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Layered semiconductor molybdenum disulfide nanomembrane based

Schottky-barrier solar cellsMariyappan Shanmugam, Chris A. Durcan and Bin Yu*

Received 21st August 2012, Accepted 27th September 2012

DOI: 10.1039/c2nr32394j

We demonstrate Schottky-barrier solar cells employing a stack of layer-structured semiconductor

molybdenum disulfide (MoS2) nanomembranes, synthesized by the chemical-vapor-deposition method,

as the critical photoactive layer. An MoS2nanomembrane forms a Schottky-barrier with a metal

contact by the layer-transfer process onto an indium tin oxide (ITO) coated glass substrate. Two

vibrational modes in MoS2nanomembranes, E12g(in-plane) and A1g(perpendicular-to-plane), were

verified by Raman spectroscopy. With a simple stacked structure of ITOMoS2Au, the fabricated

solar cell demonstrates a photo-conversion efficiency of 0.7% for110 nm MoS2 and 1.8% for 220 nm

MoS2. The improvement is attributed to a substantial increase in photonic absorption. The MoS2nanomembrane exhibits efficient photo-absorption in the spectral region of 350950 nm, as confirmed

by the external quantum efficiency. A sizable increase in MoS2 thickness results in only minor change in

MottSchottky behavior, indicating that defect density is insensitive to nanomembrane thickness

attributed to the dangling-bond-free layered structure.

Introduction

Schottky-barrier solar cells have the appealing advantage of low-

cost manufacture attributable to their simple and versatile

fabrication process and thin-film-like structure (reduced active

material consumption as compared with pn or pin junctionbulk semiconductor solar cells). Schottky-barrier solar cells rely

mostly on a single semiconductor material (either p-or n-type

doped) in association with a metal, leading to energy barrier

formation and carrier dissociation at the metalsemiconductor

(MS) interface. Various MS material systems have been used

in Schottky-barrier solar cells, including Alamorphous carbon

nitride,1 Au, Al, Mg and CaPbSe quantum dots,2 AlSnS,3 and

AuGaAs.4 Recently, carbon derivatives have attracted attention

due to their promising optical/electrical properties. Schottky

junctions made by carbon nanotubeCdSe,5 grapheneCdS

nanowire,6 graphene, carbon nanotubeSi,7,8 and grapheneSi

nanowire911 have been reported. In general, MS interfacial

behavior and carrier mobility/photon absorption in the semi-conductor material are key factors that impact the overall

performance of Schottky-barrier solar cells. While a group of

semiconductors such as boron doped diamond,12 GaAs,13,14

InP,15 SiGe,16 GaP,17 GaN,18 and SiC19 can be used, the design of

an MS system with appropriate material work function differ-

ence is an essential factor to determine the Schottky-barrier

height and efficiency of carrier separation.

The development of Schottky-barrier solar cells has been

decelerated over the past decade due to several fundamental

material challenges, including interfacial oxide layer formation

and Fermi-level pinning caused by interfacial states (which are

inevitable at MS interfaces).2024

While poor interface properties play a substantial role in theimpeded improvement of energy-conversion efficiency, searching

for a new class of semiconductor materials free of interfacial

states (hence without Fermi level pinning) would be of strategic

significance to the ultimate realization of highly efficient

Schottky-barrier solar cells.

Transition-metal dichalcogenide semiconductors exhibit two

dimensional (2D) layered lattice structures with strong in-plane

covalent bonds and weak interlayer van der Waals interaction.

Molybdenum disulphide (MoS2), a layered semiconductor, has

recently attracted attention in the fields of nanoelectronics,2530

energy storage3133 and photovoltaics.34 Nanosheets of monolayer

or multilayer MoS2can be obtained via mechanical or chemical

exfoliation,35,36similar to theway graphene is produced.Recently,direct assembly of MoS2 nanosheets on SiO2 by the chemical-

vapor-deposition (CVD) method has been demonstrated.37 MoS2exhibits direct energy bandgap (1.85 eV) in monolayer and

indirect bandgap (1.3 eV) in multilayer/bulk form, respec-

tively.38 On electronic properties, it has beenrecentlyreportedthat

MoS2 shows carrier mobility values of 200 cm2 V1 s1 in

monolayer and 517 cm2 V1 s1 in a few layers, suggesting

promising conducting capacity.39,40 Most importantly, the unique

structure of layered semiconductors provides inherent advantages

of forming a smooth surface free of any dangling chemical bonds,

potentially enabling the design of highly efficientSchottky-barrier

College of Nanoscale Science and Engineering, State University of NewYork, Albany, NY-12203, USA. E-mail: [email protected]; Fax: +1 518956-7492; Tel: +1 518 956-7492

This journal is The Royal Society of Chemistry 2012 Nanoscale, 2012, 4, 73997405 | 7399

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solar cells. Various metal contacts, including Ag, Al, Au, Co and

Fe, have been investigated for the formation of Schottky-barriers

with MoS2 bulk crystals. No Fermi level pinning was observed

even for a high density of material defect (1014 cm3).41

In this paper we demonstrate solar cells employing a layer-

structured semiconductor MoS2 nanomembranemetal (Au)

Schottky barrier. The CVD-assembled MoS2 nanomembrane

also functions as the key photo-active layer. The solar cells are

made by a simple stack structure of ITOMoS2Au. Basicmaterial properties of MoS2, major solar cell device performance

metrics, and the impact of MoS2 nanomembrane thickness are

discussed. The MoS2Au Schottky-barrier interface is charac-

terized by capacitance vs. voltage (CV) measurements.

Experimental section

Large-area (up to several square centimeters) semiconductor

MoS2nanomembranes were synthesized by the CVD approach.

Fig. 1(a) is the schematic of the CVD growth furnace system

showing Ar as a carrier gas and a sulphur (S) source flowing

towards a molybdenum (Mo)-coated SiSiO2 substrate (where

vaporsolid reaction occurs to assemble the MoS2 nano-

membrane). CVD growth of MoS2silicon coated with 100 nm of

SiO2 was used as the substrate for the pre-deposition of Mo. A

thin Mo metal layer was deposited with an approximate thick-

ness of 50 nm by the electron-beam evaporation method. The

CVD growth process started with flowing Ar of 200 sccm in thequartz-tube chamber with a pressure level of 1 Torr. The

furnace was subsequently heated up to 750 C and stabilized over

one hour. Pure S powder, placed upstream in the chamber, was

heated to just above its melting temperature (115 C). The S

vapor flowed over the Mo thin film pre-deposited on the SiO 2Si

substrate for approximately 30 minutes. Vaporsolid reaction

took place at an elevated temperature (750 C), leading to the

growth of an MoS2 nanomembrane driven by increase in

enthalpy. While the MoMo bond is relatively weak (metallic in

Fig. 1 (a) Schematic of theCVD growthsetupshowingthe reaction of sulphur vaporwith Mo filmpre-depositedon SiSiO2 substratefor thegrowth of

an MoS2nanomembrane. (b) Major process steps involved in the synthesis of the MoS2nanomembrane, layer transfer, and subsequent fabrication of a

Schottky-barrier solar cell. The KOH wet etching process was used to detach the MoS 2 nanomembrane from SiO2Si substrate. (c and d) Partially

floating MoS2nanomembrane in KOH solution. (e) Free-floating MoS2nanomembrane in KOH to be transferred onto an ITO-coated glass substrate

for solar cell fabrication.

7400 | Nanoscale, 2012, 4, 73997405 This journal is The Royal Society of Chemistry 2012

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nature), the MoS bond is a strong ionic one and therefore more

thermodynamically stable. As the temperature increases, Mo

atoms gain sufficient thermal energy (exceeding the activation

energy) and make covalent bonds with S atoms. After the

completion of CVD growth, the sample was annealed at 1000 C

for one hour to improve the crystalline quality and the unifor-

mity of the resultant nanomembrane. The major steps in the

MoS2 layer transfer sequence are depicted in Fig. 1(b) starting

from the SiSiO2substrate, Mo metal deposition, MoS2growth,and transfer to the target substrate in which the solar cells were

fabricated.

The CVD-assembled MoS2 nanomembrane was transferred

from the growth substrate (SiO2Si) to an indium tin oxide (ITO

serving as transparent electrode) coated device substrate,

involving a wet chemistry etching step of the SiO2 layer in the

growth substrate to yield a freestanding MoS2nanomembrane in

potassium hydroxide (KOH) solution. Fig. 1(c)(e) show the

optical images of the process to separate the MoS2 nano-

membrane from SiO2Si substrate. The MoS2 nanomembrane

was transferred onto the transparent conducting oxide (ITO)

substrate to fabricate the Schottky-barrier solar cell. KOH

solution with a concentration of 15 M was prepared in which theMoS2SiSiO2substrate was immersed to etch away the SiO2. A

slow and smooth KOH etching of SiO2 released the MoS2nanomembrane partially from the Si substrate. Within approx-

imately 10 minutes a completely free-floating MoS2 nano-

membrane in KOH solution was obtained. The thickness of the

MoS2 nanomembrane is determined by that of the Mo film

deposited by electron beam evaporation (it is hence scalable).

The surface morphology and thickness of the as-grown MoS2nanomembrane was characterized by Atomic Force Microscopy

(AFM, dimension 3100). Raman spectroscopy (Horiba Scien-

tific) with a laser of 532 nm was performed on the samples to

confirm the growth of MoS2 via identified vibrational modes.

After transferring the MoS2 nanomembrane onto the ITO-coated glass substrate, deposition of 50 nm of Au contact was

made by electron-beam evaporation to form a Schottky-barrier

(with MoS2). The JV characteristics of the solar cell were

measured under both dark and standard AM 1.5 illumination

conditions using an Agilent B1500A semiconductor parameter

analyzer. An Xe arc lamp was used to simulate the solar light.

The EQE characteristics of the solar cells were measured in the

wavelength range of 350950 nm using a Newport mono-

chromator (equipped with the same Xe arc lamp that was used to

measure the JV characteristics). The CV measurement was

performed on the solar cell using a B1520A-FG Multi Frequency

Capacitance Measurement Unit Module attached to an Agilent

B1500A using a frequency of 1 kHz and AC perturbation of10 mV at room temperature.

Results and discussion

Fig. 2(a) is the micrograph showing the surface morphology of

an MoS2 nanomembrane after being transferred onto the ITO

substrate, as obtained from the 2D mapping measurement of an

Atomic Force Microscope (AFM). The AFM line-scan was

carried out, from point A to point B as shown in Fig. 2(a), to

measure the physical thickness of the MoS2nanomembrane. The

step height between MoS2 and the ITO substrate, as seen in

Fig. 2(b), confirmed the thickness of the MoS2 nanomembrane to

be about 55 nm, an average value from the measured data. We

believe that the as-grown MoS2 nanomembrane on the SiSiO2substrate in our CVD process is highly uniform in thickness, as

reported by Y. Zhan et al.in a similar growth experiment.37 The

variation in AFM line-scan data (approximately 5 nm) could

arise from deformation of the MoS2 nanomembrane as a result

of the layer transfer process. We expect that micro-level wrinkles

might have been formed on the MoS2nanomembrane caused by

various types of mechanical interactions during the wet chem-

istry etching and transferring process.Material characterization using Raman spectroscopy was

conducted on two transferred MoS2 nanomembrane samples

with different physical thicknesses. While the as-assembled MoS2exhibits an average thickness of55 nm, as shown in Fig. 2(b),

stacked MoS2 nanomembranes were prepared via multiple (two

and four, respectively, in our experiment) transfers.

The samples were subsequently used for Raman spectroscopic

investigation. The expected thicknesses of the two samples are

110 nm (two transfers) and 220 nm (four transfers), respec-

tively. Prior research results show that there is a significant

difference in the signature modes in Raman spectra for

Fig. 2 (a) 2D mapping micrograph of AFM surface scan obtained on an

as-assembled MoS2 nanomembrane after transferring onto an ITO

substrate. The line-scan measurement was performed from point A to B

to evaluate the physical thickness of MoS2. (b) Measured step profile ofthe MoS2nanomembrane on ITO substrate.


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monolayer, bilayer and few-layer MoS2.37 In our experiment, a

much thicker stack of MoS2 was employed to enhance the photo-

absorption in the Schottky-barrier solar cells. Fig. 3(a) shows the

Raman spectra of the MoS2 nanomembrane samples with

thicknesses of110 nm and 220 nm, respectively. Two signif-

icant Raman active modes were identified at the wavenumbers of

383 cm1 and 408 cm1 which correspond to E12gand A1gmodes

respectively.

Fig. 3(b) displays schematically both E12g and A1g modes. Inthe E12g mode both Mo and S atoms vibrate along the in-plane

direction (yet opposite to each other), whereas in the A1g mode

the S atoms vibrate in the perpendicular-to-plane direction. The

Raman signature peaks observed in our experiment confirm both

in-plane and perpendicular-to-plane Raman active modes. As we

used relatively thick MoS2 (>100 nm), there are no blue- and red-

shifts for the E12gand A1gmodes, different from that observed in

monolayer/bilayer MoS2 nanomembranes.37 The significant

variation in the peak intensity of Raman spectra, as observed in

the two examined samples, was due to the substantial difference

in physical thickness (100 nm).

Fig. 4(a) shows the schematic cross-sectional view of the MoS2

nanomembraneAu Schottky-barrier solar cell. The MoS2nano-membrane was transferred onto an ITO substrate (serving as the

window layer through which the solar cell is illuminated). Fig. 4(b)

shows the energy band diagram of the fabricated Schottky-barrier

solar cell structure with a stack of ITOMoS2Au.

In general, if the metal work function is higher than that of the

n-type semiconductor, then an ideal Schottky-barrier could be

formed. In our experiment, Au (work function: 5.1 eV) forms abarrier on the MoS2 semiconductor (work function: 4.6 eV).

The ITO used in this study is a heavily n-type doped transparent

conductive oxide with a work function of4.5 eV. It forms an

almost perfect ohmic contact with the n-type doped MoS2nanomembrane. As shown in Fig. 4(b), the Fermi level of ITO is

above the conduction band due to its heavily doped nature.

Although layer-dependent optical properties (e.g., change of

bandgap with number of layers) has been demonstrated in ultra-

thin MoS2(from a monolayer to six layers) as reported by K. F.

Mak et al.,39 the bulk properties dominate in thicker stacks of

MoS2. In our experiment, MoS2 nanomembrane stacks

Fig. 3 (a) Raman spectra obtained on two samples of MoS2 nano-

membranes with thicknesses of110 nm and 220 nm, respectively. The

samples were prepared by multiple transfers/stacking of CVD-grown

MoS2. (b) Schematic showing the two Raman active modes corre-

sponding to the signature peaks observed in MoS2.

Fig. 4 (a) Schematic cross-sectional view of the demonstrated Schottky

barrier solar cell structure showing a stack of MoS2nanomembrane on

an ITO substrate with Au contact. (b) Energy band diagram of the solar

cell with formation of a Schottky-barrier between the MoS2 nano-

membrane and Au metal contact.


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(110 nm and 220 nm) were used for the fabrication of solar

cells. Therefore layer dependency of the optical bandgap

becomes negligible. We consider the MoS2stack with an optical

bandgap of 1.3 eV which effectively absorbs photons in the

wavelength region of 350950 nm. The Schottky-barrier solar

cells are illuminated through the ITO window layer which does

not absorb any photons in the visible light spectral region, as its

bandgap is 3.8 eV and more than 95% optically transparent.

Incident photons generate electronhole pairs in the MoS2nanomembrane and electrons are subsequently excited to the

conduction band (EC) of MoS2. As the work function difference

between Au and MoS2 results in a Schottky-barrier, the photo-

excited electrons move from the EC of MoS2 towards the Fermi

level of ITO (where they can be collected to the load). A larger work

function difference between Au and n-type MoS2would generate a

higher electric field in the depletion region of MoS2. The built-in

electric field facilitates the dissociation of the photo-excited elec-

tronhole pairs, transporting separated carriers towards ITO and

Au contacts. The processes of photo-excitation in MoS2 and elec-

tron transport from MoS2to ITO are demonstrated in Fig. 4(b).

Fig. 5(a) and (b) show the dark and illuminated JVcharac-

teristics of the fabricated Schottky-barrier solar cells with MoS2thicknesses of110 nm and 220 nm, respectively. The output

power as a function of voltage in each solar cell is shown in the

insets of Fig. 5(a) and (b), respectively. For the Schottky-barrier

solar cell with MoS2thickness of 110 nm key metricsJSC(short-

circuit current density), VOC (open-circuit voltage), PMAX(maximum power), FF (fill factor) and h (photo-conversion

efficiency) are 2.52 mA cm2, 590 mV, 1.22 104 W, 0.48 and

0.7%, respectively. For the solar cell with MoS2 thickness of

220 nm, JSC, VOC, PMAX, FFand h are 5.37 mAcm2, 597 mV,

2.99 104 W, 0.55 and 1.8% respectively. It is observed that an

increase of MoS2thickness helps to significantly enhanceJSCof

the solar cell (by 113%) due to more efficient photo-absorption in

the thicker MoS2 stack, and hence creates a noteworthy changein overall device performance. It should be noted that there is no

sizable change in VOC between solar cells with different thick-

nesses of MoS2 stacks (merely 1% change). It is evident that an

increase in thickness of the MoS2 stack would not cause a notable

change in the band structure (hence no change in the Fermi level)

due to the absence of layer-dependent properties.

Fig. 5(c) shows the schematic to compare carrier recombina-

tion in conventional and layered semiconductors. Process 1

represents optical excitation leading to electronhole pair

generation. In conventional semiconductors, the amount of

interface states (recombination centers) in the bandgap is huge

due to unsaturated chemical dangling bonds which capture

electrons from the conduction band (Process 2) and holes fromthe valence band (Process 3), leading to recombination. In

layered semiconductors, the amount of interface states is signif-

icantly less due to the self-saturated surface bonds which do not

actively participate in the recombination process. Thus, we

attribute the enhancement in solar cell performance to the

increase in MoS2thickness which improved photon absorption.

We expect that as a layered material, defect density in MoS2does

not increase drastically with increasing thickness compared to a

conventional semiconductor.

Fig. 6 shows External Quantum Efficiency (EQE) spectra of

the two Schottky-barrier solar cells with different MoS2

thicknesses in the wavelength range of 350950 nm. The

maximum EQE values of 41% and 52% were measured for solar

cells with 110 nm and 220 nm of MoS2stack, respectively, at

wavelength of 600 nm. The increase in MoS2 thickness plays a

vital role in photo-absorption. Our prior work on MoS2-based

Fig. 5 Measured dark and illuminated JV characteristics of the

Schottky-barrier solar cells with a stack of MoS2nanomembrane having

thicknesses of (a) 110 nm and (b) 220 nm. Inset of each figure showsthe power output as a function of voltage for the fabricated solar cell. (c)

Recombination process via electronhole capture by interfacial trap

states present in conventional and layered semiconductors, showing

significantly fewer traps in the latter. This leads to reduced probability of

carrier recombination in a layered semiconductor.


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solar cells34 employing a combination of monolayer/multi-layerstacks of MoS2on TiO2(serving as electron acceptor) has shown

photon absorption of MoS2 in the wavelength region of 350

750 nm. The measured EQE spectra in the extended wavelength

region, as compared with the previously reported values (up to

950 nm),34 suggests that an MoS2 nanomembrane with an

optical bandgap of 1.3 eV could effectively absorb photons in

both visible and near infrared spectral regions due to the absence

of the layer-dependent optical properties of MoS2 as governed by

its physical thickness.

Fig. 7(a) shows the MottSchottky (C2V) characteristics of

the Schottky-barrier solar cells. The C2Vcharacteristics were

used to estimate the Schottky-barrier heights associated with

each of the two fabricated solar cells. Fig. 7(b) shows schemati-cally the energy-level alignment at the MoS2Au interface under

zero, forward and reverse voltage bias conditions. Since the

MoS2 samples are n-type doped, Fermi levels in MoS2 and Au

shift upwards and downwards, respectively, with reference to the

equilibrium Fermi level at zero voltage bias, yielding

the maximum barrier height. In the case of reverse voltage bias,

the situation becomes opposite. We found that both solar cells

(with different MoS2 thickness) resulted in nearly the same

Schottky-barrier height (1.0 eV), as measured from the linear

extrapolation in the reverse-biasing region (from 1 V to 4 V). It

can be understood that a change of the MoS2should not make a

difference in the Schottky-barrier height. While we expect that

MoS2thickness (down to a few layers) may change the Schottky-barrier height, the layer dependence becomes insignificant in the

nanomembranes used in our experiment. The observed linear

behavior of the MottSchottky plots suggests that the MoS2nanomembranes possess a relatively low density of surface

defects.

J. M. Lutheret al.reported a Schottky-barrier solar cell using

PbSe quantum dots.2 The result showed significant variation in

the voltage-dependent C2 for two PbSe samples (thickness

difference > 300 nm).2 In our study, MoS2Au solar cells (with

active layer thickness difference of 100 nm) exhibit no significant

change in the C2 values with reverse bias. This could be

attributed to the layered MoS2 nanomembrane (defect-free or

with negligibly low density of defects) as compared with

conventional semiconductors in which unsaturated danglingbonds make the MS interface highly defective. In layered

semiconductors the crystal lattice planes are weakly bound by

van der Waals interactions and hence the surface exhibits self-

saturated electronic bonds which are no longer active for surface

reaction with the environment. The nature of a self-saturated

surface in layered MoS2 nanomembrane eliminates Fermi level

pinning and interfacial oxide layer formation (both could impede

the transport significantly). The voltage-bias-dependent C2

values are almost identical in both device samples (with different

thicknesses) due to negligible change in the trap states at MoS 2

Au interface where carriers are stored and released, affecting the

junction capacitance. This is a key appealing property of layered

semiconductors for solar cell applications as compared withbulk/thin film semiconductors which possess a particular type of

electronic bonding along and across the crystal planes (leading to

defective bulk and surface upon cleaving).

We project that the performance of the MoS2nanomembrane-

based Schottky-barrier solar cell could be further improved.

Layered semiconductors are typically very sensitive to growth

conditions such as temperature and pressure. Temperature fluc-

tuation across the growth furnace plays a significant role in

influencing the solidvapor reaction between pre-deposited Mo

and S vapor, and eventually the electronic quality of the MoS2nanomembrane. In addition, defects and impurities introduced

Fig. 6 Measured EQE spectra of the Schottky-barrier solar cells with a

stack of MoS2 nanomembranes having thicknesses of 110 nm and

220 nm, respectively, in the wavelength region of 350950 nm. This

confirms an optical bandgap of approximately 1.3 eV for the prepared

MoS2 nanomembranes.

Fig. 7 (a) The measured MottSchottky (C2V) plot of the Schottky-

barrier solar cells with a stack of MoS2 nanomembranes having thick-

nesses of 110 nm and 220 nm, respectively, showing a linear C2

dependency (shown in the circled region) on the applied reverse bias, VR.

(b) Schematic showing MoS2Au interfacial energy level change at zero

(equilibrium), forward, and reverse bias conditions and the effect on

Schottky barrier height.


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during the CVD growth and deformation formed during layer

transfer would all contribute to reduced carrier mobility and

degraded carrier transport properties. We expect that further

optimization of the growth process, use of a cleaner fabrication

environment, and direct assembly of MoS2 nanomembranes on

an ITO substrate would produce less defective materials, leading

to improved solar cell performance.

Conclusions

Schottky-barrier solar cells have been demonstrated using layer-

assembled, CVD-grown MoS2 semiconductor nanomembranes

as the critical photo-active layer. While relatively thick MoS2nanomembranes are used in this experiment, the material is

scalable via a controlled CVD growth process and large-area

solar cell fabrication is potentially feasible. The dependency of

solar cell performance on MoS2 thickness was evaluated. Further

improvement in solar cell performance is expected through

optimizing the growth process, eliminating structural deforma-

tion (induced during fabrication), and minimizing the amount of

surface defects and impurities. The reported results would

promote continued efforts towards developing highly efficientSchottky-barrier solar cells taking advantage of the unique

interfacial properties of layered semiconductor nanostructures.

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