WO2012038592A1

WO2012038592A1 - Thin film photovoltaic module and process for its production

Info

Publication number: WO2012038592A1
Application number: PCT/FI2011/050792
Authority: WO
Inventors: Markku Rajala
Original assignee: Beneq Oy
Current assignee: Beneq Oy
Priority date: 2010-09-23
Filing date: 2011-09-15
Publication date: 2012-03-29
Anticipated expiration: 2013-03-23
Also published as: FI20105982A0

Abstract

Photovoltaic module comprising a substrate (1), a layer of hollow particles (2), having an average outside diameter of 500 nm - 2500 nm on substrate (1), a transparent conductive oxide film (3) on substrate (1) and on the layer of hollow particles (2) and a semiconductor layer (7), capable of converting at least a fraction of infrared radiation at wavelengths 700 nm - 3000 nm into electricity.

Description

THIN FILM PHOTOVOLTAIC MODULE AND PROCESS FOR ITS PRODUCTION Field of invention

The invention relates to thin film photovoltaic (TFPV) modules, where the transparent substrate involves a layer of hollow particles deposited on the substrate and a substantially continuous transparent conductive oxide film provided on the substrate and on the particle layer. Such substrate increases light harvesting into the thin film photovoltaic module.

Background of the invention

Thin film photovoltaics (TFPV) is an important application converting solar energy into electricity. Thin-film solar cell devices (shortly: solar cells) are configured in either substrate or a superstrate structure. For superstrate configuration, the substrate is transparent and the contact is made by a conducting oxide coating on the substrate. For substrate configuration, the substrate is metal or metallic coating on a glass/polymer substrate which also acts as the contact. Typically in TFPV superstrate structures a glass substrate works as a deposition substrate onto which the

necessary layers are deposited. The layers typically include an index matching layer (typically silicon oxynitrid, silicon oxycarbide or a layered structure comprising e.g. silicon dioxide and tin oxide or titanium oxide), the transparent conductive oxide (TCO) layer, the semiconductor layer for the actual photoelectric conversion, and the back contactors. An antireflective coating may be applied on the glass to increase solar radiation harvesting.

The semiconductor of the TFPV module may be made from amorphous silicon (a-Si), micro or nanocrystalline silicon (μ/η-Si) or cadmium telluride (CdTe). The cells of fourth TFPV type, Cu-ln-Ga-Se (CIGS) thin film solar cells, are built on glass, metal foil or plastic substrates and do not require a TCO layer between the substrate and the semiconductor. The advantage of the silicon based TFPV modules is that they use about one- hundredth of the amount of silicon used by crystalline silicon PV. The a-Si solar cells unfortunately suffer from low efficiency and thus the CdTe modules, which provide the lowest cost-per-megawatt and the CIGS modules, which grant the highest efficiency are serious competitors to a-Si modules.

Different semiconductor layers with appropriately graded bandgap can be placed in tandem, or can be integrated to form a multijunction device. Theoretically, if all solar photons can be converted to electricity, 53% efficiency can be achieved with four junction devices and as the number of junctions goes to infinity, the efficiency can reach as high as 68%. Thus, stacking thin layers of Si-based semiconductor layers is a valid method for improving the competitive edge of the Si-modules. Such multi-junction cells apply either an underlying a-SiGe or μ/η-Si layer below the a-Si layer. This structure allows better light trapping and thus the efficiency of the cell increases. The TCO layer obviously needs to carry out the electrons generated in the semiconductor layer of the PV cell, and so the electrical resistivity of the TCO layer needs to be low. There is, however, an intrinsic level of the conductivity which arises from the fact that the light absorption into the TCO layer manufactured from fluorine doped tin oxide (FTO) increases together with the electrical conductivity: if the TCO layer is made very thick, the absorption increases simply due to the increased layer thickness. If the carrier concentration of the TCO layer is increased, there will be a substantial absorption at higher wavelengths due to the plasma oscillations, although the visual transmission is good. Thus the FTO layer thickness generally needs to be lower than 1 micrometer, the carrier concentration less than 3 x 10²⁰ cm^"3 and the carrier mobility around 30 cm²/Vs. This relates to a typical square resistance of 10 - 14 Ω/D.

In the Si-based thin film PV modules, the TCO layer has another function: light trapping. The light absorption of the Si-films is rather poor but can be improved by guiding the light so that it travels parallel to the semiconductor surface rather than opposite to it. This can be realized by scattering the light by the structure of the TCO- layer/semiconductor interface. A TCO made from fluorine doped tin oxide (FTO) layer may be deposited such that the grown TCO film itself provides the required surface structure for visible light scattering. This is a good solution for increasing the efficiency of an amorphous-silicon (a-Si) TFPV module.

However, scattering high wavelength solar radiation, which can be utilized by some semiconductor materials (such as GaAs, InP or micro/nanocrystalline silicon) for generating the photoelectric conversion, requires larger discontinuities at the TCO/semiconductor interface than what is achievable with the TCO surface structure only. In the prior art such a structure has been realized by depositing η/μ-particles or by growing larger inclusions with modified tin oxide layer deposition chemistry.

Although scattering also depends on the refractive index difference between the scattering center and the surrounding medium, the size of the scattering center for effectively scattering higher wavelengths should be approximately same as the wavelength of light. Thus, in order to effectively scatter high-wavelength solar radiation, the average size of the scattering center should be between 500 and 2500 nm.

"High-wavelength" solar radiation means in this application the IR-A and IR-B bands of infrared radiation as recommended by the International Commission on

Illumination (CIE). IR-A band refers to wavelength range 700 nm - 1400 nm and IR-B bands refers to wavelength range 1400 nm - 3000 nm. Later in this application we refer to these IR-bands shortly as "infrared radiation".

FTO can be manufactured from various precursors including e.g. stannic and stannous chlorides and organotin compounds such as monobityltin trichloride

(MBTC), fluorine acid, trifluoroacetic acid, etc. The characteristics of the FTO film, e.g. the surface morphology are influenced by the precursor used. The reaction chemistry is also greatly influenced by additives, e.g. the role of water vapor reaction kinetics is widely recognized.

Description of the state of the art

Patent application WO 2008/133285, Asahi glass Co., 6.1 1.2008, relates to the light emitting device comprising a substrate with a film which provides scattering, the said film containing inorganic particles which may be hollow. The publication states (par. [0027]) that the preferred particle diameter is 5 nm - 300 nm and more preferably 10 nm -100 nm, which is obvious, as the publication relates to a visible light emitting device, not to a device converting the near-IR spectrum of the solar radiation into electricity.

European patent application publication EP 1 ,445,095 A1 , Matsushita Electric Works, Ltd., 11.8.2004, describes a composite thin film-holding substrate in which a composite thin film comprising filler having a refractive index lower than that of a substrate and a binder having a refractive index higher than that of the filler is formed on a surface of the substrate. Light is efficiently scattered when passing through the composite thin film which comprises the filler and the binder. The filler can be e.g. hollow particles, especially hollow silica particles. According to the application (par.

[0058]), the average particle diameter of the fine and hollow silica particles is in the range between 5 nm and 2000 nm and preferably between 5 nm and 100 nm. The application does not teach using particle sizes between 500 nm and 2500 nm to effectively scatter near-IR radiation.

Patent application publication KR 2010/0076931 A1 , Jusung Engineering Co., 6.7.2010, describes a solar cell comprising a light scattering film, a front electrode layer and a semiconductor layer. The light scattering film may comprise hollow particles. The solar cell may be a conventional silicon-substrate-based cell or it may be a TFPV module, and the application does not specifically mention semiconductor materials which could be used to convert near-IR radiation into electricity. The application also fails to define the size of the hollow particles. The Japanese patent application JP 2001 -322552, Asahi Glass Company, Ltd.,

October 19, 2001 , describes a substrate with a transparent conductive oxide film, comprising a substrate and transparent conductive oxide film provided on the substrate and constituted by a plurality of ridges and plurality of flat portions, wherein the surfaces of the ridges and the flat portions have many continuous micron-size protrusions. The application also describes a process for producing, on a transparent substrate, discontinuous small ridges by an atmospheric pressure chemical vapor deposition (CVD) method. The structure is aimed to increase light scattering at a long wavelength, as it is effective to increase the surface roughness of the TCO film.

The term "ridge" is generally understood to imply a substantial length

associated with a raised feature. However, the application JP 2001 -322552 has no teaching of substantial length to the ridges and the micrographs presented in the application appear to show hill-like structures, i.e. hemispherical structures, not "ridges". Also the manufacturing method described would produce such

hemispherical structures, not ridges. Hemispherical structures having a size of roughly the wavelength of light do scatter light at longer wavelength region of solar radiation. As the micron-size structures are sufficiently thick, they absorb a considerable amount of light, even if they are manufactured from electrically non-conducting material.

Various other patent publications, such as US 2004/0245212, Jigjia Ji, et al., 9.12.2004; EP 1 ,500.634 A1 , Nippon Sheet Glass Co., 26.1.2005; JP 63119275, Sumitomo Bakelite Co, 23.5.1988 and EP 1 ,732,139 A1 , Kaneka Co., 13.12.2006, describe the use of particles for enhancing light scattering, but they all fail to describe the advantage of certain particle size range to scatter near-IR radiation and/or fail to describe the advantage of hollow particles in increasing light transmission.

Thus, the prior art shows technical problems of excess light absorption caused by solid (i.e. non-hollow) scattering particles and/or fails to teach the advantage of using a certain particle size range to optimally scatter near-IR solar radiation.

Summary of the invention

The aim of the current invention is to introduce a product which solves the problems of the prior art. The aim of the current invention is also to introduce a process for producing such a product.

The invented product, i.e. a thin film solar cell, comprises a substrate with a transparent conductive oxide film. A layer of hollow particles is deposited on the substrate and the layer may be at least partially embedded into the substrate. A substantially continuous transparent conductive oxide film is provided on the substrate and on the particle layer. Additionally, a refractive index matching layer may be provided on the substrate and on the particle layer and/or a diffusion barrier layer may be provided on the substrate. The aim of the layer of hollow particles is to effectively scatter light to the semiconductor layer of the thin film photovoltaic cell without increasing light absorption to the structure prior to the semiconductor (in the travelling direction of the solar radiation). The aim of the refractive index layer is to improve light harvesting into the structure. The aim of the diffusion barrier layer is to reduce alkaline ion diffusion into the transparent conductive oxide layer.

The particles or an essential amount of the particles are at least partially hollow. The material of the particles is preferably metal oxide, most preferably a metal oxide or a combination of metal oxides with low light absorption. Such metal oxides may include e.g. oxides of Mg, Ca, Sr, Ti, Zr, Hf, Ce, Zn, Al, Ga, In, Si, Ge, Sn or Pb. Most preferable metal oxides are oxides of Zn, In and Sn. The substantially continuous transparent conductive oxide film provided on the substrate and on the particle layer is beneficially provided with a surface structure where the outcroppings have a height between 30 nm and 200 nm and the distance between the adjacent outcropping tops is preferably between 30 nm and 200 nm.

For the solar cell application, the substrate with the layer of particles and the substantially continuous transparent conductive oxide film deposited on the substrate comprises a silicon layer deposited over the substantially continuous transparent conductive oxide film. The silicon layer is preferably deposited with an atmospheric pressure chemical vapor deposition (APCVD) process.

Brief description of the drawings

In the following, the invention will be described in more detail with reference to the appended schematic drawings, in which

Fig. 1 shows a schematic drawing (side view) of the invented TFPV module comprising a substrate, a layer of hollow particles deposited on the substrate, a transparent conductive oxide film provided on the substrate and on the particle layer and a semiconductor layer deposited on the transparent conductive oxide layer; and

Fig. 2 shows an embodiment of the process for manufacturing a layer of hollow particles.

For the sake of clarity, the figures only show the details necessary for understanding the invention. The structures and details which are not necessary for understanding the invention and which are obvious for a person skilled in the art have been omitted from the figures in order to emphasize the characteristics of the invention. Detailed description of preferred embodiments

Figure 1 shows a photovoltaic module comprising a substrate 1 , a layer of hollow particles 2, having an average outside diameter of 500 nm - 2500 nm on substrate 1 , a transparent conductive oxide film 3 on substrate 1 and on the layer of hollow particles 2 and a semiconductor layer 7, capable of converting at least a fraction of infrared radiation at wavelengths 700 nm - 3000 nm into electricity, on the transparent conductive oxide film 3. The photovoltaic module may optionally comprise a diffusion barrier layer 6 or an index-matching layer (not shown in the picture) . The index-matching layer and the diffusion barrier 6 should be understood as functional layers, and both functions can be integrated into a single physical layer.

The average diameter of the particle 2 is selected such that the particle layer 2 effectively scatters infrared light. Such effective scattering may be proven by measuring the haze of the substrate comprising the layer of hollow particles 2 and comparing that with the haze measured from a substrate which does not comprise the layer of hollow particles 2. The haze measurement from the substrate with particles shows a higher haze at wavelengths 700 nm - 3000 nm than the substrate without the hollow particles. The haze is preferably measured by applying the ASTM standard D 1003-00.

The preferred average particle diameter D is preferably between 500 nm and 2500 nm and more preferably between 500 nm and 1000 nm.

The material of the particles 2 is selected so that the light absorption to the particle material is minimized. Suitable materials are metal oxides, preferably oxides of Mg, Ca, Sr, Ti, Zr, Hf, Ce, Zn, Al, Ga, In, Si, Ge, Sn, P, and most preferably oxides of Zn, In and Sn. Particles 2 may also show a multilayer structure, which may be used to reduce light reflection in hollow interior of particle 2. Typically the particle 2 layer is adhered to the substrate by the TCO layer deposited on the particle layer. However, the adhesion of the particle layer 2 to the substrate 1 may still be improved by partially embedding the particles 2 into the substrate 1. This may effectively be realized by heating at least the surface of the substrate 1 after the deposition of the particle layer 2. Heating may be carried out in a furnace or by radiation or convection directed essentially towards the surface of the substrate 1. Most preferably heating is carried out with electromagnetic radiation having a wavelength of approximately 8 μηι - 12 pm, such as the radiation from a carbon dioxide (C0₂) laser. In another embodiment the particle layer adhesion is improved by coating particles 2* by a thin inorganic layer, which also covers the substrate 1 and then removing the particles 2^* which leaves the substrate covered with a continuous film with hollow particles protruding from the surface and being attached to the substrate by the essentially continuous, thin inorganic layer.

The transparent conductive oxide film 3 provided on the substrate and on the particle layer is preferably provided with a surface structure where the outcroppings 4 have a height h between 30 nm and 200 nm and the average distance S between the adjacent outcropping tops is preferably between 30 nm and 200 nm. Such a structure effectively scatters the visible light of solar radiation. The average thickness of the TCO layer, H, is preferably between 400 nm and 1000 nm.

Figure 2 shows a principle drawing of a preferred process for producing the substrate 1 of the TFPV module, with a layer of hollow particles 2 protruding from the substrate. The layer of hollow particles 2 is produced by electrospraying organic particles 2* with a diameter D* on the substrate . Such particles as well as the substrate 1 not covered by the particles are then coated by atomic layer deposition (ALD) process, which produces a thin, inorganic coating or inorganic layer. The particles 2* are covered by the coating and their effective diameter changes from D^* to D. The organic core 2^** of the particles is then removed e.g. by substrate heating (calcinations). This produces a substrate 1 with inorganic particles 2 attached on the substrate by the thin inorganic layer. The layer thickness is so low, beneficially less than 100 nm, that it does not adversely affect the properties of the substrate.

Electrospraying 100 is a process by which organic droplets 2^*, typically polymer droplets, with diameters D^* ranging from a few nanometers to several micrometers can be produced using an electrostatically driven jet of a solution. The high voltage electrostatic field 103 charges the surface of the solution 101 and induces the ejection of a liquid jet 102 through a spinneret. In a typical process, an electrical potential HV is applied between the liquid jet 102 and the grounded target substrate 1. With a suitable set of process parameters, the applied electric field 103 overcomes the surface tension of the jet 102, and charged droplets 2^* are ejected (with different process parameters it is possible to eject fibers). The droplets 2^* solidify and are collected on the substrate 1.

Inorganic hollow particles with precisely defined nanoscale walls are produced by employing atomic layer deposition (ALD) on electrosrayed polymer particles 2* and on the substrate 1. In one embodiment, hollow Al₂0₃ particles 2 were produced by first electrospraying polyvinylalcohol (PVA) particles 2*, coating the particles 2* with AI2O3 layer using AI(CH₃)3 (TMA) and H₂0 as precursors in a conventional ALD process and removing the organic part by heat treating the particles 2* in air at 400°C for several hours. It is obvious for a person skilled in the art that such a procedure may also be used to produce multilayered particles 2, each layer comprising different or similar metal oxides.

An aerosol assisted chemical vapor deposition (AACVD) process such as described in the applicant's patent application WO2009/080892 A1 , Beneq Oy, July 2, 2009, is preferably used for producing a transparent conductive oxide layer 3 on the substrate 1 and on the non-continuous layer of particles 2. A semiconductor layer capable of converting at least a fraction of infrared radiation at wavelengths 700 nm - 3000 nm into electricity is further deposited on the transparent conductive oxide layer 3. In the preferred embodiment of the invention, the TFPV module is a tandem module: an amorphous silicon-based semiconductor layer, capable of converting at least a fraction of visible (380 nm - 780 nm) light to electricity, is first deposited on the transparent conductive oxide layer, and then a micro- or nanocrystalline silicon layer, capable of converting at least a fraction of infrared (700 nm - 3000 nm or a fraction of this wavelength band) light to electricity, is deposited on the amorphous silicon layer. Various different layers may be deposited between the first (amorphous silicon) semiconductor layer and second

(micro/nanocrystalline silicon) layer.

It is possible to produce various embodiments of the invention in accordance with the spirit of the invention. Therefore, the above-presented examples must not be interpreted as restrictive to the invention, but the embodiments of the invention can be freely varied within the scope of the inventive features presented in the claims.

Claims

aims

1. Photovoltaic module comprising:

a. a substrate (1) ;

b. a layer of hollow particles (2), having an average outside diameter of 500 nm - 2500 nm on substrate (1);

c. a transparent conductive oxide film (3) on substrate (1) and on the layer of hollow particles (2); and

d. a semiconductor layer (7), capable of converting at least a fraction of infrared radiation at wavelengths 700 nm - 3000 nm into electricity.

2. The photovoltaic module of claim 1, comprising two semiconductor layers, the first one capable of converting at least a fraction of the visible light (380 nm - 780 nm) into electricity and the second one capable of converting at least a fraction of the infrared light (700 nm - 3000 nm) into electricity.

3. The photovoltaic module of claim 1 or 2, comprising a refractive index matching layer provided on the substrate (1) and on the particle (2) layer.

4. The photovoltaic module as in any of the previous claims, comprising particles (2) having at least partially a multilayer structure.

5. The photovoltaic module as in any of the previous claims, comprising hollow particles (2) which are partially embedded into the substrate (1).

6. The photovoltaic module as in any of the previous claims, comprising particles (2) comprising a metal oxide.

7. The photovoltaic module of claim 6, comprising particles (2) comprising metal oxide selected from the group consisting of oxides of Mg, Ca, Sr, Ti, Zr, Hf, Ce, Zn, Al, Ga, In, Si, Ge, Sn, Pb.

8. The photovoltaic module as in any of the previous claims, comprising a substantially continuous transparent conductive oxide film (3) with a surface structure where outcroppings (4) have a height between 30 nm and 200 nm and the distance between the adjacent outcropping tops is preferably between 30 nm and 200 nm.

9. The photovoltaic module as in any of the previous claims comprising a silicon layer (7) deposited over the substantially continuous transparent conductive oxide film.

10. A process for producing a photovoltaic module, comprising:

a. electrospraying organic particles (2^*) on a substrate;

b. coating at least the organic particles (2*) with at least an inorganic layer by atomic layer deposition method;

c. removing the organic material from the particles, thus resulting hollow particles (2);

d. depositing a substantially continuous transparent conductive oxide film (3) on the substrate (1) and on the particle (2) layer; and e. depositing a semiconductor layer (7) on the transparent conductive oxide film (3).

11. The process of claim 10, c o m p r i s i n g emoving the organic material by a heat treatment.