JP2012038783A - Photoelectric conversion element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photoelectric conversion element capable suppressing recombination of a hole and an electron that is generated in a p-type semiconductor layer by photoelectromotive force effects.SOLUTION: A photoelectric conversion element 1 comprises a semiconductor junction unit 14 in which a semiconductor layer is joined by pin-junction or pn-junction and a p-type semiconductor layer 14a serves as the light-receiving side; and a transparent electrode 12 consisting of an n-type oxide semiconductor as an electrode at the light-receiving side. A p-type transparent semiconductor layer 13 in which the energy difference between a Fermi level Efand a lower limit of a conduction band Ecis larger than that between a Fermi level Efand a lower limit of a conduction band Ecin the p-type semiconductor layer 14a, is joined between the transparent electrode 12 and the p-type semiconductor layer 14a.

Description

  The present invention relates to a photoelectric conversion element used for a solar cell.

  When a semiconductor junction unit in which a semiconductor layer is pn-junction or pin-junction is considered as a constituent unit of a photoelectric conversion portion in a solar cell, the simplest solar cell configuration is a configuration in which electrodes are provided at both ends of one semiconductor junction unit. is there. For example, a thin film solar cell including one semiconductor junction unit in which a semiconductor layer is pin-joined is divided into two types shown in FIGS. 4A and 4B depending on the order in which each semiconductor layer is stacked on a substrate serving as a support. They can be broadly classified (see, for example, Patent Document 1).

  Here, the thin film solar cell 110 in FIG. 4A is a case where a light-transmitting substrate 111 is used on the light receiving side, and a transparent electrode 112, a p-type semiconductor layer 113, an i-type semiconductor layer 114, n are formed on the substrate 111. A type semiconductor layer 115 and a back electrode 116 are stacked. On the other hand, the thin-film solar cell 120 in FIG. 4B is a case where an opaque substrate 121 is used on the side opposite to the light receiving side, and a back electrode 122, an n-type semiconductor layer 123, and an i-type semiconductor are formed on the substrate 121. The layer 124, the p-type semiconductor layer 125, and the transparent electrode 126 are stacked.

  In addition to the single-junction solar cell illustrated in FIGS. 4A and 4B, various multi-junction solar cells in which a plurality of semiconductor junction units are joined have been proposed (see, for example, Patent Document 2). Also in such a multi-junction type solar cell, the p-type semiconductor layer of the semiconductor junction unit is generally joined to the transparent electrode on the light receiving side.

In a solar cell having a general configuration as described above, an n-type oxide semiconductor such as SnO ₂ , In ₂ O ₃ , ZnO, ITO (indium tin oxide) has been used as a transparent electrode. This is because transparent electrodes are required to have high electrical conductivity suitable for taking out electrical energy from the semiconductor junction unit in addition to the translucency necessary for efficiently absorbing light to the bonded semiconductor layer. This is because the n-type oxide semiconductor satisfies both of the two conditions.

  Therefore, the junction between the transparent electrode that is an n-type oxide semiconductor and the p-type semiconductor layer is a reverse junction, which is an electrical pseudo-ohmic junction. Therefore, the transparent electrode which is an n-type oxide semiconductor is advantageous as an electrode for taking out the electric energy photoelectrically converted in the semiconductor junction unit. However, in the junction between the n-type transparent electrode and the p-type semiconductor layer, the probability that electrons generated in the p-type semiconductor layer recombine with holes increases. This will be described using the band profile of FIG. Here, FIG. 3 shows a cross-sectional view of the solar cell having the basic configuration shown in FIG. 4A, and schematically shows a band profile corresponding to this cross-sectional view.

Since electrons are more stable at lower energy levels, an electric field distribution is generated in the p-type semiconductor layer 113 in the vicinity of the interface with the transparent electrode 112 to draw electrons to the transparent electrode 112 side. That is, as shown in FIG. 3, the line indicating the energy level Ec ₁ at the bottom of the conduction band of the p-type semiconductor layer 113 has a profile that falls toward the transparent electrode 112 in the vicinity of the interface with the transparent electrode 112. . As a result, in the p-type semiconductor layer 113, the electrons absorbed in the vicinity of the interface with the transparent electrode 112 and excited from the valence band to the conduction band (shown by an arrow N) are easily drawn into the transparent electrode 112 side ( Shown, arrow M). Then, the electrons drawn to the transparent electrode 112 side re-establish holes and holes formed with the generation of electrons in the p-type semiconductor layer 113 through localized levels (broken lines in the figure) caused by defects. Will be combined. Therefore, the conventional solar cell 110 having the above configuration has room for improvement in that electrons and holes once generated by the photovoltaic effect cannot be sufficiently effectively used as electric energy.

  Therefore, in view of the above situation, the present invention is a photoelectric conversion element including a semiconductor junction unit having a p-type semiconductor layer as a light-receiving side, and a transparent electrode made of an n-type oxide semiconductor as a light-receiving side electrode. An object of the present invention is to provide a photoelectric conversion element in which electrons generated in the p-type semiconductor layer due to the photovoltaic effect are suppressed from recombining with holes.

  In order to solve the above-described problems, the photoelectric conversion element according to the present invention includes a semiconductor junction unit in which a semiconductor layer is a pin junction or a pn junction and a p-type semiconductor layer is a light receiving side. A photoelectric conversion element including a transparent electrode made of an n-type oxide semiconductor, wherein an energy difference between a Fermi level and a lower limit of a conduction band is between the transparent electrode and the p-type semiconductor layer. A p-type transparent semiconductor layer that is larger than the energy difference between the Fermi level and the lower limit of the conduction band in the semiconductor layer is joined. "

  The “semiconductor junction unit in which a semiconductor layer is pin-joined” is a joined body in which a p-type semiconductor layer, an i-type semiconductor layer, and an n-type semiconductor layer are joined in this order. The “semiconductor junction unit” is a joined body in which a p-type semiconductor layer and an n-type semiconductor layer are joined.

  In the present invention, a “p-type transparent semiconductor layer” is interposed between a transparent electrode made of an n-type oxide semiconductor and a p-type semiconductor layer, and the p-type transparent semiconductor layer and the p-type semiconductor layer are Since the relationship between the energy levels is defined, the type of semiconductor material constituting each semiconductor layer, each semiconductor layer is in an amorphous phase, or is in a microcrystalline phase or a crystalline phase It's not something to ask.

  As the translucency of the “p-type transparent semiconductor layer”, the visible light transmittance is desirably 80% or more, and more desirably 85% or more.

  In the photoelectric conversion element of the present invention having the above configuration, electrons generated in the p-type semiconductor layer are suppressed from being drawn into the transparent electrode side. The reason is as follows. In the present invention, the energy difference between the Fermi level of the p-type transparent semiconductor layer and the lower limit of the conduction band is larger than the energy difference between the Fermi level of the p-type semiconductor layer and the lower limit of the conduction band. Accordingly, when the p-type transparent semiconductor layer is bonded between the p-type semiconductor layer and the transparent electrode, the lower energy level of the conduction band is higher in the p-type transparent semiconductor layer than in the p-type semiconductor layer. As a result, in order for electrons generated in the p-type semiconductor layer to move to the transparent electrode side, the energy peak in the p-type transparent semiconductor layer must be exceeded, and it is difficult to move to the transparent electrode side. .

  Therefore, in the photoelectric conversion element of the present invention, electrons generated by light absorption in the p-type semiconductor layer are suppressed from being drawn into the n-type transparent electrode, and further, holes are introduced through the localized levels. And recombination is suppressed. Thereby, electrons and holes generated by the photovoltaic effect can be effectively used as electric energy.

  The photoelectric conversion element according to the present invention may be configured such that “the p-type transparent semiconductor layer is made of a p-type oxide semiconductor having a thickness of 10 nm to 70 nm” in the above configuration.

Examples of the “p-type oxide semiconductor” include NiO, CuAlO ₂ , CuGaO ₂ , CuCrO ₂ , CuInO ₂ , CuYO ₂ , and AgInO ₂ . A transparent thin film can be formed from these semiconductor materials.

  The reason why the thickness of the p-type oxide semiconductor as the p-type transparent semiconductor layer is set to “10 nm to 70 nm” is that a p-type oxide semiconductor having a higher electrical conductivity than an n-type oxide semiconductor has not been realized at present. It depends. Here, in order to reliably prevent the movement of electrons from the p-type semiconductor layer to the n-type transparent electrode, it is desirable that the thickness of the p-type transparent semiconductor layer (p-type oxide semiconductor) is large. On the other hand, in order to suppress the electrical resistance in the p-type transparent semiconductor layer, the thickness is desirably as small as possible. In the present invention, the thickness of the p-type transparent semiconductor layer is set in the above range, and the above conflicting requirements are harmonized, thereby being sufficient to prevent the movement of electrons from the p-type semiconductor layer to the n-type transparent electrode. While ensuring the thickness, the p-type semiconductor layer is thinned to suppress electrical resistance.

  The photoelectric conversion element according to the present invention may be configured such that “the p-type oxide semiconductor is nickel oxide” in the above configuration.

  “Nickel oxide” has a large band gap of about 4 eV. Therefore, even if the Fermi level varies depending on the carrier concentration, a p-type semiconductor in which the energy difference between the Fermi level and the lower limit of the conduction band is smaller than the energy difference between the Fermi level and the lower limit of the conduction band in nickel oxide. There is an advantage that the degree of freedom to select is high.

  As described above, as an effect of the present invention, a photoelectric conversion element including a semiconductor junction unit having a p-type semiconductor layer as a light-receiving side and a transparent electrode made of an n-type oxide semiconductor as a light-receiving side electrode, A photoelectric conversion element in which electrons generated in the p-type semiconductor layer due to the electromotive force effect are suppressed from recombining with holes can be provided.

It is the figure which made the band profile respond | correspond to sectional drawing of the solar cell using the photoelectric conversion element of one Embodiment of this invention. It is a graph which shows the electric current-voltage characteristic of the solar cell of FIG. It is the figure which made the band profile respond | correspond to sectional drawing of the solar cell of the conventional basic composition. 4A is a cross-sectional view of a conventional solar cell having a basic structure using a light-transmitting substrate as a support substrate, and FIG. 4B is a cross-sectional view of a conventional solar cell having a basic structure using a light-transmitting substrate as a support substrate. It is sectional drawing. It is sectional drawing of the solar cell using the photoelectric conversion element of other embodiment. It is sectional drawing of the solar cell of the other form using the photoelectric conversion element of FIG. FIG. 7A is a cross-sectional view of a conventional multi-junction solar cell, and FIG. 7B is a cross-sectional view of a solar cell obtained by improving the solar cell of FIG. 7A using a photoelectric conversion element of another embodiment of the present invention.

  Hereinafter, the photoelectric conversion element 1 which is one embodiment of the present invention and a solar cell 10 using the photoelectric conversion element 1 will be described with reference to FIGS. 1 and 2.

In the photoelectric conversion element of this embodiment, as shown in a cross-sectional view in FIG. 1, the p-type semiconductor layer 14a, the i-type semiconductor layer 14b, and the n-type semiconductor layer 14c are pin-joined, and the p-type semiconductor layer 14a is connected to the light receiving side. The photoelectric conversion element 1 including the semiconductor junction unit 14 and the transparent electrode 12 made of an n-type oxide semiconductor as an electrode on the light receiving side, between the transparent electrode 12 and the p-type semiconductor layer 14a, The p-type transparent semiconductor layer 13 in which the energy difference between the Fermi level Ef ₂ and the lower limit Ec ₂ of the conduction band is larger than the energy difference between the Fermi level Ef ₁ and the lower limit Ec _{1 of the} conduction band in the p-type semiconductor layer 14a. Are joined. Moreover, in the photoelectric conversion element 1 of this embodiment, the p-type transparent semiconductor layer 13 is made of a p-type oxide semiconductor having a thickness of 10 nm to 70 nm. The solar cell 10 uses a transparent substrate 11 as a support substrate, the transparent electrode 12 of the photoelectric conversion element 1 is laminated on the substrate 11, and a back surface at the end of the photoelectric conversion element 1 on the side opposite to the transparent electrode 12. In this configuration, the electrodes 15 are stacked.

  More specifically, glass or a transparent resin film can be used as the transparent substrate 11. The back electrode 15 can be made of a metal or alloy such as Ag, Al, Pt, Au, or Ni, and is particularly preferably a metal having high reflectivity such as Ag or Al.

As the transparent electrode 12, an n-type oxide semiconductor such as SnO ₂ , In ₂ O ₃ , ZnO, or ITO can be used. In order to use these oxide semiconductors as electrodes, the sheet resistance is desirably 5 to 8Ω / square. In order to increase the effective length of the light irradiated to the semiconductor junction unit 1 through the transparent electrode 12, minute irregularities can be formed on the surface of the transparent electrode 12.

NiO, CuAlO ₂ , CuGaO ₂ , CuCrO ₂ , CuInO ₂ , CuYO ₂ , AgInO ₂ or the like can be used as the p-type oxide semiconductor constituting the p-type transparent semiconductor layer 13.

  As semiconductor materials constituting the p-type semiconductor layer 14a, the i-type semiconductor layer 14b, and the n-type semiconductor layer 14c, a-Si: H, a-Si: F, a-SiC: H, a-SiC: F, Examples include a-SiGe: H, a-SiGe: F, μc-Si: H, μc-Si: F, poly-Si: H, and poly-Si: F. Further, B, Al, Ga, In, or the like can be used as the p-type dopant, and P, As, Sb, or the like can be used as the n-type dopant.

  As a method of laminating the transparent electrode 12 on the substrate 11 and further laminating the p-type semiconductor layer 14a, the i-type semiconductor layer 14b, and the n-type semiconductor layer 14c, a vacuum deposition method, a sputtering method, an ion plating method, Known thin-film manufacturing methods such as chemical vapor deposition (CVD) including plasma CVD, Cat-CVD, and thermal CVD can be used. Further, the p-type transparent semiconductor layer 13 can be formed by a sol-gel method or a spin coating method in addition to the above-described CVD method and vacuum deposition method. Further, the back electrode 15 can be stacked by a sputtering method, a vacuum deposition method, or the like.

FIG. 1 schematically shows a band profile of a solar cell 10 having the above-described configuration using the photoelectric conversion element 1 of the present embodiment in association with a cross-sectional view. In the p-type transparent semiconductor layer 13, an electric field distribution that draws electrons toward the transparent electrode 12 is generated near the interface with the transparent electrode 12, as in the conventional solar cell 110 described above. However, when the p-type transparent semiconductor layer 13 is joined between the p-type semiconductor layer 14a and the transparent electrode 12 and the Fermi levels Ef ₁ and Ef _{2 of} both layers become equal, the conduction band of the p-type transparent semiconductor layer 13 is increased. The lower limit energy level Ec ₂ is higher than the lower limit energy level Ec ₁ of the conduction band of the p-type semiconductor layer 14a. This is because the energy difference between the Fermi level Ef _{2 of} the p-type transparent semiconductor layer 13 and the lower limit Ec ₂ of the conduction band is greater than the energy difference between the Fermi level Ef ₁ of the p-type semiconductor layer 14 a and the lower limit Ec _{1 of the} conduction band. Because it is big.

  Therefore, in the solar cell 10 provided with the photoelectric conversion element 1 of the present embodiment, the electrons generated in the p-type semiconductor layer 14a do not easily move to the transparent electrode 12 side beyond the energy peak in the p-type transparent semiconductor layer 13. On the contrary, it is easy to move to the i-type semiconductor layer 14b side. This suppresses electrons generated by light absorption in the p-type semiconductor layer 14a from being drawn into the n-type transparent electrode 12 and recombining with holes through the localized levels.

In addition, the built-in potential V ₂ of the solar cell 10 is the built-in potential V ₁ of the conventional solar cell 110 that does not include the p-type transparent semiconductor layer 13 due to the presence of charges in the p-type transparent semiconductor layer 13 (see FIG. 3). Is greater. Thereby, an open circuit voltage increases combined with the above-described effect of suppressing recombination of electrons.

  Further, the junction between the p-type transparent semiconductor layer 13 and the n-type transparent electrode 12 is a reverse junction, and the electron density which is the majority carrier in the transparent electrode 12 is high and the majority carrier in the p-type transparent semiconductor layer 13 is positive. Pore density is also high. Therefore, a good ohmic junction is formed between the p-type transparent semiconductor layer 13 and the transparent electrode 12. Thereby, the electrical energy photoelectrically converted by the semiconductor junction unit 14 can be taken out smoothly.

  Note that the p-type oxide semiconductor constituting the p-type transparent semiconductor layer 13 has low electrical conductivity. However, when the film thickness is 10 nm to 70 nm, electrons move from the p-type semiconductor layer 14 a to the transparent electrode 12. It is suppressed that the electrical resistance increases while securing the action of preventing the above.

As Example 1, a solar cell having the following configuration was manufactured.
Substrate / transparent electrode / p-type transparent semiconductor layer / p-type semiconductor layer / i-type semiconductor layer / n-type semiconductor layer / back electrode In this configuration,
(Substrate) Transparent glass plate, thickness of 500mm to 5000mm
(Transparent electrode) SnO ₂ , thickness 500 nm to 1000 nm, sheet resistance 5 to 8 Ω / square, uneven texture with a width of 40 nm and a height of 30 nm is formed on the surface (p-type transparent semiconductor layer) NiO, thickness 10 nm to 70 nm, electricity Conductivity 1 × 10 ^{−6 to} 5 × 10 ⁻⁵ S / cm, visible light transmittance 85%, band gap about 4 eV
(P-type semiconductor layer) a-SiC: H, thickness 50 nm, band gap of about 2 eV, doping amount of p-type dopant 0.5% / number of SiC atoms (i-type semiconductor layer) a-Si: H, thickness 250 nm ˜500 nm, defect density 1 × 10 ¹⁶ cm ⁻³
(N-type semiconductor layer) μc-Si: H, thickness 50 nm, doping amount of n-type dopant 0.5% / number of Si atoms (back electrode) Ag / Al thin film

As Comparative Example 1, a solar cell having the following configuration was produced.
Substrate / transparent electrode / p-type semiconductor layer / i-type semiconductor layer / n-type semiconductor layer / back electrode This configuration is obtained by removing the p-type transparent semiconductor layer from the solar cell of Example 1, with reference to FIG. The configuration is the same as that of the conventional solar cell 110 described. The materials, thicknesses, and other conditions of the substrate, the transparent electrode, the p-type semiconductor layer, the i-type semiconductor layer, the n-type semiconductor layer, and the back electrode are the same as those in the first embodiment.

About the solar cell of Example 1 and Comparative Example 1, each using a solar simulator, the incident light intensity is 100 mW / cm ² , the solar radiation air mass passage condition is a reference light of AM-1.5, and the current-voltage characteristics It was measured. As a result, the solar cell of Example 1 exhibited current-voltage characteristics indicated by a solid line in FIG. 2, and the open circuit voltage was 0.8867V. The solar cell of Comparative Example 1 showed the current-voltage characteristics indicated by the broken line in FIG. 2, and the short circuit current was the same as that of Example 1, but the open circuit voltage was 0.8477V.

  As is clear from the comparison between Example 1 and Comparative Example 1, the open-circuit voltage was reduced by joining NiO, which is a p-type transparent oxide, at a thickness of 10 nm to 70 nm between the transparent electrode and the p-type semiconductor layer. It was possible to increase about 40 mV. Since the maximum output of the solar cell is represented by the product of the short-circuit current, the open-circuit voltage, and the fill factor, the solar cell of Example 1 showing the current-voltage characteristics of FIG. 2 is the conventional solar cell of Comparative Example 1. Thus, it can be said that the conversion efficiency is increased.

  Although the present invention has been described with reference to the preferred embodiments, the present invention is not limited to the above-described embodiments, and various improvements and design changes can be made without departing from the scope of the present invention. It is.

  For example, in the above, the solar cell 10 in which the photoelectric conversion element 1 of the present invention is laminated on the transparent substrate 11 from the transparent electrode 12 side is exemplified. It is not limited to this, It can also be set as the solar cell by which the photoelectric conversion element of this invention was laminated | stacked from the n-type semiconductor layer side on the opaque substrate.

  Moreover, in the above, although the photoelectric conversion element 1 provided with one semiconductor junction unit 14 was illustrated, it is not limited to this. For example, as shown in FIG. 5, a photoelectric conversion element 2 in which a plurality of semiconductor junction units 24 a and 24 b are stacked on a transparent electrode 22 and a p-type transparent semiconductor layer 23 can be formed. Then, a pair of electrodes 21 and 25 are joined to the photoelectric conversion element 2, and the wavelength of light absorbed by the plurality of semiconductor junction units 24a and 24b is set to be different, so that the tandem solar cell 20 and can do.

  Furthermore, although the case where the back surface electrode 15 was directly joined to the photoelectric conversion element 1 of this invention and the solar cell 10 was manufactured was illustrated above, it is not limited to this. For example, the semiconductor junction unit 14 of the photoelectric conversion element 1 is made of an amorphous silicon type, and the photoelectric conversion element 1 is laminated on a glass substrate 31 and a single crystal silicon type semiconductor junction unit 38 is joined as shown in FIG. Thus, the back electrode 39 can be joined to form a hybrid (HIT) solar cell 30.

  Alternatively, as shown in FIG. 7B, a back junction 97 is joined to the photoelectric conversion element 4 of the present invention after joining a semiconductor junction unit 96 having a pin junction via a transparent intermediate layer 95, so that a multijunction solar The battery 40 can be used. This is an improvement of the current thin film silicon solar cell 90 shown in FIG. 7A.

  That is, in the current thin film silicon-based solar cell 90, a glass substrate 91, an n-type transparent oxide semiconductor layer (transparent electrode) 92, a p-type a-SiC: H layer 94a, an i-type a-Si: H layer 94b, n. Semiconductor junction unit 94 comprising n-type μc-Si layer 94c, n-type transparent oxide semiconductor layer (transparent intermediate layer) 95, p-type μc-Si layer 96a, i-type μc-Si layer 96b, n-type μc-Si layer 96c A semiconductor bonding unit 96 and a back electrode 97 are sequentially bonded. The conversion efficiency of this solar cell 90 is about 15% in the case of a small area and about 9% in the case of a module.

  According to the application of the present invention, the p-type transparent semiconductor layer 43 is bonded between the n-type transparent oxide semiconductor layer (transparent electrode) 92 and the p-type a-SiC: H layer 94a. An improved solar cell 40 can be obtained. In the solar cell 40 having such a configuration, conversion efficiency is expected to be higher than that of the solar cell 90. In this case, the n-type transparent oxide semiconductor layer (transparent electrode) 92, the p-type transparent semiconductor layer 43, the p-type a-SiC: H layer 94a, the i-type a-Si: H layer 94b, and the n-type μc-Si. A portion constituted by the semiconductor junction unit 94 composed of the layer 94c corresponds to the photoelectric conversion element 4 of the present invention.

1 photoelectric conversion element 12 transparent electrode 13 p-type transparent semiconductor layer 14a p-type semiconductor layer 14 semiconductor junction unit _Ef 1, Ef ₂ Fermi level _Ec 1, Ec ₂ the lower limit of the energy level of the conduction band

Japanese Patent Laid-Open No. 5-343714 Japanese Patent Publication No. 6-50782

Claims (3)

A photoelectric conversion element including a semiconductor junction unit in which a semiconductor layer is a pin junction or a pn junction, and a p-type semiconductor layer is a light-receiving side, and a transparent electrode made of an n-type oxide semiconductor as an electrode on the light-receiving side. ,
The energy difference between the Fermi level and the lower limit of the conduction band between the transparent electrode and the p-type semiconductor layer is larger than the energy difference between the Fermi level and the lower limit of the conduction band in the p-type semiconductor layer. A photoelectric conversion element, wherein a p-type transparent semiconductor layer is bonded. The photoelectric conversion element according to claim 1, wherein the p-type transparent semiconductor layer is made of a p-type oxide semiconductor having a thickness of 10 nm to 70 nm. The photoelectric conversion element according to claim 2, wherein the p-type oxide semiconductor is nickel oxide.

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