FIELD OF INVENTION
The present invention relates to a method for detection of breaks in printed grid lines on the front grid in Si solar cells. More particularly the invention relates to a method for detection of said breaks in fine printed grid lines on front grid of Si solar cells for improving the production yield of solar cells.
BACKGROUND OF THE INVENTION
Fabrication of solar cell involves metal printing on both front and rear side. Front side is printed with narrow grid lines of silver and back side is printed with full aluminum in conventional AI-BSF technology. When the front grid line is printed, there is no tool available which quickly detects the breaks in fine printed grid lines. Discontinuity in grid lines effects the cell efficiency badly and its timely detection will lead to increase in yield. The patent described here deals with the method of detection of discontinuous grid tines.
Patent search has been conducted on the net and www.lexisnexis.com/totalpatent does not revealed any similar concept. Only one patent is found while searching with appropriate key words. Patent no US4227940A

1980-10-14 deals with Solar cell for use in concentrator while the patent described here is a method for detection of discontinuity in the printed grid lines in solar cells.
OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose a method for detection of breaks in printed grid lines on the front grid in Si solar cells, which is capable of detecting discontinuity in the printed grid lines in solar cells.
Another object of the invention is to propose a method for detection of breaks in printed grid lines on the front grid in Si solar cells, which is able to improve production yield of solar cells.
SUMMARY OF THE INVENTION
More than 92% of global solar cell shipment in 2014 is with AI-BSF technology where contacts are taken from the solar cells by printing silver and aluminum contacts on the front and back respectively. There is no tool available to quickly detect the breaks in fine printed grid lines on the front grid which can lead to cell efficiency loss.

A new concept has been conceived, The finished cell will be exposed to light and heavy photo current generated will be short circuited using printed bus bars. Using spring loaded pogo pins, voltage will be measured at multiple points of the grid lines. The difference in voltage generated of different pogo pins will be monitored using software. This will reflect that which grid lines is broken and alt the faults in the cells due to break in grid lines will be displayed graphically on a computer screen.
The solar cell will be exposed to a strong light and the heavy photo current generated will be short circuited using the printed contacts. Voltage will be measured at various points of the grid lines with respect to the printed bus bars using spring loaded pogo pins. If any grid line is broken then this will result in higher resistance and a voltage will be generated which will be sensed by the electronics and software. The software will connect only one pogo pin at a time and scan the full cell. The result will be displayed graphically on the computer screen. The colour scan of the cell will help the operator to decide whether the cell can be accepted, rejected of repaired.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: Shows a crystalline Si solar cells with three bus bars and grid lines.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
More than 46 GW solar cells are produced globally in the year 2014. Out of which >92% are produced with AI-BSF technology where contacts are taken from the solar cells by printing silver and aluminum contacts on the front and back respectively. In order to maximise the open area on the front surface of a solar cells, very fine grid and narrow bus bars are printed using very accurate printers. These contacts are fired using high temperature firing furnaces. Printing of very fine continuous lines of 30-50 micron widths of silver pastes on textured Si wafers is very tricky and any fine breaks in the continuity of the line can result in huge loss of power and cell efficiency due to increase in series resistance. If detected quickly, corrective action can be taken to reduce the loss. The cells can be retrieved also by printing the faulty cells second time.
A method has been conceived for the quick detection of the breaks in the grid lines of solar cells after the silver and aluminum contacts have been printed and fired. The cell will be exposed to light of about one sun and the heavy photo current generated will be short circuited using printed contacts. External metal contacts with the grid lines will be made using flat bottom spring loaded gold coated pogo pins. In normal condition these pogo pin will touch the grid lines but will remain open in the measurement circuitry. While the cell is in short circuit condition through the bus bars

and the rear contact, voltage will be measured between each pogo pin and the bus bar. If there are ninety grid lines, there will be 4 flat bottom pogo pins per grid line. The voltage measurement will be made using software so that at one time only one pogo pin will be connected to the measurement circuitry. This will prevent any cross connection and the results will be reliable. Let us assume that a grid line is broken at a point V as shown in fig.l, in the third grid line from the top. Due to heavy current generation, the broken grid line will offer resistance more than the other grid lines and a voltage will be developed which may be a few mV. Using software, the full cell can be scanned in about a second and a colour scan can be displayed on the computer screen showing the places where the grid lines are either broken or offering higher series resistance. This information will be colour coded so that the operator can decide that whether the cell can be accepted, rejected or suitable for reprinting.

WE CLAIM
1. A method for detection of breaks in printed grid tines on the front grid in Si solar cells comprising;
exposing printed and fired solar cells to strong light;
making external metal contacts with grid lines employing flat bottom spring loaded gold coated pogo pins wherein,
heavy photo current generated is shot circuited employing printed contacts through bus bars and rear contact when voltage is measured between each pogo pin and the said bus bar, the said voltage measurement is made employing a software so that at one time only one pogo pin is connected to the measurement circuitry preventing any cross connection and making the result reliable wherein,
employing the software, full cell is scanned in about a second when colour scan is displayed on a computer screen showing the places where the grid lines are either broken or offering higher series resistance, the said information being color coded for the operator to decide whether the cell is to be accepted or rejected or suitable for reprinting,