Soldering of Polycrystalline

Silicon Solar Cells For making DIY customised solar modules

depending on power requirementsby

manav shah (mechanical engineering student)

Solar cell basicsThe figure shows the description of different parts of the solar cell to get a understanding of its workings

Working stationAvailability of right equipment is necessary to achieve optimum results. The figure shows the workstation where the soldering process was conducted

Electrical layoutTabbing wire at negative conducting stripTabbing wire at negative conducting strip 

Efficiency – 18% minimumPower (1 cell) –1.148W

By connecting all the cells in series configurationVoltage –8.4V Current –0.5APower –4.2W

Forward diode

Busbar for series connection 

Materials requiredWooden board

Soldering iron (operating at maximum temperature of 340oc)Solder wire (40% tin and 60% lead)

Paper tape (3M)ScissorsFlux pen

Polycrystalline solar cells (MarsRock 18% having dimensions of 52mm*156mm)Tabbing wire (1cm wide)

RulerCopper gauge

Voltmeter (measuring DC voltage and current with constant frequency)Insulating wire

Methodology

Soldering of PV cells is to be performed for university project involving the integration of PV cells on flat surfaces. The procedure is devised after many trials to minimise the thermo-mechanical stresses on the cell during soldering

Step 1The cell is taken carefully from the stack. Avoid lifting the cells with fingers. Instead, rest the cell on the palm of the hand to distribute the weight evenly and prevent stresses to concentrate on a small surface area.

Step 2The cell is then placed on the board. A wooden board is used while soldering as wood has better heat dissipation property. The use of plastic board will result in uneven heat dissipation which will result in melting of the plastic while applying solder.

Step 3The cell is then taped on both ends to prevent the movement of the cell while soldering. Paper tape was used in this case due to its characteristics of easy removal and leaving minimum gum residue on the surface of the cell.

Step 4On the positive side of the cell, flux is applied on all the three conducting strips with the help of flux pen. Afterwards, small dots of solder wire (40% tin and 60% lead) are applied evenly on the entire surface of the three metallic conducting strips with the help of soldering iron. Flux application is the most important part of the process as it allows the tabbing wire to stick to the surface of the conducting strip. Not applying solder dots within few minutes after flux application will result in drying of flux on the surface which may result in metal not sticking to the surface.

Step 5A set of cells in this process refers to the two 52mm * 156mm cells connected in series. For each set, 3 pieces of 114mm tabbing wire and 6 pieces of 72mm tabbing wires are cut into pieces. The width of the tabbing wire was limited to 0.5cm which is the half of supplied width of 1cm in order to align the width of the conducting strip and the width of the tabbing wire. This promotes accurate soldering.

Step 6The temperature of the soldering iron is kept at 340c and must be placed gently on one end of the cell for few seconds for the heat to start flowing and melt the solder dots. As, the tabbing wire is coated with solder metal, heat from the soldering iron will melt the lead on the tabbing wire as well as the solder dots which results in the attachment of the tabbing wire to the surface of the conducting strip. After the surface of the wire is heated up, the soldering iron is moved evenly on the entire strip length by applying even pressure. Try to move the soldering iron over the strip in one stroke to prevent cooling of the metal before the entire strip is being soldered

Step 7The angle of the soldering iron is the vital part of the entire soldering process. The angle determines the amount of heat flow to the surface. As maximum heat flow is required, angle of 15 degrees is required for maximum heat transferred to the surface of tabbing wire which results in instant melting of the metal. Avoid touching the tabbing wire from the tip of the soldering iron as this will lead to less heat flow and uneven soldering.

15o

Step 8After the wire is soldered, do not remove the tape immediately as the wire and the cell are not completely cooled off and can lead to removal of the tabbing wire from its position. If the tabbing wire is removed slightly at a certain area, avoid repairing it immediately. After heat to the surface as part of the repairing process can heat the entire wire surface which can result in dislocation of the wire from the other areas of the strip.

The application of soldering iron results in the transferring of heat from the iron to the cell which can increase the temperature of the cell to about 80C. The most efficient option is to allow the cell to cool down completely before reapplying the iron. As the cooling of the cell is very important part of the soldering process, temperature can be determined with the help of infrared thermal camera. The figure shows the temperature after soldering the tabbing wire which is around 65C.Cell should be allowed to cool down until temperature reaches 25C-35C. The figure below shows the cell temperature has dropped from 65C to 34C

Hot cellCold cell after cooling

In case of the entire removal of the wire, the surface of the conducting strip is deposited with bunt residue from the soldering wire and flux which is hard to remove. As the reapplication of the flux and solder dots is not possible on the burnt surface, the cell is removed from its position and the same process is being implemented on a new cell (recommended) Success rate of making the perfect set for the electrical circuit is 20%. Due to the low thickness and brittle nature of the cell, the cell is prone to cracking. Occurrence of minor crack can significantly impact the electrical performance of the cell which can provide less output.

Step 9• Success rate of making the perfect

set for the electrical circuit is 20%. Due to the low thickness and brittle nature of the cell, the cell is prone to cracking. Occurrence of minor crack can significantly impact the electrical performance of the cell which can provide less output.

• If a formation of minor crack is noticed, it is not recommended to proceed forward. Do not try to repair or strengthen the cracks as the power output is being reduced after cracking

Step 10Exposure to high temperature can result in expansion of the cell. This expansion can permanently deform the cell which increases the probability of the cell while soldering. It becomes very difficult to handle and solder the cell in its deformed position due to its brittle nature.

To reduce the effect of expansion and contraction, a procedure should be set in order to improve the accuracy of soldering and minimise cell cracking

a) Positive strip ‘B1’ is soldered first with 114 mm wire and wait for the cell to cool down. The wire is placed on the solder dots and is taped on the either sides to prevent movement. The cell will expand and deform from the heat of the soldering iron.b) The cell is now flipped to its negative side and the procedure of the application of flux and solder dots is iterated to make the cell ready to solder. Then, strip ‘A3’ and ‘A2’ is soldered to balance out the expansion of the cell. During the application of the soldering iron, the cell will deform back to its previous position (flat). Allow the cell to cool before starting the next step.c) Strips ‘B3’ and ‘A1’ are then soldered respectively. The cell will expand again. At the end, solder ‘B2’ is soldered to balance the possible deformation. By following this process, the deformation will be kept to its minimum.d) Meanwhile, prepare the second cell by applying flux and solder dots and attaching the tabbing wire on conducting strips ‘D1’, ‘D2’ and ‘D3’ in a continuous process by allowing adequate cooling time in between. The cell will expand and deform significantly.e) Then, both the cells are brought side by side and the desired position is set and each corner of both the cells are taped. Gap of 10mm is kept between the two cells to allow the tabbing wire to expand and contract during heating an cooling

f) After that, both the cells are brought side by side and the desired position is set and each corner of both the cells are taped. Gap of 10mm is kept between the two cells to allow the buffer when they are placed on two sides of the absorber tube with an angle of 60o.g) Strip ‘C1’, ‘C2’, ‘C3’ are soldered without applying pressure. To prevent excess pressure from being applied, the hand is restricted from touching the ground. The sensitivity of the cell should be kept in mind as in the case of cracking of the cell during the ‘C2’ or ‘C3’ process, it is very hard to recover both the cells in the set as both the cells were connected through ‘C1’ and it cannot be reversed.

Step 11After completion of the soldering process, tapes are removed after 5 minutes in order to cool both the cells. The tapes are then discarded and are not reused in the second set as the gum on it is heated and burnt which makes the tape less sticky and its purpose is reduced to its minimum. The cell is then transported to the container by lifting the wooden board and sliding the set in the desired location.

Prototype• Working of the cells is required to be tested

under the heat of the sun to check the efficiency of the cells. A prototype of three sets was made and all the sets were connected in series. They were connected with electrical wires as they can be removed after testing with the help of copper gauge. The use of copper gauge is to extract the solder metal from the desired area by applying heat on it.• They were connected with electrical wires

as they can be removed after testing with the help of copper gauge. The use of copper gauge is to extract the solder metal from the desired area by applying heat on it.

Thank you for your time