Patent Publication Number: US-10326042-B2

Title: Highly doped layer for tunnel junctions in solar cells

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 12/404,795, filed Mar. 16, 2009. 
    
    
     U.S. GOVERNMENT RIGHTS 
     This invention was made with Government support under DE-FC36-07G017052 awarded by the Department of Energy. The Government has certain rights in this invention. 
    
    
     FIELD 
     Embodiments of the subject matter described herein relate generally to a method for improving the electrical characteristics of an interconnecting tunnel junction between adjacent solar cells and a multijunction solar cell having the improved interconnecting tunnel junction. 
     BACKGROUND 
     Multijunction solar cells are stacks of specifically oriented current generating p-n junction diodes or subcells. When electrically connected in series, current generated in one subcell is passed to the next subcell in series. Electrical characteristics of the interconnecting tunnel junction between subcells contribute to the overall efficiency of the multijunction solar cell. 
     SUMMARY 
     Presented is a method for improving the electrical characteristics of the interconnecting tunnel junction between subcells of a multijunction solar cell and a multijunction solar cell having the improved interconnecting tunnel junction. In various embodiments, the method and improved interconnecting tunnel junction comprise a narrow, delta-doped layer within the interconnecting tunnel junction that improves the current handling capability of the interconnecting tunnel junction between subcells of the multijunction solar cell. 
     The features, functions, and advantages discussed can be achieved independently in various embodiments of the present invention or may be combined in yet other embodiments further details of which can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying figures depict various embodiments of the system and method of highly doped layer for tunnel junctions in solar cells. A brief description of each figure is provided below. Elements with the same reference number in each figure indicated identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number indicate the drawing in which the reference number first appears. 
         FIG. 1  is an illustration of bandgaps of two subcells of a multijunction solar cell in one embodiment of the highly doped layer for tunnel junctions in solar cells system and method; 
         FIG. 2  is an illustration of a reversed biased junction between two subcells of a multijunction solar cell in one embodiment of the highly doped layer for tunnel junctions in solar cells system and method; 
         FIG. 3  is an illustration of an interconnecting tunnel junction between two subcells of a multijunction solar cell in one embodiment of the highly doped layer for tunnel junctions in solar cells system and method; 
         FIG. 4  is an illustration of an interconnecting tunnel junction having a delta-doped layer between two subcells of a multijunction solar cell in the highly doped layer for tunnel junctions in solar cells system and method; 
         FIG. 5  is an illustration of an interconnecting tunnel junction having a delta-doped layer and corresponding dopant concentrations in one embodiment of the highly doped layer for tunnel junctions in solar cells system and method, and 
         FIG. 6  is an block diagram of a manufacturing process for producing a multijunction solar cell having an interconnecting tunnel junction with a delta-doped layer in one embodiment of the highly doped layer for tunnel junctions in solar cells system and method. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely illustrative in nature and is not intended to limit the embodiments of the invention or the application and uses of such embodiments. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     Multijunction solar cells are constructed with a number of subcells stacked one on top of the other, each subcell being a current generating p-n junction diode. When light is incident on a subcell, photons having energies at or around the bandgap, Eg, are absorbed and converted into electrical current by the p-n junction. Photons having energies less than the bandgap are passed through the subcell to a lower subcell, while photons having energies higher than the bandgap are generally converted into excess heat. By using a topmost subcell with a comparatively high bandgap, and lower subcells of successively lower bandgaps, more of the available spectrum from the light is available to each subcell to be converted into electricity. Selection of the materials in each subcell determines the available energy to lower subcells. 
     Although for purposes of illustration and simplicity of explanation the following figures and description describe a multijunction solar cell having two cells, the system and methods described herein are equally applicable to solar cells having one, two, three or multiple cells. No limitation to a two cell multijunction solar cell is implied or intended. 
     Referring now to  FIG. 1 , in one non-limiting example, a two cell solar cell  100  having a top cell  102  comprised of Gallium Indium Phosphide (GaInP) and a bottom cell  104  comprised of Gallium Indium Arsenide (GaInAs) is presented. The cells  102 ,  104  are grown epitaxially, starting with a Germanium (Ge) substrate, and depositing layers of p-type GaInAs, n-type GaInAs, p-type GaInP, and n-type GaInP. Incident light  108  is directed at the top cell  102  of the two cell solar cell  100 . A portion of the light  108  is reflected  120 . A portion of the light  108  enters the top cell  102  and is absorbed and converted into heat  110 , especially those high energy photons  112  in the light  108  having an energy higher than the bandgap of the GaInP material, Eg&gt;1.87 eV, of the top cell  102 . High energy photons  112  having energy approximating the bandgap of the GaInP material in top cell  102 , Eg&gt;1.87 eV, are absorbed by electrons in the GaInP material. The additional energy allows electrons bound in the valence band of the GaInP crystalline lattice to move into the higher energy conduction band, creating free electrons  118  that contribute to the current generation of the two cell solar cell  100 . Low energy photons  114  have too little energy to free electrons  118  in the GaInP material and pass through the top cell  102  into the bottom cell  104 . Although the bandgap is illustrated as 1.87 eV, the bandgap for GaInP may vary from approximately 1.75 eV to approximately 1.90 eV. 
     In the bottom cell  104 , a portion of the light  108  is again absorbed and converted into heat  110 , especially those low energy photons  114  that have an energy higher than the bandgap of the GaInAs material, Eg&gt;1.39 eV, of the bottom cell  104 . Low energy photons  114  having energy approximating the bandgap of the GaInP material in bottom cell  104 , Eg&gt;1.39 eV, are converted into free electrons  118 . The remaining photons  116  pass into the substrate  106  where the remaining photons  116  generally are converted into heat  110 . Although the bandgap is illustrated as 1.39 eV, the bandgap for GaInP may vary from approximately 1.35 eV to approximately 1.43 eV. 
     Each cell  102 ,  104  is comprised of a p-n junction diode that generates current. The p-n junction diodes can be n-on-p type junction diodes, or p-on-n type junction diodes. Referring now to  FIG. 2 , two stacked cells of n-on-p type junction diodes  200  are illustrated. Each cell  102 ,  104  is comprised of an n-type doped layer  202  and a p-type doped layer  204 . Current is generated in the n-to-p direction in each cell as illustrated by the arrow I and collected through electrical connections V+ at the top of the two stacked cells of n-on-p type junction diodes  200 , and V− at the substrate  106 . By orienting the p-n junctions in both cells  102 ,  104  in the same direction, current generated by the top cell  102  passes through the bottom cell, and each cell  102 ,  104  amplifies the voltage of free electrons  118 . 
     However, the voltage differential between the p-type doped layer  202  of the top cell  102 , and the n-type doped layer  204  of the bottom cell creates a reversed biased junction  206  with a depletion region  208  relatively devoid of free elections  118 . As illustrated in the p-n junction voltage-current graph  210 , for normal operating voltages only a small amount of leakage current  212  is capable of flowing across the reversed biased junction  206 . 
     Referring now to  FIG. 3 , to increase the current carrying capability of the reversed biased junction  206 , an interconnecting tunnel junction, or ICTJ  302 , is epitaxially grown between the top cell  102  and the bottom cell  104  to create a multijunction solar cell  300 . The ICTJ  302  in the multijunction solar cell  300  comprises a highly doped p+-type layer  304  of GaInAs, GaInP, or AlGaAs, and a highly doped n+-type layer  306  of GaInAs, GaInP, or AlGaAs. Through a quantum mechanical process, the highly doped ICTJ  302  allows electrons to penetrate across the depletion region  208 , allowing an amount of tunneled current  308  to flow across the reverse-biased junction  206  that is proportional to the voltage, as illustrated in the tunnel junction voltage-current graph  310 . 
     The ICTJ  302  is constructed to pass the large amount of current that flows between the top cell  102  and the bottom cell  104 . The ICTJ  302  is optically transparent in order to pass as much light  108  as possible between the top cell  102  and the bottom cell  104 . To maximize manufacturing yields, the ICTJ  302  design is not sensitive to slight variations common in high volume manufacturing processes. To increase optical transparency, the ICTJ  302  is thin and generally has a bandgap, Eg, equal to or higher than the bottom cell to avoid capturing light  108 , specifically low energy photons  114  (shown in  FIG. 1 ), that would otherwise be converted to free electrons  118  in the bottom cell  104 . However, the thinness and bandgap, Eg, requirements for the ICTJ  302  limits the amount of dopants or intentional impurities (N A  or N D ) that can be incorporated into the ICTJ  302 . These limitations in turn limit the peak tunneling current through an approximate relationship:
 
 J peak∝exp□(− Eg 3/2 NA□ND /( NA+ND )))
 
     Where J peak  is the product of the volume and the energy of the electrons tunneling through the ICTJ  302  by quantum tunneling, Eg is the bandgap of the material used to grow the ICTJ  302 , N A  is the acceptor dopant concentration in the highly doped p+-type layer  304 , and N D  is the donor concentration in the highly doped n+-type layer  306 . Note that lower dopant concentrations and higher bandgaps reduce the peak tunneling current possible in the ICTJ  302 . 
     In terrestrial applications, incident light  108  is concentrated and focused on the multijunction solar cell  300 . This increase in concentrated illumination increases the current flowing through the ICTJ  302 . If the peak carrying capacity, J peak , is exceeded, a knee  312 , or sudden decrease, in amount of tunneled current  308  flowing across the reverse-biased junction  206  develops, as illustrated in the tunnel junction voltage-current graph  310 . In concentrator applications, the intensity of the light  108  can be the equivalent of 2000 suns or 2000 times AM1.5, a measure of spectrum and amplitude of solar radiation reaching the surface of the earth. This corresponds to a minimum J peak  of 30 A/cm 2 . The ICTJ  302  is epitaxially grown as a thin 150 A layer ( ˜ 20 atomic layers) and therefore doping levels in such thin layers are well monitored to ensure proper dopant concentrations are achieved. However doping levels can drift during production and therefore a design criteria for a J peak  Of 100 A/cm 2  is used to ensure proper yields during the manufacturing process. 
     However, it is difficult to epitaxially grow a thin 150 A layer ( ˜ 20 atomic layers) ICTJ  302 . In a typical manufacturing process, the  ˜ 20 atomic layers are deposited across a 20″ substrate millions of times. A 10% variability in thickness is a standard requirement. The ICTJ  302  thickness therefore needs to be controlled to just + or −2 atomic layers across the entire area of the 20″ substrate during manufacturing. Moreover, the peak amount of dopant (N A  or N D ) scales inversely with the band gap, Eg, of the material being doped. This limits the N A  or N D  dopant concentration to the approximately 10 19  cm −3  range for materials having band gaps of 1.8 to 1.9 eV and 10 20  cm −3  for materials having a band gap of 1.4 eV. 
     Although using materials with lower bandgaps increases the permissible doping levels, materials having lower band gaps have reduced optical transparency. Reduced optical transparency reduces both the amount of light  108  available to the bottom cell  104  and the energy in those photons that are transmitted to the bottom cell  104 , thereby reducing the energy producing capability and efficiency of the multijunction solar cell  300 . 
     Using materials with higher bandgaps improves the transparency, but manufacturing the ICTJ  302  requires tight control of doping levels to achieve the minimum J peak  of 30 A/cm 2  to 100 A/cm 2 . Direct methods of doping the ICTJ  302  layers is limited as the bulk doping properties of Group VI dopants like Te, Se, and S are limited by the presence of an atomic surface liquid layer concentration that needs to be established prior to doping and the overall solubility of a bulk mixture. These manufacturing constraints limit the peak concentrations and limits how thin the ICTJ  302  layers can be reliably grown. Group IV dopants like C, Si, Ge, and Sn will act as both donors and acceptors negating the overall dopant concentration, limiting their usefulness to the range from approximately 10 18  cm −3  to low 10 19  cm −3  concentrations. Group II dopants like Zn, Cd, and Hg tend to be mobile in the lattice, diffusing away from high concentration regions during subsequent epitaxy processes. This reduces the applicability of using Group II dopants to achieve the high dopant concentrations necessary to epitaxially grow the ICTJ  302 . 
     Referring now to  FIG. 4 , a multijunction solar cell with a delta-doped layer  400  is presented. The multijunction solar cell with a delta-doped layer  400  comprises a top cell  102 , and bottom cell  104 , and an interconnecting tunnel junction with a delta-doped layer, or δ-doped ICTJ  402 . The δ-doped layer  404  is a thin, approximately 20 A in width highly doped layer with a peak dopant concentration of 10 20  cm −3 . The δ refers to the shape of the doping profile of the δ-doped layer that approaches a Dirac delta function. A Dirac delta function, or δ, is a function that is infinite at one point and zero everywhere else. The δ-doped layer  404  is positioned in the δ-doped ICTJ  402  and adds to the effective N A  or N D  dopants, increasing the peak tunneling current of the δ-doped ICTJ  402  layer. The δ-doped layer  404  increases the tunnel current carrying capability of the δ-doped ICTJ  402  by approximately a factor of two over the ICTJ  302  of  FIG. 3  without a δ-doped layer  404 . This is also illustrated in the δ-doped tunnel junction voltage-current graph  408 , which shows the δ-doped tunnel current  406  for a reverse biased δ-doped ICTJ  402  to be approximately twice as steep as the curve representing the tunnel current  308  for an ICTJ  302  without a δ-doped layer  404 . 
     Referring now to  FIG. 5 , a δ-doped ICTJ  402  is shown in an exploded view. The highly doped p+-type layer  304  and the highly doped n+-type layer  306  adjoin to form the reversed biased junction  206 . An n-type δ-doped layer  502  is displaced within the highly doped n+-type layer  306  in close proximity to the highly doped p+-type layer  304 . Placing the n-type δ-doped layer  502  close to the highly doped p+-type layer  304  shortens the distance electrons have to travel to cross the reverse biased junction  206  and reduces the chance that electrons will recombine prior to crossing the reverse biased junction  206 , improving the peak tunneling current density. A dopant concentration chart  504  illustrates approximately the relevant dopant concentration levels for the p-dopant concentration  506 , the n-dopant concentration  508 , and n-type δ-doped concentration  510  in the δ-doped ICTJ  402 . A portion of the n-type δ-doped concentration  510  is estimated as shown by the dotted lines as the narrow width of the δ-doped layer makes concentrations difficult to measure. 
     In another embodiment, the n-type δ-doped layer  502  is directly adjacent to the highly doped p+-type layer  304 . In other embodiments, the n-type δ-doped layer  502  is centered in the highly doped n+-type layer  306 , displaced closer to the bottom cell  104 , and placed between the bottom cell  104  and the highly doped n+-type layer  306 . In another embodiment, the δ-doped ICTJ  402  contains p-type δ-doped layer (not shown) in the highly doped p+-type layer  304 . In yet another embodiment, the δ-doped ICTJ  402  utilizes both a p-type δ-doped layer in the highly doped p+-type layer  304  and an n-type δ-doped layer  502  in the highly doped n+-type layer  306 . In another embodiment, an ICTJ  302  is placed between the bottom cell  104  and the substrate  106 . In yet another embodiment, a δ-doped ICTJ  402  is placed between the bottom cell  104  and the substrate  106 . In other embodiments, one or more ICTJs  302  and/or one or more δ-doped ICTJs  402  are placed between adjacent cells or cells and other structures, including but not limited to electrical connection points and layers, in the multijunction solar cell with a delta-doped layer  400 . 
     Although in the preceding figures and description each cell  102 ,  104  has been illustrated as an n-on-p type junction diode, and the δ-doped ICTJ  402  as a p-on-n tunnel junction, this was for illustration purposes only. In other embodiments, there are a plurality of cells  102 ,  104  and each cell  102 ,  104  is separated from an adjacent cell  102 ,  104  by a δ-doped ICTJ  402 . In other embodiments, the multijunction solar cell with a delta-doped layer  400  comprises a plurality of cells  102 ,  104  that are p-on-n type junction diodes, and each cell  102 ,  104  is separated from an adjacent cell  102 ,  104  by a δ-doped ICTJ  402  that is an n-on-p tunnel junction. 
     In various embodiments, adjacent portions of the δ-doped ICTJ  402  and the cell  102 ,  104  have the same pin type doping, either both use acceptor type N A  dopants or both use donor type N D  dopants. The non-adjacent portion of the δ-doped ICTJ  402  has a complementary p/n type doping. For example, if the adjacent portion of the cell  102 ,  104  is a p-type type semiconductor material, then the adjacent portion of the δ-doped ICTJ  402  is also a p-type semiconductor material, and both comprise acceptor type N A  dopants. The other portion of the δ-doped ICTJ  402  is a complementary n-type semiconductor material, and comprises a donor type N D  dopants. The adjacent portions of the δ-doped ICTJ  402  and the cell  102 ,  104  use the same acceptor/donor type dopant, however in one embodiment the adjacent portions of the δ-doped ICTJ  402  and the cell  102 ,  104  use the same dopant, and in another embodiment the adjacent portions of the δ-doped ICTJ  402  and the cell  102 ,  104  use different dopants. In one embodiment the adjacent portions of the δ-doped ICTJ  402  and the cell  102 ,  104  use the same base semiconductor material. In one embodiment the adjacent portions of the δ-doped ICTJ  402  and the cell  102 ,  104  use the different base semiconductor materials. In one embodiment, both portions of the δ-doped ICTJ  402  use the same base semiconductor material. In one embodiment, the portions are comprised of different base semiconductor materials. 
     Referring now to  FIG. 6 , in one non-limiting example the δ-doped ICTJ manufacturing process starts by preparing  602  a Ge substrate  106 . On the Ge substrate  106 , grow epitaxially  604  a bottom cell  104  of n-on-p GaInAs. After the bottom cell  104  is complete, begin to grow  606  the highly doped n+-type layer  306  of GaInAs of the δ-doped ICTJ  402 . Then interrupt the grow  606  step for approximately one minute and deposit  608  a flow of Si 2 H 6  at 4.5×10 −2  μmol/min for a total dose of 4.5×10 −2  μmol in the vapor along with an amount of PH 3  to produce the δ-doped layer  404 . As an optional step, continue to grow  610  the remainder of the highly doped n+-type layer  306 . Then grow epitaxially  612  the highly doped p+-type layer  304  of GaInAs. And finally grow epitaxially  614  the top cell  102  of n-on-p GaInP. 
     In one embodiment, SiH 4  is used instead of Si 2 H 6 . In one embodiment, the cells  102 ,  104  are p-on-n cells and the δ-doped ICTJ  402  is an n-on-p tunnel junction. In one embodiment the δ-doped layer  404  is deposited  608  during the step of growing epitaxially  612  the highly doped p+-type layer  304  of GaInAs. In one embodiment, the deposit  608  step produces a δ-doped layer  404  using other methods and process steps for depositing silicon layers as known to those of ordinary skill. 
     In other embodiments, each of the P-dopants is selected from one of the Group II, IV or V elements, and each of the N-dopants is selected from one of the Group IV or VI elements. In still further embodiments, the substrate, subcells, and tunnel junction materials are selected each from semiconductor materials including germanium, silicon, including crystalline, multicrystalline, and amorphous silicon, polycrystalline thin films including copper indium diselenide (CIS), cadmium telluride (CdTe), and thin film silicon, and crystalline thin films including Gallium Indium Arsenide (GaInAs) and Gallium Indium Phosphide (GaInP). In other embodiments, the substrate, subcells, and tunnel junction materials are selected from the alloys GaAs, InAs, GaP, InP, AlAs, AlP, AlGaInP, AlGaP, AlInP, GaInP, AlInAs, AlGaInAs, GaInAs, GaAsP, GaInAsP, GaAsSb, GaInAsSb, AlInSb, AlGaSb, GaInNAs, GaInNAsSb, GaInNP, GaInNAs, SiGe, Ge, ErP, ErAs, ErGaAs, ErInAs. 
     The embodiments of the invention shown in the drawings and described above are exemplary of numerous embodiments that may be made within the scope of the appended claims. It is contemplated that numerous other configurations of the S-doped interconnecting tunnel junctions may be created taking advantage of the disclosed approach. It is the applicant&#39;s intention that the scope of the patent issuing herefrom will be limited only by the scope of the appended claims.