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Solar Energy 86 (2012) 220–230

Oxygen modified diamond-like carbon as window layerfor amorphous silicon solar cells

Neeraj Dwivedi a,b, Sushil Kumar a,⇑, Sukhbir Singh a, Hitendra K. Malik b

a Physics of Energy Harvesting Division, National Physical Laboratory (CSIR), K.S. Krishnan Road, New Delhi 110 012, Indiab Department of Physics, Indian Institute of Technology Delhi, New Delhi 110 016, India

Received 11 August 2011; received in revised form 20 September 2011; accepted 22 September 2011Available online 19 October 2011

Communicated by: Associate Editor Nicola Romeo

Abstract

In the present study, the effect of in situ layer-by-layer oxygen plasma treatment (OPT) on optical, nano-mechanical and electricalproperties of layer-by-layer diamond-like carbon (DLC) thin films was explored. In situ layer-by-layer OPT on layer-by-layer DLC filmsled to drastic variation of optical band gap from 1.25 eV to 2.6 eV and hardness from 16.1 GPa to 25.3 GPa. Wide band gap and theband gap feasibility over wide range may lead to its realization as p-type window layer in p–i–n solar cells and variable band gap layersin tandem solar cells. Simulations of a-Si:H based p–i–n solar cells was also carried out by considering OPT–DLC films as p-type windowlayers that yielded maximum efficiency of 8.9%. In addition, due to high hardness and other excellent nano-mechanical properties, theseOPT–DLC films can be treated as hard, protective and encapsulate layers on solar cells particularly in n–i–p configuration. It is impor-tant to mention that OPT–DLC film as p-layer can minimize the use of additional hard, protective and encapsulate layer.� 2011 Elsevier Ltd. All rights reserved.

Keywords: DLC; a-Si:H; Optoelectronic property; p-Layer; Hardness; Simulation

1. Introduction

In the recent past, owing to increasing global demand ofenergy and limitation in the availability of fossil fuels, thenon-conventional energy has become a main subject ofresearch across the world. Because of cleanness, pollutionfree and easy availability, solar energy has been found amost appropriate non-conventional energy source. It iswell known that hydrogenated amorphous carbon(a-C:H) or diamond-like carbon (DLC) exhibits uniquemechanical and tribological properties together with excel-lent biocompatibility that results in its wide spread applica-tions such as hard and protective coatings on cutting tools,automobile part, wear resistance coating on magnetic hard

0038-092X/$ - see front matter � 2011 Elsevier Ltd. All rights reserved.

doi:10.1016/j.solener.2011.09.025

⇑ Corresponding author. Tel.: +91 11 45608650; fax: +91 11 45609310.E-mail address: skumar@nplindia.org (S. Kumar).

disk and orthopedic implants (Dwivedi et al., 2011a,d;Dearnaley and Arps, 2005; Erdemir and Donnet, 2006;Robertson, 2002). DLC also exhibits outstanding electricaland optical properties (Dwivedi et al., 2011b; Kumar et al.,2010; Schwan et al., 1998) that may lead to its realizationfor optoelectronic devices such as solar cell. Recently,DLC based low emissive coating was also proposed (Mah-tani et al., 2011). Further, reports pertained to DLC basedsolar cells can be found in literature (Krishna et al., 2000;Zhu et al., 2009a,b). Still there is great scope to explorethe photovoltaic application of DLC. Among variousparameters, band gap is very important parameter thatdirectly influences the performance of solar cells. The solarspectrum covers energy range of 1–3 eV and when itexposed to p–i–n solar cells, photons with energy equaland above the band gap are absorbed within i-layer andcontribute to the efficiency. Maximum absorption andtherefore, great contribution takes place when energy of

N. Dwivedi et al. / Solar Energy 86 (2012) 220–230 221

photon matches with the band gap. Photons with energyabove the band gap although get absorbed in an activelayer of cell but they create losses due to heat generation,which is referred to as absorption losses. Just from thebeginning, crystalline silicon (band gap �1.1 eV) basedsolar cells have attracted technological and industrialattention due to their high efficiency. Owing to higher costand fixed small band gap that causes higher absorptionlosses, researchers have searched alternative solar energymaterials. Hydrogenated amorphous silicon (a-Si:H) basedp–i–n solar cells have also been studied extensively due totheir low cost, large scale deposition, low temperatureprocessing and good efficiency. Unlike crystalline silicon,a-Si:H is comparatively broader band gap material(�1.7 eV) in which a-Si:H p-layer minimizes the absorptionlosses due to the fact that photons below 1.7 eV can betransmitted to i layer. Still the band gap of a-Si:H is lowand it leads to sufficient absorption losses, while employingit as a p-layer. Moreover, a-Si:H based solar cells with p, iand n layers of a-Si:H show light induced degradation thatis referred to as Stabler–Wronski effect. The band gap of a-Si:H window layer can be tuned by carbon doping and a-SiC:H alloy with band gap �2 eV to 2.1 eV is formed thathas been employed as an excellent p-type window layer ina-Si:H based p–i–n solar cells (Kabir et al., 2010). Theuse of such a p-layer may minimize the Stabler–Wronskieffect that is generally observed in a-Si:H p–i–n solar cells.Nevertheless, a-SiC:H as p-layer also has certain problemssuch as band offset, proper stoichiometry between Si and Cand substitutional doping of C into a-Si:H during growth.Due to these difficulties, various groups introduced micro-crystalline a-Si:H (lc-Si:H) as p-type window layer toimprove the performance of a-Si:H based p–i–n solar cells.They performed experimental and theoretical investiga-tions and suggested that lc-Si:H is quite better p-type win-dow layer than a-SiC:H (Palit and Chatterjee, 1999; Rathand Schropp, 1998). Still there are several problems withlc-Si:H. In order to grow good quality lc-Si:H as p-layeron transparent conducting oxide (TCO) coated glass, thehydrogen plasma intensity should be very high otherwisethe quality of the grown lc-Si:H shall be poor and degradethe performance of the cell. On the other hand, if oneenhances the hydrogen plasma intensity then it may dam-age the TCO coating and detroit its properties. In addition,lc-Si:H as p-layer also introduce band offset. Other impor-tant wide band gap semiconducting material is zinc oxide(ZnO). However, it is quite soft and has fixed band gapof �3.3 eV, which is not desirable as window layer fora-Si:H based solar cells.

Since its band gap can be tuned over wide range (�1 eVto 4 eV), DLC can be a potential candidate as a p-layer fora-Si:H based p–i–n solar cells. The unmodified amorphouscarbon is weakly p-type in nature (Rusup et al., 2006).Recently, a-Si:H solar cell with a-C:H as p-layer was prac-tically demonstrated (Khan et al., 2003) and its spectralresponse in the range 400–500 nm was found to be betterthan a-Si:H solar cell with a-SiC:H as p-layer but at the

expense of efficiency. Some other groups also reportedamorphous carbon and doped amorphous diamond as ap-layer for a-Si:H solar cells (Lee and Lim, 1998; Tianet al., 2003; Zhu et al., 2009a,b). In addition, the role ofvariable band gap a-C:H multilayer for photovoltaic appli-cation has also been discussed (Tinchev et al., 2005). All ofthem tuned the band gap of a-C:H either by playing theprocess parameters or by doping of foreign elements. Thus,band gap tailoring properties of DLC may also lead to itsapplication in the development of multijunction, tandem,and graded band gap solar cells. In the present study, wehave modified the band gap of films by novel oxygenplasma treatment (OPT) and obtained variable band gapDLC layers. These variable band gap OPT–DLC layerscan be used as p-layers on a-Si:H based p–i–n solar cells.In addition, OPT–DLC films can also be employed in thedevelopment of tandem solar cells. In addition, we havealso performed the simulation studies for p–i–n solar cells,where these OPT–DLC films are considered as p-windowlayer.

Another existing problem with a-Si:H based p–i–n solarcells is their degraded efficiency by post-deposition contam-ination and stability under exposure of high energy radia-tion. The efficiency of a-Si:H based p–i–n solar cells gotdrastically decreased, when it was exposed in atmosphere.Since Si is very prone to form oxide, atmospheric contam-ination such as oxygen and dust particles. minimize the effi-ciency of solar cells. On the other hand, stability issue isrelated to space solar cells where solar cells embeddedspace vehicle sees high energy radiation such as Gammarays and high energy solar winds. Since a-Si:H is a softmaterial, the performance of a-Si:H based solar cells getsdegraded under this situation. However, an additionalencapsulate, hard and protective coating on solar cellscan avoid the contamination and stability problems, buthardness as well as transmission of additional coating mustbe high. ZnO and SiO2 are found to be transparent coat-ings but they are quite soft. Due to high hardness and goodtransmission, DLC films can be better encapsulate, hardand protective layer of solar cells mainly in n–i–p configu-ration. The extensive study of the stability of DLC underthe exposure of high energy radiation and its applicationfor space solar cell can also be found in literature(Litovchenko and Klyui, 2001). In view of this, we havealso studied the hardness and transmission of OPT–DLCfilms and found that it can act as an excellent encapsulate,hard and protective coating on solar cells mainly in n–i–pconfiguration where metals are used as substrate.

2. Experimental details

2.1. Deposition and oxygen plasma treatment of DLC films

Layer-by-layer DLC films, followed by in situ layer-by-layer OPT, were grown on n-type Si h1 00i wafers andcorning 7095 glasses at base pressure of 3 � 10�5 Torrusing asymmetric capacitively coupled radio frequency

Fig. 1. Schematic representation of deposition of OPT–DLC films.

222 N. Dwivedi et al. / Solar Energy 86 (2012) 220–230

(13.56 MHz)-plasma enhanced chemical vapor depositiontechnique (RF-PECVD). The schematic representation ofdesigning of layer-by-layer DLC structure along withlayer-by-layer OPT is shown in Fig. 1. Sample S-0 that pos-sesses only DLC layer is a reference sample on which noOPT was performed. However, samples S-1, S-2, S-3 andS-4 possess 1, 2, 3 and 4 layer-by-layer of DLC, respec-tively, followed by OPT on each successive DLC layer.DLC layer was grown at negative self bias of 100 V andworking pressure of 5 � 10�2 Torr that was achieved byfeeding first Ar gas which changed the pressure from3 � 10�5 to 5 � 10�4 Torr and then C2H2 that changedthe pressure from 5 � 10�4 to 5 � 10�2 Torr. The OPTon DLC layers was performed at negative self bias of100 V.

2.2. Characterizations of OPT–DLC films

The thicknesses of these films were measured by Taylor-Hobson Talystep instrument and were found to be�265 nm, �240 nm, �278 nm, �292 nm and �298 nm inS-0, S-1, S-2, S-3 and S-4, respectively. Optical properties

Table 1Process parameters used for the simulation study.

Parameters p-layerOPT–DLC

i-layer-a-Si:H

n-layer-a-Si:H

Relative permittivity (er) 4.0 11.9 11.9Electron mobility (ln, cm2/V s) 0.001 5 5Hole mobility (lp, cm2/V s) 0.0001 1 1Accepter/donor (concentration, cm�3) 1 � 1020 – 1 � 1019

Band gap (eV) 1.25–2.6 1.74 1.78Electron affinity (eV) 2.0 3.9 3.9Conduction band (con. NC, cm�3) 1 � 1020 1 � 1020 1 � 1020

Valence band (con. NV, cm�3) 1 � 1020 1 � 1020 1 � 1020

Thickness (nm) 5–20 400 30Density (g/cm3) 2.4 2.328 2.328

of these films were investigated using Shimadzu UV–Vis1601 spectrometer. Mechanical properties were measuredat indentation load of 5 mN using IBIS nanoindentationM/s Fisher Cripps Laboratory Pvt. Ltd., Australia.

2.3. Simulation details for p–i–n solar cells

In the present study, the AFORS-HET-2.4.1 softwarewas employed to simulate the different p–i–n solar cells.This software is based on one-dimensional equation andsolves various equations such as Poisson’s equation andtransport equation for electrons and holes. The OPT–DLC films with different band gaps were considered as p-layer whereas i and n layers were the a-Si:H. The open cir-cuit voltage (Voc), short circuit current (Isc), fill factor (FF)and efficiency (g) were analyzed as a function of band gapof p-layer (OPT–DLC) at different thicknesses (5–20 nm).The parameters used in the present simulation are givenin Table 1. These parameters are taken from standard ref-erences (Dao et al., 2010; Mahtani et al., 2011; Milne, 2003;Robertson, 1996; Yu et al., 2001; Zhao et al., 2008, 2009).

3. Results and discussion

3.1. OPT–DLC as a p-type window layer

Researchers have considered amorphous carbon as ap-type layer for carbon/silicon heterojunction solar cells.The wide band gap and the band gap feasibility over widerange made it a material of great utility as a p-type windowlayer. However, besides band gap higher optical transpar-ency was also found to be an essential requirement for p-layer to avoid absorption losses. Therefore, we have stud-ied the transmission of various OPT–DLC films in Fig. 2,which clearly reveals that OPT on DLC changes its trans-mission significantly. Transmission in the beginning (UV

N. Dwivedi et al. / Solar Energy 86 (2012) 220–230 223

region) of non-OPT–DLC film (S-0) was about �40% thatdrastically increased to �70% in OPT–DLC (S-1) film.However, transmission in visible region in all the filmswas found to be better. Depending upon the film thicknessthe transmission in near IR region is varied in the range86–96%. Since diamond with sp3 bonding is optically trans-parent whereas graphite with sp2 bonding is opaque,increased transmission with the OPT on DLC was due toetching of sp2 bonding, as oxygen plasma preferentiallyetches the soft graphite-like sp2 clusters. Hence, due to bet-ter optical transparency, OPT–DLC film may be consid-ered as an excellent p-layer for solar cells. It is alsointeresting to note that OPT on DLC results in removalof its brown color and makes it a colorless coating, whichcan also be used as colorless gas barrier, biocompatible andhard coating. However, in the present work, we emphasizeDLC as a potential candidate for photovoltaic application.We have also investigated the optical band gap (Eg) ofOPT–DLC films. The values of optical band gap in thesefilms was estimated using Tau plot (ahm)1/2 versus hm curveby taking the asymptotic/tangent of curve to the x-axis.The variation of band gap for various OPT–DLC samples

200 400 600 800 1000

20

40

60

80

100

Tran

smis

sion

(%)

Wavelength (nm)

S-0 S-1 S-2 S-3 S-4

Fig. 2. Transmission spectra of various OPT–DLC films.

S-0 S-1 S-2 S-3 S-4

1.2

1.5

1.8

2.1

2.4

2.7

Opt

ical

ban

d ga

p (e

V)

Samples

Fig. 3. Variation of optical band gap with different DLC samples.

is shown in Fig. 3, from which it is evident that OPT onDLC leads a drastic variation in its band gap. The unmod-ified DLC film (S-0) exhibits band gap of 2.25 eV thatabruptly decreases to 1.25 eV in S-1. This may be attrib-uted to structural rearrangement i.e. when OPT was per-formed for one time the amount of unbound hydrogen aswell as some of bonded hydrogen may come out from thestructure and hence reduces the band gap. However, whenOPT is performed more than one time (samples S-2, S-3and S-4), these films show continuous enhancement inband gap, which may be due to etching of soft graphite-likesp2 clusters (Jiang et al., 2002) and samples S-2, S-3 and S-4attains its value as 2.3 eV, 2.4 eV and 2.6 eV, respectively.It is realized that an increase in number of OPT leads torecovery of the structure of DLC films, which is confirmedby comparing the band gap of S-0 with S-2 and S-3.Observed band gap results are in good agreement withthose of hardness as the band gap varies in direct propor-tion with hardness and the observed trend is the same. It isworth noting that OPT on DLC varies its band gap overwide range of 1.25–2.6 eV. Thus, these OPT–DLC filmscan be treated as excellent p-layer for p–i–n solar cells aswell as it can be used in the development of tandem solarcells. We have also performed the simulations for a-Si:Hbased p–i–n solar cells, where OPT–DLC is considered asp-type window layer.

3.2. Simulation study

We have experimentally prepared the variable band gaplayers of DLC by plasma modifications (OPT–DLC). Nowwe simulate the p–i–n solar cells by considering OPT–DLCfilms as p-layers with the same band gaps that were realizedexperimentally. The reason for choosing only these bandgaps for p-layer of OPT–DLC is that we have experimen-tally prepared these variable band gap layers and we mayapply the same on p–i–n solar cell devices (work is in pro-gress). The simulation parameters used in the present inves-tigation are given in Table 1. Since the evaluated band gapsof different OPT–DLC films are found to be 1.25 eV,2.25 eV, 2.3 eV, 2.4 eV and 2.6 eV with such variable bandgaps, we have five different choices of p–i–n structures, asdepicted in Fig. 4.

Further, we perform the simulation for evaluating theopen circuit voltage (Voc), short circuit current (Isc), fill fac-tor (FF) and efficiency (g) for a-Si:H based p–i–n solarcells. Their variations with band gap of p-type OPT–DLC layer (thickness 5 nm) are depicted in Figs. 5 a andd. The OPT–DLC films exhibit very high optical transpar-ency despite of thicknesses in the range 240–298 nm. How-ever, we have used very low thickness of p-layer as 5 nm forsimulation. Therefore, at this thickness their transmissionmay be extremely high with negligible absorption loss onp-layer. It is evident from the figure that an initial increasein band gap from 1.25 eV to 2.25 eV leads to a drasticenhancement in Voc, which finally gets stabilized at higherband gaps (beyond 2.25 eV). It may be noted that

Fig. 4. Schematic representation of various possible p–i–n solar cell configurations with OPT–DLC as p-layer.

224 N. Dwivedi et al. / Solar Energy 86 (2012) 220–230

maximum Voc is found to be 1318 mV. Observed Isc followsthe similar trend and it increases initially with the increas-ing band gap from 1.25 eV to 2.25 eV and then becomesstable between 2.25 eV and 2.3 eV. However, it is foundto decrease at 2.4 eV and further gets increased beyond2.4 eV. The maximum value of Isc is found to be13.05 mA/cm2. In addition, the variation of fill factor(FF) with band gap is found to be non-linear; initially itsvalues drop when band gap is increased from 1.25 eV to2.25 eV and then it recovers its original values for the bandgap beyond 2.25 eV. It is important to note that the effi-ciency is continuously increased with increasing windowlayer band gap from 1.25 eV to 2.6 eV; the maximum effi-ciency of 8.9% is realized at 2.6 eV. The simulated maxi-mum efficiency 8.9% was found to be very close to DLCand a-SiC based double p-layer on Si solar cell (Lee and

1.2 1.6 2.0 2.4 2.82

4

6

826

39

52

4

8

12

1050

1200

1350

(d)

η (%

)

Band gap (eV)

(c)FF (%

)

(b)

I sc (m

A/c

m2 )

(a)V oc (m

V)

Fig. 5. Variation of Voc, Isc, FF and g with band gap of p-layer.

Lim, 1998). We have also simulated the efficiency as a func-tion of thickness of window layer of OPT–DLC of differentband gaps, as depicted in Fig. 6. Here, it is obtained thatthe efficiency is continuously decreased for the higher thick-ness from 5 nm to 20 nm at each band gap. This can beunderstood as follows. Actually the surface absorption isenhanced for the higher thickness of window layer thatincreases the absorption losses due to heat generation.Hence, the higher recombination is realized that results ina reduced efficiency. Nevertheless, at higher thicknessesthe band gap factor can be considered and most of photonswith energy above the band gap gets absorbed within thewindow layer and can create absorption loss. Hence, weobserved low efficiency at low band gap while varying awindow layer thickness from 5 nm to 20 nm.

In order to understand the basic mechanism involved inthe change in efficiency with change in band gap of p-layer,we have also shown energy band diagrams of simulated

1.21.5

1.82.1

2.42.7

234567

8

9

20 nm15 nm

10 nm

5 nm

Effic

ienc

y(%

)

Band gap (eV)

Fig. 6. Variation of efficiency with thickness as well as band gap of p–i–nsolar cell having different OPT–DLC films as p-layer.

N. Dwivedi et al. / Solar Energy 86 (2012) 220–230 225

p–i–n solar cell devices at each band gap of p-layer of thick-ness 5 nm that are depicted in Fig. 7a–e. When light isexposed on cell, e–h pairs generate in i-layer. The junctionfield separates the e� and h before recombination and elec-trons move towards n-region and holes towards p-region.When the band gap of p-layer is kept at 1.25 eV, this struc-ture shows not only low Voc and low Isc but also low effi-ciency due to introduction of high band offset and

1.25 eV 5 nm(a)

2.3 eV 5 nm(c)

2.6 eV(e)

Ec

Ev

Ev

Ec

Fig. 7. Energy band diagrams of p–i–n s

flattening of energy band over i-layer. However, FF wasfound to be high at this design of solar cell due to low Voc

and Isc values. In contrast, when the band gap of p-layeris enhanced and kept at 2.25 eV, the values of Voc, Isc andefficiency were drastically enhanced due to reduction inboth the band offset and the flattening of energy band overi-layer. The lifting of energy band of p-layer was also foundto be a possible reason in enhancement of Voc and the

2.25 eV 5 nm(b)

2.4 eV 5 nm(d)

5 nm

Ev

Ev

Ec

Ec

Ec

Ev

olar cells with OPT–DLC as p-layer.

1

2

3

4

5

Load

(mN

)

S-0 S-1 S-3 S-4 S-5

226 N. Dwivedi et al. / Solar Energy 86 (2012) 220–230

efficiency at this band gap (Lee and Lim, 1998). However,FF was decreased in this design due to increased Voc andIsc values. Further increase in p-layer band gaps saturatesVoc, oscillates Isc and enhances FF values. However, the effi-ciency remains in the same trend and continuouslyincreased with the increasing band gap of p-layer thatmay be due to a reduction in the band offset, an increasein band tilting (reduction in band flattening) and a continu-ous lifting of energy band of p-layer.

0.00 0.02 0.04 0.06 0.08 0.10 0.12

0

Dispalcement (μm)

Fig. 9. Load versus displacement curves of various OPT–DLC films.

3.3. Model: A possible mechanism of transport of low mobileholes under the action of high mobile holes

In the present simulation, we have considered OPT–DLC as a p-type window layer whereas a-Si:H as i and nlayers in p–i–n solar cell. It is to be noted that OPT–DLCfilm has very small hole mobility (lp � 10�4 cm2/Vs) thana-Si:H (lp � 1 cm2/Vs). Thus, depending upon the differ-ence in hole mobilities in these two different materials, wemade a model as depicted in Fig. 8. Since OPT–DLC hassmall hole mobility, its motion in p-layer to respective des-tination to complete the circuit should be low. We proposedthat light falls from p-layer and reaches maximum to i layerwith negligible absorption at p- layer due to its high trans-mission and very low thickness. Since absorber i layer is a-Si:H and generated e–h pairs in the action of exposure ofstream of photon (AM 1.5) have mobilities of a-Si:H, thishole mobility also influences the motion of p-layer holesand hence the efficiency of solar cell. Let h and h1 be themobilities of holes in a-Si:H and OPT–DLC layers, respec-tively. When the photons fall on i layer the electron holepairs are generated that further swept out due to built infield. Thus, generated holes (h) move toward p-layerwhereas electrons (e) towards n-layer. Since h1 has low val-ues, the corresponding holes should move with low mobilitywithin p-layer. However, generated holes (mobility h) havehigh mobility and the high drift velocity, so they may reach

Fig. 8. Schematic representation of model describing possible workingprinciple of p–i–n solar cell having different hole mobilities at p and ilayers.

p-layer rapidly and can easily collide with already existingholes (h1) of low mobile. This collision may lead to transfor-mation of energy from high mobile holes to low mobileholes. Thus, under this action the motion of low mobileholes (h1) may also get enhanced and with newly acquiredmomentum they move faster. However, they also collidewith other low mobile holes (h1) and transfer their energyto them. This process may continue and the holes (h1) pres-ent at the corner of p-layer may rapidly reach to their des-tination and may give rise to a better photo current. In allthe actions it was realized that process begins from pushingof low mobile h1 by high mobile h that further continueswith pushing of holes one another. This may thereforereferred to as hole pushing effect to enhance the photocurrent.

3.4. OPT–DLC as a hard, protective and encapsulate layer

on solar cells

We have used high resolution depth sensitive nanoin-dentation for estimating the nano-mechanical propertiesof OPT–DLC films. Load versus displacement curves atindentation load of 5 mN for samples S-0 to S-4 grownon Si substrate are shown in Fig. 9. Since hardness of thinfilms strongly depends on penetration depth, hardness wasestimated by composite hardness model. Load versus dis-placement curves were employed to estimate variousnano-mechanical parameters such as hardness (H), elasticmodulus (E), plastic resistance parameter (H/E), elasticrecovery (ER), ratio of residual displacement after loadremoval with displacement at maximum load (dres/dmax)and plastic deformation energy (Ur). Variation of H for dif-ferent OPT–DLC films is shown in Fig. 10a. H stronglydepends on nano- to microstructural defect present in thenetwork, and this should be related to the bonding betweenthe atoms and to the ability of the bonds to withstanddeformation stemming from compression, extension, bend-ing or breaking. Unmodified DLC (S-0) shows better H(23.5 GPa) that is reduced to 16.1 GPa when single timeOPT is performed on DLC (S-1). Observed significant

16

18

20

22

24

26

(a)

H (G

Pa)

SamplesS-0 S-1 S-2 S-3 S-4

S-0 S-1 S-2 S-3 S-4S-0 S-1 S-2 S-3 S-4

S-0 S-1 S-2 S-3 S-4200

220

240

260

280

300

320

(b)

E (G

Pa)

Samples

0.075

0.078

0.081

0.084

(c)

H/E

Samples

60

65

70

75

80

85

(d)

ER (%

)

Samples

Fig. 10. Variation of (a) H, (b) E, (c) H/E and (d) ER for different OPT–DLC films.

N. Dwivedi et al. / Solar Energy 86 (2012) 220–230 227

reduction in H in S-1 is attributed to structural rearrange-ment, as we have discussed in band gap section. This valueof H is found to be lowest among various samples. AgainH is increased in S-2, S-3 and S-4 samples and its valuescorresponding to these samples are found to be 16.8 GPa,19.4 GPa and 25.3 GPa. It is recognized that the DLClayer-by-layer structures followed by OPT on each layerreveal better H. Beyond S-1, the observed increased valuesof H in S-2, S-3 and S-4 samples expose that oxygen plasmastarts to etch the soft graphitic-like sp2 clusters (Jiang et al.,2002). Also the observed variation of H for different sam-ples is found to be similar to that of band gap. Thus, dueto better H, these OPT–DLC films can be used as hardand protective coatings on solar cells effectively in n–i–pconfiguration with metal substrate. This additional coatingmay protect solar cells from high energy radiation. In addi-tion, such films can also be used as encapsulate coating forthe solar cells to avoid post-preparation contaminationthat degrades the efficiency of solar cells. Besides H, wehave also investigated various nano-mechanical parame-ters. It is important to mention that self bias that is linearfunction of power influences the nano-mechanical proper-ties of DLC films significantly. As far as employing DLCas a hard and protective coating application, the self biasshould be moderate. Because such self bias decomposes

the hydrocarbon precursor sufficiently as well as reducesthe excess and unbound hydrogen, it results in higher sp3

bonding (Erdemir and Donnet, 2006). Hence, these OPT–DLC films were prepared at negative self bias of 100 V.We have also prepared DLC and modified DLC films withnegative optimized self bias of 100–150 V (Dwivedi et al.,2011a,b,c,d). Fig. 10b shows the variation of E against var-ious OPT–DLC films. It is to be noted that observed E val-ues follow the similar trend and E is found to be minimumin S-1 sample and maximum in S-4 sample. The values of E

were found to be in the range 212.7–299 GPa. Due to com-bined effect, plastic resistance parameter (H/E) was foundto be important parameter for explaining the elastic–plasticand wear resistance properties of thin films. The H/E ratiois basically related to the bulk fracture strength and itsdomain of validity for DLC based coatings varies in therange 0–0.1, where the upper and lower limits show theirelastic and elastic–plastic behaviors, respectively. In addi-tion, for high wear resistance coatings, the H/E ratio mustbe very high. The variation of H/E ratio for various OPT–DLC films is shown in Fig. 10c. Here, sample S-0 exhibitshigh value of H/E ratio that decreases for samples S-1, S-2and S-3. However, the value of H/E is drastically improvedin sample S-4 due to significant improvement in H. Thus,the sample S-4 can be used not only for hard and protective

0.15

0.20

0.25

0.30

0.35

0.40

(a)d re

s / d

max

Samples

S-0 S-1 S-2 S-3 S-4S-0 S-1 S-2 S-3 S-4

S-0 S-1 S-2 S-3 S-4

0.10

0.12

0.14

0.16

(b)

S max

(N/ μ

m)

Samples

3.0x10-10

3.3x10-10

3.6x10-10

(c)

U r (J)

Samples

Fig. 11. Variation of (a) dres/dmax, (b) Smax and (c) Ur for different OPT–DLC films.

Table 2Experimentally evaluated various nano-mechanical parameters for differ-ent OPT–DLC samples.

Samples H

(GPa)E

(GPa)H/E ER

(%)dres/dmax

Smax

(N/lm)Ur (J)

S-0 23.5 284.3 0.083 83.5 0.164 0.12 3.1 � 10�10

S-1 16.1 212.7 0.075 62.8 0.364 0.15 3.7 � 10�10

S-2 16.8 220.9 0.076 80.7 0.196 0.11 3.6 � 10�10

S-3 19.4 258.3 0.075 74.2 0.258 0.15 3.4 � 10�10

S-4 25.3 299.0 0.085 79.6 0.204 0.11 3.0 � 10�10

228 N. Dwivedi et al. / Solar Energy 86 (2012) 220–230

coating applications but also for the wear resistance coat-ing applications. It is to be noted that the observed lowvalue of H/E ratio in S-1, S-2 and S-3 samples reveal morefraction of work consumed in a plastic deformation andlarge plastic strain is expected when contacting a material.

Elastic recovery (ER) is also found to be an importantparameter for explaining the elastic–plastic behavior ofmaterials. The value of ER in these films was estimatedusing the following relation

%ER ¼ ðdmax � dresÞdmax

� 100 ð1Þ

where dmax and dres are the displacements at the maximumload and residual displacement after load removal, respec-tively. The variation of ER for different OPT–DLC films isdepicted in Fig. 10d, from which it is evident that sample S-0 exhibits high ER that is drastically decreased for S-1 sam-ple due to structural rearrangement. However, due to theuse of more layer-by-layer structure, the ER is significantlyrecovered in samples S-2 and S-4. Thus, the values of ERare found to be quite close in S-0, S-2 and S-4 samples.The dres/dmax ratio provides information similar to that ofER but with different domains of validity that vary be-tween 0 and 1, where the lower limit corresponds to fully

elastic behavior and the upper limit corresponds to rigid-plastic behavior. The variation of dres/dmax ratio for differ-ent OPT–DLC films is shown in Fig. 11a. The values ofdres/dmax are found to be quite low that reveal more elasticbehavior of films, which are in well agreement with ER re-sults. In addition, the contact stiffness (Smax) of the OPT–DLC films is also investigated. Since we have explorednano-mechanical properties of OPT–DLC films for theirpossible realization as hard and protective coating on solarcells, the investigation of Smax is an important parameter.The variation of Smax against various OPT–DLC films isshown in Fig. 11b, from which it is evident that S-1 and

N. Dwivedi et al. / Solar Energy 86 (2012) 220–230 229

S-3 films are quite stiffer than other films. The nano-mechanical property can also be explained in term of en-ergy and the variation of plastic deformation energy (Ur)for different OPT–DLC films, as depicted in Fig. 11c. Herethe value of Ur was estimated using the following relation

U r ¼1

3

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1

x0 tan2 W

s" #1ffiffiffiffiHp P 3=2 ð2Þ

where x0 is the geometry constant that attains the value of1.3 for pyramid indenter, P is the load and W is the halfangle of Berkovich indenter that has the value 65.3�. Theenergy Ur varies inversely with H and its values are foundto be quite low in the range 3 � 10�10–3.7 � 10�10 J. Theminimum value of Ur was observed in sample S-4. The esti-mated values of nano-mechanical parameters for differentOPT–DLC films are given in Table 2.

4. Conclusions

DLC layer-by-layer films with variation of layer from 1to 4 followed by OPT on each layer were grown using RF-PECVD technique at fixed working pressure of5 � 10�2 Torr and self bias of 100 V. The reference DLCsample without OPT was also grown to examine the effectof OPT on various properties of DLC films. The OPT onDLC led to wide variation of band gap from 1.25 eV to2.6 eV. Due to wide band gap and band gap feasibility overwide range, the experimentally deposited OPT–DLC maybe used as p-layer in a-Si:H based p–i–n solar cells as wellas variable band gap layers in tandem solar cells. We havealso performed simulation by considering OPT–DLC filmas p-layer and obtained maximum efficiency of 8.9% atband gap of 2.6 eV. In addition, the nano-mechanicalproperty of OPT–DLC films was estimated and it wasfound to be improved with the application of OPT. Thus,due to their improved nano-mechanical properties theOPT–DLC films may also be used as hard, protective,and encapsulate coatings on solar cells.

Acknowledgments

The authors are grateful to the Director, NationalPhysical Laboratory, New Delhi for his kind support.Authors also wish to thank Dr. O. S. Panwar and Mr. C.M. S. Rauthan for their kind support. One of authors NDacknowledges CSIR Govt. of India for providing financialassistance through SRF fellowship. This research workwas sponsored by MNRE, Govt. of India, through the pro-ject sanction no. 31/29/2010-11/PVSE.

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