What is the TPT backplane of solar cells?

Nov 03, 2021 Laminate and mid-test process of photovoltaic modules

What is the TPT backplane of solar cells?

TPT is the abbreviation for the composite material of “Tedlar film→Polyster→Tedlar film”. Tedlar is a registered trademark of DuPont. It is a polyvinyl fluoride film used on the back of the module as a backside protective packaging material.

The TPT used for packaging should have at least three layers: the outer protective layer PVF has good resistance to environmental corrosion, the middle layer polyester film has good insulation properties, and the inner layer PVF has good adhesion properties after surface treatment and EVA. The schematic diagram of the TPT backplane film and its structure is shown in Figure 1.

What is the TPT backplane of solar cells?
Figure 1 Schematic diagram of TPT backplane film and its structure

The back cover of the solar cell-the fluoroplastic film is white, which scatters the light incident to the inside of the module and improves the efficiency of the module to absorb light, so the efficiency of the module is slightly improved, and because of its higher infrared emissivity , Can also reduce the working temperature of the components, but also help to improve the efficiency of the components. Of course, this fluoroplastic film firstly meets the basic requirements of aging resistance, corrosion resistance, airtightness, and water seepage resistance required by solar cell packaging materials.

1). TPT back sheet film performance
The main properties of TPT backsheet film include mechanical properties, electrical properties, weather resistance, etc. The main performance index parameters are shown in Table 1.

Thickness/mm0.18, 0.23, 0.35
Tensile strength/(N·10mm-1)≥110
Stretching rate/%120 ~130
Tear strength/N·mm-1135 ~145
Peel strength between layers/N·cm-1≥25
Peel strength/N·cm-1≥20
Weightlessness (24h/150℃)/%≤3
Dimensional stability (0.5h/℃)/%≤3
Water vapor permeability/(g·m-2·d-1)≤2.0
Breakdown voltage/kV≥17
Anti-ultraviolet ability (60℃/1kW UV xenon lamp for 100h)No color change, stable performance
Service lifeMore than 25 years
Table 1 Performance index parameters of TPT backsheet film

2). The key points of storage and use of TPT backsheet film
(1) Keep away from light, heat, and moisture, and stack them in a flat manner. The products should not be bent or the packaging damaged.
(2) The best storage conditions: constant temperature (20℃~25℃), constant humidity (<60%). Avoid direct sunlight, keep away from heat sources, and prevent dust and fire.
(3) The shelf life of the backplane depends on different materials, the general shelf life is 12 months, and the bulk shelf life is not more than 6 months.

3). Common backplane failure modes
(1) The structural defects of the backplane itself: the service life is not up to the standard (indicated by the embrittlement, yellowing of PET, and the crack of the backplane, such as pure PET structural components, the general service life is not more than 10 years).
(2) Defects of the interlayer adhesive: delamination between the backplanes (stability of the coating process, or insufficient bonding strength of the interlayer adhesive, or rapid aging of the interlayer peeling force).
(3) EVA bonding layer defects: delamination (surface treatment problems, EVA quality problems, cross-linking degree is not up to standard), yellowing (materials are not resistant to aging, such as Toyo PVDF+W﹣PET+V﹣PET+LE LE layer is easy to aging and yellowing).

4). Backplane detection
The material of the backplane determines the service life of the components. The performance of the TPT backplane needs to be inspected to test its reliability. The main test items are shown in Table 2.

Evaluation indexTest itemsDetection method
ExteriorSize, flawsMeet the requirements of the incoming material inspection work instruction (visual inspection, ruler)
Mechanical propertiesTensile strength/elongation at breakTest according to GB/T 1040 test requirements (universal testing machine)
Electrical performanceBreakdown voltageTest according to GB/T 1408 test requirements (voltage breakdown testing machine)
ShrinkageTwo-dimensional dimensional stability150℃, 30min to measure the change of the two-dimensional size of the backplane (incubator)
PermeabilityWater permeability testTest according to ISO 15106-1 test requirements (water permeability tester)
Bonding performancePeel strength with EVA/silicone; peel strength between layersAccording to GB/T 2791 test requirements, test the weakest layer of peeling force between the back plates (tensile gauge, universal testing machine), and require it to reach the index value
reliabilityRefer to relevant IEC standardsRefer to 1EC related standards
Note: Reliability testing is mainly for material changes or regular testing; for the test items listed on the backplane, you can select and test according to the actual situation.

Table 2 TPT backplane inspection items

(1) Mechanical properties of TPT backplane. The mechanical properties of a material refer to the mechanical characteristics of the material under different environments (temperature, medium, humidity) and subjected to various external loads (tension, compression, bending, torsion, impact, alternating stress, etc.). The stress-strain experiment is the most extensive, most important and practical material mechanical property test experiment. Figure 2 and Figure 3 show the stress-strain curves under different conditions. Apply a load to the specimen at a given strain rate until the specimen breaks. Most of them use the tensile method, and use the universal testing machine for testing, as shown in Figure 4.

What is the TPT backplane of solar cells?
Figure 2 Stress-strain experimental curve of a polymer
What is the TPT backplane of solar cells?
Figure 3 Stress-strain experimental curves of samples of different materials
What is the TPT backplane of solar cells?
Figure 4 Universal testing machine

(2) Peel strength between layers. The schematic diagram of the interlayer peel strength test is shown in Figure 5.
① Test method.
(A) First cut the backing film into a size of 1cm×50cm.
(B) Peel off each layer of the backing film, and then bond the striped backing film and EVA glass together according to a certain lamination process to make a small sample.
(C) Measure the tension between the layers with a tension meter at a direction of 180° to the glass surface and at a speed of 30 cm/min.
②Requirement: The interlayer adhesion force of each layer is greater than 1N/cm.

What is the TPT backplane of solar cells?
Figure 5 Schematic diagram of interlayer peel strength test

(3) Bonding force with EVA. The schematic diagram of the adhesion test between TPT and EVA is shown in Figure 6.
①Test method:
(A) Prepare two EVA samples of the same size (greater than 300mm×400mm), a cloth-patterned tempered glass, and a target back film.
(B) Laminate and cure in accordance with the lamination sequence of “glass/EVA/EVA/back film” and a certain lamination process.
(C) After cooling the above sample, use a blade to make two fine grooves on the surface of the backing film with a distance of 10mm to form a backing film strip with a width of 10mm.
(D) Use a blade to shovel the port at one end of the backing film strip away from the EVA surface, and then use a tension meter to pull the backing film strip at a speed of 30 cm/min at a direction of 180° to the glass surface, and record the data.
②Requirement: The bonding strength is greater than 20N/cm.

What is the TPT backplane of solar cells?
Figure 6 Schematic diagram of the adhesion test between TPT and EVA

(4) Bonding force with silica gel.
①Test method:
(A) Coat a flat plate (such as aluminum alloy) with silica gel with a size of 1cm×50cm.
(B) Glue a pre-cut back film with a size of 1cm×50cm on the upper surface of the silica gel.
(C) After the silica gel is completely cured, use a tension meter to pull the backing film at a speed of 30 cm/min at an angle of 180° to the horizontal, and record the data.
②Requirement: The bonding force is greater than 30N/cm.

(5) Shrinkage rate.
①Test method: Put a piece of TPT backplane film with a size of 300mm×400mm on a clean flat glass, and put it in an oven at a temperature of 150°C for continuous baking for 30 minutes. After taking it out and cooling it, re-measure the size and calculate the shrinkage rate, as shown in Figure 7.

What is the TPT backplane of solar cells?
Figure 7 Schematic diagram of calculation of shrinkage rate of TPT backplane

(A) The transverse shrinkage rate is less than 1%.
(B) The longitudinal shrinkage is less than 1.5%.
(6) Partial discharge voltage.
①Test method:
(A) Use the back film to be tested to make a component according to the company’s conventional process.
(B) Use a high-voltage tester to test the high-voltage resistance characteristics of the above-mentioned components.
②Requirements: The high voltage resistance must be greater than 1000V.

(7) Damp heat aging test.
①Test method:
(A) Laminate and cure the backing film of the sample to be tested, the target glass, and EVA according to a certain laminating curing process to make a small sample.
(B) Put the sample into the high and low temperature test aging box, set the aging condition as: 85℃, 85%RH.
(C) Continuous aging for 1000h, of which, every 500h, take out and observe the condition of the experimental sample and make a record.
(D) Take it out after 1000h, and inspect the appearance after damp heat aging and the bonding strength between EVA and EVA.
(A) No yellowing, no embrittlement or cracking in appearance.
(B) Compared with before aging, the attenuation of bond strength must be less than 50%.
(C) The attenuation of reflectance must be less than 10%.

(8) Ultraviolet aging test.
①Test method:
(A) Use the backing film sample to be tested, the target EVA, and glass to make a small sample according to a certain lamination and curing process.
(B) Place the sample on the stage of the UV aging box.
(C) Adjust the ultraviolet radiation parameters: ultraviolet light with a wavelength of 280~385nm, the intensity does not exceed 250W/㎡.
(D) Make the total irradiance of the sample not less than 15kW/(h·㎡), among which, the radiation of 280~320mm wavelength is at least 5kW/(h·㎡)
②Requirement: The tested sample shall not be yellowish, brittle, or cracked.