JP2013012593A - Thin film photoelectric conversion device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thin film photoelectric conversion device in which an open voltage of a thin film photoelectric conversion device decreases even in the case where irregularity of a transparent electrode layer is high and a haze rate is not less than 30%, and thereby conversion efficiency does not decrease.SOLUTION: A thin film photoelectric conversion device 6 includes one or more photoelectric conversion units 3 in which a p-type semiconductor layer 31, a photoelectric conversion layer 32 of a substantially intrinsic semiconductor and an n-type semiconductor layer 33 are sequentially laminated on a transparent electrode layer 2. The transparent electrode layer 2 has a structure in which a zinc oxide 21 and an ITO (indium tin oxide) 22 are sequentially laminated from the light incident side. A haze rate of the transparent electrode layer 2 is not less than 30% and a film thickness of the ITO 22 is not more than 100 nm. The p-type semiconductor layer 31 in the photoelectric conversion unit 3 in contact with the transparent electrode layer 2 is composed of a silicon contained p-type amorphous semiconductor layer.

Description

  The present invention relates to an improvement of a thin film photoelectric conversion device, and more particularly to an improvement of a thin film photoelectric conversion device using a transparent electrode layer of zinc oxide having large irregularities. Note that the terms “crystalline” and “microcrystal” in the present specification are also used in the case of partially containing amorphous, as used in this technical field.

  In recent years, photoelectric conversion devices that convert light into electricity using the photoelectric effect inside semiconductors have attracted attention and development has been vigorously conducted. Among these photoelectric conversion devices, silicon-based thin film photoelectric conversion devices are large at low temperatures. Since it can be formed on a glass substrate or a stainless steel substrate having an area, cost reduction can be expected.

  Such a silicon-based thin film photoelectric conversion device generally includes a transparent electrode layer, one or more photoelectric conversion units, and a back electrode layer that are sequentially stacked on a transparent insulating substrate. Here, the photoelectric conversion unit generally has a p-type layer, an i-type layer, and an n-type layer laminated in this order or vice versa, and the i-type photoelectric conversion layer occupying the main part is amorphous. Is called an amorphous photoelectric conversion unit, and those having an i-type layer crystalline are called crystalline photoelectric conversion units.

  The photoelectric conversion layer is a layer that absorbs light and generates electron-hole pairs. In general, in a silicon-based thin film photoelectric conversion device, an i-type layer of a pin junction is a photoelectric conversion layer. The i-type layer which is a photoelectric conversion layer occupies the main film thickness of the photoelectric conversion unit.

  The i-type layer is an intrinsic semiconductor layer that does not ideally contain a conductivity determining impurity. However, even if a small amount of impurities is included, if the Fermi level is at the approximate center of the forbidden band, it functions as a pin junction i-type layer, which is substantially called an i-type layer. Generally, a substantially i-type layer is produced without adding a conductivity determining impurity to a source gas. In this case, a conductivity determining impurity may be included in an allowable range that functions as an i-type layer. Further, the substantially i-type layer may be formed by intentionally adding a small amount of conductivity-type determining impurities in order to remove the influence of impurities caused by the atmosphere or the underlayer on the Fermi level.

  On the other hand, the p-type or n-type conductive layer plays a role of generating a diffusion potential in the photoelectric conversion unit, and the magnitude of the diffusion potential causes an open end voltage, which is one of the important characteristics of the thin film photoelectric conversion device. The value depends. However, these conductive layers are inactive layers that do not contribute to photoelectric conversion, and light absorbed by impurities doped in the conductive layers does not contribute to power generation and is lost. Therefore, it is preferable that the thicknesses of the p-type and n-type conductive layers be as thin as possible within a range that generates a sufficient diffusion potential.

  By the way, the thin film photoelectric conversion device can make the photoelectric conversion layer thinner compared to the conventional photoelectric conversion device using bulk single crystal or polycrystalline silicon. There is a problem that it is limited by. Therefore, in order to use light incident on the semiconductor layer more effectively, the surface of the transparent electrode layer or the back electrode layer in contact with the semiconductor layer is roughened (textured), and light is scattered at the interface to scatter the light inside the semiconductor layer. In order to extend the optical path length of the light-emitting layer, a device for increasing the amount of light absorbed in the photoelectric conversion layer has been devised. This technique is called “optical confinement”, and is an important elemental technique for putting a thin film photoelectric conversion device having high photoelectric conversion efficiency into practical use.

  In order to obtain the surface uneven shape of the transparent electrode layer optimal for the thin film photoelectric conversion device, an index that quantitatively represents the uneven shape is required. Haze rate and surface area ratio (Sdr) are known as indices representing such surface irregularities.

  The haze ratio is an index for optically evaluating the surface unevenness of the transparent substrate, and is expressed by (diffuse transmittance / total light transmittance) × 100 [%] (JIS K7136). Such a haze rate can be automatically measured by a commercially available haze meter. A C light source is generally used as a light source for the measurement.

  The surface area ratio is an index that represents not only the size of the surface unevenness but also the shape of the unevenness. If the surface irregularities of the transparent electrode layer are sharpened, the open-circuit voltage and the fill factor of the thin film photoelectric conversion device may decrease, so the surface area ratio is effective as an index of the surface irregularities of the transparent electrode layer in the thin film photoelectric conversion device. . The surface area ratio is also called a developed surface area ratio, and Sdr is used as an abbreviation. For the definition of surface area ratio, KJ Stout, PJ Sullivan, WP Dong, E. Manisah, N. Luo, T. Mathia: "The development of methods for characterization of roughness on three dimensions", Publication no.EUR 15178 See EN of the Commission of the European Communities, Lucembourg, 1994.

  As a method for improving the conversion efficiency of a photoelectric conversion device, a photoelectric conversion device employing a structure called a stacked type in which two or more photoelectric conversion units are stacked is known. In this method, a front photoelectric conversion unit including a photoelectric conversion layer having a large optical forbidden band width is disposed on the light incident side of the photoelectric conversion device, and subsequently has a small optical forbidden band width (for example, Si− By arranging a rear photoelectric conversion unit including a photoelectric conversion layer (such as a Ge alloy), it is possible to perform photoelectric conversion over a wide wavelength range of incident light, and improve the conversion efficiency of the entire device by effectively using incident light Is planned.

  For example, in the case of a two-junction thin film photoelectric conversion device in which an amorphous silicon photoelectric conversion unit and a crystalline silicon photoelectric conversion unit are stacked, the wavelength of light that can be photoelectrically converted by i-type amorphous silicon (a-Si) is Although it is up to about 700 nm on the long wavelength side, i-type crystalline silicon can photoelectrically convert light having a longer wavelength of about 1100 nm. Here, in the amorphous silicon photoelectric conversion layer made of amorphous silicon having a large light absorption coefficient, a thickness of about 0.3 μm is sufficient for light absorption sufficient for photoelectric conversion. The crystalline silicon photoelectric conversion layer made of crystalline silicon having a small absorption coefficient preferably has a thickness of about 2 to 3 μm or more in order to sufficiently absorb long wavelength light. That is, the crystalline silicon photoelectric conversion layer usually needs to be about 10 times as thick as the amorphous silicon photoelectric conversion layer.

The photoelectric conversion unit as described above is formed on a transparent electrode layer. For example, a conductive metal oxide such as indium tin oxide (ITO), tin oxide (SnO ₂ ), and zinc oxide (ZnO) is formed on the transparent electrode layer. And is formed by a method such as CVD, sputtering, or vapor deposition. The transparent electrode layer desirably has an effect of increasing the scattering of incident light by having fine irregularities on the surface. By scattering incident light, the optical path length in the photoelectric conversion unit is extended, the short-circuit current density of the photoelectric conversion device is increased, and the conversion efficiency is improved. Among the transparent electrode layers, SnO ₂ is widely used in thin film photoelectric conversion devices. In recent years, ZnO, which is excellent in terms of transmittance for long-wavelength light, controllability of the haze ratio, which is an index of the light confinement effect, and resistance to hydrogen-containing plasma, is also used as a transparent electrode layer for thin film photoelectric conversion devices. It has come to be used.

  Generally, a plasma CVD (chemical vapor deposition) method is used as a method for forming the photoelectric conversion unit. As described above, the p-type semiconductor layer is preferably as translucent as possible and has high conductivity. In the photoelectric conversion unit on the transparent conductive electrode of tin oxide (SnO2), a p-type amorphous semiconductor layer containing silicon as such a p-type semiconductor layer, for example, p-type amorphous silicon or p-type amorphous silicon carbide is used. Widely adopted and high conversion efficiency is obtained. In the case of tin oxide, it is known that tin oxide is slightly reduced when a p-type amorphous semiconductor layer is formed by plasma CVD, and an ohmic contact is easily obtained. However, when plasma CVD conditions are high hydrogen dilution or high power, tin oxide is strongly reduced, and metal is deposited on the surface of the transparent electrode layer, resulting in a problem that the transmittance is reduced and the generated current is reduced. .

  On the other hand, when a zinc oxide layer is used as the transparent electrode layer, if a p-type amorphous semiconductor layer is used as the p-type semiconductor layer, the open-circuit voltage (Voc) and the fill factor (FF) of the photoelectric conversion device are used. It is known that a high conversion efficiency cannot be obtained due to a decrease in the temperature. This is because the contact resistance between the zinc oxide layer and the p-type amorphous semiconductor layer is high, and it is difficult to obtain ohmic characteristics. In the case of zinc oxide, it is considered that it is difficult to obtain ohmic characteristics with the p-type amorphous semiconductor layer because of its high resistance to plasma.

(Prior Example 1)
Non-Patent Document 1 discloses that a ohmic contact can be improved by inserting a p-type microcrystalline silicon layer between a transparent electrode layer of zinc oxide and a p-type amorphous silicon layer. The p-type microcrystalline silicon layer is considered to improve ohmic contact with zinc oxide by exhibiting a high conductivity of 1 S / cm or more. Alternatively, the p-type microcrystalline silicon is formed using a highly reducible plasma under a high hydrogen dilution condition in which the H ₂ / SiH ₄ flow rate ratio is 267 times, and the surface of zinc oxide is moderately reduced. The ohmic contact is considered to be improved.

(Prior Example 2)
Patent Document 1 discloses that an ohmic contact can be improved by forming two p-type microcrystalline silicon layers between zinc oxide and a p-type amorphous silicon carbide layer. The first p-type microcrystalline silicon layer close to zinc oxide can be easily crystallized by reducing the dopant concentration, and the crystallinity of the second p-type microcrystalline silicon layer using the first p-type microcrystalline silicon layer as a base layer can be increased. Disclosure. Patent Document 1 discloses a surface area ratio of 55% for the uneven size of zinc oxide, but does not disclose a haze ratio.

T. Roschek, Ja. Muller, S. Wieder, B. Rech, H. Wagner, 16th European Photovoltaic Solar Energy Conference Proceedings, pp. 561-564 (2000)

JP 2008-124325 A

  In order to form good ohmic characteristics between the transparent electrode layer of zinc oxide and the amorphous p-type semiconductor layer of the photoelectric conversion unit, it is possible to insert a p-type microcrystalline silicon layer between them in the preceding examples 1 and 2. However, in order to increase the light confinement effect, if the unevenness of the zinc oxide is increased so that the haze ratio is 30% or more, the open circuit voltage (Voc) is low even if the p-type microcrystalline silicon layer is used. Thus, the inventors have found that there is a problem that the conversion efficiency (Eff) is lowered. When the unevenness of the transparent electrode layer of zinc oxide is large, it is possible to form a film so as to uniformly cover the unevenness, that is, to form a so-called “coverage”, in a thin p-type microcrystalline silicon layer. This is considered to be difficult. In addition, since the p-type layer is composed of a p-type microcrystalline silicon layer and a p-type amorphous semiconductor layer, the light absorption loss of the p-type microcrystalline silicon layer increases and the short circuit current density (Jsc) is improved. There are issues that are not enough.

  In view of the above, the present invention provides a thin film photoelectric conversion device with improved conversion efficiency that suppresses a decrease in open-circuit voltage and increases a short-circuit current density by using a zinc oxide transparent electrode layer having large irregularities. With the goal.

  A thin film photoelectric conversion device according to the present invention includes one or more photoelectric conversion units in which a p-type semiconductor layer, a substantially intrinsic semiconductor photoelectric conversion layer, and an n-type semiconductor layer are sequentially stacked on a transparent electrode layer. The transparent electrode layer has a structure in which zinc oxide and ITO (indium tin oxide) are sequentially laminated from the light incident side, the haze ratio of the transparent electrode layer is 30% or more, and the film thickness of ITO is 100 nm or less. The p-type semiconductor layer in the photoelectric conversion unit in contact with the transparent electrode layer is formed of a silicon-containing p-type amorphous semiconductor layer, thereby solving the problem. By forming ITO of 100 nm or less on zinc oxide, the contact resistance at the interface between zinc oxide and ITO, and the interface between ITO and p-type amorphous semiconductor layer is lowered, and good ohmic characteristics can be obtained. The problem is solved by suppressing the decrease of the open circuit voltage. In addition, by forming the p-type amorphous semiconductor layer directly on the transparent electrode layer, it becomes possible to form the p-type layer with good coverage even on a film with large irregularities, and the reduction in Voc is suppressed. To solve. Furthermore, since p-type microcrystalline silicon is unnecessary, the light absorption loss can be reduced and the short-circuit current density can be improved.

  In general, the reduction resistance of ITO to plasma is not only lower than that of zinc oxide but also lower than that of tin oxide. Therefore, in order to avoid the problem of a decrease in transmittance of the transparent electrode layer due to strong reduction of ITO, Normally, ITO is not used when there is a photoelectric conversion unit formed by plasma CVD on a transparent electrode layer. However, in the present invention, ITO is made as thin as 100 nm or less, and the p-type layer attached on the ITO is a silicon-containing p-type amorphous semiconductor layer that can be formed under plasma conditions with relatively low reducing properties. Is suppressed from being strongly reduced, and both Voc and Jsc are improved.

  The silicon-containing p-type amorphous semiconductor layer used in the present invention can be formed of amorphous silicon or an amorphous silicon alloy containing at least one of C, O, and N.

  Further, the zinc oxide having a haze ratio of 30% or more according to the present invention can be produced by a low pressure thermal CVD method.

  The photoelectric conversion unit of the present invention can be produced by a plasma CVD method.

  In addition, the ITO used in the present invention can be manufactured by an RPD method (Reactive Plasma Deposition). ITO manufactured by the RPD method is desirable because it can have higher conductivity than ITO manufactured by a general sputtering method.

  According to the present invention, by forming an ITO and silicon-containing p-type amorphous semiconductor layer of 100 nm or less on zinc oxide having a haze ratio of 30% or more, good ohmic contact with good coverage even on zinc oxide having large irregularities. Thus, a thin film photoelectric conversion device with improved conversion efficiency can be provided by improving both Voc and Jsc.

It is typical sectional drawing which shows the thin film photoelectric conversion apparatus by one Embodiment of this invention. It is a graph which shows the relationship between the open circuit voltage (Voc) of the thin film photoelectric conversion apparatus containing Example 1, 2 and Comparative Examples 2, 4 and the haze rate in the transparent electrode layer. It is a graph which shows the relationship between the short circuit current density (Jsc) of the thin film photoelectric conversion apparatus containing Example 1, 2 and Comparative Examples 2, 4 and the haze rate in the transparent electrode layer. It is a graph which shows the relationship between the fill factor (FF) of the thin film photoelectric conversion apparatus containing Examples 1, 2 and Comparative Examples 2, 4 and the haze rate in the transparent electrode layer. It is a graph which shows the relationship between the conversion efficiency (Eff) of the thin film photoelectric conversion apparatus containing Examples 1, 2 and Comparative Examples 2, 4 and the haze rate in the transparent electrode layer.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. In each drawing of the present application, dimensional relationships such as thickness and length are appropriately changed for clarity and simplification of the drawings, and do not represent actual dimensional relationships. Moreover, in each figure, the same referential mark represents the same part or an equivalent part.

  FIG. 1 is a cross-sectional view of a thin film photoelectric conversion device substrate and a thin film photoelectric conversion device according to an example of an embodiment of the present invention. On the transparent substrate 1, the transparent electrode layer 2, the front photoelectric conversion unit 3, the rear photoelectric conversion unit 4, and the back electrode layer 5 are arranged in this order to form a thin film photoelectric conversion device 6.

  For the transparent substrate 1, a plate-like member or a sheet-like member made of glass, transparent resin or the like is mainly used. In particular, when a glass substrate is mainly used as the light-transmitting insulating substrate, it is preferable as the light-transmitting insulating substrate because of its high transmittance and low cost.

  Since the transparent substrate 1 is located on the light incident side when the thin film photoelectric conversion device 6 configured as shown in FIG. 1 is configured, for example, the transparent substrate 1 transmits more sunlight and can generate amorphous or crystalline photoelectric. In order to be absorbed by the conversion unit, it is preferably as transparent as possible, and a glass plate is suitable as the material. For the same purpose, it is desirable to apply a non-reflective coating to the light incident surface of the translucent insulating substrate so as to reduce the light reflection loss on the light incident surface of sunlight.

  Although it is possible to use a glass substrate alone as the transparent substrate 1, it is desirable that the transparent substrate 1 has a light-transmitting underlayer formed on the surface on which the transparent electrode layer is formed (not shown). By forming the transparent base layer, the adhesion of the transparent electrode layer can be improved, the uneven shape of the transparent electrode layer can be controlled, and the haze ratio or surface area ratio can be controlled to a desired value. Is possible. In this case, it is preferable that the translucent underlayer has fine surface irregularities having a root mean square roughness of 5 to 50 nm at the interface on the transparent electrode layer 2 side, and the convex portions are formed of curved surfaces. . The light-transmitting undercoat layer can be produced, for example, by applying silica fine particles having a particle diameter of 50 to 200 nm as light-transmitting fine particles together with a silicon oxide binder-forming material containing a solvent. Examples of the method for applying the coating liquid include a dipping method, a spin coating method, a bar coating method, a spray method, a die coating method, a roll coating method, and a flow coating method, but the transparent fine particles are formed densely and uniformly. For this purpose, a roll coating method is preferably used. When the coating operation is completed, the coated thin film is immediately dried by heating.

  As the transparent electrode layer 2 disposed on the transparent substrate 1, a structure in which ZnO is sequentially laminated as the first transparent electrode layer 21 and ITO is sequentially laminated as the second transparent electrode layer 22 is used. As the transparent electrode layer 2, a layer having irregularities with a haze ratio of 30% or more is used in order to enhance the light confinement effect. In order to increase the haze ratio of the transparent electrode layer 2 to 30% or more, a concavo-convex structure is mainly formed by the first transparent electrode layer 21. It is desirable to form a concavo-convex structure having a haze ratio of 30% or more with only ZnO.

  As a method of forming ZnO having an uneven structure, it can be obtained by forming unevenness by etching ZnO formed by vapor deposition or sputtering with hydrochloric acid or acetic acid. Alternatively, uneven ZnO can be directly formed by low pressure thermal CVD. In the case of the former requiring an etching process, it is difficult to form unevenness uniformly in a reproducibility or a large area, so a low pressure thermal CVD method capable of forming an uneven structure during ZnO fabrication is desirable.

  In the present invention, the term “low pressure thermal CVD method” refers to a thermochemical vapor deposition method using a gas having a pressure lower than atmospheric pressure. Low pressure thermal CVD is also called low pressure CVD or low pressure CVD (abbreviation LP-CVD), and is defined as thermochemical vapor deposition using a gas lower than atmospheric pressure. The Usually, the term “CVD” refers to “thermal CVD” except when an energy source is clearly indicated, such as “plasma CVD”, “photo CVD”, etc. It is synonymous with “thermal CVD method”. In addition, it is clear that the low pressure thermal CVD method includes an organic metal CVD method under reduced pressure (abbreviation, MO-CVD method).

Specifically, ZnO film formation by the low pressure thermal CVD method uses diethyl zinc (DEZ) or dimethyl zinc as an organic metal vapor, water as an oxidant vapor, B ₂ H ₆ as a doping gas, and H ₂ as a dilution gas. It is preferable to perform ZnO film formation by adding one or a plurality of He, Ar, and introducing the mixed gas into a vacuum chamber maintained at a pressure of 5 to 200 Pa. The substrate temperature during film formation is preferably 200 ° C. or lower, more preferably 140 ° C. or higher and 170 ° C. or lower. Specifically, the flow rate of DEZ is 10 to 1000 sccm, the flow rate of water is 10 to 1000 sccm, the flow rate of H ₂ is 100 to 10,000 sccm, and the flow rate of Ar is 100 to 10,000 sccm. B ₂ H ₆ is preferably formed into a film at 0.1% to 10% with respect to DEZ.

  The ZnO particle size is preferably about 50 to 500 nm, and a thin film having surface irregularities with an irregularity height of about 20 to 200 nm, from the viewpoint of obtaining the light confinement effect of the thin film photoelectric conversion device. The sheet resistance of ZnO is preferably 15Ω / □ or less in order to suppress resistance loss.

  The average thickness of the ZnO film is preferably 0.7 to 5 μm, and more preferably 1 to 3 μm. This is because if the ZnO film is too thin, it is difficult to sufficiently provide unevenness that effectively contributes to the light confinement effect, and it is difficult to obtain the necessary conductivity as the transparent conductive film. This is because the amount of light that passes through ZnO and reaches the photoelectric conversion unit is reduced by absorption, and the efficiency is lowered. Furthermore, when it is too thick, the film forming cost increases due to an increase in the film forming time.

  Further, the surface area ratio (Sdr) is preferably 55% or more and 200% or less under the ZnO film forming conditions. When Sdr is too large, the open circuit voltage (Voc) and the fill factor (FF) decrease, and Eff decreases. In some cases, the short circuit current density (Jsc) decreases and the conversion efficiency (Eff) decreases. When Sdr is large, Voc and FF decrease because the unevenness of the substrate for the thin film photoelectric conversion device becomes sharp, the coverage of the silicon semiconductor layer on the transparent conductive film deteriorates, and the contact resistance increases or leaks. This is thought to be due to an increase in current. The reason why Jsc decreases when Sdr is large is thought to be that the growth of the semiconductor layer on the transparent conductive film is inhibited, the film quality of the semiconductor layer is deteriorated, and loss due to carrier recombination increases. On the contrary, when Sdr is too small, the unevenness of the thin film photoelectric conversion device substrate becomes small, so that the effect of light confinement is weakened, the short-circuit current density (Jsc) is lowered, and Eff is lowered. The surface area ratio can be controlled to an optimum value by controlling the ZnO film forming conditions. For example, in the low-pressure thermal CVD method, the surface area ratio of ZnO varies greatly depending on the film forming conditions such as the substrate temperature, the raw material gas flow rate, and the pressure. Therefore, it is possible to control the surface area ratio to a desired value. It is.

  As the ITO of the second transparent electrode layer 22, the thickness of the ITO is reduced to 100 nm or less in order to suppress a decrease in transmittance due to reduction by plasma. The ITO can be formed by vapor deposition, sputtering, low-pressure thermal CVD, RPD (Reactive Plasma Deposition, reactive plasma deposition) or the like.

Specifically, in the case of the sputtering method, an In ₂ O ₃ target to which 1 to 20% by weight of SnO ₂ is added is used, and a gas in which 0.1 to 30% of oxygen is added to Ar is used for 0.1 to 0. ITO can be formed by discharging at a pressure of 5 Pa.

In particular, the RPD method is preferable because the conductivity of ITO can be improved two to three times as compared with the above sputtering method. The RPD method is also called a plasma ion plating method, and the plasma formation method is also called Uramoto Gun (Japanese Patent Laid-Open No. 55-148337). Specifically, in the RPD method, an electron beam is drawn out from the Ar plasma formed by 100 to 200 A of high-current DC discharge called a plasma gun by appropriately arranging extraction electrodes and a focusing magnet, and the tablet The material is evaporated by applying an electron beam to a material called, and a film is formed on the substrate. The tablet used was sintered In ₂ O ₃ to which SnO ₂ was added in an amount of 1 to 20% by weight in substantially the same manner as the sputtering, Ar inside the plasma gun, and oxygen in Ar in the film forming chamber was set to 0.0. A gas added with 1 to 30% is used. The pressure in the film forming chamber is 0.1 to 0.5 Pa, and ITO can be formed by evaporating the tablet material with an electron beam.

  The film thickness of ITO is preferably 5 nm to 100 nm, and more preferably 20 nm to 80 nm. In order to form ITO on ZnO with good coverage, it is desirable that the thickness of ITO is 5 nm or more. By making the film thickness of ITO 5 nm or more, ZnO is uniformly covered with good coverage, contact resistance is reduced, and FF and Voc are increased. Further, when the ITO is made thinner than 20 nm, the resistivity increases rapidly, so it is desirable to make the ITO film thickness 20 nm or more. In order to suppress absorption loss due to reduction of ITO, the thickness is desirably 100 nm or less, and more desirably 80 nm or less.

  It is an important component of the present invention that the p-type semiconductor layer of the photoelectric conversion unit in contact with the transparent electrode layer 2 is a silicon-containing p-type amorphous semiconductor layer. In order to increase the open-circuit voltage (Voc) of the thin film photoelectric conversion device, it is necessary to form a p-type semiconductor layer on the transparent electrode layer 2 with large unevenness with good coverage. The problem is solved by using an amorphous semiconductor layer. If there is a portion where the p-type semiconductor layer is not partially attached on the transparent electrode layer, Voc is significantly reduced. The reason why the coverage deteriorates when the p-type semiconductor layer is crystalline silicon is that the growth process of p-type microcrystalline silicon firstly generates seed crystals and grows crystals on it, so that it easily grows in an island shape. This is probably because when the film thickness is small, p-type microcrystalline silicon is not partially attached. In particular, when the unevenness of the transparent electrode layer, which is a base layer, is large, p-type microcrystalline silicon is likely to grow in an island shape, resulting in poor coverage and a decrease in Voc. On the other hand, by forming the p-type semiconductor layer as a silicon-containing p-type amorphous semiconductor layer, it is not necessary to generate a seed crystal in the growth process, and the p-type semiconductor has good coverage on the transparent electrode having large irregularities. It is possible to form a layer, and it is considered that Voc is improved.

  Although it is difficult to obtain ohmic characteristics when ZnO and the silicon-containing p-type amorphous semiconductor layer are in contact with each other, ohmic characteristics can be realized by disposing a thin ITO of 100 nm or less between the two.

  The silicon-containing p-type amorphous semiconductor layer can be formed of amorphous silicon or an amorphous silicon alloy containing at least one of C, O, and N. Examples of the material of the amorphous silicon alloy include amorphous silicon carbide (a-SiC), amorphous silicon oxide (a-SiO), amorphous silicon nitride (a-SiN), and amorphous silicon oxy Carbide (a-SiCO), amorphous silicon oxynitride (a-SiON), or the like is used. In particular, a-SiC and a-SiO are desirable because they have high transmittance and can easily increase conductivity.

Specifically, as the formation conditions of p-type a-SiC, SiH ₄ , H ₂ , B ₂ H ₆ and CH ₄ are used as reaction gases, the hydrogen dilution ratio H ₂ / SiH ₄ is 5 to 30 times, and CH ₄ / SiH. _{4 The} flow rate ratio is 1 to 10 times, B ₂ H ₆ / SiH _{4 The} flow rate ratio is 0.001 to 0.05 times, the pressure is 10 to 1500 Pa, the discharge frequency is 10 to 100 MHz, and the discharge power density is 10 to 30 mW / cm ² . can do.

On the other hand, as a specific example of the conditions for forming the p-type microcrystalline silicon, SiH ₄ , H ₂ , and B ₂ H ₆ are used as the reaction gas, the hydrogen dilution ratio H ₂ / SiH ₄ is 50 to 300 times, B It can be produced at a flow rate ratio of ₂ H ₆ / SiH _{4 of} 0.001 to 0.05 times, a pressure of 10 to 1500 Pa, a discharge frequency of 10 to 100 MHz, and a discharge power density of 50 to 500 mW / cm ² .

  The formation conditions of p-type a-SiC are lower than that of p-type microcrystalline silicon, and the hydrogen dilution ratio is low and the discharge power density is low. It becomes a weak plasma. For this reason, in the present invention, since a p-type amorphous semiconductor layer is formed on ITO instead of p-type microcrystalline silicon, strong reduction does not occur, and a decrease in the transmittance of ITO can be suppressed.

  The photoelectric conversion unit may be a single-junction thin-film photoelectric conversion device including only one photoelectric conversion unit, but in order to effectively use light in a wide wavelength range, a stacked type in which a plurality of photoelectric conversion units are stacked. It is desirable to use a thin film photoelectric conversion device.

  If an amorphous silicon-based material is selected as the front photoelectric conversion unit 3, it has sensitivity to light of about 360 to 800 nm, and if a crystalline silicon-based material is selected for the rear photoelectric conversion unit 4, it is longer than about 1200 nm. Sensitivity to light. Therefore, the thin-film photoelectric conversion device 6 arranged in this order from the light incident side to the front photoelectric conversion unit 3 made of amorphous silicon-based material and the rear photoelectric conversion unit 4 made of crystalline silicon-based material has a larger range of incident light. Effective use becomes possible. However, “silicon-based” materials include silicon alloy semiconductor materials containing silicon such as silicon carbide and silicon germanium in addition to silicon.

  The front photoelectric conversion unit 3 is formed, for example, by stacking semiconductor layers by plasma CVD in the order of pin layers. Specifically, for example, the p-type amorphous silicon carbide layer is the one conductivity type layer 31, the intrinsic amorphous silicon layer is the photoelectric conversion layer 32, and phosphorus as the conductivity type determining impurity atom is 0.01 atomic% or more. The doped n-type microcrystalline silicon layer may be deposited as the reverse conductivity type layer 33 in this order. In this example, an amorphous silicon photoelectric conversion unit is formed.

  The rear photoelectric conversion unit 4 is formed by stacking semiconductor layers by plasma CVD, for example, in the order of pin layers. Specifically, the p-type microcrystalline silicon layer is the one conductivity type layer 41, the intrinsic crystalline silicon layer is the photoelectric conversion layer 42, and n which is doped with phosphorus which is a conductivity type determining impurity atom is 0.01 atomic% or more. A type microcrystalline silicon layer may be deposited as the reverse conductivity type layer 43 in this order. In this example, a crystalline silicon photoelectric conversion unit is formed.

As the back electrode layer 5, it is preferable to form at least one material selected from Al, Ag, Au, Cu, Pt and Cr as at least one metal layer 52 by a sputtering method or a vapor deposition method. Further, it is preferable to form a conductive oxide layer 51 such as ITO, SnO ₂ , or ZnO as a part of the back electrode layer 4 between one or more photoelectric conversion units. The conductive oxide layer 51 enhances the adhesion between the one or more photoelectric conversion units and the back electrode layer 5, increases the light reflectance of the back electrode layer 5, and further changes the chemical change of the photoelectric conversion unit. It has a function to prevent.

  As shown in FIG. 1, two photoelectric conversion units may be used, but a thin film photoelectric conversion device including one photoelectric conversion unit, a so-called single cell may be used. In addition, a thin film photoelectric conversion device including three photoelectric conversion units, a so-called triple cell may be used, and three or more photoelectric conversion units may be stacked. For example, only an amorphous silicon photoelectric conversion unit corresponding to the front photoelectric conversion unit in FIG. 1 may be formed, and an amorphous single cell without the rear photoelectric conversion unit 4 may be used. As an example of a triple cell, there are three photoelectrics in the order of amorphous silicon photoelectric conversion unit / amorphous silicon germanium photoelectric conversion unit using amorphous silicon germanium in a substantial i layer / crystalline silicon photoelectric conversion unit. Conversion units may be stacked. Further, three photoelectric conversion units may be stacked in the order of amorphous silicon photoelectric conversion unit / crystalline silicon photoelectric conversion unit / crystalline silicon photoelectric conversion unit.

  Needless to say, the present invention is also effective in an integrated thin film photoelectric conversion device in which a series connection structure is formed on the same substrate using laser patterning. In the case of an integrated thin film photoelectric conversion device, laser patterning can be easily performed. Therefore, a structure in which light is incident from the substrate side as shown in FIG. 1 is desirable.

  Hereinafter, examples according to the present invention and comparative examples according to the prior art will be described in detail. In the drawings, the same members are denoted by the same reference numerals, and redundant description is omitted. Moreover, this invention is not limited to a following example, unless the meaning is exceeded.

Example 1
As Example 1, a thin film photoelectric conversion device 6 having the structure shown in FIG. As the transparent substrate 1, a glass substrate having a thickness of 0.7 mm was used.

A first transparent electrode layer 21 made of ZnO was formed on the transparent substrate 1 by a low pressure thermal CVD method. The first transparent electrode layer 21 was formed at a substrate temperature of 160 ° C., a pressure of 20 Pa, a vaporized diethyl zinc (DEZ) flow rate of 200 sccm, a vaporized water flow rate of 700 sccm, a diborane (B ₂ H ₆ ) flow rate of 1 sccm, and a hydrogen flow rate of 1000 sccm. .

The thickness obtained from interference of the reflection spectrum of the first transparent electrode layer 21 made of the obtained ZnO film was 2.0 μm. The sheet resistance was 13.7Ω / □. The haze ratio measured using a C light source was 30.3%. The surface area ratio (Sdr) was 129%. The Sdr in the present invention is obtained from an atomic force microscope (AFM) image obtained by observing a square region having a side of 5 μm. The non-contact mode of Nano-R system (manufactured by Pacific Nanotechnology) was used for this AFM measurement. The H concentration measured by secondary ion mass spectrometry (secondary ion mass spectroscopy, abbreviated SIMS) has a distribution in the film thickness direction, but was in the range of 9 × 10 ^{20 to} 3 × 10 ²¹ pieces / cm ³ . . SIMS used a Cs ⁺ ion source.

A second transparent electrode layer 22 made of ITO was formed on the first transparent electrode layer 21 using the RPD method. Ar was passed through the plasma gun to generate a discharge at a discharge current of 150 A, and Ar with 20% oxygen added was introduced into the deposition chamber, and the pressure was 0.3 Pa, and ITO was fabricated at a substrate temperature of 200 ° C. during deposition. The tablet used as a raw material contains SnO ₂ at 5 Wt. % In ₂ O ₃ added was used. When ITO was formed on a glass substrate under the same conditions as in Example 1, the film thickness was 80 nm, the sheet resistance was 19.3 Ω / □, and the average transmittance was 91% at a wavelength of 400 nm to 700 nm as measured by spectroscopic ellipsometry.

  The haze ratio measured in the state where the transparent electrode layer 2, that is, ZnO and ITO was laminated, was 30.1%, and the sheet resistance was 8.1Ω / □.

  On this transparent electrode layer 2, using a capacitively coupled high-frequency plasma CVD apparatus having parallel plate electrodes with a frequency of 13.56 MHz, an amorphous silicon photoelectric conversion unit 3 and a backward photoelectric conversion unit are used as the front photoelectric conversion unit. A crystalline silicon photoelectric conversion unit 4 was sequentially produced as a unit.

An amorphous silicon photoelectric conversion unit 3 was produced on the transparent electrode layer 2. First, as a silicon-containing p-type amorphous semiconductor layer, a p-type amorphous silicon carbide layer 31 was formed to a thickness of 15 nm. SiH ₄ , H ₂ , CH ₄ and B ₂ H ₆ are introduced as reaction gases, the hydrogen dilution ratio H ₂ / SiH ₄ is 10 times, the CH ₄ / SiH ₄ flow rate ratio is 2 times, and B ₂ H ₆ / SiH _4. The flow rate ratio was 0.01 times, the pressure was 100 Pa, and the discharge power density was 15 mW / cm ² . Thereafter, SiH ₄ was introduced as a reaction gas to form an amorphous silicon photoelectric conversion layer 32 having a thickness of 300 nm. Furthermore, SiH ₄ , H _2, and PH ₃ were introduced as reaction gases, and an n-type microcrystalline silicon layer 33 was formed to a thickness of 20 nm as a reverse conductivity type layer, thereby producing an amorphous silicon photoelectric conversion unit 3.

Next, a crystalline silicon photoelectric conversion unit 4 was produced. SiH ₄ , H ₂ and B ₂ H ₆ were introduced as reaction gases to form a p-type crystalline silicon layer 41 having a thickness of 10 nm as one conductivity type layer. At this time, the hydrogen dilution ratio H ₂ / SiH ₄ was 200 times, the B ₂ H ₆ / SiH ₄ flow rate ratio was 0.01 times, the pressure was 100 Pa, and the discharge power density was 75 mW / cm ² . Thereafter, SiH ₄ and H ₂ were introduced as reaction gases to form a crystalline silicon photoelectric conversion layer 42 having a thickness of 1.5 μm. Further, SiH ₄ , H _2, and PH ₃ were introduced as reaction gases to form an n-type microcrystalline silicon layer 43, thereby producing a crystalline silicon photoelectric conversion unit 4.

  After that, as the back electrode layer 5, an Al-doped ZnO film 51 having a thickness of 90 nm and an Ag film 52 having a thickness of 300 nm were sequentially formed by sputtering.

After the back electrode layer 5 is formed, the film formed on the transparent electrode layer 2 is partially removed by laser scribing and separated into a size of 1 cm ² , and the thin film photoelectric conversion device 6 (light receiving area 1 cm ² ) Was made.

The thin film photoelectric conversion device 6 (light-receiving area 1 cm ² ) obtained as described above was irradiated with AM 1.5 light at a light amount of 100 mW / cm ² and the output characteristics were measured. As shown, the open circuit voltage (Voc) is 1.342 V, the short circuit current density (Jsc) is 13.21 mA / cm ² , the fill factor (FF) is 0.711, and the conversion efficiency (Eff) is 12.60%. there were.

(Comparative Example 1)
As Comparative Example 1, a thin film photoelectric conversion device similar to Example 1 was produced. Comparative Example 1 was prepared in the same manner as in Example 1 except that the second transparent electrode layer 2 was not provided.

As shown in Table 1, when the output characteristics of the photoelectric conversion device of Comparative Example 1 were measured in the same manner as in Example 1, Voc = 0.664 V, Jsc = 10.93 mA / cm ² , FF = 0.526, Eff = 4.39%. Compared to Example 1, in Comparative Example 1, Voc was remarkably low, Jsc and FF were low, and Eff was low. This is because the contact resistance between the transparent electrode layer and the p-type amorphous silicon carbide layer is increased due to the absence of ITO.

(Comparative Example 2)
As Comparative Example 2, a thin film photoelectric conversion device similar to Comparative Example 1 was produced. Comparative Example 2 was produced in the same manner as in Comparative Example 1 except that the p-type semiconductor layer of the amorphous silicon photoelectric conversion unit was a p-type microcrystalline silicon layer. In the p-type microcrystalline silicon layer, SiH ₄ , H ₂ and B ₂ H ₆ are introduced as reaction gases, the hydrogen dilution ratio H ₂ / SiH ₄ is 200 times, and the B ₂ H ₆ / SiH ₄ flow rate ratio is 0.01. Double, pressure 100 Pa, discharge power density 75 mW / cm ^2, with a thickness of 10 nm.

As shown in Table 1, when the output characteristics of the photoelectric conversion device of Comparative Example 2 were measured in the same manner as in Example 1, Voc = 1.130 V, Jsc = 10.68 mA / cm ² , FF = 0.477, Eff = 5.75%. Although the p-type microcrystalline silicon layer is disposed between the transparent electrode layer of ZnO and the amorphous silicon carbide layer, Voc, Jsc, and FF are lower in Comparative Example 1 than in Example 1. Eff was lowered. This is presumably because the haze ratio of the ZnO transparent electrode layer was as high as 30% or more, so that the p-type microcrystalline silicon layer had poor coverage and increased contact resistance.

(Comparative Example 3)
As Comparative Example 3, a thin film photoelectric conversion device similar to Example 1 was produced. Comparative Example 3 was prepared in the same manner as in Example 1 except that the p-type semiconductor layer of the amorphous silicon photoelectric conversion unit was a p-type microcrystalline silicon layer. The p-type microcrystalline silicon layer was produced in the same manner as in Comparative Example 2.

As shown in Table 1, when the output characteristics of the photoelectric conversion device of Comparative Example 3 were measured in the same manner as in Example 1, Voc = 1.339 V, Jsc = 9.85 mA / cm ² , FF = 0.695, Eff = 9.17%. Compared to Example 1, Comparative Example 3 had a low Jsc and a low Eff. This is because when the p-type microcrystalline silicon layer is formed, the ITO dilution surface is exposed to a highly reducing plasma with many H atoms because the hydrogen dilution factor is 200 times and the discharge power density is 75 mW / cm 2. It can be said that ITO was reduced and the transmittance was lowered.

(Example 2)
As Example 2, a thin film photoelectric conversion device similar to Example 1 was produced. Example 2 was prepared in the same manner as in Example 1 except that the thickness of the first transparent electrode layer 1 made of ZnO was 2.5 μm. The sheet resistance of the first transparent electrode layer was 7.7Ω / □. The haze ratio measured using a C light source was 41.1%. The surface area ratio (Sdr) was 154%. The haze ratio measured in a state where the transparent electrode layer 2, that is, the first transparent electrode layer 21 made of ZnO and the second transparent electrode layer 22 made of ITO was laminated, was 40.2%, and the sheet resistance was 5.6Ω / □.

As shown in Table 1, when the output characteristics of the photoelectric conversion device of Example 2 were measured in the same manner as in Example 1, Voc = 1.337 V, Jsc = 14.04 mA / cm ² , FF = 0.597, Eff = 13.08%. Compared to Example 1, in Example 2, Jsc increased and Eff improved. By using the transparent electrode layer 2 having a haze ratio of 40% or more, the optical confinement effect is enhanced to increase jSC, and the contact resistance is lowered by ITO and p-type amorphous silicon carbide to reduce Voc even if the unevenness is large. And Eff improved.

(Comparative Example 4)
As Comparative Example 4, a thin film photoelectric conversion device similar to Example 2 was produced. Comparative Example 4 is the same as Example 2 except that there is no ITO second transparent electrode layer 22 and that the p-type semiconductor layer of the amorphous silicon photoelectric conversion unit is a p-type microcrystalline silicon layer. The structure was similarly produced. The p-type microcrystalline silicon layer was produced in the same manner as in Comparative Example 2.

As shown in Table 1, when the output characteristics of the photoelectric conversion device of Comparative Example 4 were measured in the same manner as in Example 1, Voc = 0.474 V, Jsc = 12.35 mA / cm ² , FF = 0.616, Eff = 3.60%. Compared to Example 2, Comparative Example 4 had significantly lower Voc and lower Eff. Since the haze ratio of the transparent electrode layer exceeds 40% and is large, the coverage of the p-type microcrystalline silicon layer is poor and the contact resistance is high, so it is considered that Voc is lowered.

(Summary of Examples 1 and 2 and Comparative Examples 2 and 4)
2 to 4 show Voc, Jsc, FF, and Eff of the thin film photoelectric conversion device with respect to the haze ratio of the transparent electrode layer, respectively. In the figure, the legendary thin film photoelectric conversion device of ZnO / ITO / a-SiC (p) has the same structure and production method as in Examples 1 and 2, and the thickness of the first transparent electrode layer 21 of ZnO is 1.5 um. The haze ratio was changed from 14.2% to 40.2% by changing to ~ 2.5 um. The point with a haze ratio of 30.1% is Example 1, and the point with a haze ratio of 40.2% is Example 2. Further, in the figure, the legendary thin film photoelectric conversion device of ZnO / uc-Si (p) / a-SiC (p) has a structure and manufacturing method similar to those of Comparative Examples 2 and 4, and the film thickness of ZnO is 1.5 um. The haze ratio was changed from 14.6% to 41.1% by changing it to ˜2.5 μm. The point with a haze ratio of 30.3% is Comparative Example 2, and the point with a haze ratio of 41.1% is Comparative Example 4.

  As can be seen from FIG. 2, in the case of a thin film photoelectric conversion device including a structure of ZnO / uc-Si (p) / a-SiC (p), Voc significantly decreases when the haze ratio increases to 30% or more. On the other hand, in the case of a thin film photoelectric conversion device including a structure of ZnO / ITO / a-SiC (p), Voc is kept almost constant even when the haze ratio is 30% or more.

  As shown in FIG. 3, ZnO / ITO / a-SiC (p) has a haze ratio of 30% or more, as compared with a thin film photoelectric conversion device including a structure of ZnO / uc-Si (p) / a-SiC (p). The Jsc of the thin film photoelectric conversion device including the structure of) is high.

  As shown in FIG. 4, ZnO / ITO / a-SiC (p) as compared with a thin film photoelectric conversion device having a haze ratio of 30% or more and including a structure of ZnO / uc-Si (p) / a-SiC (p). ) Of the thin film photoelectric conversion device including the structure of) is high.

  As shown in FIG. 5, in the thin film photoelectric conversion device including the structure of ZnO / uc-Si (p) / a-SiC (p), when the haze ratio is increased, Eff once increases, and the haze ratio is 30% or more. Eff decreases. On the other hand, in the thin film photoelectric conversion device including the structure of ZnO / ITO / a-SiC (p), Eff increases as the haze ratio increases, and Eff further increases when the haze ratio is 30% or more. A high value of Eff of 13% or more is obtained at a haze ratio of 40.2%.

DESCRIPTION OF SYMBOLS 1 Transparent substrate 2 Transparent electrode layer 3 Amorphous silicon photoelectric conversion unit 31 p-type amorphous silicon carbide carbide layer 32 Substantially intrinsic amorphous silicon photoelectric conversion layer 33 n-type microcrystalline silicon layer 4 Crystalline silicon photoelectric Conversion unit 41 p-type microcrystalline silicon layer 42 substantially intrinsic crystalline silicon layer photoelectric conversion layer 43 n-type microcrystalline silicon layer 5 back electrode layer 51 ZnO film 52 Ag film 6 thin film photoelectric conversion device

Claims (5)

In a thin film photoelectric conversion device including one or more photoelectric conversion units in which a p-type semiconductor layer, a substantially intrinsic semiconductor photoelectric conversion layer, and an n-type semiconductor layer are sequentially stacked on a transparent electrode layer,
The transparent electrode layer has a structure in which zinc oxide and ITO (indium tin oxide) are sequentially laminated from the light incident side, the haze ratio of the transparent electrode layer is 30% or more, and the film thickness of ITO is 100 nm or less. ,
The thin film photoelectric conversion device, wherein the p-type semiconductor layer in the photoelectric conversion unit in contact with the transparent electrode layer is composed of a silicon-containing p-type amorphous semiconductor layer. 2. The thin film photoelectric conversion device according to claim 1, wherein the silicon-containing p-type amorphous semiconductor layer is amorphous silicon or an amorphous silicon alloy containing at least one of C, O, and N.   3. The method for manufacturing a thin film photoelectric conversion device according to claim 1, wherein the zinc oxide is manufactured by a low pressure thermal CVD method. 4.   3. The method for manufacturing a thin film photoelectric conversion device according to claim 1, wherein the photoelectric conversion unit is manufactured by a plasma CVD method.   3. The method of manufacturing a thin film photoelectric conversion device according to claim 1, wherein the ITO is manufactured by an RPD method (reactive plasma deposition method).

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