Multi-junction solar cell

The disclosure provides a multi-junction solar cell structure and the manufacturing method thereof, comprising a first photovoltaic structure and a second photovoltaic structure; wherein at least one of the first photovoltaic structure and the second photovoltaic structure comprises a discontinuous photoelectric converting structure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to Taiwan application No. 098146648, filed Dec. 31, 2009.

TECHNICAL FIELD

The application relates to a multi-junction solar cell structure and the manufacturing method thereof.

DESCRIPTION OF BACKGROUND ART

Because the petroleum source is limited, various kinds of substitutive energy are developed extensively and turned into products. Among those, the solar cell has become the commercial products for either the industrial or the residential use, and the III-V multi-junction solar cell is mainly applied to the space industry and the industrial field because of its high conversion efficiency. The structure of this kind of solar cell is a lattice-matched triple junction Ge/GaAs/GaInP structure. Ga1-xInxP (1.85 eV; x˜0.5), which is called the top cell, has the larger band gap and is the upmost layer to absorb the photon with higher energy (the wavelength from the range of the ultraviolet to the visible light); GaAs, which is called the middle cell, has the band gap with 1.42 eV and can absorb the photon with the wavelength in the near IR range; Ge, which is called the bottom cell, has the band gap with 0.74 eV and can absorb the light passed through the upper two layers with the wavelength in the IR range. Because the absorbed spectrum range is broader, the conversion efficiency is larger than 30%.

SUMMARY OF THE DISCLOSURE

A novel multi-junction solar cell in accordance with the present application is disclosed, which has the high efficiency and could improve the heat dissipating property.

The steps of a manufacture method of a multi-junction solar cell in accordance with one embodiment of the present application include providing a growth substrate; forming a buffer layer on the growth substrate; forming a contact layer on the buffer layer; forming a first photovoltaic structure on the contact layer; forming a first tunnel junction structure on the first photovoltaic structure; forming a second photovoltaic structure on the first tunnel junction structure; forming a photon recycling layer on the second photovoltaic structure; providing a supporting body; forming a connecting layer on the supporting body; connecting the photon recycling layer and the supporting body by the connecting layer; removing the growth substrate to expose the contact layer; removing part of the contact layer to expose partial surface of the first photovoltaic structure; forming a first electrode on the contact layer, a second electrode electrically connecting to the supporting body, and an anti-reflection layer on the exposed surface of the first photovoltaic structure; wherein at least one of the first photovoltaic structure and the second photovoltaic structure comprises a discontinuous photoelectric converting structure.

In accordance with one embodiment of the present application, the discontinuous photoelectric converting structure is located in a plurality of the cavities and the cavities are defined by a patterned structure layer.

A multi junction solar cell in accordance with another embodiment of the present application includes a supporting body; a connecting layer located on a surface of the supporting body; a first electrode located on another surface of the supporting body; a photon recycling layer located on the connecting layer; a first photovoltaic structure comprising a first band gap located on another part surface of the photon recycling layer; a first tunnel junction structure located on the first photovoltaic structure; a second photovoltaic structure comprising a second band gap located on the first tunnel junction structure; a contact layer located on a part of the surface of the second photovoltaic structure and forming the ohmic contact with the second photovoltaic structure; and a second electrode located on the contact layer; an anti-reflection layer located on at least another part of the surface of the second photovoltaic structure, wherein at least one of the first photovoltaic structure and the second photovoltaic structure comprises a discontinuous photoelectric converting structure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1˜3disclose the structure and the steps of a manufacture method of a multi junction solar cell in accordance with the first embodiment of the present application, and the details are disclosed as follows:Step1: As shown inFIG. 1, a growth substrate10is provided, and the material of the growth substrate can be Ge, SiGe, or GaAs. A buffer layer11is then formed on the growth substrate10. The composition of the buffer layer11is different from but lattice-matched with the growth substrate10such as GaAs or InGaP.Step2: Forming a contact layer12on the buffer layer11. The material of the contact layer12comprises the semiconductor material such as GaAs and has a high impurity concentration, for example, higher than 1*1018cm−3.Step3: Forming a first photovoltaic structure21on the contact layer12so an ohmic contact is formed between the first photovoltaic structure21and the contact layer12. The first photovoltaic structure21with a first band gap comprises a first emitter layer211with a first electric conductivity such as n-type and a first base layer212with a second electric conductivity which is different from the first electric conductivity, such as p-type. The first emitter layer211and the first base layer212have the lattice constants matched with the growth substrate10and the materials can comprise AlInGaP (AlaInbGa(1-a-b)P; 0□a,b□1).Step4: Forming a first tunnel junction structure22on the first photovoltaic structure21, wherein the first tunnel junction structure22comprises a first tunnel junction layer221with a first electric conductivity such as p-type and an impurity concentration higher than 1*1018cm−3and a second tunnel junction layer222with a second electric conductivity such as n-type and an impurity concentration higher than 1*1018cm−3. The first tunnel junction layer221and the second tunnel junction layer222comprise high concentration of impurities and small thicknesses, for example, smaller than 500A, to form a high conductive contact structure.Step5: Forming a second photovoltaic structure31on the first tunnel junction structure22. The second photovoltaic structure31with a second band gap smaller than the first band gap comprises a second emitter layer311with a first electric conductivity such as n-type and a second base layer312with a second electric conductivity which is different from the first electric conductivity, such as p-type. The second emitter layer311and the second base layer312have the lattice constants matched with the growth substrate10and the materials can comprise GaAs.Step6: Forming a second tunnel junction structure32on the second photovoltaic structure31, wherein the second tunnel junction structure32comprises a third tunnel junction layer321with a first electric conductivity such as p-type and an impurity concentration higher than 1*1018cm−3and a fourth tunnel junction layer322with a second electric conductivity different from the first electric conductivity, such as n-type, and an impurity concentration higher than 1*1018cm−3. The third tunnel junction layer321and the fourth tunnel junction layer322comprise high concentration of impurities and small thicknesses, for example, smaller than 500A, to form a high conductive contact structure.Step7: Forming a patterned structure layer40on the second tunnel junction structure32. The patterned structure layer40comprises a pattern to define a plurality of cavities and to expose partial surfaces of the second tunnel junction structure32corresponding to the cavities.Step8: Forming a third photovoltaic structure41in the plurality of the cavities. The third photovoltaic structure is defined as a plurality of discontinuous photovoltaic regions by the patterned structure layer40. The third photovoltaic structure41with a third band gap smaller than the second band gap comprises a third emitter layer411with a first electric conductivity such as n-type and a third base layer412with a second electric conductivity which is different from the first electric conductivity, such as p-type. The third emitter layer411and the third base layer412have the lattice constants mismatched with the growth substrate10. For example, the difference of the lattice constants is larger than 1% and the materials can comprise InGaAs (IncGa(1-c)As; 0□c□1) or InGaAsP (InpGa(1-p)AsqP(1-q); 0□p,q□1).Step9: Forming a photon recycling layer51on the third photovoltaic structure41and the patterned structure layer40. The material of the photon recycling layer51comprises the material has the reflectivity higher than 70% in a specified light wavelength range and is preferred has the reflectivity higher than 70% in the light wavelength range that the third photovoltaic structure41absorbs, such as the metal material or the distributed Bragg reflector (DBR) that conforms to the mentioned condition.Step10: As shown inFIG. 2, providing a supporting body60and forming a connecting layer61on the supporting body60. The material of the connecting layer61can be metal, metal alloy, or conductive polymer material. Then, the photon recycling layer51and the supporting body60are contacted with the connecting layer61. The method of contacting can be glue bonding, solder bonding, eutectic bonding and so on.Step11: Removing the growth substrate10and the buffer layer11to expose the contact layer12. The removing method can be grounding the growth substrate10and the buffer layer11directly, etching away the buffer layer11by the etching solution to peel off the growth substrate10, or melting and decomposing the buffer layer11by laser illumination to peel off the growth substrate10.Step12: As shown inFIG. 3, a part of the contact layer12is removed to expose a part of the first photovoltaic structure21. Forming a first electrode71on the contact layer12and forming a second electrode72to electrically connect to the supporting body60. The first electrode71and the second electrode72can be a single-layer or multi-layer metal or metal alloy layers.Step13: Forming an anti-reflection layer81on the exposed surface of the first photovoltaic structure21to complete the first embodiment of the multi junction solar cell1in accordance with present application.

As shown inFIG. 3, the multi-junction solar cell1comprises a supporting body60and a connecting layer61located on one surface of the supporting body60; a second electrode72located on another surface of the supporting body60; a photon recycling layer51located on the connecting layer61; a patterned structure layer40located on a part of a surface of the photon recycling layer51to define a plurality of the cavities; a third photovoltaic structure41with a third band gap located on another part of the surface of the photon recycling layer51and in the plurality of the cavities comprising a third base layer412and a third emitter layer411; a second tunnel junction layer32located on the third photovoltaic structure41and the patterned structure layer40comprising a third tunnel junction layer321and a fourth tunnel junction layer322; a second photovoltaic structure31with a second band gap located on the second tunnel junction layer32comprising a second base layer312and a second emitter layer311; a first tunnel junction layer22located on the second photovoltaic structure31comprising a first tunnel junction layer221and a second tunnel junction layer222; a first photovoltaic structure21with a first band gap located on the first tunnel junction layer22comprising a first base layer212and a first emitter layer211; a contact layer12located on a part of a surface of the first photovoltaic structure21and forming an ohmic contact with the first photovoltaic structure21; a first electrode71located on the contact layer12; and an anti-reflection layer81located on another part surface of the first photovoltaic structure21.

When the sun light enters into the multi-junction solar cell1from the anti-reflection layer81, the light with the shorter wavelength is absorbed by the first photovoltaic structure21with a first band gap to convert into a first current, the light with the middle wavelength is absorbed by the second photovoltaic structure31with a second band gap smaller than the first band gap to convert into a second current, and the light with the longer wavelength is absorbed by the third photovoltaic structure41with a third band gap smaller than the second band gap to convert into a third current. The remaining unabsorbed light can be reabsorbed by the third photovoltaic structure41by reflecting the remaining light by the photon recycling layer51to compensate the absorbing area loss caused by the patterned structure layer40occupying a part of the surface of the third photovoltaic structure41. The patterns of the patterned structure layer40can comprise the parallel stripes4aor the interlaced stripes4bas shown inFIG. 4, and the patterns occupy about 1˜10% area of the multi junction solar cell1to make the third current produced by the third photovoltaic structure41close to or larger than one of the first current produced by the first photovoltaic structure21or the second current produced by the second photovoltaic structure31. The widths of the parallel stripes or the interlaced stripes are about 0.5 μm˜5 μm and the heights of the parallel stripes or the interlaced stripes are about 0.5 μm˜5 μm as defined by the thickness of the third photovoltaic structure41. The ratios of the heights and the widths of the parallel stripes or the interlaced stripes are about 0.1˜10, and is preferred to be about 0.5˜5. The preferred material of the patterned structure layer40is good insulating amorphous material such as oxide material or nitride material. Besides, because the third photovoltaic structure41is lattice mismatched with the growth substrate10, the thread dislocation is formed and extending easily in epitaxial growth, and the quality of the epitaxial layers and the conversion efficiency of the multi-junction solar cell are impacted accordingly. The lattice mismatch also causes the accumulation of the stress, which makes the wafer over bending and crack. The patterned structure layer40can stop the thread dislocation extending efficiently so the stress caused by the lattice mismatch can be released by forming the third photovoltaic structure41in the plurality of the cavities formed by the patterned structure layer40. The wafer bending and crack can also be eliminated.

FIGS. 5˜6disclose the second embodiment of the multi junction solar cell in accordance with the present application. The details of the structure and the steps of the manufacture method are disclosed as the following:Step1: As shown inFIG. 5, a growth substrate10is provided, and the material of the growth substrate can be Ge, SiGe, or GaAs. A buffer layer11is then formed on the growth substrate10. The composition of the buffer layer11is different from but lattice-matched with the growth substrate10such as GaAs or InGaP.Step2: Forming a contact layer12on the buffer layer11. The material of the contact layer12comprises the semiconductor material such as GaAs and has a high impurity concentration, for example, higher than 1*1018cm−3.Step3: Forming a first photovoltaic structure21on the contact layer12so an ohmic contact is formed between the first photovoltaic structure21and the contact layer12. The first photovoltaic structure21with a first band gap comprises a first emitter layer211with a first electric conductivity such as n-type and a first base layer212with a second electric conductivity which is different from the first electric conductivity, such as p-type. The first emitter layer211and the first base layer212have the lattice constants matched with the growth substrate10and the materials can comprise AlInGaP (AlaInbGa(1-a-b)P 0□a,b□1).Step4: Forming a first tunnel junction structure22on the first photovoltaic structure21, wherein the first tunnel junction structure22comprises a first tunnel junction layer221with a first electric conductivity such as p-type and an impurity concentration higher than 1*1018cm−3and a second tunnel junction layer222with a second electric conductivity such as n-type and an impurity concentration higher than 1*1018cm−3. The first tunnel junction layer221and the second tunnel junction layer222comprise high concentration of impurities and small thicknesses, for example, smaller than 500A, to form a high conductive contact structure.Step5: Forming a second photovoltaic structure31on the first tunnel junction structure22. The second photovoltaic structure31with a second band gap smaller than the first band gap comprises a second emitter layer311with a first electric conductivity such as n-type and a second base layer312with a second electric conductivity which is different from the first electric conductivity, such as p-type. The second emitter layer311and the second base layer312have the lattice constants matched with the growth substrate10and the materials can comprise GaAs.Step6: Forming a second tunnel junction structure32, wherein the second tunnel junction structure32comprises a third tunnel junction layer321with a first electric conductivity such as p-type and an impurity concentration higher than 1*1018cm−3and a fourth tunnel junction layer322with a second electric conductivity different from the first electric conductivity such as n-type and an impurity concentration higher than 1*1018cm−3. The third tunnel junction layer321and the fourth tunnel junction layer322comprise high concentration of impurities and small thicknesses, for example, smaller than 500A to form a high conductive contact structure.Step7: Forming a third photovoltaic structure90on the second tunnel junction structure32. The third photovoltaic structure90with a third band gap smaller than the second band gap comprises a third emitter layer91with a first electric conductivity such as n-type, a third base layer93with a second electric conductivity which is different from the first electric conductivity, such as p-type, and a quantum dot area92located between the third emitter layer91and the third base layer93. The third emitter layer91and the third base layer93have the lattice constants matched with the growth substrate10and the materials can comprise InGaAs (IncGa(1-c)As; 0□c□1) or InGaAsP (InpGa(1-p)AsqP(1-q); 0□p,q□1). The quantum dot area92comprises a plurality of cap layers921, a plurality of quantum well layers922, and a plurality of quantum dot layers923stacked alternately. The cap layer921can also be a barrier layer to gather the carriers (electrons or holes) in the quantum well layers922or in the quantum dot layers923, and be a flat layer to flatten the surface roughened by the quantum dot layers923and to maintain the flatness of the surface of the device. The material of the cap layer921can be the same material of the third emitter layer91, or the extrinsic or intrinsic semiconductor material with the same electrical conductivity as the third emitter layer91. The quantum well layer922is formed on the cap layer921with a band gap smaller than the band gap of the cap layer921and lattice mismatched with the growth substrate10. For example, the difference of the lattice constants is larger than 1% and the materials can comprise InGaAs (IndGa(1-d)As; 0□d□1) or InGaAsP (InrGa(1-r)AssP(1-s); 0□r,s□1). The thickness of the quantum well layer922is between 1˜10 nm and is preferred between 1˜5 nm to stop the formation and extension of the lattice defect. The quantum dot layer923is formed on the quantum well layer922by the combination of a plurality of irregularly arranged quantum dots, and is composed of substantially the same material with the quantum well layer922. The quantum dot layers923form a plurality of band gaps different from the band gaps of the quantum well layers922. As shown inFIG. 8, the plurality of the quantum dots with different sizes8a,8b,8chas the corresponding band gap Ega, Egb, Egclarger than the band gap Egdof the quantum well layer922to expand the absorbable wavelength range and to raise the conversion efficiency of the third photovoltaic structure90. The quantum dot layer923is the discontinuous photoelectric converting structure formed by the plurality of distinct quantum dots with the diameters of substantially about 1˜10 nm, which can not only raise the conversion efficiency but help to release the stress caused by the lattice mismatch.Step8: Forming a photon recycling layer51on the third photovoltaic structure41. The material of the photon recycling layer51comprises the material having the reflectivity higher than 70% in a specified light wavelength range and is preferred has the reflectivity higher than 70% in the light wavelength range that the third photovoltaic structure41absorbs, such as the metal material or the distributed Bragg reflector (DBR) that meets the mentioned condition.Step9: As shown inFIG. 6, providing a supporting body60and forming a connecting layer61on the supporting body60. The material of the connecting layer61can be metal, metal alloy, or conductive polymer material. Then the photon recycling layer51and the supporting body60are contacted with the connecting layer61. The method for contacting can be glue bonding, solder bonding, eutectic bonding and so on.Step10: Removing the growth substrate10and the buffer layer11to expose the contact layer12. The removing method can be grounding the growth substrate10and the buffer layer11directly, etching away the buffer layer11by the etching solution to peel off the growth substrate10, or melting and decomposing the buffer layer11by laser illumination to peel off the growth substrate10.Step11: As shown inFIG. 7, a part of the contact layer12is removed to expose a part of the first photovoltaic structure21. Forming a first electrode71on the contact layer12and forming a second electrode72to electrically connect to the supporting body60. The first electrode71and the second electrode72can be a single-layer, multi-layer metal or metal alloy layers.Step12: Forming an anti-reflection layer81on the exposed surface of the first photovoltaic structure21to complete the multi-junction solar cell2of the first embodiment in accordance with present application.

As shown inFIG. 7, the multi junction solar cell2comprises a supporting body60and a connecting layer61located on one surface of the supporting body60; a second electrode72located on another surface of the supporting body60; a photon recycling layer51located on the connecting layer61; a third photovoltaic structure90with a third band gap located on the photon recycling layer51comprising a third base layer93, a quantum dot area92and a third emitter layer91; a second tunnel junction layer32located on the third photovoltaic structure90comprising a third tunnel junction layer321and a fourth tunnel junction layer322; a second photovoltaic structure31with a second band gap located on the second tunnel junction layer32comprising a second base layer312and a second emitter layer311; a first tunnel junction layer22located on the second photovoltaic structure31comprising a first tunnel junction layer221and a second tunnel junction layer222; a first photovoltaic structure21with a first band gap located on the first tunnel junction layer22comprising a first base layer212and a first emitter layer211; a contact layer12located on a part of a surface of the first photovoltaic structure21and forming an ohmic contact with the first photovoltaic structure21; a first electrode71located on the contact layer12; and an anti-reflection layer81located on another part surface of the first photovoltaic structure21.

When the sun light enters the multi-junction solar cell2from the anti-reflection layer81, the light with the shorter wavelength is absorbed by the first photovoltaic structure21with a first band gap to convert into a first current, the light with the middle wavelength is absorbed by the second photovoltaic structure31with a second band gap smaller than the first band gap to convert into a second current, and the light with the longer wavelength is absorbed by the third photovoltaic structure90which comprising a discontinuous optic-electronic conversion structure formed by a quantum dot layer, with a third band gap smaller than the second band gap, to convert into a third current. The remaining unabsorbed light can be reabsorbed by the third photovoltaic structure90by reflecting the remaining light by the photon recycling layer51to compensate the absorbing area loss caused by some areas where the quantum dots are not formed in the third photovoltaic structure90. Besides, because the third photovoltaic structure90is lattice mismatched with the growth substrate10, the thread dislocation is formed and extending easily in epitaxial growth and the quality of the epitaxial layers and the conversion efficiency of the multi-junction solar cell are impacted accordingly. The lattice mismatch also causes the accumulation of the stress, which makes the wafer bending and crack. The quantum dot area92in accordance with the present application comprising the quantum dot layer923having the discontinuous photoelectric converting structure formed by the plurality of distinct quantum dots can stop the thread dislocation extending efficiently, the stress caused by the lattice mismatch could be released, and the conversion efficiency can also be enhanced by different quantum band gaps formed by the different sizes of the plurality of the quantum dots. The stack number of the quantum dot layers is about 5˜100 layers, and is preferred to have about 10˜70 layers to make the third current generated by the third photovoltaic structure90close to or larger than the first current generated by the first photovoltaic structure21or the second current generated by the second photovoltaic structure31.

FIG. 9discloses a multi junction solar cell of the third embodiment in accordance with the present application. Comparing with the multi junction solar cell2shown inFIG. 7, the quantum dot area92comprises only a plurality of cap layers921and a plurality of quantum dot layers923stacked alternately without the quantum well layers922disclosed inFIG. 7. Therefore, in the multi-junction solar cell3, the quantum dot layers923which cause the lattice mismatch are formed discontinuously in three dimensional, and the quantum well layers922covering the overall surface as shown inFIG. 7do not exist. Therefore, the stress caused by the lattice defect and the lattice mismatch can be further reduced and the optic-electric conversion efficiency is raised.

A multi-junction solar cell comprises a discontinuous photoelectric converting structure is disclosed in the present application. In the present application, the discontinuous photoelectric converting structure can be formed in at least one of the first photovoltaic structure, the second photovoltaic structure, and the third photovoltaic structure. The discontinuous photoelectric converting structure in the present application comprises but not limited to the discontinuous photovoltaic regions in the plurality of cavities defined by the patterned structure layer in accordance with one embodiment of the present application or the plurality of the quantum dots that are comprised in the quantum dot layers in accordance with another embodiment of the present application.

The principle and the efficiency of the present application illustrated by the embodiments above are not the limitation of the application. Any person having ordinary skill in the art can modify or change the aforementioned embodiments. Therefore, the protection range of the rights in the application will be listed as the following claims.