Solar Energy Materials & Solar Cells 95 (2011) 693–698

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells

0927-02

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/solmat

Surface plasmon enhanced GaAs thin film solar cells

Wen Liu a, Xiaodong Wang a,n, Yueqiang Li a, Zhaoxin Geng a, Fuhua Yang a,b, Jinmin Li c

a Engineering Research Center for Semiconductor Integrated Technology, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, Chinab State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, Chinac Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese academy of sciences, Beijing 100083, China

a r t i c l e i n f o

Article history:

Received 30 May 2010

Received in revised form

30 September 2010

Accepted 6 October 2010Available online 27 October 2010

Keywords:

Ag nanoparticles

Surface plasmon resonance

GaAs

Solar cells

48/$ - see front matter & 2010 Elsevier B.V. A

016/j.solmat.2010.10.004

esponding author. Tel.: +86 10 82305403; fax

ail address: xdwang@semi.ac.cn (X. Wang).

a b s t r a c t

As a new method to improve the light trapping in solar cells, surface plasmon resonance (SPR) has

attracted considerable attention because of its unique characteristics. Several studies have been reported

on the photocurrent improvement of Si solar cells by surface plasmons, while little research has been done

on III–V solar cells. In this work, we performed a systematic study of SPR on GaAs thin film solar cells with

different sizes of Ag nanoparticles on the surface. The nanoparticles were fabricated by annealing E-beam

evaporated Ag films in a N2 atmosphere. It was found that the surface plasmon resonance wavelength

does not undergo a simple red-shift with increasing metal thickness. It depends on the shape of the metal

nanoparticles and the interparticle spacing. It is necessary to optimize the particle size to obtain an

optimum enhancement throughout the visible spectrum for solar cells. We found that the optimum

thickness of the Ag film was 6 nm under our experimental conditions. Furthermore, from the calculation

based on the external quantum efficiency data, the short circuit current density of a GaAs solar cell with

6 nm Ag film after annealing was increased by 14.2% over that of the untreated solar cell.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

As one of the important approaches to solving future energyproblems, solar energy power generating technology has beengiven more and more attention due to the massive amount ofenergy available directly from sunlight. However, for wide spreadimplementation, the cost of manufacturing solar cells needs to bereduced by a factor of 2–5 [1]. In this case, thin film solar cells canreduce material consumption. However, unlike conventional bulksolar cells, thin film solar cells have poor light absorption becauselong path lengths of photons are required. Therefore, many studieshave been done to increase the optical absorption. In thisframework, the localized absorption of metal nanoparticles(MNPs) via surface plasmon resonance (SPR) has attractedattention because of its unique characteristics: the largeelectromagnetic field enhancement, the wavelength-selectivephoton absorption, and the adjustable resonance wavelength bychanging the material, size, shape and local dielectric environmentof the MNPs [2].

Since Ebbesen et al. [3] discovered extraordinary transmission atcertain specific wavelengths through metallic films with arrays of sub-wavelength cylindrical holes, SPR has aroused considerable interest.Surface plasmon polaritons (SPPs) are electromagnetic excitationspropagating along a metal dielectric interface. These electromagnetic

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: +86 10 82305141.

surface waves arise via the coupling of the electromagnetic fields tooscillations of the electron plasma of the metal and are evanescentlyconfined in the perpendicular direction [4]. Consequently, it is believedthat the plasmonic nanostructures can enhance light trapping, and theyhave recently been used in various photodetectors [5,6], photodiodes[7,8], and solar cells configurations [9–16], which include applicationon Si solar cells [9–13] and organic solar cells [14–16]. To ourknowledge, however, they have been little use in III–V solar cells,and only Nakayama et al. [17] researched the light absorption ofplasmonic nanoparticle enhanced GaAs thin film solar cells. They usedanodic aluminum oxide (AAO) templates to fabricate the hemisphericaland conical MNPs uniformly and systematically. However, given some of the problems of processing compatibility, this method iscomparatively complex in the primary research on the applicationof SPPs in GaAs thin film solar cells. Moreover, there are many othermethods to obtain MNPs, including annealing metal films [6,9–11],chemical synthesis [8,12], electron beam lithography [18], focused ionbeam [19] and electroless plating, as recently reported [20]. Amongthese methods, however, annealing metal films is the simplest one.Thus, in this work, we used electron beam evaporation (EBE) in additionto annealing to fabricate MNPs and conducted systematic research onthe influence of SPRs on GaAs thin film solar cells.

Furthermore, we selected different thicknesses of the Ag filmsdeposited on GaAs thin film solar cells to study the influence of theparticles size. When the size of the particles is much smaller thanthe wavelength of the incident light, there is strong absorption andweak scattering. However, as the size of the particles becomeslarger, scattering becomes dominant, which is advantageous for

Table 1The GaAs solar cell label names, corresponding treatments and measurements.

Label name Treatments Measurements

Sia Untreated EQE

Si-Agjb Ag film deposition on

corresponding Si

EQE

Si-Agj-RTA Annealing the

corresponding Si-Agj

at 260oC for 15 min

in N2

EQE

a i¼1, 2, 3, 4, 5 correspond to the five solar cell samples in the experiments.b j¼2, 4, 6, 8,12 correspond to the different deposited Ag film thickness (2, 4, 6, 8

and 12 nm).

W. Liu et al. / Solar Energy Materials & Solar Cells 95 (2011) 693–698694

light trapping. However, as the particle size increases beyondcertain limits, the contribution of retardation takes effect, andhigher order multipole excitation modes become larger, which willdecrease the efficiency of the scattering process [10]. On the otherhand, Lim [7] observed in both experiments and simulations that, atwavelengths below the nanoparticles’s surface plasmon resonancewavelength (SPRW), the photocurrent decreased because of thedestructive interference between the transmitted and scatteredfield components. However, at wavelengths above the SPRW, thephotocurrent increased because of the constructive interferencebetween the transmitted and scattered field components.Therefore, the solar cells will only suffer marginal losses whenshifting the SPRW of nanopartilces below 400 nm because therelevant spectral domain for earth bound solar cell starts at 400 nm.Very small silver particles could be able to fulfill this task, but theabsorption will dominate over the scattering for small particles, asmentioned above. Hence, it is necessary to optimize the particlesize to obtain an optimum enhancement throughout the visiblespectrum. In this study, by comparative experiments on Ag filmswith thicknesses of 2, 4, 6, 8 and 12 nm, we investigated theinfluence of Ag nanoparticles on GaAs thin film solar cells andfound that 6 nm was the optimum thickness of the Ag film for ourexperimental conditions.

2. Experiment

All of the GaAs solar cells described in this paper were grown byGenII molecular beam epitaxy on a 2 in. (1 0 0)-oriented n-typeGaAs substrate. The structure consisted of a 1-um n-type GaAsbuffer layer, a 500-nm n-type AlGaAs back surface field layer, a150-nm n-type GaAs base layer, a 50-nm p-type GaAs emitter layer,a 30-nm p-type Al0.8Ga0.2As window layer, and a 300-nm-thickp-type GaAs contact layer. Next, the GaAs solar cell slice underwenta series of processing steps. The material was first cleaned bytrichloroethylene, acetone, and alcohol, then rinsed several timeswith deionized water and blown dry with nitrogen gas. After thecleaning, groove isolation was performed and SiO2 was depositedon it by plasma chemical vapor deposition for passivation. The nextstep was the preparation of the front electrode (the pattern isshown in Fig. 1) of Ti/Pt/Au by EBE. Subsequently, the GaAs caplayer was selectively etched. Then the back electrode Ni/Au/Ge/Ni/Au was prepared by EBE, and the slice was annealed in a N2

atmosphere at 400 1C for 20 s to form the front and back ohmiccontacts. Finally, the front electrode was thickened up to 2 mm toreduce the loss due to the sheet resistance of the metal grid.

After these processes, we cut the slice into many single solarcells and selected five of them, labeled as S1, S2, S3, S4 and S5, as theexperimental samples. Then silver films with different thicknesses(2, 4, 6, 8 and 12 nm) were deposited on these solar cells by EBE

Fig. 1. The pattern and dimensions of the top electrode in the experiment.

with collecting buses protected by high temperature adhesive tape(the collecting buses were used to attach probes while testing theexternal quantum efficiencies (EQEs)). The thicknesses of the Agfilms were controlled with a calibrated quartz oscillator. Then, welabeled the five solar cells as S1-Ag2, S2-Ag4, S3-Ag6, S4-Ag8 andS5-Ag12. At the same time, five Si dummy wafers were prepared toobserve the size and shape of MNPs by scanning electron micro-scopy (SEM), and five glass dummy wafers were also prepared tomeasure the transmission spectrum using an ultraviolet doublebeam spectrophotometer. Here we selected Ag as the metal ofchoice because of its low absorption losses at optical frequenciescompared to other metals and its high scattering efficiency [6].For all cells and dummy wafers, Ag was deposited at a rate ofapproximately 0.2 A/s at 10�6 Torr, followed by rapid thermalannealing (RTA) at 260 1C for 15 min in a N2 atmosphere to form theAg nanoparticles. The five solar cell samples after annealing werelabeled as S1-Ag2-RTA, S2-Ag4-RTA, S3-Ag6-RTA, S4-Ag8-RTA, andS5-Ag12-RTA. Because Xu [21] demonstrated that the dielectricproperties of the substrates had no effect on the resonancefrequency though they had significant effects on the spectral lineshape and here we only observed the SPRW of the Ag nanoparticleswith different diameters, we could only consider the differentthicknesses of Ag films regardless of the substrate and thus labeledall of the Si samples and glass samples with Ag film thicknesses of 2,4, 6, 8 and 12 nm as Ag2, Ag4, Ag6, Ag8, and Ag12.

The EQEs of the solar cell samples before and after thedeposition of the Ag films, and after RTA treatment were testedto observe the influence of Ag nanoparticles on GaAs thin film solarcells. Table 1 summarizes all of the solar cell label names, thecorresponding treatments and measurements.

3. Results and discussion

Fig. 2 shows the SEM images of five Ag-island films before and afterannealing in a N2 atmosphere at 260 1C for 15 min with the SEM data,which was analyzed using the software ImageJ [22] as shown inTable 2. Fig. 2 and Table 2 show that the particle size became largerboth before and after the annealing with increasing metal thickness.Moreover, Fig. 2(a)–(c) show that the distribution of Ag nanoparticlesafter annealing was relatively uniform. Their shapes were relativelyspherical, but in (d) a small percentage of the nanoparticles wereslightly elongated. When the particles size became larger, as inFig. 2(e), the distribution became even more nonuniform, and theirshapes were more like ellipsoid. In Table 2, the surface coverage Z ofAg nanoparticles before annealing was increased with increase inmetal thickness, but it was not so regular after annealing. Compared tothe Z before annealing, the Z of Ag nanoparticles after annealingdecreased, which means that the height of the average particlesincreased, as discussed in Ref. [11]. Table 2 also shows that both theaverage length of the shorter and longer axes of the Ag nanoparticles

Fig. 2. SEM images of Ag nanoparticles with different thicknesses on Si substrates before and after annealing in a N2 atmosphere at 260 1C for 15 min: (a) 2, (b) 4, (c) 6, (d) 8,

and (e) 12 nm.

Table 2The surface coverage and average length of the shorter and longer axis for all Ag particles formed on Si substrates with different thicknesses before and after annealing.

SEM data Before annealing After annealing at 260 1C for 15 min

Ag2 Ag4 Ag6 Ag8 Ag12 Ag2 Ag4 Ag6 Ag8 Ag12

ga 48.6 49.0 50.0 53.6 63.0 45.7 46.0 36.6 37.0 44.5

Dminb 13.6 16.6 16.0 18.0 38.8 18.5 20.6 23.3 30.5 45.0

Dmaxc 18.8 22.0 24.2 25.0 65.7 24.2 26.0 28.7 38.1 59.5

a Surface coverage (%).b The average length of shorter axis for all particles (nm).c The average length of longer axis for all particles (nm).

Fig. 3. The transmittance spectra for different thicknesses of Ag films deposited on

glass substrates (a) before annealing and (b) after annealing.

W. Liu et al. / Solar Energy Materials & Solar Cells 95 (2011) 693–698 695

for Ag2, Ag4, Ag6, and Ag8 increased after annealing. Although theDmax for Ag12 after annealing decreased compared to that for Ag12before annealing, the Dmin increased. That agrees well with Fig. 2.

Fig. 3 illustrates the transmittance spectra for different Agdeposited thicknesses on glass substrates (a) before annealing and(b) after annealing. A dip in the transmittance shows the resonanceposition, and it is clear that the resonance curves tend to be broadenedwith increasing Ag film thickness. By comparing Fig. 3(a) with (b), it isclear that the annealing induced a red-shift of the SPRW, whichdisagrees with Nakayama et al.’ [17] work. In Ref. [17], the blue-shiftwas attributed to the improvements in the symmetry of thenanoparticle shapes. Therefore, we assume that the red-shift of theSPRW after annealing in Fig. 3 can be explained by the increase ofthe nanoparticle size after annealing, which plays a more dominantrole in inducing a red-shift than the improvements in the symmetry ofthe nanoparticle shapes. Furthermore, in Fig. 3(a), the nanoparticlesyielded red-shifted SPRW with increasing metal thickness for solarcells without annealed Ag films, while the changes of SPRW were notobvious with increase in metal thickness in Fig. 3(b) for solar cells withannealed Ag films. The SPRWs of Ag4 and Ag12 after annealingexhibited a red-shift compared to the other samples. However, theSPRWs of Ag6 and Ag8 after annealing displayed a slight blue-shiftcompared to Ag4. For Ag12, the shape of the MNPs after annealing wasmore alike ellipsoid, as shown in Fig. 2(e), which means they have alarge aspect ratio, so the plasmon resonance can be lowered infrequency [4]. To explain the behavior of Ag6 and Ag8 further,we added three other Si samples and three glass samples withthicknesses of Ag films of 10, 14 and 16 nm, which were labeled asAg10, Ag14 and Ag16 (only considering the different thicknesses ofthe Ag films). The SEM images of the Ag nanoparticles on Si substrates,which are not shown here revealed that the nanoparticles tended tobe larger and more elongated for Ag14 and Ag16. Taking 12 nm as thecritical thickness, the SPRWs exhibited a red-shift with increasing Ag

W. Liu et al. / Solar Energy Materials & Solar Cells 95 (2011) 693–698696

film thickness, as shown in Fig. 3(b). However, the changes in theSPRWs of Ag6 and Ag8 were special. This behavior can be explained asfollows according to Ref. [23]: (I) Particle shape: the shape of thenanoparticles for Ag6 and Ag8 after annealing was still spherical, asshown in Fig. 2(c) and (d), so the SPRWs did not have the obvious red-shift seen in Ag12, Ag14 and Ag16. (II) Interparticle distance: thedistance between nanoparticles for Ag6 and Ag8 clearly increasedcompared to Ag2 and Ag4, as shown in Fig. 2(a–d), so the interactionbetween particles decreased, leading to broadening and a blue-shift ofthe SPRW. Curiously, the optical character of Ag10 exhibitedabnormal behavior, which needs further investigation.

Fig. 4 shows the EQEs of GaAs solar cells in our experiments. First ofall, we must consider the influence of annealing on the solar cell thatwas not coated by nanoparticles. Therefore, we selected one solar cellsample and treated it with RTA 260 1C for 15 min in a N2 atmosphereonly. As shown in Fig. 4(a), curve 1 is the EQE before annealing, and

Fig. 4. (a) The EQEs of a GaAs solar cell sample before (curve 1) and after annealing in N2 a

solar cell itself. (b)–(d) show the EQEs for different GaAs solar cell samples in our exper

annealing Ag films at 260 1C for 15 min in N2. The EQE curves excluding the influence o

curve 2 is the EQE after annealing. By comparing these two curves, it isclear that, after annealing the solar cell itself, the EQE became lower,which may be caused by the oxidization of Al0.8Ga0.2As window layerbecause it is exposed and in a highly heated state during RTA. In ourexperiments, all of the solar cell samples were taken from the samewafer, and thus the decrease of EQE after annealing the solar cell itselfshould be the same for all other samples. In the following EQEmeasurements, we compensated this decrease for the other GaAssolar cell samples. Fig. 4(b)–(f) show the EQEs of S2, S4, S6, S8 and S12before evaporating the Ag films, after evaporating the Ag films andafter annealing the Ag films. To eliminate the influence of annealingon the solar cell itself, we corrected all of the EQE curves tested afterannealing using the data in Fig. 4(a) and labeled the associated EQEcurves as CorrS1-Ag2-RTA, CorrS2-Ag4-RTA, CorrS3-Ag6-RTA, CorrS4-Ag8-RTA, and CorrS5-Ag12-RTA, as shown in Fig. 4(b)–(f). From theseresults, it is obvious that the EQEs of all solar cells became lower after

t 260 1C for 15 min (curve 2), which confirms the influence of annealing on the GaAs

iments with different surface conditions: untreated, evaporated Ag films, and after

f RTA on the solar cell itself are also shown in (b)–(d).

Fig. 6. Photovoltaic I–V curves for S3, S3-Ag6-RTA and CorrS3-Ag6-RTA, which

excludes the influence of RTA on the solar cell itself under one-sun illumination

(AM1.5, 100 mW/cm2) using an Oriel solar simulator (Model 94041A).

W. Liu et al. / Solar Energy Materials & Solar Cells 95 (2011) 693–698 697

evaporation of the Ag films, while the EQEs of all solar cells haddifferent degrees of enhancement after annealing. Compared to theuntreated solar cells, all the EQEs of the cells after annealing the Agfilms became larger or equal at longer wavelengths but lower at shortwavelengths because the scattered field and transmitted plane werenot in phase at wavelengths shorter than the SPRW, which resulted indestructive interference. However, at wavelengths longer than theSPRW, there was constructive interference between the scattered andtransmitted field components [7]. Moreover, in Fig. 4(b)–(d), it is clearthat the EQE of S3 had the largest improvement at longer wavelengthsafter annealing the 6 nm Ag film, and the decrease at shorterwavelengths was the lowest. To make this result more visible, thecorrected EQEs of all solar cell samples after annealing Ag films werenormalized by the corresponding untreated solar cells, as shown inFig. 5.

Fig. 5 shows that S3 showed the largest improvement afterannealing the Ag film in our experiment, which can be explained asfollows: (I) The Ag nanoparticles on the S3-Ag6-RTA surface hada greater scattering cross section than the Ag nanoparticles onS1-Ag2-RTA and S2-Ag4-RTA as their larger diameters [5]. (II) TheSPRW of S3 after annealing exhibited a blue-shift, so thewavelength region of which the scattered field and transmittedplane were not in phase decreased. (III) When the size of thenanoparticles was beyond certain limits, it led to increasedretardation effect and higher order multipole excitation modes,which eventually reduced the scattering efficiency [10]. Therefore,the Ag film with a thickness of 6 nm was the optimum value in ourexperimental conditions.

Furthermore, to make the influence of SPR on GaAs thin filmsolar cells clearer, the current density versus voltage (I–V) char-acteristics of S3 were tested with and without Ag nanoparticles onits surface under a 100 mW/cm2, AM1.5 spectrum by an Oriel solarsimulator (Model 94041A), as shown in Fig. 6. The test results showthat, compared to S3, the open circuit voltage (Voc) of S3-Ag6-RTAslightly decreased from 0.662 to 0.643 V, and the short circuitcurrent density (Jsc) of S3-Ag6-RTA decreased from 9.42 to8.74 mA/cm2 because the deterioration due to RTA of the solarcell itself was not excluded. However, the fill factor (FF) of S3-Ag6-RTA increased from 68.8% to 70.5%. After adjusting the results byexcluding the RTA deterioration of the solar cell itself, the shortcircuit current increased, as shown in Fig. 6, which was inagreement with the EQE data. Moreover, the short circuitcurrent (JL) can directly be calculated from the data of one sunsolar spectrum under AM 1.5G (air mass 1.5 global) and the testdata of EQEs for S3 and CorrS3-Ag6-RTA as follows:

JL ¼ ðe=hcÞ

Z lm

0IAM1:5ðlÞ�l�EQEðlÞdl

Fig. 5. Normalized EQEs for the GaAs solar cells with the label names: CorrS1-Ag2-

RTA, CorrS2-Ag4-RTA, CorrS3-Ag6-RTA, CorrS4-Ag8-RTA, and CorrS5-Ag12-RTA,

corresponding to the samples described in Fig. 4.

where IAM1.5(l) (W/m2/nm) is the incident solar power densityunder AM 1.5G, h is Planck’s constant, e is the electron charge, and c

is the speed of light in vacuum. According to this calculation, thephotocurrent density of CorrS3-Ag6-RTA increased, although thecalculated value was 14.2% greater than that of S3.

Although the enhancement of the photocurrent density in ourexperiments was smaller than the enhancement of solar cells withantireflection coatings (the JL of the additional solar cell sampleafter the deposition of double-layer antireflection coatings ofTa2O5/SiO2 increased more than 30%), our experiments provide aconvenient method to improve the light trapping in thin film GaAssolar cells. More importantly, as now some new SPR applicationshave been used in solar cells, such as the plasmonic back structureon solar cells [9,24–26] and MNPs placed between two absorbinglayers to increase the efficiency of tandem solar cells [27,28], ourstudy provides some foreshadowing of further research on SPRs inGaAs solar cells.

4. Conclusions

We performed a preliminary investigation into the effects ofsurface plasmons in thin film GaAs solar cells by using Agnanoparticles obtained via a simple method involving the anneal-ing of metal films. Among the five GaAs solar cell samples, the shortcircuit current of the solar cell with 6 nm Ag film after annealingachieved the highest improvement compared to the untreatedsolar cell. Further calculations using the EQE data for S3 andCorrectS3-Ag6-RTA showed that the short circuit current densitywas increased by 14.2% over that of the untreated solar cell. Usingthe transmittance plots of the Ag nanoparticles on glass substratesand the SEM images of Ag nanoparticles on Si substrates, the reasonthat the 6 nm Ag film was the optimum thickness in our experi-mental conditions was analyzed. The SPRW does not undergo asimple red-shift under the same annealing conditions with increas-ing metal thickness. It depends on the shape of the MNPs and theinterparticle distance. Therefore, it is important to select theoptimum thickness of the metal film to obtain the largest lighttrapping of the solar cells. Our study provides a preliminary studyon the influence of surface plasmons on GaAs solar cells.

Acknowledgements

The authors greatly acknowledge the support from the NationalBasic Research Program of China (973 Program) under grant

W. Liu et al. / Solar Energy Materials & Solar Cells 95 (2011) 693–698698

number 2010CB933800 and the National Natural Science Founda-tion of China under grant number 60606024.

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