WO1979000813A1 - Cellules solaires a homojonction superficielle - Google Patents

Cellules solaires a homojonction superficielle Download PDF

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Publication number
WO1979000813A1
WO1979000813A1 PCT/US1979/000181 US7900181W WO7900813A1 WO 1979000813 A1 WO1979000813 A1 WO 1979000813A1 US 7900181 W US7900181 W US 7900181W WO 7900813 A1 WO7900813 A1 WO 7900813A1
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Prior art keywords
layer
gaas
shallow
homojunction
thickness
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PCT/US1979/000181
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English (en)
Inventor
R Chapman
R Mcclelland
C Bozler
J Fan
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Massachusetts Inst Technology
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
    • H01L31/06—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by potential barriers
    • H01L31/068—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by potential barriers the potential barriers being only of the PN homojunction type, e.g. bulk silicon PN homojunction solar cells or thin film polycrystalline silicon PN homojunction solar cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/547—Monocrystalline silicon PV cells

Definitions

  • This invention is in the field of homoiunction photovoltaic devices, including solar cells.
  • Solar cells have been developed for generating electrical energy directly from sunlight.
  • these cells can be classified as either heterojunction devices, which depend upon junctions such as those formed between two different semi ⁇ conductor materials or between a metal and a semi ⁇ conductor or from a metal/insulator/semiconductor sandwich, and homojunction devices which depend only upon junctions formed between layers of the same semiconductor material doped to different impurity levels to provide different electrical properties.
  • heterojunction devices which depend upon junctions such as those formed between two different semi ⁇ conductor materials or between a metal and a semi ⁇ conductor or from a metal/insulator/semiconductor sandwich
  • homojunction devices which depend only upon junctions formed between layers of the same semiconductor material doped to different impurity levels to provide different electrical properties.
  • homojunction cells using direct-gap semiconductor materials have generally exhibited dis ⁇ appointing efficiencies.
  • One reason for the relativel low efficiencies in homojunction solar cells is be- lieved to be the high absorption coefficient which is inherent in direct gap semiconductor materials such as gallium arsenide.
  • direct gap semiconductor materials such as gallium arsenide.
  • approximately half of the carriers due to AM 1 radiation are gen ⁇ erated within 0.2 um of the surface of gallium ar- senide. Therefore, for materials such as GaAs, which also has a high surface recombination velocity, most of the carriers generated by solar radiation recombine before they reach the junction causing a significant decrease in conversion efficiency.
  • Ga- ⁇ _. ⁇ Al ⁇ As gallium aluminum arsenide
  • Such cells may be referred to as heteroface cells.
  • Ga ⁇ - j Al ⁇ s/GaAs interface than at a GaAs sur ⁇ face
  • higher conversion efficiencies have been achieved.
  • Hovel and Woodall report conversion efficiencies of up to 22% for hetero- face solar cells but only up to 14% for GaAs homojunction solar cells for air mass 1 (AM 1) radiation. See Hovel and Woodall, J. M. , 12th IEEE Photovoltaic Specialists Conf., 1976 (Institute of Electrical and Electronic Engineers, New York, 1976), p. 945.
  • This invention relates to improved shallow- homojunction photovoltaic devices and methods for their fabrication. These shallow-homojunction de ⁇ vices are based upon a plurality of layers of direct gap semiconductor materials suitably doped to pro- vide an n +/p/p+ structure. An antireflection coat ⁇ ing is applied over the n top layer and the n top semiconductor layer is also limited to a thick ⁇ ness within the range which permits, upon light irradiation, significant carrier generation to
  • the top junction is referred to as a shallow- junction.
  • the antireflection layer deposited on the n + top semiconductor layer is formed by anodization.
  • the anodic layer forms an excellent antireflection coating and requires no vacuum processing. Addi ⁇ tionally, the anodization process serves to reduce the thickness of the top semiconductor layer without resulting in significant surface degradation.
  • homojunction solar cells can be provided which have conversion efficiencies approaching those obtainable with heteroface cells.
  • Fur- ther ore the use of a Ga ⁇ _ x Al x As layer is avoided which eliminates a relatively complicated and ex ⁇ pensive step in the overall fabrication of a solar cell.
  • Still another significant advantage of the photovoltaic devices described herein is the ease with which ohmic contacts can be applied to them since they have high concentrations of dopants in their outer layers. This is particularly important because shallow homojunctions can have their junction qualities destroyed by attempts to form ohmic con ⁇ tacts at elevated temperatures. With this device, ohmic contacts can be applied by directly plating a metal layer on the semiconductor surfaces without elevated temperatures so that the junction quality is preserved.
  • shallow homojunction solar cells addi ⁇ tionally possess vastly superior resistance to degradation by electron bombardment than either Ga 1 _ ⁇ Al As heterojunction cells or shallow homojunc ⁇ tion GaAs cells having a p top layer. Because of this, the shallow homojunction cells described herein have great potential for use in space applications.
  • the heavily doped p + layer additionally enables excellent solar cells to be grown on substrate mat ⁇ erials other than the host semiconductor materials, i.e., the semiconductor material which absorbs sun ⁇ light and generates electrical current.
  • Gallium arsenide solar cells have been grown on gallium arsenide and germanium substrates with equally outstanding effi ⁇ ciencies of over 20% at AM 1.
  • FIG. 1 is a cross-sectional elevational view of a typical prior art solar cell employing a thin Ga- ⁇ x
  • FIG. 2 is a cross-sectional elevation view il ⁇ lustrating one embodiment of a solar cell according to this invention
  • FIG. 3 is a cross-sectional elevation view of another embodiment of a solar cell according to this invention.
  • FIGS. 4(a) and 4(b) are schematic illustrations of the application of gold contacts to a solar cell of this invention.
  • FIGS. 5(a) and 5(b) are schematic illustrations of the application of tin contacts to a solar cell of this invention.
  • FIG. 6 is an exploded cross-sectional view illustrating the area around one contact finger of a solar cell fabricated according to this invention
  • FIG. 7 is a plot of data illustrating the quantum efficiency at varying wavelengths for a solar cell fabricated according to this invention
  • FIG. 8 is a plot comparing measured reflectivity to theoretical reflectivity for an anodic anti- reflection layer applied to a GaAs shallow-homojunc ⁇ tion device according to this invention.
  • FIG. 9 is a plot of data illustrating the quantum efficiency at varying wavelengths for a solar cell of this invention having an anodic anti- reflection coating
  • FIG. 10 is a graphical presentation of the impurity profile for a GaAs shallow-homojunction photovoltaic device of this invention grown on a germanium substrate;
  • FIG. 11 is a plot of data illustrating the
  • FIG. 12 is a plot of data for the spectral response of a GaAs solar cell of this invention and comparing internal and external quantum efficiency;
  • FIG. 13 is a plot of data illustrating the power conversion efficiency as a function of n + layer thickness for four GaAs solar cells of this invention.
  • FIG. 14 is a plot of data illustrating the decrease in Isc with increasing electron irradi ⁇ ation fluences for a GaAs shallow-homojunction solar cell of this invention
  • FIG. 15 is a plot of data illustrating the decrease in I sc , V oc , fill factor and power conversion efficiency with increasing electron irradiation fluences for another GaAs shallow- homojunction solar cell of this invention.
  • FIG. 1 A prior art homojunction photovoltaic device 10 is illustrated in FIG. 1.
  • the substrate 12 is formed from an n GaAs wafer.
  • substrate 12 might be doped to a carrier concentration of 10-***- ⁇ ⁇ -• ⁇ -10- * *- 7 carriers/cm-**-*.
  • Layer 14 is formed over layer 12 and comprises p GaAs.
  • Layer 16 is formed from p+ Ga ⁇ _ ⁇ Al ⁇ As and might have a carrier con- centration of 10----8 carriers/cm3. typical thickness for layer 16 is less than one micrometer.
  • a device of FIG. 1 can be formed by depositing a thin layer 16 of p+ Ga _ ⁇ Al ⁇ As over an n GaAs substrate and subsequently diffusing some of the p-dopants from the Ga ⁇ . j ⁇ Al ⁇ s layer into the GaAs substrate to form layer 14.
  • Ga ⁇ _ ⁇ Al ⁇ As coatings are difficult to apply using chemical vapor deposi ⁇ tion and are relatively difficult to use in the formation of an ohmic contact. Because of this, ohmic contacts are typically applied to devices such as that shown in FIG. 1 by employing vacuum coating techniques followed by alloying. These techniques are relatively expensive and detrimental to shallow-homojunctions.
  • FIG. 2 illustrates one embodiment of an im ⁇ proved photovoltaic device of this invention.
  • substrate 22 is formed from a wafer of p + GaAs.
  • Substrate 22 might be formed, for example, from GaAs suitably doped with p-dopants such as zinc, cadmium, berylium or magnesium to a carrier concentration of at least about 10 18 carriers/ cm .
  • the thickness is not critical for substrate 22, but might be between about 1 and about 500 urn.
  • Layer 24 is formed on the upper surface of layer 22 and is a layer of GaAs suitably doped with p-dopants to a carrier concentration of about 10 -10 carriers/
  • the thickness of layer 24 depends upon the minority carrier diffusion length and absorption coefficient, with a typical range for GaAs of from about 1 to about 5 um.
  • Layer 26, formed from n + GaAs, is epitaxially deposited upon layer 24.
  • Layer 26 may be formed from
  • GaAs suitably doped with n-dopants, such as sulphur, selenium or silicon to a carrier concentration of at
  • OMPI least about 10 17 carriers/cm 3 . It is critical to limit the thickness of layer 26 to one which allows sig ⁇ nificant carrier generation within layer 24. Thus, layer 26 would typically be limited to a maximum thickness of 1500 A, and preferably less. Great care is necessary to assure that such thin layers are uniform, and not all deposition techniques are suitable. The techniques believed to be suitable include chemical vapor deposition, molecular beam epitaxy, liquid phase epitaxy and ion beam implantation. In addition, if care is taken to dope the top layer 26 to a sufficiently high concentration, an ohmic contact can be formed on the surface without degrading junction character- istics.
  • the n /p/p structure has significant advantages over previously employed structures.
  • the p+ layer forms a back surface field junction with the p layer to provide high efficiency current collection.
  • the high doping level in the p + layer also simplifies the application of an ohmic contact thereto and this high doping level also allows the shallow- homojunction cell to be formed on a different sub- strate material which reduces the cost or provides other advantages.
  • the heavily doped p layer allows tunneling between any heterojunction formed between dissimilar materials thereby making ohmic contact feasible.
  • the n + top layer reduces the series resistance of solar cells having this structure. It also sim ⁇ plifies the formation of an ohmic contact to the cell because of its high doping level.
  • the p layer also provides a major advantage in the n+//p/.p+ structure.
  • Gr " ea 9 te " r cell efficiencies are possible with this structure compared to other shallow homojunction structures, such as p /n, because of the greatly increased diffusion length of minority carriers, i.e., electrons, in a p layer compared to the diffusion length of minority carriers in an n layer, i.e., holes.
  • n /p/p thus provides a shallow-homojunction device which can be manufactureed inexpensively, is capable of pro ⁇ viding high efficiencies, can be deposited on sub- ' strates formed from different materials, and has outstanding resistance to degradation by electron bombardment which is a severe problem encountered in space applications.
  • Top layer 28 is an antireflection coating which reduces the reflection of GaAs and thus increases absorption of solar energy.
  • the antireflection coating might be, for example, successive layers of transparent materials having relatively high and relatively low indices of refraction, respectively.
  • an antireflection coating might be prepared by electron-beam evaporation of titanium dioxide and magnesium fluoride. It is particularly preferred to apply the anti- reflection coating by anodization. Application of anodic coatings can be done without any vacuum processing and is an inexpensive way of producing excellent antireflection coatings. In addition, the application of an anodic layer necessarily reduces the thickness of the top GaAs layer.
  • an anodic layer typically reduces the thickness of the GaAs layer at the rate of about 2/3 part of the 5 volume of GaAs to one part by volume of anodic layer.
  • the thickness of the antireflection layer would be based upon quarterwave theory.
  • Anodic coatings can be formed by employing the device as the anode in an electrolytic cell. By proper selection and control over the cell parameters,' including the electrolyte and voltage applied, thin uniform anodic coatings can be formed.
  • Suitable electrolytes are well known and a specific one found to be suitable is a solution formed by mixing 3 grams of tartaric acid into 100 ml of water, adding sufficient NH4OH to adjust the pH to about 6.2, and then adding 250 ml of propylene glycol. The voltage applied should be sufficient to produce the anodic coating in the thickness desired.
  • direct bandgap semiconductors such as InP and CdTe are also suitable. Addition ⁇ ally, the direct bandgap semiconductor layers can be deposited on substrates formed from different materials.
  • FIG. 3 illustrates a shallow-homojunction de ⁇ vice 30 formed on p + germanium substrate 32,
  • the thickness of substrate 32 might be from 0.1 Jim to 500 urn and it might be formed from single crystal Ge " * ⁇
  • GaAs layers are then applied to Ge substrate 32, by chemical vapor deposition or other techniques, to form the desired shallow-homo- junction device from p GaAs layer 22, p GaAs layer 24 and thin n GaAs layer 26, which are similar to layers 22, 24 and 26, respectively, in FIG. 2.
  • Antireflec ⁇ tion coating 28 is subsequently applied over thin n GaAs layer 26 to complete this embodiment.
  • GaAs shallow-homojunction solar cells there is a major cost advantage possible in the manufacture of GaAs shallow-homojunction solar cells when the actual gallium arsenide employed can be minimized by depositing the cell on a substrate of less costly material, such as germanium or silicon.
  • Gallium arsenide solar cells theoretically have higher conversion efficiencies than cells formed from indirect bandgap materials, such as silicon and germanium.
  • gallium arsenide cells poten- tially should be more radiation resistant in space en ⁇ vironments.
  • Germanium for ex ⁇ ample, has higher thermal conductivity than GaAs which is an advantage in heat dissipation. Germanium also has a lower melting point and lower vapor pressure than GaAs, which might allow easier laser crystallization on a substrate such as graphite. Laser crystallized germanium having large grains would provide a good substrate for chemical vapor deposition of GaAs.
  • substrates which are different from the host semiconductor are possible because of the heavy doping of the substrate and the p layer of the device. This permits tunneling to occur around the heterojunction between the substrate and p layer so that it does not act as a barrier. Thus, a good ohmic contact can be formed.
  • Ga 1- ⁇ Al ⁇ As layer 16 in FIG. 1 which serves the function of reducing surface recombina ⁇ tion velocity of carriers generated in laye 14 upon solar irradiation. It should also be con ⁇ trasted with a typical heterojunction between dis- similar materials which is used to create a barrier to current flow in heterojunction devices. Because of these differences in purpose, solar cell 30 il ⁇ lustrated in FIG. 3, and other similar cells', will be referred to herein as shallow-homojunction solar cells formed from direct gap semiconductors deposited on different substrate materials.
  • substrate 32 has been illustrated to be single crystal Ge, other substrates could be employed. Single crystal silicon, for example, could also be employed. In fact, substrate 32 might also be formed from polycrystalline or amorphous materials, including silicon and germanium.
  • the shallow-homojunction solar cell has the structure illustrated in FIG. 3.
  • FIGS. 4(a) and 4(b) a typical application of gold contacts is illustrated.
  • the thin n + layer 26 is first anodized to form anodic coating 28 while simultaneously thinning n + layer 26.
  • the anodization potential can be set to achieve the appropriate thickness for the antireflee 4 -ion coating and n layer.
  • a photoresist mask 30 is then placed over anodic coating 28 and finger openings 32 (FIG. 4 [a]) are etched through anodic coating 28 employ ⁇ ing an etch such as dilute hydrochloric acid.
  • Gold contacts 34 are then electroplated onto n layer 26 through photoresist mask 30.
  • Photoresist mask 30 is then removed by dissolving it in acetone to produce the device of FIG. 4 (b) .
  • FIGS. 5(a) and 5(b) The application of tin contacts is illustrated in FIGS. 5(a) and 5(b).
  • Photoresist mask 30 is directly applied to thin n + layer 26 and tin con ⁇ tacts 36 are electroplated onto layer 26 through mask 30 (FIG. 5[a] ) .
  • Photoresist mask 30 is then removed and the thin n layer 26 is anodized (FIG. 5 [b]) .
  • the thin n layer 26 remains thicker under tin contacts 36 than under the remainder of anodic coating 28.
  • layer 26 has raised shoulders 38 directly beneath tin contacts 36 as can be seen in FIG. 5(b) .
  • tin contacts are also advantageous for optimizing the n + layer thickness since the anodic oxide formed on gallium arsenide can be stripped with dilue HC1 and the cell reanodized without removing the contacts. Because the thickness of the oxide layer is very uniform and easily controlled by adjusting the anodizing voltage, a series of alternating anodization and stripping steps can therefore be used for con- trolled reduction of n + layer thickness. The thickness of the gallium arsenide removed during each anodization can be accurately determined by using elliposometry to measure the anodic oxide thickness and multiplying this value by an appro- priate factor.
  • Some anodic antireflection coatings may be somewhat unstable in harsh environments. If this is a problem, it can be overcome by application of a thin (e.g., 100 A), transparent, protective coating of a material such as Si0_ or phosphosilicate glass. Such protective coatings can be applied by pyrolytic deposition techniques.
  • FIG. 6 is a cross-sectional view illustrating one finger of a solar cell having such a protective SiU2 coating 40.
  • Device fabrication is similar to that described above for FIGS. 4(a) and 4(b), except that a hydrofluoric acid etch is employed prior to the hydrochloric acid etch.
  • Contact finger 34 can be formed from gold plated to a thickness of about 4 p .
  • the back contact 42 can also be formed from plated gold.
  • the Si0_ protective, layer was described as being applied prior to contact formation, it could also be applied after the con ⁇ tacts have been formed.
  • any metal could be employed including gold, silver, platinum, tin, aluminum, copper, etc.
  • those metals can be used which form a sufficiently thick oxide layer during anodization such that current leakage through metal contacts is reduced enough to allow the semiconductor surface to be anodized.
  • Tin, aluminum and copper are examples.
  • Devices of this invention have at least one n-p homojunction and at least one other junction sufficient to increase current collection.
  • This other junction requires an impurity profile where ⁇ in the majority carriers all have the same charge and wherein an electrical field is created by the impurity profile which aids in collecting minority carriers.
  • the p layer also allows the use of substrate materials other than the host semi ⁇ conductor material. Specific examples of suitable junctions include high/low homojunctions and graded profile junctions where the impurity doping level increases with distance from the n-p junction.
  • GaAs layers were grown in an sCl 3 -Ga-H 2 system.
  • the reactor tube had an inner diameter of 55 mm, and the H 2 flow through the AsCl ⁇ evaporator and
  • OMPI over the Ga boat was in the range 300-500 cm /min.
  • the p and n dopants were introduced in the vapor phase by using (C2H5) 2 Zn and H ⁇ S, respectively.
  • the reactor tube was vertical, allowing rotation of the substrate, which resulted in greater doping uniformity in the layers.
  • Use of high purge flows allowed the reactor tube to be opened at the bottom to load and unload substrates without losing the H 2 atmosphere inside the tube.
  • the furnace could remain at growth temperature during the loading procedure, decreasing the cycle time between runs.
  • the substrate Once inside the reactor tube, the substrate could be preheated in pure H 2 just before being introduced into the reactant gas flow at the growth position.
  • n + layer 18 3 carrier concentration of 10 carriers/cm ,followed by a thin n + layer.
  • the sheet resistance of the n + layer was 70 .
  • the I-V characteristic between two ohmic contacts to the layer was measured while a channel was being etched between the contacts. When the I-V char ⁇ acteristic for back-to-back diodes was observed, etching was immediatedly stopped, and the channel depth was measured with a profilometer.
  • the n + layer thickness measured by this technique was 1300A!
  • the initial fabrication step following layer growth was the pyrolytic deposition of Si0 2 glass
  • Openings for ohmic contact fingers were etched in the glass coating using photolitho- graphic techniques. There were 10 openings, 0.5 cm long and 12 um wide, spaced 1 mm apart. The wafer was sputter-etched to remove GaAs to a depth of 40 A in the finger openings, then sputter-coated with successive layers of Au- 12% Ge (300 A) and Au (2000 A). The Au/AuGe film was defined photolithographically into 25-um-wide fingers, interconnected at one end, that overlaid the openings in the glass.
  • the contact fingers to the n + layer were electroplated with Au to a thickness of 4 um.
  • the back contact to the ⁇ + substrate was made with sputtered Au.
  • the active area of the cell was de ⁇ fined by etching a 1 cm x 0.5 cm rectangular mesa in the GaAs, and the glass layer was removed with buffered HF.
  • the fingers of the cell at this stage had a cross sectional configuration similar to that illustrated in FIG. 6, except that there was no anodic layer 28.
  • the cell was antireflec- o tion-coated with successive layers of SiO (700 A)
  • the average reflectivity measured over the 0.5-0.9 um wavelength band was less than 5%.
  • An efficiency measurement of the cell was made by using a high-pressure Xe lamp with a water filter as a simulated AM 1 solar source. The incident intensity was adjusted to 100mW/cm-2, using a, NASA standard Si solar cell, calibrated for AM 1, as a reference. The open-circuit voltage was 0.91 V, the short-circuit current 10.3 mA, and the fill factor 0.82, giving a measured conversion effi ⁇ ciency of 15.3%. When the contact area is subtracted, the corrected efficiency is 17%. The n factor at 100 mA/cm ⁇ is 1.25, as obtained from the dark I-V characteristic, indicating good material quality with long carrier diffusion lengths. The series resistance is 0.5 ⁇ .
  • the quantum efficiency of this cell as a func- tion of wavelength is shown in FIG. 7, and, as can be seen, quantum efficiency is highest at the longer wavelengths, with a gradual decrease at shorter wavelengths.
  • This cell was fabricated by sputtering and alloying techniques, which although possible because of the relatively thick n layer, are not preferred.
  • EXAMPLE 2 GALLIUM ARSENIDE SHALLOW-HOMOJUNCTION PHOTOVOLTAIC DEVICE HAVING ANODIC ANTIREFLECTION COATING
  • a photovoltaic device was prepared as in Example 1 except that the antireflection coating was an anodic coating, and all ohmic contacts were electro- - plated.
  • the GaAs layer used was grown in an AsCl3-Ga-H 2 CVD system on p + Zn-doped (100)-oriented substrate, with a carrier concentration of 10 18 cm "3 .
  • a p layer about 2 jam thick was first grown on the substrate followed by an n + layer (n-'S x lO ⁇ cm ""3 ) were doped with Zn and S, respectively, by using (C2H5)2 Zn and H 2 S sources.
  • the n + layer was anodically oxidized as follows.
  • the electrolyte solution used for anodization was prepared by mixing 3 g of tartaric acid with 100 ml of H ⁇ O, adding sufficient NH4OH to adjust the pH to about 6.2, and then adding 250 ml of propylene glycol. The final pH was 4.6-5.8.
  • Anodization of the GaAs was performed at room temperature, using a platinum wire as cathode.
  • a smooth anodic layer of uniform thickness was obtained by using a constant current source with a voltage limiter. The source was set at a current corresponding to a current density of about 750 for the GaAs anode, and the maximum output voltage was set at about 43 V.
  • the current initially remained con ⁇ stant until the voltage increased to its limiting value, after which the voltage remained constant and the current decreased. Anodization was termin ⁇ ated when the current fell to one-tenth of its ini ⁇ tial value.
  • the thickness (measured by ellipsometry using a He-Ne laser) of the anodic layer was about 20 A/V and did not depend strongly on current density. The layer produced took less than 5 minutes, was about o o
  • the anodic layer was stable up to at least 250°C in air.
  • the optical constants were mea- o sured by ellipsomtery at 4358 and 5461 A using a Hg o lamp and at 6328 A using a He-Ne laser.
  • the values of the refractive index n at these wavelengths are 1.91, 1.85 and 1.83 respectively, as shown in the inset of FIG. 8.
  • the values of extinction coefficient k are very low, and for the thickness of anodic layers used, absorption of the optical constants (and the effectiveness of the antireflection coating) is illustrated in FIG.
  • a layer of Au about 3 um thick was then electro ⁇ plated on the p + substrate as the back contact.
  • Photoresist AZ 1350J was spun on the anodic layer, and photolithographic techniques were used to etch openings for ohmic contact fingers in the anodic layer. (The anodic layer dissolves readily in AZ photoresist developer, as well as in HC1.) There were 10 openings, 0.5 cm long and 12 um wide, spaced 1 mm apart and interconnected with a bar at one end.
  • a layer of Au about 3 um thick was then electroplated into the openings, and the wafer was annealed in N2 for 1 sec at 300°C on a graphite heater strip to produce ohmic contact between the Au fingers and the n + layers.
  • the active area of the cell was defined by etching the GaAs to form a 1-cm x 0.5 cm rectangular mesa.
  • OMPI Efficiency measurements using a high-pressure Xe lamp with a water filter as a simulated AM 1 . source, were made.
  • the incident intensity was ad ⁇ justed to 100 mW/cm 2 , using a NASA-measured GaAs solar cell as a reference.
  • the cell was also measured on the roof of the laboratory at an ambient temperature of about 20°C.
  • the solar flux density measured with a pyranometer was 98 mW/cm 2 , close to AM 1 conditions.
  • the open-circuit voltage was found to be 0.97 V, the short-circuit current 25.6 mA/cm 2 , and the fill factor 0.81, giving a measured conversion efficiency of 20.5 per cent, without correcting for the area of the contact fingers.
  • the quantum efficiency of this cell as a function of wavelength is shown in FIG. 9. The quantum efficiency exceeds 90 percent at the maximum, but it decreases quite strongly at shorter wavelengths.
  • Example 1 The growth procedures and apparatus of Example 1 were used, except as noted.
  • the Ge substrates were oriented (100) 2° off toward (110) and were prepared by coating them with Si0 2 on the backside to reduce Ge autodoping of the GaAs layers during growth.
  • the electron concentration in nominally undoped layers deposited on these coated substrates was 5 x 10 15 c ⁇ tf .
  • the doping profile used for the solar cells is shown in FIG. 10.
  • the p + Ge substrate was highly doped with Ga (8 x 10- ⁇ - ⁇ cm " ) in order to overdope any As that might diffuse into the Ge during the deposition of the GaAs and to assure tunneling through any thin barriers which could arise at the heterojunction interface.
  • the p + GaAs buffer layer was highly doped with Zn, again to assure tunneling and also to overdope Ge diffusing into the GaAs during growth.
  • the change in hole con ⁇ centration from 5 x 10 18 cm ⁇ 3 in the buffer layer to 1 x 10 cm in the active layer provided a backsurface field to increase the collection effi ⁇ ciency.
  • the n + layers were doped to a carrier con- centration of 5 x lO ⁇ cm"" 3 with sulfur, had a sheet resistivity in the range 45-100 tyO, and their electron mobility was ' -'lOOO cm /V-sec.
  • An AR coating was produced on the n + layer by anodic oxidation, which consumed a thickness of GaAs equal to 0.66 times the thickness of oxide produced.
  • the anodizing solution was prepared by adding 3 g tartaric acid to 100 ml H 2 0, adjust ⁇ ing the pH to 6.2 with NH 4 0H, and adding 250 ml propylene glycol.
  • the thickness of the oxide layer was proportional to the limiting voltage used for anodization.
  • the thickness required o for an optimum AR coating was 850 A, which was obtained for a limiting voltage of 43V.
  • Elec ⁇ troplated Au formed ohmic contacts with a spec ⁇ ific resistance of 8 x 10 ⁇ 5 A-cm 2 .
  • Electroplated Sn also formed ohmic contacts, although their resistance was not measured. Sn had the advan-
  • the n + layer was thicker under the Sn contacts than under the anodic oxide, so that there was a larger separation of the metal from the p-n junction than with Au contacts. Because the n + layer is so thin, the increased separation was believed to be better. for device yield and reli ⁇ ability.
  • a mesa etch of the GaAs was used to define the active area of the cells, and the back contact to the Ge substrate was made by Au plating. No alloy- ing or vacuum processing was used in cell fabrica ⁇ tion.
  • n + layer thickness Measurements of spectral response as a function of n + layer thickness were made on small cells, 0.05 cm 2 in area, having two Sn contact fingers 0.5 mm apart connected to a Sn bar at one end.
  • the n + layer which was initially 2000A thick with a sheet resistance of 45 St/U, was thinned by alter ⁇ nate anodization and stripping.
  • the external quantum efficiency which is the ratio of the number of carriers collected (I /q) to the number of incident photons, was measured after each of three anodizations at 43 V, so that the cells were antireflection-coated during each measure ⁇ ment.
  • the values of I sc and incident photon flux were measured as a function of wavelength in a spectrometer which was arranged so that all tHe light fell between the two contact fingers.
  • the results for cell 1 are given in FIG. 11, which shows that thinning the n + layer results in a marked improvement in quantum efficiency, espec ⁇ ially at shorter wavelengths. This is expected because of the high absorption coefficients for GaAs (10 4 -10 5 cm -1 ) , which increase with decreas ⁇ ing wavelength, and the high surface recombina- tion velocity, which is believed to be around 10 cm/sec.
  • the power conversion efficiency for each n + layer thickness is also given in FIG. 11. These values were measured with the cell fully illuminated by a simulated AMI source, with no correction made for the finger area.
  • FIG. 12 shows the final spectral response of cell 2, which was fabricated next to cell 1.
  • the n + layer was slightly thinner than that of cell 1, and the response was therefore slightly improved at the short wavelengths.
  • the curve for internal quantum efficiency which is the ratio of I c ./q to the rate at which photoms enter the semiconductor, was obtained from the measured external efficiency by correcting for the spectral reflectivity of the AR-coated cell. This curve indicates that the cell design is very near the optimum.
  • FIG. 13 The AMI power efficiencies of cells 1 and 2 and two other small cells fabricated side by side on the same wafer are plotted in FIG. 13 as a function of n + layer thickness.
  • cells 2, 3 and 4 were first anodized to 10, 20 and 30 V, respect ⁇ ively, after which the oxide was stripped. All 4 cells were then anodized together to 43V to pro ⁇ vide an AR coating, stripped, anodized to 43 V, stripped again and once more anodized to 43 V. Efficiency measurements were made after each 43 V anodization. After the third such anodization, the efficient of cell 3 had dropped to 10% while cell 4 had essentially no sensitivity to light.
  • Table 1 lists the measured values of V , I sc , fill factor and efficiency, as well as the initial sheet resistance and the total anodization voltage.
  • the sheet Re ⁇ sistance value gives some indication of the initial thickness of the n + layer; a value of 100 S ⁇ O corres- o ponds to approximately 1200 A.
  • the total anodization voltage is the sum of the limiting voltages used in a series of anodization-strip steps where the thin- t ⁇ ing ratio . is 20 A/V. For each cell the final ano ⁇ dization was carried out at 43 V to provide the AR coating.
  • the conversion efficiency values, not corrected for contact areas, are all in the 17-20% range.
  • a GaAs shallow-homojunction solar cell prepared according to Example 1 and having a thin ⁇ layer of about 1000A without an antireflection coating thereon was subjected to electron fluences of 0, 8 x 10 , 3.2 x 10 15 and
  • FIG. 14 is a plot of I /(I sc ) 0 here sc )o is the original short circuit current before electron bom ⁇ bardment.
  • I the short circuit current
  • FIG. 15 illustrates corresponding data for a GaAs shallow-homojunction solar cell having a o thin n + layer of about 1400 A with an anodic AR- coating under electron fluences ranging from 5 x 10 to 7 x 10 electron/cm .
  • This cell was also measured under simulated AM 0 conditions at the above range of dosages.
  • short circuit current (I ) decreased by about 20% at 7 x 10 15 electron/cm 2 , the open circuit voltage
  • V also decreased gradually.
  • the fill factor, oc ff actually increased slightly before it began to decrease with increasing fluences.
  • the conversion efficiency * h of the cell which was about 14% at AM 0 before electron irradiation, still had over 60%. of its original efficiency after being bombarded with
  • This invention has industrial applicability in the fabrication of solar cells, particularly solar cells for use in space applications.

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Abstract

Ameliorations aux cellules solaires a homojonction superficielle formees par une pluralite de couches de materiau semi-conducteur a espace d'interaction directe tel que GaAs, ainsi que leur fabrication. Les cellules solaires a homojonction superficielle possedent une structure n+/p/p+ (26, 24, 22) dans laquelle la couche superieure n+ (26) est limitee en epaisseur, ce qui permet d'avoir une activite porteuse importante au niveau de la couche semi-conductrice inferieure (24). Un revetement antireflechissant (28) est applique sur la couche superieure n+ (26), application realisee, de preference, par traitement anodique. Ces cellules solaires peuvent etre produites sur des substrats relativement peu couteux tels que le silicium ou le germanium.
PCT/US1979/000181 1978-03-22 1979-03-21 Cellules solaires a homojonction superficielle WO1979000813A1 (fr)

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WO1987001513A2 (fr) * 1985-09-09 1987-03-12 Hughes Aircraft Company Systeme de cellules solaires a l'arseniure de gallium
US4753683A (en) * 1985-09-09 1988-06-28 Hughes Aircraft Company Gallium arsenide solar cell system
US5286698A (en) * 1989-04-25 1994-02-15 University Of Notre Dame Du Lac Metal oxide catalysts

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US10154923B2 (en) 2010-07-15 2018-12-18 Eyenovia, Inc. Drop generating device

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US3982265A (en) * 1975-09-19 1976-09-21 Bell Telephone Laboratories, Incorporated Devices containing aluminum-V semiconductor and method for making
US4128843A (en) * 1977-10-14 1978-12-05 Honeywell Inc. GaP Directed field UV photodiode

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US3982265A (en) * 1975-09-19 1976-09-21 Bell Telephone Laboratories, Incorporated Devices containing aluminum-V semiconductor and method for making
US4128843A (en) * 1977-10-14 1978-12-05 Honeywell Inc. GaP Directed field UV photodiode

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Conference Record, Eleventh IEEE Photovoltaic Specialists Conference, issued 1975, J. MANDELKORN et al, Advances in The Theory And Application of BSF Cells, pages 36-39. *
Conference Record, Thirteenth IEEE Photovoltaic Specialists Conference, issued 1 September 1978, J.C.C. FAN et al, High-Efficiency GaAs Shallow-Homojunction Solar Cells, pp 953-955. *
Conference Record, Twelfth IEEE Photovoltaic Specialists Conference, issued 1976, H.J. HOVEL et al, Improved GaAs Solar Cells with Very Thin Junctions, pages 945-947. *
IBM Technical Disclosure Bulletin, Vol. 19, No. 6, issued November 1976, H.J. HOVEL et al, High-Efficiency Ga1-xA1x AS-GaAS Solar Cells, page 2289. *
IBM Technical Disclosure Bulletin, Vol. 19, No. 7, issued December 1976, H.J. HOVEL et al, Anodized GaAs Solar Cells, page 2808. *
See also references of EP0011629A4 *

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1987001513A2 (fr) * 1985-09-09 1987-03-12 Hughes Aircraft Company Systeme de cellules solaires a l'arseniure de gallium
WO1987001513A3 (fr) * 1985-09-09 1987-05-21 Hughes Aircraft Co Systeme de cellules solaires a l'arseniure de gallium
US4753683A (en) * 1985-09-09 1988-06-28 Hughes Aircraft Company Gallium arsenide solar cell system
US5286698A (en) * 1989-04-25 1994-02-15 University Of Notre Dame Du Lac Metal oxide catalysts

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SE7909584L (sv) 1979-11-20
JPS55500168A (fr) 1980-03-27
EP0011629A4 (fr) 1980-10-17
CA1137604A (fr) 1982-12-14
EP0011629A1 (fr) 1980-06-11

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