Solar cell stack

Description

[0001] The invention relates to a stack of solar cells.

[0002] To achieve the highest possible efficiency in converting sunlight into electrical energy with solar cells, multi-junction solar cells made of different semiconductor materials have been investigated for several years [W. Guter, Optimization of III-V based high-performance solar cells, Dissertation, University of Konstanz, Faculty of Physics, 2011]. Multi-junction solar cells divide the incident light into several stacked sub-solar cells with different bandgap energies. To achieve maximum efficiency, the semiconductor materials and their bandgap energies must be matched so that each of the electrically series-connected sub-solar cells preferably generates the same current.

[0003] Provided the semiconductor materials of the individual subcells of the stack have the same lattice constant, the subcells can be fabricated using an epitaxial process. A lattice-matched quadruple solar cell made of AlInGaP, InGaAs, InGaNAs, and Ge is known from Meusel et al., III-V MULTIJUNCTION SOLAR CELLS - FROM CURRENT SPACE AND TERRESTRIAL PRODUCTS TO MODERN CELL ARCHITECTURES, 5th WCPEC, 2010, Valencia, 1AP.1.5. Due to crystal quality issues, particularly with the InGaNAs subcell, only insufficient efficiencies are achieved.

[0004] If the subcells have different lattice constants, a first alternative involves inserting metamorphic buffer layers between two subcells. From Guter et al., DEVELOPMENT, QUALIFICATION AND PRODUCTION OF SPACE SOLAR CELLS WITH 30% EOL EFFICIENCY, European Space Power Conference, 2014, Noordwijkerhout, The Netherlands, a sequence of subcells made of AlInGaP, AlInGaAs, InGaAs, a metamorphic buffer, and Ge is known. Furthermore, from Cornfeld et al., Development of a four sub-cell inverted metamorphic multi-junction (IMM) highly efficient AM0 solar cell, 35th IEEE PVSC, 2010, Honolulu, USA, another sequence of subcells made of InGaP, GaAs, with a first metamorphic buffer, a first InGaAs subcell, a second metamorphic buffer, and a second InGaAs subcell is known.

[0005] A solar cell stack with a buffer and varying doping concentrations is known from US patent 2011 / 220190 A1. Further multi-junction solar cells, some with buffers, are described in Hector Cotal et al.: "III_V multijunction solar cell for concentrating photovoltaics". Energy & Environmental Science, Vol. 2, No. 2, January 2009, page 174, ISSN: 1754-5692, and from Li KL et al: "Effects of Si doping on the strain relaxation of metamorphic (AL)GaInP buffers grown on GaAs substrates", Applied surface science, Vol. 288, pages 482-487, ISSN: 0169-4332, and from Tangring et al: "A study of the doping influence on strain relaxation of graded composition in GaAs Layers grown on molecular beam epitaxy", Journal of crystal growth, Elsevier, Amsterdam, NL, Vol. 311, No. 7, March 15, 2009, pages 1684-1687, ISSN 00022-0248, and from DE 10 2013 000882 B3, and from V. Klinger et al: "Determination of hardness and Young's modulus for important III_V compound semiconductors", Thin solid films, Vol. 548, 29.August 2013, pages 358-365, ISSN 0040-6090, known.

[0006] Furthermore, US patent 2004 / 0079408 A1 discloses a III-V metamorphic multi-junction solar cell with a Ge substrate subcell, wherein the metamorphic buffer is formed between the Ge subcell and a subsequent InGaAs subcell. It is known that the lattice constant of the buffer layers increases from the first subcell to the second subcell, and that the doping of the buffer influences the hardness of the buffer layers and the relaxation behavior.

[0007] Against this background, the object of the invention is to provide a device that further develops the state of the art.

[0008] The problem is solved by a stack of solar cells having the features of claim 1. Advantageous embodiments of the invention are the subject of dependent claims.

[0009] According to the invention, a solar cell stack is provided, comprising a first semiconductor solar cell, wherein the first semiconductor solar cell has a pn junction made of a first material with a first lattice constant, and a second semiconductor solar cell, wherein the second semiconductor solar cell has a pn junction made of a second material with a second lattice constant, and the first lattice constant is at least 0.008 Å smaller than the second lattice constant, and a metamorphic buffer, wherein the metamorphic buffer is formed between the first semiconductor solar cell and the second semiconductor solar cell, and the metamorphic buffer comprises a sequence of three layers and the lattice constant increases in the sequence towards the second semiconductor solar cell, and the lattice constants of the layers of the metamorphic buffer are larger than the first lattice constant, wherein two layers of the buffer are doped.and where the difference in dopant concentration between the two layers is greater than 4E17 cm-3<.

[0010] For clarity, it should be noted that the first semiconductor solar cell has a smaller lattice constant compared to the second semiconductor solar cell. The term "subcell" will be used synonymously with "semiconductor solar cell" in the following text. It is understood that the subcell with the highest bandgap energy is located on the side of the solar cell stack facing the sun. The term "stacked integrated multi-junction solar cell" will also be used synonymously with "solar cell stack" in the following text.

[0011] After the light passes through the semiconductor solar cell with the largest bandgap energy, the unabsorbed light, i.e., the longer wavelength range, strikes the semiconductor solar cell with the smaller bandgap energy. If a substrate is present for the solar cell stack, it is always connected to the cell with the smallest bandgap energy. It should be noted that the two semiconductor solar cells are monolithically integrated together.

[0012] In this context, the term dopant concentration is understood to mean the average value of the dopants in the respective layer of the buffer.

[0013] One advantage is that the solar cell stack according to the invention exhibits high efficiency by stacking two solar cells with different lattice constants on top of each other using a metamorphic buffer. Investigations have shown that doping in the buffer region significantly reduces the number of dislocations, i.e., the doped layers of the metamorphic buffer become harder. Furthermore, with a high-quality metamorphic buffer, i.e., one that suppresses the propagation of dislocations from the buffer into the surrounding active regions, the bandgap energies can be selected such that the overall efficiency of the solar cell stack according to the invention is increased compared to a stack of lattice-matched solar cells.

[0014] In a further development, at least one layer of the metamorphic buffer has a fourth lattice constant, wherein the fourth lattice constant is larger than the lattice constant of the second semiconductor solar cell. It is preferred that the lattice constant of the layers of the metamorphic buffer increases by at least 0.003 Å from layer to layer in the direction of the second semiconductor solar cell.

[0015] In a preferred embodiment of the metamorphic buffer, the two immediately successive layers have different concentrations of dopants.

[0016] In a further embodiment, the difference in the concentration of the dopants between two immediately successive layers of the buffer is greater than a factor of five, preferably greater than a factor of seven, and most preferably greater than a factor of ten. It is understood that the layers of the metamorphic buffer have either n-type or p-type doping.

[0017] Preferably, the metamorphic buffer contains Zn and / or C and / or Mg and / or Be as p-doping agents and Si and / or Te and / or Se as n-doping agents. Investigations have shown that the concentration of the dopants within the respective layer of the metamorphic buffer is essentially constant, preferably constant.

[0018] According to the invention, the metamorphic buffer comprises at least five layers, preferably at least six layers.

[0019] In a preferred embodiment, two layers of the metamorphic buffer have the same concentration of dopants. Preferably, the two layers are bonded together.

[0020] In another embodiment, in a sequence of several layers of metamorphic buffer, the concentration of the dopants first increases from layer to layer and then decreases again. It is advantageous if the concentration of the dopants within the metamorphic buffer layers does not exceed 1E19 cm⁻³, most preferably 0.5E19 cm⁻³.

[0021] It is understood that the metamorphic buffer layers are not part of a pn junction of a tunnel diode.

[0022] In a particularly advantageous embodiment, Ge or GaAs is provided as the substrate layer, wherein the first semiconductor solar cell and / or the second semiconductor solar cell consist of the materials Ge and / or GaAs and / or InGaAs and / or InGaP.

[0023] In a further development, all solar cells of the stack are monolithically integrated and / or an optical back mirror is incorporated. It is understood that the optical back mirror is located below the semiconductor solar cell with the lowest bandgap energy. Preferably, the optical back mirror is made of a metal. The back mirror allows the unabsorbed photons to be reflected back into the semiconductor solar cell with the lowest bandgap energy, thus increasing the optical path.

[0024] The invention is explained in more detail below with reference to the drawings. Similar parts are labelled with identical designations. The illustrated embodiments are highly schematic; that is, the distances and the lateral and vertical extents are not to scale and, unless otherwise indicated, do not exhibit any derivable geometric relationships to one another. The drawings show: Figure 1a shows a first embodiment of a metamorphic buffer in connection with a dual semiconductor solar cell, Figure 1b shows the lattice constant curve for a dual semiconductor solar cell, as illustrated in the Figur 1a Figure 1c: Doping profile for the first embodiment of the buffer, shown in the Figur 1a Figure 2a shows a second embodiment of the metamorphic buffer, Figure 2b shows the doping profile for the second embodiment of the metamorphic buffer, as illustrated in the Figur 2aFigure 2c shows the hardness curve for the second embodiment of the buffer, as illustrated in the Figur 2a Figure 3a shows a third embodiment of the metamorphic buffer, and Figure 3b shows the doping profile for the third embodiment of the metamorphic buffer, as illustrated in the Figur 3a Figure 3 shows an alternative doping profile for the third embodiment of the buffer, illustrated in the Figur 3a ,

[0025] The illustration of Figur 1a shows a solar cell stack 10 with a first semiconductor solar cell 20 and a second semiconductor solar cell 30 and a metamorphic buffer 40 formed between the semiconductor solar cell 20 and a second semiconductor solar cell 30.

[0026] The first semiconductor solar cell 20 has a pn junction made of a first material with a first lattice constant a1 and a first band gap energy Eg1. The second semiconductor solar cell 30 has a pn junction made of a second material with a second lattice constant a2 and a second band gap energy Eg2. The second lattice constant a2 is larger than the first lattice constant a1. Sunlight passes through the second semiconductor solar cell 30, then through the buffer, and subsequently reaches the first semiconductor solar cell 20. In an embodiment not shown, sunlight passes through the first semiconductor solar cell 20, then through the buffer, and subsequently reaches the second semiconductor solar cell 30.

[0027] The buffer 40 in this case has a sequence of five layers 41, 42, 43, 44, and 45, wherein the first layer 41 of the buffer 40 is arranged between the first semiconductor solar cell 20 and the second layer 42 of the buffer 40, the third layer 43 of the buffer 40 is arranged between the second layer 42 of the buffer 40 and the fourth layer 44 of the buffer 40, the fourth layer 44 of the buffer 40 is arranged between the third layer 43 of the buffer 40 and the fifth layer 45 of the buffer 40, and the fifth layer 45 of the buffer 40 is arranged between the fourth layer 44 of the buffer 40 and the second semiconductor solar cell 30. In an embodiment not shown, the metamorphic buffer 40 has six or more layers. In another embodiment, also not shown, the buffer 40 has three or four layers.

[0028] The illustration of Figur 1bshows the curve of the lattice constant A in the unit Å for the solar cell stack of the dual semiconductor solar cell 10, shown in the Figur 1a The following only describes the differences from the embodiment shown in the figure of the Figur 1a , explained. The first semiconductor solar cell 20 has the first lattice constant a1, where the first lattice constant a1 is constant for the entire first semiconductor solar cell 20.

[0029] At the transition from the first semiconductor solar cell 20 to the first layer 41 of the buffer 40, the lattice constant A jumps to a lattice constant ap1 of the first layer 41, where the lattice constant ap1 of the first layer 41 is larger than the lattice constant a1 of the first semiconductor solar cell 20.

[0030] At the transition from the first layer 41 to the second layer 42, the lattice constant A jumps to a lattice constant ap2 of the second layer 42 that is larger compared to the lattice constant ap1 of the first layer 41, with the lattice constant A increasing by at least 0.003 Å from layer to layer of the buffer 40.

[0031] At the transition from the second layer 42 to the third layer 43, the lattice constant A jumps to a lattice constant ap3 of the third layer 43 that is larger compared to the lattice constant ap2 of the second layer 42.

[0032] At the transition from the third layer 43 to the fourth layer 44, the lattice constant A jumps to a lattice constant ap4 of the fourth layer 44 that is larger compared to the lattice constant ap3 of the third layer 43.

[0033] At the transition from the fourth layer 44 to the fifth layer 45, the lattice constant A changes to a smaller lattice constant ap5 of the fifth layer 45 compared to the lattice constant ap4 of the fourth layer 44, where the lattice constant ap5 of the fifth layer 45 is equal to the second lattice constant a2 of the second semiconductor solar cell 30. Since the lattice constant of the fourth layer 44 is the largest within the buffer 40, and in particular larger than the second lattice constant a2, a high degree of buffer relaxation is achieved. Furthermore, the lattice constant a1 of the first semiconductor solar cell 20 is at least 0.008 Å smaller than the lattice constant a2 of the second semiconductor solar cell 30.

[0034] The illustration of Figur 1c shows the doping profile D in the unit N / cm³ for the layers of the metamorphic buffer 40, as depicted in the Figur 1aThe following only describes the differences from the embodiment shown in the figure of the Figures 1a and 1b , explained. It should be noted that if the doping D is changed between the layers of the buffer, the difference in concentration must be at least greater than 4E17 N / cm 3<.

[0035] Starting from an initial low concentration or an intrinsic concentration of dopants dp1 in the first layer 41 of the buffer 40, the doping D jumps at the transition from the first layer 41 to the second layer 42 to a doping dp2 of the second layer 42 that is larger compared to the doping dp1 of the first layer 41.

[0036] At the transition from the second layer 42 to the third layer 43, the doping D jumps to a doping dp3 of the third layer 43 that is larger compared to the doping dp2 of the second layer 42.

[0037] At the transition from the third layer 43 to the fourth layer 44, the doping concentration D jumps to a higher concentration dp4 in the fourth layer 44 compared to the doping concentration dp3 in the third layer 43. After this, the doping concentration dp5 in the fifth layer 45 remains at the level of the fourth layer 44. In other words, there is no difference in the concentration of dopants between the fourth layer 44 and the fifth layer 45, and dp4 = dp5.

[0038] The illustration of Figur 2a Figure 1 shows a second embodiment of the metamorphic buffer 40. Only the differences from the previous embodiments are explained below. The buffer 40 again has a total of five layers 41, 42, 43, 44 and 45.

[0039] The illustration of Figur 2b shows the doping profile D for the second embodiment of the metamorphic buffer 40, as depicted in the Figur 2aThe following explains only the differences from the preceding embodiments. The doping D increases in steps of 1E16 N / cm³ < < ΔD <5e17 N / cm³ < from one layer of buffer 40 to the next, up to and including the fourth layer 44. The doping dp4 and dp5 for the last two layers 44 and 45 of buffer 40, however, remain constant.

[0040] The illustration of Figur 2c shows the hardness H curve for the second embodiment of the buffer 40, as depicted in the Figur 2a The hardness H increases abruptly from the first layer 41 to the second layer 42 and from the second layer 42 to the third layer 43 and from the third layer 43 to the fourth layer 44 of the buffer 40, whereby the hardness H remains constant for the last two layers 44 and 45 of the buffer 40.

[0041] The illustration of Figur 3aFigure 1 shows a third embodiment of the metamorphic buffer 40. Only the differences from the previous embodiments are explained below. In this embodiment, the fourth layer 44 and the fifth layer 45 are each slightly thicker than the other layers of the buffer 40.

[0042] The illustration of Figur 3b shows the doping profile D for the third embodiment of the metamorphic buffer, as depicted in the Figur 3aThe differences from the preceding embodiments are explained below. The doping concentration D increases in steps of 1E16 N / cm³ < < ΔD <5e16 N / cm³ < from one layer of buffer 40 to the next, up to and including the third layer 43. From the third layer 43 to the fourth layer 44, the doping concentration increases by at least five times the concentration of the dopant in the third layer 43; preferably, the doping concentration increases by at least a factor of 10 from the third layer 43 to the doping concentration dp4 of the fourth layer 44.

[0043] The illustration of Figur 3c shows the course of the alternative doping D for the third embodiment of the metamorphic buffer, as depicted in the Figur 3a. Only the differences from the preceding embodiments are explained below. The doping D is consistently low or intrinsic from the first layer 41 up to and including the third layer 43 and, starting from the third layer 43 to the fourth layer 44, exhibits a jump to a higher concentration of at least 1E18 N / cm³.

Claims

1. Solar cell stack (10) comprising - a first semiconductor solar cell (20), wherein the first semiconductor solar cell (20) has a p-n transition of a first material with a first lattice constant (a1), and - a second semiconductor solar cell (30), wherein the second semiconductor solar cell (30) has a p-n transition of a second material with a second lattice constant (a2), and - the first lattice constant (a1) is smaller than the second lattice constant (a2) by at least 0.008 Å, and - Ge or GaAs is provided as substrate layer and the first semiconductor solar cell (20) and / or the second semiconductor solar cell (30) consists or consist of the materials Ge and / or GaAs and / or InGaAs and / or InGaP, and - the semiconductor solar cells (20, 30) are monolithically integrated, and - a metamorphic buffer (40), wherein the metamorphic buffer (40) is formed between the first semiconductor solar cell (20) and the second semiconductor solar cell (30), and - the metamorphic buffer (40) comprises at least five layers (41, 42, 43, 44, 45), - with respect to the metamorphic buffer (40) in the case of the sequence of three layers (41, 42, 43) the lattice constants (ap1, ap2, ap3) increase in the direction of the second semiconductor solar cell (30), - the lattice constants (ap1, ap2, ap3, ap4, ap5) of the layers of the metamorphic buffer (40) are greater than the first lattice constant (a1), characterised in that - two layers of the metamorphic buffer have a doping (D), wherein the difference in the dopant concentration between the two layers is greater than 4E17 cm-3, and - the concentration of the dopant (D) over the sequence of the plurality of layers of the metamorphic buffer (40) initially increases from layer to layer and subsequently decreases again or initially increases and is constant in the case of the last two layers (44, 45) of the metamorphic buffer (40).

2. Solar cell stack (10) according to claim 1, characterised in that a layer (44) of the metamorphic buffer (40) has a fourth lattice constant (ap4) and the fourth lattice constant (ap4) is greater than the lattice constant (ap2) of the second semiconductor solar cell (20).

3. Solar cell stack (10) according to one of the preceding claims, characterised in that the lattice constant (A) of the metamorphic buffer (40) increases in the direction of the second semiconductor solar cell (30) from layer to layer by in each instance at least 0.0003 Å.

4. Solar cell stack (10) according to any one of the preceding claims, characterised in that in the case of two directly successive layers of the metamorphic buffer (40) the two layers have a different concentration of the dopant (D).

5. Solar cell stack (10) according to any one of the preceding claims, characterised in that in the case of two directly successive layers of the buffer (40) the difference in the concentration of the dopant (D) is greater than the factor five.

6. Solar cell stack (10) according to any one of the preceding claims, characterised in that the layers of the metamorphic buffer (40) have an n-doping or a p-doping.

7. Solar cell stack (10) according to any one of the preceding claims, characterised in that in the metamorphic buffer (40) Zn and / or C and / or Mg and / or Be is or are provided as p-dopant and Si and / or Te and / or Se is or are provided as n-dopant.

8. Solar cell stack (10) according to any one of the preceding claims, characterised in that the concentration of the dopants within the respective layer of the metamorphic buffer is constant.

9. Solar cell stack (10) according to any one of the preceding claims, characterised in that the metamorphic buffer (40) comprises more than five layers.

10. Solar cell stack (10) according to any one of the preceding claims, characterised in that the metamorphic buffer layers (40) are not part of a pn transition of a tunnel diode.

11. Solar cell stack (10) according to any one of the preceding claims, characterised in that the concentration of the dopants (D) within the metamorphic buffer layers does not exceed 1E19 cm-3.

12. Solar cell stack (10) according to any one of the preceding claims, characterised in that an optical rear-side mirror is formed.

13. Solar cell stack (10) according to claim 12, characterised in that the optical rear-side mirror is arranged below the semiconductor solar cells (20, 30) with the smallest band gap energy and is made from a metal.