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EVA VS. TPO IN THE CONTEXT OF MANUFACTURING SMART WIRE PV MODULES

Aug. 22, 2018
Blog Post
by FreeVolt

EVA
The reliability and outdoor performance of a PV module is one of the end user’s biggest concerns because the investment is a medium to long term investment and therefore has to last over the years. Even the highest efficiency module needs to last 20 to 40 years in outdoor conditions. Here, extended climatic chamber tests required for IEC certification, damp heat (DH, 85% relative humidity, 85°C) and thermal cycling (TC, -40°C to 85°C in 6 hours) are presented. Damp heat is critical for the encapsulation material and the cell technology, whereas thermal cycling is vital when it comes to the reliability of interconnection technology.

The most popular encapsulant for photovoltaic module design has long been the copolymer ethylene vinyl acetate (EVA). This type of module has been operational in the field for over 30 years, even though many failures have been discovered, observed and investigated. Failure mechanisms connected to EVA are often attributed to moisture penetration into the module through the backsheet and the bulk of the encapsulant. When subjected to water and/or ultraviolet (UV) radiation exposure, EVA decomposes to produce acetic acid, which accelerates metallization corrosion. Under outdoor conditions, EVA suffers yellowing, browning and delamination, which cause considerable power loss. Among the observed failures, there is clear evidence of delamination and yellowing, which lead to a total measured power loss of 15%. Potential Induced Degradation (PID) has also been linked to EVA formulation and identified as a critical aspect of PV module system reliability.

The durability of EVA is mainly influenced by the additive elements which has been considerably improved in recent years. Many solutions have been presented with regard to the degradation problem of yellowing, but other degradation reactions (acetic acid production) still remain for this type of encapsulant. The use of cross-linking additives in EVA encapsulants also creates issues in terms of both module processing time and material storage. Encapsulants (such as cross-linked EVA) with a large number of additives that cause an increased likelihood of bubble formation during lamination process, especially in glass/glass PV modules design.

TPO
Investigating this challenge many PV experts have come to conclusions that thermoplastic polyolefin encapsulants (TPO) with water absorption less than 0.1% and no (or few) cross-linking additives have proved to be the best option for long-lasting PV modules.

These new generation encapsulation materials are now starting to enter the market because of their high electrical resistivity and their hydrolysis resistance. The fast curing and the additive-free physical cross-linking make them suitable for continuous lamination processing and they also demonstrate low water diffusivity and absorption as well as good UV stability. No formation of acetic acid has been observed during weathering. These properties make TPO encapsulants an interesting candidate for long-lasting PV modules.

TPO may need more time to flow and fill the gaps between the cells. Any remaining unlaminated patches (incomplete melting of the encapsulant) between the cells might then lead to a delamination issue. To avoid this issue, longer process times may be necessary with these encapsulants especially with traditional laminators. Our internal tests have shown that in damp-heat conditions (DH: 85°C, 85% RH), glass/glass PV modules based on Smart Wire Connection Technology and a TPO encapsulant endure 7000h (seven times the IEC test standard) without noticeable power degradation. In contrast, PV modules based on standard ribbon connection technology, laminated with EVA encapsulant for glass/glass and glass/backsheet lay-ups exhibit power losses of 19.5% and 40%, respectively, after the same length of time in DH conditions. An electroluminescence analysis revealed degradation due to moisture ingress and corrosion of the cells for the module with the EVA encapsulant, while no degradation issues were observed for the module with the TPO encapsulant. Glass/glass PV modules based on Smart Wire Connection Technology and TPO encapsulant also achieve successful results in extended thermal- cycling tests (TC: –40°C/+85°C), with a power output degradation of only 2.5% after 800 cycles (eight times the IEC standard).

Moreover, the lower volume resistivity of TPO encapsulants (ρv = 1014–1017Ωcm, compared with ρv = 1014Ωcm for EVA) also guarantees lower PID (Potential Induced Degradation)

Encapsulation – Damp Heat Tests
Standard module design comprises an EVA encapsulation layer and a PET or PET-Tedlar based back sheet. This sandwich has the disadvantage of degradation when modules are exposed to moisture. This effect is shown in the charts below, where power loss data from solar modules made with standard c-Si cells and encapsulated with materials such as TPU, TPO and EVA were collected. A strong power loss in damp heat conditions for a module with EVA was observed. This power degradation occurring between 2000 and 6000 hours of DH for modules encapsulated with EVA material is classic, as shown in the following charts. The EVA quality is usually the main criterion for the timing of the degradation. The superior performance of liquid silicone and TPO compared with EVA is mainly related to their very low water vapour transmission rate (WVTR) and water absorption values. The liquid silicone and TPO encapsulant absorbed less moisture than EVA. TPO material can therefore offer solid protection against potential induced degradation (PID). TPO has been chosen as the current solution for SWCT as it provides a more reliable performance and is available as a foil at a low cost. This TPO material provides also a perfect combination with HJT cells since we do not observe any degradation up to 5000 h in damp heat.

Connection technology – Thermal Cycling Tests 
The critical test for the interconnection technology is the thermal cycling test. Figures below present the results of internal tests based on HJT and standard cells that were connected with various technologies. Smart Wire Connection Technology combined with TPO encapsulant provided excellent resistance to thermal cycling test conditions, as no power degradation was observed on HJT modules following more than 800 cycles (four times the IEC criteria). This clearly demonstrates the superior performance of SWCT compared to other connecting technologies even for low temperature Ag paste metallization scheme. SWCT provide also excellent durability for standard cell with up to 400 cycles without losses. The modules will remain in the chamber until a failure appears.

Conclusions
Non-cross-linked (or slightly cross- linked) TPO encapsulants yield the best results with regard to processability of the SWCT glass/glass and glass/ backsheet module design. Thanks to the high-reliability properties of TPO encapsulants (low WVTR, low water absorption and small number of additives), more reliable PV modules passing extended IEC tests (7000h in DH and 800 cycles in TC) can be obtained.

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