JPH0262482B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a hydrogen-containing silicon layer for use in electronic devices incorporating a layer of silicon or one of its alloys, particularly solar cells that can be used to convert photon radiation into electrical energy. The present invention relates to a manufacturing method.

Crystalline silicon has generally been used as a material for conventional solar cells, but although it has good cell characteristics, it has the disadvantage of high manufacturing costs. On the other hand, regarding amorphous silicon, which has been attracting attention recently,
Although it has the advantage of being able to be made thin due to its high light absorption and can be produced using a low-temperature process, it requires long-term growth due to its slow growth rate, and there are also problems with its reliability in long-term use. Furthermore, it has a narrow wavelength sensitivity range in terms of the sunlight spectrum, so it cannot be said to be an optimal material.

An object of the present invention is to provide a solar cell having characteristics that have the advantages of both amorphous silicon and microcrystalline silicon by using a silicon film containing both amorphous silicon and microcrystalline silicon. This film is a material with wavelength sensitivity close to the sunlight spectrum and high light absorption, and because it has small localized levels and high mobility, it exhibits high photoelectric conversion efficiency and exhibits long-term stable characteristics. .

In order to use amorphous silicon as a solar cell, it is essential to include hydrogen in the film to impart photoconductivity, and the film is usually formed by a glow discharge method. The formation temperature is 200 to 300°C, and those formed at temperatures higher than this have an extremely low hydrogen content and are not suitable for use as solar cells.
In addition, it is known that hydrogen is released from silicon films that contain hydrogen during heat treatment at temperatures of 300°C or higher. Therefore, hydrogen-containing silicon films are 300℃
It has been believed that heating above this level has negative effects.

However, when hydrogen plasma treatment was performed after forming the silicon film, the higher the treatment temperature, the better, and sufficient photoconduction was obtained up to a maximum temperature of 550°C. On the other hand, if a silicon film is formed by vacuum evaporation or chemical decomposition or plasma decomposition under reduced pressure, a film containing both amorphous silicon and microcrystalline silicon can be formed depending on the relationship between pressure and formation temperature. It is easy to mold. When the film thus formed is treated with hydrogen plasma at a temperature near the microcrystalization temperature, it exhibits high photoconductivity and light absorption close to that of amorphous silicon, and its wavelength sensitivity is between that of crystalline silicon and amorphous silicon. value. Furthermore, the film is much more reliable than that formed at low temperatures by the glow discharge method. The temperature for microcrystallization varies slightly depending on the silicon film formation method and conditions, but it is 480°C to 540°C for vacuum evaporation and reduced pressure plasma decomposition methods, and 560°C for chemical decomposition methods under reduced pressure. The temperature was between 600℃ and 600℃. The light absorption rate of a silicon film depends on the size of crystal grains. wavelength
The absorption coefficient value at 0.5μm is
1.1×10 ⁴ cm ^-1 and not doped with impurities 250
Amorphous silicon vacuum-deposited at ℃ is 2×10 ⁵ cm ^-1
On the other hand, silicon microcrystallized at 500℃ has a temperature of 4×
It was 10 ⁴ cm ^-1 . As the formation temperature increases, this value approaches that of crystalline silicon and decreases to 550
It was 2.5×10 ⁴ cm ^-1 at ℃. That is, if the film is formed at a temperature higher than this temperature, the solar cell will require a film thickness of several μm or more, and the advantage will be small.

The effect of hydrogen plasma treatment after film formation depends on the crystallinity of the film, and the smaller the crystal grains, the better the low temperature treatment. This effect can be confirmed by measuring the amount of hydrogen in the film by infrared absorption measurement or the like and by changing the conductivity.

For films containing microcrystalline silicon, the most effective temperature range was ±50°C of the microcrystalization temperature T, but not exceeding 550°C. Furthermore, if this hydrogen plasma treatment is once exposed to the atmosphere after film formation, the silicon in the surface layer will be oxidized and cannot bond with hydrogen, reducing its effectiveness.

Example 1 A silicon film was deposited on a heated quartz substrate by irradiating crystalline silicon, which was a deposition source, with an electron beam under a pressure of 10 ^-10 Torr. At this time, the substrate temperature
At 450°C, the deposited silicon film is amorphous, but at 500°C it becomes microcrystalline. First, amorphous silicon was deposited to a thickness of 0.5 μm at a rate of 1 nm per second at a temperature of 450 °C, and then microcrystalline silicon was deposited to a thickness of 0.5 μm at a rate of 1.5 nm per second at a temperature of 500 °C. . Next, hydrogen plasma treatment was performed at 460°C and a pressure of 0.1 Torr for 30 minutes. As a result, while the dark conductivity is 3×10 ^-9 Ω ^-1・cm ^{-1 ,} the light irradiation conductivity at AM1 intensity is 5×10 ^-4 Ω ^-1・cm ^-1 and 10 ⁵
It was found that the change was more than double, and that enough free carriers were generated that could be used as solar cells.
Also, the absorption coefficient value for a wavelength of 0.5 μm is 6.5×10 ⁴
cm ^-1 and showed an intermediate value between amorphous silicon and crystalline silicon.

Example 2 FIG. 1 is an overall view of a reduced pressure plasma CVD apparatus for forming a silicon film. The reaction tube 1 is heated from the outside by a furnace 2, and a stainless steel substrate 3 is held by an electrode 4. Reference numeral 5 indicates a high frequency power supply, and reference numeral 6 indicates a vacuum pump. After drawing the inside of the reaction tube 1 into a high vacuum, the temperature below which microcrystallization occurs
SiH ₄ and PH ₃ at a temperature of 300℃ SiH ₄ : PH ₃ =
The flow rate was 10:1, and the total pressure was 1 Torr. In this state, a high frequency voltage was applied to the electrode 4 to cause plasma discharge inside the reaction tube. First, after growing an n-type layer to a thickness of 30 nm on the substrate 3, the PH ₃ was stopped.
After adjusting the pressure to 1 Torr, add a non-doped layer of 300n.
I grew m. Next, the temperature in the reactor was set to 500℃, near the temperature of microcrystalization, and a non-doped layer was subsequently formed.
It was grown to 400nm. Finally at this temperature
SiH ₄ and B ₂ H ₆ were flowed at a ratio of SiH ₄ :B ₂ H ₆ =20:1, and a 10 nm p-type layer was grown at a pressure of 1 Torr.
In this state, the reaction tube was evacuated to a high vacuum, hydrogen was flowed in, the pressure was set to 0.1 Torr, plasma was discharged, and 15
Hydrogen plasma treatment was performed for 1 minute.

After taking out from the reactor, a transparent conductive film was deposited on the surface to form a solar cell, and under simulated sunlight of AM1, the open circuit voltage was 0.81 V, and the short circuit current was 7.8 mA/
^cm2 was obtained. Furthermore, for the solar spectrum
It exhibited sensitivity to wavelengths from 350 nm to 900 nm, and exhibited characteristics intermediate between crystalline silicon solar cells and amorphous solar cells.

Example 3 This will be explained using FIG. 2. Film forming temperature 460-485
A large number of samples prepared by changing the temperature within a range of .degree. C. were subjected to plasma treatment, and the photoconductivity of the samples was measured, and the results were as shown in FIG. It can be seen from FIG. 2 that the higher the film-forming temperature, the higher the microcrystal content, and the more pronounced the effect of hydrogen plasma treatment on photoconductivity becomes.

Example 4 This will be explained using FIG. 3. A large number of samples prepared in the same manner as in Example 1 were subjected to hydrogen plasma treatment at different heating temperatures. Its temperature range is 10~
The temperature is 550℃. The conductivity of these samples (without light irradiation) was measured, and the results were as shown in FIG. The inventors of the present invention found from FIG. 3 that the conductivity (without light irradiation) decreased significantly in the range of 430 to 550°C, that is, the effect of hydrogen plasma treatment was significant. This temperature range is a range in which it was considered impossible to form a hydrogen-containing silicon film based on conventional knowledge.

The present invention is useful in chemical or plasma decomposition methods.

According to the present invention, it is possible to produce a solar cell that takes advantage of the advantages of both amorphous silicon solar cells and crystalline silicon solar cells. That is, since the light absorption is large, the film thickness may be several micrometers or less, and by changing the ratio of the thickness of the amorphous layer to the microcrystalline layer depending on the application, a film with arbitrary wavelength sensitivity can be obtained. In terms of photoelectric conversion efficiency, by microcrystallizing the surface-side junction layer, the localized level density in the depletion layer is reduced, making it possible to achieve a current density close to that of a crystalline silicon solar cell, resulting in high conversion efficiency. Furthermore, since the device is subjected to heat treatment near the microcrystallization temperature, the reliability of the device is significantly improved.

[Brief explanation of the drawing]

FIG. 1 is an overall view of a low pressure plasma CVD apparatus for forming a silicon film. FIG. 2 is a diagram showing the results of Example 3. Figure 3 shows Example 4.
FIG. 1...Reaction furnace, 2...Heating furnace, 3...Substrate, 4
...High frequency electrode, 5...High frequency power supply.

Claims (1)

[Claims] 1. Production of a hydrogen-containing silicon layer characterized by subjecting a silicon layer formed under reduced pressure, in which amorphous silicon and microcrystalline silicon coexist, to hydrogen plasma treatment at 430 to 550°C before exposing it to the atmosphere. Method.