High Voltage Stress Impact on P Type Crystalline Silicon PV Module

doi:10.4236/epe.2013.57049

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Energy and Power Engineering, 2013, 5, 455-458

http://dx.doi.org/10.4236/epe.2013.57049 Published Online September 2013 (http://www.scirp.org/journal/epe)

High Voltage Stress Impact on P Type Crystalline Silicon

PV Module

Han-Chang Liu*, Chung-Teng Huang, Wen-Kuei Lee, Mei-Hsiu Lin

Green Energy and Environment Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan

Email: *[email protected]

Received July 18, 2013; revised August 18, 2013; accepted August 25, 2013

which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

The effects of the high voltage stress and other environmental conditions on crystalline silicon photovoltaic module

performance have not been included in the IEC 61215 or other qualification standards. In this work, we are to evaluate

the potential induced degradation on p type crystalline silicon PV modules by three cases, one case is in room tempera-

ture, 100% relative humidity water bath, another is in room temperature, the front sheet coverage with aluminum foil

and the other is in the 85˚C, 85% relative humidity climate chamber. All the samples are applied with the −1000 V bias

to active layers, respectively. Our current-voltage measurements and electroluminescence results showed in these mod-

ules power loss of 37.74%, 11.29% and 49.62%, respectively. These test results have shown that among high voltage

effects the climate chamber is the harshest and fastest test. In this article we also showed that the ethylene vinyl acetate

volume resistivity and soda-lime glass ingredients are important factors to PID failure. The high volume resistivity

which is more than 1014 Ω·cm and Na less contents glass will mitigate the PID effect to ensure PID free.

Keywords: Potential Induced Degradation; High Voltage; Volume Resistivity

1. Introduction

In photovoltaic (PV) solar modules, reliability is the very

important issue for solar power performance, as light

induced degradation is a well-known phenomenon. It has

long been included in the performance guarantees offered

by producers in the industry or the calculations of project

developers and system operators. Light induced degrada-

tion can cause an approximate 2% decrease in system

performance in the first few hours of operation of any

new PV installation. In 2005, a new form of performance

degradation began to be noticed and now called potential

induced degradation (PID) [1] which is high voltage

stress effect in negative potential field relative to ground.

With the more and more growing PV system and in-

creasing system voltages the PID effects are more seri-

ously and the leakage currents are the characterizations.

Possible pathways for the leakage currents from the en-

capsulated cell to the frame are described by J. A. del

Cueto [2]. The domain pathway to cause PID is via the

front sheet as glass to the frame. Higher leakage currents

can be caused by water entering the solar module causing

the encapsulation material to become more conductive.

So far the potential degradation mechanism is not

monitored by the typical PV tests listed in IEC 61215 [3].

Some researchers were trying to find out it. It is known

that metal ions such as Na+ formed from the oxides of the

module glass can drift toward the cell if the cell is biased

negatively [4]. Recently P. Hacke et al. found the in-

creased Na concentrations in the surface and sub-surface

area of PID affected samples were shown by secondary

ion mass spectroscopy (SIMS) [5] and Na precipitates

were found on the surface of such samples [6]. M.

Schütze et al. show that PID can also be caused by other

ions usually not present in photovoltaic modules indicat-

ing that the chemical nature of the ions is not relevant for

PID [7]. The Dr. Liu et al. in their study directly verified

that the PID caused by Na+ from their saline water bath

experiment [8]. Until now, there is no agreement with

evidence that observed metal concentration increasing in

the vicinity or inside the cell is responsible for shunting

of PID affected modules. The effect of the high voltage

and other environmental conditions on module perform-

ance has not been included in the IEC61215 or other

qualification standards. In this work we discussed the

PID effects in different environmental conditions. The

case one is 100% relative humidity (RH) water bath, an-

other is covered with conductive aluminum foil and the

other is tested in the 85˚C/85% RH chamber to provide

*Corresponding autho

H.-C. LIU ET AL.

456

the different test methods for PV manufacturer to ensure

their solar cell modules to have PID free.

2. Experimental

Three commercial silicon PV modules with 6 × 10 mc-Si

solar cells were applied to the PID test. Three methods

are applied to the high voltage test, one is in the water

baths with room temperature and 100% RH environ-

mental factors for 168 hr, another one is in room tem-

perature and full area coverage with aluminum foil of the

modules’ front surface for 168 hr [9], the third method is

using 85˚C and 85% RH damp heat chamber for 48 hr

[10], then application of −1000 V between the cells (via

the junction box) and the aluminum frame, respectively.

In addition, a 4 × 3 mini module which without soda-

lime glass was made and applied −1000 V between the

ells (via the junction box) and the aluminum frame to

identify the PID formation issue.

For characterization of these modules prior and after

the PID test a Berger PSS30 flash tester [11] and a high

resolution electroluminescence (EL) camera were used.

The volume resistivity test was according to ASTM-

D257 [12] “Standard Test Methods for DC Resistance or

Conductance of Insulating Materials” using 500 ± 5 V as

the applied direct voltage and charged with 60 sec to

measure the volume resistivity variety.

3. Results and Discussion

Three commercial silicon PV modules were produced in

order to compare different options for PID test conditions.

The degree of PID was measured in terms of the standard

test condition (STC) module power as shown in Table 1.

In Table 1 all the samples after PID testing are shown

the power loss more than 5% during 168 hr or 48 hr test.

From our results the PID test under water bath had about

three times power loss than that was covered with alu-

minum foil that means water acceleration the PID rate. In

another method by chamber test we find the same result

as test under water bath. The chamber PID test reducing

70% time is more effective than water bath.

Table 1. PID test data under different conditions.

Test time Voc Isc Vmp Imp PmaxΔPmax

Module

NO [hr] [V] [A] [V] [A] [W] [%]

0 36.670 8.290 29.160 7.700 224.430

Water

bath 168 34.184 8.356 21.774 6.410 139.737

−37.737

0 36.690 8.420 28.860 7.770 224.310

Al foil

168 36.562 8.467 27.372 7.270 198.988

−11.289

0 37.410 8.530 30.071 8.024 241.284

Chamber

48 33.600 8.203 23.381 5.199 121.551

−49.623

Furthermore we used electroluminescence technology

to check these cells performance. Figure 1 was EL im-

age after PID test in water bath condition.

We can see that serious dark areas more than two

strings after PID test, relevant the Voc decreased about

2.5 V and power loss 38% that means weakening of

cell’s depletion zone, resulting in a reduction of Voc and

Pmax. In the Figure 2, there was EL image after PID test

coverage with aluminum foil.

From the Figure 2 there are random dark areas distri-

bution in the EL images, and relevant Voc decreased

slightly about 0.1 V and loss power 11%. Compared to

the two testing results humidity played an important role

on acceleration PID testing. Humidity can enhance the

leakage currents flow from module-cells through module

insulation and packaging materials, to the module frames,

to earth-ground via module supports, moreover moisture

entrancing the module can induce encapsulant degrada-

tion and reduce insulation of encapsulant to form PID

(a) (b)

Figure 1. EL images of a module (a) pre and (b) post PID

test in water bath.

(a) (b)

Figure 2. EL images of a module (a) pre and (b) post PID

test with front sheet coverage with aluminum foil.

H.-C. LIU ET AL. 457

prone, the encapsulant especially meaning ethylene vinyl

acetate (EVA). Figure 3 was EL image after PID test in

85˚C/85% RH climate chamber which also had the seri-

ously dark areas after 48 hr PID test. In the chamber ex-

cept moisture we added heat to accelerate the PID test. It

can more quickly and effectively check the photovoltaic

solar cell module whether it have PID prone.

Furthermore we studied the volume resistivity relative

to PID effect. Two different EVA films A and B were

aged in 85˚C and 85% RH climate chamber monitored

one week as showed in Figure 4.

The film A showed that volume resistivity from 1014

order decreased to 1013 order, opposite to A the film B

appearance good performance during aged period. From

the results we can deduce that increasing EVA film con-

ductivity caused the module to PID prone. The Figure 4

also showed that the excellent encapsulant can restrain

moisture entrance to module and mitigate PID occur-

rence. Because no prediction of PID effect without test-

ing is possible so far, hence test can protect manufacturer

from massive customer complains.

In order to study the PID mechanism we designed a 4

× 3 mini module without soda-lime glass as front sheet

for the PID testing. The testing was performed in the

(a) (b)

Figure 3. EL images of a module (a) pre and (b) post PID

test in 85˚C/85% RH climate chamber.

Figure 4. The volume resistivity varies of EVA film A and B

after 85˚C and 85% RH climate chamber test.

climate chamber with 85˚C and 85% RH conditions ap-

plying −1000 V until 48 hr. The test result was showed in

Table 2.

There is only 1% power loss after PID test which

shows good performance than that with soda-lime glass

specimens testing previously. EL image without glass

module had no obvious difference before and after PID

test as showed in Figure 5 that means the module was

PID free.

From the result we can speculate some compounds in

the soda-lime glass which also plays important role for

PID effect. The compounds especially sodium ions can

migrate easily from glass to cell if strong negative elec-

tric field exists between cell and module aluminum frame.

Above all result, we can suppose the PID process as fol-

lows: moisture permeate into module which causes EVA

degradation to become more conductive and helpful Na

migration to cell surface or P/N junction, finally causes

the cell shunting to loss output power.

4. Conclusions

The lifetime of PV modules are reduced by various deg-

radation factors such as: harsh environments, high sys-

tem voltages and material failures over long periods of

Table 2. PID test data without Soda-lime glass module.

Test TimeVocIsc Vmp Imp PmaxΔPmax

Module

NO. [hr] [V][A] [V] [A] [W]%

0 7.4858.552 5.799 7.888 45.742

Without

Soda-lime

glass 48 7.4808.449 5.767 7.856 45.305

−0.955

(a)

(b)

Figure 5. EL images of a module (a) pre and (b) post PID

test without Soda-lime glass module.

H.-C. LIU ET AL.

458

operation. In this article we investigated the high system

voltage effect or PID effect. Our PID test results showed

that PID in chamber is the harshest and fastest test

method than in water bath and coverage with aluminum

foil. Our results also verified that the volume resistivity

and soda-lime glass ingredients are important factors to

PID failure.

Nowadays, PID effects are focused by every PV

manufacturer to assure their products PID free. In the

future how to reduce module PID effect to increase reli-

ability and performance will lead PV modules to more

competitiveness and wide acceptance of PV technolo-

gies.

5. Acknowledgements

The financial support provided by Bureau of Energy

(contract No.: 102-D0305) is gratefully acknowledged.

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