JP2805353B2 - Solar cell - Google Patents

Description

Description: TECHNICAL FIELD [0001] The present invention scatters incident light to effectively utilize light absorbed by an active layer, and at the same time, reduces retention due to unevenness of constituent members and reduces reliability. The present invention relates to an improved high-efficiency solar cell in which a reduction in durability and durability is prevented by a buffer layer.

[Description of Prior Art]

In a solar cell using a light-reflective substrate, a method for improving the efficiency by forming the light-reflective surface as a rough surface having irregularities and increasing the path length of light having a low absorption wavelength is disclosed in, for example, USP 4,126,150. No. RCA (RCA), column 7, lines 3-8, and is also described in JP-A-56-152276 (Teijin). Further, JP-A-59-104185
Publication No. (Exxon Research and Engineering Company) details the optical effect of a roughened substrate.

Furthermore, Journal of Applied Physics, Vol. 62, No. 7, 2016
The page (Thomas C. Paulick. Oct '87) mathematically deals with the optical reflection characteristics of amorphous silicon solar cells using silver texture.

As a method for forming unevenness, wet etching in Japanese Patent Application Laid-Open No. 54-153588 (National Patent Development Corporation) is used.
No. 159383 (Energy Conversion Devices) discloses sand blasting, facet forming and co-evaporation methods, and Japanese Unexamined Patent Publication No. 59-14682 (Electrolytic foil industry etc.) discloses aluminum roughening by direct current electrolytic etching or chemical etching. However, in Japanese Patent Application Laid-Open No. 59-82778 (Energy Conversion Devices), the sputter etching method and the sand blast method are described in Japanese Patent Application Laid-Open No. 59-82778.
No. 104185 discloses a lithography method, a transparent conductor deposition method by thermal decomposition spray, an ion beam simultaneous deposition method, and an etching method.

In addition, a material which can easily form irregularities by nature is disclosed in Japanese Patent Application Laid-Open No. Sho 58-180069 (Director of the Industrial Technology Institute) and a metal reflective layer provided thereon.
No. 74 (Chairman of Industrial Technology) ceramic substrate.

On the other hand, when forming a Schottky junction or PIN junction on a reflective substrate, the advantage of arranging a cermet layer on the reflective substrate is to prevent the yield from dropping due to pinholes, etc. SERI Report SAN-1286-8 (Carlson et al. Oct
1978.EY-76-C-03-1286).

Further, in forming a solar cell on a reflective substrate, a method in which a transparent conductive layer is interposed in order to prevent short circuits due to scratches and protrusions is disclosed in JP-A-56-69875 (Fuji Electric). Have been. A similar one is disclosed in JP-A-58-35988 (Taiyo Yuden).

Further, a combination of these and providing a transparent electrode layer on a reflective surface having irregularities is disclosed in the above-mentioned JP-A-58-159383 (Energy Conversion Devices). The transparent electrode layer disclosed in this publication prevents a substance on a reflection surface (reflector) having irregularities from diffusing into the PIN layer to prevent deterioration of characteristics.

Also, in the aforementioned JP-A-59-104185 (Exxon Research and Engineering), in a reflective solar cell sandwiched between transparent conductive films, an optical path is formed as a rough surface with either one of the surfaces of one TCO. Techniques have been disclosed that extend and improve collection efficiency in long wavelength regions.

In addition to these, JP-A-60-84888 (Energy
Conversion device) is provided with a barrier layer for preventing a short circuit between upper and lower electrodes due to a pinhole or a projection.

However, these prior arts do not
As a reflective substrate for a silicon solar cell, it was still insufficient in several points described below.

Light having a wavelength that is partially absorbed by the optically active layer of an amorphous silicon solar cell, the rest of which is transmitted and further reflected by the substrate, is reflected with a larger scattering angle as the absorption is smaller, in other words, as the wavelength is longer. It is preferable to increase the amount of light absorbed by the optically active layer. However, in general, the scattering angle becomes smaller as the wavelength becomes longer on a specific uneven surface, so that an optimization method is required for the pitch and shape of the uneven surface.

The distribution in the thickness direction of the photocarriers generated by the reflected light in the optically active layer of the solar cell also becomes a problem. That is, the optical carrier is generated by the incident light and the light that is reflected and absorbed, and produces a distribution in the thickness direction. Since the carrier reach distance (distance in which carriers are transported by an electric field) differs greatly between electrons and holes of normally generated photocarriers, for example, in a solar cell having a PIN structure, it is better to increase the carrier generation density closer to the P layer. However, the desired carrier generation distribution differs depending on the layer structure of the solar cell, for example, the collection efficiency is high. The unevenness of the reflective substrate should be determined with this in mind. For example, in the case of the above-described PIN type solar cell (when light is incident from the P layer side), the scattering angle of reflected light is particularly large. In this case, the carrier distribution on the N layer side increases, which does not necessarily lead to a great improvement. The angle of the reflected light should be determined in consideration of the running property of the solar cell, the internal electric field, and the electrical properties of other materials.

Reflective substrates, in which light reflects at an angle, are usually achieved with mechanical asperities, often resulting in reduced yield of solar cells formed thereon. In particular, in the case of amorphous silicon, the thickness of the optically active layer is on the order of the wavelength of light (which is not so different from the size of the irregularities), and the diffusion length of the carrier is extremely short. Is thinned to 300 mm or less, mechanical and electrical defects occur in the thin layer due to the unevenness of the underlying substrate, leading to a decrease in open short-circuit voltage and a decrease in yield due to short circuit. It is effective to provide a buffer layer in order to prevent this, but as this buffer layer, the solar cell functions in the film thickness direction and has an electric resistance ground having a short-circuit prevention effect, and is optically effective. A material that is transparent or substantially transparent to reflected light or promotes optical scattering is required in connection with a solar cell. Supplementing the electric resistance value corresponding to the film thickness direction, the electric resistance of the buffer layer increases and the internal resistance of the solar cell increases equivalently when approaching the resistance value at the time of operation of the solar cell. When the electrical resistance of the buffer layer is small, the effect as the buffer layer cannot be achieved, and the performance degradation due to the short circuit cannot be prevented. Moreover,
Since the film thickness is different between the projecting portion and the concave portion, there is necessarily an appropriate value to cover both.

The demand for manufacturing costs is severe, not only for those used in consumer equipment but also for solar cells for electric power. In order to reduce the manufacturing cost, easiness in each manufacturing process is required. For example, no matter how much the reflection characteristics are improved, it is not realistic if a substrate is created using lithography and the effect of the substrate is not visually improved as an overall efficiency. .

Further, it is preferable that the range of selection of the base material, the electrode, and the optically active material is large. Although this is not always necessary for a specific solar cell, it is extremely significant when the solar cell is comprehensively viewed.

As described above, a non-light-transmitting substrate on which a solar cell (especially, a solar cell based on Amorphous Silicon) suitably operates as an integral body can effectively prevent light partially absorbed by the optically active layer. Reflection at an angle, reflection of light as a carrier distribution that contributes to collection efficiency due to the traveling of the optical carrier due to the reflected light, and adoption of the substrate does not lead to a decrease in open-circuit voltage or an increase in short-circuit current. Conversely, it is desired to contribute to the prevention of short circuits.

[Object of the invention]

An object of the present invention is to solve the problems in the conventional solar cell, achieve the above-described objects, and provide a desired solar cell.

[Structure and effect of the invention]

The present invention has been completed by the inventors of the present invention on the basis of the knowledge obtained through experiments described later, with intensive studies to achieve the above object.

The present invention provides a solar cell having the configuration described below. A solar cell provided according to the present invention is provided on a substrate in contact with a light-reflective metal layer, is substantially transparent to light reflected from the light-reflective metal layer, and is capable of conducting a short circuit current. In a solar cell having a layer and a photovoltaic layer that generates photovoltaic light with respect to incident light, the light-reflective metal layer reflects light partially absorbed by the photovoltaic layer. It has surface irregularities with a height difference greater than the wavelength, and all or a part of the material constituting the surface irregularities is made of polycrystalline Ag or Al or an Al-Si alloy or an alloy thereof, and the particle size of the polycrystalline Is the maximum height difference of the surface unevenness, the buffer layer has a surface unevenness having a height difference smaller than the wavelength of light partially reflected by the light reflective metal layer and absorbed by the photovoltaic layer. Having all or part of the material constituting the buffer layer is polycrystalline
It is made of ZnO, ZnS or ZnSe, characterized in that the grain size of the polycrystal is at least as large as the height difference of the surface irregularities, and the thickness of the buffer layer is 0.2 to 14 μm.

The solar cell of the present invention is characterized in that the difference in surface irregularities of the light-reflective metal layer is 0.1 to 5 μm.

The photovoltaic layer is mainly made of a material containing one or more of Si, Ge, C, B, and P.

In the present invention, the difference in height of the surface irregularities refers to the average of the difference in height between the convex portions that are high portions and the concave portions that are low portions of the unevenness generated on the surface of the light reflective metal layer or the buffer layer. Shall be.

The present invention, a light reflective metal layer, a combination of various materials of the buffer layer and the surface properties, by changing the value of resistance, etc. to create a solar cell, the following findings are obtained as a result of intensive study, It has been found that a solar cell manufactured under these conditions provides several steps of improvement over the conventional one.

FIG. 1 shows the structure of a solar cell as an application example of the present invention.

By the way, as the base material applicable to the present invention, glass materials: metal materials such as aluminum, copper, nickel, cobalt, chromium, iron, zinc, lead and alloy materials thereof (such as stainless steel): silicon, germanium, alumina , Toria, magnesia, beryllia, silicon nitride, boron nitride, silicon carbide, etc .; ceramic materials: polymer materials such as polyimide, polyamide, polyethylene and the like.

The surface of these substrate materials is polished or roughened as appropriate in applying the present invention.

Polishing is mechanical polishing, electrolytic combined polishing, chemical polishing,
Reactive ion etching (RIE) can be used. On the other hand, sandblasting, chemical etching, dry etching and the like are used for roughening.

The surface of these substrate materials is appropriately subjected to a conductive treatment when the present invention is applied. As the conductive treatment, a metal or oxide conductor (ITO, ZnO, or the like) may be deposited, or an impurity may be doped by a diffusion method or an implantation method.

As the material used for the light reflective metal layer of the present invention,
Silver, silicon, aluminum or their alloys or iron,
Alloys with copper, nickel, chromium, and molybdenum are applicable. Among them, silver, aluminum, and aluminum silicon alloy are preferable.

The size of the crystal grains of the light reflective metal layer in the present invention,
Regarding the relationship with the height difference of the surface irregularities, a range was obtained in which a favorable solar cell was obtained as a result of producing a film by changing the producing conditions and producing a solar cell using the film.

In forming the light-reflective metal layer having surface irregularities in the present invention, the crystal grain size of the other crystal film and the height difference of the surface irregularities depend on the temperature, surface state, deposition rate, heating time, and deposition method of the substrate. Different, especially when the substrate temperature is 100 ° C
It is not unusual for the state of the film to be significantly different due to the difference. Therefore, it was determined by the following trial process performed by the present inventors.

First, the temperature set by the temperature controller is set to room temperature, and a desired deposition material and apparatus are used to select a typical parameter for vapor deposition of the material and vapor deposition is performed. Next, the temperature at which unevenness starts to be generated by heating the deposited film is recorded. Further, a new substrate is prepared and the temperature controller is set slightly higher than the recorded temperature. Thus, vapor deposition is performed again under the same conditions. The height difference of the unevenness is measured with a scanning electron microscope SEM and a surface roughness meter. If the size is larger than the desired size, increase the vapor deposition conditions. This was repeated to determine the temperature and the corresponding deposition conditions.

A schematic diagram of a solar cell having an improved substrate made by the method of the present invention is shown in FIG.

First, after forming a light-reflective metal layer on a substrate, the film material of the buffer layer having a thickness of 0.2 to 14 μ, which will be described in detail later, is ZnO, the height difference of the surface irregularities is 0.5 μm, and the grain size is 0.5 μm. μ, the film thickness is set to 1.2 μm, and the film is formed according to a desired method and conditions described below. Then, as a photovoltaic layer 104 under the method and conditions described later, an n-type Si semiconductor layer is formed by an apparatus illustrated in FIG. After forming an i-type Si semiconductor layer and a p-type Si semiconductor layer, ITO was deposited at 800 と し て as a transparent electrode 105, and a grid electrode 106 was formed thereon, thereby forming a PIN solar cell.

In this experiment, the created solar cell was AM1.0 (100 mW / cm ² )
As a result of the measurement under irradiation, it is clear that the conversion efficiency has a clearly optimum and suitable range in the particle size of the light-reflective metal layer polycrystalline film and the height difference of the surface irregularities as shown in FIG. There was found.

That is, from the results of this experiment, it is preferable that the size of the reflective crystal grains be 5 μm from 0.01 μm, more preferably 1 μm than 0.1 μm.

Regarding the crystallinity of the light reflective metal particles in the present invention,
Those having at least one directionality are preferable, and those having biaxial directionality in the substrate surface are more preferable. The evaluation of the crystallinity was performed by X-ray diffraction TEM (transmission electron beam) observation and SEM (scanning electron microscope) observation.

From the results of the above solar cell that provides a better efficiency, the average height difference of the surface irregularities of the light-reflective metal layer in the present invention is preferably 0.1 μm to 5 μm, more preferably 0.5 μm to 2 μm. It is good to be selected from among. Further, it is more preferable to select the wavelength in relation to the wavelength of light partially absorbed and transmitted by the photovoltaic layer. In other words, by making the average height difference of the surface irregularities larger than the wavelength of the light partially absorbed and transmitted by the photovoltaic layer, the collection efficiency in a long wavelength region can be improved.

The height difference between the surface irregularities of the light-reflective metal layer can be evaluated by observing images with a surface roughness meter and a scanning electron microscope (SEM).

In forming the light-reflective metal layer having surface irregularities in the present invention, a sputtering method, an electron beam evaporation method, a resistance heating evaporation method, or the like that can control the temperature of the substrate can be used.

FIG. 4 is a schematic structural view of a planar type DC magnetron sputtering apparatus which is a kind of sputtering method. Planar type
The advantage of using DC magnetron sputtering is that high-speed sputtering can be realized with a small device.
RF magnetron type can also be realized.

In FIG. 4, reference numeral 401 denotes a vacuum vessel, and the heating plate 403 is a insulator.
Located at 402. A heater 406 and a thermocouple 404 are embedded in the heating plate 403, and are controlled to a predetermined temperature by a temperature controller 405. The heat uniform pair 407 transmits heat uniformly to the base 408, and specifically, aluminum foil or copper foil is used. The base 408 is supported by being pressed against the heating plate 403 by the base holder 409. A target 410 is disposed facing the base 408, and the target 410 is
Plasma space with magnet 414 on the back installed on 412
A magnetic field can be formed at 425. In order to cool the target heated during sputtering, cooling water from a cooling water introduction pipe 414 is introduced to the back surface of the target. The introduced water is discharged from the cooling water discharge pipe. The target 410 is supplied from a sputtering power source via the target base 212.
DC voltage is applied. The sputtering gas is supplied with argon and oxygen from the mass flow controllers 420 and 421, respectively, and the internal pressure can be monitored by the vacuum gauge 423.
The entire vacuum vessel 401 is connected to an exhaust system by a main valve 224 connected to an acid.
Is brought into a vacuum state through.

FIG. 6 shows a schematic configuration diagram of an electron beam evaporation machine. The temperature of the substrate 608 is controlled in the same configuration as in the sputtering apparatus of FIG. The deposition material is usually placed in the form of pellets and placed in the crucible 658, and the electron beam from the filament 663 is applied with a magnetic field by a magnet (not shown) while an acceleration voltage is applied to the hearth 659 from the electron beam acceleration voltage source 661. And is heated by colliding with the material. The heated material is converted into a vapor to form a substrate 608.
A shutter 657 is provided to control the on / off state of the adherence.

FIG. 7 shows a schematic configuration diagram of a resistance heating vapor deposition apparatus. The temperature of the substrate 708 is controlled by the same configuration as in the sputtering apparatus of FIG. Filament 771,
Filament support 772, heating power supply 773 and shutter 757
And constitute a well-known heating system.

Optimum expression of the light-reflective metal layer of the present invention As a result of an intensive study using the above-described apparatus on the conditions under which the height difference of the unevenness can be obtained, the substrate temperature and the deposition time among the deposition conditions were optimized. It turned out that it could be fulfilled.

First, the substrate temperature, when the substrate temperature is changed from room temperature to 240 ° C., the height difference of the surface irregularities of the light-reflective metal layer is less than 0.1 μm, and the substrate temperature is preferably set to 250 ° C. or more. Average height difference of surface irregularities is 0.5μ
m to 5 μm. As for the vapor deposition time, it was necessary that the time for keeping the substrate at that temperature (including the heat treatment time after vapor deposition) was 30 minutes or more.

The buffer layer in the present invention is selected from a material containing a transparent conductive oxide of ZnO, a material such as ZnS, ZnSe, etc., which has an increased conductivity by adding impurities to a wide energy gap semiconductor material.

The thickness d of the buffer layer is selected in a range determined by the absorption coefficient α for the wavelength of the light to be reflected and the electric conductivity σ. That at least it is selected so as to satisfy _{d ≦ 1 / α AσR SH ≦} d ≦ σR SR. Here, A is the cell aperture ratio, R _SH is the parallel resistance per unit area of the applied solar cell, and R _SR is the series resistance per unit area of the applied solar cell. As an example of an amorphous silicon PIN solar cell, A = 10 ^-7 , R _SH = 10 kΩ / cm ² , R _SR
= 4Ω / cm ² and α = 7 × 10 ² cm ⁻¹ , if the electric conductivity of the buffer layer is σ2 × 10 ⁻² S / cm, then 0.2 ≦ d ≦ 14.3 [μm] according to the above equation. To be selected.

In the experiment of the present invention, the same P was used while keeping the crystal grain size of the polycrystalline film of the light-reflective metal layer used in the above-described embodiment at 0.5 μ.
It was made with solar cells with IN layer structure. Solar AM1.0 (100mW
/ cm ² ), as shown in FIG.
% In a solar cell using ZnO as a buffer layer such that an efficiency of not less than 0.2% can be obtained, good characteristics are obtained when the film thickness is in the range of 0.2 ≦ d ≦ 14 μm. It has been found that at 8 μm, most preferably at 0.4 to 5 μm, a solar cell with an efficiency of 11% or more can be obtained. Similar results were obtained when a buffer layer was used as another material.

That is, after the light is incident, the light reflecting metal layer reflects a part of the surface unevenness smaller than the wavelength of the light that is partially absorbed by the photovoltaic layer as reflected by the light reflecting metal layer. A good solar cell was obtained when the size was maximum and the height difference of the surface unevenness was 0.2 to 14 μm.

Buffer layer in the present invention is already vacuum resistance heating evaporation, sputtering, other than electron beam heating evaporation, CVD,
It can be formed by spinner coating, dipping, or the like.

The unevenness in the buffer layer is formed by the same method as in the case of the light-reflective metal layer.

As for the conditions under which the optimum height difference of the surface unevenness of the buffer layer of the present invention can be obtained, as a result of an intensive study using the above-described apparatus, particularly, among the deposition conditions, it is necessary to optimize the substrate temperature and the deposition time. Turned out to be able to meet.

First, the temperature of the substrate is maintained. By maintaining the temperature of the substrate on which the light-reflective metal layer is formed at 250 ° C. or more and 350 ° C. or less during the formation of the buffer layer, the height difference of the surface unevenness of the buffer layer is 0.4 to 5
μ, which indicates that good solar cell characteristics can be obtained as described above. As for the vapor deposition time, it was necessary that the time for keeping the substrate at that temperature (including the time for heat treatment after vapor deposition) was 40 minutes or more.

As an optically active layer of a solar cell applicable in the present invention, a hetero face type, PN
Type, PIN (or NIP) type, etc., and from the characteristics of the phase as the material, amorphous, polycrystalline, microcrystalline, or a mixture or composite thereof, silicon, germanium, thin film diamond, A group IV semiconductor such as silicon carbide, or a group III-V or group II-VI semiconductor or a crystal thereof can be applied. It is also possible to combine these,
A better effect can be obtained when used in a so-called stacked cell.

Among these materials, examples using amorphous silicon, which is one of the constitutions of the present invention, will be described. FIG. 6 is a schematic structural view of a plasma CVD apparatus for forming an amorphous silicon film shown in FIG.

Reference numeral 501 denotes a vacuum vessel, and a heating plate 503, a thermocouple 504, a heater 506, a temperature controller 505, and a heat uniform material 507 are a second container.
The temperature of the substrate 508 is controlled similarly to the sputtering apparatus shown in the figure. Substrate 5
08 is grounded similarly to the vacuum vessel 501,
The counter electrode 570 provided opposite to the
An RF power of 3.56 MHz is supplied to decompose the gas from the mass flow controllers 550 to 556 to deposit a film on the substrate 508. Main valve connected to the exhaust system in the container
A constant low pressure is maintained via 324. Usually, a capacitance manometer 560 is used for measuring the pressure. The supplied gas is selected according to the desired film properties. For example, in the case of a-Si, SiH ₄ , and in some cases, SiF ₄ , S
i ₂ H _6, Ar, are used also added such as H _2. In the case of n-type a-Si, PH _{3 is used} , and in the case of p-type s-Si, BF ₃ is used.

In addition, as the transparent conductive film on the light incident side and the technology of the current collecting electrode, those known so far are used.

〔Example〕

Hereinafter, the present invention will be described in more detail using examples.

Example 1 A solar cell having the configuration shown in FIG. 1 was produced.

A substrate 101 was prepared by polishing the surface of a stainless steel (SUS304) plate having a thickness of 0.8 mm to 0.04 s and ultrasonically cleaning with 1,1,1-trichloroethane for 10 minutes.

On the substrate 101, a target type silver target purity 99.99% sputter gas argon sputter pressure 5 × 10 ^-3 Torr argon flow rate 25sccm sputter current 0.15A sputter rate 100Å / min sputter time The light-reflective metal layer 102 was formed under the condition of a substrate temperature of 300 ° C. and a heat treatment time of 2 hours for 30 minutes.

As a result of image observation by SEM, inclined rod-shaped particles having a size of 0.8 to 1.2 × 2 to 4 μm were uniformly distributed. When the substrate temperature is sputtered at room temperature, an extremely smooth surface (2
(Surface unevenness is not recognized in the SEM image of 10,000 times), it can be presumed that these particles are probably crystal grains. Moreover, the coordination direction of the rod-shaped particles (longitudinal direction of the particles)
Is limited in almost two directions in the plane of the substrate surface, and indicates a correlation with the crystal plane of stainless steel (austenite phase).

Next, a buffer layer 103 was formed on the light-reflective metal layer 102 using the same sputtering apparatus used to form the light-reflective metal layer 102.

At this time, the deposition conditions were as follows: target zinc oxide target purity 99.99% sputtering gas argon / oxygen sputtering pressure 5 × 10 ^-3 Torr argon flow rate 25 sccm oxygen flow rate 0.1 sccm sputtering current 0.8 A sputtering rate 170 ス パ ッ タ / min sputtering time 1 hour Substrate The temperature was 300 ° C. SEM observation revealed surface irregularities with a height difference of 3000 °.

Using the substrate thus prepared, a ni-pa-Si solar cell was produced using the apparatus shown in FIG.

First, the N-type doped layer of 200Å by the glow discharge method, discharge frequency 13.56MHz discharge power density 0.03 W / cm ² pressure 0.2Torr gas flow rate _{Ar 38sccm SiH 4 0.2sccm H 2 0.4sccm} PH 3 0.4sccm deposition rate 1.5Å / sec Deposition temperature 250 ° C, and then a non-doped layer of 5500Å was formed by the same glow discharge method, discharge frequency 13.56MHz discharge power density 0.03W / cm ² pressure 0.2Torr gas flow rate SiH ₄ 1sccm H ₂ 49sccm Deposition rate: 2Å / sec Deposition temperature: 245 ° C. A P-type doped layer of 100 さ ら に was further grown by the glow discharge method at a discharge frequency of 13.56 MHz, a discharge power density of 0.4 W / cm ^{2, a} pressure of 0.2 Torr, and a gas flow rate of SiH _4. 0.1 sccm H ₂ 70 sccm BF ₃ 0.05 sccm Deposition rate 1Å / sec Deposition temperature 240 ° C. The photovoltaic layer 104 having a PIN structure was obtained.

Subsequently, the ITO layer 1 was formed by the electron beam heating evaporation method shown in FIG.
06 is deposited at 700 ° and a silver grid electrode 106 having a thickness of 1 μm is formed by electron beam heating deposition.
I got

When the carrier absorption spectrum of the thus obtained solar cell was measured, it was found that the structure of the following 802 to 805 was obtained as shown at 801 in FIG. 803: The light reflecting metal layer forming temperature is 250 ° C., the height difference of the light reflecting metal layer unevenness is 1000 ° (average value), the buffer layer forming speed is 1/5, and the buffer layer unevenness is Height difference 8000Å on average
804: A light-reflective metal layer forming temperature of 70 ° C and a smooth surface of the light-reflective metal layer, 805: A light-reflective metal layer forming temperature of 70 ° C and a buffer layer forming temperature of
Compared to the case where the light reflective metal layer and the buffer layer were both smooth surfaces at 220 ° C, the collection efficiency was improved, especially for light around 700 nm. AM1.0 of sample 801 at this time (100 mW / cm
² ) The efficiency under sunlight was 9.8%.

In addition, the survival rate of a plurality of cells having an area of 1 cm ² (percentage of all cells excluding unusable cells after shunting) was examined.
The solar cell having the substrate structure according to the present invention has a remarkable improvement effect as compared with those without the buffer layer (30 to 50%) and those with the substrate surface roughened by 1 s by sandblasting (20 to 30%). Have. At the same time, a slight improvement in the open-circuit voltage was observed, indicating that there was an effect on the short circuit of the cell.

Example 2 The sputtering temperature of the buffer layer in Example 1 was set at 250 ° C. to 400 ° C.
A solar cell was prepared in the same manner as in Example 1 except that the temperature was changed at ° C.

FIG. 9 shows the tendency of the size d of the thickness of the buffer layer when the temperature is used as a parameter and the efficiency η of the solar cell corresponding thereto. The wavy portion of the curve corresponding to efficiency indicates that the measurement is inaccurate due to the increase in shunt current due to the short circuit. As can be seen from this figure, when the size d of the buffer layer exceeds 0.1, the efficiency is improved, and when the size d exceeds 1 micron, it is the opposite effect.

Example 3 The light-reflective metal layer in Example 1 was used in the crucible used. Molybdenum Evaporation material Silver Evaporation material form Chunk Evaporation material purity 99.99% Electron beam current 60 mA Pressure 5 × 10 ⁻⁶ Torr Average deposition rate 20 ° / sec Substrate temperature 300 ° C. A solar cell was manufactured in the same configuration as in Example 1 except that the solar cell was formed by the electron beam heating vapor deposition shown in FIG. 6 under certain conditions. A light-reflective metal layer composed of silver crystal grains having a size of 0.6 to 0.9 micron was confirmed, and the solar cell had a collection efficiency spectrum in the long wavelength region that was longer than that having a smooth metal reflection surface without crystal grains. We saw a 10% improvement.

Example 4 The light-reflective metal layer in Example 1 was used in a crucible. Molybdenum Evaporation material Chromium Evaporation material form Pellet Evaporation material purity 99.99% Electron beam current 120 mA Pressure 5 × 10 ⁻⁶ Torr Average deposition rate 10Å / sec Substrate temperature 370 ° C. A solar cell was manufactured in the same configuration as in Example 1 except that the solar cell was formed by the electron beam heating vapor deposition shown in FIG. 6 under certain conditions. A light-reflective metal layer composed of crystal grains of chromium having a size of 0.6 to 0.9 micron was confirmed, and the solar cell was more efficient in the long wavelength region of the collection efficiency spectrum than one having a smooth metal reflective surface without crystal grains. We saw a 5% improvement.

Example 5 The light-reflective metal layer in Example 1 was formed by using a filament tungsten evaporating material aluminum evaporating material form wire evaporating material purity 99.99% filament current 50A pressure 5 × 10 ^-6 Torr average deposition rate 15Å / sec substrate temperature 420 ° C. A solar cell was produced in the same configuration as in Example 1 except that the solar cell was formed by resistance heating evaporation shown in FIG. 0.6
A light-reflective metal layer composed of aluminum crystal grains having a size of about 0.9 μm was confirmed. The solar cell was about 7 times longer in the long wavelength region of the collection efficiency spectrum than a solar cell having a smooth metal reflection surface without crystal grains. Seen a percent improvement. As a result AM1.0
(100 mW / cm ² ), a photoelectric conversion efficiency of 11% was obtained.

Example 6 Instead of the stainless steel substrate used in Example 1, a SUS410 having the same stainless steel but having a martensite phase was used, and a solar cell was produced in the same manner and under the same conditions as in Example 1 except for the above.

As compared with a substrate having the same substrate and having a smooth light-reflective metal layer, the collection efficiency spectrum in the long wavelength region showed an improvement of about 10%.

Example 7 The same as Example 1 except that 7059 glass (alkali-free glass) was used instead of the stainless steel substrate used in Example 1, and that a silver layer as a light-reflective metal layer was deposited by 3 μm. A solar cell was made according to the method and conditions.

The electrical characteristics of the solar cell between the grid electrode on the light incident side and the light-reflective metal layer were measured and compared with those with a smooth light-reflective metal layer. Showed about 10% improvement.

Example 8 The conditions for forming the buffer layer in Example 1 were set. The target ITO target purity was 99.99%. Sputtering gas Argon / oxygen Sputtering pressure 5 × 10 ⁻³ Torr Argon flow rate 25 sccm Oxygen flow rate 0.1 sccm Sputtering current 0.8 A Sputtering rate 170 ° / min Sputtering A solar cell was formed by the same method, conditions and apparatus as in Example 1 except that the substrate temperature was 270 ° C. for 1 hour.

As compared with the same substrate having a smooth light-reflective metal layer, the collection efficiency spectrum in the long wavelength region showed an improvement of about 9%.

[Summary of effects of the invention]

As mentioned above, the solar cell according to the invention has the advantages outlined below.

(I) reflects light partially absorbed by the optically active layer at an effective angle; (ii) reflects light as a carrier distribution in which traveling of the optical carrier due to the reflected light contributes to collection efficiency; (iii) The use of the substrate does not lead to a decrease in the open-circuit voltage or an increase in the short-circuit current, but preferably contributes to the prevention of the short-circuit. And the solar cell according to the present invention that provides these various effects can be manufactured by a simple method, and
Widely applicable to substrate materials, electrodes and optically active layer materials.

[Brief description of the drawings]

FIG. 1 is a structural diagram of a solar cell having an improved substrate according to the present invention. FIG. 2 is a diagram showing the relationship between the grain size of the light-reflective metal layer polycrystalline film according to the present invention and the height difference of surface irregularities, and the conversion efficiency. FIG. 3 is a diagram showing the relationship between the film thickness d of the buffer layer polycrystalline film according to the present invention and the height difference between the surface irregularities and the conversion efficiency. FIGS. 4, 6 and 7 are schematic structural views showing examples of a sputtering apparatus, an electron beam evaporation apparatus, and a resistance heating evaporation apparatus used in producing a light-reflective metal layer having surface irregularities in the present invention. . FIG. 5 is a schematic structural diagram showing an example of a plasma CVD apparatus suitable for forming an amorphous silicon film in the present invention. FIG. 8 is a comparison diagram of the photocarrier collection efficiency showing the improvement effect of the solar cell in the example of the present invention. FIG. 9 is a diagram showing a difference in height of surface irregularities of the buffer layer and a change in solar cell efficiency with respect to temperature in the example of the present invention. In the figure, 101: base, 102: light-reflective metal layer, 103: buffer layer, 104: photovoltaic layer, 105: transparent electrode, 106: grid electrode, 401, 501, 601, 701: vacuum vessel, 403, 503, 603, 703 ... heating plate, 405, 505, 605, 705 ... temperature controller, 408, 508, 608, 708 ... base, 410 ... target, 413 ... magnet, 416 ... sputter power supply, 422, 622 ... ... ion gauge, 423,623 ... vacuum gauge, 424, 524, 624, 724 ... ... main valve, 440,441,550 to 556,560… manometer, 562… RF power supply, 657,757… shutter, 658… crucible, 659… hearth, 663… filament, 664… power supply, 661… electron beam acceleration voltage source, 771… Filament, 773 ... Heating power supply.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Mitsuyuki Niwa 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (56) References JP-A-58-159383 (JP, A) JP-A Sho 61-288473 (JP, A) JP-A-1-154570 (JP, A) JP-A-1-137675 (JP, A) JP-A-59-213174 (JP, A) JP-A-1-63150 (JP, A) U) (58) Field surveyed (Int. Cl. ⁶ , DB name) H01L 31/04

Claims (3)

(57) [Claims] 1. A buffer layer provided on a substrate in contact with a light-reflective metal layer, substantially transparent to light reflected from the light-reflective metal layer, and capable of flowing a short-circuit current, and incident light. And a photovoltaic layer that generates photovoltaic power with respect to the photovoltaic layer, wherein the light-reflective metal layer has a height difference larger than the wavelength of light that is partially reflected and absorbed by the photovoltaic layer. Having a surface irregularity having, all or a part of the material constituting the surface irregularity is made of polycrystalline Ag or Al or an Al-Si alloy or an alloy thereof, and the grain size of the polycrystalline is at most the surface irregularity. The buffer layer has surface irregularities having a height difference smaller than the wavelength of light partially reflected by the light-reflective metal layer and absorbed by the photovoltaic layer, and the buffer layer All or part of the material constituting polycrystalline ZnO or ZnS or ZnSe, the particle size of the polycrystalline is small Also the magnitude of the difference in height of the surface irregularities, and solar cells the thickness of the buffer layer is characterized in that it is a 0.2～14Myuemu. 2. The method according to claim 1, wherein the height difference of the surface irregularities of the light-reflective metal layer is
The solar cell according to claim 1, which has a thickness of 0.1 to 5 m. 3. The solar cell according to claim 1, wherein the photovoltaic layer is mainly made of a material containing at least one of Si, Ge, C, B, and P.