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Contract No. DE-AC36-99-GO10337 

F.J. Pern and K. Touryan 

National Renewable Energy Laboratory

Zh. Panosyan 

The State Engineering University of Armenia 

A.A. Gippius 

P.N. Lebedev Physical Institute of the Russian Academy of Science 

J.A. Kontsevoy 

Pulsar Institute 

Prepared for the 31st IEEE Photovoltaic Specialists 

Conference and Exhibition 

Lake Buena Vista, Florida 

January 3–7, 2005 

February 2005 • NREL/CP-520-37374 

Diamond-Like Carbon Coatings 

as Encapsulants for 

Photovoltaic Solar Cells
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DIAMOND-LIKE CARBON COATINGS AS ENCAPSULANTS 

FOR PHOTOVOLTAIC SOLAR CELLS 

F.J. Pern,1 Zh. Panosyan,2 A.A. Gippius,3 J.A. Kontsevoy,

4

 K. Touryan,1 S. Voskanyan,2 and Y. Yengibaryan2

1

 National Renewable Energy Laboratory (NREL), 1617 Cole Blvd., Golden, CO 80401 2

 Department of Physics, the State Engineering University of Armenia, Yereven, Armenia 3

 P. N. Lebedev Physical Institute of the Russian Academy of Sciences, Moscow, Russia 4

 "Pulsar" Institute, Moscow, Russia 

 

ABSTRACT

High-quality single layer and bilayer diamond-like

carbon (DLC) thin films are fabricated by two technologies,

namely, ion-assisted plasma-enhanced deposition (IAPED) 

and electron cyclotron resonance (ECR) deposition.

Deposition on various substrates such as sapphires and 

solar cells has been performed at low substrate

temperatures (50o

~80oC). The two deposition technologies 

allow good control over the growth conditions to produce 

DLC films with desired optical properties, thickness, and 

energy bandgap. The bilayer structured DLC can be

fabricated by using IAPED for the bottom layer followed by

ECR for the top layer, or just by IAPED for both layers with 

different compositions. The DLC films have shown good

spatial uniformity, density, microhardness, and adhesion

strength. They exhibit excellent stability against attack by

strong acids, prolonged damp-heat exposure at 85o

C and 

85% relative humidity, mechanical scratch, ultrasonication,

and irradiation by ultraviolet (UV), protons, and electrons. 

When deposited on crystalline Si and GaAs solar cells in 

single layer and/or bilayer structure, the DLC films not only 

serve as antireflection coating and protective encapsulant,

but also improve the cell efficiencies. 

INTRODUCTION 

Diamond-like carbon (DLC) coatings have received

increasing attention in the past decade for their unique

physical and electrical properties. They may have great

potential in various applications, as reviewed

independently by Lifshitz [1] and Grill [2] in 1999. For

example, Park and Chin employed DLC to protect 

polycarbonate sheets from radiation-induced degradation

[3]. Litovchenko and Klyui demonstrated that crystalline-Si

(c-Si) solar cells deposited with N-doped DLC films by the

r.f. glow-discharge method are useful for space 

applications [4]. Applebaum et al. investigated the

electron-damaging effects on DLC-coated c-Si solar cells

[5]. For DLC coatings to be practically useful for space or

terrestrial solar cells, two issues have to be resolved. The 

first issue is the potential reflection loss resulting from the

large refractive index of the DLC coatings, and the second

issue is the need to resist the irradiation encountered in

space. These issues are addressed in this project by using 

a bilayer structure of DLC coatings. In this design, the

refractive index, energy bandgap, and thickness of the two

DLC layers are the key factors that require good control in

film growth. The first (bottom) DLC layer, adjacent to the 

p-n junction of a solar cell, should meet the conditions of

antireflection and be thin enough that the requirements for

the energy bandgap can be relaxed. This, in turn, can 

minimize the transmission loss. The second (top) DLC

layer should have a lower refractive index to minimize 

reflection losses at the surface of the structure, a large

energy bandgap to minimize absorption loss, and a

thickness large enough to provide adequate radiationstopping power. This project has focused on the

developments of two DLC deposition technologies for

single layer and bilayer DLC films with controllable energy

bandgap and refractive index. The effect of deposition 

conditions, different gas mixtures, physical properties, and

the ability of the DLC films to withstand weathering

including UV, electron, and proton irradiations, are studied.

The developed deposition technologies and related

parameter controls have been successfully employed to

produce high-quality DLC films serving as both

antireflectance coatings and protective encapsulants for cSi and GaAs solar cells. These coatings also enhance cell

efficiencies. 

EXPERIMENTAL

IAPED is the main technique developed in this

project for the growth of DLC films [6-8], The Kashtan

IAPED system, manufactured by the S. A. Vekshinsky 

Institute of Vacuum Technique, uses a reaction chamber

that is modified with a radical ion source to initiate 

decomposition of hydrocarbons. The ion source with a

cold cathode provides ionization of the gas by means of

electron impact. The feed-gas mixture (e.g., C7H8, Ar, N2) 

consisted of 55% Ar, with the remaining 45% divided 

between C7H8 and N2. The DLC films were grown typically 

with ion energy between 20 and 140 eV, because ion

energy higher than 140 eV would degrade solar cell

properties and ion energy lower than 20 eV would produce

films with refractive index too high to be useful for

antireflection coatings. Plasma current was between 0.20

1 
and 0.80 mA/cm2

 to ensure proper deposition efficiency

and reduce defects in the DLC films. A specially designed 

substrate holder with a complex rotating capability allowed

film nonuniformity of less than 5% within an area ≥ 110

cm2

 at relatively low deposition temperatures (50o~80oC).

The other technique developed for DLC film growth is

ECR deposition using a custom-built system. In this 

technique, the strength of the applied magnetic field is 

selected so that the resulting frequency of electron

gyration is equal to that of the microwave frequency.

When the two frequencies are matched, the plasma 

density is dramatically increased by the enhanced

absorption of microwave energy by the plasma. Single

layer DLC films from either method and bilayer films from 

subsequent depositions first by IAPED and then by ECR

were prepared. The DLC films were characterized for their

transmission, reflectance, Raman, adhesion strength, 

morphology, and resistance to strong acids, damp-heat

exposure, and irradiations of UV, electrons, and protons. 

Fig. 1. Cross-section of PV cell with contact grid and 

bilayer DLC film encapsulant.

0

10

20

30

40

50

60

70

80

90

100

250 350 450 550 650 750 850 950 1050 1150 1250 1350 1450

Wavelength (nm)

Reflectance (

%)

1

2

3 4

6 5

1. Polished Si wafer 

2. DCL_CD-1 (0.12 μm) 

3. DLC_CF-3 (0.20 μm)

4. DLC_CE-2 (0.24 μm) 

5. DLC_Red#1 (0.54 μm) 

6. DLC_Blue#1 (0.84 μm)

RESULTS AND DISCUSSION 

As stated in the Introduction, two issues have to be

resolved for the DLC films to be practically useful for

space or terrestrial solar cells: (1) the potential reflection

loss resulting from the large refractive index of the DLC

coating and (2) the resistance to the irradiation in space. 

These issues are addressed in this project by using a

bilayer structure of DLC coatings as shown in Fig. 1. The 

first layer (adjacent to the p-n junction of the PV cell)

should possess the properties that meet the conditions of

antireflection, that is 

Fig. 2. Reflectance spectra of single layer DLC films of 

various thickness deposited by IAPED on crystalline-Si 

wafers.

nDLC nDLC nSi = ⋅ (1) (2) 

and 

d n DLC( ) DLC( ) / 1 1 = λ 4 ,

where λ is the wavelength corresponding to maximal 

antireflection, and nDLC(1) and NDLC(2) are the refractive

indexes of the two DLC layers.

Accordingly, the bottom layer (DLC1) should possess

a high refractive index and a thickness of 60~80 nm. The 

minimal thickness of this layer makes the requirements for

the energy bandgap less severe than those for the second

layer. The top layer (DLC2) should possess a low

refractive index to minimize reflection losses at the surface 

of the structure with a large energy bandgap to minimize

absorption losses and a thickness high enough to provide

adequate resistance to radiation. The bilayer-structured

DLC can be fabricated by using IAPED for the bottom (first)

layer followed by ECR for the top (second) layer, or just by

IAPED with different compositions for both layers. 

Optically, the DLC film’s thickness affected its

transmittance (not shown) and reflectance spectra. The

latter is illustrated in Fig. 2 for some DLC films of various 

thicknesses deposited on Si substrates by IAPED. As 

seen, optimal reduction in reflectance loss can be

obtained with adequate film thickness. Table 1 illustrates

the effects of deposition conditions, concentration of C7H8,

Table 1. Refractive indexes (n) and thickness (d) of DLC films grown by IAPED using C7H8 deposited under various 

conditions of anode-cathode voltage, Uac, current density, bias voltage, Ub, and average ion kinetic energy, <Ek>. 

No. of Si Uac Iac Jp Ub Pressure C7H8 <Ek> Thickness R. I. 

 sample (kV) (mA) (mA/cm2

) (V) (pascal) (%) (eV) d (nm) n

 85 2.5 30 0.20 -300 4x10-3 35 90 240 1.48 

 78 2.6 35 0.25 -350 6x10-3 28 100 250 2.00 

 79 2.8 40 0.30 -400 8x10-3 24 140 290 2.10 

 86 2.2 80 0.60 -250 9x10-3 18 60 185 2.40 

 95 2.3 100 0.65 -300 1x10-2 12 65 196 2.45 

 93 2.4 120 0.80 -350 3x10-2 15 70 200 2.35 

99 1.5 45 0.35 -20 2x10-2 10 20 194 2.55 

98 1.8 50 0.40 -50 5x10-2 8 25 192 2.60 

91 2.0 60 0.45 -100 7x10-2 4 35 190 2.57 

2 
and film thickness on the refractive index, n, of DLC films

deposited by IAPED on monocrystalline Si wafers. These 

results clearly demonstrate that, by controlling the various

parameters, either single layer or bilayer DLC films of 

continuously varying compositions can be conveniently

grown by the IAPED method. On the other hand, using 

different reactant species, from ketones to alcohols, DLC

films of different energy bandgaps, 2.4 eV < Ebg < 3.0 eV, 

and refractive indexes, 1.54 < n < 2.05, were obtained by

the ECR method, as shown in Table 2. 

Results of Raman shift analysis for these films in Fig.

3 show a broad emission peak centered at ~1535 cm-1

compared to the sharp peak at 1332 cm-1 for sp3

 of a pure

diamond, confirming that the DLC films are primarily 

amorphous, but with a predominant characteristic of sp3. 

An increase in the cell efficiency on c-Si and GaAs solar

cells was obtained when they were coated with DLC films.

The current-voltage and power-voltage curves for a c-Si

cell before and after a single layer DLC film was deposited

is shown in Fig. 4. If the surface of c-Si cell was first

cleaned with ions, a higher increase of cell efficiency was

obtained with DLC coating (figure not shown). The

increase in the cell efficiency was attributed primarily to

the effect of anti-reflection of the DLC coating, although 

contribution by reduced surface boundary recombination

loss was also possible. For the GaAs cell sample, the 

efficiency improvements are shown in Table 3. In this case, 

the first layer DLC coating was applied by IAPED on the 

GaAs cell whose original ZnS AR coating was removed,

resulting in a cell increase from 9.26% to 11.0%. With

second DLC layer deposited by ECR, the cell efficiency

increased further to 12.5%, higher than the 12.2% with

ZnS AR coating. This demonstrates the feasibility of

depositing bilayer-structured DLC coating by the two 

deposition technologies. The bilayer-structured DLC

coating was also fabricated by using IAPED method with

continuous composition gradient. The DLC coatings have

also been applied successfully to thin-film amorphous Si

cells and mini-modules. These results will be presented

elsewhere. 

Table 2. Energy gap and refraction index of DLC films 

grown by ECR technique with various reactant species 

Reactant Energy Gap Refractive 

Species Ebg (eV) Index, n

CH3COCH3 2.85 1.6 

C2H5OC2H5 2.65 1.65 

CH3COC2H5 2.4 1.75 

C2H5OH 2.9 1.54 

C4H9OH 2.5 2.05 

CH3OH 3.0 1.54 

C3H7OH 3.0 1.6

Table 3. Cell parameters for a GaAs solar cell without

and with DLC coating (DLC1 layer by IAPED, 60 nm

thick; DLC2 layer by ECD, 900 nm thick) 

Solar Cell Isc Voc Imax Vmax Eff. 

+ Coating (mA) (mV) (mA) (mV) (%)

Original Cell 82 960 76 760 12.2 

with ZnS AR 

No AR 62 940 56 760 9.26 

(ZnS removed)

DLC1 74 960 68 760 11.0 

DLC1+DLC2 84 960 80 740 12.5

0

500

1000

1500

2000

2500

3000

3500

200 400 600 800 1000 1200 1400 1600 1800

Raman Shift (cm-1)

Raman Intensity (cps)

Si

Diamond

DLC on Si

1

2

3 4

5

6 1. Diamond

2. Polished Si wafer 

3. DCL_CD-1 (0.12 μm) 

4. DLC_CE-2 (0.24 μm) 

5. DLC_Blue#1 (0.84 μm)

6. DLC_Red#1 (0.54 μm) 

Fig. 3. Raman spectra comparing the DLC deposited

on Si wafers as in Fig. 1 with a blank Si wafer and a

diamond reference.

0

400

800

1200

1600

2000

0 100 200 300 400 500 600

Voltage, V (mV)

Current, I (mA)

0

150

300

450

600

750

900

Po

wer (m

W)

1

2

3

4

1. I without DLC

2. I with DLC 

3. P without DLC 

4. P with DLC

The DLC films were very resistant to the attacks of 

high humidity at high temperature and mineral acids such

as HCl, HNO3, and H2SO4, as demonstrated in Table 4 for 

Si cells with DLC coatings. In comparison, ZnS is easily 

attacked by mineral acids. The DLC films on Si wafers 

showed virtually no change in reflectance after 762h of 

damp-heat exposure at 85o

C and 85% relative humidity

(RH). The DLC films deposited on Si wafer or cell were

very resistant to mechanical scratch and ultrasonication

tests, no scratch-off or peel-off was observed. They also

exhibited excellent stabilities against UV, proton, and

electron irradiations, as illustrated in Figs. 5 and 6, 

showing very little change in the spectral efficiency of Si

cells when protected by 1.5-μm DLC films (Fig. 5) and 

small change in the transmittance spectrum of 0.51-μm 

DLC films on sapphire (Fig. 6) before and after irradiation

by protons and electrons, respectively. 

Fig. 4. Current-voltage and power-voltage curves for a

~100-cm2

 c-Si solar cell without (1, 3) and with (2, 4) a 

DLC coating by IAPED. 

3 
CONCLUSIONS 

High-quality DLC films were obtained using ionassisted plasma-enhanced and electron cyclotron

resonance deposition technologies. The two technologies

allowed DLC film fabrications at low temperatures with

good spatial uniformity. Single and bilayer DLC films were

obtained with good control of the deposition parameters

and feed-gas mixture for desired transmittance,

reflectance, thickness, and refractive index that can be

fine-tuned in the range of 1.5~2.6, and an energy bandgap

of up to 4.0 eV. The DLC films, when deposited on

crystalline Si wafers, have shown excellent stability 

against mineral acids and prolonged damp heat exposure,

high adhesion strength, and excellent stability against

irradiations. Serving as both an antireflection coating and

a protective encapsulant, the DLC films enhanced the

efficiency of c-Si and GaAs solar cells. 

ACKNOWLEDGEMENTS 

The authors thank Drs. M.-J. Seong and S.-J. Yoon

at NREL for the Raman characterization. This work was 

supported by the U.S. Department of Energy under the 

Initiatives for Proliferation Prevention program (IPP) 

Contract No. AAT-1-31902-01.

REFERENCES 

[1] Y. Lifshitz, "Diamond-Like Carbon–Present Status," 

Diamond and Related Materials, 8, 1999, pp. 1659-1676. 

[2] A. Grill, "Diamond-Like Carbon: State of the Art," ibid.,

8, 1999, pp. 428-434. 

[3] K.J. Park and E.Y. Chin, "Effect of Diamond-Like

Carbon Thin Film Deposition on the Resistance of

Polycarbonate to Radiation-Induced Degradation,"

Polymer Degradation and Stability, 68, 2000, pp. 93-96. 

[4] V.G. Litovchenko and N.I. Klyui, "Solar Cells Based 

on DLC Film – Si Structures for Space Application," Solar

Energy Materials and Solar Cells, 68, 2001, pp. 55-70. 

[5] J. Applebaum, N. Croitoru, L. Klibanov, and D. 

Scheinman, "The Effect of Electron Damage on Silicon

Solar Cells Coated with Diamond-Like Carbon Films,"

Prog. Photovolt: Res. Appl., 8, 2000, pp. 571-578. 

[6] K. Kassabian, Zh. Panosyan, A. Stepanyan, G. 

Torosyan, and Y. Yengibaryan, "Low-Temperature

Technology of Obtaining the Protecting Layer From

Diamond-Like Film," Proceeding of the 6th Applied 

Diamond Conference, Auburn, Alabama, USA, 2001, pp.

431-435. 

[7] Y. Yengibaryan, S. Voskanyan, A. Voskanyan, A. 

Stepanyan, and Zh. Panosyan, "Growth of Diamond-Like

Carbon Films on the Si Surface of Solar Cell." Proceeding

of the 3rd National Conference of "Semiconductor

Microelectronics," Sevan, 2001, pp. 267-271.

[8] Y.A. Kontsevoy, P.B. Constantinov, and S.L. 

Shevchuk, "Studies of DLC films Grown with Ion Beam

Technique." Russian Conference on "Micro- and 

Nanoelectronics – 2001," Zvenigorod, 2001, p. 44.

Fig. 5. Spectral efficiency of three Si PV cells without

and with 1.5-μm DLC coating exposed to two levels of

proton irradiation, 1014/cm2

 20 keV and 1013/cm2 150

keV.

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

450 550 650 750 850 950 1050

Wavelength (nm)

A

mpere per Watt

1

2

4

5

1. No DLC with H+

2. DLC1 without H+

3. DLC2 without H+

4. DLC1 with H+

5. DLC2 with H+

H+ Irradiations:

20 keV 1014/cm2 

50 keV 1013/cm2

3

0

10

20

30

40

50

60

70

80

90

0

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8

Photon Energy (eV)

Transmittance (

%)

DLC1_Before

DLC2_Before

DLC1_After

DLC2_After

Electron Irradiation:

1 MeV 9x1014/cm2

Fig. 6. Transmission spectra of two 0.51-μm DLC

films on sapphire substrates before and after

irradiation by 9×1014/cm2

 1-MeV electrons. 

10

Table 4. Efficiencies of c-Si PV cells with DLC films 

grown at various conditions before and after 

weathering at 80o

-90°C and 90% RH for 20 h (W.T.) 

and chemical stability tests* (C.T.) 

Cell <Ek> C7H8 d Cell Efficiency (%) 

ID (eV) (%) (nm) Initial After After 

 W.T. C.T. 

30 55 22 80 9.87 9.85 9.78 

41 70 16 80 9.71 9.75 9.58 

42 65 13 75 8.90 8.93 8.78 

43 60 12 85 9.27 9.23 9.28 

44 75 10 85 9.11 9.18 9.14 

45 65 14 80 8.82 8.90 8.80

* Chemical stability tests included sequentially: 

1. concentrated HNO3 acid, 30 min, 25°C 

2. diluted (1%) HNO3 acid, 1 h, 25°C 

3. concentrated H2SO4 acid, 30 min, 25°C 

4. diluted (1%) H2SO4 acid, 1 h, 25°C 

5. saturated solution of NaCl (sea fog simulation), 40 

h, 25o

-30°C 

4 
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4. TITLE AND SUBTITLE

Diamond-Like Carbon Coatings as Encapsulants for Photovoltaic 

Solar Cells 

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NREL/CP-520-37374 

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DO890210 

6. AUTHOR(S) 

F.J. Pern, Zh. Panosyan, A.A. Gippius, J.A. Kontsevoy, K. Touryan, 

S. Voskanyan, and Y. Yengibaryan 

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

 National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 

Department of Physics, the State Engineering University of Armenia, 

Yereven, Armenia 

P. N. Lebedev Physical Institute of the Russian Academy of Sciences, 

Moscow, Russia 

 Pulsar Institute, Moscow, Russia 

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14. ABSTRACT (Maximum 200 Words)

High-quality single-layer and bilayer diamond-like carbon (DLC) thin films are fabricated by two technologies, namely, 

ion-assisted plasma-enhanced deposition (IAPED) and electron cyclotron resonance (ECR) deposition. Deposition on 

various substrates, such as sapphires and solar cells, has been performed at low substrate temperatures (50o

~80oC). 

The two deposition technologies allow good control over the growth conditions to produce DLC films with desired optical 

properties, thickness, and energy bandgap. The bilayer-structured DLC can be fabricated by using IAPED for the bottom 

layer followed by ECR for the top layer, or just by IAPED for both layers with different compositions. The DLC films have 

shown good spatial uniformity, density, microhardness, and adhesion strength. They exhibit excellent stability against 

attack by strong acids, prolonged damp-heat exposure at 85o

C and 85% relative humidity, mechanical scratch, 

ultrasonication, and irradiation by ultraviolet (UV), protons, and electrons. When deposited on crystalline Si and GaAs 

solar cells in single-layer and/or bilayer structure, the DLC films not only serve as antireflection coating and protective 

encapsulant, but also improve the cell efficiencies. 

15. SUBJECT TERMS

PV; diamond-like carbon (DLC); thin films; ion-assisted plasma-enhanced deposition (IAPED); electron cyclotron 

resonance (ECR); ultraviolet (UV); antireflection coating; protective encapsulant; 

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