Solar Energy Materials & Solar Cells 102 (2012) 44–49

Contents lists available at SciVerse ScienceDirect

Solar Energy Materials & Solar Cells

0927-02

http://d

n Corr

E-m

manasr

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

Surface plasmon enhanced intermediate band based quantum dots solar cell

Jiang Wu a,n, Scott C. Mangham a, V.R. Reddy a, M.O. Manasreh a,n, B.D. Weaver b

a Department of Electrical Engineering, University of Arkansas, Fayetteville, AR 72701, USAb Naval Research Laboratory, Code 6816, 4555 Overlook Ave, SW, Washington, DC 20375, USA

a r t i c l e i n f o

Article history:

Received 5 October 2011

Received in revised form

25 March 2012

Accepted 28 March 2012Available online 14 April 2012

Keywords:

Surface plasmon

Quantum dot

Solar cell

Intermediate band

Metallic nanoparticles

48/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.solmat.2012.03.032

esponding authors. Tel.: þ1 479 575 5444; fa

ail addresses: jxw009@uark.edu (J. Wu),

eh@uark.edu (M.O. Manasreh).

a b s t r a c t

Surface plasmon enhancement effect is investigated in InAs/GaAs quantum dots solar cell. A nontrivial

enhancement was observed in the photocurrent, quantum efficiency, and the spectral response of the

solar cell. The surface plasmon was generated in gold or silver nanoparticles synthesized by a chemical

reduction method. The coupling of the metallic nanoparticles to the surface of the solar cell was

achieved by utilizing dithiol ligands in conjunction with thermal annealing process. The enhancement

was observed in the entire spectral range covered by the solar cell (visible and near infrared spectral

regions). This enhancement is attributed to photon scattering and trapping by the surface plasmon

generated in the metallic nanoparticles.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Solar energy harvesting has been under intensive researchactivities in recent years. Among various photovoltaic technolo-gies, intermediate band in solar cells is a promising approach andreported to achieve a power conversion efficiency on the order of�60% [1,2]. Quantum dots with three-dimensional confinementare reported as an example for the high efficiency intermediateband solar cells [3]. Even though intermediate band assistedmulti-band transitions has been theoretically demonstrated toachieve significant enhancement in power conversion efficiency,only partial successes have been demonstrated experimentallyfrom quantum dots based intermediate band solar cells. Martıet al. [4] observed photocurrent produced from two-photonabsorption process in an InAs quantum dot solar cell. Hubbardet al. [5] and Laghumavarapu et al. [6] demonstrated improvedphotocurrent in quantum dot solar cells with strain-compensa-tion layers. By optimizing InAs coverage and strain-balancelayers, Bailey et al. [7] achieved enhanced photocurrent whileobtained an open circuit voltage near 1.0 V. Despite of thesereported successes, the photocurrent generated from subbands isvery small (�1% of the current generated from the interband) [4]in the quantum dots solar cells. It is critical to achieve practicalintermediate band solar cells where the current is generated

ll rights reserved.

x: þ1 479 575 7967.

through the subband energy levels and can be substantiallyenhanced while output voltage is preserved [8].

Numerous efforts have been devoted to surface plasmonenhanced solar cell using metal nanostructures, which hold thepromise to achieve broadband full spectrum absorption enhance-ment in solar cells [9–11]. In particular, metallic nanoparticleshave been proposed to enhance absorption in quantum dots forpractical intermediate band solar cells [12]. The photon scatteringby near field potential in metallic nanoparticles in close vicinity ofquantum dots was reported an enhancement of about two ordersof magnitude [12]. It is a challenging problem to couple metallicnanoparticles to InAs quantum dots matrixed in GaAs p–njunction by using thin film technologies. However, there areseveral ways to incorporate surface plasmon in photovoltaic cellsfor light trapping and scattering from metallic nanoparticles atthe solar cell surface [10]. This coupling is an attractive approachdue to its simplicity and compatibility of large scale production.

In this work, we report on experimental measurements on thesurface plasmon enhancement effect in InAs quantum dots solarcell with the emphasis on the photocurrent, quantum efficiency,spectral response, and the power conversion efficiency.

2. Experiment

Gold and silver metallic nanoparticles were investigated asbeing the source surface plasmon. These nanoparticles weresynthesized by using the reduction method as schematicallyshown in Fig. 1(a) and (b) shows the absorption spectra of theAu and Ag nanoparticles dispersed in toluene, which were

300 400 500 600 700 8000.0

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orba

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(a.u

.)

Wavelength (nm)

Au NPs in Toluene

Ag NPs in Toluene

Aqueous solutionof metal salt

Add phasetrans fer reagent

Addreducingagent

Extract

Fig. 1. (a) Schematic presentation of metallic nanoparticle synthesis. (b) Absorption spectra of Au and Ag nanoparticles dispersed in toluene. SEM images of Au and

Ag nanoparticles deposited on GaAs surfaces by using dithiol linkers. (c) as deposited Au nanoparticles. (d) Au nanoparticles after annealing for 10 min at 170 1C in vacuum.

(e) as deposited Ag nanoparticles. (f) Ag nanoparticles after annealing for 10 min at 170 1C in vacuum. The scale bars in the SEM images are 1 mm.

J. Wu et al. / Solar Energy Materials & Solar Cells 102 (2012) 44–49 45

measured by using Cary 500 UV–vis-NIR spectrophotometer. TheAu and Ag nanoparticles show peaks in the spectra at 527 nm and423 nm, respectively. These peaks confirm the presence of surfaceplasmons. The observation of these plasmonic peaks is necessarybefore proceeding with next step of coupling the metallic nano-particles to the surface of the solar cell.

In general, the direct deposition of metallic nanoparticles onthe surface of semiconductors lacks the control over the densityand uniformity of the metallic nanoparticles. Before coupling themetallic nanoparticles to the surface of the GaAs top contact layerof the InAs quantum dots solar cell, the surface was chemicallyetched by using HCl for 2 min to remove native oxide and thentreated with organic dithiols. The dithiol molecules have two –SHfunctional groups and the –SH functional group adsorbs readily ongold, silver, and GaAs surfaces [13–15].The dithiol molecules act aschemical linkers (ligands) between the metallic nanoparticles andthe solar cell surfaces. For surface functionalization, 0.5 ml of1,3-propanedithiol was dissolved in 10 ml of ethanol and stirredthoroughly to form a homogeneous solution. The freshly etchedquantum dot solar cell was immersed in propanedithiol solutionfor 5 h. After dithiol functionlization, the surface was thoroughlywashed with ethanol to remove excess dithiol. Once, the solar cellwas dried completely with nitrogen gas, it was immersed in eitherthe gold or the silver nanoparticles solution for over 1 h. Then thedevices were annealed at 170 1C in a rough vacuum to remove thechemical linkers and to form a strong physical intimate couplingbetween the metallic nanoparticles and the solar cell surface.Fig. 1(c), (d), (e), and (f) show the scanning electron microscope(SEM) images of the metallic nanoparticles deposited on GaAssurfaces before and after thermal annealing in vacuum. The

nanoparticle diameter is measured to be less than 5 nm for Auand �10 nm for Ag. However, the surface morphology of Au andAg nanoparticles is quite different: Au nanoparticles show goodmonolayer coverage on GaAs surface as shown in Fig. 1(c) whileAg nanoparticles have a more random distribution as shown inFig. 1(e). The random distribution of Ag nanoparticles may be dueto their large size which results in partial anchoring of the Agnanoparticles over GaAs surface. In order to have favorable lightscattering, large size nanoparticles are preferred [9,12]. In additionto the removal of dithiol linker molecules, the thermal annealingin vacuum has caused coalescence of these closely packed metallicnanoparticles and created larger metallic nanoparticles, as shownin Fig. 1(d) and (f).

The quantum dots solar cell structure was grown on a pþ-GaAs(100) substrates by molecular beam epitaxy technique. The solar cellstructure begun with a 500 nm lightly doped p-type GaAs buffer.Following the buffer, a 1 mm thick GaAs base region was grown withZn doped to 1�1017 cm�3. Subsequently, the substrate was cooleddown for growth of InAs quantum dots. The growth of quantum dotswas accomplished by deposition of 2 monolayer (ML) InAs usingtraditional Stranski–Krastanov growth mode. The InAs quantum dotswere capped with 5 nm GaAs quantum well and sandwiched by 4 MLAl0.3Ga0.7As fence barriers in order to eliminate charge trapping [16].The quantum dot layers were then followed by 20 nm GaAs spacerand repeated for another nine periods. The InAs quantum dot layerswere doped with [Si]¼1�1017 cm�3. After ten periods of quantumdot layers, another 30 nm undoped GaAs spacer was grown and theemitter was formed by depositing 100 nm n-GaAs with [Si]¼2�1018 cm�3 and 30 nm p-Al0.8Ga0.2As with [Si]¼2�1018 cm�3.Finally, a 50 nm nþ-GaAs top contact layer was grown with Si doped

Metal Nanoparticles

InAs Quantum Dot Solar Cell

X10

50 nm n+ GaAs30 nmn AlGaAs100 nm n GaAs30 nm i GaAs20 nm i GaAs4 ML i AlGaAs5 nm i InGaAs2 ML i InAs QD4 ML i AlGaAs1000 nm p GaAs Base500 nm p GaAs Bufferp+ GaAs substrate

0 10 20 30 40 500

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ber

of P

artic

les

Diameter (nm)

0 50 100 150 2000

5

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of P

artic

les

Diameter (nm)

Fig. 2. (a) Schematic of metallic nanoparticles attached to a quantum dot solar cell surface with 1,3-propanedithiol. The quantum dot solar cell structure is also given

on the left. (b) SEM images of Au deposited on quantum dot solar cell surface after thermal annealing. (c) Size distribution of Au nanoparticle in (b). (d) SEM images

of Ag deposited on quantum dot solar cell surface after thermal annealing. (e) Size distribution of Ag nanoparticle in (d). The scale bar is 400 nm in (b) and 1 mm in (d).

J. Wu et al. / Solar Energy Materials & Solar Cells 102 (2012) 44–4946

to 5�1018 cm�3. Standard lift-off procedures were used to fabricatesolar cells with size of 3�3 mm2. The bottom contact metallizationconsisted of 70 nm AuGe/20 nm Ni/150 nm Au and the top contactconsisted of 200 nm AuZn/100 nm Au. All metals were deposited byan Edward 306 electron beam evaporator and rapid thermal annealedin N2 ambiance. The device structure and a schematic of nanoparticlemodified cell are presented in Fig. 2(a). The metallic nanoparticleswere deposited on the solar cell surfaces by using the same dithiollinker procedures. The solar cells modified with Au and Ag nanopar-ticles are referred as sample A and B, respectively. The SEM images ofmetallic nanoparticles on quantum dot solar cell surface after thermalannealing and their size distributions are show in Fig. 2(b), (c), (d),and (e). The Au nanoparticles show relatively uniform distributionwith an average particle diameter around 10 nm. On the other hand,Ag nanoparticles demonstrate a large diameter distribution from 30to 150 nm. The current density–voltage (I–V) characteristics with andwithout nanoparticles were measured using a Keithley 4200 semi-conductor parameter analyzer and a Newport AM1.5 Global solarsimulator (300 mW/cm2). Quantum efficiency measurements wereperformed using a Newport-Oriel IQE 200 system. The photoresponsespectra of solar cells were measured using a Bruker IFS 125HRFourier-transform infrared spectrometer with a broadband visible-near infrared source. A low-noise current preamplifier, StanfordResearch System SR570, was interfaced with the spectrometer.

3. Results and discussion

By depositing the metallic nanoparticles on solar cell surfaces,the photocurrent was notably improved. Fig. 3(a) and (b) show theI–V characteristics of quantum dots solar cell before and after thecoupling of the metallic nanoparticles.. The short circuit currentdensity increases from Jsc¼56.0 mA/cm2 for the unmodified to67.2 mA/cm2 for device A, which is coupled to Au nanoparticles.Similarly the short circuit current density increases from Jsc¼

55.6 mA/cm2 for the unmodified to 64.4 mA/cm2 for modifieddevice B with Ag nanoparticles. The metallic nanoparticle modifica-tion increases the power conversion efficiency from Z¼8.0% to 9.5%for device A. Similarly, the power conversion efficiency increases

from Z¼8.0% to 8.9% for device B. However, after introducingmetallic nanoparticles, the fill factor (FF) was reduced from 0.535to 0.52 for device A and from 0.536 to 0.51 for device B. This slightreduction in the FF may be due to the increase of surface sheetresistance caused by the chemical linkers. The open circuit voltageof �0.81 V remains almost unchanged and therefore the enhance-ment in the solar power conversion is mainly due to the improve-ment in short circuit current. The enhancement however, isattributed to the incident light scattering and subsequent internallight trapping by the localized surface plasmon [17]. The surfaceplasmon generated in the metallic nanoparticles can significantlyenhances light scattering with scattering cross section many timeslarger than the geometrical cross section of the particles [18,19].

External quantum efficiency (EQE) as well as the photo-response measurements were made to investigate the spectralresponse of the solar cell and to confirm the plasmonic effects onthe absorption enhancement after coupling the metallic nano-particles to the solar cell surfaces. Fig. 3(c) and (d) show the EQE

measurements from 350 nm to 1200 nm for devices A and Bbefore and after nanoparticles deposition The enhancement in theEDE occurs across the entire wavelength range (500–800 nm). Theenhancement appears to be mainly from the GaAs p–n junctionand there is no observable enhancement from the InAs quantumdots at wavelength longer than 870 nm. This is due to the factthat EQE measurement is undertaken under monochromic lightwithout bias light. The photocurrent generated from InAs quan-tum dots arise from two groups of optical transitions. The firstgroup is from InAs hole energy levels to InAs conduction bandelectron energy levels and the second group from electron energylevels to GaAs continuum. In order to generate photocurrent, bothtransitions have to simultaneously occur in the quantum dots.The photoresponse to a broadband illumination would be insight-ful to investigate the effects of metallic nanoparticles, which willbe discussed in the following section. The EQE of device Aincreases by 21% overall after coupling the device to the Aunanoparticle, while the EQE of device B increases by 15% overallafter coupling the device to the Ag nanoparticles.

In order to further understand the EQE spectral improvement,the spectra obtained after coupling the devices to the metallic

20

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%)

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%)

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Fig. 3. J–V characteristics of quantum dot solar cells: (a) sample A before and after Au nanoparticle deposition; (b) sample B before and after Ag nanoparticle deposition.

External quantum efficiency spectra of quantum dot solar cells. (c) sample A before and after Au nanoparticle deposition. (d) sample B before and after Ag nanoparticle

deposition.

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Fig. 4. EQE ratio between as-is cells and cells after metallic nanoparticle deposi-

tion. The half-filled blue circles are the ratio for sample A with Au nanoparticle

modification and the red squares are for sample B with Ag nanoparticle

modification. (For interpretation of the references to color in this figure legend,

the reader is referred to the web version of this article).

J. Wu et al. / Solar Energy Materials & Solar Cells 102 (2012) 44–49 47

nanoparticle are overlaid on the spectra of the pristine devices asshown in Fig. 4. Both Au and Ag nanoparticles deposition show asimilar broadband enhancement. However, device A exhibits astronger enhancement as compared to device B. Similar behavioris also observed in the I–V measurements. It is worth noting thatthe Ag nanoparticle density is much lower than Au nanoparticledensity. The estimated nanoparticle density on devices A and Bare 1.4�1012 cm�2 and 2.1�1011 cm�2, respectively. Althoughnear density of the Ag nanoparticles is about an odred ofmagnitude smaller the the density of the Au nanoparticles, the

enhancement in solar cell EQE is comparable in both devices. Thisobservation is not surprising because of the following facts: First,Ag exhibits a higher efficiency of plasmon excitation over Au [20]and second, extinction cross-section for Ag nanoparticles is largerthan that of the Au nanoparticles under the same conditions[20,21]. Furthermore, the Ag nanoparticles prepared in this studyhave a much larger diameter than Au nanoparticles, as shown inFig. 2, and thus Ag nanoparticles have larger dipole scatteringcross section. The enhancement in the EQE spectra exhibitwavelength-dependency. The maximum enhancement is locatedat around 640 nm for device A and around 610 nm for device B.The lowest enhancement is observed for both devices A and B,which is due to destructive interference between the scatteredand incident fields reduces the magnitude of the transmitted fieldbelow localized surface plasmon resonance. [17,22].

Mie theory can be invoked to qualitatively interpret theobserved plasmon enhancement in quantum dots solar cell fromscattering with metallic nanoparticles. From Mie theory, the totallight extinction consists of scattered light and the absorption andthe corresponding absorption and scattering cross sections Cabs

and Csca are calculated based on the polarizability parametera and can be written as [23]

Cabs ¼2pl

Im a½ �pr3 ð1Þ

Csca ¼1

6p2pl

� �4

aj j2pr6 ð2Þ

where l is the light wavelength and r is the particle radius. Atplasmon resonance, polarizability parameter a is maximum andnanoparticles have maximum extinction cross-section near plas-mon resonance frequency. From the above equation the scatteringproperty of nanoparticles is strongly dependent on the particlesize. In the electrostatic approximation limit, metallic nano-particles carry a single dipole mode, the Mie extinction coefficient

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Phot

ores

pons

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PL at 300 K

GaAs

1

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ores

pons

e (a

.u.)

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Fig. 5. Photoresponse spectra of samples A and B before and after metallic

nanoparticle depositions. The green curve is the photoluminescence spectrum of

the quantum dot solar cell structure measured at 300 K. Each number in the figure

represents a peak observed in the photoresponse spectra with the arrow indicat-

ing the peak position. The full spectrum photoresponse spectra are included in

the inset.

J. Wu et al. / Solar Energy Materials & Solar Cells 102 (2012) 44–4948

mainly consists of absorption. With increasing diameters, thescattering cross section becomes dominate. This again explainswhy sample B with low density Ag nanoparticles shows compar-able enhancement with sample A with high Au nanoparticles. TheEQE enhancement ratio peaks at 640 nm (Au) and 610 nm (Ag) arered-shifted from absorption peaks of colloidal nanoparticle solu-tions as shown in Figs. 1 and 4. The discrepancy may result fromthe fact that toluene is the surrounding dielectrics of colloidalnanoparticles. In reality, the nanoparticles are placed in theinterface between GaAs and air, which results in maximumscattering at longer wavelengths. The inter-particle interactionof the high density nanoparticle arrays may also modify the idealMie scattering model. Moreover, the thermal annealing hasmodified the nanoparticle morphology as shown in Fig. 2. Theincrease of nanoparticle size and shape factor after annealing alsoleads a red-shift of plasmon resonance peaks.

So far, the effects of surface plasmon have only been seen on thewavelength region where optical transitions carried out in GaAs. Inorder to probe the plasmon effects on InAs quantum dots solar cellswere measured at room temperature to obtain zero-bias photo-current response. Fig. 5 shows the photoresponse spectra of bothsample A and B before and after coupling the the metallic nano-particle.. These photoresponse measurements reveal additionalphotocurrent generated from InAs quantum dots. In the nearinfrared region, the photoresponse of the quantum dot solar cellscovers from 870 nm to nearly 1200 nm below GaAs bandgap. Aphotoluminescence spectrum was also plotted in Fig. 5 where twopeaks are observed in the spectral region of 1050–1100 nm. Thepeaks labeled 1, 2 and 3 observed in the photoresponse spectrumare related to interband transitions within the InAs quantum dots.However, transitions related to the wetting layer can not be ruledout. After coupling the metallic nanoparticles, the photoresponsespectra of both devices exhibit a small enhancements, which wouldbe expected as the metallic nanoparticles are not optimized at thiswavelength region. In addition, the growth quality of the materialsfrom which the devices were fabrciated may also contribute to thelow intensity spectral response in the mid-infrared spectral regionbecause of the following reasons. First, the density of quantum dotsis generally low (1010–1011 cm�2). Second, the strained quantum

dots along with Al-containing barrier may introduce various types ofdefects. Interestingly, the photoresponse spectra of both devicesshow a blue-shift after the metallic nanoparticle coupling (about�6 nm for Au nanoparticles and �3 nm for Ag nanoparticles). Theshift may be caused by plasmon exciton coupling in quantum dotsthrough incoherent and coherent interactions between the metallicnanoparticles and the InAs quantum dots [24]. This leads to theconclusion that the photocurrent enhancement in the InAs/GaAsquantum dot solar cell may involve both scattering and local fieldenhancement effects [25]. The photocurrent from InAs quantumdots is still weaker comparing to the photocurrent from the p–njunction GaAs, as shown in the inset of Fig. 5. In order tosignificantly enhance the photocurrent from the InAs quantum dots,the solar cell as well as metallic nanoparticle structures have to beoptimized by considering the following two ideas. First, the metallicnanoparticle need to be deposited in the close vicinity of the InAsquantum dots. This could be achieved by using a thin emitterquantum dot solar cell. Second, the plasmon resonance energy needto be tuned to match the transitions in the quantum dots in order toobtain a strong coupling and near field absorption enhancement.

4. Conclusions

In conclusion, Au and Ag nanoparticles were coupled throughsurface functionalization of InAs/GaAs quantum dots solar cell,which lead to an overall power conversion efficiency enhance-ment from 8.0% to 9.5% for Au nanoparticles and to 8.9%, for theAg nanoparticles. Both Au and Ag nanoparticles have significantlyenhanced the photocurrent and quantum efficiency of the solarcells. The metallic nanoparticles have served as efficient lightscatters and resulted in a broadband absorption enhancement.The interaction between quantum dots and plasmon at nearinfrared wavelength region was observed, which suggests thatlocalized surface plasmon could be employed to enhance theperformance of the quantum dots solar cells and hence realizepractical intermediate band solar cells.

Acknowledgment

This work was supported by the Air Force Office of 199Scientific Research (Grant no. FA9550-10-1-0136), the NSF-EPS-CoR program (Grant no. EPS-1003970), and NASA (Grant no.242026-1BBX11AQ36A).The work at the Naval Research Lab wassupported by a grant from the Naval Research Office.

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