Closed-space sublimation process for production of CZTS thin-films

In one embodiment, a method includes depositing a CZT(S, Se) precursor layer onto a substrate, introducing a source-material layer comprising Sn(S, Se) into proximity with the precursor layer, and annealing the precursor layer in proximity with the source-material layer in a constrained volume.

TECHNICAL FIELD

This disclosure generally relates to the manufacturing of photovoltaic devices, and in particular to the production of photovoltaic devices from copper, zinc, tin, and sulfur/selenium (CZTS).

BACKGROUND

A typical photovoltaic cell includes a p-n junction, which can be formed by a layer of n-type semiconductor in direct contact with a layer of p-type semiconductor. The electronic differences between these two materials create a built-in electric field and potential difference. When a p-type semiconductor is placed in intimate contact with an n-type semiconductor, then a diffusion of electrons can occur from the region of high electron-concentration (the n-type side of the junction) into the region of low electron-concentration (the p-type side of the junction). The diffusion of carriers does not happen indefinitely, however, because of an opposing electric field created by the charge imbalance. The electric field established across the p-n junction induces separation of charge carriers that are created as result of photon absorption. When light is incident on this junction, the photons can be absorbed to excite pairs of electrons and holes, which are “split” by the built-in electric field, creating a current and voltage.

The majority of photovoltaic cells today are made using relatively thick pieces of high-quality silicon (approximately 200 μm) that are doped with p-type and n-type dopants. The large quantities of silicon required, coupled with the high purity requirements, have led to high prices for solar panels. Thin-film photovoltaic cells have been developed as a direct response to the high costs of silicon technology. Thin-film photovoltaic cells typically use a few layers of thin films (≦5 μm) of low-quality polycrystalline materials to mimic the effect seen in a silicon cell. A basic thin-film device consists of a substrate (e.g., glass, metal foil, plastic), a metal-back contact, a 1-5 μm semiconductor layer to absorb the light, another semiconductor layer to create a p-n junction and a transparent top conducting electrode to carry current. Since very small quantities of low-quality material are used, costs of thin-film photovoltaic cells are lower than those for silicon.

The two primary technologies in the thin-film solar space are copper indium gallium sulfur/selenide (CIGS) and cadmium telluride (CdTe). CIGS and CdTe photovoltaic cells have lower costs per watt produced than silicon-based cells and are making significant inroads into the photovoltaic market. However, CIGS and CdTe technologies are likely to be limited by the potential higher costs, lower material availability, and toxicity of some of their constituent elements (e.g., indium, gallium, tellurium, cadmium).

DESCRIPTION OF EXAMPLE EMBODIMENTS

CZTS Materials Generally

In particular embodiments, a thin-film in a photovoltaic cell may be manufactured using copper zinc tin sulfur/selenide (CZTS). CZTS materials have a favorable direct band gap (1.45 eV), a large absorption coefficient (>104cm−1), and are formed entirely from non-toxic, abundant elements that are produced in large quantities. CZTS also shares a number of similarities with CIGS as the equipment and processes used for deposition of these two materials are very similar. CZTS materials can be synthesized through sold-state chemical reactions between Zn(S, Se), Cu2(S, Se), and Sn(S, Se)2.FIG. 1illustrates an isothermal phase diagram for SnS—Cu2S—ZnS systems at 670 K. As illustrated in the phase diagram, Cu2ZnSnS4forms in this system in region101, while Cu2ZnSn3S8forms in region102.

In particular embodiments, the CZTS fabrication processes may consist of two main steps. First, a precursor containing a combination of the constituent elements (copper, zinc, tin, sulfur, and selenium) may be deposited onto a substrate to form a precursor layer. Any suitable combination of the constituent elements may be used. The substrate is typically coated with a suitable electrode material. Deposition of the precursor layer may be performed using any suitable thin-film deposition process, such as, for example, chemical-vapor deposition, evaporation, atomic-layer deposition, sputtering, particle coating, spray pyrolysis, spin-coating, electro-deposition, electrochemical deposition, photoelectrochemical deposition, hot-injection, chemical-bath deposition, spin coating, another suitable deposition process, or any combination thereof. Second, the precursor may be annealed at high temperature (approximately >400° C.) to form the CZTS crystalline phase.

CZTS is unstable at high temperatures and thus ideal compositional stoichiometries are difficult to maintain. Furthermore, the annealing conditions used to form the crystalline phase may create electronic defects in the film. At temperatures greater than 450° C., crystalline CZTS can decompose and volatile constituent materials may evaporate from the film. In particular embodiments, CZTS may decompose according to the following reaction scheme:
Cu2ZnSn(S,Se)4Cu2(S,Se)(s)+Zn(S,Se)(s)+Sn(S,Se)(g)+S2(g).

In this reaction, tin sulfide and sulfur gas are evaporated from a crystalline CZTS film at high temperature, creating electronic defects that are detrimental to device performance. This means that for the reaction to proceed in the forward direction, Sn(S, Se) and/or S2gas must be evaporated from the film. Evolution of a gaseous phase in a reaction must also lead to an increase in the total pressure of the system. Although this disclosure describes a particular decomposition reaction for CZTS, this disclosure contemplates any suitable decomposition reaction for CZTS.

In particular embodiments, a fabrication apparatus may deposit a CZTS precursor layer onto a substrate. The precursor layer may comprise Cu, Zn, Sn, and one or more of S or Se.FIG. 2illustrates example precursor layer architectures. In each example, the precursor layer is deposited on a suitable substrate.FIG. 2Aillustrates an example precursor layer comprising of film layers of copper, zinc, and tin. In order to form a suitable CZTS material, one or more of sulfur or selenium may later be deposited onto the precursor layer, such as, for example, during a separate deposition step or during annealing.FIG. 2Billustrates an example precursor layer comprising film layers of CuaSb/CuaSeb, where approximately 0.5≦a≦2 and approximately b=1, ZncSd/ZncSed, where approximately 0.5≦c≦2 and approximately d=1, and SneSf/SneSef, where approximately 0.5≦e≦2 and approximately f=1. The use of sulfide and selenide layers can be used to control the sulfur-to-selenium ratio in the precursor layer. InFIGS. 2A and 2B, the film layers may be deposited sequentially, with minimal mixing between the film layers. The layers inFIGS. 2A and 2Bmay be arranged in any suitable order, may have any suitable thickness, and each layer may have a different thickness. The thickness of the layers inFIGS. 2A and 2Bmay be used to control the composition of the initial precursor film and the final post-annealing film.FIG. 2Cillustrates an example precursor layer comprising a mixture of copper, zinc, tin, sulfur, and selenium. Any suitable combination of these elements may be used. As another example, the precursor layer may comprise approximately 5-50 atomic % Cu, approximately 5-50 atomic % Zn, approximately 5-50 atomic % Sn, approximately 5-50 atomic % S, and approximately 5-50 atomic % Se. As yet another example, the precursor layer may comprise CuxZnySnz(SαSe1−α)β, where approximately 0.5≦x≦3, approximately y=1, approximately 0.5≦z≦3, approximately 0≦α≦5, and approximately 0≦β≦5.FIG. 2Dillustrates an example precursor layer comprising a CZTS crystalline film (Cu2ZnSn(S, Se)4). For example, the crystalline film may be deposited using physical-vapor deposition at high-temperature such that the crystalline phase is formed during deposition.FIG. 2Eillustrates an example precursor layer comprising nanoparticles of the constituent elements (Cu, Zn, Sn, S, Se) or compounds of the constituent elements (e.g., ZnS, SnS, ZnSe, SnSe). AlthoughFIG. 2illustrates particular precursor layers with particular compositions and architectures, this disclosure contemplates any suitable precursor layers with any suitable compositions or architectures. For example, additional constituents such as alkali metal salts, antimony, bismuth, another suitable constituent, or any combination thereof may be added to the precursor layer to enhance its properties (e.g., grain size) or performance. As another example, to improve the electrical properties of the precursor layer or to optimize the subsequent annealing process, the precursor layer may contain up to approximately 20 atomic % of one or more of Al, Si, Ti, V, Zn, Ga, Zr, Nb, Mo, Ru, Pd, In, Sn, Ta, W, Re, Ir, Pt, Au, Pb, or Bi.

In particular embodiments, the precursor layer may be annealed at high-temperature while controlling the stoichiometry of the layer and reducing or suppressing the decomposition of the CZTS material. CZTS films manufactured in this way may be device-quality, that is, the film may be incorporated into a photovoltaic device and used to generate electricity from light at a reasonable efficiency. The decomposition of CZTS at high temperature may be reduced or suppressed by controlling the formation of gaseous Sn(S, Se) and/or S2during the annealing process. For example, if the partial pressure of gaseous Sn(S, Se) and/or S2in the annealing apparatus is maintained at or above the equilibrium vapor pressure of the gaseous component, the decomposition of the CZTS film can be suppressed or even reversed. This may be achieved, for example, by annealing the CZTS film in a constrained volume where the partial pressure of gaseous Sn(S, Se) and/or S2can be controlled.

Separated Layers in a Constrained Volume

In particular embodiments, a CZTS film may be manufactured by annealing the film in a constrained volume.FIG. 3illustrates an example closed-space sublimation apparatus300. Apparatus300includes a heater310, a first substrate320, a second substrate330, a precursor layer340, and a source-material layer350. Heater310may be any suitable heating source. Heater310can provide heat via conduction, convection, radiation, or any combination thereof. For example, heater310may be a belt furnace that provides heat via a combination of conduction, convection, and radiation. First substrate320and second substrate330may be any suitable substrate capable of withstanding high temperatures and/or pressures. First substrate320and second substrate330may provide structural support for the film stack. For example, first substrate320or second substrate330may be soda-lime glass, a metal sheet or foil (e.g., stainless steel, aluminum, tungsten), a semiconductor (e.g., Si, Ge, GaAs), a polymer, another suitable substrate, or any combination thereof. Precursor layer340may be any suitable CZTS material, such as, for example, the CZTS materials described previously. In particular embodiments, precursor layer340comprises Cu, Zn, Sn, and one or more of S or Se. In alternative embodiments, precursor layer340comprises Cu, Zn, and Sn. S or Se may later be deposited onto the precursor layer in order to make a suitable CZTS material. Precursor layer340may be deposited on first substrate320. Source-material layer350may be a film layer comprising Sn and one or more of S or Se. For example, source-material layer350may comprise 50% tin and 50% sulfur. As another example, source-material layer350may comprise 30-70% tin and 30-70% sulfur. As yet another example, source-material layer350may comprise 30-70% tin, 30-70% sulfur, and 30-70% selenium. As yet another example, source-material layer350may comprise Cu(S, Se)2. Source-material layer350may be any suitable thickness. In particular embodiments, source-material layer350may have a thickness of approximately 100 nm to approximately 5000 nm. For example, source-material layer350may have a thickness of 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, or 1000 nm. In particular embodiments, source-material layer350may be deposited on second substrate330. In alternative embodiments, source-material layer350may be deposited onto precursor layer340. Apparatus300may be capable of performing high-pressure, high-temperature processes. The reaction conditions in apparatus300may be precisely controlled, monitored, and adjusted to optimize the reaction yield and sample uniformity. Apparatus300may be a constrained volume, with minimal dead space in the reaction chamber. AlthoughFIG. 3illustrates a particular arrangement of heater310, first substrate320, second substrate330, precursor layer340, and source-material layer350, this disclosure contemplates any suitable arrangement of heater310, first substrate320, second substrate330, precursor layer340, and source-material layer350. For example, apparatus300may include a flexible continuous web that carries the individual components into the reaction chamber. Moreover, althoughFIG. 3illustrates a particular number of heaters310, first substrates320, second substrates330, precursor layers340, and source-material layers350, this disclosure contemplates any suitable number of heaters310, first substrates320, second substrates330, precursor layers340, and source-material layers350. For example, apparatus300may include multiple precursor layers340or source-material layers350.

In particular embodiments, apparatus300may introduce a source-material layer350into proximity with the precursor layer340. Any suitable mechanism may be used to introduce source-material layer350into proximity with precursor layer340. For example, sheets coated with precursor layer340and source-material layer350may be manually inserted into the reaction chamber of a closed-space sublimation apparatus (e.g., apparatus300) such that precursor layer340and source-material layer350are directly facing each other in the reaction chamber. In particular embodiments, precursor layer340and source-material layer350may be separated from each other by a specified distance. The surface of precursor layer340may be substantially parallel to source-material layer350. For example, precursor layer340and source-material layer350may be separated from each other by approximately 0.01 mm to approximately 5 mm. As yet another example, precursor layer340and source-material layer350may be in contact or substantially in contact with each other. In particular embodiments, source-material layer350may be introduced over precursor layer350. For example, precursor layer340may be manually inserted into the reaction chamber of apparatus300such that precursor layer340is substantially lying in a horizontal position. Source-material layer350may then be manually inserted into the reaction chamber of apparatus300such that source-material layer350is also substantially lying in a horizontal position above precursor layer340. In particular embodiments, the source-material layer350may be deposited onto precursor layer340. Deposition of source-material layer350may be performed using any suitable thin-film deposition process, such as, for example, chemical-vapor deposition, evaporation, atomic-layer deposition, sputtering, particle coating, electro-deposition, another suitable deposition process, or any combination thereof. For example, a sheet coated with precursor layer340and source-material layer350(which is deposited over precursor layer340) may be manually inserted into the reaction chamber of a closed-space sublimation apparatus (e.g., apparatus300). Although this disclosure describes introducing source-material layer350over precursor layer340in a particular manner, this disclosure contemplates introducing source-material layer350over precursor layer340in any suitable manner.

In particular embodiments, apparatus300may anneal precursor layer340in the presence of source-material layer350. The annealing may be performed in a constrained volume under isochoric, isobaric, isothermal, or other suitable conditions. The annealing may be performed at any suitable pressure. For example, annealing may occur under vacuum, under partial vacuum, at atmospheric pressure, or with an overpressure of gas. During annealing, the tin, sulfur, and selenium in source-material layer350will decompose at high temperatures, creating an atmosphere above the CZTS film that has a high concentration of SnS gas, SnSe gas, sulfur gas (S2or S8), selenium gas, or any combination thereof. As source-material layer350decomposes into gaseous components, the constrained volume in apparatus300may create an overpressure of the SnS gas, SnSe gas, sulfur gas (S2or S8), selenium gas, or any combination thereof. In particular embodiments, the CZTS decomposition reaction may be further controlled by adding SnS gas, SnSe gas, sulfur gas (S2or S8), selenium gas, or any combination thereof to apparatus300to control the partial pressure of each gas. By maintaining relatively high partial pressures of these gases, the decomposition of precursor layer340at high temperatures may be reduced or suppressed by shifting the equilibrium of the CZTS decomposition reaction, such that it is slowed or even reversed. Thus, the CZTS precursor can be annealed at high temperature without any decomposition. In particular embodiments, other gaseous components may be added to apparatus300during annealing. For example, the atmosphere during annealing may comprise H, He, N2, O2, Ar, H2S, Kr, H2Se, Xe, another suitable gas, or any combination thereof. In particular embodiments, the total pressure of the gas atmosphere in apparatus300may range from, for example, 10−8Pa to approximately 107Pa. In particular embodiments, apparatus300may heat precursor layer340to a first temperature of approximately 350° C. to approximately 700° C. during annealing. Heaters210may heat the system using any suitable type of heating, such as, for example, conduction, convection, radiation, or any combination thereof. For example, precursor layer340may be heated to a first temperature of 350° C., 360° C., 380° C., 400° C., 420° C., 440° C., 460° C., 480° C., 500° C., 520° C., 540° C., 560° C., 580° C., 600° C., 620° C., 640° C., 660° C., 680° C., or 700° C. Precursor layer340may then be held at the first temperature for 5 minutes to 120 minutes. Precursor layer340may then be cooled to a second temperature of approximately 20° C. to approximately 100° C. In particular embodiments, precursor layer340and source-material layer350may be compressed during annealing. For example, precursor layer340and source-material layer350may be placed substantially in contact with each other and then laterally compressed, such as, for example, by applying mechanical force via a weight, a vice, hydraulics, another suitable apparatus, or any combination thereof. In particular embodiments, precursor layer340may comprise Cu, Zn, and Sn. One or more of S or Se may then be deposited onto precursor layer340during annealing. For example, one or more of S or Se may be deposited from source-material layer350onto precursor layer340during annealing. As source-material layer350is heated during annealing, source-material layer350may decompose to form sulfur and selenium gas, which may then be deposited onto precursor layer340. Although this disclosure describes annealing precursor layer340in a particular manner, this disclosure contemplates annealing precursor layer340in any suitable manner.

FIGS. 4A-4Gillustrate example annealing temperature profiles. In particular embodiments, apparatus300may anneal a CZTS layered structure by using pulsed annealing, flash annealing, laser annealing, furnace annealing, lamp annealing, another suitable annealing process, or any combination thereof. Annealing may be performed using a light source (e.g., a halogen lamp or a laser), resistive heaters, lasers, another suitable heating source, or any combination thereof. The heating may be effected either directly onto the surface of a film layer or via a back substrate.FIGS. 4A-4Gillustrate example plots of temperature as a function of time (T=f(t)) during annealing of the layered structure. InFIG. 4A, the temperature of the layered structure is first increased from T0to T1at a temperature ramp rate (increase rate) of (T1−T0)/(t1−t0), followed by a decrease to T0at a cooling rate of (T0−T1)/(t2−t1). InFIG. 4B, the temperature of the layered structure is first increased from T0to T1at a ramp rate that decreases with increasing temperature, followed by a decrease to T0at a cooling rate at a cooling rate that is initially fast and decreases with decreasing temperature. InFIG. 4C, the temperature of the layered structure is first increased from T0to T1with a temperature ramp rate of (T1−T0)/(t1−t0). The temperature of the layered structure is then held at approximately T1for a time (t2−t1) before subsequently reducing the temperature to T0with a cooling rate of (T0−T1)/(t3−t2). InFIG. 4D, the layered structure is first preheated to a temperature T1before increasing the temperature of the layered structure from T1to T2with a temperature ramp rate of (T2−T1)/(t2−t1). The temperature of the layered structure is then held at approximately T2for a time (t3−t2) before subsequently reducing the temperature to T0with a cooling rate of (T0−T2)/(t4−t3). InFIG. 4E, the layered structure is annealed using a step-wise temperature profile, where the layer structure is first heated to T1with a ramp rate of (T1−T0)/(t1−t0), held at approximately T1for a time (t2−t1), then heated to T2with a ramp rate of (T2−T1)/(t3−t2), held at approximately T2for a time (t4−t3), and so on until a target temperature Tnis reached. InFIG. 4F, the temperature of the layered structure is first increased from T0to T1with a temperature ramp rate of (T1−T0)/(t1−t0), held at approximately T1for a time (t2−t1), followed by step-wise cooling where the layered structure is cooled to T2at a rate (T2−T1)/(t3−t2), held at approximately T2for a time (t4−t3), and so on until a target temperature T0is reached. InFIG. 4G, the layered structure is heated from T0to Tnusing the step-wise heating method described with reference toFIG. 4E, held at approximately Tnfor a time (tn+1−tn), and then cooled to T0using the step-wise cooling method described with reference toFIG. 4F. AlthoughFIGS. 4A-4Gillustrates and this disclosure describes particular annealing temperature profiles, this disclosure contemplates any suitable annealing temperature profiles.

FIG. 5illustrates an example method500for producing a CZTS thin-film by annealing a precursor layer340and a source-material layer350in a constrained volume. The method may begin at step510, where precursor layer340is deposited onto first substrate320. Precursor layer340may comprise Cu, Zn, Sn, and one or more of S or Se. At step520, source-material layer350may be introduced over precursor layer340. Source-material layer350may comprise Sn and one or more of S or Se. At step530, apparatus300may anneal precursor layer340in proximity with source-material layer350Annealing may be performed in a constrained volume. Particular embodiments may repeat one or more steps of the method ofFIG. 5, where appropriate. Although this disclosure describes and illustrates particular steps of the method ofFIG. 5as occurring in a particular order, this disclosure contemplates any suitable steps of the method ofFIG. 5occurring in any suitable order. For example, method500may be repeated multiple times with repeated deposition of precursor layers to provide a multi-layered variable or graded band gap absorber. Moreover, although this disclosure describes and illustrates particular components, devices, or systems carrying out particular steps of the method ofFIG. 5, this disclosure contemplates any suitable combination of any suitable components, devices, or systems carrying out any suitable steps of the method ofFIG. 5.

FIG. 6illustrates an example method600for producing a CZTS thin-film by depositing a source-material layer350onto a precursor layer340. The method may begin at step610, where precursor layer340is deposited onto first substrate320. Precursor layer340may comprise Cu, Zn, Sn, and one or more of S or Se. At step620, source-material layer350may be deposited onto precursor layer340. Source-material layer350may comprise Sn and one or more of S or Se. At step630, apparatus300may anneal precursor layer340and source-material layer350. Annealing may be performed in a constrained volume. Particular embodiments may repeat one or more steps of the method ofFIG. 6, where appropriate. Although this disclosure describes and illustrates particular steps of the method ofFIG. 6as occurring in a particular order, this disclosure contemplates any suitable steps of the method ofFIG. 6occurring in any suitable order. Moreover, although this disclosure describes and illustrates particular components, devices, or systems carrying out particular steps of the method ofFIG. 6, this disclosure contemplates any suitable combination of any suitable components, devices, or systems carrying out any suitable steps of the method ofFIG. 6.

Annealing with a Controlled Overpressure

In particular embodiments, a CZTS film may be manufactured by controlling the pressure of decomposition gasses formed during annealing.FIG. 7illustrates an example tube-furnace apparatus700. Apparatus700includes a heating coil710, a substrate720, a precursor layer740, a gas inlet760, and a gas outlet770. Heating coil710may be any suitable heating source. Heater710can provide heat via conduction, convection, radiation, or any combination thereof. For example, heater710may be a belt furnace that provides heat via a combination of conduction, convection, and radiation. Substrate720may be any suitable substrate capable of withstanding high temperatures and/or pressures. Substrate720may provide structural support for the film stack. For example, substrate720may be soda-lime glass, a metal sheet or foil (e.g., stainless steel, aluminum, tungsten), a semiconductor (e.g., Si, Ge, GaAs), a polymer, another suitable substrate, or any combination thereof. Precursor layer740may be any suitable CZTS material, such as, for example, the CZTS materials described previously. In particular embodiments, precursor layer740comprises Cu, Zn, Sn, and one or more of S or Se. In alternative embodiments, precursor layer740comprises Cu, Zn, and Sn. S or Se may later be deposited onto the precursor layer in order to make a suitable CZTS material. Precursor layer740may be deposited on substrate720. Gas inlet760and gas outlet770may be any suitable gas flow control elements. For example, gas inlet760or gas outlet770may be a control valve, a variable-speed pump, a pressure-relief valve, a mass-flow controller, a throttle valve, another suitable gas flow control element, or any combination thereof. Gas inlet760and gas outlet770may be used to provide a gaseous phase to apparatus700and to control the pressure of the gaseous phase over time. The gaseous phase my comprise SnS gas, SnSe gas, sulfur gas (S2or S8), selenium gas, or any combination thereof. Gas inlet760may be able to precisely control the partial pressure of each component of the gaseous phase. Gas inlet760and gas outlet770may also be used to provide a carrier gas to apparatus700. Apparatus700may be capable of performing high-pressure, high-temperature processes. The reaction conditions in apparatus700may be precisely controlled, monitored, and adjusted to optimize the reaction yield and sample uniformity. Apparatus700may be a constrained volume, with minimal dead space in the reaction chamber. AlthoughFIG. 7illustrates a particular arrangement of heating coil710, substrate720, precursor layer740, gas inlet760, and gas outlet770, this disclosure contemplates any suitable arrangement of heating coil710, substrate720, precursor layer740, gas inlet760, and gas outlet770. For example, apparatus700may include a flexible continuous web that carries the individual components into the tube furnace. Moreover, althoughFIG. 7illustrates a particular number of heating coils710, substrates720, precursor layers740, gas inlets760, and gas outlet770, this disclosure contemplates any suitable number heating coils710, substrates720, precursor layers740, gas inlets760, and gas outlet770. For example, apparatus700may include multiple gas inlets760and gas outlets770, allowing for more precise spatial control of the partial pressure of each component of the gaseous phase.

In particular embodiments, apparatus700may anneal precursor layer740in the presence of a gaseous phase. Apparatus700may be used to anneal a CZTS film without decomposition of the crystalline CZTS phase. In particular embodiments, precursor layer740may be introduced into apparatus700. Gas outlet770may then pull a full or partial vacuum in the tube-furnace. Gas outlet770may then be closed, such as, for example, with a control valve, and gas inlet760may then be used to provide a gaseous phase comprising Sn and one or more of S or Se. Gas inlet760may provide a gaseous phase comprising Sn and one or more of S or Se. Gas inlet760may be used to create an overpressure of the SnS gas, SnSe gas, sulfur gas (S2or S8), selenium gas, or any combination thereof. Controlled quantities of each component of the gaseous phase can be introduced into the tube-furnace until a specified partial pressure of each component is reached. Gas inlet760may then be closed and precursor layer740may then be annealed. The annealing may be performed in a constrained volume under isochoric, isobaric, isothermal, or other suitable conditions. The annealing may be performed at any suitable pressure. For example, annealing may occur under vacuum, under partial vacuum, at atmospheric pressure, or with an overpressure of gas. In particular embodiments, the partial pressure of a particular component of the gaseous phase may range from approximately 0 atm to approximately 10 atm. During annealing, gas inlet760and gas outlet770may be used to continuously control the partial pressure of each component of the gaseous phase by controlling the inlet and outlet gas flow rates. In particular embodiments, the partial pressure of each component of the gaseous phase may be kept approximately constant over substantially all of the surface of precursor layer740. Minimizing concentration variations across the surface of precursor layer740during annealing may improve the properties or performance of precursor layer740. In particular embodiments, the partial pressure of one or more components of the gaseous phase may be kept constant during substantially all of the annealing process. In alternative embodiments, the partial pressure of one or more components of the gaseous may vary over time during the annealing process, while still maintaining a partial pressure that is approximately spatially-constant over the surface of precursor layer740. For example, the gaseous phase may initially have a partial pressure of S2gas of p0, and the partial pressure may be ramped down to p1over time (t1−t0) at a rate of (p1−p0)/(t1−t0). By maintaining relatively high partial pressures of these gases, the decomposition of precursor layer740at high temperatures may be reduced or suppressed by shifting the equilibrium of the CZTS decomposition reaction, such that it is slowed or even reversed. Thus, the CZTS precursor can be annealed at high temperature without any decomposition. In particular embodiments, the gaseous phase may also comprise a carrier gas to facilitate transport of the gaseous phase in apparatus700. The carrier gas may comprise H, He, N2, O2, Ar, H2S, Kr, H2Se, Xe, another suitable gas, or any combination thereof. In particular embodiments, the partial pressure of the carrier gas may range from approximately 0 atm to approximately 1 atm. In particular embodiments, apparatus700may anneal according to one or more of the annealing temperature profiles described previously, such as, for example, an annealing temperature profile described with respect to apparatus300or illustrated inFIG. 4. In particular embodiments, precursor layer740may comprise Cu, Zn, and Sn. One or more of S or Se may then be deposited onto precursor layer740during annealing. For example, one or more of S or Se may be deposited from the gaseous phase onto precursor layer740during annealing. As the gaseous phase is heated during annealing, gaseous sulfur or selenium from the gaseous phase may be deposited onto precursor layer740. Although this disclosure describes annealing precursor layer740in a particular manner, this disclosure contemplates annealing precursor layer740in any suitable manner.

FIG. 8illustrates an example method800for producing a CZTS thin-film using a controlled overpressure. The method may begin at step810, where precursor layer740is deposited onto substrate720. Precursor layer740may comprise Cu, Zn, Sn, and one or more of S or Se. At step820, precursor layer740may be annealed in the presence of a gaseous phase comprising Sn and one or more of S or Se. The partial pressure of each component of the gaseous phase may be approximately constant over substantially all of the surface of precursor layer740for substantially all of the duration of annealing Annealing may be performed in a constrained volume. Particular embodiments may repeat one or more steps of the method ofFIG. 8, where appropriate. Although this disclosure describes and illustrates particular steps of the method ofFIG. 8as occurring in a particular order, this disclosure contemplates any suitable steps of the method ofFIG. 8occurring in any suitable order. Moreover, although this disclosure describes and illustrates particular components, devices, or systems carrying out particular steps of the method ofFIG. 8, this disclosure contemplates any suitable combination of any suitable components, devices, or systems carrying out any suitable steps of the method ofFIG. 8.

Properties of CZTS Materials

The properties of CZTS thin-films manufactured using some of the disclosed embodiments are described below and illustrated inFIGS. 9-13.

FIG. 9illustrates an x-ray diffraction pattern of a CZTS thin-film. The diffraction pattern shows the primary peaks for CZTS and can be used to establish that the film has the correct crystal structure.

FIG. 10illustrates a scanning electron microscopy image of a CZTS thin-film. The SEM image shows that the CZTS thin-film has relatively large grains and minimal defects (e.g., cracks, pores).

FIG. 12illustrates current-voltage measurements of various CZTS thin-films. Sample A was deposited at high temperature and Sample B was deposited at room temperature and annealed using the annealing processes described previously. Sample A was observed to be tin poor due to loss of tin sulfide and had considerably reduced efficiency.

FIG. 13illustrates an external quantum efficiency measurement of a CZTS-based photovoltaic cell. The best efficiency achieved using this methodology was 9.3%, which is either comparable with or, in most cases, exceeds what is possible with other deposition and annealing methods.

CZTS Device Stack

FIG. 14illustrates an example CZTS device stack1400. A CZTS film layer produced by one of the methods described previously may be incorporated into the example device structure illustrated inFIG. 14. Device stack1400includes a substrate1420, an electrical contact1422, a light-absorbing layer1440, a semiconductor layer1482, a conducting layer1486, and a metal grid1490. One or more layers of device stack1400may be deposited using one or more of chemical-vapor deposition, evaporation, atomic-layer deposition, sputtering, particle coating, spray pyrolysis, spin-coating, electro-deposition, electrochemical deposition, photoelectrochemical deposition, hot-injection, another suitable deposition process, or any combination thereof. AlthoughFIG. 14illustrates a particular arrangement of substrate1420, electrical contact1422, light-absorbing layer1440, semiconductor layer1482, conducting layer1486, and metal grid1490, this disclosure contemplates any suitable arrangement of substrate1420, electrical contact1422, light-absorbing layer1440, semiconductor layer1482, conducting layer1486, and metal grid1490. For example, the position of semiconductor layer1482and light-absorbing layer1440may be switched, such that semiconductor layer1482may be deposited on substrate1420and light-absorbing layer1440may be deposited on semiconductor layer1482. Moreover, althoughFIG. 14illustrates a particular number of substrates1420, electrical contacts1422, light-absorbing layers1440, semiconductor layers1482, transparent conducting layers1486, and metal grids1490, this disclosure contemplates any suitable number of substrates1420, electrical contacts1422, light-absorbing layers1440, semiconductor layers1482, transparent conducting layers1486, and metal grids1490. For example, device stack1400may include multiple light-absorbing layers1440and semiconductor layers1482, forming multiple p-n junctions. In addition, U.S. application Ser. No. 12/953,867, U.S. application Ser. No. 12/016,172, U.S. application Ser. No. 11/923,036, and U.S. application Ser. No. 11/923,070, the text of which are incorporated by reference herein, disclose additional layer arrangements and configurations for photovoltaic cell structures that may be used with particular embodiments disclosed herein.

In particular embodiments, substrate1420may be any suitable substrate capable of withstanding high temperatures and/or pressures. Substrate1420may provide structural support for the film stack. For example, substrate1420may be soda-lime glass, a metal sheet or foil (e.g., stainless steel, aluminum, tungsten), a semiconductor (e.g., Si, Ge, GaAs), a polymer, another suitable substrate, or any combination thereof. In particular embodiments, substrate1420may be coated with an electrical contact1422. Electrical contact1422may be any suitable electrode material, such as, for example, Mo, W, Al, Fe, Cu, Sn, Zn, another suitable electrode material, or any combination thereof. If substrate1420is a non-transparent material, then conducting layer1486may be transparent to allow light penetration into the photoactive conversion layer. In particular embodiments, substrate1420may be replaced by another suitable protective layer or coating, or may be added during construction of a solar module or panel. Alternatively, device stack1400may be deposited on a flat substrate (such as a glass substrate intended for window installations), or directly on one or more surfaces of a non-imaging solar concentrator, such as a trough-like or Winston optical concentrator.

In particular embodiments, light-absorbing layer1440may be a CZTS thin-film as described herein. Light-absorbing layer1440may also be another suitable material, such as CIGS or CdTe. Light-absorbing layer1440may be either a p-type or an n-type semiconductor layer. In particular embodiments, device stack1400may include multiple light-absorbing layers. The plurality of light-absorbing layers may vary between CZTS thin-films and other types of thin-films, such as CIGS or CdTe thin-films. Although this disclosure describes particular types of light-absorbing layers1440, this disclosure contemplates any suitable type of light-absorbing layer1440.

In particular embodiments, conducting layer1486may be a transparent conducting oxide, such as, for example, ZnO/Al, In2O3/Sn, another suitable transparent conducting oxide, or any combination thereof. In particular embodiments, conducting layer1486may be replaced by metal grid1490. Metal grid1490may be deposited using screen-printing. Metal grid1490may be arranged in a grid (e.g., fingers and busbars) on one side (or both sides) and a full area metal contact on the other side. Additional layers, such as anti-reflection coatings may also be added.

The layers of device stack1400may be deposited using any suitable process. In particular embodiments, the one or more layers of device stack may be deposited (e.g., by conventional sputtering or magnetron sputtering) in vacuum or in an atmosphere that includes at least one of the following gases: Ar, H, N2, O2, H2S, and H2Se. In particular embodiments, one or more of the layers of the multilayer structures described above may be doped (e.g., up to approximately 4 atomic %) with at least one of the following elements: Na, P, K, N, B, As, and Sb.

Miscellaneous

Herein, “or” is inclusive and not exclusive, unless expressly indicated otherwise or indicated otherwise by context. Moreover, “and” is both joint and several, unless expressly indicated otherwise or indicated otherwise by context. Furthermore, “a”, “an,” or “the” is intended to mean “one or more,” unless expressly indicated otherwise or indicated otherwise by context.

This disclosure encompasses all changes, substitutions, variations, alterations, and modifications to the example embodiments herein that a person having ordinary skill in the art would comprehend. Similarly, where appropriate, the appended claims encompass all changes, substitutions, variations, alterations, and modifications to the example embodiments herein that a person having ordinary skill in the art would comprehend. Moreover, this disclosure encompasses any suitable combination of one or more features from any example embodiment with one or more features of any other example embodiment herein that a person having ordinary skill in the art would comprehend. Furthermore, reference in the appended claims to an apparatus or system or a component of an apparatus or system being adapted to, arranged to, capable of, configured to, enabled to, operable to, or operative to perform a particular function encompasses that apparatus, system, component, whether or not it or that particular function is activated, turned on, or unlocked, as long as that apparatus, system, or component is so adapted, arranged, capable, configured, enabled, operable, or operative.