FIELD OF INVENTION
The present invention relates to the development of a test apparatus for quick detection of shunts present in industrial silicon solar cells and a method for the same. More particularly, the invention relates to a low cost technique adapting temperature-sensitive liquid crystal (LC) sheets in the set up.
BACKGROUND AND PRIOR ART OF THE INVENTION
The crystalline silicon (c-Si) solar cell technology still remains the mainstay of the PV industry, as nearly 90% of market share is maintained by c-Si technology. In a quest to produce low-cost solar electricity through crystalline silicon solar cell technology, two-pronged approach is being adopted when new manufacturing plants with multi megawatt capacity are being added worldwide, efforts are underway to enhance the efficiency of crystalline silicon solar cells to generate more power with the same input materials and process cost.
The power output of solar cells is critically dependent on its performance parameters, such as, output current, voltage and fill factor (FF). The FF of solar cell is intrinsically controlled by the presence of series and shunt resistance. For an optimum performance of solar cells, the series resistance has to be as low as possible and the

shunt resistance as high as possible. Typical values are 1 Ohm-cm2 for series resistance and 1000 Ohm-cm2 for shunt resistance.
The series resistance of the cell is minimized by providing high conductance contact materials such as silver paste and the optimum contact firing process. Further, to reduce the series resistance of the front grid lines without loosing more on contact coverage, high aspect ratio of the contact fingers is maintained.
The shunt resistance in solar cells is from two physical origins, viz., material and process induced. Material induced shunts are: strong recombination sites at grown-in defects e.g., metal decorated small angle grain boundaries, grown-in macroscopic SiNx inclusions and inversion layers crossing the wafer (carbon induced). Typical process induced shunts present in crystalline silicon cells are, viz., bus bars, grid fingers, scratches, cracks, aluminum paste contamination etc. Depending on the extent and type of shunts present, performance of the solar cell is affected drastically.
Lock-in Thermography is undoubtedly the most sensitive technique for detection of shunt as it allows temperature differences in the milli Kelvin range and detects the relevant shunts at the working point of the solar cell. However, the high price of IR lock-in camera still prevents its widespread use.

Hence, there exists a need to provide a low cost technique for quick detection of shunts present in silicon solar cells in an industrial environment.
OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose a test apparatus and a method for quick detection of shunts present in industrial silicon solar cells which is of low cost technique.
Another object of the invention is to propose a test apparatus and a method for quick detection of shunts present in industrial silicon solar cells which helps in troubleshooting the solar cell process by directly assigning the cause to a specific process step and thereby prevent its recurrence in the next batch of cells.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.l - shows the test set-up for shunt detection according to the invention. Fig.2 - shows light I-V (current voltage) curves of good and shunted solar cell. Fig.3a and 3b - show respectively photographs of LC sheets for cells without shunt and with shunt under reverse bias condition.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
As shown in Fig.l, the set up essentially consists of a metallic plate which acts as a vacuum chuck (1) for the test cell, a set of probes (2) for making electrical contacts to the cell (S), a DC power supply (3) (Voltage range: 0-25 V, Current range: 0-5A), a vacuum generator based on venturi effect or a vacuum pump and a temperature sensitive LC sheet (4).
The test set-up employed for quick detection of shunts in industrial solar cells (S) is carried out with commercially available temperature sensitive liquid crystal sheet (4) which is in firm physical contact with the test cell (S) under reverse bias condition.
For detection of shunt, the solar cell (S) under test is placed on a vacuum chuck (1) and is reverse biased with typical applied voltage between -2V and -10V. The LC sheet (4) is placed on the cell (S) in close physical contact. Under the reverse bias condition, regions of the cells with low values of shunt resistance start getting heated up locally due to the 'hotspot' phenomenon. The LC sheet (4) detects these local hotspots instantaneously and manifests by a change of colour. The colour pattern generated on the LC sheet (4) is recorded with a digital camera and transferred to a PC for comparison and future reference.

The change in colour of the regions of the liquid crystal sheet directly corresponds to the shunted regions of the cell (S). The picture of the LC sheet (4) is captured under this condition.
The power output of the solar cell critically depends on the completeness of edge isolation process. The application of this invention is made to check quickly the completeness of the edge isolation process in solar cells by simply looking at the change in colour of the liquid crystal sheet along the edges of the cell.
As shown in Fig.2, the curves give light I-V characteristics of a cell without shunt and other with edge shunting. The presence of edge shunts in the cell has reduced the cell voltage drastically without affecting the current.

WE CLAIM
1. A test apparatus (A) for quick detection of shunts present in industrial silicon
cells and a method for the same comprising:
a metallic plate acting as a vacuum chuck (1) for the test cell (S);
a set of probes (2) for making electrical contact to the cell;
a DC power supply (3) for applying reverse bias to the cell (S);
a vacuum generator based on venturi effect or vacuum pump;
characterized in that a temperature sensitive liquid crystal sheet (LC) (4) is in
firm physical contact with the test cell (S) under reverse bias condition for
detecting the local hotspots by manifestation of change of colour.
2. A test apparatus (A) as claimed in claim 1, wherein the DC power supply has a voltage range of 0-25V and current range 0-5A.
3. A test apparatus (A) as claimed in claim 1, wherein the change in colour of liquid crystal sheet (4) directly corresponds to the shunted regions of the cell.
4. A method for quick detection of shunts present in industrial silicon cells comprising:
placing the solar cell (S) under test on a vacuum chuck (1); applying voltage to reverse bias the cell (S);

placing the LC sheet (4) on the cell (S) in firm physical contact; characterized in that under the reverse bias conditions, the cells (S) with low values of shunt resistance get heated up locally when the local hotspots are detected by the LC sheet (4) instantaneously by a manifestation of a change of colour and the colour pattern generated on the LC sheet is recorded with a digital camera and transferred to a PC for comparison and future reference.
5. The method as claimed in claim 1, wherein the applied voltage is between -2V to -10V to reverse bias the test cell (S).

A test apparatus (A) for quick detection of shunts present in industrial silicon solar cells and a method for the same consists of a metallic plate acting as a vacuum chuck (1) for the test cell (S), a set of probes (2) for making electrical contact to the cell (S), a DC power supply (3) for applying reverse bias to the cell, a vacuum generator based on venturi effect or vacuum pump, wherein a temperature sensitive liquid crystal (LC) sheet (S) is in firm physical contact with the test cell (S) under reverse bias condition for detecting the local hotspots by manifestation of change of colour. The solar test cell (S) is placed on a vacuum chuck (1) and is reverse biased with applied voltage from DC power (3) supply when the LC sheet (4) is placed in firm contact with the test cell (S). Under the reverse bias condition, the cells (S) with low values of shunt resistance starts getting heated up locally creating hotspots. The LC sheet (4) detects these local hotspots instantaneously by a manifestation of change of colour. The colour pattern generated on the LC sheet (4) is recorded with a digital camera and transferred to a PC for comparison and future reference.