ieee pvsc-40 highly abrasive resistant ar coating

7/21/2019 IEEE PVSC-40 Highly Abrasive Resistant AR Coating

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A Highly Abrasive-Resistant, Long-Lasting Anti-Reflective

Coating for PV Module Glass

Sergiu C. Pop¹, Venkata Abbaraju¹, Brenor Brophy², Y. Sam Yang², Sina Maghsoodi²,Peter Gonsalves²

¹Yingli Green Energy Americas, San Francisco, CA 94108, USA²Enki Technology, San Jose, CA 95131, USA

Abstract — More than 4% of incident light is reflected fromthe front cover glass of photovoltaic (PV) modules. The industry-wide trend to cost-effectively increase the efficiency of PVmodules has driven the widespread adoption of anti-reflectivecoated (ARC) glass. The most common deposition methods forthese anti-reflective (AR) coatings are wet sol-gel processes, witha small minority of glass manufacturers using vacuum-basedsputtering. Most commercial sol-gel coatings consist of singlelayer porous silica and are highly transmissive across a broadrange of the solar spectrum. The mechanical strength of thesecoatings is generally derived from high temperature treatmentduring the tempering of the glass. However, as the PV industryincreasingly focuses on project levelized cost of electricity(LCOE), PV module manufactures and system owners areseeking ARC glass with increased durability and long-termperformance, particularly for systems operating in medium tohigh soiling environments, where PV modules are subjected toairborne particle abrasion and repeated washing. In this work,we report durability test results for a new, low temperaturecurable sol-gel AR coating from Enki Technology, showingsignificantly improved abrasion-resistance compared to moretraditional AR coatings. The new coating is denser than thetraditional coatings and chemically derives its abrasion-resistance at relatively low process temperatures without

sacrificing the optical performance. The samples under test inthis work were subjected to extensive testing following theindustry-standard abrasion test method in EN1096.2 andaccelerated environmental test procedures in IEC61215. Ourresults show increased coating longevity for the new coating, up

to four times greater than existing coatings with comparableoptical performance. This increased mechanical strength directlytranslates to reduced risk of handling during manufacturing and

installation, lower O&M costs, increased energy yield andreduced warranty costs.

Index Terms — amorphous materials, glass, materials

reliability, photovoltaic cells.

I. I NTRODUCTION

Greater than 4% of normally incident light is lost due to

reflection at the interface of air and PV module glass. As the

industry has driven towards higher efficiencies at ever lower

costs it has widely adopted ARC for PV module front cover

glass. It is estimated that greater than 70% of silicon PV

modules now ship with ARC glass [1].

While a small minority of suppliers still provide vacuum

based AR coatings, the vast majority of traditional coatings

are based on single layer, porous silica, wet sol-gel technology

[2], [3]. These coatings typically derive their mechanical

strength through a high temperature sintering step that occurs

when the PV cover glass is tempered.

As the PV industry has grown PV module manufacturers

and system owners and operators are increasingly focused on

project LCOE. With growing experience in the long-term field

performance of these coatings they are seeking ARC glass

with increased durability and long-term performance, particularly for systems operating in medium to high soiling

environments, where PV modules are subjected to airborne

particle abrasion and repeated washing [4], [5].

II. EXPERIMENTAL DESIGN

We evaluated three traditional AR coatings and the new AR

coating on three different PV module glass substrates (A, B,

and C). The traditional coatings were received as coated from

the manufacturer on tempered 30x30cm glass sheets.

Uncoated samples of the same base glass were coated with the

new coating. For this work the new coating was prepared by

hydrolysis and partial condensation of organo-tri-alkoxy-

silanes and tetra-alkoxy-silanes. The resulting gel-free

homogenous solutions of the high silanol containing co-

polymers of silsesquioxane have long term shelf life stability

at room temperature. The sols were coated on pre-cleaned

substrates by flow coating technique, dried and cured at 300

°C. The resulting thin-film was a homogenous, uniform,

transparent, defect-free, crack-free dense coating with

thickness in the range of 70-120 nm and excellent AR

properties.

Optical reflection was measured on coated sheets using a

Konica Minolta CM-2600d spectrophotometer. Based on the

difference in reflection spectra over 360~780nm between an

uncoated control and the coated sample a percentage increase

in transmission of solar weighted photons is calculated using

the AM1.5 spectrum [6]. This metric is used to measure

optical performance of the samples.

Six substrates (3 traditional, 3 new) were then submitted to

abrasion testing according to specification EN1096.2. This test

consists of 1000 strokes using a Taber linear abrader with a

14.5mm diameter, 10mm thick, medium density felt disk with

4N of force applied, at 30 strokes/minute and a stroke length



of approximately 10cm. The disk is rotated

on every second stroke. Optical reflection

every 200 strokes in the center of the abra

reduction in optical performance is calculated.

To test for reliability, five full size 60-cell

solar panels were tested at Underwriter L

with the three most relevant tests for solar

IEC61215 guidelines: Damp Heat (DH, 100

and 85% relative humidity), Temperaturecycles between -40 °C and +85 °C) and Humi

10 cycles between -40 °C and +85 °C).

uncoated, a second was coated with a traditi

equivalent to the coating on substrate C, and t

coated with the new coating. These reliability

the power fall by no more than 5%, and t

significant visual defects on the glass surface.

III. R ESULTS AND DISCUSSION

A. Optical Performance

A.1. Reflectance

Fig. 1 shows the initial optical performance

percentage increase of transmittance of solar

of the traditional coatings versus the ne

observed that the traditional coating on sub

highest optical performance. In the other

substrates A and B respectively, the opti

recorded for the traditional coatings are

compared to the new coating. As is sh

properties of the new coating does not depen

the substrate type.

Fig. 1. Initial average optical performance expretransmission of solar weighted photons

By measuring optical performance during t

is possible to emulate how the coating wear

allows us to distinguish between a gradual

decline in performance as opposed to a cat

Fig. 2 shows a measure of the linearity of th

the optical performance during the abrasion te

bout 22 degrees

as re-measured

ed area and the

.

multi-crystalline

boratories (UL)

glass, under the

hours at 85 °C

Cycle (TC, 200dity Freeze (HF,

One panel was

onal AR coating

hree panels were

tests require that

hat there are no

S

expressed as the

eighted photons

coating. It is

strate C has the

two cases, on

cal performance

slightly lower

wn the optical

significantly on

ssed as % increase

e abrasion test it

over time. This

and proportional

strophic failure.

e degradation of

st. By measuring

the abrasion resistance every 20

whether a coating will slowly wea

comparable to the linear warranty

manufacturers, or whether the coa

fitting a line between measuremen

strokes, a metric of linearity can b

types the new coating shows val

linear) compared to traditional c

higher degree of homogeneity of th

Fig. 2. Optical performance degraabrasion testing

The total degradation experien

shown in Fig. 3 There is a very cl

traditional coatings on different

coating. In each case the traditional

is a few times higher than the ne

substrate C the traditional coat

degradation, while the new film de

The traditional coating C had t

performance as shown in Fig. 1

performance is correlated to the lar

optical performance is caused by

which often has poor adhesion and

Fig. 3. Post abrasion weighted trreduction in optical performance due

strokes, we can predict

away over time, at a rate

offered by all solar panel

ing is easily removed. By

s of abrasion loss per 200

deduced. For all substrate

es closer to unity (more

oatings, attributable to a

new AR coating.

ation linearity (R²) during

ced by each substrate is

ear difference between the

substrates and the new

coating’s degradation rate

film. Remarkably, for the

ing suffered the highest

radation was minimal.

e greatest pre-test optical

However, this high initial

e degradation rate because

a highly porous coating

nhomogeneity.

nsmission values (top) andto the abrasion test (bottom).



The red line is the test fail criteria (-0.5%) showincoatings failed the test

Another interesting observation is the app

been the types of films (traditional and ne

type substrate (Fig. 3). The degradation rate

films increases from substrate A to C while t

the new film decreases, as demonstrated by t

3.TABLE I

ABRASION TEST R ESULTS

Substrate A Substrate B

Trad. New Trad. New

Initial 2.26 2.29 1.75 2.07

Post 1.15 1.89 0.29 1.76

Delta -1.11 -0.40 -1.46 -0.31

Result Fail Pass Fail Pass

R 2 0.75 0.87 0.93 0.99

According to industry specifications, a

optical performance of more than 0.5% is conTable I shows the initial and post-abrasio

experimental samples. As a result, all tra

apparently fail the industry test specification

passes independently of the glass substrate typ

A.2. Film thickness and refractive index

The uniformity of film thickness and refra

measured at nine locations on each glass samp

samples, with new and traditional coatings w

optical characterization using an ellipsometer.

Similar to the reflection results, the fil

refractive index values shows an increased st

sample when comparing the new againstcoating. The film thickness of the new coatin

stable value around 100 nm regardless of th

type used. For each sample the uniformity is

across the three types of glass. In case o

coating the film thickness of the different s

large, with median values from 87 nm to ab

new coating shows a comparably wide ra

substrate, however this is attributed to t

method used which inherently leaves a thin t

the direction of flow, and was used for this st

that all traditional

arent correlation

) with the glass

of the traditional

e degradation of

e red line in Fig.

Substrate C

Trad. New

3.35 1.85

1.29 1.77

-2.06 -0.08

Fail Pass

0.70 0.74

degradation of

sidered a failure.values for all

itional coatings

while new film

e.

ctive index were

le. Both types of

ere subjected for

thickness and

bility within the

the traditional, Fig. 4, shows a

e glass substrate

also very similar

f the traditional

bstrates is quite

ut 128 nm. The

ge within each

e flow coating

o thick profile in

dy only.

Fig. 4. Box plot of coating thickneover nine points for each sample

TABLE

FILM THICKNESS

Substrate A Subs

Trad. New Trad.

Mean 102.2 102.5 128.3

Median 94.1 102.9 128.5

St. Dev 29.0 18.6 7.4

Min 71.8 72.0 112.5

Max 167.6 127.4 135.5

The refractive index, shown in

consistency than the thickness. In t

refractive index of the new coati

manner almost the same value, 1.

used. The absolute numbers for th

from 1.36 to 1.43 presumably d

structure. The new coating alsdistribution within each substra

substrates. The traditional coatin

among the glass samples when the

compared and substrates A an

distribution within the substrate.

ss (nm) uniformity measured

II

NIFORMITY

rate B Substrate C

New Trad. New

100.8 91.7 98.9

101.8 86.6 98.7

17.0 12.1 15.1

71.4 78.3 76.6

120.0 117.9 120.3

Fig. 4, reveals even more

rms of absolute values, the

ng shows in a consistent

4, regardless the substrate

traditional coatings varies

e to their differing pore

o exhibits a very tightte and across all three

s exhibit a larger spread

three different vendors are

C show a very wide



Fig. 5. Box plot of coating refractive index uniformity measuredover nine points for each sample

TABLE III

FILM R EFRACTIVE I NDEX U NIFORMITY

Substrate A Substrate B Substrate C

Trad. New Trad. New Trad. New

Mean 1.36 1.44 1.43 1.44 1.36 1.44

Median 1.34 1.44 1.43 1.43 1.34 1.44

St. Dev 0.055 0.008 0.009 0.011 0.026 0.013

Min 1.31 1.43 1.42 1.43 1.33 1.43

Max 1.47 1.45 1.46 1.46 1.40 1.47

B. Reliability Test Data

Five modules were tested under damp heat (DH),

temperature cycle (TC) and humidity freeze (HF) cycles at

UL. These modules consisted of three modules with the new

coating, one module with a traditional coating and one

uncoated control module. All five had power losses less than

5%, which is the IEC requirement; however, there werenoticeable differences between the modules. The modules

with the new coating appeared defect free after cleaning with

water, as opposed to the traditional and control modules, as

shown by the images in Figs. 6 and 7. It can be seen that after

both DH and HF, the modules with the new coating showed

no discoloration, pockmarking, scratches or streaks, as

opposed to both the traditional coated and uncoated modules.

Fig. 8 shows the visual results after temperature cycling.

Normally this test would not be expected to cause damage to

an AR coating given the close CTE match between silica

coatings and glass. However, in this test while both the

uncoated module and the modules with the new coating are

unaffected, the module with the traditional coating showssome visual defects.

These test results demonstrate both how traditional AR

coatings while meeting the electrically specifications in

IEC61215, are marginal for visual defect performance.

Furthermore they show the greatly improved durability of the

new coating and its ability to protect the glass surface.

Traditional New Uncoated

Fig. 6. Images showing visual appearance of modules after 1000hours of damp heat testing according to specification IEC61215. Thenew coated modules show no visual defects, whereas both thetraditional coating and uncoated modules show streaking.


Fig. 7. Images showing visual appearance of modules after 10cycles of humidity freeze testing according to specificationIEC61215. The new coated modules show no visual defects, whereas

both the traditional coating and uncoated module show spotting andstreaking.


Fig. 8. Images showing visual appearance of modules after 200cycles of temperature cycling according to specification IEC61215.The new coated modules and the uncoated module show no visualdefects, whereas the traditional coated module show spotting and

streaking.

The degradation induced by a test such as DH to the solar

glass typically affects coated glass more than uncoated glass.

The streaking and haziness seen in the glass create aesthetic

defects that do not necessarily directly affect the measured ISC

of the module. The percentage change of ISC and PMAX of the

modules after the DH test are displayed in Fig. 9. As can be

seen in Fig. 9, the ISC of the module with the traditional

coating (which was similar to that of substrate C above)

started higher, but dropped by 0.75% after the DH test. The

new coating as well as the uncoated glass had no reduction in

ISC (within measurement noise). Similarly, the PMAX of the

modules, which is a test of the uniformity of the defects on the

string of cells, shows a much larger decrease for the module

with the traditional coating compared to the new coating or the

uncoated module. More specifically, the module with the new

coating displayed on average a lower loss than even the

control glass, signifying an added protective capability of this

coating against moisture.



Fig. 9. Percentage change of module ISC andhours of DH testing. The module with the traditi

both the greatest reduction in ISC and PMAX herethe new coating shows the lowest reduction and

protective effect compared to the uncoated module.

C. Greater kWh gains vs. traditional coatingsThere is little industry consensus on

equivalent performance correlates to the

performed in this work (or indeed to any la

test). Therefore, we can only model field p

relative basis in undetermined time units. T

coating may last 4 times longer than the ot

know if that is 4 months vs 1 month or 20 ye

In order to quantify the kWh gain prov

coatings, we performed an energy predictio

10MW system. In the simulation we assum

in Tariff (FIT) value of $443/MWh, and

1.84% from AR coating versus no coati

module degradation considered in the calcul

The net present value (NPV) for the traditi

versus non-coating was found to be $20,

calculated value for new coated system ve

was $186,623 which translates to an approxi

relative gain.

We conclude that the incorporation of new

into a 10MW system brings significant ben

with the size of the project.

C.1. Predicted lifetime gains

The traditional coated modules show a sli

starting value. However, considering the factdurable with rapid wear, the predicted AR

just in five time units.

The new AR coated modules has a good

with a higher durability over time and the we

which provides a better power prediction. In

gain reaches 0% in 23 time units which tra

times longer life than traditional coatings.

PMAX after 1000nal coating showss the module withappears to have a

.

how the field

abrasion testing

b based abrasion

erformance on a

hat is to say one

er, but we don’t

ars vs 5 years.

ided by the AR

simulation of a

d an initial Feed

system gain of

ng. The annual

tions was 0.5%.

nal ARC system

676. The same

sus non-coating

ately eight times

film technology

fits which scale

ghtly higher AR

that they are lessgain reaches 0%

R starting value

ar is more linear

his case, the AR

slates to a four

Fig. 10. Optical performance decline

IV. SUMMARY AND

Three traditional AR coatings f

tested for durability against a ne

Technology. The new coating is s

more durable than the traditional c

modeled in a representative ene

which shows an increase in durab

increase in the net present value

coatings are the layer of the solar

subjugated to the stresses of t

coatings with high durability ar

deployed in the field with predi

decades.

R EFERENC

[1] International Technology Roadm2013 Results.[2] C. Ballif, J. Dicker, D. Borchert,

industrial porous SiO2 antireflect photovoltaic module properties iyearly energy yield gain” Solar Evol. 82, pp. 331-344, 2004.

[3] R. Prado, G. Beobide, A. Marcaid“Development of multifunctioreflection coatings with enhanced

Energy Materials & Solar Cells, v[4] K. Midtdal, B. P. Jelle, “Self-clea

of-the-art review and future rese Materials & Solar Cells, vol. 109,

[5] E. Klimm, T. Lorenz, K.-A. Weiss,solar glass influence the degree ofPV modules drastically?” in the 2Conference, 2013.

[6] http://rredc.nrel.gov/solar/spectra/

f ARC as a function of time.

ONCLUSION

or PV module glass were

w AR coating from Enki

hown to be up to 4 times

atings. These results were

gy prediction simulation,

ility resulting in an 8 fold

of the coating. Since AR

panel that is most heavily

he external environment,

crucial for PV systems

cted lifetimes of multiple

ES

p for Photovoltaics (ITRPV),

. Hofmann, “Solar glass withon coating: measurements of provement and modelling ofergy Materials & Solar Cells,

, J. Goikoetxea, A. Aranzabe,al sol–gel coatings: Anti- self-cleaning capacity’, Solarol. 94, pp. 1084-1088, 2010.ing glazing products: A state-rch pathways”, Solar Energy

pp. 126–141, 2013.“Can anti-soiling coating on

performance loss over time ofth European PV Solar Energy

m1.5/

ieee pvsc-40 highly abrasive resistant ar coating

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