Field of invention

The invention relates to substrate for thin film photovoltaic modules, the substrate involving a substantially continuous transparent conductive oxide film provided on the substrate. The transparent conductive oxide film is machined so that it comprises essentially discrete wells with a depth of at least 50% of the thickness of the TCO film. The combined area of the cross sections of the wells is at least 30% of the area of the surface of the substrate. Such a 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. In the TFPV modules 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 oxynitride or silicon oxycarbide), 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 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 providing the lowest cost-per-megawatt and the CIGS modules granting the highest efficiency are serious competitors to a-Si modules.

Stacking thin layers of Si-based semiconductor layers is a valid method for improving the competitive edge of the a-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 most common TCO material in TFPV is fluorine doped tin oxide

(FTO), which 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 on the reaction kinetics is widely recognized.

Description of the state of the art

US patent US 4,808,462, Feb. 28, 1989, Glasstech Solar, Inc., describes a solar cell substrate comprising a glass substrate and a transparent electrically conductive layer formed thereon, said conductive layer having a plurality of polygonal projections, having approximate diameters of from 0.1 μηι to 0.3 μηι and height/diameter ratios of at least 0.6. The transparent conductive layer should have a sheet resistance value less than 300 Ω/ο, preferably from 4 to 10 Ω/ο. This range is desirable particularly in view of securing high transmittance and avoiding ohmic losses in a large area substrate. The transparent conductive material formed on glass substrate is preferably made of a transparent metal oxide having a desired conductivity, such as tin oxide doped with fluorine or antimony or indium oxide doped with tin. Atmospheric pressure chemical vapor deposition process used in the production of the transparent conductive oxide automatically produces a textured oxide layer. US patent US 5,102,721 , Apr. 7, 1992, Solarex Corporation, describes a textured layer of tin oxide on a vitreous substrate in which the thickness and the degree of texture of the layer can be controlled independently of one another.

After the deposition of the TCO layer, often a scribing process is carried out to scribe the uniform TCO-layer on the glass substrate to define edges of each individual cell. The purpose of this so-called P1 scribing process is to completely cut through the TCO. Scribing is most often carried out through the glass substrate and the P1 process needs a laser wavelength that transmits through the glass but is strongly absorbed by the TCO. Near-infrared lasers are most often used for the purpose. Since the TCO layer is very thin, typically less than one micrometer, it can be completely ablated by moderate laser power levels.

Thus, as such, there exists technology for TCO layer ablation. E.g. United States Patent Application Publication US 2010/0167431 A1 , Hironaru

Yamaguchi, et al., 1.6.2010, describes a laser processing apparatus for processing thin films. The publication states that in techniques of manufacturing thin-film solar cells which use amorphous silicon or microcrystal silicon, a sheet electricity-generating layer and transparent conductive film are separated like islands and divided into cells and the cells are connected in series in order to obtain a high voltage. In this process, the spacing between cells for division should be minimized to reduce area loss. As a technique for dividing the film into cells, microfabrication by laser light is useful. However, the use of laser light for dividing the film into the cells may cause such problems as low fabrication yields and failures to achieve specific characteristics. If the laser processing accuracy is low, insulation between cells may be inadequate, resulting in a failure to attain a specific voltage level and a decline in electricity generation efficiency. The laser processing accuracy is considered to be influenced by inter-pulse variation in laser light intensity, film thickness, substrate undulation and so on. Thus, in brief, the publication describes scribing of a transparent conductive film for the purpose of creating separate islands of photovoltaic structures that can be connected in series to obtain a higher cell voltage. Said publication does not teach anything on the role of the indentations in increasing the transmittance of light into the photovoltaic device. In other words, the publication does not teach producing essentially discrete wells to the TCO film. The publication is also silent on the balance of the well characteristics related to of light transmittance through the TCO film (requiring wells with large cross section) and current carrying capabilities across the TCO film (requiring wells with small cross section).

The TCO layer absorbs light. Light absorption depends greatly on the properties of the TCO layer, especially on the electrical carrier concentration in the TCO layer. The light absorption also depends on the wavelength of light and it is pronounced at higher wavelengths, especially at near-infrared wavelengths. Thus, especially multi-junction solar cells suffer from light absorption into the TCO layer. This is a serious problem of the prior art.

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 product.

The invented product comprises a substrate with a transparent

conductive oxide (TCO) film. The film has essentially discrete wells reaching at least halfway through the TCO film. "Essentially discrete" may e.g. mean that the wells are not cut all the way through the TCO layer, but still the light

transmittance through the layer is remarkably higher through the wells than through the other parts of the TCO film owing to the thinned nature of the TCO layer at the location of the wells. In most cases it is beneficial that the wells extend completely through the TCO film. The well shape is not limited, but beneficially the cross sectional area of the well is an essentially round spot with an area of less than 3000 μιη ² , which corresponds to roughly 60 μιη diameter of a round spot. The total area covered by the wells should be such that the sunlight entry into the PVTF cell is considerably increased, but the conductivity of the TCO layer is not radically decreased. In practice this means that the total aggregate area of the cross sections of the wells at the surface of the TCO film should be between 30% and 70% of the area of the surface of the substrate, and also smaller cross sectional areas of the essentially discrete wells are preferred. It is surprisingly found that a cross sectional proportion of the wells of 30%-70% yields optimal cell performance in terms of high light transmittance and low ohmic losses. Preferably, the total aggregate area of the cross sections of the wells at the surface of the TCO film is between 40% and 60% of the area of the surface of the substrate, and most preferably, the total aggregate area of the cross sections of the wells at the surface of the TCO film should be between 45% and 55% of the area of the surface of the substrate.

The wells are preferably manufactured by laser ablation. The properties of the TCO layer, especially the carrier concentration in the TCO layer has a drastic effect on the coupling of the laser light into the TCO layer. The laser light should beneficially not couple into the glass which means that the laser wavelength should be between 400 nm and 10 000 nm. In a preferred

embodiment the laser wavelength should be between 980 nm and 1550 nm and in the most preferred embodiment the laser wavelength should be 1050 nm - 1100 nm. The laser light only couples effectively into the TCO layer, if the carrier concentration in the layer is high enough. In practice, the carrier concentration should be of 5 x 10 ¹⁹ cm ^"3 or higher. If partial ablation of the TCO layer is preferred, then it is advantageous to have a layered TCO film structure, with at least two films, the first film having a carrier concentration of 5 x 10 ¹⁹ cm ^"3 or higher and the second film having a carrier concentration of 1 x 10 ²⁰ cm ^"3 or less, and the carrier concentration of the first film being higher than the carrier concentration of the second film.

The laser light may be focused on TCO layer preferably from either side of the substrate. If a complete ablation of the well material is favored then it is advantageous to focus the laser light into the TCO layer through the glass substrate which allows a suction of the ablated TCO material near the substrate surface and thus re-deposition of the ablated material is essentially avoided.

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 of the invented substrate; and Fig. 2 shows a schematic drawing of the invented process for manufacturing the invented substrate.

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 schematic drawing of the invented substrate 1 for thin film photovoltaic (TFPV) modules. Figure 1A shows the top view and Fig. 1 B shows the side view as cut through the line A-A of Fig. 1A. Glass substrate 2 is first coated with an index matching and/or sodium diffusion barrier layer 4. The first transparent conductive oxide layer 3a is deposited on layer 4. The first TCO layer 3a is manufactured so that its carrier concentration is 1 x 10 ²⁰ cm ^"3 or less, preferably less than 1 x 10 ¹⁹ cm ^"3 and obviously less than the carrier

concentration of the layer 3b. Such a low carrier concentration may be produced by manufacturing a TCO from tin oxide or slightly fluorine doped tin oxide (FTO). The low carrier concentration makes layer 3a transparent to the laser light used in machining the TCO layer 3b. The second TCO layer 3b is deposited on the first TCO layer 3a, and the carrier concentration in the second TCO layer 3b is 5 x 10 ¹⁹ cm ^"3 or higher, preferably 1 -3 x 10 ²⁰ cm ^"3 , as such a carrier concentration provides good current conductance but has low light absorption for visible light. The TCO layers 3a and 3b are preferably produced by using monobutyltin trichloride (MBTC) for the tin precursor and trifluorineacetic acid (TFA) for the fluorine precursor.

At least the transparent conductive oxide layer 3b is processed so that the discrete wells 5 extend at least halfway through the TCO film. Preferably the wells reach completely through the processed TCO film 3b. This allows light to propagate through the substrate 1 to the semiconductor layer of the thin film photovoltaic (TFPV) module. In order to achieve a substantial improvement in light propagation, the individual wells 5, which may have a free form as shown in Figure 3A, should have a cross sectional area of at least 3000 micrometers square (μΐη ² ), and the aggregate cross sectional area of the wells should be at least 30% of the total substrate area to enable good light transmittance through the TCO film. At the same time, to enable high enough current carrying capability, aggregate cross sectional area of the wells should be at most 70% of the total substrate area.

Figure 2 shows the schematic drawing for manufacturing the wells 5. A pulsed laser 6 is focused on the TCO film 3b through glass 2, and the layers 4 and 3a below the TCO layer 3b to be ablated. As the laser light needs to propagate through different layers, the laser light needs to have such a wavelength, that laser light is essentially absorbed only to the TCO layer 3b. Thus the laser wavelength is preferably between 980 nm and 1550 nm and most preferably about 1064 nm. The laser unit needs to have a suitable output power, typically at least around 10 W at 1064 nm. The pulse energy should be reasonably high, typically from tens to hundreds μϋ (microjoules) and a high repetition rate, up to 100 kHz (for pico/nanosecond pulses). In the embodiment shown in Figure 2, the laser is scanned at the direction of the arrow at a rate of 1100 mm/s, the repetition rate is about 30 kHz and the pulse length is about 1 ns. This produces a row of spherical wells to the TCO layer 3b. 1064 nm lasers and laser movement systems for applying such machining are commercially available and such lasers are frequently used for scribing the TCO layer of the photovoltaic substrate.

It is obvious for a person skilled in the art that the laser light may be focused on the TCO layer also from the side of the TCO layer of the substrate and not through the glass substrate 2. In such an embodiment various other laser wavelengths than the ones described above may be used.

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.