Patent Application
20100154882
This application claims the priority benefit of Taiwan application serial no. 97150530, filed on Dec. 24, 2008. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.
1. Field of the Invention
The present invention is related to a solar cell capable of achieving the optimum utility rate of the sunlight.
2. Description of Related Art
The solar energy is a non-polluting and inexhaustible energy source. When petrochemical energy sources confront with problems such as pollution and shortage, the mass has gradually focused on the issue of how to utilize the solar energy source efficiently. As solar cells can convert solar energy into electric energy directly, the solar cells have become a key point currently in terms of utilizing the solar energy.
A known solar cell converts light energy into electrical energy with a P-I-N junction structure. Specifically, the solar cell includes a front contact, a P-type semiconductor layer, an intrinsic layer (i.e., an I layer), an N-type semiconductor layer and a back contact stacked on one another. The intrinsic layer serves as the primary area which generates pairs of electrons and holes. The P-type semiconductor layer and the N-type semiconductor layer above and under the intrinsic layer form a strong electric field, which then causes the electrons and holes to separate from each other, thus generating currents.
However, sunlight may be reflected at interfaces of the solar cell (e.g., the interface between the front contact and the P-type semiconductor layer, the interface between the P-type semiconductor layer and the intrinsic layer or the interface between the intrinsic layer and the N-type semiconductor layer), such that the solar cell cannot effectively utilize the sunlight, thereby resulting in low short current density and poor efficiency of the solar cell.
Accordingly, the present invention is directed to a solar cell capable of increasing light transmittance and reducing light reflection occurring at interfaces of the solar cell.
As embodied and broadly described herein, the present invention provides a solar cell, which includes a front contact, a first conductive type layer, an intrinsic layer, a second conductive type layer and a back contact stacked on one another. The solar cell is characterized by the first conductive type layer being a material layer with low refractive index, and a refractive index of the material layer with low refractive index is lower than 3.
Based on the above, a material layer of low refractive index having a refractive index lower than 3 is disposed as the first conductive type layer in the solar cell according to the present invention, so that light transmittance can be increased and light reflection occurring at interfaces can be reduced. Therefore, the solar cell achieves the optimum utility rate of sunlight, which in turn enhances a short circuit current density (Jsc) and efficiency of the solar cell.
In order to make the aforementioned and other objects, features and advantages of the present invention more comprehensible, several embodiments accompanied with figures are described in detail below.
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.
Referring to
In this embodiment, the refractive index of the front contact 102 is lower than or equal to 1.8, and the thickness thereof is 60-140 nm, for example. The material of the front contact 102 is transparent conductive oxide (TCO), for example, which can be selected from ITO, ZnO, AlZnO, SnO2 and In2O3. The refractive index of the intrinsic layer 106 is 4, the thickness thereof is 5-30 nm, and the material thereof is a-Si or hydrogenated amorphous silicon (a-Si:H), for example. The second conductive type layer 108 is a P-type layer, the refractive index thereof is 4.5, and the thickness thereof is 50 μm-150 μm, for example. The material of the second conductive type layer 108 is single-c-Si or poly-c-Si, for example. The material of the back contact 110 is transparent conductive oxide (TCO) or a metal layer, and the material of the metal layer is Al, Ag, Mo, Cu or other suitable metals or alloys, for example.
It should be noted that in the solar cell 100, the refractive indexes of the front contact 102, the material layer 104 with low refractive index serving as the first conductive type layer, the intrinsic layer 106 and the second conductive type layer 108 as stacked increase in sequence. As a result, incident light is prevented from being reflected at interfaces, e.g., the interface between the front contact 102 and the material layer 104 of low refractive index, the interface between the material layer 104 with low refractive index and the intrinsic layer 106, and the interface between the intrinsic layer 106 and the second conductive type layer 108. Moreover, the present embodiment is exemplified by the material layer 104 with low refractive index serving as an N-type layer and the second conductive type layer 108 being a P-type layer. However, according to another embodiment (not shown), the material layer 104 of low refractive index can also be a P-type layer, and the second conductive type layer 108 can be an N-type layer. In other words, the material layer 104 with low refractive index having a refractive index lower than 3 in the solar cell 100 can serve as an N-type layer or a P-type layer.
It should be pointed out that in the solar cell 100 shown by
In the foregoing embodiment, the refractive index of the material layer 104 of low refractive index serving as the first conductive type layer is lower than 3, and the refractive indexes of the front contact 102, the material layer 104 with low refractive index, the intrinsic layer 106 and the second conductive type layer 108 increase in sequence. As a result, light transmittance is enhanced and occurrence of light reflection at the interfaces is reduced (e.g., the interface between the front contact 102 and the material layer 104 with low refractive index, the interface between the material layer 104 of low refractive index and the intrinsic layer 106, and the interface between the intrinsic layer 106 and the second conductive type layer 108). Hence, the solar cells 100 and 200 achieve the optimum utility rate of sunlight, which in turn enhances short circuit current densities (Jsc) and efficiencies of the solar cells 100 and 200.
It is to be noted that in the foregoing embodiment the material of the intrinsic layer 106 is a-Si, and the material of the second conductive type layer 108 is silicon. Hence, the solar cell 100 is a hetero -junction solar cell. In the conventional hetero-junction solar cell, since an absorption coefficient of a-Si is higher than the absorption coefficient of silicon, sunlight is largely absorbed by an a-Si layer before it enters a silicon layer such that photoelectric current of the hetero-junction solar cell is significantly reduced. However, the structure of the solar cell in the present invention significantly enhances transmittance of sunlight and reduces light reflection at the interfaces so that more sunlight reaches the silicon layer to solve the aforementioned problems, thereby enhancing the utility rate of sunlight, the short circuit current density and the efficiency of the hetero-junction solar cell.
Several experiment examples are described below to prove the efficacy of the present invention.
In order to compare the influence of the material layer of low refractive index in the short circuit current density (Jsc) and the refractive index, a solar cell having a material layer with low refractive index (n) lower than 3 is manufactured first. The solar cell includes a 140-nm TCO layer serving as the front contact (n=1.8-2), a 60-nm N-μc-SiOx layer (n=2-2.3) serving as the N-type layer, a 30-nm a-Si layer (n=4) serving as the intrinsic layer, a 150-nm P-type silicon layer (n=4.5) serving as the P-type layer and a metal layer serving as the back contact.
In addition, another solar cell is manufactured as a comparison example, and the solar cell of the comparison example differs from that of the experiment example only in that a 60-nm N-type a-Si layer (n=4.5) is used as the conventional N-type layer in the solar cell of the comparison example.
Afterwards, light of different wavelengths irradiates the solar cells of the present invention and the comparison example through the front contact, and then the refractive indexes of the solar cells are measured and the short circuit current densities (Jsc) thereof are computed.
It is known from
According to the present experiment example, the thicknesses of the TCO layers serving as the front contacts in the solar cells of the present invention and the comparison example are reduced to 60 nm, the materials and parameters of the remaining layers are all the same as those described in the experiment example 1.
Afterwards, light of different wavelengths irradiates the solar cells of the present invention and the comparison example through the front contact, and then quantum efficiencies (QE) of the solar cells are measured and the short current densities (Jsc) thereof are computed. The quantum efficiencies varying with different wavelengths are shown by the curve diagram of
It is known from
It is known from the foregoing experiment examples that the material layer with low refractive index having a refractive index lower than 3 in the present invention reduces the probability of light reflection and enhances the short circuit current density (Jsc) in the solar cell. Moreover, the thickness reduction of the TCO layer allows the solar cell of the present invention to obtain a higher Jsc and higher efficiency.
In summary, the solar cell of the present invention has the material layer of low refractive index having a refractive index lower than 3 to serve as the first conductive type layer, and the refractive indexes of the front contact, the material layer of low refractive index, the intrinsic layer and the second conductive type layer increase in sequence, for example. Consequently, sunlight transmittance is increased and light reflection at the interfaces is reduced (e.g., the interface between the front contact and the material layer of low refractive index, the interface between the material layer of low refractive index and the intrinsic layer, and the interface between the intrinsic layer and the second conductive type layer). Therefore, the solar cell achieves the optimum utility rate of sunlight, which in turn enhances the short current density (Jsc) and efficiency. Additionally, the solar cell structure of the present invention can be applied to a hetero-junction solar cell to solve the problem of insufficient photoelectric current in the heterojunction cell because light is largely absorbed by the a-Si layer, thus enhancing the utility rate of sunlight, the short circuit current density and efficiency of the hetero-junction solar cell.
It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.
| Number | Date | Country | Kind |
|---|---|---|---|
| 97150530 | Dec 2008 | TW | national |
