Method and device for thin-film ablation of a substrate

Details Method and device for thin-film ablation of a substrate Method and device for thin-film ablation of a substrate

2001-10-05

2003-05-20

Elve, M. Alexandra (Department: 1725)

Electric heating

Metal heating

By arc

C219S121680, C219S121670

Reexamination Certificate

active

06566628

ABSTRACT:

Foreign priority benefits are claimed under 35 U.S.C. §119(a)-(d) or 35 U.S.C. §365(b) of German application number 19915640.9, filed Apr. 7, 1999, German application number 19927529.7, filed Jun. 16, 1999, and German application number 19933703.9, filed Jul. 19, 1999.
FIELD OF THE INVENTION
The invention relates to a method and device for thin-film ablation of a substrate, for example, in this case for delamination of laminated ceramics or glasses. More particularly the invention is intended for surface layer ablation of a thin-film solar cell. In addition the invention relates to a method for encapsulating a thin-film solar module comprising a thin-film laminated substrate.
BACKGROUND OF THE INVENTION
Conventional solar modules of crystalline silicon are based on wafer fabrication with subsequent electrical wiring, resulting in relatively small power units of approximately 1 W, silicon wafers then needing, as a rule, to be circuited into 50-100 W modules.
As an alternative to these conventional solar modules thin-film solar cells have become known on the basis of micrometer film thicknesses. The substantial elements of a thin-film solar cell are shown in FIG.
2
and consist of a p
junction between the absorber layer and the window layer.
Unlike conventional silicon wafer wiring, thin-film cells can have integrated circuitry. Following the individual coating steps on the total surface area, the back electrode, the sandwiched cell, and the front electrode are sliced into longitudinal strips. Staggering these three slices relative to each other forms an electrical connection between cells adjoining the front and back electrode. Slicing may be done by scribing or laser slicing, permitting cost-effective fabrication of a standard solar module for 12 V applications on a size of approximately 0.5×0.5 m
2
, for example.
The useful life of such a solar module is substantially dictated by how well the thin-film layers are protected from the weather and environment. Achieving a long life of 30 years and more requires the thin-film layers to withstand extreme exposure to sunlight, moisture and air pollutants. The requirements as to moisture stability and voltage strength can thus only be satisfied when the thin-film solar cell is sufficiently encapsulated and the current-carrying components are adequately insulated electrically. For this purpose the current-carrying components of a thin-film solar cell are encapsulated and laminated. Encapsulation is achieved by coating the substrate with the current-carrying film layers, delaminating the surface portion of the substrate, and then depositing a laminate on the complete film-layer. This results in laminate and substrate being surface bonded and resistant to corrosion, thus reliably protecting internal portions from moisture degradation.
To make this clear,
FIG. 1
is a diagrammatic representation of a section through an encapsulated solar cell, showing how patterned film-layers
3
deposited on the substrate
4
are delaminated in the surface areas
5
and encapsulated by a laminate layer
2
. Deposited over the laminate layer
2
is a layer of window glass
1
.
One problem in such an encapsulating of a thin-film solar cell is in delaminating the thin-film surface layers. Conventional methods of delamination, such as for instance sandblasting or delamination by grinding, unavoidably also result in damage to the substrate surface and in the formation of microcracks therein. The great differences in temperature in a thin-film module once in operation and the resulting tensile stresses add to the risk of fracture and ultimately to the solar cell being damaged due to cracks forming in the surface. Accordingly, surface delamination needs to be undertaken with particular care in a surface usually a few millimeters to centimeters wide.
Chemical methods of ablation are known in principle for machining solar cells which, however, have the drawback of lengthy machining times and involve complicated steps in machining.
This is why removing the surface areas as described continues to be done by mechanical methods such as grinding or sandblasting since these methods permit precisely defining material ablation. In addition to the damage done to the substrate surface and the formation of microcracks as already described, these methods have the additional drawback that the workpiece as a rule needs to be subsequently chemically cleaned in an ultrasonic bath since the plume of the ablated film-layers soils the module undesirably.
It is known to produce the steps as described above for forming integrated circuits for a thin-film solar module by a laser beam for interconnecting the resulting individual strips in series.
Known, for example, from U.S. Pat. No. 4,734,550 is a method for machining thin-film layers with a laser, one special application cited being machining photoelectric film-layers generated by thin-film technology. The objective of the machining method is to notch the thin-film layers by directing the machining beam on the surface of each workpiece so that the power distribution applied at the time remains as homogenous as possible. This is achieved by using a rectangularly focused beam having a flat intensity distribution. In addition, the beam is controlled and steered such that the overlapping portions of the rectangularly focused beam results in an intensity distribution as uniform as possible. This method is said to permit notching to a precise and even depth for series circuiting solar cells.
The objective of all methods for series circuiting solar cells is to attain a track width as narrow as possible for minimum electric power degradation of the solar module, track widths of the order of 50 &mgr;m being cited, for example, in U.S. Pat. No. 4,734,550.
Supplementary thereto, optimum power densities are cited in Nakano, S., et al., “Laser Patterning Method for Integrated Type a-Si Solar Cell Submodules, in: Jap. J. of Appl. Phys. Vol. 25, No. 12, 1986, pages 1936-1943 for laser beam machining of solar modules in obtaining series circuiting. In this case, power densities of the order of 1.10
6
W/cm
2
are cited as being suitable for obtaining a track width of 50 &mgr;m.
Since, as aforementioned, the delamination methods for series circuiting has the objective of attaining a minimum track width, they are unsuitable for surface delamination of solar modules, since for this application surface strips a few millimeters wide need to be delaminated. This is why the aforementioned mechanical methods are used, now as then, despite the aforementioned drawbacks, for surface delamination.
U.S. Pat. No. 5,151,135 discloses a device for cleaning the surfaces of solar cells and other means, it comprises a laser resonator for generating a light pulse machining beam having a pulse duration smaller than 100 ns and a pulse energy density in the range of 0.1 J/cm
2
to 10J/cm
2
, an optical system for imaging the machining beam generated by the laser resonator on the surface to be cleaned over a surface area in the range of 1 mm
2
to a few cm
2
with a substantially homogenous power distribution, a first positioner for a predefined relative movement between the surface to be cleaned and the machining beam, and including a controller for signaling the first positioner such that each unit of surface area is beamed with a substantially constant amount of energy.
Japanese Patent No. 10 052 780A discloses a device for tin-film ablation of a thin-film solar cell comprising a laser resonator for generating a light pulse machining beam, an optical system comprising a lens and a pin-hole diaphragm for imaging the machining beam generated by the laser resonator on the surface to be machined over a surface area in the range of a 1 mm
2
to cm
2
with a substantially homogenous power distribution, a first positioner for a predefined relative movement between the workpiece and the machining beam, and including a controller for signaling the first positioner such that each unit of surface area is beamed with a substantially constant amount of energy.
SUMMARY

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