Patent Application
20100126579
This application claims the priority benefit of Taiwan application serial no. 97145207, filed on Nov. 21, 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 relates to a solar cell having a reflective structure.
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.
However, the current amorphous silicon (a-Si) film products used in a solar cell have two problems. One is irradiation stability and the other is that only the sunlight with a wavelength less than 800 nm can be absorbed.
Therefore, other methods capable of enhancing the utilization efficiency are required. Currently, it is believed that the most preferred solution for enhancing the efficiency lies in decreasing a thickness of a hydrogenated amorphous silicon (a-Si:H) photoelectric conversion layer and staking micro-crystalline (μc-Si) film solar cells capable of absorbing lights with a long wavelength into a tandem solar cell.
Accordingly, it can lower the light absorbance to decrease a thickness of the photoelectric conversion layer. However, as for a conventional tandem solar cell, since the refraction index coefficients of both cells are quite close and both of the cells are silicon layers with a refraction index of about 4, no reflective interface will be formed when light passes through an interface between the two cells, and only when the light reaches a silver layer, the light is reflected to a μc-Si layer of a bottom cell.
Accordingly, the present invention is directed to a solar cell having a reflective structure, which can reflect lights directly to an amorphous silicon (a-Si) layer, thereby decreasing a thickness of a photoelectric conversion layer.
The present invention is further directed to a solar cell having a reflective structure, which can increase a light flux and absorption of a photoelectric conversion layer, so as to increase a short circuit current density (Jsc).
As embodied and broadly described herein, the present invention provides a solar cell having a reflective structure, which includes a front contact, a P layer, an intrinsic layer (I layer), an N layer, and a back contact that are stacked together. The solar cell is characterized in that the N layer is a layer of low refraction index, and a refraction index of the layer of low refraction index is lower than that of the I layer.
The present invention further provides a solar cell having a reflective structure, which includes a front contact, a P layer, an I layer, an N layer, and a back contact that are stacked together. The solar cell having the reflective structure is characterized in that the N layer is a multi-layer structure formed by several films in such a manner that films with low refraction indexes and films with high refraction indexes are stacked alternately. In the multi-layer structure, a film in contact with the I layer is a film of low refraction index. A refraction index of the film of low refraction index is lower than that of the I layer.
Based on the above, the present invention provides an N layer of low refraction index in a solar cell structure, which can increase the reflection of an interface, decrease the thickness of an I layer in an a-Si solar cell, achieve the highest utilization efficiency of the sunlight, thereby increasing a Jsc and enhancing the element efficiency.
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.
Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.
Referring to
As the layer of low refraction index 108 in the first embodiment can reflect lights back to the I layer 106, a light gain of the lights reflected back to the I layer 106 is further utilized, so that the I layer 106 further absorbs the lights with a waveband in the visible region once again, so as to generate more photoelectric current. In addition, the layer of low refraction index 108 is, for example, microcrystalline structure.
Referring to
In the second embodiment, since reflective interfaces in multiple layers are provided (for example, an interface between the I layer 106 and the film of low refraction index 204, an interface between the film of low refraction index 204 and the film of high refraction index 206, and an interface between the film of high refraction index 206 and the back contact 110), a light flux and absorption of a photoelectric conversion layer can be increased, thereby increasing the Jsc.
Although the multi-layer structure 202 in
Referring to
The layer of low refraction index 108 in the third embodiment can reflect lights back to the I layer 106, so as to prevent an N/P layer in a conventional tandem solar cell with an intermediate layer from absorbing the reflective lights once again. Moreover, with the layer of low refraction index 108 having a reflective function, the intermediate layer in the conventional tandem solar cell can be omitted.
Referring to
In the fourth embodiment, the refraction indexes of the films of low refraction index 406 and 410 are, for example, less than or equal to 2.1. The film of high refraction index 408 may be selected to be made of a material for an N layer in the prior art. In other words, in the multi-layer structure 404, materials of the films stacked alternately may be selected from a group consisting of N—SiOx, N—SiCx, N—SiNx, N-a-Si, and N-μc-Si. For example, when the films of low refraction index 406 and 410 are N—SiOx layers, the electrical conductivity of the films of low refraction index 406 and 410 is at least greater than 10−7 S/cm. The film of high refraction index 408 may be an N-a-Si layer, and the electrical conductivity thereof is about 10−4 S/cm. Each layer in the multi-layer structure 404 is completed in the same manufacturing process.
In the fourth embodiment, since reflective interfaces in multiple layers (such as an interface between the I layer 106 and the film of low refraction index 406, an interface between the film of low refraction index 406 and the film of high refraction index 408, an interface between the film of high refraction index 408 and the film of low refraction index 410, and an interface between the film of low refraction index 410 and the bottom cell P layer 306) are provided, the light flux and absorption of the photoelectric conversion layer can be increased, thereby increasing the Jsc. In addition, at least one of the films of low refraction index 406 and 410 is microcrystalline structure, for example. Although the multi-layer structure 404 in
Several experiments are listed below to demonstrate the efficacies of the present invention.
In order to prove that the demonstrative N—SiOx in each of the above embodiments of the present invention is a suitable material with a low refraction index, the N—SiOx is compared with a conventional N layer material used in a top cell of a tandem solar cell, that is, N type hydrogenated amorphous silicon (N-a-Si:H), and a conventional N layer material used in a bottom cell of a tandem solar cell, that is, N type micro-crystalline silicon (N-μc-Si:H).
As seen from the curve diagram (shown in
As seen from the curve diagram (shown in
In order to prove that the N layer of the present invention further contributes to increasing the light reflection, a glass of 1.1 mm is used as a substrate. An a-Si (n=4) of 300 nm is stacked on the substrate as an I layer, and then micro-crystalline SiOx (n=2.1) of 100 nm with a low refraction index is stacked on the I layer as an N layer of the present invention.
In addition, a contrast example is manufactured. In the same way as that described above, a glass of 1.1 mm is used as a substrate. An a-Si (n=4.5) of 300 nm is stacked on the substrate as an I layer. Difference from the above manner, a μc-Si of 30 nm is then stacked on the I layer as a conventional N layer.
Subsequently, lights with different wavelengths are irradiated from the glass towards the N layer, and then, the reflectivity of the N layers is measured and shown in
In order to compare the Jsc of the present invention with that of a conventional single junction solar cell, first of all, a solar cell with a layer of low refraction index of the present invention is manufactured. The solar cell includes a TCO as a front contact, a P layer of 100 nm, an I layer (n=4) of 350 nm, an N—SiOx layer (n=2) of 100 nm with a low refraction index as an N layer of the present invention, a TCO as a back contact, and a silver layer.
In addition, a contrast example is manufactured. The only difference between the contrast example and the above solar cell lies in that an a-Si (n=4.5) of 30 nm is used as a conventional N layer.
Then, lights of different wavelengths are irradiated from the front contact into the solar cells, and then, the quantum efficiency (QE) of the N layers is measured and shown in
In order to compare the Jsc of a tandem solar cell of the present invention with that of a conventional tandem solar cell, first of all, the tandem solar cell (Example 1) with a layer of low refraction index of the present invention is designed, which includes: a TCO as a front contact; a top cell containing a P layer of 10 nm, an I layer (n=4) of 280 nm, and an N—SiOx layer (n=2) of 44 nm with a low refraction index; a bottom cell containing a P layer of 10 nm, a μc-Si of 3 μm as an I layer, and a conventional N layer of 20 nm; a TCO as a back contact; and a silver layer. Herein, the N—SiOx layer is microcrystalline structure as show in nano beam diffi action (NBD) pattern of
In addition, a tandem solar cell (Example 2) having a multi-layer structure with a reflective effect of the present invention is designed. The difference between Example 2 and Example 1 lies in that, the N layer of the top cell is replaced with a multi-layer structure formed by an N—SiOx layer (n=2) of 55 nm, an N-a-Si layer (n=4.5) of 27.5 nm, and an N—SiOx layer (n=2) of 55 nm which contains microcrystalline structure, and the I layer of the bottom cell is replaced with a μc-Si of 4 μm.
The corresponding contrast example includes a TCO as a front contact; a top cell containing a P layer of 10 nm, an I layer (n=4) of 280 nm, and a conventional N layer (amorphous silicon, n=4) of 30 nm; a bottom cell containing a P layer of 10 nm, a μc-Si of 2 μm as an I layer, and a conventional N layer of 20 nm; a TCO as a back contact; and a silver layer.
Then, lights of different wavelengths are simulated to be irradiated from the front contact into the tandem solar cell, and then the quantum efficiency (QE) is measured and shown in
In order to further illustrate the thickness changes of an I layer in a bottom cell and a top cell, a curve diagram of QE variation is obtained and shown in
Then, a tandem solar cell having a layer of low refraction index is designed, which includes a TCO as a front contact; a top cell containing a P layer of 10 nm, an I layer (n=4) of 230 nm and an N—SiOx layer (n=2) of low refraction index of 44 nm; a bottom cell containing a P layer of 10 nm, a μc-Si of 2.5 μm as an I layer, and a conventional N layer of 20 nm; a TCO as a back contact; and a silver layer. Herein, the N—SiOx layer is microcrystalline structure as show in
Then, a tandem solar cell having a multi-layer structure with a reflective effect is designed. The differences between the tandem solar cell and Example 1 lie in that, the thickness of the I layer in the top cell is reduced to 190 nm; the N layer is replaced with a multi-layer structure containing an N—SiOx layer (n=2) of 55 nm, an N-a-Si layer (n=4.5) of 27.5 nm, and an N—SiOx layer (n=2) of 55 nm; the thickness of the I layer in the bottom cell is changed to 3.3 μm. Herein, the N—SiOx layer is microcrystalline structure as show in
In the tandem solar cell, the N layer is designed into a multi-layer structure, and the current of the top cell is fixed. The thickness of the I layer in the top cell may be decreased by 32%. As long as the thickness of the I layer in the bottom cell is increased by about 39%, a current match between the top cell and the bottom cell can be achieved.
As known from Experiment 4 and Experiment 5, the light deterioration can be alleviated by means of fixing the current of the top cell and reducing the thickness of the top cell (Staebler Wronski Effect, briefly referred to as SWE effect). Meanwhile, a fill factor may be added, and the thickness increment of the bottom cell may be reduced. Thus, it is a more preferable manner to fix the current of the top cell and reduce the thickness of the top cell than that of fixing the thickness of the top cell.
To sum up, the present invention has an N layer with reflection and conductivity functions, which is capable of reflecting lights back to the I layer, and utilizing the gain of the lights reflected to the I layer to enable the amorphous silicon film to absorb the lights with wavelengths in the visible region once again, so as to reduce the thickness of the I layer while achieving the same efficiency as a thicker one. Alternatively, an I layer with the same thickness can be employed, and the N layer film with reflection and conductivity functions of the present invention can produce more photoelectric current. Moreover, when the present invention is applied to a tandem solar cell, an N layer with reflection and conductivity functions can be employed to design a single-layer or multi-layer structure in the top cell. The reflection at the designed structure interfaces can be utilized to increase the light flux and absorption of the photoelectric conversion layer, thereby increasing the Jsc.
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 |
|---|---|---|---|
| 97145207 | Nov 2008 | TW | national |
