The experimental objectives of this report are: (1) compare the performance of modules exposed to high temperature and humidity; (2) determine the effects of different encapsulants on long term stability of CIGSS modules; and (3) analyze failure modes to determine areas in need of improvement

Kempe, M. D.

Tarrant, D.

Terwilliger, K.

UNT Digital Library

Stress Induced Degradation Modes in CIGSS MinimodulesMike Kempe1, Kent Terwilliger1, and Dale Tarrant21 National Renewable Energy Laboratory (NREL), 1617 Cole Boulevard, Golden, CO 804012 Shell SolarNREL/PR-520-43310Presented at the 33rd IEEE Photovoltaic Specialist Conference held May 11-16, 2008 in San Diego, CaliforniaExperimental Objectives? Compare the performance of modules exposed to high temperature and humidity.? Determine the effects of different encapsulants on long term stability of CIGSS modules.? Analyze failure modes to determine areas in need of improvement.Experimental Setup? Systematically changed:? (1) Encapsulant (EVA or GE RTV615 silicone)? (2) Front-sheet (Glass or Tefzel)? Samples exposed to:? (1) 85C/85% RH in air.? (2) 85C/0% RH in air.  (Dew point ~ -40C)? Used 4 or 5 replicates. 4*23+2=34 samples.? Initial average cell parameters:? Voc=0.538 V? Jsc=32.8 mA/cm2? FF=65.7%? η=11.59% Stress at 85°C and 85% RH Causes Rapid Degradation3002001000-100-200-300Current (mA)1086420-2-4Voltage (V)   Shell Solar CIGSS #32     Laminated with GE RTV615     Dow Corning 1200 Primer   Tefzel Frontsheet   Stressed at 85oC/85% RH   0 h   1 h   103 h   199 h   457 h-300-200-1000100200300Current (mA)1086420-2-4Voltage (V)   Shell Solar CIGSS #25     Laminated with EVA     Glass Frontsheet   Stressed at 85oC/85% RH   0 h   1 h   103 h   199 h   457 h   935 hEVA with Glass Front-SheetSilicone with Tefzel Front-SheetGlass Slows down the degradation but does not prevent it.Infrared Images Shows a Striped Pattern+81 mA, 9.6 VForward BiasSilicone encapsulant with a Tefzel Front-Sheet.  Module #32  457 h of 85 ºC and 85% RH-0.24 mA, -9.6 VReverse BiasNo Signs of ShuntsWeakDiodes (i.e. small areawith low Voc )IR Heat Pattern Indicates High Resistance ZnOWarmerAreaIR image of module under forward bias of 9.6V and 81 mA.Schematic representation of CIGSS cell interconnection schemeHeating not symmetric around the scribe and therefore is not due to resistance in interconnection scribes.ZnOCIGSSMoGlassP1  P2  P3 P1  P2  P3Heating Caused Principally by Recombination Current ⎥⎥⎦⎤⎢⎢⎣⎡ −= ⎟⎠⎞⎜⎝⎛1exp nkTqVojj00.10.20.30.40.50.60 0.2 0.4 0.6 0.8 1Distance Across CellAbsorber Voltage Drop 0123456789Current Density Through the AbsorberZnOCIGSSMoGlassP1  P2  P3 P1  P2  P385°C and 0% RH Exposure Causes Voc and FF Losses4002000-200Current (mA/cm2)1086420-2-4Voltage (V)   Shell Solar CIGSS #13     Laminated with EVA     Glass Frontsheet   Stressed at 85oC/0% RH   0 h   1 h   103 h   199 h   457 h   935 h   1574 h   2287 h   3607 h   4663 h   6463 h   8767 h6005004003002001000-100-200-300Current (mA)1086420-2-4Voltage (V)   Shell Solar CIGSS #29    Laminated with GE RTV615    Dow Corning 1200 Primer    Tefzel Frontsheet   Stressed at 85oC/0% RH   0 h   1 h   103 h   199 h   457 h   935 h   1574 h   2287 h   3607 h   4663 h   6463 h   8767 hSilcone EVA Silicone encapsulated cells performed better.  They had better fill factors and less roll over.An Analysis of Variance Indicates Statistical SignificanceTwo factor ANOVA for samples exposed to 8767 h of 85ºC and 0% RH. “F ratio” is the ratio of the uncertainty between treatments to the sample set uncertainty.  “Probability” is the chance of getting this F ratio if the two treatments were actually equivalent.“Encapsulant*Front-sheet” indicates the probability that interactions between treatments significantly affect the results.  Resistances determined from inverse slope.Silicone Encapsulated Cell Have Lower FF and “Rs “ LossesResistances were inferred from the inverse slope.  The horizontal lines for each data set correspond to the 95% confidence interval for the magnitude of the changes.The large horizontal line spanning the plots is the grand mean for the data set.  Efficiency Change-6-5-4-3-2-10E/G E/T Si/G Si/TFF Change-25-20-15-10-50E/G E/T Si/G Si/TRseries,light Change05101520E/G E/T Si/G Si/TRshunt,dark Change050000100000150000E/G E/T Si/G Si/T8767 hours of 85ºC and 0% RHAt 8767 h 85C and 0% RH ZnO Resistance Has Increased Forward Bias Module #159.5 V, 31 mA, 15 s8767 h 85°C and 0% RHEVA/TefzelRseries,light Change (ohm)05101520E/G E/T Si/G Si/TIR Images Distinguish Weak Diodes from ShuntsFlashlightIllumination5 s forward Bias9.3 V 153 mASilicone/Tefzel after 2290 h 85ºC and 0% RH. #295 s Reverse Bias9.3 V 0.53 mAIR images are made by subtracting image values before and after application of voltage.Many Weak Diodes Are Located on the P1 scribeFlashlight IlluminationThe flashlight illuminated the side of the cell just outside the image to ensure that we were not just seeing a reflection.ZnOCIGSSMoGlassP1   P2   P3Silicone/Tefzel after 2290 h 85ºC and 0% RH. #29Weak Diodes Principally Located at P1 Scribes or Cell EdgeWeak-diode P1 Weak-diode CellWeak-diode EdgeSilicone/Tefzelafter 2290 h 85ºC and 0% RH. 9.3V and 153 mAApplied for 20 sModule #29The Number of Weak Diodes Barely Changed90 mA, 9.6 V20s2290 h Exposure50 mA, 9.5 V20s8770 h ExposureModule #30, Silicone/Tefzel at 85°C/0% RHForward BiasReverse Bias2.3 mA, -9.5V20s8770 h ExposureShunts Shunts and Weak Diodes2 mA, -9.6V20s2290 h ExposureDiodes are Weaker but Shunting is UnchangedVoc Change per Cell (V)-0.045-0.04-0.035-0.03-0.025-0.02-0.015-0.01-0.0050E/G E/T Si/G Si/T8770 h exposure to 85°C and 0% RHRshunt,Light Change Module (ohm)-2000-1500-1000-50005001000E/G E/T Si/G Si/TVocRshunt,lightSimilar change in Voc across all sample sets. No statistically significant change in Shunts.Conclusions? Exposure to 85°C and 85% RH for 457 h or 935 h:? Large increases in ZnO resistance.? Some Voc losses.? Exposure to 85°C and 0% RH for 8767 h:? Small increases in ZnO resistance.? Some Voc losses.? No shunting change.? Silicone encapsulated cells performed better than EVA.? EVA produced greater losses in FF and series resistanceAcknowledgements? The authors would like to thank Tom McMahon for helping with the infrared imaging and David Albin for help with computer code. ? This work was supported by the U.S. Department of Energy under Contract No. DE-AC36-99GO10337 with the National Renewable Energy Laboratory.

Stress Induced Degradation Modes in CIGSS Minimodules

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Stress Induced Degradation Modes in CIGSS Minimodules

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