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Research Papers

Thermal Repair of Incomplete Back Contact Insulation (P1) in Cu(In,Ga)Se2 Photovoltaic Thin-Film Modules

[+] Author and Article Information
B. Misic

IEK-5 Photovoltaik,
Forschungszentrum Jülich,
Jülich 52425, Germany
e-mail: b.misic@fz-juelich.de

B. E. Pieters, U. Rau

IEK-5 Photovoltaik,
Forschungszentrum Jülich,
Jülich 52425, Germany

1Corresponding author.

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING ENERGY CONSERVATION. Manuscript received February 24, 2015; final manuscript received July 19, 2015; published online September 2, 2015. Assoc. Editor: Wojciech Lipinski.

J. Sol. Energy Eng 137(6), 061004 (Sep 02, 2015) (7 pages) Paper No: SOL-15-1044; doi: 10.1115/1.4031214 History: Received February 24, 2015; Revised July 19, 2015

We investigate the repair of interruptions in the back contact (P1) scribing line between two Mo back electrodes by thermally induced fractures. The fractures occur during Cu(In,Ga)Se2 (CIGS) absorber co-evaporation, as it is applied in CIGS thin-film module manufacturing and can effectively repair line interruptions of up to about 70 μm. Additionally, we present that a thermal treatment after P1 laser scribing and before CIGS co-evaporation can repair even interruptions of up to 1 mm. The fractures which are required for insulation are only of approximately 4 μm width which indicates the potential for further reduction of the interconnection width in CIGS modules and therefore improvement of the electrical module efficiency.

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Figures

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Fig. 4

Schematic setup for the measurement of inherent P1 line interruptions. Each Mo back electrode is contacted with one pin, and the pins are connected to a multiplexer which routes all pairs of neighboring Mo electrodes. For the sake of simplicity, only eight Mo electrodes are shown, while there are 136 in a real CIGS thin-film module. A SMU is connected to the multiplexer and the measurement result is displayed in the HMI. A moveable microscope is used in order to image the line interruption at the cell transition that is identified by the SMU to be defective. The correspondent microscope image is also shown in the HMI.

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Fig. 3

First-order approximation of thermally induced mechanical stress σMo imposed by the glass onto the Mo layer. The yield strength σyieldMo required for irreversible deformation is exceeded at approximately ΔT=350 K, and the ultimate tensile strength σUTSMo required for creation of fractures at approximately ΔT=400 K. The actual CIGS deposition takes place at approximately ΔT=650 K with room temperature as reference.

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Fig. 6

Inherent thermal repair of incomplete P1 scribing. Microscope images show the defect position before CIGS deposition (second column) and after finishing the CIGS module (third column). The EL-images (fourth column) show sufficient electrical insulation for defect lengths up to Ldefect≈ 63 μm. In each EL image, the repaired defect position is located in the centre between the second and third cell. For Ldefect≈ 88 μm, the characteristic P1 EL defect pattern appears. All EL images are recorded at jEL=8.3 mA/cm2.

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Fig. 2

Schematic square material composite consisting of Mo layer on float glass substrate, as assumed in the first-order approximation. The thermally induced elongation of glass imposes the stress σMo onto the Mo layer into x- and y-directions, and the Mo layer imposes σglass onto the glass substrate.

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Fig. 1

Microscope images of (a) a smooth Mo surface before CIGS deposition, (b) a slightly rugged Mo surface after CIGS deposition, and (c) a Mo surface after CIGS deposition clearly showing dislocations or stretch marks. No systematics could be found in order to explain the differences between (b) and (c). All images are recorded from glass side and from the same CIGS module.

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Fig. 5

Schematic drawing of the preparation and positioning of the tape that absorbs the laser beam and causes the P1 line interruption. (a) Thin slices of width w are cut from the tape which has an initial height (industrially manufactured) of hslice=18 mm. (b) Afterward, the initial slice is cut into three subslices, each of height hsubslice=6 mm. (c) The three subslices are placed on the module perpendicularly to the positions of the P1 laser lines (which are scribed afterward). The height of 6 mm determines that each subslice must lead to exactly one P1 line interruption. The width w of the subslice finally determines the P1 interruption length.

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Fig. 7

Intentional thermal repair of incomplete P1 scribing. We deliberately implement P1 scribing interruptions of up to 1 mm length. Afterward, the glass substrate passes the CIGS co-evaporation chamber and temperature ramp but with empty precursor gas tanks. In (a)–(e), five P1 scribing interruptions are recorded from the Mo film side. Fractures are visible for interruptions of up to 940 μm. (a)–(e) are recorded with perpendicular microscope illumination. (f) shows the left end of the scribing line in (e), but with a ring illumination which highlights the inclined edges of the laser line and of the fracture. The setup in (f) allows to determine the fracture width as approximately 4 μm.

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