JP4780930B2 - Method for manufacturing photoelectric conversion device - Google Patents

Description

  The present invention relates to a photoelectric conversion device such as a solar cell using an amorphous Si semiconductor thin film containing at least Si (silicon) and H (hydrogen), and a photovoltaic device using the photoelectric conversion device.

  Devices using amorphous Si-based films represented by hydrogenated amorphous silicon (hereinafter abbreviated as a-Si: H) are very diverse today, but in recent years the world has been facing energy and global environmental problems. Solar cells have been attracting particular attention as interest in

  The feature of amorphous Si in solar cells is that the light absorption coefficient is very large compared to crystalline Si (including microcrystalline Si film), so the film thickness required for the photoelectric conversion layer is the same as that of crystalline Si. It can be a fraction (0.5 μm or less), and when forming a film by a CVD method (chemical vapor deposition method) such as PECVD method (plasma CVD method), it is much more productive than using crystalline Si. It is that it is excellent in. However, on the other hand, it has been known for a long time that amorphous Si generally has a photodegradation phenomenon called Staebler-Wronski effect, and solar cells using this film have an efficiency of about 10-15% by light irradiation. We continue to have the problem of falling.

Although the mechanism of the photodegradation phenomenon has not yet been clarified, it is believed that the phenomenon is related to hydrogen, and several approaches have been attempted so far. That is, [1] reduce the Si—H ₂ (die hydride) bonding state in the film (increase the Si—H (monohydride) bonding state ratio), [2] reduce the hydrogen concentration in the film, [3] film For example, fine crystal grains are appropriately contained therein. [1] and [2] have overlapping portions.

[1] is based on the fact that a film having a high Si—H ₂ density in the film has a large photodegradation. Regarding a specific method for reducing the Si—H ₂ density in the film, research has been conducted by many organizations under various hypotheses. For example, the origin of the Si—H ₂ bonding state in the film is considered to be in the high-order silane molecules in the gas phase is sterically hindered disturbing the film growth reaction by incorporated into the film, or film-forming surface, SiH ₂ molecules involved in reaction for producing the high-order silane molecules There is something that tries to reduce the generation of. Specifically, in the PECVD method, a low electron temperature plasma such as VHF plasma is used, the hydrogen dilution rate is optimized (minimizing the electron temperature), and the substrate temperature is raised to some extent (the upper limit is 350 ° C. Attempts have been made to promote the hydrogen desorption reaction from the surface of film formation, and a-Si: H with a stabilization efficiency of 8.2% and a photodegradation rate of 12% under the condition of a film formation rate of 2 nm / s. Single element characteristics are obtained (see Non-Patent Document 1). However, the efficiency and the light deterioration rate are still not sufficient.

  As an example of [2], a method of extracting excess hydrogen from a film growth surface layer while alternately repeating film deposition and hydrogen plasma treatment in film formation by PECVD is proposed (Patent Document 1, Patent Document 1). 2 and 3). However, with this method, the hydrogen concentration in the film is not sufficiently reduced as expected, and it is difficult to obtain a high film forming speed because the film deposition and hydrogen plasma treatment are repeated many times. The problem is that it is very inferior in productivity. Moreover, high efficiency and low light deterioration rate have not been achieved at the same time.

  [3] uses an amorphous Si film which is supposed to be obtained in the vicinity of the phase boundary region between amorphous Si and microcrystalline Si, and has recently begun to attract attention. This film has not yet been given a definitive name and is called “Protocrystalline Si: H” (see Non-Patent Document 2: Wronski et al.) Or “nanocrystal-embedded amorphous” by research institutions. “Silicon (crystals with fine grain size dispersed in an amorphous matrix)” (see Non-Patent Document 3: AIST), or “Nanostructure-controlled Si film (crystalline silicon nanoclusters in a-Si: H Is well-dispersed) ”(see Non-Patent Documents 4 and 5: Kyushu Univ.), But the common concept is“ having amorphous film characteristics, but photodegradation It is an amorphous Si film containing nano-sized crystal grains that is greatly reduced (or hardly photo-degraded).

  In the present specification, when it is particularly desired to designate a film containing nano-sized crystal grains among the amorphous Si films, it will be referred to as a “nanocrystal grain-containing amorphous Si film” in this specification. To do.

  By the way, there are at least two reasons why the nanocrystalline grain-containing amorphous Si film exhibits a very small photodegradation rate (or hardly undergoes photodegradation). (1) Since nano-sized crystal grains are scattered and distributed in the film, carrier recombination occurs preferentially in the crystal grains and recombination in the amorphous region is reduced (a-Si: H film). (2) The nano-sized crystal grains in the film are considered to be caused by the weak Si-Si bond in the amorphous network being cut by the energy released during carrier recombination) This is because the network structure of the amorphous Si film itself is less likely to be optically deteriorated due to the distributed distribution (due to strain relaxation, etc.), but in any case, it exists in the film. Nanosized crystal grains are considered to play an important role.

However, even in the solar cell using the nanocrystalline grain-containing amorphous Si film, although the reduction in the photodegradation rate is certainly observed, the initial efficiency is still low (for example, non-patent) (In Reference 3, the initial efficiency is 6.28% and the stabilization efficiency is 6.03%).
JP-A-5-166733 JP-A-6-120152 JP 2002-9317 A Applied Physics Vol.71 No.7 (2002) p.823 Twenty-Ace I Triple E Photovoltaic Specialists Conference 2000, p.750 The 63rd Autumn Meeting of Applied Physics (2002) p.838 (25p-ZM-3) The 63rd Autumn Meeting of Applied Physics (2002) p.27 (25p-B-4) The 63rd Autumn Meeting of Applied Physics (2002) p.845 (24p-ZM-9)

  As described above, in the method [1], the stabilization efficiency and the light degradation rate are still not sufficient. In the method [2], the hydrogen concentration in the film is not sufficiently reduced as expected, and the film deposition rate and the hydrogen plasma treatment are repeated many times. It is difficult to obtain, and has the problem that productivity is remarkably bad. Also, high efficiency and low light degradation rate cannot be achieved at the same time. Further, in the method [3], although a reduction in the light degradation rate is recognized, the initial efficiency is still low.

  Accordingly, the present invention has been made in view of the above-described problems, and an object of the present invention is to provide a high-efficiency and low light deterioration rate photoelectric conversion using an amorphous Si semiconductor film having high quality and excellent light stability. It is to implement | achieve a converter and a photovoltaic device using the same.

In order to achieve the above object, the method for producing a photoelectric conversion device of the present invention contains at least silicon and hydrogen, and the ratio of the TA mode scattering peak intensity to the TO mode scattering peak intensity obtained by the Raman scattering spectrum is 0. .35 or less , the half-width of the scattering peak of the TO mode obtained by the Raman scattering spectrum is 64 to 65 cm ⁻¹ or less, and the Si— with respect to the sum of the existence density of the Si—H bonding state and the existence density of the Si—H ₂ bonding state. When the ratio of the density of H-bonded states is 0.95 or more, the crystallization rate defined by the Raman scattering spectrum is 30% or less, and the crystalline silicon grains are included, the maximum grain size is 5 nm or less. For the semiconductor multilayer film of the photoelectric conversion device including a semiconductor multilayer film including a semiconductor thin film made of crystalline silicon as a photoactive portion, Light irradiation intensity 100 mW / cm ² using a solar like, and performing light irradiation treatment light irradiation time 200 hours, and with respect to the temperature T of 40 to 50 ° C. under conditions of T ± 2 ° C..

According to the photoelectric conversion device of the present invention, the ratio I _TA / I _TO of the TA mode scattering peak intensity I _TA to the TO mode scattering peak intensity I _TO containing at least silicon and hydrogen and obtained by the Raman scattering spectrum is 0.35. A semiconductor multilayer film including a semiconductor thin film made of the following amorphous silicon is provided, and the semiconductor multilayer film is irradiated with a light irradiation intensity of 100 mW / cm ² using pseudo sunlight, a light irradiation time of 200 hours, and 40 Since the open-circuit voltage after the light irradiation treatment performed under the condition of T ± 2 ° C. for the temperature T of ˜50 ° C. is higher than the open-circuit voltage before the light irradiation treatment, the SRO is higher than that of the conventional amorphous Si film. It is possible to provide a photoelectric conversion device typified by a solar cell having an amorphous Si film having greatly improved high quality and high light stability and having a high efficiency and a greatly reduced light deterioration rate.

The photoelectric conversion device of the present invention includes an amorphous Si semiconductor thin film containing at least silicon and hydrogen and having a half-value width of a scattering peak of a TO mode obtained by a Raman scattering spectrum of 65 cm ⁻¹ or less. The semiconductor multilayer film is provided with a light irradiation intensity of 100 mW / cm ² using pseudo-sunlight for the semiconductor multilayer film, a light irradiation time of 200 hours, and a temperature T of 40 to 50 ° C. T ± 2 ° C. Since the open-circuit voltage after the light irradiation process performed under the conditions is higher than the open-circuit voltage before the light irradiation process, the SRO is greatly improved compared to the conventional amorphous Si film, and the light quality and the light stability are improved. It is possible to provide a photoelectric conversion device typified by a solar cell having an amorphous Si film and having a high efficiency and a significantly reduced light deterioration rate.

Further, according to the photoelectric conversion device of the present invention, the Si—H bonding state relative to the sum of the existence density of the Si—H bonding state and the existence density of the Si—H ₂ bonding state in the amorphous Si semiconductor thin film. Since the ratio of abundance density is 0.95 or more, it can be an amorphous Si film having high quality and high photostability with further improved SRO compared to the conventional amorphous Si film, A photoelectric conversion device having excellent characteristics using such an amorphous Si film can be realized.

  Furthermore, according to the photovoltaic device of the present invention, since the photoelectric conversion device is used as a power generation means and the generated power of the power generation means is supplied to a load, it is possible to provide a highly efficient photovoltaic power generation device. it can.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of a photoelectric conversion device of the present invention and a photovoltaic power generation device using the photoelectric conversion device will be described in detail.

  In order to obtain the amorphous Si film of the present invention, a Cat-PECVD method (cathode-type plasma CVD method with a built-in thermal catalyst; Japanese Patent Application No. 2001-313272, Japanese Patent Application No. 2001-293031), which is a kind of PECVD method, An example formed using the reference will be described below. As used herein, the term “amorphous Si film” refers to a film whose crystallization rate falls within the range of 0 to 60% by Raman scattering spectroscopy (that is, the amorphous Si film is not non-crystalline). Contrary to that, the crystalline Si film means that the crystallization rate falls within the range of 60 to 100%.

<Cat-PECVD method>
When the Cat-PECVD method is used, the amorphous Si film of the present invention uses a VHF band (27 MHz or higher: usually about 40 to 80 MHz) as a plasma excitation frequency, and Ta (tantalum) provided inside the cathode. The temperature of the thermal catalyst made of a high melting point material such as W (tungsten) or C (carbon) is 1400 to 2000 ° C., the H ₂ / SiH ₄ gas flow ratio is 2 to 20, the substrate temperature is 100 to 350 ° C., gas It is obtained under conditions where the pressure is set to 13 to 665 Pa and the VHF plasma power density is set to 0.01 to 0.5 W / cm ² . Here, H ₂ and SiH ₄ are guided through different gas introduction paths in a separated state until they are released into the film forming space, and the H ₂ gas introduction path includes a part of the path. Although the thermal catalyst provided in the cathode is disposed, the thermal catalyst is not disposed in the SiH ₄ gas introduction path. In this way, only H ₂ is heated and activated by the thermal catalyst, so that film deposition and consumption are performed in the gas introduction path until SiH ₄ is decomposed and activated by the thermal catalyst and released into the film forming space. Thus, the effect of using the thermal catalyst described later can be obtained.

In the Cat-PECVD method, a high-order silane formation reaction (SiH ₂ insertion reaction into Si _n H _m molecules), which is an exothermic reaction, is suppressed by a gas heating effect resulting from the use of a thermal catalyst, so that the film formation space The density of higher order silane molecules in can be kept low, and the incorporation of higher order silane molecules into the film and the occurrence of steric hindrance on the film forming surface are reduced. As a result, it is possible to form an amorphous Si film in which the existence density of Si—H ₂ bonding states in the film is greatly reduced.

Further, since the gas heating effect also has an effect of lowering the electron temperature of the plasma, SiH ₂ generation by the one-electron collision reaction between the source gas SiH ₄ and the electrons in the plasma space (also for the generation of SiH ₃ by the one-electron collision reaction). Compared with this route, higher order silane generation reaction (SiH ₂ insertion reaction into Si _n H _m molecule) can be suppressed also by this route.

  Further, the Cat-PECVD method can promote the generation of hydrogen radicals, so the hydrogen abstraction reaction from the surface of the film can be promoted and the hydrogen concentration in the film can be effectively reduced.

In addition, Si clusters (higher order silanes) are formed in the gas phase under film forming conditions in which the hydrogen dilution ratio (H ₂ / SiH ₄ gas flow rate ratio) is increased, the thermal catalyst temperature Tcat is increased, or the VHF power is increased. The growth of molecules is said to be promoted by the generation of the above-mentioned gases, which are molecules grown to some extent and usually have a size of up to about 10 nm, including those having an amorphous structure and those having a crystal structure. With the heating effect and the hydrogen radical generation promoting effect, it is possible to control the cluster size and density, and further promote the crystallization of the cluster while suppressing the growth to particles and powder. That is, when the Si clusters are taken into the film, a high-quality nanocrystal-grain-containing amorphous Si film in which the size and density of the crystal grains contained in the film are controlled can be formed.

  Due to the effects described above, it is possible to obtain a high-quality amorphous Si film excellent in light stability used in the present invention.

  In Cat-PECVD method, crystallization can be promoted by the effect of introducing a thermal catalyst even under the film forming conditions that were difficult with the conventional PECVD method. Therefore, in terms of controlling the crystallization rate described below. Particularly preferred. Here, the origin of the crystallization region in the film is (1) due to the crystallization reaction on the surface of the film formation, and (2) Si fine particles (nanometer size crystal clusters) generated in the plasma space. There are at least two types of those caused by incorporation into the film.

<Substrate temperature>
The substrate temperature is in the range of 100 ° C to 350 ° C. This is because if the substrate temperature is lower than 100 ° C., the hydrogen extraction effect does not work effectively and the hydrogen concentration in the film cannot be reduced effectively, and if the substrate temperature is higher than 350 ° C., the hydrogen from the film growth surface cannot be reduced. This is because the detachment of the metal becomes remarkable and the dangling bond density in the film increases, and a high-quality film cannot be obtained.

<Raman scattering characteristics>
When describing the structure of an amorphous Si film, the degree of order is often handled, and in particular, the short range order (SRO) has a very close relationship with film quality and light stability. It is said that. As a representative method for evaluating the vibration dynamics of an amorphous network reflecting this SRO, Raman scattering spectroscopy is known.

In general, Raman scattering spectrum of amorphous system Si film, TO (horizontal optical vibration) band having a peak near 480 cm ^-1, LO (longitudinal optical vibrations) having a peak near 385cm ^-1 band, 305 cm ^-1 It consists of an energy band of an LA (vertical acoustic vibration) band having a peak in the vicinity and a TA (lateral acoustic vibration) band having a peak in the vicinity of 160 cm ⁻¹ . Table 1 shows the Raman scattering measurement spectrum of the Si film corresponding to the film used in the device of the present invention and the comparative conventional device, obtained by separating the four energy bands (modes) by the Gaussian type + Lorentzian type approximate curve. The TA / TO scattering peak intensity ratio (I _TA / I _TO ), which is the ratio of the TA mode scattering peak intensity (I _TA ) to the TO mode scattering peak intensity (I _TO ), and the TO mode scattering peak half width ( It is shown HW _tO). Here, the Raman spectrum was measured using a Renishaw Ramanscope System 1000 using a He—Ne laser (wavelength 632.8 nm) as excitation light.

It is known that the distribution width Δθ of the Si bond angle θ has a positive correlation with the TA / TO scattering peak intensity ratio (hereinafter, I _TA / I _TO ) or the TO band half width (HW _TO ). It has been. Here, regarding I _TA / I _TO , as shown in Table 1, in the films used for the conventional elements 1 and 2, I _TA / I _TO = 0.38 (I _TA / I of the conventional amorphous Si film) _It can be seen that _TO takes a value larger than 0.35), whereas the film used in the present invention elements 1 to 4 has a value of 0.35 or less, which is greatly reduced from the conventional value. Also, look at the _{HW TO,} 66cm in film used in the conventional elements 1 ^-1, whereas a 68cm ^-1, the film used in the present invention device with narrowed than this 65cm ^-1 It turns out that it is the following.

  The above shows that the variation Δθ in the Si bond angle of the film used in the element of the present invention is smaller than that of the conventional one, that is, the SRO is improved. This structural change (improvement) related to SRO is considered to be the main cause of realizing a highly photostable photoelectric conversion element in which the light deterioration rate of the present invention is greatly reduced.

Incidentally, SRO improving the above effect, although I _{TA /} I _TO is as observed in the case of 0.35 or less, more preferably 0.25 or less.

<ESR spin density>
In the amorphous Si film used in the element of the present invention, the Si dangling bond density, which is the ESR spin density measured by the electron spin resonance method, is about 3 × 10 ¹⁵ cm ⁻³ before light irradiation (initial stage). The film used for the conventional device had a value exceeding 5.0 × 10 ¹⁵ cm ⁻³ . When the film has a dangling bond density exceeding 5.0 × 10 ¹⁵ cm ⁻³ , the recombination current in the same layer increases when applied to the active layer, and high device characteristics cannot be obtained. JES-RE3X manufactured by JEOL Ltd. was used for ESR spin density measurement.

<FT-IR characteristics (abundance ratio of Si-H bonded state)>
Further, in the amorphous Si film used in the element of the present invention, the Si—H bond state of the Si—H bond state existence density σ _S—H evaluated by FT-IR (Fourier transform infrared spectroscopy) analysis. the sum of the density sigma _S-H and the density of _{Si-H 2} bonds state _{σ Si-H2 (σ S-} H + σ Si-H2) for the ratio _{(σ S-H / (σ} S-H + σ Si-H2) Hereinafter referred to as Si-H bond state existence ratio) was 0.97 or more. As already described in the description of the prior art, this Si—H bond state existence ratio has a large correlation with light deterioration, and the higher this ratio (the more dominant the Si—H bond), the light deterioration. Has been reported to be smaller. In the Si film used for the element of the present invention, the Si-H bonding state existence ratio obtained by the conventional plasma CVD method is significantly higher than about 0.90, and the effect of suppressing the photodegradation appears to be remarkable 0.95 or more The value of is obtained. Here, specific results are shown in Table 2. Table 2 shows the results obtained by the Cat-PECVD method with the catalyst body temperature as a parameter. From the table, the Si-H bond state existence ratio is 0.95 or more (in this example, there are only two examples, 0.967 of the present invention film 1 and 0.997 of the present invention film 2, but 0.934 of the conventional film 2 and the present invention film 1) In the middle of 0.967 (0.951), it seems that there is an inflection point. Note that FTIR-8300 manufactured by Shimadzu Corporation was used for the measurement of FT-IR characteristics. The table also shows that the Cat-PECVD method is a very suitable method for this control.

<Hydrogen concentration in membrane>
Further, the hydrogen concentration in the amorphous Si film used in the element of the present invention was evaluated and calculated to be about 5 at% by FT-IR analysis, and the conventional film was evaluated and calculated to be about 10%. At this time, the hydrogen concentration in the film was calculated using an absorption peak area in the vicinity of 630 cm ⁻¹ and A value = 1.6 × 10 ¹⁹ cm ⁻² . As described above, the Si film used in the element of the present invention has a low hydrogen concentration of 7 at% or less, which is considered to contribute greatly to the high light stability of the element of the present invention. When the hydrogen concentration in the film is larger than 7%, the existence density of the Si—H ₂ bonded state in the film is increased, and the photodegradation rate is increased.

<Optical band gap>
Further, the optical band gap energy Eg.opt of the amorphous Si film used in the element of the present invention was evaluated to be around 1.6 eV by a so-called cube root plot, and the conventional film was about 1.8 eV. As described above, the optical band gap energy Eg.opt of the Si film used in the element of the present invention is narrowed to 1.7 eV or less, and it is confirmed that the Si film is an amorphous Si film excellent in long wavelength sensitivity characteristics. It was done. When the optical band gap energy Eg.opt is larger than 1.7 eV, when the film is used for the photoactive layer, long-wavelength light cannot be sufficiently absorbed and photoelectrically converted, resulting in a decrease in photocurrent, and device characteristics. Invite the decline.

<Band gap adjusting element>
The band gap energy Eg of the amorphous Si film can be adjusted to some extent by the hydrogen concentration in the film. However, if it is desired to adjust Eg while keeping the hydrogen concentration in the film low, an Eg adjusting element is added. That's fine. Specifically, a gas containing molecular formula such as Ge or Sn for narrow gap and C, N or O for wide gap may be introduced into the film forming space. That is, when Ge is contained, GeH ₄ (H includes deuterium D), Ge _n H _{2n + 2} (n is a positive integer, the same shall apply hereinafter), GeX ₄ (X is a halogen element), etc. contain Sn. _SnH 4 in the case of (H contains deuterium _{D), Sn n H 2n +} 2, SnX 4 (X is halogen), _SnR 4 (R is an alkyl group) and the like, CH ₄ in the case of incorporating the C (H includes deuterium D), C ₂ H ₂ , C _n H _{2n + 2} , C _n H _2n , CX ₄ (X is a halogen element), etc., and N ₂ , NO _n , N _{When 2} O, NH ₃ or the like is contained, O ₂ , CO, CO ₂ , NO _n , N ₂ O, H ₂ O or the like may be introduced.

<Crystal grain size>
Here, when the amorphous Si film used in the present invention is a nanocrystal grain-containing amorphous Si film containing nano-sized crystalline Si grains, the nano-size contained in the amorphous Si film The maximum grain size of the crystal grains is 1 nm to 5 nm, more preferably 1.5 nm to 3 nm. If the maximum particle size is smaller than 1 nm, the number of Si atoms constituting the film is too small to realize a significant reduction in band gap energy relative to the amorphous Si film, and substantially the same film characteristics as the amorphous Si film. End up. Further, when the maximum particle size exceeds 5 nm, it becomes difficult to disperse and distribute in the amorphous Si film at an appropriate distance interval. The effect of improving the quality matrix network structure (such as SRO described later) is reduced.

<Crystalization rate>
When the amorphous Si film is a nanocrystal grain-containing amorphous Si film, the control of the crystallization rate expressed as the volume fraction occupied by the nanocrystal grains in the film is controlled by the hydrogen dilution rate ( H ₂ / SiH ₄ gas flow ratio), thermal catalyst temperature (Tcat), VHF power density, film forming gas pressure, and substrate temperature can be freely performed in the range of 0 to 100%, but crystallization The rate is adjusted within a range of 30% or less, and more preferably within a range of 10% or less. When the crystallization rate exceeds 30%, the characteristics as an amorphous system are not sufficiently exhibited, and the deterioration of the characteristics cannot be ignored.

  At this time, it is possible to further improve the efficiency of the photoelectric conversion element (not shown) by causing the crystallization rate to be inclined, stepped, or periodic in the film thickness direction. That is, in the case of a gradient distribution or a stepped distribution, the crystallization rate is increased from the light incident side (front surface side) toward the back surface side. By doing so, it is possible to obtain a band profile in which the optical band gap energy decreases from the front side to the back side, and more efficient than when the optical band gap energy is constant in the film thickness direction. Photoelectric conversion (that is, higher efficiency) becomes possible. In the case of a periodic distribution, the length of one cycle is 100 nm or less, preferably 10 nm or less. In this way, a band profile in which the optical bandgap energy is periodically modulated can be obtained, and more efficient photoelectric conversion (that is, compared to the case where the optical bandgap energy is constant in the film thickness direction) High efficiency). In particular, when the period length is 10 nm or less, a quantum size effect (formation of a quantum well structure) starts to appear, and higher efficiency can be achieved. Here, the control of the crystallization rate (that is, the reduction of the crystallization reaction on the film forming surface and the control of the size and density of the nanoclusters in the gas phase) is preferably performed by controlling the temperature of the thermal catalyst. . That is, the temperature control of the thermal catalyst can be performed very quickly and with high accuracy, which is very suitable for controlling the crystallization rate at high speed and with high accuracy during film formation.

The crystallization rate in the present specification is defined as crystal phase peak intensity / (crystal phase peak intensity + amorphous phase peak intensity) in the spectrum obtained by Raman scattering spectroscopy. a peak intensity at a peak intensity + 520 cm ^-1 in crystalline phase peak intensity = 500~510cm ^-1, also intended to peak intensity in the definition of an amorphous phase peak intensity = 480 cm ^-1. The crystallization ratios shown in Table 1 were evaluated by the Raman scattering spectroscopy.

<Photoelectric conversion device>
Next, a photoelectric conversion element such as a solar cell which is the photoelectric conversion device of the present invention will be described in detail. As described above, the photoelectric conversion device of the present invention contains at least silicon and hydrogen, and the ratio of the TA mode scattering peak intensity I _TA to the TO mode scattering peak intensity I _TO obtained by the Raman scattering spectrum is 0.35 or less. A semiconductor multilayer film including at least one semiconductor thin film made of amorphous silicon as a photoactive portion is provided. The semiconductor multilayer film has a light irradiation intensity of 100 mW / cm ² using pseudo-sunlight, a light irradiation time. The deterioration rate of the conversion efficiency of photoelectric conversion before and after the light irradiation treatment is 7% or less by the light irradiation treatment performed under the condition of T ± 2 ° C. for 200 hours and the temperature T of 40 to 50 ° C., and The open circuit voltage after the light irradiation process is higher than the open voltage before the light irradiation process. Alternatively, as described above, an amorphous Si semiconductor thin film containing at least silicon and hydrogen and having a half-value width of a scattering peak of TO mode obtained by a Raman scattering spectrum of 65 cm ⁻¹ or less is used as a photoactive portion. A semiconductor multilayer film including a layer is provided. The semiconductor multilayer film has a light irradiation intensity of 100 mW / cm ² using pseudo sunlight, a light irradiation time of 200 hours, and a temperature T of 40 to 50 ° C. T ± 2 The deterioration rate of the conversion efficiency of photoelectric conversion before and after the light irradiation treatment is 7% or less due to the light irradiation treatment performed under the condition of ° C, and the open voltage after the light irradiation treatment is the open voltage before the light irradiation treatment. Higher. As described above, in these photoelectric conversion elements, the Si—H bond state relative to the sum of the density of Si—H bond state and the density of Si—H ₂ bond state in the amorphous Si semiconductor thin film. It is more desirable that the ratio of the existence density of the carbon atom be 0.95 or more.

  FIG. 1 shows an example of a super straight type tandem thin film Si (silicon) solar cell element as a photoelectric conversion element of the present invention. In the figure, 1 is a translucent substrate, 2 is a first electrode as a surface electrode, 3 is a semiconductor multilayer film, 31 is a first semiconductor junction layer, 32 is a second semiconductor junction layer, and 31a is a first semiconductor junction layer. A p-type semiconductor layer in the semiconductor junction layer, 31b is a photoactive layer in the first semiconductor junction layer, 31c is an n-type semiconductor layer in the first semiconductor junction layer, and 32a is p in the second semiconductor junction layer. 32b is a photoactive layer in the second semiconductor junction layer, 32c is an n-type semiconductor layer in the second semiconductor junction layer, and 4 is a second electrode as a back electrode. A thick arrow indicates the incident direction of light (hν).

  Here, an intermediate layer made of a transparent conductive film, a thin metal layer, or silicide may be inserted between the semiconductor layer 31c and the semiconductor layer 32a (not shown).

  In order to manufacture such a photoelectric conversion element, first, a translucent substrate 1 is prepared. Here, as the translucent substrate 1, a plate material or a film material made of glass, plastic, resin, or the like can be used.

Next, the 1st electrode 2 which is a surface electrode is formed on the translucent board | substrate 1. FIG. A known oxide transparent conductive film can be used as the first electrode 2. Specifically, materials such as SnO ₂ that is tin oxide, ITO that is indium-tin oxide, and ZnO that is zinc oxide can be used. The film thickness of the transparent conductive film is adjusted in the range of about 60 to 600 nm in consideration of the antireflection effect and low resistance. As a guide for lowering the resistance, it is desirable that the sheet resistance is about 10Ω / □ or less. The transparent conductive film is exposed to an active hydrogen gas atmosphere due to the use of SiH ₄ and H ₂ when a Si film is subsequently formed on the film. It is desirable to form an excellent ZnO film at least as the final surface. At this time, the thickness of the ZnO film is in the range of 10 to 100 nm. As a method for forming the transparent conductive film, known techniques such as a thermal CVD method, a vapor deposition method, an ion plating method, a sputtering method, a spray pyrolysis method, and a sol-gel method can be used.

  In addition, if a concavo-convex structure is formed on the surface of the glass substrate by a method such as RIE treatment or blasting before forming the transparent electrode, and then the transparent electrode is formed on the concavo-convex structure surface, the light scattering effect is enhanced and the light confinement effect is promoted. It is suitable for improving efficiency.

  Next, the semiconductor multilayer film 3 is formed. The semiconductor multilayer film 3 includes a first semiconductor junction layer 31 and a second semiconductor junction layer 32. As a manufacturing method, in addition to the known PECVD method and Cat-CVD method, the present inventors have already disclosed a Cat disclosed in Japanese Patent Application Nos. 2000-130858, 2001-293031, and 2002-38686. -PECVD method can be used.

  First, a first semiconductor junction layer 31 is formed on the first electrode 2. The semiconductor junction layer 31 is a pin junction in which a p-type semiconductor layer 31a, a photoactive layer 31b (substantially i-type layer), and an n-type semiconductor layer 31c are sequentially stacked.

As the p-type semiconductor layer 31a, an amorphous Si film or a crystalline Si film can be used. The film thickness is adjusted in the range of about 2 to 100 nm depending on the material. The doping element concentration is about 1 × 10 ¹⁸ to 1 × 10 ²¹ / cm ³ and is substantially p ⁺ type. If an appropriate amount of a gas containing C (carbon) such as CH ₄ in addition to a gas such as SiH ₄ and H ₂ used for film formation and B ₂ H ₆ as a doping gas is mixed, the Si _x C _1-x film is formed. It is obtained and is very effective for forming a window layer with little light absorption loss, and is also effective for reducing a dark current component for improving an open circuit voltage. In addition to C, the same effect can be obtained by mixing an appropriate amount of a gas containing O (oxygen) or a gas containing N (nitrogen).

For the substantially i-type semiconductor layer that is the photoactive layer 31b, the above-described high-quality, high-light-stable amorphous Si film formed by the Cat-PECVD method is used, and the film thickness is 0.1. Adjust within the range of ~ 0.5μm. At this time, the film formation conditions are: plasma excitation frequency 60 MHz, thermal catalyst temperature 1600-2000 ° C., H ₂ / SiH ₄ flow rate ratio 5-10, substrate temperature 200-250 ° C., gas pressure 133-200 Pa, The VHF power density is set to 0.1 to 0.3 W / cm ² . By adopting such film forming conditions, the above-described high-quality and light-stable amorphous Si film can be formed at a high film-forming speed of 2 nm / sec. Here, since the non-doped film usually shows a slight n-type characteristic, in this case, the non-doped film is adjusted so as to be substantially i-type by slightly containing the p-type doped element. Can do. In some cases, n ^- type or p ^- type may be used for the purpose of fine adjustment of the internal electric field strength distribution.

For the n-type semiconductor layer 31c, an amorphous Si film or a crystalline Si film can be used. The film thickness is adjusted in the range of about 2 to 100 nm depending on the material. The doping element concentration is about 1 × 10 ¹⁸ to 1 × 10 ²¹ / cm ³ and is substantially n ⁺ type. A Si _x C _1-x film can be obtained by mixing an appropriate amount of a gas containing C (carbon) such as CH ₄ in addition to gases such as SiH ₄ and H ₂ used for film formation and PH ₃ as a doping gas. Therefore, it is possible to form a film with little light absorption loss and to reduce dark current components for improving the open circuit voltage. In addition to C, the same effect can be obtained by mixing an appropriate amount of a gas containing O (oxygen) or a gas containing N (nitrogen).

In order to further improve the junction characteristics, a substantially i-type non-single crystal Si is formed between the p-type semiconductor layer 31a and the photoactive layer 31b or between the photoactive layer 31b and the n-type semiconductor layer 31c. A layer or a non-single-crystal Si _x C _1-x layer may be inserted as a buffer layer (not shown). At this time, the thickness of the insertion layer is about 0.5 to 50 nm.

  Here, in order to realize good recombination characteristics at the junction between the first semiconductor junction layer 31 and the second semiconductor junction layer 32 described below (that is, to achieve electrical connection characteristics like ohmic contact). Therefore, in the n-type semiconductor layer 31c included in the first semiconductor junction layer 31 and the p-type semiconductor layer 32a included in the second semiconductor junction layer 32 to be described later, at least a portion where they are in contact has a crystallization ratio. It is desirable to keep it high. At this time, when the crystallization rate is 60% or more, tunnel junction characteristics can be obtained, and good reverse junction recombination characteristics can be realized. In the reverse crystalline junction, a microcrystalline Si layer of the same conductivity type is formed about 5 to 30 nm following the n-type semiconductor layer 31c, and then a microcrystalline Si layer of reverse conductivity type is formed about 5 to 30 nm. Subsequently, the p-type semiconductor layer 32a is formed, and the n-type semiconductor layer 31c and the p-type semiconductor layer 32a may be formed by inserting a dedicated crystalline Si layer (not shown). ).

In addition, in order to realize the ohmic contact electrical connection characteristics between the first semiconductor junction layer 31 and the second semiconductor junction layer 32, a transparent conductive film, a thin metal layer, or a thin silicide layer (silicon and metal (Alloy layer) can be used as a conductive intermediate layer (not shown). Here, as the transparent conductive film, SnO ₂ that is tin oxide, ITO that is indium-tin oxide, ZnO that is zinc oxide, or the like can be used. When a transparent conductive film is used here, an optical effect (reflection and transmission characteristics) can also be introduced by the presence of the transparent conductive film, which is very excellent in terms of high efficiency. That is, by adjusting the transparent conductive film thickness, the short wavelength light is reflected by the transparent conductive film and preferentially reenters the first semiconductor bonding layer 31, and the long wavelength light has the same antireflection effect. According to the principle, it can be preferentially confined in the second semiconductor junction layer 32, and more efficient photoelectric conversion of light energy becomes possible. Further, when a thin metal layer or silicide layer is used, it is possible to expect an effect that the ohmic contact between the first semiconductor junction layer 31 and the second semiconductor junction layer 32 can be realized more reliably and simply with a high yield.

  Next, a second semiconductor junction layer 32 is formed on the first semiconductor junction layer 31. The second semiconductor junction layer 32 comprises a pin junction in which a p-type semiconductor layer 32a, a photoactive layer (substantially i-type semiconductor layer) 32b, and an n-type semiconductor layer 32c are sequentially stacked.

Here, for the p-type semiconductor layer 32a, an amorphous Si film or a crystalline Si film can be used. The film thickness is adjusted in the range of about 2 to 100 nm depending on the material. The doping element concentration is about 1 × 10 ¹⁸ to 1 × 10 ²¹ / cm ³ and is substantially p ⁺ type. Note that Si _x C _1-x can be obtained by mixing an appropriate amount of a gas containing C (carbon) such as CH ₄ in addition to gases such as SiH ₄ and H ₂ used for film formation and B ₂ H ₆ which is a doping gas. A film is obtained, which is very effective for forming a window layer with little light absorption loss, and also effective for reducing dark current components for improving the open circuit voltage. In addition to C, the same effect can be obtained by mixing an appropriate amount of a gas containing O (oxygen) or a gas containing N (nitrogen).

In addition, for the substantially i-type semiconductor layer that is the photoactive layer 32b, the above-described high-quality, high-light-stable amorphous Si film formed using the Cat-PECVD method is used. The thickness is adjusted in a range of about 0.5 to 1 μm, and when a crystalline Si film typified by a microcrystalline Si film is used, the thickness is adjusted in a range of about 1 to 3 μm. At this time, in the case of using the above-described high quality and high photostable amorphous Si film, the film forming conditions are as follows: the plasma excitation frequency is 60 MHz, the thermal catalyst temperature is 1600 to 2000 ° C., and the H ₂ / SiH ₄ flow rate. When the ratio is 5 to 10, the substrate temperature is 200 to 250 ° C., the gas pressure is 133 to 200 Pa, the VHF power density is 0.1 to 0.3 W / cm ² , and a crystalline Si film is used, The H ₂ / SiH ₄ flow rate ratio is 5 to 20, the gas pressure is 200 to 665 Pa, and the VHF power density is 0.2 to 0.5 W / cm ² . By using such film forming conditions, the above-described high-quality and high-light-stable amorphous Si film or crystalline Si film can be formed at a high film forming speed of 2 nm / sec. Here, since the non-doped film usually shows a slight n-type characteristic, in this case, the non-doped film may be adjusted so as to be substantially i-type by slightly containing a p-type doping element. it can. In some cases, n ^- type or p ^- type may be used for the purpose of fine adjustment of the internal electric field strength distribution. Further, when the crystalline Si film is used for the photoactive layer 32b, the surface shape after the film formation as an aggregate of (110) -oriented columnar crystal grains is a spontaneous structure suitable for light confinement. It is desirable to have an uneven structure.

For the n-type semiconductor layer 32c, an amorphous Si film or a crystalline Si film can be used. The film thickness is adjusted in the range of about 2 to 100 nm depending on the material. The doping element concentration is about 1 × 10 ¹⁸ to 1 × 10 ²¹ / cm ³ and is substantially n ⁺ type. A Si _x C _1-x film can be obtained by mixing an appropriate amount of a gas containing C (carbon) such as CH ₄ in addition to gases such as SiH ₄ and H ₂ used for film formation and PH ₃ as a doping gas. Therefore, it is possible to form a film with little light absorption loss and to reduce dark current components for improving the open circuit voltage. In addition to C, the same effect can be obtained by mixing an appropriate amount of a gas containing O (oxygen) or a gas containing N (nitrogen).

  In order to further improve the junction characteristics, a substantially i-type non-single-crystal Si layer is buffered between the p-type semiconductor layer 32a and the photoactive layer 32b or between the photoactive layer 32b and the n-type semiconductor layer 32c. It may be inserted as a layer (not shown). At this time, the thickness of the insertion layer is about 0.5 to 50 nm.

Next, a metal film is formed as the second electrode 4 serving as the back electrode. As this metal film material, it is desirable to use Ag (silver) which is excellent in conductive characteristics and light reflection characteristics in the Si medium. However, Al (aluminum) which is a cheaper material can be expected, and characteristics equivalent thereto can be expected. It is also possible to use it. Of course, a laminate of Ag and Al or an alloy material containing Ag and Al may be used. Further, the second electrode 4 has a two-layer structure, the first layer on the semiconductor layer side is a metal film containing Ag or Al, and the second layer stacked on the first layer is more than Ag. An inexpensive metal material layer may be used. As the film forming method, known techniques such as vapor deposition, sputtering, ion plating, screen printing, and plating can be used. At this time, the film thickness is about 0.1 μm or more.

It is more preferable that the second electrode 4 has a structure (not shown) in which a transparent conductive film / a metal film are stacked in this order from the side in contact with the semiconductor layer. Thus, by inserting the transparent conductive film between the semiconductor layer and the metal film, it is possible to suppress the phenomenon that the metal film component diffuses into the semiconductor layer and deteriorates the device characteristics. In addition, if an appropriate uneven structure is provided on the transparent conductive film forming surface, light is effectively scattered, so that the light confinement effect effective for improving the efficiency of the solar cell can be enhanced. Here, as the transparent conductive film material, SnO ₂ , ITO, ZnO or the like can be used as described above, and as a film forming method, a CVD method, a vapor deposition method, an ion plating method, a sputtering method, a spray method, In addition, a known technique such as a sol-gel method can be used.

<Element characteristics>
The characteristics (initial characteristics and characteristics or rate of change after the light irradiation test) of the device manufactured as described above are shown in Tables 1 and 2 in comparison with the conventional example. Here, the light irradiation test is performed under the condition of irradiating AM1.5 simulated sunlight (solar simulator light) continuously for 200 hours at a light irradiation intensity of 100 mW / cm ² at a temperature of 48 ± 2 ° C. Shall. Further, the initial state before light irradiation is realized according to the annealing method shown in the international standard IEC1646. The stabilization efficiency in Table 1 is the efficiency measured after the light irradiation test. In Table 2, Jsc is the short-circuit current density, Voc is the open circuit voltage, FF is the fill factor, and the rate of change thereof. Is defined as {value after light irradiation test (stabilized value) −value before light irradiation test (initial value)} / initial value. Note that the elements shown in this table are those in the case where the photoactive layer 32b of the second semiconductor junction layer 32 is formed of a microcrystalline Si film.

  As can be seen from Tables 1 to 3, in the present invention elements 1 to 4, since the above-described high-quality and high-light-stable amorphous Si film was used, the photoactive layer 31b was formed at a high film-forming speed. Nevertheless, it has a high initial conversion efficiency, and in particular, the light deterioration rate is extremely lower than those of the conventional elements 1 and 2, and the characteristic is greatly reduced to a value of 7.0% or less.

  As for the open circuit voltage Voc, the change rate from the initial Voc in the light irradiation test of the conventional element shows a negative value (deterioration), whereas the change rate of the present invention element is positive. The value of is shown. This Voc characteristic is one of the important components that the element of the present invention exhibits an extremely low light deterioration rate characteristic, and the physical reason is not yet clear, but the above-described high quality / high light stability characteristic. It is presumed that some cause is required for the film physical properties of the amorphous Si film.

  Also, the fill factor F.I. F. As for the conventional element, the initial F. F. The change rate from -8% is -8% (deterioration rate 8%), whereas the change rate is -6% or less (deterioration rate 6% or less) in the element of the present invention. It shows reduced characteristics.

In addition, from the same table, it can be seen that when the intensity ratio of the TA peak intensity to the TO peak intensity of the Raman scattering spectrum is 0.35 or less, or the half width of the TO mode is 65 cm ⁻¹ or less, the light deterioration rate can be reduced. Recognize.

  Although the results of the tandem element are shown in the present embodiment, the same light irradiation test is performed before and after a single junction element using an amorphous Si film formed under the same film forming conditions as a photoactive layer. It is confirmed that it shows the trend of characteristics.

<Photovoltaic generator>
The photoelectric conversion device described above can be used as a power generation unit, and a photovoltaic power generation device configured to supply the generated power from the power generation unit to a load can be obtained. That is, one or more of the above-described photoelectric conversion devices (in the case of a plurality of photoelectric conversion devices, these are connected in series, parallel or series-parallel) are used as power generation means (in the case of a plurality of photoelectric conversion devices, these are connected in series, A generator connected in parallel or in series and parallel may be used as the power generation means), and the generated power may be supplied directly from the power generation means to the DC load.

  In addition, the DC power generated by the power generation means described above is converted into appropriate AC power by power conversion means such as an inverter, and the converted power is converted into an AC load such as a commercial power system or various electric devices. You may comprise the photovoltaic device which can be supplied to.

  Furthermore, it is also possible to use such a power generation device as a photovoltaic power generation device such as a solar power generation system of various aspects by installing it on a roof or wall of a building with good sunlight.

<Generalization>
In the above description of the embodiment, an example in which the amorphous Si film of the present invention is applied to a tandem solar cell having two semiconductor junctions in a semiconductor multilayer film has been described. The crystalline Si film is a single-junction solar cell (not shown) having one semiconductor junction, a triple-junction solar cell (not shown) having three semiconductor junctions, or a larger number of semiconductors. The present invention can also be applied to a multi-junction solar cell (not shown) having a junction, and the same effect can be obtained.

  Further, although the solar cell in which the semiconductor junction layer pin is formed in the order of pin from the light receiving surface side has been described, the same effect can be obtained also in the solar cell formed in the order of nip from the light receiving surface side.

  Moreover, although the super straight type solar cell in which light is incident from the substrate side has been described, the same effect can be obtained for a substrate type solar cell (not shown) in which light is incident from the semiconductor film side. In the case of the substrate type, the substrate is not limited to the light-transmitting substrate, and a light-impermeable substrate such as stainless steel may be used, and the first electrode is made of a metal material, and the second electrode The electrode is made of a translucent material.

  Moreover, although the case where the amorphous Si film of the present invention was formed using the Cat-PECVD method which is a kind of PECVD method has been described, the manufacturing method is not limited to this, and the film forming conditions are adjusted. This does not exclude the possibility of formation by the conventional PECVD method. Similarly, the possibility of formation by a CVD method other than the PECVD method represented by the Cat-CVD method (catalytic CVD method) by adjusting the film forming conditions is not excluded.

  Furthermore, the amorphous Si film of the present invention can be applied to general photoelectric conversion devices such as photodiodes, phototransistors, and optical sensors in addition to solar cells.

<Summary>
As described above, according to the manufacturing method of a photoelectric conversion device of the present invention, at least silicon and containing hydrogen, scattering peak intensity of TA mode for scattering peak intensity I _TO the TO mode obtained by Raman scattering spectrum I _TA ratio I _TA / I _TO is 0.35 or less, half-width of scattering peak of TO mode obtained by Raman scattering spectrum is 64 to 65 cm ⁻¹ or less, existence density of Si—H bonded state and Si—H ₂ bonded state Si- for the sum of abundance density
When the ratio of the density of H-bonded states is 0.95 or more, the crystallization rate defined by the Raman scattering spectrum is 30% or less, and the crystalline silicon grains are included, the maximum grain size is 5 nm or less. For the semiconductor multilayer film of the photoelectric conversion device provided with the semiconductor multilayer film including a semiconductor thin film made of crystalline silicon as a photoactive part, the light irradiation intensity using pseudo sunlight is 100 mW / cm ² , the light irradiation time The light irradiation treatment performed under the condition of T ± 2 ° C. for 200 hours and a temperature T of 40 to 50 ° C. has a deterioration rate of photoelectric conversion efficiency of 7% or less before and after the light irradiation treatment, and the light irradiation Since the open-circuit voltage after the treatment is higher than the open-circuit voltage before the light irradiation treatment, the amorphous Si having a high quality and high light stability in which the SRO is greatly improved as compared with the conventional amorphous Si film. High efficiency light with film Rate can be provided a photoelectric conversion equipment typified by significantly reduced solar cells.

Further, according to the photoelectric conversion device of the present invention, the Si—H bonding state relative to the sum of the existence density of the Si—H bonding state and the existence density of the Si—H ₂ bonding state in the amorphous Si semiconductor thin film. Since the ratio of abundance density is 0.95 or more, it can be an amorphous Si film having high quality and high photostability with further improved SRO compared to the conventional amorphous Si film, A photoelectric conversion device having excellent characteristics using such an amorphous Si film can be realized.

  Furthermore, according to the photovoltaic device of the present invention, since the photoelectric conversion device is used as a power generation means and the generated power of the power generation means is supplied to a load, it is possible to provide a highly efficient photovoltaic power generation device. it can.

It is sectional drawing explaining embodiment of the photoelectric conversion apparatus of this invention.

Explanation of symbols

1: Substrate 2: First electrode 3: Semiconductor multilayer film
31: First semiconductor junction layer
31a: p-type semiconductor layer
31b: Photoactive layer
31c: n-type semiconductor layer
32: Second semiconductor junction layer
32a: p-type semiconductor layer
32b: Photoactive layer
32c: n-type semiconductor layer 4: second electrode

Claims (1)

The ratio of the TA mode scattering peak intensity to the TO mode scattering peak intensity obtained by the Raman scattering spectrum is 0.35 or less , and the half width of the TO mode scattering peak obtained by the Raman scattering spectrum is at least 0.35. 64 to 65 cm ⁻¹ or less, the ratio of the existence density of the Si—H bond state to the sum of the existence density of the Si—H bond state and the existence density of the Si—H ₂ bond state is 0.95 or more, and the Raman scattering spectrum When the crystallinity defined by is 30% or less and includes crystalline silicon grains, a semiconductor multilayer film including a semiconductor thin film made of amorphous silicon having a maximum grain size of 5 nm or less as a photoactive portion is provided. A light irradiation intensity of 100 mW / cm ² using pseudo sunlight and a light irradiation time of 2 for the semiconductor multilayer film of the photoelectric conversion device provided 00 hours, and a manufacturing method of a photoelectric conversion device comprising a TURMERIC line light irradiation treatment with T ± 2 ° C. conditions with temperature T of 40 to 50 ° C..

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