Patent Publication Number: US-2015068581-A1

Title: Fabrication Method for Multi-junction Solar Cells

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of, and claims priority to, PCT/CN2013/078965 filed on Jul. 8, 2013, which claims priority to Chinese Patent Application No. CN 201210249856.3 filed on Jul. 19, 2012. The disclosures of these applications are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     The present disclosure relates to a multi junction solar cell, which pertains to the semiconductor material field. 
     Solar cell, as a new and practical way of generating energy, has drawn more attentions in recent years. It is a semiconductor device that converts solar energy into electric energy with photovoltaic effect, and has become one of the most effective approaches for green energy for reducing dependence on coal, petroleum and natural gas. Amongst all new energies, solar cell is one of the most ideal renewable energy sources, its full and effective development has become an important energy strategic policy of sustainable development for all countries in the world. In recent years, multi junction component solar cells, which represent the third-generation photovoltaic power technology, has attracted more attentions due to their high photoelectric conversion efficiency. 
     GaInP/GaAs/Ge three junction solar cells can achieve as high as 41.8% photoelectric conversion efficiency under concentrated conditions. However, mismatching of short-circuit currents of top cells between InGaP and GaAs, resulting from the Ge bottom cell absorbing too many low-energy photon, tends to prevent the traditional GaInP/GaAs/Ge three junction solar cell from being the most optimized combination in terms of efficiency for. Ideally, current matching is available in the three junction cell if Ge is replaced by material of 1 eV energy gap. The alternative 1 eV In 0.3 Ga 0.7 As, despite of a 2.14% lattice mismatch with GaAs, has relatively high cost due to complex process after inverted growth. 
     SUMMARY 
     According to a first aspect of the present disclosure, a fabrication method for high effective multi junction solar cells is provided, comprising (1) providing a Ge substrate for semiconductor epitaxial growth; (2) growing an emitter region on the Ge substrate (as the base) to form a first subcell with a first band gap; (3) forming a second subcell with a second band gap larger than the first band gap and matching lattice with the first subcell over the first subcell via MBE; (4) forming a third subcell with a third band gap larger than the second band gap and matching lattice with the first and second subcells over the second subcell via MOCVD; and (5) forming a fourth subcell with a fourth band gap larger than the third band gap and matching lattice constants with the first, second and third subcells over the third subcell via MOCVD. 
     According to a second aspect of the present disclosure, an epitaxial growth system of solar cell, comprising an MOCVD reaction chamber, an MBE reaction chamber and a pre-processing chamber, wherein, the MOCVD reaction chamber and the MBE reaction chamber share the pre-processing chamber and are mutually connected via a channel. A transmission device is provided inside the channel. 
     Through a designed combination of MOCVD and MBE crystal growth methods, an in-situ growth of required solar cell structure is available in different growing chambers, thus guaranteeing sample surface cleanliness and improving lattice quality. 
     The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this specification, together with the embodiments, are therefore to be considered in all respects as illustrative and not restrictive. In addition, the drawings are merely illustrative, which are not drawn to scale. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating the relationship between 1 eV GaInNAsSb band gap and lattice constant. 
         FIG. 2  is a schematic diagram of an epitaxial growth device of solar cells according to this disclosure. 
         FIG. 3  is a distribution diagram of band gaps of a high effective five-junction solar cell according to this disclosure. 
         FIG. 4  is a process flow diagram disclosed in Embodiment 2. 
         FIG. 5  is a process flow diagram for epitaxial growth of a second subcell according to this disclosure. 
         FIG. 6  is a structure diagram of a multi junction solar cell disclosed in Embodiment 2. 
         FIG. 7  is a process flow diagram disclosed in Embodiment 3. 
         FIG. 8  is a structure diagram of a multi junction solar cell disclosed in Embodiment 3. 
     
    
    
     IN THE DRAWINGS 
       100 ,  110 : first subcell;  101 ,  111 : p-type Ge substrate;  102 ,  112 : first subcell emitter region;  103 ,  113 : first subcell window layer;  200 ,  210 : second subcell;  201 ,  211 : second subcell back surface field layer;  202 ,  212 : second subcell base;  203 ,  213 : second subcell emitter region;  204 ,  214 : second subcell window layer;  300 ,  310 : third subcell;  301 ,  311 : third subcell back surface field layer;  302 ,  312 : third subcell base;  303 ,  313 : third subcell emitter region;  304 ,  314 : third subcell window layer;  400 ,  410 : fourth subcell;  401 ,  411 : fourth subcell back surface field layer;  402 ,  412 : fourth subcell base;  403 ,  413 : fourth subcell emitter region;  404 ,  414 : fourth subcell window layer;  510 : fifth subcell;  511 : fifth subcell back surface field layer;  512 : fifth subcell base;  513 : fifth subcell emitter region;  514 : fifth subcell window layer;  611 : tunnel junction between the first and second subcells;  612 : tunnel junction between the second and third subcells;  613 : tunnel junction between the third and fourth subcells;  614 : tunnel junction between the fourth and fifth subcells;  700 ,  710 : cap layer;  800 : device system;  810 : MOCVD reaction chamber;  820 : MEB reaction chamber;  830 : pre-processing chamber;  840 : vacuum channel. 
     DETAILED DESCRIPTION 
       FIG. 1  is a relational graph of 1 eV GaInNAsSb band gap and lattice constant. According to the figure, a 1 eV GaInNAs(Sb) subcell is inserted into the traditional GaInP/GaAs/Ge three junction solar cell to form a four-junction solar cell to achieve cell current matching. The lattice matching between GaInNAs(Sb) and GaAs also increases photoelectric conversion efficiency of the solar cell. 
     In a conventional epitaxial growth process, a high-quality lattice GaInNAs(Sb) material is obtained through MBE growth method. However, the MBE epitaxial growth method, despite its low growth rate, requires high vacuum and low temperature condition, which is not applicable for most material (e.g., S, P). To solve the above problems, the embodiments disclose an epitaxial growth system for multi junction solar cells that integrates the MOCVD system and the MBE system (connected by a vacuum channel) in one pre-processing chamber. A transmission device is provided in the vacuum channel for epitaxial wafer transmission between the MOCVD system and the MBE system during epitaxial growth. 
     According to some embodiments described below, an epitaxial growth system is provided to fabricate high-effective multi junction solar cells. 
     A four junction solar cell is prepared by the epitaxial growth system according to some embodiments, comprising: 
     On a p-type Ge substrate, growing an n-type GaAs as the emitter region in the MBE growing chamber and the Ge substrate serves as a base, constituting a first subcell with a first band gap (0.65-0.70 eV). 
     Growing a GaInNAs(Sb) second subcell with a second band gap (0.95-1.05 eV) larger than the first band gap and matching lattice with the first subcell over the first subcell via MBE epitaxial growth. 
     Transmitting the first and second subcells to the MOCVD growing chamber through the transmission device for further growth. 
     Growing a third subcell with a third band gap (1.35-1.45 eV) larger than the second band gap and matching lattice with the first and second subcells over the second subcell via MOCVD. 
     Growing a fourth subcell with a fourth band gap (1.86-1.95 eV) larger than the third band gap and matching lattice with the first, second and third subcells over the third subcell via MOCVD. 
     Form a high-doped cap layer over the fourth subcell. 
     A fifth junction solar cell is prepared by the epitaxial growth system according to some embodiments, comprising: 
     On a p-type Ge substrate, growing an n-type GaAs as the emitter region in the MBE growing chamber and the Ge substrate serves as a base, constituting a first subcell with a first band gap (0.67-0.70 eV). 
     Growing a GaInNAs(Sb) second subcell with a second band gap (0.95-1.05 eV) larger than the first band gap and matching lattice with the first subcell over the first subcell via MBE epitaxial growth. 
     Transmitting the first and second subcells to the MOCVD growing chamber through the transmission device for further growth. 
     Growing a third subcell with a third band gap (1.40-1.42 eV) larger than the second band gap and matching lattice with the first and second subcells over the second subcell via MOCVD. 
     Growing a fourth subcell with a fourth band gap (1.60-1.70 eV) larger than the third band gap and matching lattice with the first, second and third subcells over the third subcell via MOCVD. 
     Growing an Al x Ga y In 1-x-y P fifth subcell with a fifth band gap (1.90-2.10 eV) larger than the fourth band gap and matching lattice with the first, second, third and fourth subcells over the fourth subcell via MOCVD. 
     Form a highly-doped cap layer over the fifth subcell. 
     More specially, in some embodiments, the growth method for the GaInNAs(Sb) second subcell comprises: forming a back surface field layer via MOCVD over the first subcell; forming a GaInNAs(Sb) base and an emitter region on the back surface field layer via MBE; and forming a window layer over the emitter region via MOCVD, thus constituting a second subcell. 
     Refer to Embodiments 1-3 for more details. 
     Embodiment 1 
       FIG. 2  discloses an epitaxial growth system  800  for multi junction solar cells. The epitaxial growth system  800  has an MOCVD system, an MBE system and a pre-processing chamber  830 , wherein, the MOCVD reaction chamber  810  and the MBE reaction chamber  820  share the pre-processing chamber  830 . The vacuum channel  840  connects the MOCVD reaction chamber  810  and the MBE reaction chamber  820  with vacuum degree maintained below 1×10 −6  Pa. A transmission device is provided in the vacuum channel to transmit the epitaxial wafer between the MOCVD system and the MBE system during epitaxial growth. 
     In the epitaxial growth system, the MOCVD reaction chamber  810  and the MBE reaction chamber  820  are arranged in a same pre-processing chamber. A transmission device is provided to realize conversion between the MOCVD growth and the MBE growth in a same pre-processing chamber during epitaxial growth via program control. The combination of MOCVD and MBE crystal growth methods makes an in-situ growth of required solar cell structure available in different growing chambers, thus preventing sample surface oxidation and adsorption pollution and guaranteeing sample surface cleanliness. 
     Embodiment 2 
       FIG. 4  discloses a process flow diagram of a fabrication method for four-junction solar cells. 
     Step S 11 : provide a Ge substrate. Select 140 μtm p-type Ge substrate  101  with doping concentration of 2×10 17 cm −3 -5×10 17 cm −3 . 
     Step S 12 : select the Ge substrate as the base to form a first subcell  100 . In the MOCVD growing chamber, form an n-type GaAs with doping concentration of 2×10 18 cm −3  and thickness of 100 nm over the substrate  101  surface via epitaxial growth to serve as the first subcell emitter region  102 . Form an InGaP material layer with thickness of 25 nm and doping concentration of 1×10 18 cm −3  over the n-type GaAs layer  102  via epitaxial growth to serve as a window layer  102 . Select the p-type Ge substrate as the base to constitute a first subcell. 
     In the MOCVD growing chamber, form a high-doped p++/n++-GaAs tunnel junction  601  with thickness of 50 nm and doping concentration as high as 2×10 19  cm −3  over the first subcell. 
     Step S 13 : form a GaInNAs(Sb) second subcell  200  over the tunnel junction  601 . Referring to  FIG. 5 , it comprises Step S 13   a -S 13   e.  S 13   a : In the MOCVD growing chamber  810 , grow a p-type InGaP with thickness of 50 nm and doping concentration about 1×10 18  cm −3  as the back surface field layer  201  over the tunnel junction  601 . S 13   b : samples after growth, via the pre-processing chamber  830  and the vacuum channel  840 , are transmitted to the MBE growing chamber  820 . S 13   c : In the MBE growing chamber  820 , form a Ga 0.92 In 0.08 N 0.02 As 0.97 Sb 0.01  second subcell over the back surface field layer  201 . Preferably, thickness of the base  202  is 3000 nm and doping concentration is 5×10 17 cm −3 ; the emitter region  203  is 200 nm thick with doping concentration of 2×10 18 cm −3 . S 13   d : samples after growth, via the pre-processing chamber  830  and the vacuum channel  840 , are transmitted back to the MOCVD growing chamber. S 13   e : In the MOCVD growing chamber, form an n-type InGaP window layer  204  with thickness of 25 nm and doping concentration about 1×10 18 cm −3  over the emitter region  203 . 
     In the MOCVD growing chamber, grow a high-doped p++/n++-GaAs tunnel junction  602  with thickness of 50 nm and doping concentration as high as 2×10 19  cm −3  over the second subcell. 
     Step S 14 : form a third subcell  300  over the second subcell via MOCVD. 
     In the MOCVD growing chamber, grow a p+-InGaP material layer with thickness of 50 nm and doping concentration of 1-2×10 18  cm −3  over the tunnel junction  602  to serve as the back surface field layer  301 ; grow an n-type GaAs material layer with thickness of 2 μm and doping concentration of 1-5×10 17  cm −3  over the back surface field layer  301  to serve as second subcell base  302 ; grow an n+-Ga(In)As material layer with thickness of 100 nm and doping concentration about 2×10 18  cm −3  over the base  302  to serve as the emitter region  303 ; form an n-type InGaP window layer  304  with thickness of 25 nm and doping concentration about 1×10 18  cm −3  over the emitter region  303 . 
     In the MOCVD growing chamber, grow a high-doped p++/n++-InGaP tunnel junction  603  with thickness of 50 nm and doping concentration as high as 2×10 19  cm −3  over the third subcell. 
     Step S 15 : form a fourth subcell  400  over the third subcell via MOCVD. In the MOCVD growing chamber, form a p-AlGaInP material layer with thickness of 100 nm and doping concentration of 1-2×10 18  cm −3  via epitaxial growth over the tunnel junction  603  to serve as the back surface field layer  401 ; grow a p+-GaInP material layer with thickness of 1000 nm and doping concentration of 5×10 17  cm −3  over the back surface field layer  401  to serve as the base  402 ; grow an n+-GaInP material layer with thickness of 100 nm and doping concentration of 2×10 18  cm −3  over the base  402  to serve as the emitter region  403 ; form an n-type AlGaInP window layer  504  with thickness of 25 nm and doping concentration about 1×10 18  cm −3  over the emitter region  403 . 
     In the MOCVD growing chamber, grow a high-doped n++-GaAs material layer with thickness of 500 nm and concentration of 1×10 19  cm −3  on the top of the fourth cell as the cap layer  700 . 
     Lastly, take latter processes as AR coating evaporation on sample surface and metal electrode preparation to complete the required solar cell. Referring to  FIG. 6  for the structural section view. 
     Embodiment 3 
     A fabrication method for high-effective five junction solar cells, comprising the following steps: 
     Step S 21 : provide a Ge substrate. Select 140 μm p-type Ge substrate  111  with doping concentration of 2×10 17 cm −3 -5×10 17 cm −3 . 
     Step S 22 : select the Ge substrate as the base to form a first subcell  110 . In the MOCVD growing chamber, form an n-type GaAs material layer with doping concentration of 2×10 18  cm −3  and thickness of 100 nm over the substrate  111  surface via epitaxial growth to serve as the first subcell emitter region  112 . Form an InGaP material layer with thickness of 25 nm and doping concentration of 1×10 18  cm −3  over the n-type GaAs layer  112  via epitaxial growth to serve as a window layer  113 . Select the p-type Ge substrate as the base to constitute a first subcell. 
     In the MOCVD growing chamber, form a high-doped p++/n++-GaAs tunnel junction  611  with thickness of 50 nm and doping concentration as high as 2×10 19  cm −3  over the first subcell. 
     Step S 23 : form a GaInNAs(Sb) second subcell  210  over the tunnel junction  611 . In the MOCVD growing chamber  810 , grow a p-type InGaP with thickness of 50 nm and doping concentration about 1×10 18  cm −3  as the back surface field layer  211  over the tunnel junction  611 . Samples after growth, via the pre-processing chamber  830  and the vacuum channel  840 , are transmitted to the MBE growing chamber  820 . In the MBE growing chamber  820 , form a Ga 0.92 In 0.08 N 0.02 As 0.97 Sb 0.01  second subcell over the back surface field layer  211 . Preferably, thickness of the base  212  is 3000 nm and doping concentration is 5×10 17  cm −3 ; the emitter region  213  is 200 nm thick with doping concentration of 2×10 18  cm −3 . Samples after growth, via the pre-processing chamber  830  and the vacuum channel  840 , are transmitted back to the MOCVD growing chamber. In the MOCVD growing chamber, form an n-type InGaP window layer  214  with thickness of 25 nm and doping concentration about 1×10 18  cm −3  over the emitter region  213 . 
     In the MOCVD growing chamber, grow a high-doped p++/n++-GaAs tunnel junction  612  with thickness of 50 nm and doping concentration as high as 2×10 19  cm −3  over the second subcell. 
     Step S 24 : form a third subcell  310  over the second subcell via MOCVD. In the MOCVD growing chamber, grow a p+-InGaP material layer with thickness of 50 nm and doping concentration of 1-2×10 18  cm −3  over the tunnel junction  612  to serve as the back surface field layer  311 ; grow an n-type Ga(In)As material layer with thickness of 2 μm and doping concentration of 1-5×10 17  cm −3  over the back surface field layer  301  to serve as the base  312 ; grow an n+-Ga(In)As material layer with thickness of 100 nm and doping concentration about 2×10 18  cm −3  over the base  312  to serve as the emitter region  313 ; form an n-type InGaP window layer  314  with thickness of 25 nm and doping concentration about 1×10 18  cm −3  over the emitter region  313 . 
     In the MOCVD growing chamber, grow a high-doped p++/n++-InGaP tunnel junction  613  with thickness of 50 nm and doping concentration as high as 2×10 19  cm −3  over the third subcell. 
     Step S 25 : form a fourth subcell  400  over the third subcell via MOCVD. In the MOCVD growing chamber, form a p-AlGaInP material layer with thickness of 100 nm and doping concentration of 1-2×10 18  cm −3  via epitaxial growth over the tunnel junction  613  to serve as the back surface field layer  411 ; grow a p+-AlxGa1-xAs material layer with thickness of 1000 nm and doping concentration of 5×10 17  cm −3  over the back surface field layer  411  to serve as the base  402 ; grow an n+-AlxGa1-xAs material layer with thickness of 100 nm and doping concentration of 2×10 18  cm −3  over the base  402  to serve as the emitter region  413 ; form an n-type AlGaInP window layer  414  with thickness of 25 nm and doping concentration about 1×10 18  cm −3  over the emitter region  413 . 
     In the MOCVD growing chamber, grow a high-doped p++/n++-AlGaAs tunnel junction  614  with thickness of 50 nm and doping concentration as high as 2×10 19  cm −3  over the fourth and fifth subcells. 
     Step S 26 : form a fifth subcell  500  over the fourth subcell via MOCVD. In the MOCVD growing chamber, form a p-AlGaAs material layer with thickness of 100 nm and doping concentration of 1-2×10 18  cm 3  via epitaxial growth over the tunnel junction  614  to serve as the back surface field layer  511 ; form a p+-AlxGayIn1-x-yP material layer with thickness of 500 nm and doping concentration of 1-5×10 17 cm −3  over the back surface field layer  511  via epitaxial growth to serve as the base  512 ; grow an n+-AlxGayIn1-x-yP material layer with thickness of 50 nm and doping concentration about 2×10 18  cm −3  over the base  512  to serve as the emitter region  513 ; form an n-type AlGaAs window layer  903  with thickness of 25 nm and doping concentration about 1×10 18  cm −3  over the emitter region  513 . 
     In the MOCVD growing chamber, grow a high-doped n++-GaAs material layer with thickness of 500 nm and concentration of 1×10 19  cm −3  on the top of the fourth cell as the cap layer  710 . 
     Lastly, take latter processes as AR coating evaporation on sample surface and metal electrode preparation to complete the required solar cell. Referring to  FIG. 7  for the structural section view. 
     According to the embodiment, a Ge/GaInNAs(Sb)/InGaAs/AlGaAs/AlGaInP five junction solar cell is provided and the band gap distribution is as shown in  FIG. 3 . In comparison with the four junction solar cell, the five-junction solar cell refines the absorption spectrum to facilitate current matching, wider spectrum absorption scope and higher efficiency. 
     The solar cells can be used in a solar energy generation system, which may include a number of the solar cells according to embodiments disclosed here.