DETERMINATION OF MOISTURE INGRESS THROUGH VARIOUS ENCAPSULANTS IN GLASS/GLASS LAMINATES

Jane Kapur1, Kristof Proost2, C. Anthony Smith3

1. E. I. DuPont de Nemours & Co., Inc., Wilmington, DE 19880 2. E. I. DuPont de Nemours & Co., Inc., Mechelen, Belgium

3. E. I. DuPont de Nemours & Co., Inc., Washington, WV 26181

ABSTRACT Encapsulant materials can provide protection and electrical isolation of the solar components in photovoltaic (PV) modules from the environment. However, some photovoltaic devices are sensitive to low levels of moisture and the ingress of water into a module can decrease its performance significantly during the lifetime of a module. In glass/glass PV modules, the moisture penetrates through the encapsulant to the module’s metal components and degradation can occur. In this study, we have developed and validated methods to determine moisture ingress in situ in a laminate. Water permeability and equilibrium moisture level (which are temperature dependent) through the encapsulant may affect the corrosion of metal components. We have measured the moisture ingress through an encapsulant material from the glass edge towards the center by an in-situ Fourier

transform infrared (FTIR) spectroscopy technique after damp-heat exposure. The FTIR measurements were performed on glass / encapsulant / glass laminates that were weathered at various times at elevated temperatures and humidity. The moisture level in the encapsulant can be determined by integration of the IR band between 1880 and 1990 nm. This peak surface was compared to a calibration curve, which was obtained using laminates with known encapsulant moisture levels (determined by Karl-Fischer titration). The moisture migration through an encapsulant material from the edge was also measured using an ASTM D7191 moisture analysis method [1]. The measurements were made on Al foil/encapsulant/Al foil laminates that were exposed to 85°C and 85%RH (damp heat). The experimental data correlated well to a Fickian diffusion model.

INTRODUCTION The reduction of moisture penetration through the encapsulant in a photovoltaic module is critical in certain solar cell technologies. Moisture can corrode the metal components in the module and/or increase the conductivity of the encapsulant, which may result in an increase in the current leakage of the PV module [2]. Various other defects, such as delamination, bubble formation and edge cloud, are all known to be influenced by the presence of moisture [3]. Photovoltaic modules and

encapsulants are affected by elevated levels of moisture with respect to mechanical integrity, electrical properties such as resistivity, current leakage and overall cell efficiency and these factors been the subject of many studies [4,5]. It is important to understand the factors influencing the rate and magnitude of moisture ingress into PV module encapsulants. Under accelerated-aging conditions, such as damp heat exposure (85°C and 85 % RH), the rate of moisture ingress is significantly increased. Photovoltaic modules are exposed to 1000 hours of damp heat as stated in IEC 61215 or IEC61646 standards and must retain a certain percentage of their initial power output in order to pass this part of the standard. In this work, we have tested various commercial and developmental encapsulants to elucidate the rate of moisture ingress through polymeric encapsulants laminated between impermeable substrates. The encapsulants in this study are made from EVA -- ethylene vinyl acetate polymer), DuPont PV5200 -- polyvinylbutyral (PVB), and DuPont PV5300 – an ionomer; additional developmental encapsulants were also investigated in this study. In order to measure the rate of moisture ingress, we have developed two primary methods to measure the moisture entering the edge of a glass/glass laminate that simulates a double glass photovoltaic module.

EXPERIMENTAL PROCEDURE - FTIR METHOD Glass laminates are exposed to damp heat conditions of 85°C and 85%RH. The 20 x 20 cm glass laminates are made in an autoclave process and are comprised of a 3mm glass, encapsulant, and 3mm glass. With a Perkin Elmer BX FTIR spectrophotometer, the following transmission spectrum spanning from 1600 to 2200 nm was obtained and is shown in Figure 1 (in this case of a PVB laminate: 2 lites of 3mm clear glass with 0.76mm PVB interlayer):

Figure 1: FTIR spectrum in the near infrared (NIR) region of polyvinyl butyral resin (PVB). A ratio (R) between the height of the CH peak (a) and the height of the moisture peak (b) is calculated for the PVB laminate (R=b/a). The height of the moisture band is taken between the maximum of the peak and the baseline that connects the two minima of the peak. Spectra are obtained for 6 standards with known moisture content and over the range of interest. These are laminates with known moisture content determined by Karl-Fischer titration (ASTM D6869 for polymers). By means of linear regression, a calibration curve is calculated using these 6 standards.

OBTAINING THE MOISTURE INGRESS PROFILE A special specimen mount was created to allow for accurate positioning of the glass laminate in the spectrophotometer compartment. In this way, the beam location could be well-defined and the sample could be translated laterally to make various measurements starting at the edge and inward. The position was indexed from the edge and measured using a standard sliding caliper scale. To establish the efficacy of the methodology, measurements of samples were conducted for two different encapsulant materials known to have substantially different moisture diffusion coefficients and equilibrium moisture levels. Glass-glass samples (3mm clear annealed glass) were prepared using DuPont PV5200 and PV5300 encapsulants and standard processing conditions. Two environmental exposure conditions were then evaluated; South Florida outdoor weathering and full water immersion (maintained at room temperature). Weathering conditions over the three-year exposure period averaged 28°C and 82% relative humi dity according to instrumentation at the weathering site. The moisture profile data collected is displayed in Figure 2.

Figure 2: Moisture profile measured using NIR technique developed for in-situ glass/glass PV module of ingress distance and concentration. Temperature is well-known to have a significant effect on moisture diffusion. The graph below shows the weight gain for a given encapsulant material (in 3-mm thick plaque form, not laminated to glass) as a function of temperature immersed in water.

Figure 3: Encapsulant weight gain as a function of temperature at 100% relative humidity conditions. (Not laminated) The net influence of temperature on moisture absorption necessitates very accurate control and monitoring of chamber temperature to obtain reproducible test results. Many approaches in measuring diffusion rates for different materials exist but are often complicated or difficult to measure over a wide range of temperatures. Since typical WVTR equipment is very sensitive to film thickness, control of thickness uniformity and difficulty making measurements at elevated temperatures, the determination of the moisture profile within a glass laminate affords an alternative methodology to better understand moisture ingress behavior.

A moisture profile was generated by putting a glass–encapsulant–glass sample in the damp heat chamber at 85oC and 85% RH. The sample was taped on the bottom and the top with a moisture-barrier tape to ensure that the moisture is entering the encapsulant in one direction. After a given time (typically 1000 hours), the sample was taken out of the damp heat chamber and the moisture levels were then measured every centimeter, starting from the edge of the laminate shown in Figure 2 below.

Figure 4: Diagram of Glass/Glass Laminate and location of FT-IR measurements for moisture determination. The advantage of this method is that moisture measurements can be made in-situ in a glass laminate and it can be performed quickly when the moisture peak is distinguishable. A peak integration method was also employed for situations where the moisture peak was not easily resolved, either due to low intensity and/or broadness of the peak. The moisture peak in some polymer resin systems is not well-defined and can be spread over the range of 1925 to 2100 nm. In this case, the peak intensity is integrated [Fig. 1 (c)] across that wavelength range yielding much better sensitivity for making determinations of moisture levels below 0.08% moisture. A calibration curve was generated in the same manner as for the peak intensity method described above. EXPERIMENTAL PROCEDURE - ASTM D7191 METHOD An alternate method was also developed for measurement at low moisture levels (i.e., below 0.1%) or in the case for EVA where the spectral moisture peak coincides with the characteristic peaks of the polymers making the NIR method more difficult and less reliable. The moisture migration through an encapsulant material from the edge can also be measured by a relative humidity sensor method, ASTM D7191. The measurements were made on aluminum foil/encapsulant/aluminum foil laminates that were exposed to damp heat for two- and six-week time periods. The foil laminates were prepared on a Meier

vacuum laminator (Icolam® 10/08) using standard lamination conditions producing an extrusion-coated Al foil/Encapsulant/extruded-coated Al foil structure. The aluminum foil was extrusion-coated with a 25um layer of Nucrel® acid copolymer resin [DuPont] which provides good adhesion to the foil and the various encapsulants of interest. Sample strips (1.27cm in width) were die-cut from the foil laminates. The moisture level of the sample was determined using a Computrac® Vapor Pro Rx® moisture analyzer (Model CT-100) made by Arizona Instruments. This technique was calibrated to samples tested using ASTM D-6869 Karl-Fischer titration method (Figure 5). The weight of the foil was subtracted from the foil laminate weight to determine the moisture level in the encapsulant.

Figure 5: Calibration Curve between Karl-Fischer (ASTM D6869) and Computrac Vapor Pro ASTM D7191 method. The moisture ingress through PV5200 (PVB) laminates determined by FTIR and ASTM D7191 methods is shown in Figure 6. Both methods correlate very well after the Vapor Pro measurements have been calibrated.

Figure 6: Moisture Ingress after 2 weeks of damp heat exposure through DuPont PV5200 encapsulant using

FTIR and ASTM D7191 method (before and after calibration).

RESULTS AND DISCUSSION We compared the moisture ingress of several commercial PVB materials laminated between two lites of glass. The laminates were exposed to 1000 hours of damp heat. The measurements were determined by the FTIR method (Figure 7). The difference in moisture levels among the PVB sheets is minimal. After 1000 hours of accelerated-aging, the moisture levels do not change beyond 6 cm into the laminate. We also measured the moisture ingress through the PV5300 encapsulant which is ionomer-based. The moisture penetrates approximately 5 cm into the laminates after 3000 hours of accelerated aging. The moisture levels are much lower than that of PVB.

Figure 7: Moisture ingress after 1000h of damp heat exposure through four commercially available PVBs--DuPont PV5200 encapsulant and three PVBs obtained from other sources-in glass/glass laminates.

Figure 8: Moisture Ingress through PV5300 Encapsulant in glass/glass laminate after various times in damp heat (measured by FTIR method). The moisture ingress through EVA after damp heat (85°C/85%RH) exposure was measured at 2 and 6-week periods. Data is compared to a Fickian Diffusion model in Figure 9. The moisture ingress through the edge of a polymer laminated between two lites of glass can be modeled as Fickian diffusion through semi-infinite media. The solution to this problem is given by Crank [6] in the following equation:

)4/()( DtxerfcCCCC isi ⋅−=− (1) where C is the moisture fraction, Ci is the initial concentration of moisture in the encapsulant, Cs is the surface concentration at the laminate edge or equilibrium moisture concentration of the encapsulant x is the distance from the laminate edge, D is the diffusion coefficient, and t is the exposure time. The equation is valid at relatively short times (t) and short distances from the laminate edge (x<<L, where L is the length of the laminate). Kempe [7] reports that the D is 6.3 x 10-6 cm2/s and the surface concentration is 0.0055 at 85C at high humidity. The experimental data compares well to the model indicating that the diffusion of water vapor in EVA is Fickian.

Figure 9: Moisture Ingress through EVA encapsulant after 2 and 6 weeks of damp heat (85C/85%RH) exposure compared to the Fickian Diffusion model where the diffusion coefficient of water vapor is D = 6.3 x 10-6 cm2/s and the surface concentration, Cs = 0.0055 (taken from literature [7]). We have also measured the ingress of moisture through developmental encapsulants. Results show that the moisture level is lower than that of commercial EVA encapsulants after six weeks of damp heat exposure. The experimental data can be fitted to Eq. 1 to determine the diffusion coefficient of water vapor in the developmental encapsulants. The diffusion coefficient of DuPont PV5300 is approximately 4 times slower than that of EVA. Figure 10 indicates that PV5300 and the developmental encapsulants can significantly slow the migration of water vapor (compared to EVA) into a photovoltaic module.

Figure 10: Moisture Ingress after 1000hr of damp heat exposure.

CONCLUSIONS We have developed two methods to accurately measure moisture ingress through encapsulants over time. The methods have been correlated to the Karl-Fischer titration method. The methods have been shown to be effective in measuring moisture ingress profiles for various encapsulant materials and characterize their performance as a function of temperature and various environmental exposure conditions. This methodology can be applied to the development of enhanced encapsulant materials possessing both lower moisture ingress rates and equilibrium levels.

ACKNOWLEDGEMENTS The authors would like to thank Douglas Evans, Michael Evans, John Jensen and Joseph Winward for their help in completing this work.

REFERENCES [1] ASTM D7191-05 “Standard Test Method for Determination of Moisture in Plastics by Relative Humidity Sensor”, Annual book of ASTM Standards, 2008, vol. 08.03, American Society for Testing and Materials, West Conshohocken, PA. [2] M.A. Quintana, D.L King, T.J McMahon, C.R. Osterwald, Photovoltaic Specialists Conference, Conference Record of the Twenty-Ninth IEEE, 2002, pp. 1436 - 1439. [3] B.C. Wong, “Shattering Old Myths about Defect Formation in Lamination Glass”, Glass Processing Days, 1997, pp. 13 -15. [4] D.L. King, M. A. Quintana, J.A. Kratochvil, D.E. Ellibee and B. R. Hansen, Prog. Photovolt. Res. Appl. 8, 2000, pp. 241-256. [5] G.J. Jorgensen, K.M. Terwilliger, J.A. DelCueto, S.H. Glick, M.D. Kempe, J.W. Pankow, F.J. Pern, T.J. McMahon, Solar Energy Materials & Solar Cells 90 (2006) pp. 2739 - 2775. [6] J. Crank, The Mathematics of Diffusion, Clarendon Press, Oxford, 1975. [7] M.D. Kempe, Solar Energy Materials & Solar Cells 90, 2006, pp. 2720 - 2738.