Formation of solar cells on foil substrates

ES 2 342 091 T3

DESCRIPTION

Solar cell formation on metallic foil substrates.

Field of the invention

The present invention relates to the manufacture of photovoltaic devices, and more specifically to the processing and annealing of absorbent layers for photovoltaic devices.

Background of the invention

Efficient photovoltaic devices, such as solar cells, have been manufactured using absorbent layers made of alloys containing Group IB, IIIA and VIA elements, for example copper alloys with indium and / or gallium or aluminum, and selenium and / or sulfur. Such absorbent layers are often called CIGS layers and the resulting devices are often called CIGS solar cells. The CIGS layer can be deposited on a substrate. It would be desirable to fabricate such an absorbent layer on an aluminum foil substrate, because aluminum foil is relatively inexpensive, lightweight, and flexible. Unfortunately current techniques for depositing absorbent CIGS layers are incompatible with the use of aluminum foil as a substrate.

Typical deposition techniques include evaporation, sputtering, chemical vapor deposition, and the like. These deposition processes are typically carried out at elevated temperatures and for prolonged periods. Both of these factors can result in damage to the substrate on which the deposition is occurring. Such damage can be generated directly from changes in the substrate material after exposure to heat, and / or from undesirable chemical reactions driven by the heat of the deposition process. Therefore, very robust substrate materials are typically required for the fabrication of CIGS solar cells. These limitations have excluded the use of aluminum and aluminum foil-based metal foils.

An alternative deposition approach is solution-based printing of CIGS primer materials onto a substrate. Examples of solution-based printing techniques are described, for example, in PCT Published Application WO 2002/084 708 and commonly issued US 2005-0 183 767. Advantages of this deposition approach include both deposition temperature relatively less as the speed of the deposition process. Both of these advantages serve to minimize the potential for heat-induced damage to the substrate on which the deposition is forming.

While solution deposition is a relatively low temperature stage in CIGS solar cell manufacturing, it is not the only stage. In addition to deposition, a key step in the manufacture of CIGS solar cells is the selenization and annealing of the CIGS absorbent layer. Selenization introduces selenium into the absorbent layer of bulk CIG or CI, where the element is incorporated into the structure, while annealing provides the absorbent layer with the appropriate crystalline structure. In the prior art, selenization and annealing have been carried out by heating the substrate in the presence of H vapor.₂Se or Se, and keeping this incipient absorbent layer at elevated temperatures for extended periods of time.

While the use of Al as a substrate for solar cell devices would be desirable due to both the low cost and lightweight nature of such a substrate, conventional techniques that effectively anneal the absorbent layer of CIGS also heat the substrate to elevated temperatures, which results in damage to Al substrates. There are several factors that result in degradation of the Al substrate upon prolonged exposure to heat and / or selenium-containing compounds for prolonged periods. First, upon prolonged heating, discrete layers within a Mo-coated Al substrate can melt and form an intermetallic back contact for the device, which decreases the expected electrical functionality of the Mo layer. Second, the interfacial morphology of the Mo layer is altered during heating, which can negatively affect subsequent CIGS grain growth through changes in nucleation patterns that can arise on the surface of the Mo layer. Third, after extended heating, Al can migrate to the absorbent layer CIGS, impairing the function of the semiconductor. Fourth, impurities that are typically present in Al foil (e.g. Si, Fe, Mn, Ti, Zn, and V) can move along with mobile Al that diffuses into the solar cell after prolonged heating, thereby that can impair both the electronic and optoelectronic functions of the cell. Fifth, when Se is exposed to Al for relatively long periods and at relatively high temperatures, aluminum selenide can form, which is unstable. In humid air, aluminum selenide can react with water vapor to form aluminum oxide and hydrogen selenide. Hydrogen selenide is an extremely toxic gas, the free formation of which can represent a safety risk. For all these reasons, high temperature deposition, annealing and selenization are therefore not very viable for substances made of aluminum or aluminum alloys.

Due to long-term elevated temperature deposition and annealing steps, CIGS solar cells cannot be efficiently fabricated on aluminum substrates (for example, flexible metal foils composed of Al and / or Al-based alloys), and instead they must be manufactured from heavier substrates made of more robust (and more expensive) materials such as stainless steel, titanium or molybdenum sheet metal,

ES 2 342 091 T3 glass substrates, or glass coated with metal or metal oxides. Thus, even though CIGS solar cells based on aluminum foil would be lighter, more flexible and cheaper than stainless steel, titanium or molybdenum foil, glass substrates, or metal coated glass substrates. or with metallic oxides, current practice does not allow aluminum foil to be used as a substrate.

Document WO03 / 007 386 discloses a photovoltaic device, comprising: an electrically conductive aluminum foil substrate; at least one electrically conductive base electrode layer comprising a layer of molybdenum; an adhesion layer located between the aluminum foil substrate and the electrode layer, said adhesion layer comprising chromium; and an absorbent layer containing one or more elements of groups IB, IIIA and VIA disposed on the aluminum foil substrate.

Summary

It is an object of the present invention to provide an improved manufacturing process for protecting the aluminum substrate during manufacturing.

The present invention discloses an improved method of forming an absorbent layer of a photovoltaic device, according to claim 1. Other advantageous embodiments are set forth in the dependent claims.

Brief description of the drawings

The descriptions of the present invention can be easily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:

Figure 1 is a schematic cross-sectional diagram illustrating the manufacture of an absorbent layer in accordance with one embodiment of the present invention.

Description of the specific realizations

Although the following detailed description contains many specific details for illustrative reasons, one skilled in the art will appreciate that many variations and alterations to the following details are within the scope of the invention. Accordingly, exemplary embodiments of the invention described below are set forth without any loss of generality to the claimed invention and without imposing limitations thereon.

Embodiments of the present invention allow the fabrication of CIGS absorbent layers on aluminum foil substrates. In accordance with embodiments of the present invention, an incipient absorbent layer containing Group ID and IIIA elements formed on an aluminum substrate by solution deposition can be annealed by rapid heating from room temperature to a plateau temperature range of between about 200 ° C and about 600 ° C. The temperature is kept in the plateau range for about 2 minutes to about 30 minutes, and then reduced. Alternatively, in an example that is not part of the invention, the annealing temperature could be modulated to oscillate within a temperature range without staying at a particular plateau temperature.

Figure 1 depicts a partially fabricated photovoltaic device 100, and a rapid heating unit 110; The device generally includes an aluminum foil substrate 102, an optional base electrode 104, and an incipient absorbent layer 106. The aluminum foil substrate 102 can be from about 5 microns to about one hundred or more microns in thickness, and of any suitable width and length. The aluminum foil substrate 102 may be made of aluminum or an aluminum-based alloy. Alternatively, the aluminum foil substrate 102 may be made by metallizing a polymeric foil substrate where the polymer is selected from the group of polyesters, polyethylene naphthalate, polyetherimides, polyethersulfones, polyether ether ketone, polyimides and / or combinations of the above. By way of example, the substrate 102 may be in the form of a long sheet of aluminum foil suitable for processing in a roll-to-roll system. The base electrode 104 is made of an electrically conductive material compatible with the processing of the incipient absorbent layer 106. By way of example, the base electrode 104 may be a layer of molybdenum, for example, about 0.1 to 25 microns thick, and more preferably about 0.1 to 5 microns thick. The base electrode layer can be deposited by sputtering or evaporation, or alternatively by chemical vapor deposition (CVD), atomic layer deposition (ALD), sol-gel coating, electroplasty and Similar.

Aluminum and molybdenum can interdiffuse into each other, with damaging electronic and / or optoelectronic effects in device 100. To inhibit such interdiffusion, an interfacial layer 103, intermediate, can be incorporated between sheet metal substrate 102 made of aluminum and the base electrode 104 of molybdenum. The interfacial layer can be formed of compounds such as nitrides (including tantalum nitride, tungsten nitride, and silicon nitride), oxides and / or carbides. The thickness of this layer can vary from 10 nm to 50 nm, and more preferably from 10 nm to 30 nm.

ES 2 342 091 T3

The incipient absorbent layer 106 includes material containing Group 1B, IIIA, and (optionally) VIA elements. Preferably, the copper (Cu) of the absorbent layer is the element of group IB, gallium (Ga) and / or indium (In) and / or aluminum can be the elements of group IIIA, and selenium (Se) and / or sulfur (S) the elements of the group VIA. The group VIA element can be incorporated into the incipient absorbent layer 106 when initially deposited in solution or during subsequent processing to form a final absorbent layer from the incipient absorbent layer 106. The incipient absorbent layer 106 can be about 1000 nm thick when deposited. The subsequent rapid thermal process and the incorporation of elements of the group VIA can change the morphology of the resulting absorbent layer, so that it increases its thickness (for example, at most to about twice the thickness of the incipient layer under certain circumstances).

Fabrication of the absorbent layer on the aluminum foil substrate 102 is relatively straightforward. First, the incipient absorbent layer is deposited on the substrate 102 directly on the aluminum or on a higher layer such as the electrode 104. By way of example, and without loss of generality, the incipient absorbent layer is deposited in the form of a film of a solution-based precursor material, containing nanoparticles that include one or more elements of groups IB, IIIA and (optionally) VIA. Examples of such films are described in such solution-based printing techniques, for example in commonly issued US Pat. 2005-0 183 767, entitled "SOLUTION-BASED FABRICATION OF PHOTOVOLTAIC CELL" and also in PCT Publication WO 02/084 708 entitled "METHOD OF FORMING SEMICONDUCTOR COMPOUND FILM FOR FABRICATION OF ELECTRONIC DEVICE AND FILM PRODUCED BY SAME ”(method of forming a semiconductor composite film for the manufacture of electronic device and film produced by it).

Alternatively, but not included in the invention, the incipient absorbent layer 106 may be manufactured by a sequence of atomic layer deposition reactions or by any other process normally used to form such layers. Atomic layer deposition of absorbent layers IB-IIIA-VIA is described, for example, in commonly issued US 2005-0 186 342, entitled "FORMATION OF CIGS ABSORBER LAYER MATERIALS USING ATOMIC LAYER DEPOSITION AND HIGH THROUGHPUT SURFACE TREATMENT ON COILED FLEXIBLE SUBSTRATES ”(formation of absorbent layer materials CIGS using atomic layer deposition and high performance surface treatment on flexible spiral substrates).

The incipient absorbent layer 106 is then flash annealed, and / or the substrate 102, from room temperature to an average plateau temperature range of between about 200 ° C and about 600 ° C, with the heating unit. 110. The heating unit 110 preferably provides sufficient heat to rapidly increase the temperature of the incipient absorbent layer 106 and / or the substrate 102 (or a significant part thereof) for example, to between about 5 ° C / s and about 150 ° C. / s. By way of example, heating unit 110 may include one or more infrared (IR) lamps that provide sufficient radiant heat. As an example, 8 IR lamps rated at about 500 W each positioned approximately 1/8 "(3.1750 mm) to 1" (24.4 mm) from the surface of the substrate 102 (4 above and 4 below the substrate, all directed towards the substrate) can provide enough radiant heat to process a substrate area of ​​about 25 cm² per hour in a 4 ”tube furnace. The lamps can be activated gradually in a controlled manner, for example at a rate of rise of about 10 ° C / s. Those skilled in the art will be able to devise other types of heat source configurations that can be used as a heating unit 110. For example, in the roll-to-roll manufacturing line, heating and other processes can be accomplished by using IR lamps 25.4mm (1 ”) apart along the length of the processing region, with IR lamps positioned homogeneously both above and below the substrate, and where both IR lamps both above and below the substrate are directed towards the substrate. Alternatively, the IR lamps could be positioned just above or just below substrate 102, and / or in configurations that increase lateral heating from the camera side toward the substrate 102 side.

The absorbent layer 106 and / or the substrate at 102 are maintained in the average plateau temperature range for between about 2 minutes and about 30 minutes. For example, the temperature can be kept in the desired range by reducing the amount of heat from the heating unit 110 to a suitable level. In the example of IR lamps, the heat can be reduced simply by unplugging the lamps. Alternatively, the lamps can be actively cooled. The temperature of absorbent layer 106 and / or substrate 102 is subsequently reduced to a suitable level, for example, by further reducing or turning off the heat supply from heating unit 110.

In embodiments of the invention, two or more discrete or continuous annealing steps are carried out sequentially, in which group VIA elements such as selenium or sulfur are incorporated in a second or final step. For example, the incipient absorbent layer 106 can be exposed to H₂Be gaseous, H₂S gaseous or Se vapor before, or during, flash heating or rapid thermal processing (RTP). In this embodiment, the relative shortness of exposure allows the aluminum substrate to better resist the presence of gases and vapors, especially at high thermal levels.

Once the incipient absorbent layer 106 has been annealed, additional layers can be formed to complete the device 100. For example, a window layer is typically used as a bonding partner for the absorbent layer. By way of example, the binding partner layer can include cadmium sulfide (CdS), zinc sulfide (ZnS), or zinc selenide (ZnSe) or some combination of two or more of these. Layers of these materials can be deposited,

ES 2 342 091 T3 for example by chemical bath deposition, chemical surface deposition, or aerosol pyrolysis, up to a thickness of between about 50 m and about 100 nm. Furthermore, a transparent electrode, for example a conductive oxide layer, can be formed in the window layer by sputtering, vapor deposition, CVD, ALD, atomic layer electrochemical epitaxy, and the like.

Embodiments of the present invention overcome the disadvantages associated with the prior art by rapidly thermally processing incipient CIGS absorbent layers deposited, or otherwise formed, on aluminum substrates. Aluminum substrates are much cheaper and lighter than conventional substrates. Therefore, solar cells based on aluminum substrates can have a lower cost per watt and a shorter energy payback period, compared to conventional silicon-based solar cells. Additionally, aluminum substrates allow for a flexible form factor that enables both high performance roll-to-roll printing during solar cell manufacturing, as well as faster and easier installation processes during solar system and module installation.

Embodiments of the present invention allow the fabrication of inexpensive and lightweight photovoltaic devices on aluminum substrates. Flash heat processing / rapid heat processing processing of incipient absorbent layer 106 allows proper annealing and incorporation of group VIA elements without damaging or destroying the aluminum foil substrate 102. The plateau temperature range is sufficiently below the melting point of aluminum (about 660 ° C) to avoid damaging or destroying the aluminum foil substrate. The use of aluminum foil substrates greatly reduces the material costs of photovoltaic devices, for example solar cells, made on such substrates, thereby reducing the cost per watt. Economies of scale can be obtained by processing the aluminum foil substrate in roll-to-roll form, with the various layers of the photovoltaic devices being built on the substrate as it passes through a series of deposition, annealing, and other process steps. .

While the foregoing is a complete description of the preferred embodiment of the present invention, it is possible to use various alternatives, modifications, and equivalents. Therefore, the scope of the present invention should not be determined by referring to the above description but by referring to the appended claims, with the full scope of their equivalents.

References cited in description

The list of references cited by the applicant is for the convenience of the reader only. It is not part of the European Patent document. Although special care has been taken in compiling the references, errors or omissions cannot be ruled out and the EPO disclaims all responsibility in this regard.

Patent documents cited in the description • WO 2002 084 708 A [0004] • US 2005 0 183 767 A [0004] [0017] • WO 03 007 386 A [0008] • WO 02 084 708 A [0017] • US 2005 0 186 342 A [0019]