Qualification of conductive adhesives for photovoltaic application - accelerated ageing tests

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7th International Conference on Silicon Photovoltaics, Silicon PV 2017 7th International Conference on Silicon Photovoltaics, Silicon PV 2017

Qualification of conductive adhesives for photovoltaic application Qualification of conductive adhesives for photovoltaic application accelerated ageing tests The 15th International Symposium on District Heating and Cooling accelerated ageing tests a a a, Luciana Pitta Bauermann , Sandorthe Stecklum Daniel Philippaa, Torsten a, Esther Fokuhl ausing a, Assessing the feasibility of heat demand-outdoor a a a b b Luciana Pitta Bauermann , Esther Fokuhl , Sandor Stecklum Daniel Philipp , Torsten Geipel a, Achim Krafta, Ulrich Eitnera, Thomas Fischerb, Dieter Breitenbücherb temperature function for aEitner long-term forecast Geipel , Achim Kraft , Ulrich , Thomasdistrict Fischer ,heat Dieterdemand Breitenbücher a a

Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstrasse 2, 79110 Freiburg, Germany b Fraunhofer Institute Systems Heidenhofstrasse 2,Germany 79110 Freiburg,cGermany Fa.for Teamtechnik, Planckstrasse 71691 Freiberg, a,b,c aSolar Energy a ISE,40, b b Fa. Teamtechnik, Planckstrasse 40, 71691 Freiberg, Germany

I. Andrić a

*, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Correc

IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b

Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France Abstract c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France Abstract In order to achieve a fast and reliable qualification of promising conductive adhesives for solar cells, different accelerated ageing In to achieve a fast and with reliable qualification of promising solar cells, different testorder sequences, accompanied analyses on material level are conductive conducted. adhesives Damp-heatfor followed by damp heataccelerated at a currentageing range test with analyses material are conducted. Damp-heat followed damp heat at a current range ofAbstract 8 sequences, A; humidityaccompanied freeze and thermal cycling.onThe testinglevel procedure in the herein presented work isbyadapted to identify advantages of 8 also A; humidity freeze cycling. The testingwith procedure in the hereinThe presented workadhesives is adaptedreplace to identify and weaknesses ofand the thermal materials in combination the entire design. conductive the advantages lead-based and also weaknesses of the are in combination with entire potential design. conductive adhesives replace lead-based soldering thereby networks allowing amaterials lower processing temperature. Another material costs is the use of double District heating commonly addressed in thethe literature as one The ofreduction the mostofeffective solutions forthe decreasing the soldering thereby allowing a busbars. lower temperature. Another potential of material costs thethrough use of double redundant fingers instead offrom In this sector. way, a These considerably smaller amount of silver iswhich needed. Theisinterconnectors greenhouse gas emissions the processing building systems require highreduction investments are returned the are heat redundant of redundant busbars. Infingers this way, atheconsiderably smaller amount silver is needed. The interconnectors are glued double with conductive adhesive. Anotherofheat advantage of the gluing is thecould possibility of sales.between Duefingers totheses theinstead changed climate conditions and building renovation policies, demand in the future decrease, glued between theses double redundant fingers with the conductive adhesive. Another advantage of the gluing is the possibility of using groove structured interconnectors that allow an increase in power of the solar module by reflecting light back to the solar prolonging the investment return period. using groove structured interconnectors that allow an increase in power of the solar module by reflecting light back to the solar cell. The main scope of this paper is to assess the feasibility of using the heat demand – outdoor temperature function for heat demand cell. As major degradation the performance of the modules containing conductive redundant and groove forecast. The districtindicator of Alvalade, located in Lisbon (Portugal), was used as a caseadhesives, study. The district fingers is consisted of 665 As major degradation the performance thetypology. modules containing conductive adhesives, redundant fingers and groove structured interconnectors is measured before andof after the exposition of the solar module to different accelerated tests buildings that vary inindicator both construction period and Three weather scenarios (low, medium, high) and ageing three district structured interconnectors is measured before and after the exposition of the solar module to different accelerated ageing and compared to the reference solar module with flat interconnectors soldered on busbars. After the accelerating ageing test damp renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values tests were and compared to results the flatviability interconnectors soldered on busbars. Aftertwo the accelerating ageing test damp heat (DH), there wasreference already clearmodule difference in the conductive adhesives. For module compared with from aasolar dynamic heatwith demand model, of previously developed and validated byconductive the authors.adhesives, heat already clear difference in aviability of power the conductive adhesives. For conductive module power lossesthere werewas comparable the weather reference, for third one losses amounted 14 %.two loss in power is caused by an The(DH), results showed that whenatoonly change is considered, the margin of errortocould beThe acceptable foradhesives, some applications power losses were comparable to the reference, for a third one power losses amounted to 14 %. The loss in power is caused bythe an increase in serial resistance. Analyses of the adhesives indicate a relation between the degradation in damp-heat and (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation increase in serial resistance. Analyses of the adhesives indicate a relation between the degradation in damp-heat and the adsorption of humidity of the material. Different subsequent aging tests (after DH) showed the feasibility of this innovative scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). adsorption of the weaknesses material. Different subsequent aging tests DH) showed feasibility this conductive innovative technology, buthumidity also unveiled compared towithin a traditional soldered busbar technology. Two ofthat theof three The valueofof slope coefficient increased on average the range of(after 3.8% up to 8% perthedecade, corresponds to the technology, also unveiled weaknesses compared traditional soldered busbar technology. the three conductive adhesives the 61215 criteria in termstoduring ofapower after damp heat, humidity andofthermal cycling. decrease have inbut thefulfilled number ofIEC heating hours of 22-139h theloss heating season (depending onfreeze theTwo combination of weather and adhesives have fulfilledconsidered). the IEC 61215 in terms power loss after increased damp heat,for humidity freeze thermal cycling. on the renovation scenarios On criteria the other hand, of function intercept 7.8-12.7% perand decade (depending ©coupled 2017 The Authors. The Published bysuggested Elsevier Ltd. scenarios). valuesby could be used to modify the function parameters for the scenarios considered, and © 2017 The Authors. Published Elsevier Ltd. © 2017 The Authors. Published by Elsevier Ltd. of SiliconPV 2017 under responsibility of PSE AG. Peer review by the scientific conference committee improve thebyaccuracy of heatconference demand estimations. Peer review the scientific committee of SiliconPV 2017 under responsibility of PSE AG. Peer review by the scientific conference committee of SiliconPV 2017 under responsibility of PSE AG. Keywords: conductive adhesives; accelerated ageing tests; cell interconnection; lead-free © 2017 The Authors. Published by Elsevier Ltd. Keywords: conductive adhesives; accelerated ageing tests; cell interconnection; lead-free

Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.

1876-6102 2017demand; The Authors. Published Elsevier Ltd. Keywords:©Heat Forecast; Climatebychange 1876-6102 The Authors. Published by Elsevier Ltd. Peer review©by2017 the scientific conference committee of SiliconPV 2017 under responsibility of PSE AG. Peer review by the scientific conference committee of SiliconPV 2017 under responsibility of PSE AG.

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer review by the scientific conference committee of SiliconPV 2017 under responsibility of PSE AG. 10.1016/j.egypro.2017.09.266



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1. Introduction Avoiding the soldering process at solidification temperatures around 180°C by using alternative interconnection methods is an interesting opportunity in the effort to reduce manufacturing costs. Especially thinner cells are more sensitive to thermal-mechanical stresses that can induce micro cracks. A promising solution which does not require temperatures above 150 °C are conductive adhesives, with their ability to enable low-stress, lead-free interconnections. Low process temperatures along with sufficient electrical conductivity and durability could lead to low-stress and reliable interconnections [1]. Several accelerated ageing tests, accompanied by analyses are applied in order to investigate the behavior of conductive adhesives used in combination with a double redundant finger layout compared to soldering. Nomenclature DH TC HF ECA EVA PET HAc

damp heat thermal cycling humidity freeze electrical conductive adhesives ethylene vinyl acetate polyethylene terephthalate acetic acid

2. Experimental part 2.1. Conductive adhesives The conductive adhesives used here are provided from different manufacturers and are selected in terms of processing parameters as well as cost aspects (lower silver content), see table 1. Table 1. Overview of used conductive adhesives.

Conductive adhesive A B C

Ag (wt %) 25-50 10-40 50-100

Cu (wt %) 40-70 -

2.2. Cell layout With the use of conductive adhesives a new cell layout can be tested. The busbars, which require a high amount of silver, are replaced by thin double redundant fingers. The conductive adhesives were used on Si solar cells, with and without busbars (Fig. 1). Where no busbar was present, the interconnectors were glued on redundant fingers, silver grids perpendicular to silver fingers. The soldered reference samples have a cell design with busbars.

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Fig. 1: Scheme of solar module with busbar (left) and with double redundant fingers (right).

2.3. Interconnectors Due to the low process temperature of the conductive adhesives and absence of pressing the interconnectors in this new layout, it is possible to use groove structure interconnectors (Fig. 2). With the use of these new interconnectors, the performance of the solar module could be optimized.

Fig. 2: Cross section of a grooved interconnector on a redundant double finger glued with conductive adhesive.

2.4. Tests and experiments For the tests on mini-modules the soldered reference and the conductive adhesive samples are manufactured to 1or 2-cell modules using semi-automatic processing (soldering, curing). After the interconnection a standard lamination process is performed using EVA encapsulation and PET-based back sheet materials. The solar modules are stressed under accelerated ageing tests listed in table 2. The designed stress tests are based on the photovoltaic standard IEC 61215. Table 1. Ageing tests applied on the solar modules containing conductive adhesives, as well as soldered reference. Test

Description

acetic acid exposure

1 M HAc RT conditions, 400 h/ 0,1 mM HAc, 100 h

damp-heat (DH)

1000 h, 85 °C. 85 % rh

damp-heat with current (DH @8A)

1000 h, 85 °C. 85 % rh @ 8 A

humidity freeze (HF)

20 h, 85 °C. 85 % rh; 10 min, -40 °C, x 10 cycles

thermal cycling (TC)

10 min, -40 °C; 10 min, 85 °C, x 200 cycles

2.5. Characterization ATR-IR is used to investigate the chemical degradation of the conductive adhesives after exposure to acid (1 M HAc). The conductive adhesives were screen printed on a glass and dipped in 1 M HAc for 100 h. The loss in performance of the solar modules is investigated with I/V curves and determination of the serial resistance Rs.



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Light microscope images of the joint cross section extracted from the solar modules are used to clarify the mechanical damage after the ageing tests. 3. Results 3.1. Acetic acid exposure The direct acetic acid exposure on the conductive adhesives intends to accelerate possible chemical reaction occurring in contact with the degradation product of the encapsulation material EVA. The chemical reaction of the conductive adhesives is investigated by ATR-IR spectroscopy (Fig. 3). For none of the used adhesives degradation in form of corrosion and visible changes could be observed. However adhesives of type ‘A’ showed comparably large water absorption in contact with the acidic solution.

Fig. 3. ATR-IR spectra of two conductive adhesives showing different behavior in moisture uptake after 1 M HAc exposure.

3.2. DH In order to accelerate ageing in the solar module, additional stresses as high temperature or electrical current are needed. This is firstly investigated by a standard DH test. The results show significant performance losses only for the conductive adhesive group A (Fig. 4). The power degradation is caused by the increase in serial resistance, which indicates a damage in the electrical contact between the solar cell and the interconnection. This might be explained by the water uptake of the conductive adhesive A (compare point 3.1). This water uptake can cause a decrease in the electrical conductivity of the adhesive as well as disrupt the mechanical contact between silver finger and interconnector.

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The conductive adhesives B and C show a negligible loss of power after DH, in the range of the soldered reference.

Fig. 4: Loss in Performance (left) and increase in serial resistance (right) of the solar modules with conductive adhesives as well as soldered references before and after 1000 h DH.

3.3. DH-TC The tests TC and HF are expected to be especially demanding for the conductive adhesives pre-stressed by DH. Due to the fast change in temperature over a wide range, there is a local high mechanical stress due to the different coefficients of thermal expansion of the polymeric and metallic components. Fig. 5 shows the loss in performance and its correlation with the increase in serial resistance of the solar modules after the sequential test DH-TC. The modules with adhesive A show again much higher losses in performance. Probably these modules were already damaged from the DH test. The conductive adhesives B and C show negligible losses, in the same range as the reference. 3.4. DH-HF The loss in performance after the sequence DH followed by HF is also negligible as can be seen in Fig. 5. The loss in performance for the conductive adhesives is higher than the reference module but still lower than 1 %. 3.5. DH @ 8 A To investigate possible galvanic corrosion of the metal filling of the adhesives, DH with an additionally applied current of 8 A is performed. This test results in the highest performance loss for the solar modules with conductive adhesives B and C (Fig. 5). Especially for the conductive adhesive B, which contains two metals (Ag and Cu), galvanic corrosion under current is expected. The serial resistance is again the parameter responsible for the loss in performance. The corrosion of the adhesives will be analyzed in more detail soon.



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Fig. 5: Loss in performance (left) and increase in serial resistance (right) of the solar modules with conductive adhesives as well as soldered references after the sequential tests TC, HF and DH @8A.

4. Conclusions The experiments showed the basic suitability of the conductive adhesives in combination with the double redundant finger layout. However considering all conducted tests, none of the used adhesives shows a stability which is comparable to a traditional soldered busbar layout. The ECA A showed relatively strong degradation when exposed to hot and humid climate (DH). This sample displayed a stronger humidity absorption when exposed to acetic acid. This correlation between these two observations needs to be verified in further experiments., as water is mentioned by many authors as the main cause for the deterioration of the conductive adhesives. The swelling of the organic part of the adhesive causes a mechanical stress in the glued area as well as changes in its adhesion due to a possible water film formation. If additionally the adhesive suffers a crack, the water transport increases considerably. In subsequent aging tests the conductive adhesives B and C perform very well after the stress tests DH-TC and DH-HF, whereas they showed a relatively high power loss in DH - DH @ 8A. This sequential test can be used to detect corrosion especially in adhesives containing two different metals, while the role of the reverse current has to be further investigated in ongoing work. 5. References [1] Jha SC, Forster JA, Breit HF. U.S. Patent 5 310 520,1994.