Non-Destructive and Destructive Evaluation of Field Aged Photovoltaic Module

Dinesh SoundararajanASU Photovoltaic Reliability Laboratory (ASU-PRL)

• Introduction• Methodology

– Non Destructive Methods• Result

– Destructive Methods• Result

• Conclusion

Slide 2

Outline

Introduction• Solder bond failure is one of key degradation issues

• Solder bond degradation leads to higher series resistance (Rs) at metal circuit

• The power directly depends on Fill Factor (FF) which in turn is affected by series resistance (Rs)

• Destructive and Non Destructive Tests were conducted on 23Y ARCO Solar M65 module of 30 cells exposed in Xoxocotla, Mexico (Hot-Humid)

Methodology

• Solder bond degradation is analyzed through Non-Destructive and Destructive methods.• Destructive methods enables cell level analysis • Non Destructive Methods:

– IV & DIV Curve Tests – IR & EL Tests– Reflectance Test (Cell level)

• Destructive Methods: - DIV Curve Test (Cell level)- IR Test (Cell level)

Non-Destructive MethodsIV Curve Test• IV Curve Test is the most basic test to determine the

performance of a PV module Normalized IV Curve (STC – 1000 W/m2 / 25⁰C / 1.5 AM)

Slide 6

DIV Curve Test

• The dark IV test is done in a dark chamber at ambient temperature of 25 C ⁰• DC voltage is increased in steps until 1.25 times Voc is reached

DIV Curve Test

0 5 10 15 20 250 5 10 15 20 250.0001

0.001

0.01

0.1

1

10

0

0.2

0.4

0.6

0.8

1

1.2Dark IV Curve

Semi-Log

Linear

Voltage (V)

Sem

i-Log

Cur

rent

(A)

Line

ar C

urre

nt (A

)

Result: Performance Analysis

Isc (A) Voc (V) Imp (A) Vmp (V) FF (%) Rs (Ω) Pm (W)

Rated

at STC

3.25 18.00 2.95 14.60 73.00 0.64 43.00

Normalized

to STC

1.79 15.78 1.08 8.56 32.70 5.23 9.3

Percentage

change

- 44.92% - 12.33% - 63.38% - 41.37% - 55.21% + 717.19% - 78.37%

Result: Performance Analysis

Imp Vmp FF Rs Pm

-200.00%

-100.00%

0.00%

100.00%

200.00%

300.00%

400.00%

500.00%

600.00%

700.00%

800.00%

- 63.38%- 41.37% - 55.21%

+ 717.19%

- 78.37%

Percentage change between rated and tested value

IR Image Analysis

• The IR image of the module shows the presence of various hot spots in the module mostly along the solder bonds

Ambient - 28⁰C

EL Image Analysis

• EL image shows poor contact or inactive regions as dark lines, areas.

• Completely dark cells are dead cells.

Reflectance Analysis• Reflectance is caused by browning and delamination.• 82% of cells between median to worst reflectance value – Edge

cells

Result – Reflectance analysis• Edge cells reflected the light 33.6% more than Non-Edge Cells

Delamination - Image processing (MatLab)

• The gray area are the delamination of encapsulant due to moisture penetration

• It increases the reflectance rate• Delamination in edge cells is much higher than non-

edge cells

Destructive Methods

• Analyzing the performance at the cell level we could determine the causes and thus the extent of degradation more accurately

• Cell location based analysis helps in focusing on the often over looked areas for reliability issues

• Destructive tests enable analysis at cell level

Preparation for Destructive Testing

• Backsheet is cut in a specific area with access to solder ribbons in small squares using a single edge blade

Preparation for Destructive Testing Contd..

• It is then heated using an air heat gun to break the adhesive bonds between the backsheet and the cell encapsulant

• The backsheet (made of three layers) is removed

• Then the encapsulant is also removed exposing the ribbons

Preparation for Destructive Testing Contd..

• Finally the positive ribbon beneath the cell and the negative ribbon beneath the adjacent cell is soldered to the ribbon leads

Cell level IR Test

• Cell level IR Test is done in dark room to capture resistive heating alone

Result – Cell level IR Test

• Tcenter < Tedge < Tsolder in almost every cell of the module • The series resistance of solder increases due to moisture

induced degradation.

Cell level DIV Curve Test

• It is done using a cell level DIV tester integrated with a solar simulator

• Through cell level DIV tests the series resistance Rs, shunt resistance Rsh and Fill Factor FF (%) of each cell are found

Result – Cell level DIV Tests

• Non-Edge cells has 1.7% lower series resistance, 31% higher shunt resistance, 7% higher FillFactor than edge cells

Result – FF Vs Rsh Vs Rs Vs Tedge, solder, center

• Fill Factor FF dropped with increasing Tcenter, Tedge and Tsolder and importantly dropped drastically with slight increase of series resistance Rs but FF increased with increasing shunt resistance Rsh.

Conclusion

Test module - 23 years aged Arco M65 moduleSite - Xoxocotla, Mexico (Hot-Humid) • 717.19% increase in series resistance (Rs), 73.8% drop in

maximum power (Pmax) which is -3%/year• Very high power loss is due to harsh Hot-Humid climate at

Xoxocotla, Mexico• 33.6% higher reflectance in edge cells than non-edge cells• 1.7% lower Series resistance (Rs) in non-edge cells than

edge-cells• Degradation is due to moisture penetration through the

insulation along frame and permeation through backsheet.