WO2011087895A2

WO2011087895A2 - Photovoltaic module and method for making the same

Info

Publication number: WO2011087895A2
Application number: PCT/US2011/000026
Authority: WO
Inventors: David A. Strickler; Douglas M. Nelson
Original assignee: Pilkington Group Limited
Priority date: 2010-01-14
Filing date: 2011-01-06
Publication date: 2011-07-21
Also published as: WO2011087895A3

Abstract

A photovoltaic module includes a glass substrate, a dielectric metal oxide interlayer formed over the glass substrate, a transparent conductive metal oxide layer formed over the dielectric metal oxide interlayer, and a photovoltaic structure formed over the transparent conductive metal oxide layer. Additional structure may be formed over the photovoltaic layers including another transparent conductive metal oxide layer. A process for forming the same is also included.

Description

TITLE

PHOTOVOLTAIC MODULE AND METHOD FOR MAKING THE SAME

BACKGROUND OF THE INVENTION

The invention relates to photovoltaic modules, specifically thin-film photovoltaic modules.

A thin-film photovoltaic module comprises a sequence of coating layers on a substrate which convert light into electricity. A conventional thin-film photovoltaic module is shown in Fig. 1 , and includes a photovoltaic module 50 formed over a substrate 52 and with a TCO layer 58 in contact or in close proximity with a photovoltaic (PV) structure and a back contact layer. Both the TCO layer 58 and the two PV layers 54, 56 may be capable of absorbing light.

When a photon is absorbed in the PV layers 54, 56, the energy of the absorbed photon creates an electron-hole pair. Under the influence of the electric field created by the junction between the two PV layers 54, 56, the electrons and holes move into the external electrical circuit and thereby produce electric power. However, some photons may be absorbed in one of the PV layers at a distance from the junction between the PV layers 54, 56. Electron-hole pairs created outside the region of the junction are more likely to recombine than to move into the external electric circuit. Thus, conversion efficiency is reduced. Accordingly, it is preferable that the photons are absorbed in the PV layer 54, 56 as close to the PV junction as possible. To facilitate such photon absorption, it is highly desirable that the thickness of the PV layers 54, 56 is minimized.

Known photovoltaic cell 50 may also comprise a second TCO layer 60.

The first and second TCO layers 58, 60 allow electric current to flow from the PV layers 54, 56 to an external electrical circuit.

As shown in Fig. 2, in known PV modules, in at least an upper surface of the first TCO layer 58, a large range of vertical deviations may exist. The average of the absolute values of these vertical deviations is often referred to as the average surface roughness, Ra, of the surface. The difference between the maximum and minimum vertical deviations is often referred to as the maximum profile height of the surface, Rt. When the thickness of the first PV layer 54 is not greater than the difference between the maximum and minimum vertical deviations, the TCO layer 58 will interact with the second PV layer 56. This interaction may result in electrical short circuits, reductions in the photovoltaic cell's power output, and/or heat generation within the cell, and thus is highly undesirable.

Accordingly, in order to reduce the thickness of the PV layer 54 without incurring the above-mentioned problems, the amplitude of the maximum and minimum vertical deviations in the upper surface of the first TCO layer 58 should be minimized. Therefore, it would be desirable to provide a photovoltaic cell having a first TCO layer upper surface which provides such improvement and a method for forming same, meeting the above-noted criteria.

SUMMARY OF THE INVENTION

In accordance with the present invention, a photovoltaic module is provided. The photovoltaic module of the invention includes an interlayer having an Ra of < 20 A formed over an upper surface of a glass substrate. At least one transparent conductive metal oxide layer (TCO) is formed over the interlayer. The TCO layer has a thickness of at least 3500 A. Finally, at least one photovoltaic structure is formed over the at least one TCO layer.

In another embodiment, the photovoltaic module includes an interlayer having an upper surface and having an Rt of < 170 A formed over a surface of a glass substrate. Further, at least one TCO layer having an upper surface is formed over the interlayer. The at least one TCO layer has a thickness of at least 3500 A. Finally, at least one photovoltaic structure is formed over the at least one TCO layer.

In another aspect of the invention, a process for forming a coated glass substrate for a photovoltaic cell is provided. The process includes providing a moving glass substrate surrounded by an atmosphere at essentially

atmospheric pressure. The process also includes delivering a precursor gas mixture at a predetermined temperature to a location adjacent the moving glass substrate. The precursor gas mixture is introduced into a vapor space above the moving glass substrate, wherein a single or multi-layer interlayer is formed on the substrate. The interlayer has one or more of an Rt < 170 A and an Ra of ≤ 20 A. The process also includes forming at least one TCO layer over the interlayer. The at least one TCO layer has a thickness of at least 3500 A.

BRIEF DESCRIPTION OF THE DRAWINGS

The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description when considered in light of the accompanying drawings in which:

Fig. 1 is a cross-sectional view of a photovoltaic module known in the art;

Fig. 2 is an inset of Fig. 1 ;

Fig. 3 is a cross-sectional view of an embodiment of the photovoltaic module of the invention; and

Fig. 4 is an inset of Fig. 3. DETAILED DESCRIPTION OF THE INVENTION

It is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes described in the following specification are simply exemplary embodiments of the inventive concepts. Hence, specific dimensions, directions, or other physical characteristics relating to the embodiments disclosed are not to be considered as limiting, unless the claims expressly state otherwise.

The present invention is generally practiced in connection with the formation of a photovoltaic module 10 on a glass substrate 12. The glass substrate 12 may be a continuous sheet of glass or glass ribbon formed during float glass manufacturing.

As shown in Fig. 3, the photovoltaic module 10 of the present invention comprises at least one interlayer 22 formed over the substrate 12. The at least one interlayer 22 may comprise at least one dielectric metal oxide layer 20 and at least one dielectric oxide layer 14. The at least one dielectric oxide layer 14 is preferably an oxide of silicon. A preferred oxide of silicon for use in the present invention is SiO₂ having a thickness of between 200 A and 400 A, and most preferably having a thickness of about 250 A. The dielectric metal oxide layer 20 of the interlayer 22 may be a transparent metal oxide, for example, at least one of T1O2, AI2O3, Ga₂0₃, MgO, or ZnO or may be a transparent mixed metal oxide, for example, Ti02:Si02, Zn₂SnO₄, or ZnSn0₃.

In an embodiment, the at least one dielectric oxide layer 14 of the interlayer 22 may be formed over the substrate 12 and is separated from the substrate 12 by a dielectric metal oxide layer 20. The at least one dielectric oxide layer 14 may be formed over or directly onto the substrate 12. The at least one dielectric metal oxide layer 20 may preferably be deposited at a thickness of between 100 A and 300 A, and most preferably at a thickness of about 250 A.

It has surprisingly been discovered that the roughness of the upper surface of_the at least one dielectric metal oxide layer 20 of the interlayer 22 has an effect on the roughness of the layers subsequently formed over it.

Thus, regardless of its thickness, the present invention provides for a reduction in the Ra and/or the Rt of the dielectric metal oxide layer 20 of the interlayer 22 over those previously known. Accordingly, the dielectric metal oxide layer 20 of the interlayer 22 preferably has an Ra of < 20 A. Even more preferably, the dielectric metal oxide layer 20 has an Ra of < 15 A. In another aspect of the invention, the Rt of the dielectric metal oxide layer 20 is preferably < 170 A.

Even more preferably, the dielectric metal oxide layer 20 has an Rt of < 160 A.

The photovoltaic module 10 of the present invention also comprises at least one TCO layer 26 formed over the dielectric metal oxide layer 20.

Generally, the at least one TCO layer 26 is in contact with the at least one dielectric oxide layer 14. As shown in Figs. 3 and 4, the at least one TCO layer 26 is generally in contact with a photovoltaic structure 32, for example a PV semiconductor layer such as denoted herein by the reference numeral 16. The at least one TCO layer 26 may be made conductive by doping at least a portion of the TCO layer with a metal such as aluminum or with a non-metal such as fluorine or nitrogen. A preferred doped transparent conductive metal oxide is SnO₂:F.

The thickness of the at least one TCO layer 26 may selectively vary depending on the composition and the desired conductivity of the layer. However, it is preferable that the at least one TCO layer 26 has a thickness of at least 3500 A. The at least one TCO layer 26 preferably has an Ra of < 170 A. The at least one TCO layer 26 also preferably has an Rt of < 1600 A. More preferably, the at least one TCO layer 26 has an Rt of < 1500 A.

The photovoltaic module 10 of the present invention comprises one or more photovoltaic semiconductor layers 16, 18, collectively forming at least one photovoltaic structure 32. The at least one photovoltaic structure 32 may be formed over the at least one TCO layer 26. However, those skilled in the art would appreciate that additional layers may be formed between the at least one TCO layer 26 and the at least one photovoltaic structure 32. Thus, at least one high resistivity transparent layer (not shown) may be formed between the at least one TCO layer 26 and the at least one photovoltaic structure 32. The high resistivity transparent layer may comprise, as examples, T1O2, ZnSnO3, SnO2, or Zn₂SnO4.

The semiconductor layers 16, 18 of the photovoltaic structure 32 may each be formed at a thickness of at least 10,000 A. However, it may be preferable that one or both of the layers 16, 18 be formed at a thickness of less than 10,000 A, even for example, less than 5,000 A. The at least one photovoltaic structure 32 may comprise, for example, compounds of silicon or cadmium, such as amorphous and crystalline silicon, and cadmium telluride or cadmium sulfide.

Additional layers may be formed over the at least one photovoltaic structure 32, such as protective overcoat layers, dielectric layers, and/or additional TCO layers.

The present invention also includes a method for forming a coated glass substrate for the photovoltaic module 10. The glass substrate 12 may be stationary or moving when one or more of the layers of the photovoltaic module 10 is formed. Preferably, as many layers as possible are formed when the glass substrate is moving through the float glass manufacturing process.

Where a float glass manufacturing process is utilized as a means for practicing the method of the present invention, the float glass manufacturing line more particularly comprises a canal section along which molten glass is delivered from a melting furnace, to a float bath section wherein the continuous glass ribbon is formed in accordance with the well known float process.

Preferably, the glass produced is a soda-lime-silica glass. The glass ribbon advances from the bath section through an adjacent annealing lehr and a cooling section.

The float bath section includes a bottom section within which a bath of molten tin is contained, a roof, opposite sidewalls, and end walls. The roof, side walls, and end walls together define an enclosure in which a non-oxidizing atmosphere is maintained to prevent oxidation of the molten tin. Additionally, gas distributor beams may be located in the float bath section. The gas distributor beams in the bath section may be employed to form the dielectric metal oxide layer 20, the at least one dielectric oxide layer 14, the at least one TCO layer 26, and/or the at least one photovoltaic structure 32. Alternatively, forming the dielectric metal oxide layer 20, the at least one dielectric oxide layer 14, the at least one TCO layer 26, and the at least one photovoltaic structure 32 may take place further along the glass manufacturing line, for example in the gap between the float bath and the annealing lehr, or in the annealing lehr. However, not all of the layers comprising the photovoltaic module 0 need to be formed during the glass manufacturing process in order to practice the present invention. The at least one photovoltaic structure 32 may especially be formed in a process which is not a part of the float glass manufacturing process.

Preferably, the process of the present invention comprises a chemical vapor deposition (CVD) process. More preferably, the process of the invention comprises an atmospheric chemical vapor deposition process.

In a CVD process according to the invention, the precursor gas mixture is preferably premixed prior to contacting the substrate. For example, the dielectric metal oxide layer 20 of the interlayer 22 may be formed by volatilizing and premixing a precursor compound, such as an organotitanium compound, with an inert gas. Examples of suitable organotitanium precursor compounds include titanium ethoxide, titanium methoxide, and titanium isopropoxide.

Those skilled in the art would appreciate that the present invention is not limited to only the above-noted titanium precursors. Thus, the present invention may be another chlorine free organotitanium compound. The inert gas may be helium, nitrogen, argon, or combinations thereof. An oxidant, such as oxygen or ethyl acetate, may also be added to the precursor gas mixture. The precursor gas mixture may be delivered at a temperature below the thermal decomposition temperature of the precursor compound to a location adjacent the substrate 12. The precursor gas mixture may be introduced into a vapor space above the substrate 12. Generally, the substrate 12 is at a temperature above 750°F when the precursor gas mixture is introduced into the vapor space above it. Preferably, the temperature of the substrate 12 is between about 840°F - 1300°F when the precursor gas mixture is introduced into the vapor space above it.

At the time of deposition, the substrate 12 may be surrounded by an atmosphere at essentially atmospheric pressure. Generally, a suitable non- oxidizing atmosphere, generally nitrogen or a mixture of nitrogen and hydrogen in which nitrogen predominates, is maintained in the float bath enclosure to prevent oxidation of the tin bath. The atmosphere gas is admitted through conduits operably coupled to a distribution manifold. The non-oxidizing gas is introduced at a rate sufficient to compensate for normal losses and maintain a slight positive pressure, on the order of about 0.001 to about 0.01 atmosphere above ambient atmospheric pressure, so as to prevent infiltration of outside atmosphere. For purposes of the present invention, the above-noted pressure range is considered to constitute normal atmospheric pressure. Heat for maintaining the desired temperature regime in the tin bath and the enclosure is typically provided by radiant heaters within the enclosure. The atmosphere within the lehr is typically atmospheric air, as the cooling section is not enclosed and the glass ribbon is open to the ambient atmosphere. Ambient air may be directed against the glass ribbon as by fans in the cooling section. Heaters may also be provided within the annealing lehr for causing the temperature of the glass ribbon to be gradually reduced in accordance with a predetermined regime as it is conveyed therethrough.

Gas distributor beams are positioned in the float bath to deposit the various layers on the glass ribbon. The gas distributor beam is one form of reactor that can be employed in practicing the process of the present invention. A conventional configuration for the distributor beams suitable for supplying the precursor materials in accordance with the invention is an inverted, generally channel-shaped framework formed by spaced inner and outer walls and defining two enclosed cavities. A suitable heat exchange medium is circulated through the enclosed cavities in order to maintain the distributor beams at a desired temperature.

The precursor gas mixture is supplied through a fluid cooled supply conduit. The supply conduit extends along the distributor beam and admits the gas through drop lines spaced along the supply conduit. The supply conduit leads to a delivery chamber within a header carried by the framework.

Precursor gases admitted through the drop lines are discharged from the delivery chamber through a passageway toward a coating chamber defining a vapor space opening onto the glass where they flow along the surface of the glass.

Baffle plates may be provided within the delivery chamber for equalizing the flow of precursor materials across the distributor beam to assure that the materials are discharged against the glass in a smooth, laminar, uniform flow entirely across the distributor beam. Spent precursor materials are collected and removed through exhaust chambers along the sides of the distributor beam.

Various forms of distributor beams used for chemical vapor deposition are suitable for the present method and are known in the prior art. In one such alternative distributor beam configuration, the precursor gas mixture is introduced through a gas supply duct where it is cooled by cooling fluid circulated through a plurality of ducts. The gas supply duct opens through an elongated aperture into a glass flow restrictor.

The gas flow restrictor comprises a plurality of metal strips longitudinally crimped in the form of a sine wave and vertically mounted in abutting

relationship with one another extending along the length of the distributor. Adjacent crimped metal strips are arranged "out of phase" to define a plurality of vertical channels between them. These vertical channels are of small cross- sectional area relative to the cross-sectional area of the gas supply duct, so that the gas is released from the gas flow restrictor at substantially constant pressure along the length of the distributor. The precursor gas mixture is released from the gas flow restrictor into the inlet side of a substantially U-shaped guide channel generally comprising an inlet leg of a coating chamber which opens onto the hot glass substrate to be coated, and at least one exhaust leg, whereby spent precursor gas mixture is withdrawn from the glass. The rounded corners of the blocks defining the coating channel promote a uniform laminar flow of coating parallel to the glass surface across the glass surface to be coated.

It must be noted that the process conditions are not sharply critical for the successful combining and delivering of vaporized reactants according to the present invention. The process conditions described hereinabove are generally disclosed in terms which are conventional to the practice of the present invention. Occasionally, however, the process conditions as described may not be precisely applicable for each compound included within the disclosed scope. Those compounds for which this occurs will be readily recognizable by those ordinarily skilled in the art. In all such cases, either the process may be successfully performed by conventional modifications known to those ordinarily skilled in the art, e.g., by increasing or decreasing temperature conditions, by varying rates of combination of the reactants, by routine modifications of the vaporization process conditions, etc., or other process conditions which are otherwise conventional will be applicable to the practice of the invention.

EXAMPLES

The following examples are for illustrative purposes only and are not to be construed as a limitation on the invention.

For Table 1 and Table 2, all roughness measurements were collected by performing atomic force microscopy (AFM) surface scans on 5 micron X 5 micron areas of each sample and all layer thicknesses were measured using profilometry. Table 1 summarizes the dielectric metal oxide layer 20 upper surface Ra and Rt of the present invention and those known in the prior art. Table 2 summarizes the effect of the Ra and Rt of the dielectric metal oxide layer 20 on the Ra and Rt of a TCO layer such as TCO layer 26. Unless otherwise noted, Ra and Rt measurements are in Angstroms. In Table 1 , Examples 1 and 2 illustrate the Ra and Rt for the upper surface of a dielectric metal oxide layer 20 of the present invention formed on the surface of a moving glass substrate 12. In Examples 1 and 2, the dielectric metal oxide layer 20 is Ti0₂ and has a thickness of approximately 200 A. The dielectric metal oxide layer 20 was deposited at a rate of approximately 80

A/sec. Comparative Example 3 illustrates the Ra and Rt for the first surface of a dielectric metal oxide layer formed on a moving glass substrate known in the art. In Comparative Example 3, the dielectric metal oxide layer is Sn0₂ and has a thickness of approximately 250 A.

Table 1 : Dielectric metal oxide layer first surface Ra and Rt

In Table 2, Examples 4, 5, and 6 illustrate the Ra and Rt for the upper surface of a TCO layer such as TCO layer 26 for use in practicing the present invention. In Examples 4, 5, and 6 a moving glass substratel 2 was utilized. A dielectric metal oxide layer 20 was formed above the moving glass substrate. The dielectric metal oxide layer 20 is Ti0₂ and has a thickness of approximately 250 A. A dielectric oxide layer 14 was formed over the dielectric metal oxide layer. The dielectric oxide layer 14 is SiO₂ and has a thickness of

approximately 250 A. A TCO layer 26 was formed over the dielectric oxide layer 14. The TCO layer 26 is SnO₂:F and has a thickness of approximately 4000 A.

Comparative Examples 7, 8, 9, 10, 11 , and 12 are also provided to illustrate the Ra and Rt for the upper surface of a TCO layer known in the art. In Examples 7, 8, 9, 10, 11 , and 12, a moving glass substrate 52 was utilized. A dielectric metal oxide layer 62 was formed above the moving glass substrate. The dielectric metal oxide layer 62 was SnO₂ at a thickness of approximately 250 A. A dielectric layer was formed over the dielectric metal oxide layer 62. The dielectric oxide layer 64 was SiO₂ having a thickness of approximately 250 A. A dielectric metal oxide layer 62 was formed over the dielectric oxide layer 64. The TCO layer 58 was Sn02:F having a thickness of approximately 4000 A. Table 2: TCO layer first surface Ra and Rt

Thus, as illustrated in Table 1 , the present invention provides a dielectric metal oxide layer 20 having a decreased surface roughness, even when the dielectric metal oxide layer 20 is formed at high deposition rates. Further, Table 2 illustrates that the formation of the dielectric metal oxide layer 20 of the invention results in the formation of a TCO layer 26 with a similarly decreased surface roughness which is an improvement over those known in the art. Thus, the present invention allows the thickness of a photovoltaic structure 32 to be reduced.

The invention has been disclosed in what is considered to be its preferred embodiments. It must be understood, however, that the specific embodiments are provided only for the purpose of illustration, and that the invention may be practiced otherwise than as specifically illustrated without departing from its spirit and scope.

Claims

WHAT WE CLAIM:

1. A photovoltaic module, comprising:

a glass substrate having a surface;

a dielectric metal oxide interlayer having an Ra of < 20 A formed over the surface of the glass substrate;

at least one transparent conductive metal oxide layer formed over the dielectric metal oxide interlayer, wherein the at least one transparent conductive metal oxide layer has a thickness of at least 3500 A; and at least one photovoltaic layer formed over the at least one transparent conductive metal oxide layer.

2. The photovoltaic module defined in claim 1 , wherein the dielectric metal oxide interlayer comprises T1O2.

3. The photovoltaic module defined in claim 1 , wherein the dielectric metal oxide interlayer is one chosen from the group consisting of ZnO,

Zn₂SnO , and ZnSnO₃.

4. The photovoltaic module defined in claim 1 , wherein the thickness of the dielectric metal oxide interlayer is < 250 A.

5. The photovoltaic module defined in claim 1 , wherein the Rt of the at least one transparent conductive metal oxide layer is < 1600 A.

6. The photovoltaic module defined in claim 1 , wherein the at least one transparent conductive metal oxide layer has an Ra of < 170 A.

7. The photovoltaic module defined in claim 1 , wherein the at least one transparent conductive metal oxide layer is a doped metal oxide.

8. The photovoltaic module defined in claim 1 , wherein the at least one transparent conductive metal oxide layer is a metal oxide of Sn, Zn, or Ti.

9. The photovoltaic module defined in claim 1 , further comprising a dielectric oxide layer formed over the dielectric metal oxide interlayer, wherein the dielectric oxide layer is composed of S1O2.

10. A photovoltaic module, comprising:

a glass substrate having a surface;

an interlayer comprising a dielectric metal oxide layer and a dielectric oxide layer having an Rt of < 170 A, formed over the surface of the glass substrate;

at least one transparent conductive metal oxide layer formed over the interlayer, wherein the at least one transparent conductive metal oxide layer has a thickness of at least 3500 A; and

at least one photovoltaic structure formed over the at least one transparent conductive metal oxide layer.

11. The photovoltaic module defined in claim 10, wherein the dielectric metal oxide layer has an Rt of < 160 A.

12. The photovoltaic module defined in claim 10, wherein the dielectric oxide layer is an oxide of silicon and has a thickness of at least 100 A.

13. The photovoltaic module defined in claim 10, wherein the at least one photovoltaic structure comprises at least two photovoltaic semiconductor layers and wherein the Rt of the at least one transparent conductive metal oxide layer does not exceed the thickness of the adjacent photovoltaic semiconductor layer.

14. A process for forming a coated glass substrate for a photovoltaic module comprising:

providing a moving glass substrate;

premixing a precursor gas comprising a metal containing compound and an inert gas;

delivering the precursor gas mixture at a temperature below the thermal decomposition temperature of the metal containing compound to a location adjacent the moving glass substrate, wherein the substrate is surrounded by an atmosphere at substantially atmospheric pressure; introducing the precursor gas mixture into a vapor space above the moving glass substrate wherein a dielectric metal oxide interlayer is formed on the substrate, the dielectric metal oxide interlayer having an Rt < 170 A; and

forming at least one transparent conductive metal oxide layer over the dielectric, wherein the at least one transparent conductive metal oxide layer has a thickness of at least 3500 A.

15. The process defined in claim 14, wherein the metal containing compound is an organotitanium compound.

16. The process defined in claim 14, wherein the deposition rate of the dielectricjnetal oxide interlayer is > 50 A/sec.

17. The process defined in claim 14, further comprising forming at least one photovoltaic structure formed over the at least one transparent conductive metal oxide layer.

18. The process defined in claim 15 wherein the temperature of the moving glass substrate is above 750°F when the precursor gas mixture is introduced into the vapor space above the glass substrate.

19. The process defined in claim 18, wherein the temperature of the moving glass substrate is between about 840°F - 1300°F when the precursor gas mixture is introduced into the vapor space above the glass substrate.

20. The process defined in claim 14, further comprising doping at least a portion of the at least one transparent conductive metal oxide layer.

21. The process defined in claim 15, wherein the precursor gas mixture further comprises an oxygen containing compound and the dielectric metal oxide layer comprises T1O2.