Plasmon-Induced Photon Manipulation by AgNanoparticle-Coupled Graphene Thin-Film: Light Trappingfor Photovoltaics

Pooja Kanade & Pankaj Yadav & Manoj Kumar &

Brijesh Tripathi

Received: 7 July 2014 /Accepted: 25 August 2014# Springer Science+Business Media New York 2014

Abstract This paper reports plasmon-enhanced light trans-mission through the Ag nanoparticle-coupled graphene thin-film deposited on fluorine-doped tin oxide (FTO) glass sub-strates. An experimental set-up is developed to quantify thephoton enhancement due to metal nanoparticle-embeddedgraphene layer. The results show that a specific distributionof graphene nanosheets over Ag nanoparticle-deposited FTOglass can trap maximum normally incident light for photovol-taic applications. An enhancement of 6.35 % in the currentdensity of silicon solar cell (kept under Ag-coated FTO glass)is observed for 30-μL graphene dispersion deposited on theFTO/Ag (3 nm). The results indicate the possibility of mini-mizing the reflection of incident radiation by combining plas-monic oscillations of metal nanoparticles with grapheneplasmonics, which can be useful for optoelectronic devices,radiation sensors, and various types of photovoltaic cells.

Keywords Ag plasmon . Si solar cell . Light trapping .

Graphene . Photonmanipulation

Introduction

In conventional silicon solar cells, the light trapping is achievedby (a) texturing the surface [1, 2] and (b) applying the anti-reflection coatings [3]. In recent times, plasmonic architectureemploying metal nanoparticles (NPs) has received a lot ofattention from the research community as an alternative

approach to enhance light trapping in photovoltaic (PV) devicesthrough surface plasmon resonance (SPR) [4–6]. In SPR, thescattered light from sub-wavelength metallic nanoparticlesplaced at silicon-air interface is directed towards high dielectricconstant material (Si) [7, 8]. This phenomena is investigated fora wide range of metal nanoparticles (e.g., Ag, Al, Cu) toimprove the performance of solar cells, LEDs, and sensors[9–11]. It is also observed that the presence of nanoparticlesleads to a severe degradation due to absorption by the nanopar-ticle in their metallic interband transition and destructive inter-ference between incident and scattering light within a certainshorter wavelength side of the localized surface plasmon reso-nance (LSPR) [12–16]. Other researchers have shown that theabsorption in nanostructures is of parasitic nature, so it is chal-lenging to increase the short-circuit current density (JSC) of solarcells by more than 2.5 % using plasmonic processes [17].

Graphene, a two dimensional carbonmaterial, has potentialapplication in solar cells due to its high transmittance andconductivity [18]. Recently, Chen et al. have showed 7.2 %enhancement in the current density of textured screen-printedsolar cells using Al nanoparticles and wrinkle-like graphenesheets [12].

On the other hand, transparent conducting oxides (TCOs)are the foundation for many technical and commercial de-vices, such as liquid crystal flat panel displays [19], transpar-ent thin-film transistors (TTFTs) [20], solar cells [21], gassensors, and optoelectronic applications [22]. Over the years,indium-doped tin oxide (ITO) thin films have been the essen-tially favored TCO thin films in industry. However, the defi-ciency in supply of rare and expensive indium in ITO [22] is acause of apprehension. Several attempts have been made tobuild a potential substitute for ITO. Reasonably low-pricedfluorine-doped tin oxide (FTO) is found to be a good alterna-tive due to its better heat resistivity [23]. FTO glass substrateshave been widely used in different type of solar cells, e.g.,dye-sensitized solar cell [24, 25], perovskite-based solar cell

P. Kanade : P. Yadav :B. Tripathi (*)School of Solar Energy, Pandit Deendayal Petroleum University,Gandhinagar 382007, Indiae-mail: brijesh.tripathi@sse.pdpu.ac.in

M. Kumar :B. TripathiSchool of Technology, Pandit Deendayal Petroleum University,Gandhinagar 382007, India

PlasmonicsDOI 10.1007/s11468-014-9790-4

[26], thin-film solar cells [27], etc. These devices can befurther benefited if the transmission of incident light throughthese substrates can be enhanced [28, 29].

In this paper, plasmon-enhanced light transmission throughthe Ag nanoparticle-coupled graphene thin-film deposited onFTO glass substrates is reported. An experimental set-up isdeveloped, which uses commercially available silicon solarcell, to quantify the photon enhancement due to Agnanoparticle-coupled graphene layer.

Experimental

Glass substrates coated with FTO having sheet resistance ≈7 Ω/□ were cleaned sequentially by ultrasonic treatment indetergent, de-ionized water, acetone, and isopropyl alcoholand dried with nitrogen before film preparation. A very thinsilver layer was deposited on FTO glass substrate (SigmaAldrich) using silver wire of purity 99.9998 % (Alfa Aeser)by thermal evaporation method (base pressure ∼1×10−6 mbar,BC300, HHV make). Thickness was recorded from the thick-ness monitor supplied with the thermal evaporation system.The deposition was followed by vacuum annealing of thesamples at 250 °C for 2 h. The monolayer graphene (G)dispersion consisting of graphene flakes (1 mg/L, SKUPGF-1-50, Graphene Supermarket) was used to deposit a thinlayer of graphene on the Ag-deposited FTO samples (FTO/Ag/G). The average particle size of monolayer grapheneflakes was ≈550 nm and average flake thickness was≈0.35 nm. The samples were characterized by scanning elec-tron microscopy (ULTRA, Carl Zeiss) for topographic infor-mation. Optical properties of the samples were investigated byUV–vis spectrophotometer (ISR-2600Plus, Shimadzu). Apolycrystalline silicon wafer-based solar photovoltaic (PV)cell of size 10 mm×10 mm was used to characterize the role

of silver-coupled graphene (Ag/G) over FTO glass substrate inlight trapping as per the scheme shown in Figs. 1 and 2. Thephotovoltaic characteristics of solar cells were measured un-der AM1.5G spectrum using a solar simulator (Class AAA,PEC Inc., USA). The power of the simulated light was cali-brated to be 100 mW/cm2 by using a reference Si photodiodeequipped with an IR cut-off filter. A current–voltage curvewas obtained by applying an external bias to the cell andmeasuring the generated photocurrent with a digital sourcemeter (U2722A, Agilent, USA).

Theoretical Background

The terminal equation for current (I) and voltage (V) of thesolar cell is mentioned below as described by many groups[30, 31]:

I ¼ IPH−IS expq V þ IRSð ÞkBTCA

� �−1

� �− V þ IRSð Þ

.RSH ð1Þ

where IPH represents the photocurrent generated due toillumination, IS represents the reverse saturation current, qrepresents the electron charge (≈1.6×10−19 C), kB representsthe Boltzmann’s constant, A represents the ideality factor, RSrepresents the series resistance, and RSH represents the shuntresistance of the solar cell.

Generally, for the solar PV cells, IPH≫IS, so in Eq. (1), thesmall diode and ground-leakage currents can be ignored underzero-terminal voltage. Therefore, the short-circuit current(ISC) is approximately equal to the photocurrent. The expres-sion for IPH is given by Eq. (2) [30, 31]:

IPH ¼ ISCλ ð2Þ

where λ=Incident radiation in kW/m2 received by the solarcell and ISC is the measured short-circuit current under

Fig. 1 The scheme of the experiment to explore the light trapping properties of Ag/G

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AM1.5G spectrum (100 mW/cm2). An enhancement in λ isexpected due to plasmon-induced scattering by Ag nanopar-ticles or by a combination of Ag NPs and graphene flakesdeposited on the FTO glass substrate and is dependent on theirsize and distribution. Assuming that the Ag nanoparticles areof spherical shape with radius R, a point dipole model de-scribes the scattering of light up to good extent. The diameterof the particle must be well below the wavelength of light. Thescattering and absorption cross sections (Cscat and Cabs, re-spectively) are related to the particle size and wavelength (λ)of incident light as given by [32]:

Cscat ¼ 1

6π2πλ

� �4

αj j2 ð3Þ

Cabs ¼ 2πλIm α½ � ð4Þ

where

α ¼ 3Vεp.εm−1

εp.εm þ 2

24

35 ð5Þ

represents polarizability of the particle. Here, V representsparticle volume (function of particle radius), εp representsdielectric function of the particle and εm represents the dielec-tric function of the embedding medium. Therefore, scatteringcross section increases with increase in particle size, as a resultmore fraction of light is scattered towards the medium withhigher refractive index. The size of Ag nanoparticles can beoptimized by controlling thickness of the Ag thin-film duringdeposition process.

Results and Discussion

Effect of Ag Nanoparticles and Graphene Flakes on the SolarSpectrum

The surface morphology of FTO glass substrates and Agnanoparticle layer on FTO (≈3 nm) produced by the methodof metal island growth using thermal evaporation is shown inFig. 3a, b, respectively. The Ag nanoparticles are formed dueto the re-crystallization and coalescence process. The differ-ence in the contrast of Fig. 3a, b is due to the different chargingof conducting surfaces. A detailed morphological analysis ofAg nanoparticle layers is presented in our previous article [32,

Fig. 2 The scheme of the experiment to fabricate Ag NP-coupled graphene layer

(a) (b)

(c) (d)

Fig. 3 SEM and EDS images. aGlass/FTO. b Glass/FTO/Ag(3 nm). cGlass/FTO/Ag (3 nm)/G(30 μL). d EDS image of Glass/FTO/Ag (3 nm)/G (30 μL)

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33]. Figure 3c shows the SEM morphology of graphene overAg nanoparticle-coated FTO substrate. The graphene flakesare seen lying randomly in a wrinkled manner on the surface,which indicates that the effective cross section of the flakesinteracting with incident light is less than their average size(≈550 nm). The sharp edges of the flakes seen in Fig. 3c areexpected to scatter a greater amount of incident light. Theenergy dispersive spectrograph (EDS) of FTO/Ag/G sample isshown in Fig. 3d. The strongest peak of Sn is observed due totin oxide (F:SnO2) layer on glass. A peak of Si and O isattributed to the glass (SiO2) substrates. The observed Agand C peaks are due to the presence of Ag nanoparticle layerand graphene flakes on the FTO substrate.

The effect of Ag layer thickness on the optical transmissionwas investigated byUV–vis measurements as shown in Fig. 4.The two resonance minima peaks at ≈364 and ≈420 nmwave-lengths are observed in the transmission spectra. The shorterwavelength minimum at ≈364 nm corresponds to the

quadrupole resonance whereas a wavelength minimum at≈420 nm corresponds to the dipolar resonance of Ag nano-particles. The observed dipolar resonance signifies the absorp-tion and scattering property possessed by the deposited Agnanoparticles.

Similar quadrupolar and dipolar resonance peaks in trans-mission spectra of Ag nanoparticle layer over various sub-strates were reported by other researchers [14, 34, 35]. Withincrease in the thickness of Ag nanolayer (3 to 7 nm), redshifting of dipolar resonance peak is observed (Fig. 4). How-ever, the quadrupole resonance peaks show a thickness-independent nature, which signifies that peak positions areoriginated due to the near lying interband transition edges inAg nanoparticles [34]. A distinct nature of the two peaks isclearly visible up to ≈5-nm thickness, but the dipolar reso-nance peak tends to vanish as the film thickness is increased,which signifies the disappearance of scattering effects. Bycomparing the transmission spectra of the 3- and 7-nm layer,it can be concluded that the enhanced transmission for the 3-nm layer for all the wavelengths probed here is due to the nearfield effect among the Ag nanoparticles. With increase in thethickness of Ag nanolayer (from 3 to 7 nm), the agglomerationsize begins to increase which results in closely distributednanoparticles. The agglomeration size is critical for plasmonic

Fig. 4 Transmission spectra of Ag-deposited FTO glass for various layerthicknesses

Fig. 5 Effect of Ag layer on the solar spectrum (downloaded fromASTM) for various Ag layer thicknesses

Table 1 Effect of Ag layer on the transmission and irradiance

Sampledescription

% transmission (at550 nm)

% increase in irradiance

FTO Baseline (82.38) Baseline (472.91 W/m2)

FTO/Ag (3 nm) 90.23 +9.09

FTO/Ag (4 nm) 90.13 +8.17

FTO/Ag (5 nm) 90.04 +7.53

FTO/Ag (7 nm) 86.22 +4.12

Fig. 6 Transmission spectra of Ag-deposited FTO glass coated withdifferent amounts of graphene

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effects, which is clearly indicated by the optical experimentsreported here and also in previous work [33]. For the 7-nmlayer, a lower transmission is observed due to larger nanopar-ticle agglomeration size, which reflects or absorbs greateramount of incident light. The transmission data is used toexplore the effect of Ag layer on the AM1.5 solar spectrumpassing through the FTO/Ag sample as shown in Fig. 5.Dependent on the transmission nature of Ag nanoparticle-coated FTO glass, a modified solar spectrum (=T(λ)×AM1.5spectrum, T being transmission coefficient) is received by thesilicon solar cell placed below the FTO glass. The transmis-sion data at 550 nm for various thicknesses of Ag layer andcorresponding change in solar irradiance (in percentage withrespect to the FTO baseline) is listed in Table 1.

In order to explore the properties of graphene overFTO glass, a layer of graphene dispersion was depositedon FTO with an amount of 10 μL. The amount of mono-layer graphene flakes was increased by adding 10 μL onthe same sample for each measurement to vary thegraphene amount in the range of 10 to 50 μL. The opticaltransmission data for these FTO/G samples are presentedin Fig. 6. The transmission data shows a maximum trans-mission of incident radiation for 60-μL graphene sampledue to better scattering effects achieved for the graphenedistribution. Just beyond 80-μL graphene dispersion, a

lower transmission is observed for most part of the visiblespectrum as shown in Fig. 6.

The modified spectrum is generated using the transmissionprofile of FTO/G samples as shown in Fig. 7. A comparisonamong the curves and data listed in Table 2 shows maximumirradiance for 60-μL graphene dispersion coated over FTOglass substrate.

The enhanced transmission observed is due to the incidentlight-induced surface plasmon resonance of Ag nanoparticlesand graphene monolayer. Due to the size, shape, and distribu-tion of Ag nanoparticles and monolayer graphene flakes,resonance can be quite broad, which has also been shown byother groups [28, 29, 32]. An excited nanoparticle/flake emitsin a range of directions, but the vast majority of the flux goestowards the solar cell because of the high refractive index ofFTO.

Effect of Ag/Graphene on the I-V Curve of Solar Cell

The current–voltage characteristics of the solar cell placedbelow Ag-coated FTO glass is shown in Fig. 8, and the key

Fig. 7 Effect of graphene on the solar spectrum (downloaded fromASTM) for various amounts of graphene dispersion

Table 2 Effect of graphene dispersion on the transmission and irradiance

Sampledescription

% transmission (at550 nm)

% increase in irradiance

FTO Baseline (81.70) Baseline (472.91 W/m2)

FTO/G (20 μL) 82.62 +1.35

FTO/G (40 μL) 83.78 +2.73

FTO/G (60 μL) 84.62 +3.63

FTO/G (80 μL) 84.60 +3.62

Table 3 Solar cell performance parameters for Ag layer on FTO glass

Sampledescription

%increaseinirradiance

ISC(mA)

VOC

(V)FF(%)

Efficiency(%)

Currentenhancement(%)

FTO Baseline 23.43 0.56 74.54 9.78 Baseline

FTO/Ag(3 nm)

9.09 24.27 0.559 73.20 9.95 3.59

FTO/Ag(4 nm)

8.17 24.20 0.558 73.68 10 3.29

FTO/Ag(5 nm)

7.53 24.17 0.558 73.84 9.9 3.16

FTO/Ag(7 nm)

4.12 23.61 0.558 73.95 9.7 0.77

Fig. 8 Current–voltage characteristics of silicon solar cell kept belowFTO glass with varying thicknesses of Ag layer

Plasmonics

parameters are listed in Table 3. The influence of the Agnanoparticles layer on the performance of the solar cell wasinvestigated through the relationship between ISC enhance-ment and the film thickness. For all the Ag films ranging from≈3–7 nm, the 3-nm layer provides maximum short-circuitcurrent. As expected from the transmission data, the largestenhancement in photovoltaic performance was achieved forthe 3-nm layer with a 3.6 % relative increase in ISC (from23.43 to 24.27 mA), which is significantly greater than theparasitic enhancements (given as 2.5 % by [12]). The energyconversion efficiency increases considerably from 9.78 to9.95 % as listed in Table 3. The enhancement in ISC isattributed to the forward scattering of incident light facilitatedby the small Ag nanoparticles (3-nm layer), which results inhigher transmission towards the silicon solar cell over thecomplete wavelength range. With an increase in the Aglayer thickness, a decrease in ISC is due to the extinctionof incident light through the bigger agglomerates becauseof lower availability of radiation to the silicon solar cell.The short-circuit current plotted as a function of Ag layerthickness is shown in Fig. 9, and a trend-line is fitted withthe following quadratic relationship between these

parameters:

ISC mAð Þ ¼ −0:059x2 þ 0:444xþ 23:43 ð6Þwhere, x denotes the thickness of Ag layer deposited on the

FTO glass. Corresponding data for the solar cell under Ag-coated FTO glass (FTO/Ag) is given in Table 3.

With intent of additional improvement in solar cell perfor-mance, the Ag nanoparticles (FTO/Ag (3 nm)) are coupledwith graphene sheets as shown in the last step of Fig. 2.Figure 10 and Table 4 presents the current–voltage character-istics and corresponding key parameters of silicon solar cell.The incorporation of Ag-coupled graphene clearly demon-strates a significant enhancement of 6.35 % in the short-circuit current as compared to the cell under Ag (3 nm) layer.A maximum ISC is obtained for a configuration of FTO/Ag(3 nm)/G (30 μL) against 60-μL graphene dispersion (highesttransmission in Fig. 6). The difference in observed and ex-pected results in terms of graphene amount is due to anoptimal surface coverage of Ag (3 nm)/G (30 μL), whichproduced a balance between light trapping and light blocking.A similar conclusion over the improvement of ISC with incor-poration of graphene in silicon solar cell was observed byother researchers [12]. The basic mechanism for the ISC im-provement is explained as follows: (1) the lower refractive

Fig. 10 Current–voltage characteristics of silicon solar cell kept belowAg (3 nm)/FTO with varying amounts of graphene dispersion

Fig. 9 Short-circuit current variation with respect to the thicknesses ofAg layer

Table 4 Photovoltaic performance parameters under Ag-coupledgraphene nanoparticles

Sampledescription

ISC(mA)

VOC

(V)FF(%)

Efficiency(%)

Currentenhancement (%)

FTO/Ag (3 nm) 24.27 0.559 73.20 9.95 Baseline

FTO/Ag (3 nm)/G(10 μL)

25.08 0.564 74.73 10.57 3.34

FTO/Ag (3 nm)/G(20 μL)

25.37 0.562 74.06 10.56 4.53

FTO/Ag (3 nm)/G(30 μL)

25.81 0.568 73.94 10.84 6.35

FTO/Ag (3 nm)/G(40 μL)

25.51 0.567 74.18 10.73 5.11

FTO/Ag (3 nm)/Gn50 μL)

25.43 0.562 74.66 10.67 4.78

Fig. 11 Short-circuit current variation with respect to the amount ofgraphene dispersion on FTO/Ag (3 nm)

Plasmonics

index of graphene, than that of the underlying layer below600 nm acts as an extra anti-reflection mechanism, whichcauses an increase in photocurrent, and (2) the wrinkle struc-ture of graphene also improves the light scattering towards thesolar cell [12]. The short-circuit current is plotted as a functionof used amount of graphene dispersion on FTO/Ag (3 nm)shown in Fig. 11, and a trend-line is fitted with the followingquadratic relationship between these parameters:

ISC mAð Þ ¼ −0:001g2 þ 0:094g þ 24:12 ð7Þ

where, g denotes the amount of graphene dispersion overthe Ag layer (3 nm) deposited on the FTO glass.

For FTO/Ag samples, the experimental results show that ahigher transmission could be observed for 3-nmAg thickness.For FTO/G samples, the results show that the maximumtransmission of incident radiation could be observed for60-μL graphene sample, but for FTO/Ag/G samples, thegraphene dispersion is 30 μL for a better current. The ob-served results can be attributed to the electromagnetic cou-pling of Ag nanoparticles and graphene, which lowers therequirement of graphene dispersion from 60 to 30 μL. Asimilar effect of electromagnetic coupling between grapheneand Au nanoparticles has been observed by other researchers[36], which shifts the resonance wavelength in a complexmanner and related to the relative permittivity of the surround-ing media.

Conclusions

This paper reports plasmon-enhanced light transmissionthrough the Ag/Graphene composite thin-film deposited onFTO glass substrates. An experimental set-up is developed toquantify the photon enhancement due to metal nanoparticle-embedded graphene layer. The results show that a specificdistribution of graphene nanosheets over Ag nanoparticle-deposited FTO glass can trap maximum normally incidentlight for photovoltaic applications. An enhancement of6.35 % in the current density of silicon solar cell (kept underAg-coated FTO glass) is observed for 30-μL graphene disper-sion deposited on the Ag (3 nm)/FTO. The results indicate thepossibility of minimizing the reflection of incident radiationby combining plasmonic oscillations of metal nanoparticleswith graphene plasmonics, which can be useful for optoelec-tronic devices, radiation sensors, and various types of photo-voltaic cells.

Acknowledgments The authors would like to acknowledge the finan-cial support from the Office of Research and Sponsored Programs(ORSP), Pandit Deendayal Petroleum University, Gandhinagar forconducting research work reported in this article.

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