1. Technical Field
This disclosure relates to photovoltaic modules and methods of manufacturing photovoltaic modules and, more particularly, to photovoltaic modules and methods of manufacturing photovoltaic modules in which mechanical distortion in the modules is substantially reduced or eliminated.
2. Discussion of the Related Art
One type of flexible photovoltaic (PV) module is formed as a thin-film device on a polymeric substrate. An example of such devices is the Copper-Indium-Gallium-Selenide (CIGS) device. CIGS devices present many challenges in terms of the thin-film deposition processes, device patterning, and final assembly/packaging. Polymer substrates are of great significance since high-temperature variations of the material are adequate to accommodate CIGS processing while the material maintains its dielectric properties, which enables monolithic integration without any additional insulating films.
A fundamental challenge in flexible CIGS devices is in the deposition of a metallic back contact onto the polymer prior to the deposition of the CIGS p-type absorber layer. This back contact makes ohmic contact to the CIGS and allows for current to flow through the device and be collected through interconnects to the leads attached to the electrical load. Thus, this back contact, which is usually a metal, must maintain high electrical conductivity, both before and after device processing. It must also survive the deposition environment for the subsequent thin film deposition steps.
According to a first aspect, a polymer substrate and back contact structure for a photovoltaic element is provided. The structure includes a polymer substrate having a device side at which the photovoltaic element can be located and a back side opposite the device side. A layer of dielectric is formed at the back side of the polymer substrate. A metal structure is formed at the device side of the polymer substrate.
According to another aspect, a photovoltaic element is provided. The photovoltaic element includes a CIGS photovoltaic structure and a polymer substrate having a device side at which the CIGS photovoltaic structure can be located and a back side opposite the device side. A layer of dielectric is formed at the back side of the polymer substrate. A metal structure is formed at the device side of the polymer substrate between the polymer substrate and the CIGS photovoltaic structure.
According to another aspect, a method for forming a photovoltaic element includes the following steps: (1) disposing a first adhesion layer on a back side of a polymer substrate; (2) disposing a dielectric layer on the first adhesion layer; (3) after the step of disposing the dielectric layer, disposing a metal structure on a device side of the polymer substrate, the device side being opposite of the back side; and (4) disposing a CIGS photovoltaic structure on the metal structure.
According to another aspect, a method for forming a photovoltaic element includes the following steps: (1) disposing a dielectric layer on a back side of a polymer substrate; (2) disposing a metallic film layer on a device side of the polymer substrate, the device side being opposite of the back side; (3) disposing a molybdenum cap layer on the metallic film layer at least partially using a vacuum-based sputter deposition process at a pressure of less than 20 millitorr; and (4) disposing a CIGS photovoltaic structure on the molybdenum cap layer.
According to another aspect, a method for forming a photovoltaic element includes the following steps: (1) disposing a backside metal layer on a back side of a polymer substrate using a vacuum-based sputter deposition process at a pressure of less than 6 millitorr; (2) disposing a metallic film layer on a device side of the polymer substrate, the device side being opposite of the back side; (3) disposing a molybdenum cap layer on the metallic film layer; and (4) disposing a CIGS photovoltaic structure on the molybdenum cap layer.
According to another aspect, a photovoltaic element includes a polymer substrate having a device side and a back side opposite the device side. A dielectric layer is disposed on the back side of the polymer substrate, and a metallic film layer is disposed on the device side of the polymer substrate. A molybdenum cap layer is disposed on the metallic film layer, and the molybdenum cap layer has a density of at least 85% of the bulk density of molybdenum. A CIGS photovoltaic structure is disposed on the molybdenum cap layer.
The foregoing and other features and advantages will be apparent from the more particular description of preferred aspects, as illustrated in the accompanying drawings, in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale. In the drawings, the thickness of layers and regions may be exaggerated for clarity.
For CIGS devices, molybdenum (Mo) has been a common choice of material for a back contact, regardless of the substrate. While Mo can be deposited in a straightforward manner using DC sputtering or other thin film deposition methods, the wide range of stress states possible with sputtering can particularly complicate deposition onto flexible substrates, particularly those that do not exhibit significant stiffness, such as polymers. Unlike rigid substrates where the film stresses can readily be borne by the substrate, film stresses can have a significant impact upon the life, surface topology, and physical properties of flexible substrates, particularly substrates made from polymers. This class of substrates, while exhibiting excellent dielectric properties that allow monolithic integration, also typically exhibits high and inconsistent thermal expansion coefficient compared to the metals and semiconductors of the CIGS layer stack. Thus, there exist extrinsic stresses that combine with intrinsic stresses that can warp, wrinkle, distort and otherwise diminish the integrity of these flexible substrates. In addition, the electrical and mechanical properties of a back contact also affect the device performance and adhesion.
According to the present disclosure, a multilayer approach using two or more different metals in the back contact is used to replace the prior Mo film deposited onto both sides of a high-temperature polymeric substrate. According to the disclosure, the polymeric substrate can be, for example, polyimide, polybenzobisoxazole (PBO), insulated metal foils, or other such material for flexible, monolithically integrated CIGS modules using a high-temperature CIGS deposition process, such as multi-source evaporation. Unlike prior processes which use Mo films on both sides of the polymer in order to balance the stresses of this process, along with subsequent CIGS, CdS and TCO depositions, according to some exemplary embodiments, a stress-balanced back contact is formed using a dielectric film on the back side of the polymer substrate, a primary high-conductivity but low-modulus and low-cost metallic film layer, for example, aluminum (Al), applied to the front side of the polymer, followed by a thin cap of Mo over the Al film layer. The Mo may be disposed onto the Al with or without added oxygen.
Referring to
Mo presents a challenge in that, not only can the material exhibit dramatically different inherent stresses due to variations in process parameters, but mismatches in the coefficient of thermal expansion (CTE) between Mo and the underlying substrate coupled with high-temperature processing, the stiffness of the substrate, and ultimately, the mechanical properties of the subsequent films, can all lead to large stresses in the resultant multilayer construction. Mo can be deposited in various intrinsic stress states ranging from tensile to compressive in nature, as shown in
Table 1 illustrates the challenge in depositing a metal, particularly Mo, onto a high-temperature polymeric substrate. Both Mo and Al have a much higher modulus by an order of magnitude than the polymer, while the thermal expansion may be a closer match between Al and the polymer than Mo. More importantly, the yield stress of the Al is much lower than Mo, and the stress at 5% elongation of the polymer is closer to Al than Mo. Finally, the ultimate stress of the Mo is nearly twice that of the polymer.
In accordance with some exemplary embodiments, the overall stress state in the polymer is reduced, and, as a result, a more planar, wrinkle-free substrate is provided. Because Mo is used for a proper interface to CIGS, but is a major reason for the high stresses in the substrate, according to the inventive concept, its use has been minimized to the minimum required to mask the work function of the underlying primary metallic film, as shown in Table 2. In some exemplary embodiments, the primary metallic film of choice is aluminum (Al), although formulations using copper (Cu) and other highly electrically conductive materials, for example, brass or bronze, can be used. The CIGS device relies on the proper work function of its metallic back contact to function properly. While it is possible to use metallic foils (without insulting layers) with subsequent Mo deposition to mask the work function of the metal foil substrate, the inherent stiffness of the non-polymeric substrates enables the ability to apply greater Mo film thicknesses without the Mo stress overwhelming the substrate. With the polymeric process according to embodiments of the inventive concept, and their lower mechanical properties, the desirable masking effect by the Mo of the work function of the underlying primary thin film back contact material (Al, Mo, etc.) is carefully balanced with the high stresses in Mo that can increase with greater Mo thickness. Furthermore, the use of metallic foils without insulating layers precludes the straightforward ability to integrate monolithically the photovoltaic device, and as such, limit device construction to discrete individual cells.
The AlMo stack of some exemplary embodiments provides several advantages over conventional single or multi-layer Mo back contacts.
Because Mo that is sufficiently thick to provide adequate electrical conductivity on polymer contributes adversely to the stress state in the photovoltaic stack, minimizing the Mo content of the device back contact allows for another material, other than Mo, to serve as a back-side film, according to exemplary embodiments. According to the present disclosure, by eliminating dependence upon Mo on the back-side film, and by minimizing it in the back contact, significant advantages over the prior art are realized.
According to the exemplary embodiments, elimination of a metal back side film and replacing it with a dielectric layer provides thermal management in the device, in addition to stress management, as described herein in detail. Heating of substrates in a vacuum includes conductive heating (direct contact to a substrate) and/or radiative heating (energy radiating from one source to another). Radiative heating is the most common means of transferring thermal energy to the substrate, but the degree to which energy is conveyed is dependent upon the substrate's absorptivity (ability to absorb energy) and emissivity (ability to radiate heat into the environment). Metals typically have lower emissivity than, for example, oxide films; thus, metal surfaces do not give up their heat as easily as oxides. Thus a polymer coated with metal on both sides can trap the heat within the sandwiched polymer substrate. In a vacuum, a surface coated with a high-emittance coating, such as an oxide or nitride, can provide radiative cooling to that surface and the substrate. A cooler back side coating and substrate helps to keep the substrate from degrading and embrittling during high device-side temperatures, and thus enables higher device-side temperatures that can lead to higher quality solar absorber layers.
Applicant has additionally determined that it is desirable that Mo cap layer 18 be relatively dense to minimize diffusion of metal, such as aluminum or copper, from metallic film layer 16 into CIGS layer 20. For example, in some embodiments, Mo cap layer 18 has a density of at least 85% of the bulk density of molybdenum so that Mo cap layer 18 acts as a diffusion barrier, thereby potentially enabling aluminum, copper, or other metal, of metallic film layer 16, to be disposed adjacent to Mo cap layer 18 without significant diffusion of the metal through Mo cap layer 18. High density of Mo cap layer 18 is obtained, for example, by using a low-pressure vacuum-based sputter deposition process to deposit Mo cap layer 18. For example, in a particular embodiment, Mo cap layer 18 is deposited by a vacuum-based sputter deposition process at a pressure of less than 20 millitorr (mTorr), preferably at less than 6 mTorr, to obtain high density of Mo cap layer 18.
In some embodiments, Mo cap layer 18 includes a plurality of sublayers, where a sublayer closest to metallic film layer 16 has a high density, and one or more other sublayers further from metallic film layer 16 have lower densities. For example,
Moreover, Applicant has additionally determined that it may be desirable to deposit dielectric layer 12 before metal film layer 16 and Mo cap layer 18 (or bilayer Mo cap layer 418) in embodiments including optional adhesion layer 13. In particular, adhesion layer 13 is typically at least slightly electrically conductive, and presence of adhesion layer 13 may therefore cause arcing if metallic film layer 16 and/or Mo cap layer 18 are deposited by a sputter process. Deposition of dielectric layer 12, however, insulates adhesion layer 13. Thus, deposition of dielectric layer 12 before depositing metallic film layer 16 and Mo cap layer 18 reduces the likelihood of arcing during sputter deposition of metallic film layer 16 and Mo cap layer 18.
While it is desirable for the backside layer to be dielectric, in some alternate embodiments, dielectric layer 12 is replaced with a backside metal layer, where the backside metal layer balances stress resulting from layers on the device side of polymer substrate 14. For example,
Exemplary embodiments have been described herein. For example, exemplary embodiments have been described in terms of specific exemplary polymeric substrates and particular exemplary coatings or layers. It will be understood that the exemplary embodiments relate to stress balancing to provide a back contact in an improved photovoltaic device. Therefore, the present disclosure is applicable to other substrate materials and other back side coatings or layers. In fact, the disclosure may be applicable to structures with alternate substrates and the elimination of back side coating altogether.
Various features of the present disclosure have been described above in detail. The disclosure covers any and all combinations of any number of the features described herein, unless the description specifically excludes a combination of features. The following examples illustrate some of the combinations of features contemplated and disclosed herein in accordance with this disclosure.
In any of the embodiments described in detail and/or claimed herein, the photovoltaic element can comprise a CIGS structure.
In any of the embodiments described in detail and/or claimed herein, the dielectric can comprise at least one of SiO2, Al2O3, and silicone resin.
In any of the embodiments described in detail and/or claimed herein, a thin adhesion layer can be disposed between the layer of dielectric and the back side of the polymer substrate.
In any of the embodiments described in detail and/or claimed herein, the adhesion layer can comprise at least one of Mo, Cr, and Ti.
In any of the embodiments described in detail and/or claimed herein, the metal structure can comprises a first metal layer, the first metal layer comprising at least one of aluminum, brass, bronze and copper. The metal structure is optionally disposed on the polymer substrate after the dielectric layer is disposed on the polymer substrate.
In any of the embodiments described in detail and/or claimed herein, the metal structure can further comprise a layer of molybdenum formed over the first metal layer. The layer of molybdenum optionally has a density of at least 85% of the bulk density of molybdenum. The layer of molybdenum is optionally formed at least partially using a vacuum-based sputter deposition process at a pressure of less than 20 mTorr. The layer of molybdenum optionally includes a plurality of sublayers, where a sublayer closest to the first metal layer is formed using a vacuum-based sputter deposition process at a pressure of less than 20 mTorr, and one or more sublayers further from the first metal layer are formed using a vacuum-based sputter deposition process at a pressure of greater than that used to form the sublayer closest to the first metal layer.
In any of the embodiments described in detail and/or claimed herein, the metal structure can further comprise a thin adhesion layer disposed between the first metal layer and the device side of the polymer substrate.
In any of the embodiments described in detail and/or claimed herein, the thin adhesion layer can comprise at least one of molybdenum, aluminum, titanium and chromium.
In any of the embodiments described in detail and/or claimed herein, the metal structure can further comprise a thin adhesion layer in contact with the device side of the polymer layer.
In any of the embodiments described in detail and/or claimed herein, the thin adhesion layer can comprise at least one of molybdenum, aluminum, chromium, titanium nitride (TiN), a metal oxide, and a metal nitride.
In any of the embodiments described in detail and/or claimed herein, the dielectric layer may be replaced with a backside metal layer formed using a vacuum-based sputter deposition processes at a pressure of less than 6 mTorr. The backside metal layer can be formed of a Mo layer, where the Mo layer is optionally 10% to 30% oxygen in the ambient gas during the deposition process.
While the present disclosure makes reference to exemplary embodiments, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present disclosure.
This application is a continuation-in-part of U.S. non-provisional application Ser. No. 14/198,209 filed Mar. 5, 2014, which is a continuation of U.S. non-provisional application Ser. No. 13/572,387 filed Aug. 10, 2012, which claims the benefit of priority to U.S. Provisional Patent Application No. 61/522,209 filed Aug. 10, 2011. Each of the above-mentioned applications is incorporated herein by reference.
Number | Date | Country | |
---|---|---|---|
61522209 | Aug 2011 | US |
Number | Date | Country | |
---|---|---|---|
Parent | 13572387 | Aug 2012 | US |
Child | 14198209 | US |
Number | Date | Country | |
---|---|---|---|
Parent | 14198209 | Mar 2014 | US |
Child | 14210209 | US |