Patent Publication Number: US-2010126579-A1

Title: Solar cell having reflective structure

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     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. 
     BACKGROUND OF THE INVENTION 
     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. 
     SUMMARY OF THE INVENTION 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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. 
         FIG. 1  is a schematic cross-sectional view of a solar cell having a reflective structure according to a first embodiment of the present invention. 
         FIG. 2  is a schematic cross-sectional view of a solar cell having a reflective structure according to a second embodiment of the present invention. 
         FIG. 3  is a schematic cross-sectional view of a solar cell having a reflective structure according to a third embodiment of the present invention. 
         FIG. 4  is a schematic cross-sectional view of a solar cell having a reflective structure according to a fourth embodiment of the present invention. 
         FIG. 5  is a curve diagram of refraction indexes of N—SiOx, N-a-Si:H, and N-μc-Si:H changing with wavelengths. 
         FIG. 6  is a curve diagram of extinction coefficients of N—SiOx, N-a-Si:H, and N-μc-Si:H changing with wavelengths. 
         FIG. 7  is a curve diagram of a reflectivity of an N layer of the present invention and that of a conventional N layer changing with wavelengths. 
         FIG. 8  is a curve diagram of a QE of a solar cell in the present invention and that of a conventional single junction solar cell changing with wavelengths. 
         FIG. 9  is a curve diagram of QEs of Example 1, Example 2, and a contrast example of the present invention changing with wavelengths. 
         FIG. 10A  is a curve diagram of a QE of the contrast example in  FIG. 9  changing with wavelengths. 
         FIG. 10B  is a curve diagram of a QE of a tandem solar cell having a layer of low refraction index in Experiment 5 changing with wavelengths. 
         FIG. 10C  is a curve diagram of a QE of a tandem solar cell having a multi-layer structure with a reflection effect in Experiment 5 changing with wavelengths. 
         FIG. 11  is a Nano beam diffraction Pattern of N—SiOx Layer. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     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. 
       FIG. 1  is a schematic cross-sectional view of a solar cell having a reflective structure according to a first embodiment of the present invention. 
     Referring to  FIG. 1 , a solar cell  100  having a reflective structure in the first embodiment includes a front contact  102 , a P layer  104 , an intrinsic layer (I layer)  106 , a layer of low refraction index  108  to serve as an N layer, and a back contact  110 . A refraction index of the layer of low refraction index  108  is lower than that of the I layer  106 . Moreover, a difference between the refraction index of the layer of low refraction index  108  and that of the I layer  106  is, for example, greater than or equal to 1.5. In the first embodiment, the refraction index of the layer of low refraction index  108  is, for example, less than or equal to 2.1, and a thickness thereof is about less than 150 nm. The layer of low refraction index  108  may be made of a material selected from a group consisting of N type silicon oxide (N—SiOx), N type silicon carbide (N—SiCx), N type silicon nitride (N—SiNx), N type amorphous silicon (N-a-Si), and N type micro-crystalline silicon (N-μc-Si). For example, when the layer of low refraction index  108  is N type silicon oxide (N—SiOx) layer, an electrical conductivity of the layer of low refraction index  108  is at least greater than 10 −7  S/cm. In addition, a metal layer  112 , for example, silver, is generally disposed on a back surface of the back contact  110 . 
     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. 
       FIG. 2  is a schematic cross-sectional view of a solar cell having a reflective structure according to a second embodiment of the present invention. Reference numbers the same as those of  FIG. 1  are used in  FIG. 2  to refer to the same elements. 
     Referring to  FIG. 2 , a solar cell  200  having a reflective structure in the second embodiment includes a front contact  102 , a P layer  104 , an I layer  106 , a multi-layer structure  202  to serve as an N layer, and a back contact  110 . The multi-layer structure  202  is formed by several films in such a manner that films with low refraction indexes and films with high refraction indexes are stacked alternately. Furthermore, a film in contact with the I layer  106  in the multi-layer structure  202  is a film of low refraction index  204 . The refraction index of the film of low refraction index  204  is lower than that of the I layer  106 . The difference between the refraction index of the film of low refraction index  204  and that of the I layer  106  is, for example, greater than or equal to 1.5. In the second embodiment, the refraction index of the film of low refraction index  204  is, for example, less than or equal to 2.1. A material of the film of low refraction index  204  may be obtained with reference to the material of the layer of low refraction index  108  in the first embodiment. For example, when the film of low refraction index  204  is an N—SiOx layer, the electrical conductivity of the film of low refraction index  204  is at least greater than 10 −7  S/cm. Additionally, the layer of low refraction index  204  is, for example, microcrystalline structure. In the multi-layer structure  202 , a film in contact with the back contact  110  is a film of high refraction index  206 . The refraction index of the film of high refraction index  206  is generally greater than that of the back contact  110 , so that the film of high refraction index  206  may be selected to be made of an N type hydrogenated amorphous silicon (N-a-Si:H) that serves as an N layer in the prior art. In addition, the difference between the refraction index of the film of low refraction index  204  and that of the film of high refraction index  206  is, for example, greater than or equal to 1.5. 
     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  FIG. 2  only has one film of low refraction index  204  and one film of high refraction index  206 , the number of films in the multi-layer structure  202  may still be increased depending on the design requirements. However, the thickness of the multi-layer structure  202  is preferably less than 150 nm. A metal layer  112  is generally disposed on a back surface of the back contact  110 . 
       FIG. 3  is a schematic cross-sectional view of a solar cell having a reflective structure according to a third embodiment of the present invention. Reference numbers the same as those of  FIG. 1  are used in  FIG. 3  to refer to the same elements. 
     Referring to  FIG. 3 , a solar cell  300  having a reflective structure in the third embodiment is a tandem solar cell. The tandem solar cell has a set of top cells  302  and a set of bottom cells  304 . Each of the top cells  302  in the tandem solar cell includes a P layer  104 , an I layer  106 , and a layer of low refraction index  108  to serve as an N layer that are stacked together as that mentioned in the first embodiment. Each of the bottom cells  304  includes a bottom cell P layer  306 , a bottom cell I layer  308 , and a bottom cell N layer  310  that are stacked together. The bottom cell P layer  306  of the bottom cell  304  is in contact with the layer of low refraction index  108  of the top cell  302 . A back contact  110  is disposed on a back surface of the bottom cell N layer  310  of the bottom cell  304 , and furthermore, the metal layer  112  is generally disposed on the back surface of the back contact  110 . 
     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. 
       FIG. 4  is a schematic cross-sectional view of a solar cell having a reflective structure according to a fourth embodiment of the present invention. Reference numbers the same as those of  FIG. 3  are used in  FIG. 4  to refer to the same elements. 
     Referring to  FIG. 4 , a solar cell  400  having a reflective structure in the fourth embodiment is also a tandem solar cell, which includes a set of top cells  402  and a set of bottom cells  304 . Each of the bottom cells  304  includes a bottom cell P layer  306 , a bottom cell I layer  308 , and a bottom cell N layer  310  that are stacked together as mentioned in the third embodiment. Each of the top cells  402  includes a P layer  104 , an I layer  106 , and a multi-layer structure  404  to serve as an N layer that are stacked together as mentioned in the first embodiment. The multi-layer structure  404  is 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  404 , a film in contact with the I layer  106  is a film of low refraction index  406 , and then a layer of high refraction index  408  and another film of low refraction index  410  are further stacked. The refraction indexes of the films of low refraction index  406  and  410  are lower than that of the I layer  106 , the bottom cell P layer  306 , and the film of high refraction index  408  respectively. In the embodiment, the difference between the refraction indexes of the films of low refraction index  406  and  410  and that of the I layer  106  and the film of high refraction index  408  are greater than or equal to 1.5. 
     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  FIG. 4  is formed by two films of low refraction index  406 ,  410  and one film of high refraction index  408  and serves as an N layer, the number of films in the multi-layer structure  404  may still be increased depending on the design requirements. However, the total thickness of the multi-layer structure  404  is preferably less than 150 nm. 
     Several experiments are listed below to demonstrate the efficacies of the present invention. 
     Experiment 1 
     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  FIG. 5 ) of refraction indexes of the N—SiOx, N-a-Si:H, and N-μc-Si:H changing with wavelengths, a refraction index of the demonstrative N—SiOx in each embodiment of the present invention is lower than that of the N-a-Si:H and the N i  μc-Si:H. 
     As seen from the curve diagram (shown in  FIG. 6 ) of extinction coefficients of the N—SiOx, N-a-Si:H, and N-μc-Si:H changing with wavelengths, an extinction coefficient of the demonstrative N—SiOx in each embodiment of the present invention is also lower than that of the N-a-Si:H and the N-μc-Si:H. 
     Experiment 2 
     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  FIG. 7 . From  FIG. 7 , it can be seen that the reflectivity of the N layer at wavelengths between 550 nm and 800 nm is significantly greater than that of the conventional N layer, so that the film of low refraction index of the present invention is capable of increasing the light reflection. 
     Experiment 3 
     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  FIG. 8 . As seen from  FIG. 8 , when the conventional N layer in the solar cell is replaced with the N—SiOx layer of low refraction index, the QE is increased in wavelengths between 550 nm and 750 nm, and thus the Jsc is increased by 0.5%. 
     Experiment 4 
     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  FIG. 11 . 
     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  FIG. 9 . As seen from  FIG. 9  that, in the tandem solar cell, compared with the contrast example, Example 1 achieves a significant increase of the light reflection at the wavelengths between 500 nm and 700 nm, and meanwhile, the QE of the top cell increases and the QE of the bottom cell decreases. As for Example 2 with a fixed thickness of the I layer of the top cell, the Jsc of the top cell increases from 8.05 mA/cm 2  to 8.87 mA/cm 2 , that is, the Jsc is increased by 10%. Meanwhile, the thickness of the I layer of the bottom cell is increased by about 50%, and a current match between the top cell and the bottom cell is achieved. 
     Experiment 5 
     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  FIG. 10A  through simulation based on the contrast example in Experiment 4. 
     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  FIG. 11 . Subsequently, a curve diagram of the QE variation is obtained and shown in  FIG. 10B  through simulation. 
     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  FIG. 11 . Subsequently, a curve diagram of the QE variation is obtained and shown in  FIG. 10C  through simulation. 
     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.