Abstract:
A tandem solar cell structure includes a substrate, a conductive layer, a bottom solar cell combination and a top solar cell, The bottom solar cell combination includes a plurality of solar cell units and is disposed on the substrate. A conductive layer is disposed between the top solar cell and the bottom solar cell combination. The top solar cell is connected to one of the solar cell units in series. A wide energy distribution of the solar radiation can be absorbed through the tandem solar cell structure. The electrical series connection of the top solar cell and the solar cell units of the bottom solar cell combination reduces current mismatch between the top and bottom cells and enhances the overall system open circuit voltage due to more units in the bottom cell combination. The efficiency of the tandem solar cell structure is therefore improved considerably.

Description:
RELATED APPLICATIONS 
       [0001]    The application claims priority to Taiwan Application Serial Number 101116684, filed May 10, 2012, which is herein incorporated by reference. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present disclosure relates to a tandem solar cell structure and fabrication method thereof. More particularly, the present disclosure relates to a tandem solar cell structure that can reduce current mismatch between a top solar cell and a bottom solar cell combination and can enhance the open circuit voltage (Voc). 
         [0004]    2. Description of Related Art 
         [0005]    A demand on exploiting new energy sources increase dramatically in that the energy shortage issue is getting more and more serious. The energy of a solar radiation from the sun to the surface through the atmosphere of the earth is about 1.8×10 14  kW. Such energy is around one hundred thousand times of than the annual worldwide demand in electric power. Efficiently utilizing the solar energy will be of great help in solving the issue of energy shortage. 
         [0006]    A solar cell is an energy conversion device. The purpose of the solar cell is to convert a solar energy to an electrical energy. In principle, an electricity of a solar cell is generated based on the photovoltaic effect. A solar cell is basically consisting of a p-type and an n-type semiconductor. When a solar radiation is incident to the solar cell, the energy higher than a bandgap of the semiconductor is absorbed. As such an electron-hole pair is generated, and thus an electric current. 
         [0007]    The spectrum of the solar radiation ranges from 0.3 micron (μm) to a few microns, which is equal to an energy distribution from 0.4 eV (Electronic Volt) to 4 eV. In this regard, the solar radiation distributes in a wide range. A conventional solar cell structure is made of silicon (Si)-based materials. The bandgap energy of Si is about 1.1 eV in the room temperature. Only the solar energy larger than 1.1 eV can be absorbed by the solar cell structure, and the solar energy lower than 1.1 eV cannot be absorbed, which leads to low photoelectric conversion efficiency. To address this issue, a tandem solar cell structure is disclosed. The concept of the tandem solar cell structure is to combine two semiconductor devices which have different bandgap energy into one solar cell structure. Therefore, different energy regions of the solar radiation can be absorbed by the two semiconductors having different bandgap energy and the photoelectric conversion efficiency can be enlarged. Although the bandwidth distribution of energy absorption can be enlarged, there exists a current density mismatch between the top solar cell and the bottom solar cell of the tandem solar cell structure. Such current mismatch issue will still lead to low photoelectric conversion efficiency. 
       SUMMARY 
       [0008]    According to one aspect of the present disclosure, a tandem solar cell structure is provided. A bottom solar cell combination and a top solar cell are disposed on a substrate. A conductive layer is disposed between the top solar cell and the bottom solar cell combination. The bottom solar cell combination comprises a plurality of solar cell units, and the solar cell units are connected in series with each other. The top solar cell is only connected to one of the solar cell units in series. The top solar cell has bandgap energy higher than bandgap energy of the solar cell units of the bottom solar cell combination. 
         [0009]    According to another aspect of the present disclosure, a fabrication method applicable to the tandem solar cell structure of the present disclosure is provided. The fabrication method comprises the following steps: forming a solar cell body; cutting the solar cell body into a plurality of solar cell units, wherein a gap formed between each of the solar cell units; connecting the solar cell units with each other in series to form the bottom solar cell combination; forming a top solar cell and connecting the top solar cell with one of the solar cell units in series. 
         [0010]    According to still another aspect of the present disclosure, the substrate is a transparent substrate or a bendable substrate. Besides, the substrate is made of glass, metal or organic materials. 
         [0011]    According to still another aspect of the present disclosure, the method of forming the solar cell units can be laser scribing, chemical etching or reactive ion etching. 
         [0012]    According to still another aspect of the present disclosure, the solar cell units of the bottom solar cell combination are made of III-V group semiconductor compounds, II-VI group semiconductor compounds, organic semiconductor compounds, nanoscale materials, CIGS (CuInGaS)-based materials or CIS (CuInSe)-based materials. 
         [0013]    According to still another aspect of the present disclosure, the conductive layer is a tunneling-junction layer, and the conductive layer is made of transparent oxide materials or thin and transparent metal materials. 
         [0014]    According to still another aspect of the present disclosure, the top solar cell is made of a-Si-based materials, CGS (CuGaSe)-based materials or a-Si/μc-SiC multi-junction structures. 
         [0015]    According to still another aspect of the present disclosure, the series connection between each solar cell units is an electrical connection. The electrical connection is performed by applying a conductive substrate or by the following steps: depositing an insulting material to the gaps between each solar cell units; cutting the insulating material to form a filling space and filling a conductive material into the filling space. 
         [0016]    According to still another aspect of the present disclosure, a large solar cell module is formed by repeatedly connecting the tandem solar cell structures in series, wherein the series connection of the tandem solar cell structure is performed by connecting a negative electrode of the respective solar cell units of one of the tandem solar cell structures to a positive electrode of the respective solar cell units of another one of the tandem solar cell structures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The disclosure can be more fully understood by reading the following detailed description of the embodiment, with reference made to the accompanying drawings as follows: 
           [0018]      FIG. 1  shows an elementary method of fabricating a tandem solar cell structure according to the embodiment of the present disclosure; 
           [0019]      FIG. 2A  is a schematic view showing a first process step of the tandem solar cell structure according to the embodiment of the present disclosure; 
           [0020]      FIG. 2B  is a schematic view showing a second process step of the tandem solar cell structure according to the embodiment of the present disclosure; 
           [0021]      FIG. 2C  is a schematic view showing a third process step of the tandem solar cell structure according to the embodiment of the present disclosure; 
           [0022]      FIG. 2D  is a schematic view showing a fourth process step of the tandem solar cell structure according to the embodiment of the present disclosure; 
           [0023]      FIG. 2E  is a schematic view showing a fifth process step of the tandem solar cell structure according to the embodiment of the present disclosure; 
           [0024]      FIG. 2F  is a schematic view showing a sixth process step of the tandem solar cell structure according to the embodiment of the present disclosure; 
           [0025]      FIG. 2G  is a schematic view showing a seventh process step of the tandem solar cell structure according to the embodiment of the present disclosure; 
           [0026]      FIG. 2H  is a schematic view showing an eighth process step of the tandem solar cell structure according to the embodiment of the present disclosure; 
           [0027]      FIG. 2I  is a schematic view showing a ninth process step of the tandem solar cell structure according to the embodiment of the present disclosure; 
           [0028]      FIG. 3  shows an application method of the tandem solar cell structure; and 
           [0029]      FIG. 4  shows n improvement of the tandem solar cell structure. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    Reference will now be made in detail to the present embodiments of the disclosure, 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. 
         [0031]    Referring to  FIG. 1 , a tandem solar cell structure  110  comprises a substrate  111 , a bottom solar cell combination  112 , a conductive layer  113  and a top solar cell  114 . The bottom solar cell combination  112  is disposed on the substrate  111  The bottom solar cell combination  112  comprises a solar cell unit  115  and a solar cell unit  116 . The solar cell unit  115  and the solar cell unit  116  are formed by cutting a solar cell body (not labeled). The conductive layer  113  is disposed between the top solar cell  114  and the solar cell unit  115 . The top solar cell  114 , the solar cell unit  115  and the solar cell unit  116  are connected in series electrically. The top solar cell  114  has higher bandgap energy value, and the solar cell unit  115  and the solar cell unit  116  have lower bandgap energy value. A solar radiation incident to the top solar cell  114  and the short wavelength region of the solar radiation is absorbed by the top solar cell  114 . The long wavelength region of the solar radiation passes through the top solar cell  114  and is absorbed by the bottom solar cell combination  112 . According to one embodiment of the disclosure, the top solar cell  114  covers the bottom solar cell combination  112  so that the usage efficiency of the solar radiation can be enlarged. The solar combination cell  112  is composed of a solar cell unit  115  and a solar cell unit  116 , and the solar cell unit  115  and the solar cell unit  116  both has small contact area. As the current density is inverse proportional to the contact area, the current density of the bottom solar cell combination  112  is increased, thus the current mismatch between the top solar cell unit  114  and the bottom solar cell combination  112  can be reduced. Therefore, the short circuit current of the tandem solar cell structure  110  minimize the influence of the smaller short circuit current of the top solar cell  114 , meanwhile, the open circuit voltage is increased owing to the series connection of the bottom solar cell combination  112 . Consequently, the photoelectric conversion efficiency of the tandem solar cell structure can be enhanced. 
         [0032]    A tandem solar cell structure for real case and fabrication method thereof is descried in the following embodiments. The tandem solar cell structure comprises an amorphous Silicon based p-i-n type top solar cell and a CIGS based p-n type bottom solar cell combination. 
         [0033]    Referring to  FIG. 2A , a Mo conductive metal layer  212  is deposited on a substrate  211  as a back-side electrode. A p-type CIGS layer  213  is then deposited on the Mo conductive metal layer  212 . An n-type CdS layer  214  is deposited on the p-type CIGS layer  213  in order to form a p-n junction. 
         [0034]    Referring to  FIG. 2B , a laser scribing method is applied to the structure of  FIG. 2A . A notch with a width W1 is formed in position P1 by cutting through the CdS layer  214  and the GIGS layer  213 . 
         [0035]    Referring to  FIG. 2C , a laser scribing method is applied to the structure of  FIG. 2B . A notch with a width W2 is formed in position P2 by cutting through the Mo conductive metal layer  212 . 
         [0036]    Referring to  FIG. 2D , a first ZnO layer  215  is deposited on the structure of  FIG. 2C . 
         [0037]    Referring to  FIG. 2E , a laser scribing method is applied to the structure of  FIG. 2D . A notch with a width W3 is formed in position P3 by cutting through the first ZnO layer  215 . 
         [0038]    Referring to  FIG. 2F , a transparent high conductive first ZnO:Al layer  216  is deposited on the structure of  FIG. 2E . 
         [0039]    Referring to  FIG. 2G , a removable mask  220  is formed on the first ZnO:Al layer  216  of the right side of the solar cell body (not labeled). A laser scribing method is applied to cut through the first ZnO:Al layer  216  and a notch with a width W4 is formed in position P4. A second transparent and high conductive ZnO:Al layer  217  is deposited on the first ZnO:Al layer  216  of the left side of the solar cell body (not labeled). 
         [0040]    Referring to  FIG. 2H , a removable mask  230  is formed on the second ZnO:Al layer  217 , and a second ZnO layer  218  is deposited. 
         [0041]    Referring to  FIG. 21 , an a-Si based p-i-n type solar cell structure  219  is deposited on the structure of  FIG. 2H . Thus, a tandem solar cell structure  210  is formed. 
         [0042]    Referring to  FIG. 3 , a transparent high conductive ZnO:Al layer  312  is deposited on the a-Si based p-i-n type solar cell structure  219 , and an optical transmission layer  313  is deposited on the ZnO:Al layer  312 . The optical transmission layer  313  is made of glass or plastic, and the optical transmission layer  313  is equal to or larger than the a-Si based p-i-n type solar cell structure  219  in dimensions. A solar radiation incident through the optical transmission layer  313 . The short wavelength region of the solar radiation is absorbed by the a-Si based p-i-n solar cell structure  219  and the long wavelength region of the solar radiation is absorbed by the CIGS based p-n type solar cell (not labeled) which is composed of the p-type CIGS layer  213  and n-type CdS layer  214 . An inner current generated by the Photovoltaic Effect, and the inner current path is  410 . In real application, an outer device  500  is connect to the Mo conductive metal layer  212  and the ZnO:Al layer  312 . The outer current path is  420 . 
         [0043]    Referring to  FIG. 4 , an improvement of the tandem solar cell structure is provided. An transparent high conductive ZnO:Al layer  311  is further deposited between the second ZnO:Al layer  217  and the a-Si based p-i-n solar cell structure  219  of the tandem solar cell structure  210  of  FIG. 21 . A current loss can be reduced by the ZnO:Al layer  311  when the current pass through a hetero-junction. 
         [0044]    Moreover, one of the embodiments shows a fabrication method to form a large solar cell module. The fabrication method of forming the large solar cell module comprises, connecting a top solar cell to a bottom solar cell units in series in order to form a tandem solar cell unit; coating a bottom electrode layer on the selected large area substrate; cutting the large area substrate in the number of pre-selected for the total number of tandem cell units; coating a tandem solar cell which not comprise a top transparent electrode layer on the large area substrate; cutting the tandem solar cell which not comprise the top transparent electrode layer into the bottom electrode slightly beside the previous cut of the bottom electrode layer; coating a transparent electrode layer on the pre-cut tandem solar cell and cutting the tandem cell structure comprise the as-coated transparent electrode layer into the bottom electrode layer. Therefore, a negative electrode of a first tandem solar cell unit is connected with a positive electrode of a second tandem solar cell unit, and a large solar cell module has selective electric voltage and power is formed by inter-connection mechanism, In other words, the concept of “nested” type of configuration in the complete electrical inter-connection is different from the conventional mechanism used in thin film solar cell module manufacturing processes. 
         [0045]    The present disclosure provides a tandem solar cell structure that can reduce current mismatch and having high photoelectric conversion efficiency. The tandem solar cell structure comprises a top solar cell and a bottom solar cell combination. A solar radiation is incident to the top solar cell having larger bandgap energy, so the shorter wavelength region of the solar radiation is absorbed. Then, the longer wavelength region of the solar radiation passes through a tunneling conductive layer and reaches the bottom solar cell combination of the tandem solar cell structure. The longer wavelength region of the solar radiation is absorbed by the bottom solar cell combination. Therefore, a broadband of energy of the solar radiation can be absorbed. The bottom solar cell combination is consisting of a plurality of solar cell units which are cut from a standalone solar cell body. As the intensity of a current is inverse proportional to the dimension of a contact area, the current mismatch between the top solar cell and the bottom solar cell combination can be compensated by the solar cell units with small contact areas. Thus, the photoelectric conversion efficiency can be enhanced. And, owing to the solar cell units in the present disclosure is cut from a standalone solar cell body, manufacturing cost can be economized and fabrication efficiency can be increased. Furthermore, a large solar cell module can be formed by a series connection of each tandem solar cell structure. 
         [0046]    It will be apparent to those ordinarily skilled in the art that various modifications and variations can be made to the structure of the present disclosure without departing from the scope or spirit of the present disclosure. In view of the foregoing, it is intended that the present disclosure cover modifications and variations of the present disclosure provided they fall within the scope of the following claims.