RU2610225C1 - Four-junction solar cell - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: four-junction solar cell includes, successively grown on a substrate (1) made of p-Ge, four sub-cells (2), (3), (4), (5), connected to each other by tunnel p-n junctions (6, 7, 8), a metamorphous gradient buffer layer (9) between first (2) and second (3) sub-cells and a contact layer (10). The first sub-cell (2) includes a substrate (1) made of p-Ge, a layer (11) made of n-Ge, a wide-bandgap layer (12) made of n-GalnP and a buffer layer (13) made of n-GaInAs, the second sub-cell (3) includes a layer (14) made of p-GalnAs and a layer (15) made of n-GaInAs, the third sub-cell (4) includes a layer (16) made of p-AIGalnAs and a layer (17) made of n-AlGaInAs, and the fourth sub-cell (5) includes short-period superlattices of A2B6 of p- and n-type (18, 19). The second, third and fourth sub-cells of the multi-junction solar cell are matched on the lattice constant with In_xGa_1-xAs (x=0.25-0.3), and the first (Ge) sub-cell of the multi-junction solar cell is pseudomorphous with the substrate (1) made of p-Ge.

EFFECT: four-junction solar cell is easier to make and has higher bandgap of the upper wide-bandgap sub-cell, which provides high efficiency of converting the short-wave part of solar radiation.

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Description

The present invention relates to the field of solar energy, and more specifically to the design of multi-junction solar cells based on semiconductor compounds of groups A3B5 and A2B6, which are used to convert the energy of solar radiation into electrical energy.

From the current level of technology it is known that, in accordance with the fundamental principles of thermodynamics established in the works of Shockley and Kuesye (W. Shockley, HJ Queisser, J. Appl. Phys., V. 32, p. 510, 1961) and Henry (CH Henry, J. Appl. Phys. V. 51, p.4494, 1980), the efficiency of conversion of solar energy into electrical single-junction solar cells cannot exceed 31% under standard solar illumination (AM 1,5 Global, “ 1 Sun ”) and 37% in a thousandfold concentrated sunlight (“ 1000 Suns ”). Opportunities to overcome the fundamental limit of the conversion efficiency of solar radiation by a single junction cell are associated with the use of multi-junction solar cells, including two or more tunnel-coupled photodiode pn junctions (subcells) made of semiconductors with different bandgaps (Hutchby, JA, Markunas, RJ, Bedair, SM, Proceedings of the 18th IEEE Photovoltaic Specialists Conference, Las Vegas, USA. IEEE, New York, p. 20, 1985). Each photoactive p-n junction of a multi-junction structure converts only part of the solar spectrum, which allows realizing transformations of the solar spectrum close to optimal conditions and significantly increasing efficiency. For structures of a three-junction solar cell pseudomorphically grown on a Ge substrate in which all pn junction materials have close lattice constants: Ge with a band gap Eg of 0.67 eV, Ga (In) As with Eg ~ 1.4 eV and InGaP with Eg ~ 1.85 eV, and individual photoactive pn junctions are connected electrically using tunnel pn junctions and optically through wide-gap GaInP and AlInP layers, an efficiency approaching 42% has been demonstrated (M.A. Green, K. Emery, Y. Hishikawa, W. Warta, and ED Dunlop, Prog. Photovolt: Res. Appl., V. 21, p. 1, 2013). The main advantage of pseudomorphic structures of solar cells lies in the possibility of forming layers of materials with a relatively low density of defects. Their main drawback is the lack of the possibility of optimizing the band gap of solid solutions forming p-n junctions, since they are rigidly fixed by the condition of equality of the lattice constant. An alternative approach is to use a metamorphic structure (M.A. Green, K. Emery, D.L. King, Y. Hisikawa, and W. Warta, Prog. Photovoltaics Res. Appl. 14, 45, 2006). The main difference lies in the presence of a metamorphic buffer layer between the narrowest Ge-based transition and the subsequent GaInAs-based transition, which allow one to grow a GalnAs transition with a substantially larger In composition, the band gap of which (1.18 eV) is closer to optimal than InGaAs with Ge lattice constant. Despite a markedly higher defect density, solar cells of this type show high efficiency (up to 41.1%), comparable to the efficiency of pseudomorphic structures (DJ Friedman, J.M. Olson, and S. Kurtz, in Handbook of Photovoltaic Science and Engineering, 2nd ed., Edited by A. Luque and S. Hegedus, John Wiley and Sons, Chichester, 2011). An even greater efficiency of photoelectric conversion (~ 44%) (spectrum AM 1.5, 942 of the sun) was obtained in the metamorphic 3-junction solar cell GaInP / GaAs / GaInAs [M.A. Green, C. Emery, Y. Hishikawa, W. Warta, and E.D. Dunlop, Prog. Photovolt: Res. Appl. 21, 1 (2013)]. The current record photoelectric conversion efficiency (~ 45.7%) (spectrum AM1.5, 234 suns) was obtained in the inverse metamorphic four-junction solar cell GaInP / GaAs / GaInAs / GaInAs (NREL press release NR-4514, 16 December 2014 ) However, a further increase in the conversion efficiency of the four-junction solar cell cannot be performed using compounds of the A3B5 group due to the absence of direct-gap compounds with a sufficiently large band gap in this group.

A four-junction solar cell is known (application CN 102569475, IPC H01L-031/0725; H01L-031/0735; H01L-031/18, published July 11, 2012) grown on an InP substrate and containing two subelements based on pn junctions InGaAs and InGaAsP, isoperiodic to the InP substrate, the In _x Ga _1-x P gradient metamorphic buffer layer with a smooth composition change and with a lattice constant varying from 0.58 nm to 0.566 nm, and two subelements based on pn junctions InAlGaAs and InGaAsP with a lattice constant consistent from the lattice constant at the surface of the InGaP gradient buffer layer. In this case, the band gap of the subelements are respectively in the range of 0.72-0.76 eV, 1-1.1 eV, 1.35-1.42 eV and 1.85-1.92 eV.

The disadvantages of the known four-junction solar cell are the need to remove the substrate during post-growth operations and the insufficient conversion efficiency of the short-wavelength part of the solar radiation spectrum, since the band gap of the upper subcell does not exceed 1.92 eV.

A multi-junction solar cell is known (PCT application WO 2014078664, IPC H01L 31/04; H01L 31/18, published May 22, 2014) containing subcells based on pn junctions of group IV elements (Ge, SiGe) and pn junctions based on materials of the A3B5 group, such as (Al) InGaP, (Al) GaAs, InGa (As) P, Al (In) GaAs, Ga (In) As, GaInNAsSb, etc. A multi-junction solar cell contains the first group of one or more sub-cells matched by the lattice constant with the first substrate, and a second group of one or more subelements matched along the lattice constant with the second substrate, while the second group subelements is connected with the first group of subelements through a special wafer bonding technology.

The disadvantages of the known multi-junction solar cell are the complexity of post-growth operations and low conversion efficiency of the short-wave part of the spectrum of solar radiation.

A four-junction solar cell is known (PCT application WO 2009067347, IPC 01L 21/20, H01L 21/36, H01L 29/20, H01L 29/22, published May 28, 2009) containing four subcells successively grown on a GaSb substrate interconnected tunnel pn junctions, and a contact layer. Each subelement includes a pn junction formed by two layers of compounds A3B5 or A2B6, where the layers are doped with p- and n-type conductivity, respectively. The first (adjacent to the substrate) subelement includes p- and n-layers of GaSb (band gap Eg1 = 0.72 eV), the second subelement includes p- and n-layers Al _x Ga _1-x AS _y Sb _1-y (Eg2 = 1.32 eV), the third subelement includes p- and n-layers Zn _x Cd _1-x Se _y Te _1-y (Eg3 = 1.71 eV), and the fourth (upper, closest to the surface of the structure) subelement includes p - and n-layers of ZnTe (Eg1 = 2.27 eV). Subelements are matched along the lattice constant with the substrate or pseudomorphic to the substrate.

The disadvantages of the known four-junction solar cell are the low doping levels of p-type or n-type quaternary ZnCdSeTe solid solutions enriched with CdSe or ZnTe respectively to achieve the required band gap, and especially the impossibility of achieving high levels of doping of n-type ZnTe when grown by standard epitaxial methods . An additional problem in the technological implementation of ZnCdSeTe quaternary solid solutions is the difficulty in obtaining layers of a given composition due to the presence of two volatile components Se and Te in the solid solution. It should also be noted the high cost of GaSb and InAs substrates, on which these prototype solar cells can be implemented.

A four-junction solar cell is known (application EP2672528, IPC G01R-031/26; H01L-031/04; H01L-031/0687, published December 11, 2013), which coincides with the present invention by the largest number of essential features and adopted as a prototype. The prototype solar cell contains a substrate (GaAs) and four subcells based on pn junctions InGaP, GaAs, InGaAs and InGaAs with bandgaps of 1.9 eV, 1.4 eV, 1.0 eV and 0.7 eV, respectively, connected between tunnel pn junctions. To coordinate the lattice constants of the various subcells, the structure of the known four-junction solar cell contains two metamorphic gradient buffer layers grown between the GaAs and InGaAs subelements and between the two InGaAs subelements. The structure of the inverted solar cell is grown in the reverse order, i.e. starting with a sub-element with a maximum band gap and ending with a sub-element with a minimum band gap, after which the substrate is separated. Accordingly, the first (adjacent to the substrate) subelement includes p- and n-layers of InGaP (band gap Eg1 = 1.9 eV), the second subelement includes p- and n-layers of GaAs (Eg2 = 1.4 eV), the third subelement includes p- and n-layers of InGaAs (Eg3 = 1.0 eV), and the fourth (lower) subelement includes p- and n-layers of InGaAs (Eg4 = 0.7 eV).

The disadvantages of the known four-junction solar cell are the presence of two regions of generation of structural defects due to the use of two metamorphic gradient buffer layers, the complexity of post-growth operations associated with the removal of the substrate, and the insufficient conversion efficiency of the short-wave part of the solar radiation spectrum.

The objective of the present invention was to develop such a four-junction solar cell that would be simpler to manufacture and have a higher band gap (more than 2 eV) of the upper wide-band sub-cell, which is necessary to obtain a high conversion efficiency of the short-wave part of solar radiation.

The problem is solved in that the four-junction solar cell includes four sub cells successively grown on a p-Ge substrate, interconnected by tunnel pn junctions, a contact layer, and a metamorphic gradient buffer layer between the first and second sub cells. The first sub-element includes a p-Ge substrate, an n-Ge layer, a wide-gap window layer of n-GaInP and a buffer layer of n-GaInAs. The metamorphic gradient buffer layer includes a p-Ga _1-x In _x As layer with x = 0.01 at the beginning of growth and up to x = 0.30-0.40 in the near-surface region of the layer. The second subelement includes a p-GaInAs layer and an n-GaInAs layer. The third sub-element includes a layer of p-AlGaInAs and a layer of n-AlGaInAs. The fourth subelement includes short-period p- and n-type A2B6 superlattices (CP) with an effective band gap of Eg4 = 2.1-2.2 eV and a total thickness of 0.4-0.5 μm, each of which consists of alternating layers CdSe with a thickness of 2.5 ÷ 4.5 monomolecular layers and ZnSe or ZnS _y Se _1-y , where y = 0.05-0.40, with a thickness of 2.5 ÷ 10 monomolecular layers. The second, third and fourth subelements are lattice-constant matched with In _x Ga _1-x As, where x = 0.25-0.30, and the first subelement is pseudomorphic to the p-Ge substrate.

The structure of this four-junction solar cell contains an In _x Ga _1-x As metamorphic gradient buffer layer grown between the Ge and InGaAs sub-cells to pair the first sub-cell (Ge) with the remaining sub-cells of the four-junction solar cell, with the top three pn-based subcells transitions InGaAs, InAlGaAs and short-period superlattices of A2B6 compounds are coordinated with each other along the lattice constant. Using the metamorphic gradient buffer layer In _x Ga _1-x As allows us to construct a four-junction solar cell structure with optimally selected bandgaps, which potentially allows us to realize a solar cell with an efficiency of converting solar energy into electrical energy of more than 50%.

To change the lattice constant during the transition from the first subelement based on the Ge pn junction to the second subelement based on the GaInAs pn junction, the structure contains a metamorphic p-In _x Ga _1-x As buffer layer with a linear profile of composition change.

New in this four-junction solar cell is the simultaneous fabrication of the substrate and the first sub-element from germanium layers, and the fourth element in the form of short-period superlattices of p- and n-type A2B6 compounds with an effective band gap of Eg4 = 2.1-2.2 eV and a total thickness 0.4-0.5 μm, each of which consists of alternating layers of CdSe with a thickness of 2.5 ÷ 4.5 monomolecular layers and ZnS _y Se _1-y with a thickness of 2.5 ÷ 10 monomolecular layers, where y = 0- 0.4.

The total thickness of 0.4-0.5 microns of short-period p- and n-type superlattices, determined mainly by the thickness of the p-region of the base (0.4-0.45 microns), is determined by the penetration depth of the short-wave part of solar radiation into the semiconductor (for small the total thickness of CP is the insufficient efficiency of light absorption, and with a large thickness - an increase in the growth time and resistance of the structure without increasing the efficiency). The thickness range of the CP-forming CdSe and ZnS _y Se _1-y layers is due to the necessity of matching the average lattice constant of CP with the lattice constant of the second subelement based on the pn junction In _x Ga _1-x As (with Eg2 ~ 1.0-1.1 eV and x = 0.25-0.3) to satisfy the condition of pseudomorphic growth, i.e. so that the thickness of each CP layer is less than the critical thickness of the pseudomorphic growth h _cr , depending on the mismatch of the lattice constant of this layer with the lattice constant of the second subelement based on the pn junction In _x Ga _1-x As (x = 0.25-0.3), and the need to ensure effective vertical transport of carriers in the superlattice, determined by the effective width of the mini-zone of heavy holes.

Calculations show that these conditions correspond to the range of thicknesses of CdSe layers in 2.5 ÷ 4.5 monomolecular layers and ZnS _y Se _1-y in 2.5 ÷ 10 monomolecular layers at y = 0-0.4. The choice of y = 0-0.4 is due to the fact that for y> 0.4 for the formation of CP (CdSe / ZnS _y Se _1-y ), isoperiodic to In _x Ga _1-x As (x = 0.2-0 , 3) the use of molecular beam epitaxy requires the use of a low (less than 250 ° C) growth temperature, which is determined by the coefficient of sulfur incorporation under stoichiometric conditions on the growth surface [SV Ivanov, SV Sorokin, PS Kop'ev, JR Kim, HD Jung and HS Park, J. Crystal Growth 159, 16 (1996)], or use far from optimal conditions for strong enrichment of the surface with Zn atoms in the MPE of ZnS _y Se _1-y layers, which leads to a decrease in morphology and an increase in the number of defects in the SR. The choice of the effective gap band gap CP Eg4 = 2.1-2.2 eV follows from a certain range of CP parameters (CdSe / ZnS _y Se _1-y ).

The introduction of the metamorphic In _x Ga _1-x As gradient metamorphic gradient buffer layer into the design of the four-junction solar cell makes it possible to optimize the structure of the four-junction solar cell from the point of view of selecting the bandgap widths of the subcells. In this case, when growing the metaphoric buffer layer p-InxGa1-xAs, it is necessary to use a linear profile of compositional variation, which is determined by the possibility of calculating stresses (strains) on the surface of the metamorphic buffer layer with a high degree of accuracy for the subsequent coupling of the second, third and fourth (upper) subelements with metamorphic buffer layer along the lattice constant.

The present invention is illustrated by drawings, where:

in FIG. 1 is a schematic sectional view of a true four junction solar cell;

in FIG. Figure 2 shows the parametric dependence of the effective band gap and the mini-band width of heavy holes on the layer thickness and on the S content for the CdSe / ZnSSe superlattice isoperiodic to In _x Ga _1-x As (x = 0.3), (MS is a monomolecular layer , CP is the superlattice);

при условии оптимизации по ширине запрещенной зоны 3-го перехода

для нескольких значений

; на нижней части фиг. 3 приведены соответствующие зависимости

от

.in FIG. Figure 3 shows the dependences of the efficiency of a four-junction solar cell on the band gap of the second transition calculated for the spectrum of AM1.5: 500 Sun

subject to optimization over the band gap of the 3rd transition

for multiple values

; on the bottom of FIG. 3 shows the corresponding dependencies

from

.

) и ZnS_ySe_1-y толщиной в 2,5÷10 мономолекулярных слоя (6,7÷28

), при содержании серы y=0-0,4. Четырехпереходной солнечный элемент содержит метаморфный градиентный буферный слой 9 In_xGa_1-xAs (с x, изменяемым от x=0,01-0,05 до x=0,28-0,38) с линейным профилем изменения состава, выращиваемый между субэлементом 2 на основе Ge и субэлементом 3 InGaAs с целью изменения постоянной решетки. Каждый субэлемент (кроме субэлемента 2 на основе Ge) также может включать слой тыльного барьера и слой широкозонного окна (на чертеже не показаны).The real four-junction solar cell (see Fig. 1) is grown on p-Ge substrate 1 and contains four subcells 2, 3, 4, 5, interconnected by tunnel pn junctions 6, 7, 8, a metamorphic gradient buffer layer 9, and a contact layer 10. The first sub-element 2 includes a p-Ge substrate 1, an n-Ge layer 11, a wide-gap window layer of n-GaInP 12 and a buffer layer 13 of n-GaInAs, the second sub-element 3 includes a p-GaInAs layer 14 and a layer 15 of n-GaInAs, the third sub-element 4 includes a layer 16 of p-AlGaInAs and a layer 17 of n-AlGaInAs, and the fourth sub-element 5 includes short-period CP 18, 19 A2B6, respectively p-type and n-type with an effective band gap Eg4 = 2.1-2.2 eV and a total thickness of 0.4-0.5 μm, each of which consists of alternating layers (not shown in the drawing) of CdSe with a thickness in 2.5 ÷ 4.5 monomolecular layers (7.6 ÷ 13.7

) and ZnS _y Se _1-y with a thickness of 2.5 ÷ 10 monomolecular layers (6.7 ÷ 28

), with a sulfur content of y = 0-0.4. The four junction solar cell contains a 9 In _x Ga _1-x As metamorphic gradient buffer layer (with x varying from x = 0.01-0.05 to x = 0.28-0.38) with a linear composition change profile grown between Ge-based sub-element 2 and InGaAs sub-element 3 in order to change the lattice constant. Each sub-element (except Ge-based sub-element 2) may also include a back barrier layer and a wide-gap window layer (not shown in the drawing).

Short-period CPs 18 and 19, respectively, of p-type and n-type from CdSe / ZnS _y Se _1-y layers can be grown in a wide range of effective values of the band gap. The use of superlattices 18, 19 CdSe / ZnS _y Se _1-y compared with the layers of bulk ZnCdSSe solid solutions allows one to change the effective band gap by changing the thicknesses of the CdSe and ZnS _y Se _1-y layers forming CP 18, 19 without changing the composition ZnS _y Se _1-y layers. The use of CP 18, 19 CdSe / ZnS _y Se _1-y , in comparison with the layers of bulk ZnCdSSe solid solutions, also allows us to achieve sufficient levels of p-doping (at least 10 ¹⁷ cm ^-3 ) and n-doping (up to 10 ¹⁹ cm ^{- 3} ) when using the method of molecular beam epitaxy (MPE), which is confirmed by measurements of the doping level of test structures with CP CdSe / ZnS _y Se _1-y . Subelements 3, 4, 5 of a multi-junction solar cell are pseudomorphic to In _x Ga _1-x As (x = 0.2-0.3) metamorphic buffer layer 9 (matched to it by the lattice constant), subelement 2 is pseudomorphic to substrate 1 of p- Ge.

The composition and thicknesses of the layers of CP CdSe / ZnS _y Se _1-y 18, 19 when growing the upper fourth sub-cell of 5 four-junction solar cells are selected from the condition of matching the lattice constant with In _x Ga _1-x As (x = 0.2-0.3 ), i.e. so that tensile strains in ZnS _y Se _1-y layers exactly compensate for compressive strains in CdSe layers. The ratios of the thicknesses and compositions of the layers forming CP 18, 19, at which CP 18, 19 are consistent over the lattice period with In _x Ga _1-x As (x = 0.2-0.3), are found from the condition that the average lattice constant CP 18, 19 and the lattice constant In _x Ga _1-x As (x = 0.2-0.3) ( a _InGaAs ) taking into account the difference in the elastic properties of the layers forming CP 18, 19.

:For CP 18, 19 CdSe

:

where a _ZnSSe , a _CdSe are the lattice constants of bulk (unstressed) layers of ZnS _x Se _1-x and CdSe, respectively, CP 18, 19, m;

a _SL is the average lattice period CP, m;

a _InGaAs is the lattice period of the In _x Ga _1-x As layer (x = 0.2-0.3), m;

и

- толщины соответствующих ненапряженных слоев ZnS_xSe_1-x и CdSe CP 18, 19, м;

and

- thicknesses of the corresponding unstressed layers ZnS _x Se _1-x and CdSe CP 18, 19, m;

G _ZnSSe and G _CdSe are the shear moduli of the constituent layers ZnS _x Se _1-x and CdSe CP 18, 19, Pa.

For epitaxial growth on a substrate 1 with orientation (001), the shear moduli can be expressed in terms of the elastic constants c ₁₁ and c ₁₂ :

и

- модули упругости соответствующих слоев, Па.where i means ZnS _x Se _1-x or CdSe,

and

are the elastic moduli of the corresponding layers, Pa.

); толщина слоев ZnS_ySe_1-y варьируется в диапазоне 2,5÷10 мономолекулярных слоя (6,7÷28

); период CP 18, 19 CdSe/ZnS_ySe_1-y составляет 1,5÷4 нм; оптимальная суммарная толщина слоев CP 18, 19 верхнего (четвертого) субэлемента 5 четырехпереходного солнечного элемента составляет ~400-500 нм, что соответствует 100÷330 периодам CP 18, 19.When choosing the thickness of the CdSe and ZnS _y Se _1-y layers forming CP 18, 19, in addition to condition (1) and the required effective band gap, Eg4 = 2.1-2.2 eV, it is also necessary that the thickness of each CdSe layer and ZnS _y Se _1-y was less than the critical thickness h _{cr of} pseudomorphic growth, depending on the mismatch of the lattice constant of a given layer with the lattice constant In _x Ga _1-x As (x = 0.2-0.3) ( a _InGaAs ). For the considered range of compositions, these thicknesses are no more than 10-12 monomolecular layers. In addition, it is necessary to ensure effective vertical transport of carriers in CP 18, 19, therefore, the effective width of the mini-zone of heavy holes in CP 18, 19 should be at least 10-15 meV. The dependence of the effective band gap on the width of the mini-band of heavy holes for CP 18, 19 CdSe / ZnS _y Se _1-y , isoperiodic to the lattice constant In _x Ga _1-x As (x = 0.3) at T = 300 K, presented in FIG. 2. Summing up the above requirements and taking into account that ZnS _x Se _1-x solid solutions with a sulfur content of y ~ 0.4 are technologically feasible during MPE growth, we obtain the parameters of the layers CP 18, 19 CdSe / ZnS _y Se _1-y : sulfur content in layers CP 18, 19 varies in the range y ~ 0-0.4; the thickness of the CdSe layers lies in the range 2.5 ÷ 4.5 monomolecular layers (7.6 ÷ 13.7

); the thickness of the ZnS _y Se _1-y layers varies in the range of 2.5 ÷ 10 monomolecular layers (6.7 ÷ 28

); the period of CP 18, 19 CdSe / ZnS _y Se _1-y is 1.5 ÷ 4 nm; the optimal total thickness of the layers CP 18, 19 of the upper (fourth) subcell 5 of the four junction solar cell is ~ 400-500 nm, which corresponds to 100 ÷ 330 periods of CP 18, 19.

In FIG. Figure 3 shows the theoretically calculated dependences of the efficiency of a four-junction solar cell on the band gap Eg2 of the second sub-element 3 for the spectrum AM1.5: 500 of the suns corresponding to the standard spectrum of solar radiation on the earth under conditions of high radiation concentration for several values of the band gap of the 4th transition subelement 5 (Eg4).

Each point of this dependence was optimized in terms of the band gap of the 3rd transition - sub cell 4 (Eg3), so that the maximum of each dependence corresponds to the maximum achievable efficiency of the four junction solar cell at a selected value of Eg4 and under the condition Eg1 = 0.67 eV. The corresponding dependences of Eg3 (Eg2) are shown in the lower part of FIG. 3. The values of the remaining parameters correspond to the best possible quality of materials of subelements 2, 3, 4, 5 with a minimum number of defects.

Example 1. A four-junction solar cell was grown on a p-Ge substrate and containing four subcells interconnected by pn tunnel junctions and a metamorphic In _x Ga _1-x As gradient buffer layer between the first (based on Ge) and the second (based on InGaAs) subcells, and contact layer. The first sub-element includes a p-Ge substrate, an n-Ge layer (Eg1 = 0.67 eV) ~ 200 nm thick, a wide-gap window layer of n-GaInP 100 nm thick and a buffer layer of n-GaInAs 1000 nm thick, the second sub-element includes a layer of p-GaInAs and a layer of n-GaInAs (Eg2 = 1.04 eV) with a thickness of 3400 nm and 100 nm, respectively, the third sub-element includes a layer of p-AlGaInAs and a layer of n-AlGaInAs (Eg2 = 1.5 eV) with a thickness of 1000 nm and 50 nm, respectively, and the fourth subelement includes p-type and n-type short-period CP A2B6 with an effective band gap of Eg4 = 2.17 eV and with a total thickness of 0.45 μm, each of which consists of layers of CdSe with a thickness of 3.8 monomolecular layers and ZnSe with a thickness of 8 monomolecular layers. The structure of the four-junction solar cell contains an In _x Ga _1-x As metamorphic gradient buffer layer with a linear compositional variation profile 1000 nm thick (with x varying from x = 0.07 to x = 0.37) with a linear compositional variation profile grown between the first sub element based on Ge and the second sub element based on InGaAs. The first subelement was formed by gas phase epitaxy from organometallic compounds (MOVPE), the second, third and fourth subelements, as well as the In _x Ga _1-x As metamorphic gradient buffer layer, were grown by molecular beam epitaxy (MPE). The Ge Pn transition was realized due to the diffusion of phosphorus into the p-Ge substrate during the formation of the GalnP wide-gap window layer. Ga, In, Al, As were used as sources of molecular beams when growing the second, third subelements and metamorphic gradient buffer layer; Si and Be, respectively, were used as n- and p-type dopant materials. When growing the fourth subelement, Zn, Cd, Se were used as sources of molecular beams; ZnCl ₂ and plasma-activated N _2, respectively, were used as n- and p-type dopant materials. The doping levels of p- and n-type superlattices were p ~ 10 ¹⁷ cm ³ and n ~ 2 × 10 ¹⁸ cm ^-3 , respectively.

Example 2. A four-junction solar cell was grown on a p-Ge substrate and containing four subcells interconnected by pn tunnel junctions and a metamorphic In _x Ga _1-x As gradient buffer layer between the first (based on Ge) and the second (on based InGaAs) subcells, and contact layer. The first sub-element includes a p-Ge substrate, an n-Ge layer (Eg1 = 0.67 eV) ~ 200 nm thick, a wide-gap window layer of n-GaInP 100 nm thick and a buffer layer of n-GaInAs 1000 nm thick, the second sub-element includes a layer of p-GaInAs and a layer of n-GaInAs (Eg2 = 1.0 eV) with a thickness of 3400 nm and 100 nm, respectively, the third sub-element includes a layer of p-AlGaInAs and a layer of n-AlGaInAs (Eg2 = 1.5 eV) 1000 nm and 50 nm thick, respectively, and the fourth subelement includes p-type and n-type short-period A2B6 CPs with an effective band gap of Eg4 = 2.09 eV and with a total thickness of 0.45 μm, each of which consists of uyuschihsya CdSe layer thickness of 4.0 monomolecular layer and ZnS _y Se _1-y thickness of 4.0 monomolecular layer, where y = 0,40. The structure of the four-junction solar cell contains a metamorphic In _x Ga _1-x As gradient buffer layer 1000 nm thick with a linear profile of compositional variation (with x varying from x = 0.05 to x = 0.30), grown between the first sub cell based on Ge and second subcell based on InGaAs. The first subelement was formed by gas phase epitaxy from organometallic compounds (MOVPE), the second, third and fourth subelements, as well as the metamorphic In _x Ga _1-x As gradient buffer layer, were grown by the MPE method. The sources of molecular beams and the materials of the dopant were the same as in Example 1. The doping levels of p- and n-type superlattices were p ~ 10 ¹⁷ cm ^-3 and n ~ 2 × 10 ¹⁸ cm ^-3, respectively.

CP CdSe / Zn (S) Se of both fabricated solar cells are represented by circles in FIG. 2. Effective doping of p- and n-type conductivity of short-period CP CdSe / ZnS _y Se _1-y , as well as the possibility of reducing the band gap of the upper wide-gap subelement to a level below 2.2 eV, provides a high solar cell efficiency due to efficient conversion shortwave part of solar radiation. Additionally, the use of short-period alternating-stressed CP CdSe / ZnS _y Se _1-y instead of the layer of ZnCdSSe solid solution allows increasing the critical thickness of pseudomorphic growth.

Claims (1)

A four-junction solar cell, including four subcells successively grown on a p-Ge substrate, interconnected by pn tunnel junctions, a contact layer and a metamorphic gradient buffer layer between the first and second subcells, the first subcell including a p-Ge substrate, a layer of n -Ge, a wide-gap window layer of n-GaInP and a buffer layer of n-GaInAs, a metamorphic gradient buffer layer includes a layer of p-Ga _1-x In _x As with x = 0.01 at the beginning of growth and up to x = 0 , 30-0.40 in the near-surface region of the layer, the second sub-element includes a p-GaInAs layer and an n-GaInAs layer, the third subelement includes a p-AlGaInAs layer and an n-AlGaInAs layer, and the fourth subelement includes p-type and n-type A2B6 short-period superlattices with an effective band gap of Eg4 = 2.1 -2.2 eV and with a total thickness of 0.4-0.5 μm, each of which consists of alternating CdSe layers with a thickness of 2.5 ÷ 4.5 monomolecular layers and ZnSe or ZnS _y Se _1-y , where y = 0.05-0.40, with a thickness of 2.5 ÷ 10 monomolecular layers, with the second, third and fourth subelements matched according to the lattice constant with In _x Ga _1-x As, where x = 0.25-0.30, and the first subelement is pseudomorphic to the p-Ge substrate .

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