WO1994027328A1 - Process for at least partially converting semi-conductor films of the i-iii-vi2-type - Google Patents

Process for at least partially converting semi-conductor films of the i-iii-vi2-type Download PDF

Info

Publication number
WO1994027328A1
WO1994027328A1 PCT/US1994/004245 US9404245W WO9427328A1 WO 1994027328 A1 WO1994027328 A1 WO 1994027328A1 US 9404245 W US9404245 W US 9404245W WO 9427328 A1 WO9427328 A1 WO 9427328A1
Authority
WO
WIPO (PCT)
Prior art keywords
film
compound semiconductor
additional element
iii
semiconductor
Prior art date
Application number
PCT/US1994/004245
Other languages
French (fr)
Inventor
James H. Ermer
Dale E. Tarrant
Original Assignee
Siemens Solar Industries International, Inc.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Siemens Solar Industries International, Inc. filed Critical Siemens Solar Industries International, Inc.
Publication of WO1994027328A1 publication Critical patent/WO1994027328A1/en

Links

Classifications

    • 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
    • H01L31/072—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 heterojunction type
    • H01L31/0749—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 heterojunction type including a AIBIIICVI compound, e.g. CdS/CulnSe2 [CIS] heterojunction solar cells
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
    • C30B29/10—Inorganic compounds or compositions
    • C30B29/46—Sulfur-, selenium- or tellurium-containing compounds
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B33/00—After-treatment of single crystals or homogeneous polycrystalline material with defined structure
    • 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
    • H01L31/0248—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 characterised by their semiconductor bodies
    • H01L31/0256—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 characterised by their semiconductor bodies characterised by the material
    • H01L31/0264—Inorganic materials
    • H01L31/032—Inorganic materials including, apart from doping materials or other impurities, only compounds not provided for in groups H01L31/0272 - H01L31/0312
    • H01L31/0322—Inorganic materials including, apart from doping materials or other impurities, only compounds not provided for in groups H01L31/0272 - H01L31/0312 comprising only AIBIIICVI chalcopyrite compounds, e.g. Cu In Se2, Cu Ga Se2, Cu In Ga Se2
    • 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
    • Y02E10/541—CuInSe2 material PV cells

Definitions

  • the present invention relates generally to a process for forming compound semiconductors and, more particularly, to a process for at least partially converting compound semiconductor films of the I-III-VI 2 type to semiconductors of the same type with different composition.
  • Thin film I-III-VI 2 absorbers such as CuInSe 2 are promising materials for high power, low cost photovoltaic applications. Because bandgaps for I-III-VI 2 absorbers range from below one electron volt (ev) for CuInTe 2 to above 2.5 ev for AgGaS 2 , it is possible to optimize such materials by carefully controlling their chemical compositions. Both the bandgap and lattice constants of such materials can be adjusted by changing the relative amounts of their constituent elements, permitting I-III-VI 2 absorbers to be used in both single junction and tandem junction photovoltaic modules. The concept of controlling the makeup of a I-III-VI 2 absorber by varying its chemical makeup is disclosed in the following publications: N. Roy, et al.
  • composition of a "multinary" semiconductor i.e., a compound semiconductor having at least three constituent elements
  • the composition of the compound semiconductor is determined by the materials included at the time the compound is initially formed.
  • substitution of one constituent element for another in the process of formation can have a deleterious effect on certain properties of the resulting film, including its grain structure and uniformity.
  • the inclusion of relatively high amounts of sulfur in copper indium diselenide and other multinary compounds tends to make the deposited films nonuniform over their surface. While the reasons for this are not entirely understood, the problem may result from differences in the crystal growth mechanisms caused by the addition of sulfur. Therefore, it is desirable in many applications to provide a process for forming multinary semiconductor compounds which are uniform over a large area and have improved physical structures.
  • the process of the present invention is capable of producing multinary semiconductors having a wide variety of constituents and composition profiles, including graded absorber regions, with high film uniformity. It is possible according to the invention to convert part or all of what will later be the absorber layer without affecting the remainder of the semiconductor film. Thus, the advantageous properties of a sulfur-rich region can be obtained on a selective basis in the absorber.
  • the foregoing advantages are achieved by: depositing or otherwise providing a film including a first compound semiconductor having constituent elements chosen from each of the chemical groups I, III and VI; exposing at least one surface of the film to a substance containing at least one additional element chosen from the chemical groups I, III and VI; and heating the film to a preselected temperature in the presence of that substance to cause the additional element to be exchanged for at least a portion of one or more of the constituent elements of the film; whereby the first compound semiconductor is at least partially converted to a second compound semiconductor of different composition.
  • the additional element can be a group I element, such as silver, a group III element, such as gallium; a group VI element, such as sulfur, or a combination of such elements.
  • the additional element is introduced in the form of a gas.
  • the additional element can be sulfur introduced in the form of hydrogen sulfide gas.
  • An important feature of the present invention is the fact that conversion takes place after the first I-III-VI 2 - type compound semiconductor film is formed. Additional elements enter the film during heat treatment and substitute themselves for constituent elements of the same chemical group, altering the composition and material properties of the affected region.
  • the result of the process can be a partial conversion to a given depth of the film, possibly resulting in a quaternary or other multinary compound, or may extend throughout the film such that the film is fully converted to a different semiconductor material.
  • the limited conversion process is particularly well-suited for creating graded absorber layers which increase the open circuit voltage of a resulting photovoltaic cell without unduly reducing short circuit current.
  • FIGURE 1 is a schematic representation of a photovoltaic cell constructed from a I-III-VI 2 -type semiconductor film formed in accordance with an illustrative embodiment of the present invention
  • FIGURE 2 is schematic representation of a portion of the photovoltaic cell of FIGURE 1 in the process of formation
  • FIGURE 3 is a simplified block diagram of an illustrative form of the process of the present invention
  • FIGURE 4 is a graph showing the results of an Auger depth profile of an absorber structure of the type illustrated in FIGURE 2, in which the composition of the absorber is graded to achieve a high efficiency
  • FIGURE 5 is a graph of the current voltage characteristic of a photovoltaic cell of the type illustrated in FIGURE 1, including calculated values for open circuit voltage (V oc ) , short circuit current (J sc ) , fill factor (FF) and efficiency (Eff)
  • FIGURE 6A is a graph illustrating the relationship between efficiency and sulfur content (in percent) for a photovoltaic cell of the type depicted in FIGURE 4;
  • FIGURE 6B is a graph showing the dependence of open circuit voltage (V oc ) as a function of relative sulfur content
  • FIGURE 6C is a graph showing the dependence of short circuit current (J sc ) on relative sulfur content.
  • FIGURE 7 is a graph of the spectral response of a copper-multinary structure having a graded absorber, as compared to copper-multinary devices having homogeneous high and low bandgap absorbers, respectively.
  • a photovoltaic device 10 constructed according to an illustrative embodiment of the present invention has a p- type compound semiconductor layer 12 made up of a bulk region 14 and a converted surface region 16.
  • the converted surface region 16 includes part or all of the "absorber" region (not separately shown) of the semiconductor layer 12, and forms a heterojunction 18 with an n-type semiconductor layer 20, such as cadmium sulfide (CdS) or zinc oxide (ZnO) .
  • the compound semiconductor layer 12 is formed on a back contact 22 which is a thin film of molybdenum (Mo) or other suitable metal deposited on a substrate 24.
  • Contact to the n-type layer 20 is made by a transparent conductive oxide layer 26 and, in some device configurations, a plurality of conductive grid lines 28.
  • light (hv) enters the device through the transparent oxide layer 26 and passes through the n-type layer 20 and into the p-type compound semiconductor layer 12. Virtually all of such light is absorbed within the absorber portion of the p-type layer 12, which extends up to about 10,000 angstroms into the p-type layer.
  • the "converted" surface region can extend through part or all of the absorber region, as described in more detail below.
  • the light absorbed in this region generates free carriers which are separated by the built-in electric field resulting from the p-n heterojunction 118, causing current to flow through an external circuit connected between the back contact 22 and the conductive grid lines 28.
  • the basic components of the photovoltaic device 10 and known methods for fabricating them are described in U.S. Patent No. 4,798,660, issued January 17, 1989 to Ermer et al., and U.S. Patent No. 5,045,409, issued September 3, 1991 to Eberspacher et al., the specifications of which are incorporated herein by reference for all purposes.
  • FIGURE 2 illustrates a portion of the photovoltaic device 10 at the stage in its fabrication when the surface region 16 of the semiconductor layer 12 is converted to a different compound semiconductor by the process of the present invention.
  • the p-type layer 12 is formed on the back electrode 22 prior to conversion. This can take place either just prior to conversion or far in advance of it. Either way, the upper surface of the layer 12 is eventually exposed to a substance containing at least one additional element chosen from the chemical Groups I, III and VI and heated to a preselected temperature in the presence of the substance to cause at least a portion of one of the constituent elements of the original layer to be displaced by the additional element.
  • the number of atoms entering the layer 12 and the depth to which they penetrate are dependent on the characteristics of the additional element and the substance containing it, as well as the concentration of the substance and the temperature and duration of the heat treatment step.
  • the entire layer 12 can be converted by the process of the invention, if desired, it is desirable in most cases to convert something less than the entire absorber region. This creates a graded concentration profile which provides a high bandgap and the attendant high open circuit voltage (V oc ) .
  • the conversion may also neutralize "dangling bonds" at the surface region in much the same way that hydrogen passivates dangling bonds in thin film silicon-containing alloys.
  • the process of the invention is illustrated in its simplest form in the flow chart of FIGURE 3, wherein the I- III-VI 2 semiconductor layer 12 is deposited at S10, is exposed at its upper surface to an additional Group I, III or VI element, such as gallium or sulfur, at S12, and is annealed in the present of the additional element at S14 to exchange the additional element with the corresponding constituent element of the original film.
  • the step S10 can include formation of the I-III-VI 2 semiconductor just before the exchange takes place, or the original layer can be formed well in advance of the conversion.
  • Step S12 can be accomplished by exposure of the film to a gas, such as hydrogen sulfide, which contains the additional element.
  • the additional element can be applied in any other convenient way, such as by depositing a layer of solid material on the upper surface of the semiconductor layer 12, exposing the film to a vapor of the material itself, or applying an organic compound, such as a mercaptan, in the case of sulfur. All that is required is that the element be available to enter the film in place of one of the constituents of the original film when the composite structure is subjected to heat at S14.
  • the concentration of sulfur in the atmosphere is preferably between 5 percent and 50 percent of the gaseous atmosphere, the remainder being nitrogen, argon or other suitable carrier gas.
  • the concentration of hydrogen sulfide is most preferably approximately 30 percent for purposes of the invention.
  • the temperature of conversion is preferably at least 450 degrees in order to dissociate the hydrogen sulfide gas and release sulfur into the layer 12, and more preferably between approximately 475 and 550 degrees C.
  • the sulfidation process typically lasts between 5 and 50 minutes, and most preferably approximately 20 minutes. Under these circumstances the depth of sulfur within the layer 12 is typically between 100 and 5000 angstroms, most preferably approximately 1500 angstroms, to provide a graded bandgap structure.
  • FIGURE 4 illustrates an Auger depth profile of the structure of FIGURE 2 after partial conversion of the layer 12.
  • the time designated "0" at the left hand side of the graph indicates the concentrations of elements at the upper surface of the region 12, as detected by Auger analysis. It can be seen that the concentrations of sulfur and selenium are approximately equal, making the material a "sulfoselenide" at its surface. Extending downwardly from the surface (moving from left to right on the time scale of FIGURE 4) , the sulfur concentration is graded rapidly down and the selenium concentration increases rapidly until the sulfur signal essentially disappears (approximately 20 minutes into the analysis) .
  • FIGURE 5 is the measured current-voltage characteristic of the graded cell of FIGURE 4, in which the open circuit voltage (V oc ) and the short circuit current (J sc ) yield an efficiency of 15.1 percent. The significance of this in comparison to a similar cell containing sulfur homogeneously throughout its thickness is illustrated in the graph of
  • FIGURE 7 shows that the graded sulfur device has a quantum efficiency similar to that of a cell that does not contain sulfur, and much greater than a homogeneous Cu(InGe) (Se, S 2 ) cell.
  • The" optimal sulfur content within the absorber region for a particular temperature of heat treatment can be obtained from FIGURES 6A, 6B and 6C, which illustrate the efficiency, open circuit voltage and short circuit current, respectively, at relative sulfur contents ranging from approximately 25 percent up to 100 percent. It should be noted in connection with these graphs, however, that the sulfur content for these purposes is defined as the concentration of sulfur in an absorber normalized to the absorber with the highest measured sulfur concentration. Thus, 100 percent total sulfur concentration does not represent stoichiometric CuInS 2 . Keeping this in mind, FIGURES 6A-6C show that the optimal relative sulfur content is between 40 and 55 percent.
  • the process of the present invention is applicable to the formation of a wide variety of I-III-VI 2 -type semiconductor compounds, including by way of example, compounds in which the group I element is copper and/or silver, the group III element is indium and/or gallium, and the group VI element is selenium and/or sulfur.
  • Such compounds can also include one or more other elements, such as those of chemical group IV, in addition to the group I, III, VI components without departing from the scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.

Landscapes

  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • General Physics & Mathematics (AREA)
  • Physics & Mathematics (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Power Engineering (AREA)
  • Microelectronics & Electronic Packaging (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • Electromagnetism (AREA)
  • Inorganic Chemistry (AREA)
  • Computer Hardware Design (AREA)
  • Sustainable Energy (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Photovoltaic Devices (AREA)

Abstract

A compound semiconductor film of the I-III-VI2 type is exposed to a substance containing at least one additional element chosen from the chemical groups I, III and VI and is heated to a preselected temperature in the presence of the substance to exchange the additional element for at least a portion of one or more of the original constituent elements of the film. The result is a second compound semiconductor of the same type but having a different composition.

Description

PROCESS FOR AT LEAST PARTIALLY CONVERTING SEMI-CONDUCTOR FILMS OF THE I-III-VI2 TYPE
BACKGROUND OF THE INVENTION The present invention relates generally to a process for forming compound semiconductors and, more particularly, to a process for at least partially converting compound semiconductor films of the I-III-VI2 type to semiconductors of the same type with different composition.
Thin film I-III-VI2 absorbers, such as CuInSe2, are promising materials for high power, low cost photovoltaic applications. Because bandgaps for I-III-VI2 absorbers range from below one electron volt (ev) for CuInTe2 to above 2.5 ev for AgGaS2, it is possible to optimize such materials by carefully controlling their chemical compositions. Both the bandgap and lattice constants of such materials can be adjusted by changing the relative amounts of their constituent elements, permitting I-III-VI2 absorbers to be used in both single junction and tandem junction photovoltaic modules. The concept of controlling the makeup of a I-III-VI2 absorber by varying its chemical makeup is disclosed in the following publications: N. Roy, et al. "The Diode Current Mechanism in CuInSe2/(CdZn)S Heterojunctions". Proceedings of the 20th IEEE Photovoltaic Specialists Conference, September 26-30, 1988, pp 1618-1623; J.R. Tuttle, "Experiments on the modification of the Bi-layer Structure in CdS/CuInSe2 Devices", Proceedings of the 20th IEEE Photovoltaic Specialists Conference, 1988, pp 1525-1530; G.B. Turner et al., "Band Discontinuity and Bulk vs. Interface Recombination in CdS/CuInSe2 Solar Cells", Proceedings of the 20th IEEE Photovoltaic Specialists Conference, 1988, pp 1457-1460; R.J. Schwartz, "The Use of CuIn^GaxSez Layers to Improve the Performance of CuInSe2 Cells", Proceedings of the 21st IEEE Photovoltaic Specialists Conference, 1990, pp 570- 574; J.J. Loferski et al., "Investigation of Thin Film Cadmium Sulfide/Mixed Copper Ternary Heterojunction Photovoltaic Cells", 13th IEEE Photovoltaic Specialists Conference, 1978, pp 190-194; J.R. Tuttle et al., "Physical, Chemical and Structural Modifications to Thin-Film CuInSe2- Based Photovoltaic Devices", 22nd IEEE Photovoltaic Specialists Conference, 1991, pp 1062-1067.
The foregoing references describe or suggest that the composition of a "multinary" semiconductor, i.e., a compound semiconductor having at least three constituent elements, can be altered by incorporation of gallium, sulfur or other suitable elements in the absorber layer to raise the bandgap of the material and thereby raise the open circuit voltage of a resulting device. In each case, however, the composition of the compound semiconductor is determined by the materials included at the time the compound is initially formed. Unfortunately, substitution of one constituent element for another in the process of formation can have a deleterious effect on certain properties of the resulting film, including its grain structure and uniformity. For example, the inclusion of relatively high amounts of sulfur in copper indium diselenide and other multinary compounds tends to make the deposited films nonuniform over their surface. While the reasons for this are not entirely understood, the problem may result from differences in the crystal growth mechanisms caused by the addition of sulfur. Therefore, it is desirable in many applications to provide a process for forming multinary semiconductor compounds which are uniform over a large area and have improved physical structures.
SUMMARY OF THE INVENTION
The process of the present invention is capable of producing multinary semiconductors having a wide variety of constituents and composition profiles, including graded absorber regions, with high film uniformity. It is possible according to the invention to convert part or all of what will later be the absorber layer without affecting the remainder of the semiconductor film. Thus, the advantageous properties of a sulfur-rich region can be obtained on a selective basis in the absorber.
The foregoing advantages are achieved by: depositing or otherwise providing a film including a first compound semiconductor having constituent elements chosen from each of the chemical groups I, III and VI; exposing at least one surface of the film to a substance containing at least one additional element chosen from the chemical groups I, III and VI; and heating the film to a preselected temperature in the presence of that substance to cause the additional element to be exchanged for at least a portion of one or more of the constituent elements of the film; whereby the first compound semiconductor is at least partially converted to a second compound semiconductor of different composition. The additional element can be a group I element, such as silver, a group III element, such as gallium; a group VI element, such as sulfur, or a combination of such elements. In one embodiment, the additional element is introduced in the form of a gas. The additional element can be sulfur introduced in the form of hydrogen sulfide gas.
An important feature of the present invention is the fact that conversion takes place after the first I-III-VI2- type compound semiconductor film is formed. Additional elements enter the film during heat treatment and substitute themselves for constituent elements of the same chemical group, altering the composition and material properties of the affected region. The result of the process can be a partial conversion to a given depth of the film, possibly resulting in a quaternary or other multinary compound, or may extend throughout the film such that the film is fully converted to a different semiconductor material. The limited conversion process is particularly well-suited for creating graded absorber layers which increase the open circuit voltage of a resulting photovoltaic cell without unduly reducing short circuit current. BRIEF DESCRIPTION OF THE DRAWINGS The above and other features of the present invention may be more fully understood from the following detailed description, taken together with the accompanying drawings, wherein similar reference characters refer to similar elements throughout and in which:
FIGURE 1 is a schematic representation of a photovoltaic cell constructed from a I-III-VI2-type semiconductor film formed in accordance with an illustrative embodiment of the present invention;
FIGURE 2 is schematic representation of a portion of the photovoltaic cell of FIGURE 1 in the process of formation;
FIGURE 3 is a simplified block diagram of an illustrative form of the process of the present invention; FIGURE 4 is a graph showing the results of an Auger depth profile of an absorber structure of the type illustrated in FIGURE 2, in which the composition of the absorber is graded to achieve a high efficiency; FIGURE 5 is a graph of the current voltage characteristic of a photovoltaic cell of the type illustrated in FIGURE 1, including calculated values for open circuit voltage (Voc) , short circuit current (Jsc) , fill factor (FF) and efficiency (Eff) ; FIGURE 6A is a graph illustrating the relationship between efficiency and sulfur content (in percent) for a photovoltaic cell of the type depicted in FIGURE 4;
FIGURE 6B is a graph showing the dependence of open circuit voltage (Voc) as a function of relative sulfur content;
FIGURE 6C is a graph showing the dependence of short circuit current (Jsc) on relative sulfur content; and
FIGURE 7 is a graph of the spectral response of a copper-multinary structure having a graded absorber, as compared to copper-multinary devices having homogeneous high and low bandgap absorbers, respectively. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, specifically FIGURES 1 and 2, a photovoltaic device 10 constructed according to an illustrative embodiment of the present invention has a p- type compound semiconductor layer 12 made up of a bulk region 14 and a converted surface region 16. In the illustrated embodiment, the converted surface region 16 includes part or all of the "absorber" region (not separately shown) of the semiconductor layer 12, and forms a heterojunction 18 with an n-type semiconductor layer 20, such as cadmium sulfide (CdS) or zinc oxide (ZnO) . The compound semiconductor layer 12 is formed on a back contact 22 which is a thin film of molybdenum (Mo) or other suitable metal deposited on a substrate 24. Contact to the n-type layer 20 is made by a transparent conductive oxide layer 26 and, in some device configurations, a plurality of conductive grid lines 28. Thus, light (hv) enters the device through the transparent oxide layer 26 and passes through the n-type layer 20 and into the p-type compound semiconductor layer 12. Virtually all of such light is absorbed within the absorber portion of the p-type layer 12, which extends up to about 10,000 angstroms into the p-type layer.
The "converted" surface region can extend through part or all of the absorber region, as described in more detail below. The light absorbed in this region generates free carriers which are separated by the built-in electric field resulting from the p-n heterojunction 118, causing current to flow through an external circuit connected between the back contact 22 and the conductive grid lines 28. The basic components of the photovoltaic device 10 and known methods for fabricating them are described in U.S. Patent No. 4,798,660, issued January 17, 1989 to Ermer et al., and U.S. Patent No. 5,045,409, issued September 3, 1991 to Eberspacher et al., the specifications of which are incorporated herein by reference for all purposes. FIGURE 2 illustrates a portion of the photovoltaic device 10 at the stage in its fabrication when the surface region 16 of the semiconductor layer 12 is converted to a different compound semiconductor by the process of the present invention. Thus, the p-type layer 12 is formed on the back electrode 22 prior to conversion. This can take place either just prior to conversion or far in advance of it. Either way, the upper surface of the layer 12 is eventually exposed to a substance containing at least one additional element chosen from the chemical Groups I, III and VI and heated to a preselected temperature in the presence of the substance to cause at least a portion of one of the constituent elements of the original layer to be displaced by the additional element. The number of atoms entering the layer 12 and the depth to which they penetrate (indicated by a broken line in FIGURE 2) are dependent on the characteristics of the additional element and the substance containing it, as well as the concentration of the substance and the temperature and duration of the heat treatment step. Although the entire layer 12 can be converted by the process of the invention, if desired, it is desirable in most cases to convert something less than the entire absorber region. This creates a graded concentration profile which provides a high bandgap and the attendant high open circuit voltage (Voc) . When the I-III-VI2 semiconductor layer 12 is formed of CuInSe2 and the additional element is sulfur or gallium, the conversion may also neutralize "dangling bonds" at the surface region in much the same way that hydrogen passivates dangling bonds in thin film silicon-containing alloys. The process of the invention is illustrated in its simplest form in the flow chart of FIGURE 3, wherein the I- III-VI2 semiconductor layer 12 is deposited at S10, is exposed at its upper surface to an additional Group I, III or VI element, such as gallium or sulfur, at S12, and is annealed in the present of the additional element at S14 to exchange the additional element with the corresponding constituent element of the original film. As described above, the step S10 can include formation of the I-III-VI2 semiconductor just before the exchange takes place, or the original layer can be formed well in advance of the conversion. Step S12 can be accomplished by exposure of the film to a gas, such as hydrogen sulfide, which contains the additional element. Alternatively, the additional element can be applied in any other convenient way, such as by depositing a layer of solid material on the upper surface of the semiconductor layer 12, exposing the film to a vapor of the material itself, or applying an organic compound, such as a mercaptan, in the case of sulfur. All that is required is that the element be available to enter the film in place of one of the constituents of the original film when the composite structure is subjected to heat at S14.
In a preferred embodiment in which the semiconductor layer 12 is CuInSe2 and sulfur is introduced from hydrogen sulfide gas placed in contact with the layer 12, the concentration of sulfur in the atmosphere is preferably between 5 percent and 50 percent of the gaseous atmosphere, the remainder being nitrogen, argon or other suitable carrier gas. The concentration of hydrogen sulfide is most preferably approximately 30 percent for purposes of the invention. The temperature of conversion is preferably at least 450 degrees in order to dissociate the hydrogen sulfide gas and release sulfur into the layer 12, and more preferably between approximately 475 and 550 degrees C. The sulfidation process typically lasts between 5 and 50 minutes, and most preferably approximately 20 minutes. Under these circumstances the depth of sulfur within the layer 12 is typically between 100 and 5000 angstroms, most preferably approximately 1500 angstroms, to provide a graded bandgap structure.
FIGURE 4 illustrates an Auger depth profile of the structure of FIGURE 2 after partial conversion of the layer 12. Thus, the time designated "0" at the left hand side of the graph indicates the concentrations of elements at the upper surface of the region 12, as detected by Auger analysis. It can be seen that the concentrations of sulfur and selenium are approximately equal, making the material a "sulfoselenide" at its surface. Extending downwardly from the surface (moving from left to right on the time scale of FIGURE 4) , the sulfur concentration is graded rapidly down and the selenium concentration increases rapidly until the sulfur signal essentially disappears (approximately 20 minutes into the analysis) . Thus, sulfur is exchanged for selenium near the surface, raising the bandgap and possibly passivating any dangling bonds. The resulting body is much more uniform and much higher in quality than when sulfur is introduced during layer formation as disclosed in the prior art. FIGURE 5 is the measured current-voltage characteristic of the graded cell of FIGURE 4, in which the open circuit voltage (Voc) and the short circuit current (Jsc) yield an efficiency of 15.1 percent. The significance of this in comparison to a similar cell containing sulfur homogeneously throughout its thickness is illustrated in the graph of
FIGURE 7. FIGURE 7 shows that the graded sulfur device has a quantum efficiency similar to that of a cell that does not contain sulfur, and much greater than a homogeneous Cu(InGe) (Se, S2) cell. The" optimal sulfur content within the absorber region for a particular temperature of heat treatment can be obtained from FIGURES 6A, 6B and 6C, which illustrate the efficiency, open circuit voltage and short circuit current, respectively, at relative sulfur contents ranging from approximately 25 percent up to 100 percent. It should be noted in connection with these graphs, however, that the sulfur content for these purposes is defined as the concentration of sulfur in an absorber normalized to the absorber with the highest measured sulfur concentration. Thus, 100 percent total sulfur concentration does not represent stoichiometric CuInS2. Keeping this in mind, FIGURES 6A-6C show that the optimal relative sulfur content is between 40 and 55 percent.
From the above, it can be seen that the disclosed process for conversion of I-III-VI2-type semiconductors enables higher quality and/or more uniform layers to be deposited and permits the formation of high efficiency photovoltaic devices having high open circuit voltage.
While the preferred embodiment has been described and illustrated, various substitutions and modifications may be made thereto without departing from the scope of the invention. For example, the process of the present invention is applicable to the formation of a wide variety of I-III-VI2-type semiconductor compounds, including by way of example, compounds in which the group I element is copper and/or silver, the group III element is indium and/or gallium, and the group VI element is selenium and/or sulfur. Such compounds can also include one or more other elements, such as those of chemical group IV, in addition to the group I, III, VI components without departing from the scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.

Claims

WHAT IS CLAIMED IS:
1. A process for at least partially converting a compound semiconductor film of the I-III-VI2 type to a another compound semiconductor comprising: providing a film including a first compound semiconductor having constituent elements chosen from each of the chemical groups I, III and VI; exposing at least one surface of the film to a substance containing at least one additional element chosen from the chemical groups I, III and VI; and heating the film to a preselected temperature in the presence of said substance to exchange said at least one additional element for at least a portion of one or more of the constituent elements of said film; whereby the first compound semiconductor is at least partially converted to a second compound semiconductor of different composition.
2. The process of claim 1 wherein: the step of providing said film comprises forming said film on a substrate.
3. The process of claim 1 wherein: a graded bandgap is created in the compound semiconductor film as a result of said at least partial conversion.
4. The process of claim 1 wherein: said at least one additional element is chosen from the chemical group VI.
5. The process of claim 4 wherein: the first compound semiconductor includes selenium; and said at least one additional element is sulfur.
6. The process of claim 5 wherein: said preselected temperature is greater than 450 degrees C.
7. The process of claim 6 wherein: said preselected temperature is at least approximately 500 degrees C.
8. The process of claim 1 wherein: the process of exposing at least one surface of the film to at least one additional element comprises exposing said surface to a gas containing said at least one additional element.
9. The process of claim 8 wherein: said gas contains hydrogen.
10. The process of claim 9 wherein: the first compound semiconductor includes selenium; said at least one additional element is sulphur; and said gas is hydrogen sulphide.
11. The.process of claim 10 wherein: the step of providing said film comprises forming the film on a substrate by a process in which at least one layer containing elements chosen from each of the chemical groups I and III is heat treated in the presence of hydrogen selenide within a reaction chamber; and the steps of exposing the film to a substance and heating the film to a preselected temperature take place in said chamber, as well.
12. The product formed by the process of claim 1.
13. A process for at least partially converting a copper-multinary semiconductor film to a another copper- multinary semiconductor Comprising: providing a film including a first copper-multinary semiconductor having constituent elements chosen from each of the chemical groups I, III and VI; exposing at least one surface of the film to a substance containing at least one additional element chosen from the chemical groups I, III and VI; and heating the film to a preselected temperature in the presence of said substance to exchange said at least one additional element for at least a portion of one or more of the constituent elements of said film; whereby the first copper-multinary semiconductor is at least partially converted to a second copper-multinary semiconductor of different composition.
14. The process of claim 13 wherein: the first compound semiconductor includes selenium; and said at least one additional element includes sulfur.
15. The process of claim 13 wherein: the first compound semiconductor includes indium; and said at least one additional element includes gallium.
16. The process of claim 13 wherein: the first compound semiconductor includes indium and selenium; and said at least one additional element includes gallium and sulphur.
17. A process for at least partially converting a compound semiconductor film of the Cu-In-Se2 type to a another compound semiconductor comprising: providing a film including a first compound semiconductor containing Cu, In and Se; exposing at least one surface of the film to a substance containing at least one additional element chosen from the chemical groups I, III and VI; and heating the film to a preselected temperature in the presence of said substance to exchange said at least one additional element for at least a portion of one or more of the constituent elements of said film; whereby the first compound semiconductor is at least partially converted to a second compound semiconductor of different composition.
PCT/US1994/004245 1993-05-07 1994-04-18 Process for at least partially converting semi-conductor films of the i-iii-vi2-type WO1994027328A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US5840493A 1993-05-07 1993-05-07
US08/058,404 1993-05-07

Publications (1)

Publication Number Publication Date
WO1994027328A1 true WO1994027328A1 (en) 1994-11-24

Family

ID=22016609

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/US1994/004245 WO1994027328A1 (en) 1993-05-07 1994-04-18 Process for at least partially converting semi-conductor films of the i-iii-vi2-type

Country Status (1)

Country Link
WO (1) WO1994027328A1 (en)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1548845A1 (en) * 2003-12-22 2005-06-29 Scheuten Glasgroep Process for treating of powder particles
US8546176B2 (en) 2010-04-22 2013-10-01 Tsmc Solid State Lighting Ltd. Forming chalcogenide semiconductor absorbers

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4335266A (en) * 1980-12-31 1982-06-15 The Boeing Company Methods for forming thin-film heterojunction solar cells from I-III-VI.sub.2
EP0195152A1 (en) * 1985-03-22 1986-09-24 Siemens Solar Industries L.P. Process of forming a compound semiconductive material
EP0318315A2 (en) * 1987-11-27 1989-05-31 Siemens Solar Industries L.P. Process for making thin film solar cell

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4335266A (en) * 1980-12-31 1982-06-15 The Boeing Company Methods for forming thin-film heterojunction solar cells from I-III-VI.sub.2
EP0195152A1 (en) * 1985-03-22 1986-09-24 Siemens Solar Industries L.P. Process of forming a compound semiconductive material
EP0318315A2 (en) * 1987-11-27 1989-05-31 Siemens Solar Industries L.P. Process for making thin film solar cell

Non-Patent Citations (3)

* Cited by examiner, † Cited by third party
Title
J.J. LOFERSKI ET AL.: "Inves- tigation of Thin Film Cadmium, Sulfide/Mixed Copper Ternary Heterojunction Photovoltaic Cells", THIRTEENTH IEEE PHOTOVOLTAIC SPECIALISTS CONFERENCE - 1978, June 1978 (1978-06-01), pages 190 - 194 *
J.R. TUTTLE ET AL.: "Experi- ments on the Modification of the Bi-Layer Structure in CdS/CuInSe2 Devices", TWENTIETH IEEE PHOTOVOLTAIC SPECIALISTS CONFERENCE - 1988 VOLUME II, September 1988 (1988-09-01), pages 1525 - 1530 *
J.R. TUTTLE ET AL.: "Physical, Chemical and Structural Modifications to Thin-Film CuInSe2-based Photovoltaic Devices", TWENTY SECOND IEEE PHOTO- VOLTAIC SPECIALISTS CONFERENCE - 1991, VOLUME II, October 1991 (1991-10-01), pages 1062 - 1067, XP010039374, DOI: doi:10.1109/PVSC.1991.169374 *

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1548845A1 (en) * 2003-12-22 2005-06-29 Scheuten Glasgroep Process for treating of powder particles
WO2005064691A1 (en) * 2003-12-22 2005-07-14 Scheuten Glasgroep Method for treating powder particles
CN100461460C (en) * 2003-12-22 2009-02-11 绍于腾玻璃集团公司 Method for treating powder particles
US8546176B2 (en) 2010-04-22 2013-10-01 Tsmc Solid State Lighting Ltd. Forming chalcogenide semiconductor absorbers

Similar Documents

Publication Publication Date Title
US5141564A (en) Mixed ternary heterojunction solar cell
Herberholz et al. Prospects of wide-gap chalcopyrites for thin film photovoltaic modules
US5078804A (en) I-III-VI2 based solar cell utilizing the structure CuInGaSe2 CdZnS/ZnO
US6092669A (en) Equipment for producing thin-film solar cell
JP3249407B2 (en) Thin-film solar cells composed of chalcopyrite-based multi-compound semiconductor thin-film light-absorbing layers
Nakada et al. Improved Cu (In, Ga)(S, Se) 2 thin film solar cells by surface sulfurization
AU604774B2 (en) Improved photovoltaic heterojunction structures
Ferhati et al. Exceeding 30% efficiency for an environment-friendly tandem solar cell based on earth-abundant Se/CZTS materials
US20140124011A1 (en) Heat Treatment Process and Photovoltaic Device Based on Said Process
JPH08330614A (en) Thin film solar cell and manufacture thereof
US20140246087A1 (en) Photoelectric conversion element and solar cell
Shafi et al. A numerical simulation for efficiency enhancement of CZTS based thin film solar cell using SCAPS-1D
Konagai et al. Development of Cu (InGa) Se2 thin film solar cells with Cd-free buffer layers
US20190341506A1 (en) Doping and passivation for high efficiency solar cells
CN103117323B (en) Photo-electric conversion element and solar cell
Nakada et al. Improved efficiency of Cu (In, Ga) Se/sub 2/thin film solar cells with chemically deposited ZnS buffer layers by air-annealing-formation of homojunction by solid phase diffusion
JP3311873B2 (en) Manufacturing method of semiconductor thin film
JPH07122762A (en) Thin film photovoltaic device
GB2384621A (en) II-VI and III-V thin film photovoltaic devices
JP3408618B2 (en) Solar cell manufacturing method
US8373062B2 (en) Solar cell, and method for producing the solar cell
WO1994027328A1 (en) Process for at least partially converting semi-conductor films of the i-iii-vi2-type
JP2000012883A (en) Manufacture of solar cell
JP3414814B2 (en) Solar cell and manufacturing method thereof
EP3903352A1 (en) Photovoltaic devices and methods of forming the same

Legal Events

Date Code Title Description
AK Designated states

Kind code of ref document: A1

Designated state(s): FI JP

AL Designated countries for regional patents

Kind code of ref document: A1

Designated state(s): AT BE CH DE DK ES FR GB GR IE IT LU MC NL PT SE

121 Ep: the epo has been informed by wipo that ep was designated in this application
122 Ep: pct application non-entry in european phase