formed by changing the germanium flow rate ratio (GeH ₄ / SiH ₄ + GeH ₄ ) from 5% to 50% and changing the composition ratio of Ge. The microcrystalline SiGe film formed under these conditions has a signal intensity from the Ge—Ge bond of 30% or more compared to the signal intensity from the Si—Si bond observed by Raman spectroscopy. % Or less. And the photovoltaic apparatus which used this microcrystal silicon germanium film | membrane for the photoactive layer was created. FIG. 5 shows the change in conversion efficiency measured for these photovoltaic devices under AM-1.5, 100 mW / cm ² light irradiation. From FIG. 5, it can be seen that good values are obtained when the Ge composition ratio is between 20 atomic% and 40 atomic%.
[0028]
Next, a second embodiment of the present invention is shown in FIG. FIG. 6 is a sectional view showing a photovoltaic apparatus according to the second embodiment of the present invention. In addition, the same code | symbol is attached | subjected to the same part as above-mentioned embodiment, and description is abbreviate | omitted. This embodiment has a structure in which semiconductor layers having a nip structure are stacked in several stages. That is, a highly reflective metal film 2 and a transparent conductive film 3 are provided on a support substrate 1, and an n-type microcrystalline Si film 4 (4a), an i-type semiconductor film 5 (5a), and a p-type semiconductor film 6 (6a) are provided thereon. ) In several layers in this order.
[0029]
In the embodiment shown in FIG. 6, an n-type microcrystalline Si film 4a, an i-type amorphous Si film 5a, and a p-type amorphous SiC film 6a are formed on the incident side of the photovoltaic element of the embodiment shown in FIG. This is a structure in which photovoltaic elements are stacked. The p-type amorphous SiC film 6a and the i-type amorphous Si film 5a are formed by parallel plate type RF plasma CVD at 13.56 MHz. The rest is the same as the above-described embodiment.
[0030]
In the second embodiment described above, a short-circuit current of 12 mA / cm ² , an open-circuit voltage of 1.30 V, a fill factor of 0.71, and a conversion efficiency of 11% were shown under the same measurement conditions as in the first embodiment. This is also the same value as the photovoltaic element formed under the same conditions except that the microcrystalline SiGe active layer was changed to microcrystalline Si, and the effect of the present invention was shown.
[0031]
The present invention has a structure in which a nip structure semiconductor layer is formed on a substrate as in the first embodiment, and a nip structure semiconductor layer on the substrate as in the second embodiment. Of course, the present invention can be applied not only to a photovoltaic device having a structure in which two layers are formed but also to a stacked photovoltaic device having a structure having three or more layers. Furthermore, the present invention can of course be applied to a photovoltaic device of a type in which light enters from the opposite direction to the above embodiment, that is, a type of light that enters from the substrate side.
[0032]
【The invention's effect】
As described above, according to the present invention, a photovoltaic device with good conversion efficiency can be obtained by using microcrystalline silicon germanium having a small thickness for the photoactive layer.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing a photovoltaic device according to an embodiment of the present invention using a microcrystalline silicon germanium (SiGe) film as a photoactive layer.
FIG. 2 is a Raman spectrum obtained by measuring a microcrystalline SiGe film having a Ge composition ratio of 30 atomic% according to an embodiment of the present invention by Raman spectroscopy.
FIG. 3 is a Raman spectrum obtained by measuring a microcrystalline SiGe film having a Ge composition ratio of 30 atomic% formed for comparison by Raman spectroscopy.
FIG. 4 is a characteristic diagram in which the conversion efficiency of a photovoltaic device using microcrystalline SiGe in which the signal from the Ge—Ge bond is changed with respect to the signal intensity from the Si—Si bond is used for the photoactive layer.
FIG. 5 is a characteristic diagram obtained by measuring the conversion efficiency of a photovoltaic device using microcrystalline SiGe having a changed Ge composition ratio as a photoactive layer.
FIG. 6 is a sectional view showing a photovoltaic element according to a second embodiment of the present invention.
FIG. 7 is a cross-sectional view showing the structure of a conventional photovoltaic device.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Support substrate 2 High reflection metal film 3 Transparent conductive film 4 n-type microcrystalline Si film 5 i-type microcrystalline SiGe film 6 p-type microcrystalline Si film 7 Surface transparent conductive film 8 Comb electrode

Claims (1)

The composition ratio of germanium is 20 atom% or more and 40 atom% or less, and the signal intensity from the bond between germanium and germanium is 35% or more and 55% or less with respect to the signal intensity from the silicon-silicon bond observed by Raman spectroscopy. A photovoltaic device, characterized in that microcrystalline silicon germanium having a signal intensity from a bond of silicon and germanium between the two signal intensities is used as a photoactive layer, and the film thickness is 1 μm or less.

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