FIELD OF INVENTION
The present invention generally relates to thin film solar cell modules used as solar photovoltaic. More particularly, the invention relates to a process and a device to burn the shunt paths in thin film solar cell modules adapting a multiple pulse reverse bias mending technique.
BACKGROUND OF THE INVENTION
Thin film technologies to produce solar cells can offer cost advantages on account of low material requirement and low energy consumption for their production process. These technologies are based on deposition of thin films of semiconductor materials such as amorphous silicon (a-Si), cadmium telluride (CdTe) and copper indium diselenide (CIS) on inexpensive substrates like glass, stainless steel and polymer films.
In thin film solar cell manufacturing process, the active semiconducting layers are deposited on a various substrates like glass, SS sheet, Polymer sheet etc. Typically a transparent conducting oxide (TCO) layer is deposited on a cleaned glass substrate followed by patterning of the TCO layer by laser scriber. The patterned TCO substrate is then loaded into a vacuum deposition system where

active semiconductor layers are deposited. The active layers are deposited in a vacuum deposition chamber. After the active layer deposition process, a metal contact layer deposited followed by patterning of the metal layer by selective printing and etching. In these process steps some particulates get embedded in the layers leading to creation of shunt paths for the current in the module. The shunts are formed mainly during the deposition process of active layers as some dust particles generated in the deposition system (which is inherent of the system) get trapped in the layer leading to shunts. Another possible source for creation of the shunts in thin film solar cell modules is debris generated during laser scribing process. Although the process steps are fine tuned to minimize the shunt paths, however it cannot be totally eliminated during the manufacturing process.
The large area of the solar cells modules, very low thicknesses of the thin films and number of process steps involving laser scribing of various layers are responsible for the generation of pin holes leading to electrical shunt paths. Removal of these shunt paths improves the efficiency of the modules. These shunts can be removed by burning those using high currents in the reverse bias mode. Hence, the only solution is to detect the shunts in the finished module and burn them effectively.

EP 1670 067 A2 discloses a reverse bias mending technique which is suitable for a semi finished solar cell module, where positive and negative contacts of the cells are formed by delineating the metal contact layers using laser scribing technique such that the positive and negative contacts can be easily accessed. However, this method is not suitable for a module configuration where the back contact is delineated by selectively etching the back contact metal layer after printing etch resistant paste.
OBJECTS OF THE INVENTION
It is therefore an object of this invention to propose a process to burn the shunt paths in thin film solar cell modules produced by laser scribing and selective printing & etching techniques, which improves the cell efficiency.
Another object of the invention is to propose a device to burn the shunt paths effectively generated during production of thin film solar cell modules.
SUMMARY OF THE INVENTION:
According to the invention, there is provided an improved process and device to effectively burn the shunt paths in thin film solar cell modules. The invention adapts a reverse bias technique, using multiple electrical pulses and probes. Multiple pulse reverse bias device of the invention uses the

probes which make contacts with the positive and negative contacts of a cell through the printed masking layer. A fixed forward bias is applied. The leakage current in the forward bias is measured. If the leakage current exceeds the predetermined value, the multiple reverse bias pulses, as per the required number of pulses and pulse duration are applied. A provision has been made in the device to vary the number of pulses as well as the pulse duration as per the requirement.
The reverse bias mending technique according to the invention is used for burning of shunts in thin film solar cell modules for removing unwanted leakage current paths in individual cells in a module, thereby enhancing the over all efficiency of module.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1- shows a device to burn the shunt paths in thin-film solar cell modules according to the invention.

DETAIL DESCRIPTION OF THE INVENTION
Accordingly, there is provided a process and a device for implementing the inventive reverse bias technique, using multiple modules (1). As shown in fig. 1, a jig (2) is placed on the module (1) for proper position of a plurality of probes (3). A fixed forward bias voltage of 1.2V is applied across the tips of the probe
(3) suitable for double junction amorphous silicon solar cell. The leakage current in a cell of the module (1) is measured with the help of a digital ampere meter
(4) in dark condition. If the leakage current exceeds the value of 10mA for the cell area of 180 cm2, a selector switch (5) is changed from forward to reverse mode without removing the probes (3) from the cell (1). The device then applies heavy current in the reverse bias. The reverse voltage to be applied can be adjusted by varying the voltage knob of a regulated DC supply (6). The reverse voltage is selected between 5.0-5.5V for the a-Si double junction cell which is measured through a digital voltmeter (8). The device has the capability to apply multiple pulses via a pulse width generator (9) having adjustable control (7). The pulse width on-time can be varied from 80 ms to 1000 ms and number of pulses can be varied from 1 to 12.

According to the invention, the shunts in the modules (1) are burst by applying to the modules, multiple electrical pulses , using the probes (3), in reverse bias condition after the printing of an etch resist paste and selective etching of the metal contact on the modules . When the pulses are applied, a large amount of current flows through the shunts which in turn generate intense heat across the shunts which burn the shunts leading to improvement in module efficiency.
According to the invention, the solar cells in the module (1) can be mended even after coating the last protective layer of the etch resist paste on the negative terminal of the cells.
Typical results obtained after burning of shunts according to the invention are given in the Table-1. The multiple reverse bias mending technique yielded more than 1% (absolute) improvement in the module efficiency.

Table-1: Typical results obtained after burning of shunts according to the invention.

Pulse-width time= 90 minutes

We Claim:
1. A process for burning the shunt paths generated in the manufacture of thin film solar cell modules, being manufactured by carrying-out the steps of:
- depositing on a clean glass substrate a transparent conducting oxide (TCO) layer followed by patterning the TCO-layer by laser scriber;
- loading the patterned TCO-substrate into a vacuum deposition chamber,
- depositing semiconductor layers on the substrate inside the vacuum deposition chamber; and
- depositing a metal layer which is patterned adapting a masking and etching technique, the process for burning the shunt-paths generated during the manufacturing steps of the modules, comprising the steps of:
- disposing a plurality of probes on the modules such that the probes are enabled to contact the positive and negative contacts of a cell;
- applying a fixed forward bias voltage across the tips of the probes corresponding to that of a double-junction amorphous silicon cell;

- measuring the leakage current of the forward bias in the cell; and
- changing the mode of the pulses from forward to reverse bias pulses without removing the probes from the module, if the leakage current exceeds a predetermined value.

2. The process as claimed in claim 1, wherein the predetermined value of the leakage current is 10mA for a cell area of 180 cm2.
3. The process as claimed in claim 1, wherein the fixed forward bias voltage is 1.2V.
4. The process as claimed in claim 1 or 3, wherein the reverse voltage is selected between 5.00 to 5.50 volt.
5. The process as claimed in claim 4, wherein the reverse voltage is applied through multiple reverse bias pulses.
6. The process as claimed in claim 5, wherein the pulse-width on-time is variable between 80 ms to 1000ms, and wherein the number of pulses is variable between 1 to 12.

7. A device to effectively burn the shunt paths generated during production
of thin-film solar cell modules, comprising:
- a plurality of probes (3) positionable on at least one solar cell module (1), the accuracy for positioning of the probes (3) being maintained by adapting a jig (2);
- a regulated variable DC power supply source (6) for application of a fixed forward bias voltage across the probes (3);
- a digital ampere meter (4) for measuring the leakage current in the cells of the atleast one module (1);
- a selector switch (5) for changing the mode from forward bias voltage to reverse bias voltage, in case the measured leakage current exceeds a predetermined value; and
- a pulse-width generator (9) for generating multiple pulses for applying multiple pulses to burn the shunt-paths.
8. The device as claimed in claim 7, comprising a digital volt meter (8) for
measuring the reverse bias voltage.

9. The device as claimed in claim 7, comprising an adjustable control (7) for
adjusting the on-time of the pulse width including adjustment in the number
of pulses.
10. A process for burning the shunt paths generated in the manufacture of thin film solar cell modules as substantially described and illustrated herein with reference to the accompanying drawings.
11. A device to effectively burn the shunt paths generated during production of thin-film solar cell modules as substantially described and illustrated herein with reference to the accompanying drawings.

The invention relates to a process for burning the shunt paths generated in the manufacture of thin film solar cell modules, being manufactured by carrying-out the steps of depositing on a clean glass substrate a transparent conducting oxide (TCO) layer followed by patterning the TCO-layer by laser scriber; loading the patterned TCO-substrate into a vacuum deposition chamber, depositing semiconductor layers on the substrate inside the vacuum deposition chamber; and depositing a metal layer which is patterned adapting a masking and etching technique, the process for burning the shunt-paths generated during the manufacturing steps of the modules, comprising the steps of disposing a plurality of probes on the modules such that the probes are enabled to contact the positive and negative contacts of a cell; applying a fixed forward bias voltage across the tips of the probes corresponding to that of a double-junction amorphous silicon cell;measuring the leakage current of the forward bias in the cell; and changing the mode of the pulses from forward to reverse bias pulses without removing the probes from the module, if the leakage current exceeds a predetermined value.