INTERCONNECTMATERIALS
with rubber like flexibility. The test was conducted in accordance with ASTM D903 with a pull rate of 305 mm/s. Tin silver coated ribbon with a width of
2.5 mm was used. The ECAs were each cured for 1
⁄2
hr at 150°C to ensure complete cure. Figure 4 shows a plot of peel strength vs. displacement where the adhesive with rubber like elasticity exhibits superior peel strength despite having a lower modulus at ambient temperature. In addition to the high peel adhesion values observed, the failure mode was also desirable. The failure mode was characterized as “cohesive”, where adhesive was still attached to each interface, as shown in Figure 5.
Experimental
Although the solar cell is encapsulated, polymeric materials are unable to provide a complete hermetic seal from moisture permeation. Moisture will diffuse into the cell, and certainly the possibility exists that moisture may not easily diffuse out of the cell. In order to exasperate the effect of moisture, the adhesive and ribbons were exposed to 85°C/85% humidity (damp heat conditioning) without encapsulation. Essentially, the ribbon adhesive assembly was exposed directly to the damp heat conditions without any barriers.
A test scheme was developed in which the ribbons are joined with ECA in series creating 12 ribbonadhesive interfaces. The ribbons were affixed to a sheet of Kapton in the pattern shown in
Figure 7. Series resistance. Rubber like vs. rigid
Testing was performed for 1000 hours. Sanding of the ribbon to remove any oxide was performed where the probes were placed to ensure a good connection to the ribbon. The series connection of several interfaces amplifies any corrosion signal.
Both the rigid and the rubber like ECA were chosen for the initial damp heat evaluation in the series resistance scheme.
Figure 6. Series resistance scheme
Figure 6, and the adhesive was dispensed and cured. One hour at 150°C was chosen as the cure condition to ensure complete cure of the ECAs. The resistance was measured by utilizing a Keithley 2750 4 point probe multimeter at the points identified in figure 6. The measurement will take into account resistance of the ribbons, the 6 adhesive beads, and most importantly 12 ribbon adhesive interfaces.
After an initial resistance value was taken, the test pieces were placed in the damp heat chamber at 85°C/85% RH conditions. The parts were removed about every 150 hours and resistance was tested before placing samples back into the chamber.
Initial rubber like heat damp test The chart in figure 7 shows the rapid escalation of the series resistance with the rigid material, while the rubberlike material exhibits contact resistance that increases steadily over the 1000 hour test period. After the 1000 hours the rigid material exhibited a total resistance change of about 10,000%, while the rubber like ECA underwent a resistance change of about 400%. The % change in resistance is based on the initial series resistance values. The initial values were in the range of 40 to 60 mohm. The performance disparity is not a function of whether the material is rigid or rubber like. Permeability of water vapor is related to both the diffusion rate and the solubility of the molecule in the polymer (in this case the rubber like ECA). The simplified relation is shown below:
P = D χ S (6) 23
Figure 8. Improved rubber like series resistance
www.solar-pv-management.com Issue III 2011
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48