Potential Induced Degradation of Photovoltaic Modules ... · Potential Induced Degradation of...

Potential Induced Degradation of Photovoltaic Modules:

Influence of Temperature and Surface Conductivity

By

Faraz Ebneali

April 2012

Overview

Goals

Introduction to Photovoltaic Reliability

Why PID?

PID Definition

IEC standard

Parameters influencing PID High Voltage in the String

Project Setup

Characterization of samples and diagnostic tools

Findings Positive Bias

Negative Bias

Regeneration Bias

Conclusion and Recommendation

2

Goals Study High voltage, Temperature and humidity (surface conductivity) effects on

crystalline silicon modules (Poly -Si and Mono-Si)

To compare the effect of temperature on reaction rate and degradation level

To find out the correlation between leakage current and power reduction

To compare and evaluate the rate of degradation among Fresh, Thermal Cycling

(TC) and Damp Heat (DH) samples

To understand what type of samples are suitable for PID test

Level of Electrochemical Corrosion and shunting within the interface

To analyze the power reversibility of the samples after degradation

To estimate the internal series resistance (Rs) of the samples before and after PID

stress test

To verify and confirm previous PRL PID studies

3

Reliability in PV Industry

• The first period : Eliminate by Qualification test

• The Second Period : Low failure rate

• The last Period : End of the useful life

• PID test : decreases early field failure rate and increases the normal life cycle

4

Why PID?

Oh no! Our modules are down 40%, we think it is potential–induced

degradation” ‐anonymous module manufacturer, 2010. [1]

Australia’s silent module killer (due to high temperatures and humidity,

specially in the north of the country)

But what conditions actually cause the effect to occur?

Ideal conditions for PID;

1. High voltage (600 -1500 V)

2. Temperature

3. Surface Conductivity (Humidity)

5

PID free modules!

• PID could jeopardize performance of the PV module.

• Degradation due to PID is nothing new but has been resurfaced in recent years; more sensitive modules , more system voltage

Lack of standard test

Q-Cells PID Test Conditions:

• 25°C, damp module surface (what %RH?)

• 600 Volt (what polarity?)

• 300 hours test

Tainergy PID Test Conditions:

• 25°C, (what %RH?)

• -1000 Volt

• 140 hours test

6

IEC61215 standard

Current Standard !

• Qualification tests like Thermal Cycling, Damp Heat and …

• Require testing for safety issues at high voltage conditions.

Inclusion of PID in IEC standard

• Need to standardize the PID test that completely evaluate the durability of

the modules under stressed conditions.

• Early detection of damage with applying both negative and positive

potential and other conditions.

• Reasonable PID test in case of cost, time and stress level.

7

PID Definition

• The potential difference between the frame and the cells results in a leakage

current (charge transferred or polarization)

• Polarization : Charge accumulate in the encapsulation! Shunting, FF, Short and open circuit

• Flow of leakage current from active cell layer through the encapsulation

and the glass along the surface to the frame [2]

8

Parameters influencing on PID effects

• Cell Level

1. Anti reflective coating

2. Emitter depth

3. Type of base doping

• Module Level

1. Encapsulant material

2. Module design (frame, mounting, isolation)

• System Level

1. Voltage (1000V)

2. Humidity (85%RH)

3. Temperature (85°C)

4. Grounding connected with frame [3]

9

High voltage in the string [4]

1.Transformerless Inverter (no transformer): Lighter, cheaper and higher efficiency

2.Transformer-based Inverter: PID effect can be prevented by grounding the negative string,

however high potential may cause new failure mechanisms

10

Transformless Inverters [5]

• PV Offset box (SMA in cooperation with Evergreen)

• Operates at nigh (2 mA Max and 600 V)

• Still can not reverse the electrochemical degradations

11

Two main failure modes in PID Electrochemical Reaction : Electricity can enable a non-spontaneous chemical

reaction to occur and create an electrolytic corrosion.

Electrolysis;

• Voltage

• Electrolyte solution (Water Vapor)

• Cell = anode

• Frame = cathode

Leading to degradation of the material such as silicon nitride (ARC), EVA and the

active cell.

Polarization: Leakage current causes the charge to accumulate in the

encapsulation material via cell, causing polarization, not only lowering the parallel

resistance but the FF and the output. This is associated with significant shunting of

individual cells.

12

[6]

Indoor test to simulate field condition

• To simulate the open-air conditions (Field environment)

1. Test Duration: 35 h

2. Voltage: ±600 V

3. Chamber Temperature : 60°C and 85°C

4. Chamber Humidity : 85%

13

System Setup and Components

PV Samples

DAS

Grounding

Point

Resistors

Power Supply

14

System Setup

PRL complete PID stress test package

• Precise

• Automated (2 round per day)

• Durability (18AWG)

• Upgradable (2kV)

• Safe (Ground Fault Protection)

PID Channel Monitor

15

7.5 kV Insulator

Process Flow

Goal

Project 1 and 2:

Share similar characteristics.

To discover the effect of

polarity on PV samples and

study the temperature

variation effect on the

amount of Leakage current

(LC)

Project 3 and 4 :

Monitor and analyze the

effect of temperature

16

Procedure Description

I. Phase-I Positive Bias (Negatively grounded): +600V

Apply +600V to cell active layer 5h intervals at 85°C or 60°C and 85%RH (excluding 1 hour ramp rate) 35hours ( 7-PID cycles-Characterization after 2nd ,4th and 7th cycle )

II. Phase-II Negative Bias (Positively Grounded): −600V

Apply -600V to cell active layer 5hours at 85°C or 60°C and 85%RH (excluding 1 hour ramp rate)

Phase-III: Regeneration Process

+600V is applied to the active layer of Phase-II modules

17

Arrhenius behavior

Arrhenius plots are used

to analyze the effect of

temperature on the rates

of chemical reactions

The rate of the aging

can be approximated by

using Arrhenius

equation

y = -8.2745x + 26.819

y = -7.4607x + 23.991

y = -5.7052x + 18.735

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

2.8 2.9 3.0 3.1 3.2 3.3

Lea

ka

ge

curr

ent (µ

A)

1000/T(K)

Project 1-Arrhenius plot, Cycle 7 (+ Bias, 35h) at 85°C/85%RH

DH

TC

Fresh

25°C

45°C

75°C

18

Arrhenius behavior

y = -7.2263x + 23.911

y = -7.3277x + 24.121

y = -7.6797x + 25.12

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

2.8 2.9 2.9 3.0 3.0 3.1 3.1 3.2 3.2 3.3 3.3

Lea

ka

ge

curr

ent (µ

A)

1000/T(K)

Project 1-Arrhenius plot, Cycle 7(-Bias, 35h) at 85°C/85%RH

DH

TC

Fresh

25°C 45°C 75°C

19

Arrhenius Plot

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

2.8 2.9 3.0 3.1 3.2 3.3

Lea

ka

ge

curr

ent (µ

A)

1000/T(K)

Project 3, Arrhenius plot- Cycle 2(+Bias, 10h) at 85°C/85%RH

DH

TC

Fresh

25°C 75°C 45°C

20

Ea Summary

21

These (~.6 ev to.8 eV) correspond closely to the activation energy for current conduction through

soda-lime silicate glasses. In the presence of high RH, the top surface becomes coated with a

macroscopic layer of water, is fairly conductive, and behaves as an equipotential that approximates

earth ground potential [2].

Phase-I Positive Bias (Negatively grounded): +600V

• Total coulombs transferred vs. Fraction of power remaining in project

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

Before Round 2 Round 4 Round 7 Coulombs

Fra

ctio

n o

f P

ow

er R

ema

inin

g

Test Hours

Project 1 (+Bias, 85°C/85%RH)-Power Performance

DH-Coulombs TC-Coulombs Fresh-Coulombs TC DH Fresh

No sign of degradation

22



0

0.2

0.4

0.6

0.8

1

1.2

1.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1


Fra

ctio

n o

f P

ow

er R

ema

inin

g

Test Hours



23



0

0.2

0.4

0.6

0.8

1

1.2

1.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1


Fra

ctio

n o

f P

ow

er R

ema

inin

g

Test Hours



24

Phase-II Negative Bias (Positively Grounded): −600V


0

0.2

0.4

0.6

0.8

1

1.2

0.5

0.6

0.7

0.8

0.9

1.0

1.1


Fra

ctio

n o

f P

ow

er R

ema

inin

g

Test Hours

Project 1 (-Bias, 85°C/85%RH)-Power Degradation


25



0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1


Fra

ctio

n o

f P

ow

er R

ema

inin

g

Test Hours

Project 2 (-Bias, 60°C/85%RH)-Power Degradation


26


I-V curves at Standard Test Condition (STC) Power Performance

Negative bias caused drop in power output

Increase in PID stress test increase the power loss due to shunting and polarization effect

Small changes in Voc but significant FF drop

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

0 10 20 30 40 50 C

urr

ent

Voltage

Project 1 (DH, -Bias)-STC Power Comparison

Pre-Test

10h

20h

35h

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

0 10 20 30 40 50

Cu

rren

t

Voltage

Project 1 (Fresh , -Bias)-STC Power Comparison

Pre-Test

10h

20h

35h

Vmp, Imp Dropped

Vmp, Imp Dropped

27


Due to ease of ionization of the Ag anode in positive bias the LC is greater than

other conditions

Ionic migration leading to a leakage current through the wet encapsulant medium

[1]

28

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Coulombs transferred

Coulombs Transfered after 35 hours Test

+Bias, 85C

-Bias, 85C

+Bias, 60C

-Bias, 60C

Correlation of power and coulombs transferred in

negatively biased projects

0.00

0.20

0.40

0.60

0.80

1.00

1.20

0.00

0.20

0.40

0.60

0.80

1.00

DH Fresh TC

Co

ulo

mb

s T

ran

sfer

red

Fra

ctio

n o

f P

ow

er R

ema

inin

g

Fraction of Power Remaining vs. Coulombs Transferred, Project 1 ( -Bias, 85°C)

Fraction of Power Remaining Total Coulombs Transferred

29

Correlation of power and coulombs transferred in

negatively biased projects

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.00

0.20

0.40

0.60

0.80

1.00

Fresh DH TC

Co

ulo

mb

s T

ran

sfer

red

Fra

ctio

n o

f P

ow

er R

ema

inin

g

Fraction of Power Remaining vs. Coulombs Transferred, Project 1 ( -Bias, 60°C)

Fraction of Power Remaining Total Coulombs Transferred

30


The positive ion migration mechanism causes surface polarization of the

modules in negatively biased samples. This affect is reversible to some extent.

There are two ways that the effected modules can be recovered ;

Heat Recovery

Reverse Polarity

0.50

0.60

0.70

0.80

0.90

1.00

1.10

Neg35h Round 2 Round 4

Fra

ctio

n o

f P

ow

er R

ema

inin

g

Test Hours

Project 1 (Regeneration, 85°C/85%RH, +600 V)

TC

Fresh ~100% gained original power

DH Broke

31


0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

1.10

Before Neg-35 Round 2 Rround 4 Round 7

Fra

ctio

n o

f p

ow

er R

ema

nin

g

Test Hours

Project 2, (Regeneration, 60°C/85%RH)

DH

TC

Fresh Up to 98% Power recovery

32

Power Regeneration (STC)

0

20

40

60

80

100

120

140

160

180

200

0 10 20 30 40 50

Cu

rren

t

Voltage

Project 2, DH module (Regeneration, +Bias, 60°C) STC Power Comparison

Pre-Test-P(W)

Neg 35h-P(W)

Reg+5h-P(W)

Reg+10h-P(W)

Reg+20h-P(W)

Reg+35h-P(W)

33

Back to Original Power

Internal Series Resistance (Rs)

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

Pre-Rs (P1) Post-Rs (P1) Pre-Rs (P2) Post-Rs (P2)

Res

ista

nce

(Ω

)

Pre vs. Post Test *

Project 1(+Bias, 85°C/85%RH)-Series Resistance

DH

TC

Fresh

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

Pre-Rs (P1) Post-Rs (P1) Pre-Rs (P2) Post-Rs (P2)

Res

ista

nce

(Ω

)

Pre vs. Post Test *

Project 1(-Bias, 85°C/85%RH)-Series Resistance

DH

TC

Fresh

Pre and Post PID test “Rs”

calculated using IEC

60891 standard

*P1= Procedure 1

*P2= Procedure 2 34

Internal Series Resistance

0.00

0.50

1.00

1.50

2.00

2.50

3.00

0.50

0.60

0.70

0.80

0.90

1.00

1.10

Before Round 2 Rround 4 Round 7 Series Resistance

(Ω)

Fra

ctio

n o

f P

ow

er R

ema

inin

g

Test Hours

Project 1 (-Bias, 85°C/85%RH)

Pre-Rs Post-Rs TC DH Fresh

Post-Test Rs Pre-Test Rs

35

Electroluminescence Image

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

1.10

Before Neg-35 Reg+10 Reg+20 Reg+35

Fra

ctio

n o

f P

ow

er R

ema

inin

g

Test Hours

Project 2, DH Module (Regeneration, +Bias)-Power Regeneration

Cell shunting

36

Electroluminescence Image

1- Moisture penetration

2- Less resistive path to ground

37

No longer contributes to short circuit current production

Infrared Scan

38

Fresh

DH

TC

Proje

ct 2 a

fter 1

0 h

ou

rs

test

(-b

ias,

85

°C/8

5%

RH

)

Infrared Scan

39

Fresh

DH

TC

Proje

ct 2 a

fter 2

0 h

ou

rs

test

(-b

ias,

85

°C/8

5%

RH

)

Infrared Scan

40

Fresh

DH

TC

Proje

ct 2 a

fter 3

5 h

ou

rs

test

(-b

ias,

85

°C/8

5%

RH

)

Infrared Scan Summary

1. Project 1 (-bias, 85°C/85%RH)

2. Project 2 (-bias, 60°C/85%RH)

3. Project 1 (+bias, 85°C/85%RH)

1

3

2

41

Summary

42

Carbon paste

Summary

43

Carbon paste

Summary

44

Carbon paste

Summary of Findings Positive Bias

• Positive potential bias does not seem to affect the performance, irrespective of pre-history (fresh, TC or DH) and surface conductivity (conductive carbon or humidity) of the modules.

Negative Bias

• In the negative bias modules that were subjected to TC tests did not show power output deterioration when humidity (water film) was used for the surface conductivity; however, in the same projects DH and fresh modules lost up to 50% of their original power. Therefore degradation in negative bias depends on pre-history (fresh, TC or DH) and surface conductivity

– Phase Transition of components and materials, cure EVA and back sheet

– That is in contrast to the previous study

Regeneration Bias

• Reverse polarity can recover modules’ power to approximately their original power

EL Image

• Dead cells and shunting effects were observed in negative PID test samples

IR Scan

• Majority of samples in both polarities have an elevated average operating module temperature after 35 hours of PID testing

Damp Heat samples

• DH samples experienced more noticeable amounts of charge transfer than other samples (DH = 4.59, fresh =3.43 coulombs transferred)

Correlation of Power loss and charge transferred in Negative Bias

• Larger amount of charge transferred, larger degradation and vice versa

45

Further investigation

Why thermal cycling samples of this study show resistance to

the PID? (but not in previous study)

Increase 35 hours stress test (7 cycles) to more cycles; it

allows better auditing methods for any type of phase transition

or new mechanisms in the cell and polymeric materials.

Large sample size but from the same model

System level study

46

Acknowledgements

• Thankful to my advisor Dr. Govindasamy Tamizhmani for introducing me to

the world of Photovoltaic industry and specifically reliability of photovoltaic.

His encouragements and supports in various ways. And thanks to Joseph

Kuitche for discussing valuable information with me in relevance to the study.

• I gratefully thank Dr. Narcio F. Macia and Dr. Bradely Rogers for their time and

constructive comments on this paper.

• Thanks to Dr.Peter Hacke, NERL, for sharing his experiences regarding this

study.

• Regards and blessings to my parents, who without them it was impossible to

make it to this point. Their kind and unconditional support in all aspects during

the completion of my master’s studies.

And to all my PRL students (current and previous) specially;

Sandyha Goranti

Sai Tatapudi

Kolapo Olakonu

Meena Vemula

Annie

Jose Cano

Saurabh

Surya Narayana

End

References

[1] Peter Hacke “Considerations for a Standardized Test for Potential‐Induced

Degradation of Crystalline SiliconPV Modules” February 29, 2012 NREL/PR-5200-

54581

[2] J.A. del Cueto and S.R. Rummel “ Degradation of Photovoltaic Modules Under High

Voltage Stress in the Field” 2010

[3] Simon Koch “Potential Induced Degradation (PID) effects on crystalline solar

modules” 2011

[4] Ivo Kastle “Dealing with high voltage stress “ PV Magazin , 2011

[5] www.SMA.de “PV OFFSET BOX Night-time discharging of PV panels” SMA Solar

Technology

[6] P. B. Ghate “Electromigration-induced failures in VLSI interconnects” texas

instruments incorporated, “Thin Film Electromigration" Workshop, International

Reliability Physics Symposium, March 30, 1982.

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