Solar cell

Details Solar cell Solar cell

2000-02-29

2001-07-10

Diamond, Alan (Department: 1753)

Batteries: thermoelectric and photoelectric

Photoelectric

Cells

C136S252000, C136S262000, C136S263000, C136S264000, C257S043000, C257S428000, C257S461000, C257S464000

Reexamination Certificate

active

06259016

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a solar cell, in particular, a solar cell including a compound semiconductor layer comprising at least one element from each of groups Ib, IIIb and VIb as the light-absorption layer.
2. Description of the Prior Art
CuInSe
2
and Cu(In, Ga)Se
2
are compound semiconductors (having a chalcopyrite structure) comprising at least one element from each of groups Ib, IIIb and VIb. Thin-film solar cells using a CuInSe
2
film (hereinafter, also referred to as a CIS film) or a Cu(In, Ga)Se
2
film (hereinafter, also referred to as a CIGS film) as the light-absorption layer have a high energy conversion efficiency, which does not deteriorate due to irradiation or the like. Therefore, such a thin film solar cell has received attention.
In the solar cell, theoretically, when the band gap of the light-absorption layer is in the range from 1.4 eV to 1.5 eV, the highest conversion efficiency can be obtained. In the case of the solar cell including a CIGS film, it is possible to control the band gap by changing the ratio of Ga and In. When the ratio Ga/(In+Ga) by number of atoms is in the range from 0.5 to 0.8, the band gap is 1.4 eV to 1.5 eV.
However, in the current CIGS solar cells, the highest conversion efficiency can be obtained when the band gap of the CIGS film is in the range from 1.2 eV to 1.3 eV (corresponding to the range of the ratio Ga/(In+Ga) by number of atoms from 0.2 to 0.3). In the current CIGS solar cell, contrary to the theory, even if the band gap is broadened by increasing the Ga concentration, the conversion efficiency is reduced.
The CIGS solar cell with high conversion efficiency that has been reported has a heterojunction of a CdS film as the window layer and a CIGS film as the light-absorption layer. On the other hand, in recent years, a CIGS solar cell without CdS has received attention for environmental reasons. As a result, several CIGS solar cells using a ZnO based semiconductor as the window layer instead of CdS, have been reported. However, these cells have a lower conversion efficiency than that of cells comprising the CdS film. When a ZnO based semiconductor is used as the window layer, especially the open-circuit voltage is low.
SUMMARY OF THE INVENTION
Therefore, with the foregoing in mind, it is an object of the present invention to provide a solar cell that comprises no CdS film in the semiconductor layer forming a pn junction and has a high efficiency.
In order to achieve the above-object, a first solar cell of the present invention includes a first semiconductor layer that is n-type and a second semiconductor layer that is p-type, the first and second semiconductor layers forming a pn junction, wherein the first semiconductor layer is free from Cd, the second semiconductor layer is a light-absorption layer, the band gap Eg
1
of the first semiconductor layer and the band gap Eg
2
of the second semiconductor layer satisfy the relationship: Eg
1
>Eg
2
, and the electron affinity &khgr;
1
(eV) of the first semiconductor layer and the electron affinity &khgr;
2
(eV) of the second semiconductor layer satisfy the relationship: 0≦(&khgr;
2
−&khgr;
1
)<0.5. This embodiment provides a solar cell that comprises no CdS film in the semiconductor layer forming a pn junction and has high efficiency.
In the first solar cell, it is preferable that the first semiconductor layer is formed closer to the side from which light is incident than the second semiconductor layer is. This embodiment allows loss of incident light to be reduced.
The first solar cell preferably further includes a third semiconductor layer between the first semiconductor layer and the second semiconductor layer, and it is preferable that the band gap Eg
3
of the third semiconductor layer and the band gap Eg
2
satisfy the relationship: Eg
3
>Eg
2
. This embodiment provides a solar cell having particularly high efficiency.
In the first solar cell, it is preferable that the third semiconductor layer is made of a semiconductor selected from the group consisting of an n-type semiconductor and a highly resistive semiconductor. This embodiment can reduce damage to the second semiconductor layer in the process of forming the first semiconductor layer and provides a satisfactory pn junction, so that a solar cell having particularly high efficiency can be obtained.
In the first solar cell, it is preferable that the electron affinity &khgr;
3
(eV) of the third semiconductor layer and the electron affinity &khgr;
2
satisfy the relationship: (&khgr;
2
−&khgr;
3
)≧0.5, and the thickness of the third semiconductor layer is not more than 50 nm. This embodiment provides a solar cell having a high conversion efficiency, because carriers tunnel through the third semiconductor layer and are transported.
In the first solar cell, it is preferable that the third semiconductor layer comprises an oxide comprising Zn and at least one element selected from group IIIb or a chalcogenide comprising Zn and at least one element selected from group IIIb.
The first solar cell preferably further includes an insulating layer between the first semiconductor layer and the second semiconductor layer, and it is preferable that the band gap Eg
INS
of the insulating layer and the band gap Eg
2
satisfy the relationship Eg
INS
>Eg
2
. This embodiment provides a solar cell having a particularly high efficiency.
In the first solar cell, it is preferable that the electron affinity &khgr;
INS
(eV) of the insulating layer and the electron affinity &khgr;
2
satisfy the relationship: (&khgr;
2
−&khgr;
INS
)≧0.5, and the thickness of the insulating layer is not more than 50 nm. This embodiment provides a solar cell having a high conversion efficiency, because carriers tunnel through the third semiconductor layer and are transported.
In the first solar cell, it is preferable that the insulating layer is made of at least one insulator selected from Al
2
O
3
, Ga
2
O
3
, Si
3
N
4
, SiO
2
, MgF
2
and Mgo.
In the first solar cell, it is preferable that the second semiconductor layer further includes an n-type semiconductor layer or a highly resistive semiconductor layer on the surface thereof on the side of the first semiconductor layer. This embodiment provides a solar cell having a high conversion efficiency, because the pn junction is formed in the second semiconductor layer so that the defect density at the junction interface can be reduced.
In the first solar cell, it is preferable that the second semiconductor layer is a compound semiconductor layer comprising at least one element from each of groups Ib, IIIb, and VIb. This embodiment provides a solar cell including a compound semiconductor of a chalcopyrite structure having less light-induced degradation as the light-absorption layer. Throughout this specification, “groups Ib, IIIb, VIb, and IIa” refer to “groups 1B, 3B, 6B and 2A” of the periodic table of elements according to the old IUPAC recommendation before 1985.
In the first solar cell, it is preferable that the first semiconductor layer is made of a compound comprising Zn. This embodiment provides a solar cell that is free from CdS in the semiconductor layer forming the pn junction and has a particularly high conversion efficiency.
In the first solar cell, it is preferable that the compound of the first semiconductor layer is an oxide comprising Zn and at least one element selected from group IIa, or a chalcogenide comprising Zn and at least one element selected from group IIa.
In the first solar cell, it is preferable that the compound of the first semiconductor layer comprises an oxide expressed by a general formula Zn
1−x
A
x
O (where element A is at least one selected from Be, Mg, Ca, Sr and Ba, and 0<X<1) as the main component. This embodiment allows the electron affinity of the first semiconductor layer to be changed by changing the element A and X depending on the second semiconductor layer and thus provides a solar cell having a particularly high conversion efficiency.
In th

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