Significant Broadband Photocurrent Enhancement by Au-CZTS Core-Shell Nanostructured Photocathodes

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received: 09 December 2015

accepted: 03 March 2016

Published: 21 March 2016

# Significant Broadband Photocurrent Enhancement by Au- CZTS Core-Shell Nanostructured Photocathodes

Xuemei Zhang 1,* , Xu Wu 1,* , Anthony Centeno 2 , Mary P. Ryan 1 , Neil M. Alford 1 , D. Jason Riley 1
& Fang Xie 1

Copper zinc tin sulfide (CZTS) is a promising material for harvesting solar energy due to its abundance
and non-toxicity. However, its poor performance hinders their wide application. In this paper gold (Au)
nanoparticles are successfully incorporated into CZTS to form Au@CZTS core-shell nanostructures.
The photocathode of Au@CZTS nanostructures exhibits enhanced optical absorption characteristics
and improved incident photon-to-current efficiency (IPCE) performance. It is demonstrated that
using this photocathode there is a significant increase of the power conversion efficiency (PCE) of a
photoelectrochemical solar cell of 100% compared to using a CZTS without Au core. More importantly,
the PCE of Au@CZTS photocathode improved by 15.8% compared to standard platinum (Pt) counter
electrode. The increased efficiency is attributed to plasmon resonance energy transfer (PRET) between
the Au nanoparticle core and the CZTS shell at wavelengths shorter than the localized surface plasmon
resonance (LSPR) peak of the Au and the semiconductor bandgap.

Solar energy is the most abundant and sustainable natural energy source that could facilitate global sustainability.
Photovoltaic cells in particular are a promising solar energy conversion technology that can directly convert solar
light energy into electricity. Solar cells based on copper zinc tin sulfide (CZTS), a quaternary semiconducting
compound, have been receiving increased interest as CZTS is composed of only abundant and non-toxic elements. Indeed it is estimated that only 1% of the global reserves of Cu, Sn, Zn and S could produce enough energy
to power the world 1 .

CZTS is a promising light absorbing material 2–5 with a high absorption coefficient (α - 10 4 cm −1 ) and a direct
bandgap of 1.4–1.6 eV 6,7 . Due to the p-type nature of CZTS, one area of interest is the incorporation of CZTS
thin films as photocathodes in regenerative photoelectrochemical solar cells, such as dye-sensitized solar cells
(DSSCs). In traditional DSSCs, platinum (Pt)-loaded conductive glass has been widely used as the standard
counter electrodes 8 . However, high cost and low abundance greatly limit the large-scale use of Pt in DSSCs 9 .
Photocathodes made from low-cost semiconductors as an alternative to Pt counter electrodes have attracted
considerable attention, not only because the substitution of Pt may reduce the manufacturing cost of the devices,
but also because the double-photoelectrode cells have the potential to harvest more light and give rise to higher
current densities. To date, CZTS thin films have been successfully employed as photocathodes in photoelectrochemical solar cells 10–13 and have shown increased cell efficiency relative to devices fabricated using Pt counter
electrodes.

Plasmonic nanostructures such as copper, silver and gold have demonstrated enhanced performance for fluorescence based detection 14–16, as well as for a photoanode in a photoelectrochemical cell 17–21 . Three possible
enhancement mechanisms have been discussed in the literature 17,22–24 . These are plasmon resonance energy transfer 24 (PRET), scattering effects and hot-carrier injection where the charge carrier gains sufficient kinetic energy to
transfer from the metal nanoparticle to the conduction band of the semiconductor. In PRET the plasmonic metal
absorbs photons and transfers the absorbed energy to the semiconductor _via_ dipole-dipole coupling.

1Department of Materials, Imperial College London, London, United Kingdom, SW7 2AZ. 2Malaysia-Japan
International Institute of Technology, University of Technology Malaysia International Campus, 54100, Kuala
Lumpur, Malaysia. * These authors contributed equally to this work. Correspondence and requests for materials
[should be addressed to F.X. (email: f.xie@imperial.ac.uk)](mailto:f.xie@imperial.ac.uk)

Figure 1. TEM image as-synthesized particles: ( a ) Au nanoparticles; ( b ) wurtzite CZTS nanoparticles;
( c ) TEM image of particles synthesized by reacting the original amount of metal precursors and thiol in the
presence of Au nanoparticles; ( d ) TEM image of particles synthesized by reacting half amount of the original
amount of metal precursors and thiol in the presence of Au nanoparticles.

In this paper we consider whether the incorporation of metal nanoparticles into a CZTS photocathode can
lead to an enhanced efficiency of the double-photoelectrode electrochemical solar cell. We demonstrate that
coupling of the Au@CZTS photocathode with a dye-sensitized TiO2 photoanode and iodide/triiodide electrolyte
leads to a cell that is 100% more efficient than a device fabricated in the same manner but with a photocathode
made of CZTS only. Furthermore, the cell with Au@CZTS photocathode performs _ca._ 15.8% more efficient than
using platinum as a counter electrode. To interrogate the mechanism for the enhanced cell performance electrochemical impedance spectroscopy experiments are undertaken. The optoelectronic effects of plasmonic metal
nanoparticles in CZTS are discussed.

Results and Discussion
XRD patterns of samples synthesized by reacting Cu-Zn-Sn metal precursor and thiol in/without the presence of
Au nanoparticles are shown in Fig. S1 (Supplementary). The diffraction pattern of the sample prepared without
Au can be identified as wurtzite CZTS. The pattern of the sample synthesized in the presence of Au consists of
diffraction peaks from both Au and wurtzite CZTS. The diffraction peaks in both patterns are relatively broad,
indicating the formation of nanosized particles.

To check if the particles prepared in the presence of Au have a core-shell structure or are just a mixture of
individual Au and CZTS particles the morphology of as-prepared particles was investigated by TEM microscopy.
TEM images of bare Au nanoparticles and pristine wurtzite CZTS nanoparticles are shown in Fig. 1a,b as references, respectively. TEM image of the sample synthesized by reacting metal precursors and thiol in the presence
of Au nanoparticles (Fig. 1c) reveal that the resulting particles are not a mixture of individual Au particles and
wurtzite CZTS particles but have a core-shell structure. As shown in the image, the gold core, of size 10–15 nm,
is surrounded by a shell of CZTS which has a rice-like shape. With the aim of adjusting the thickness of the shell
the amount of metal precursors was changed. Nanoparticles were synthesized when the metal precursors were
reduced to half of the original concentration and the TEM image of the resultant material is shown in Fig. 1d.
With the reduced precursor concentration most gold cores have incomplete shells, which is in contrast with the
complete encapsulation of gold cores (Fig. 1c) when the samples were prepared at higher precursor concentration. This indicates that a suitable concentration of metal precursor is necessary for the formation of complete
core-shell nanostructures.

To investigate the formation process of Au@CZTS core-shell nanostructure, aliquots were taken out from the
reaction flask at different times after the injection of the sulfur source (thiol) into the mixture of Cu-Zn-Sn metal
precursor and Au nanoparticles. The temperature was raised from 120 °C to 270 °C after the injection. TEM images

Figure 2. UV-Vis absorption spectra of as-synthesized nanoparticles suspended in toluene. Inset shows the
plotting of (Ahν ) 2 _vs._ hν based on the UV-Vis absorption spectrum of CZTS which gives an estimation of its
bandgap.

of the particles extracted from the system at 2, 8, 18 and 30 minutes (end of the reaction) are shown in Fig. S2
(Supplementary). After 2 minutes (Fig. S2a), bare Au nanoparticles along with amorphous chunks of precursor
are observed because the temperature of the system, 150 °C, was not thermodynamically high enough for the
nucleation of CZTS. After 8 minutes (Fig. S2b), the temperature had reached 250 °C and core-shell nanostructure
had formed. Prolonging reaction to 18 minutes and 30 minutes increases the average thickness of CZTS shell
(Fig. S2c, d).

The optical property of the Au@CZTS core-shell nanostructure was investigated using UV-Vis spectroscopy
(Fig. 2). Spectra of bare Au nanoparticles and pure wurtzite CZTS nanoparticles are both displayed for comparison. The spectrum of Au nanoparticles has a plasmonic peak centred at 530 nm; wurtzite CZTS shows a broad
absorption extending to the edge of near infrared. The spectrum of Au@CZTS core-shell nanoparticles shows two
features, a broad absorption and an Au plasmonic peak. The Au plasmonic peak in Au@CZTS nanoparticles shifts
to _ca._ 680 nm, relative to 530 nm in the bare Au nanoparticles. This is consistent with the change in refractive
index of the medium surrounding the gold, from toluene (~1.5) to CZTS (~2.5).

The bandgap of the wurtzite CZTS nanocrystals can be estimated by Tauc plot based on its UV-Vis absorption
spectrum 25 . The estimated bandgap is _ca._ 1.51 eV by extrapolating the linear region of a plot of ( _Ahν_ ) 2 vs _hν_ as
shown in inset in Fig. 2. For Au@CZTS nanoparticles, the Au plasmon resonance precludes bandgap estimation
from Tauc plot, however, similarity in the absorption edge indicates Au@CZTS has a similar bandgap as the pristine wurtzite CZTS material.

Au@CZTS thin films and CZTS thin films were made by spray-coating nanoparticle-toluene dispersion on
clean substrates. The morphology of the thin films was observed by SEM (Fig. S3). Large cracks as wide as 300 nm
can be observed in the films fabricated from pristine CZTS nanocrystals (Fig. S3a). The films made from Au@CZTS
nanoparticles exhibit a more continuous surface, although narrower cracks are still visible in the film (Fig. S3b).
This corresponds with the observation made by Aydil that thin films made from nanoparticles with larger size
tend to have fewer cracks 26 . At higher magnification (Fig. S3c), it can be seen that the surface of the film is covered
with Au@CZTS particles with a size of 30–50 nm, which agrees with the observed particle dimension observed in
the TEM images (Fig. 1c). The cross-sectional image on the crack-free area of the film shows that the nanoparticles are densely packed (Fig. S3d).

Au@CZTS thin films and CZTS thin films fabricated on FTO glass substrates were used as photoelectrodes
in the three-electrode cell. Their IPCE curves are shown in Fig. 3a. Compared with the CZTS thin film, the Au@
CZTS photocathode exhibits an enhanced IPCE across the entire wavelength range. It can be seen that there is a
distinct peak between 600–750 nm which corresponds to the plasmon resonance peak of the gold core embedded
in the CZTS matrix, as shown in the UV-Vis absorption spectrum (Fig. 2).

Transient photocurrent of an Au@CZTS photocathode was recorded in the same electrolyte under modulated
60 mW cm −2 radiation from a xenon lamp (Fig. 3b). The transient photocurrent has a negative sign which reflects
the p-type nature of CZTS. At the onset of illumination the photocurrent increases slowly. The response (solid
curve in Fig. 3b) fits a function which is the sum of two exponential components, with characteristic time of 18 s
and 77 s. Chronoamperometric responses with two temporal components have been reported in studies of plasmonic photoanodes 27 . A maximum current of 540 μA cm −2 under illumination from a Xe lamp of 60 mW cm −2
indicates a higher photon-to-current conversion efficiency under more intense radiation than that observed in
the previous monochromatic IPCE measurement, suggesting that charge carrier recombination _via_ trap states
occurs at low light intensity.

Au@CZTS thin films and pristine wurtzite CZTS thin films (on FTO glass substrates) were employed as counter electrodes in photoelectrochemical solar cells as illustrated in Fig. 4a. A commercial platinum (Pt) counter

Figure 3. ( a ) IPCE curves of CZTS and Au@CZTS thin films under illumination from monochromatic light;
( b ) transient photocurrent of Au@CZTS thin film.

Figure 4. ( a ) Illustration of a photoelectrochemcal solar cell with a photocathode made from wurtzite CZTS/
Au@wurtzite CZTS nanoparticles; ( b ) J–V curves of solar cells fabricated with different counter electrodes: Pt,
CZTS and Au@CZTS.

electrode was also used as a comparison. Current-voltage (J–V) curves of three solar cells are shown in Fig. 4b and
the key parameters summarized in Table 1.

As shown, in Table 1, solar cells fabricated with wurtzite CZTS thin films as the photocathodes both have
higher short circuit photocurrent than a cell containing a Pt counter electrode. This is because CZTS acts as
an absorbing layer and provides extra charge carriers, contributing to the photocurrent. However, the wurtzite
CZTS photocathodes give rise to a slightly lower open circuit voltage and poorer fill factor, resulting in a lower

Counter electrode VOC (V) JSC (mA/cm 2 ) FF η (%)

Pt 0.70 16.59 0.57 6.64

CZTS 0.67 18.18 0.31 3.72

Au@CZTS 0.70 20.25 0.54 7.69

Table 1. Key parameters of DSSCs extracted from Fig. 4.

Figure 5. ( a ) Calculated absorption and scattering efficiency for a 15 nm diameter Au sphere embedded in
wurtzite CZTS; ( b ) schematic of the charge carrier generation in Au@CZTS nanoparticles under illumination in
two routes: Route 1 in red shows charge generation directly by incident light and Route 2 in blue shows charge
generation by energy transfer from LSPR of Au core to CZTS _via_ PRET.

cell efficiency compared with the cell consisting of a Pt counter electrode. On the other hand, solar cells fabricated
with Au@CZTS films as the photocathodes, have similar open circuit voltage and fill factor to cells prepared using
a Pt counter electrode. Solar cells with the Au@CZTS photocathodes possess higher short circuit current than
the classic cell with a Pt counter electrode. The higher short circuit current means that the cell with an Au@CZTS
counter electrode had an efficiency 15.8% higher than an identical cell with a Pt counter electrode, and twice that
of pure wurtzite CZTS.

It should also be noted that kesterite phase CZTS counter electrodes have been reported with higher short
circuit photocurrents than Pt 12 . In this work we attempted to synthesize Au@kesterite CZTS thin films but to date
the quality of the particles have been poor. At present reasons are not clear, but the growth of CZTS on the surface
of the Au may be considered as epitaxial and we think that there is a mismatch between the lattice parameter of
the Au and kesterite CZTS. This is an area of continuing investigation which we shall endeavour to present results
on in the future.

To analyze the IPCE and associated UV-Vis curves further electromagnetic modelling was carried out using
the Finite Difference code MEEP (MIT Electromagnetic Equation Propagation) 28 . The analysis consisted of calculating the absorption and scattering efficiency 29 between 350–1000 nm, for a 15 nm diameter Au sphere surrounded by CZTS, using both the Finite Difference Time Domain (FDTD) and Frequency Domain (FDFD)
solvers. The scattering efficiency of an object much smaller than the wavelength of the incident wave, is defined
as the power scattered by the object divided by the power density of the incident wave on the cross-section. The
absorption efficiency is found by considering the power dissipated, rather than scattered. (The calculation method
and details of the electromagnetic modelling are more fully described in the supplementary information). The
calculations showed the scattering efficiency to be very low across the spectrum compared to the absorption
efficiency. The results are shown in Fig. 5a. It can be seen that there is a large absorption peak at 620 nm. It is also
observable that there is higher absorption efficiency for wavelengths below 700 nm. The prepared CZTS has a
bandgap of 1.51 eV, corresponding to a wavelength of 821 nm. The absorption peak at 620 nm of the Au nanoparticles means that there is a likelihood of non-radiative transfer of energy from the LSPR dipole of Au nanoparticles to the transition dipole of CZTS semiconductor to induce charge separation by plasmon resonant energy
transfer (PRET). The strength of the PRET depends on the spectral overlap of the semiconductor’s band edge
(absorption band) with the Au nanoparticle absorption and the distances between the two dipoles. It is noted
that there is a large increase in IPCE below 500 nm for the Au@CZTS thin film (Fig. 3a), compared to the CZTS
film. Previous literature has shown that there is coherent PRET at wavelengths below that of the donor dipole, the
LSPR of the Au@CZTS 24 . This arises as there is still strong absorption of photons by the Au at these wavelengths
and there remains an overlap with the absorption spectra of the CZTS. The enhancement at short wavelengths is
consistent with the work recently reported by Li _et al._ 24 We also note that non-plasmonic enhancement of photocurrent, due to Au nanoparticles mixed with Copper (Cu) based nanocrystals, has recently been reported 30 . This
is due to the effect of an enhanced magnetic field on the Cu-based delofassite nanocrystals. Therefore, this is not
the enhancement mechanism in CZTS.

CZTS, as a new solar cell material, has recently attracted a lot of research interest due to its ability to generate
stable and high quality photocurrent from solar illumination. The efficient separation of excited electrons from
holes over CZTS could reduce the radiative recombination of excitons and generate photocurrent for external
circuitry 31 . The introduction of Au core into the CZTS can clearly generate more photoelectrons for both photovoltaic and catalytic hydrogen production applications 31 . A proposed pathway for photocurrent generation in
the photoelectrochemical cells is described as follows (shown in Fig. 5b). When the Au@CZTS particles were
employed as a photocathode material in the photoelectrochemical solar cells, light which passed through the
photoanode and electrolyte could excite electrons from the valence band of CZTS into the conduction band
(Route 1 in Fig. 5b), which contributed higher photocurrent to the circuit compared with Pt counter electrode.
Furthermore, the Au nanoparticles can transfer energy to CZTS via PRET (Route 2 in Fig. 5b), contributing extra
charge carriers and hence a higher photocurrent relative to pure CZTS photocathode. The produced photoelectrons on the sulphide surface are captured by oxidized species in the electrolyte, while the holes are transported
to the external circuit to complete the photoelectrochemical cycle. Electrochemical impedance spectroscopy
(EIS) studies suggest that the enhancement arises due to increased absorption at the photocathode rather than
increased scattering (Details of the EIS is given in the Supplementary). This is consistent with the electromagnetic
modelling data where very low scattering efficiency was predicted across the spectrum.

Conclusion
We have demonstrated significant and broadband IPCE enhancement of the photoelectrode containing Au@
CZTS core-shell nanostructures, compared to CZTS without Au core. When the Au@CZTS photocathode was
employed as the counter electrode in a photoelectrochemical solar cell, the power conversion efficiency of the cell
was doubled, compared to an identical cell with CZTS as a counter electrode. Moreover, the power conversion
efficiency of the Au@CZTS photocathode was 15.8% higher than a standard Pt counter electrode. In summary,
we circumvent the limitations of CZTS by introducing the novel Au@CZTS core-shell nanostructrure, potentially
paving the way for large-scale use of CZTS in solar cells.

Methods
Synthesis of Au@CZTS Nanoparticles and CZTS Nanoparticles. The Au@CZTS core-shell nanoparticles were synthesized adapting a reported method from literature 31 . To start with, Cu-Zn-Sn metal precursor solution was prepared in a three-neck bottle attached to a Schlenk line. In a typical synthesis, 1.8 mmol
copper acetylacetonate (Cu(acac)2), 1.2 mmol zinc acetate (Zn(OAc)2) and 1.0 mmol tin chloride (SnCl2) were
mixed with 5 cm 3 oleylamine. The mixture was then heated to 120 °C under argon and stirred for 30 minutes to
dissolve the metal salts. Au nanoparticles as the core of the heterostructures were prepared separately on a hot
plate. 0.05 mmol HAuCl4·3H2O was dissolved in 5 cm 3 oleylamine in a glass vial, heated to 120 °C and stirred for
10 minutes for Au nanoparticle growth. The Au nanoparticles were quickly injected into the Cu-Zn-Sn metal
precursor in the flask. The mixture was stirred for 5 minutes to allow homogenous dispersion of Au nanoparticles in the precursor solution. 0.05 cm 3 1-dodecanethiol (1-DDT) and 0.35 cm 3 _tert_ -dodecanethiol ( _t_ -DDT) was
then injected into the mixture simultaneously and temperature was increased to 270 °C in _ca._ 10 minutes. After
30 minutes, the heating mantle was removed and the mixture was cooled down to room temperature. The nanoparticles were separated from the liquid phase by centrifuging followed by washing with toluene and isopropanol
three times. The nanoparticles were dispersed in toluene by sonication for future use. In order to obtain different
thicknesses of CZTS on the Au core the amounts of Cu, Zn, and Sn salts were varied at constant mole ratios for
the preparation of Cu-Zn-Sn precursor solution.

To investigate the influence of Au incorporation on the properties of nanoparticles, pristine CZTS nanocrystals were synthesized as a blank control using an identical procedure to the above but omitting the addition of Au
nanoparticles and using a reaction time of 1 hr.

Thin film Fabrication. Au@CZTS/CZTS nanoparticles were dispersed in toluene under sonication to
form a suspension which had a concentration of 10–20 mg cm −3 . Nanoparticle suspension was then sprayed
onto FTO glass substrates, pre-cleaned using UV-Ozone, by an airbrush with an argon flow at a spray rate of
0.1–0.3 cm 3 min −1 . During the spray-coating, the nozzle to substrate distance was maintained at 10–15 cm and the
substrate temperature was _ca._ 100 °C. The films were dried in a vacuum oven at 160 °C for 2 hr. The pristine and
core-shell nanoparticle films which were only dried in the oven without heat treatment were labelled as ‘CZTS’
and ‘Au@CZTS’, respectively.

Characterization
The morphology of nanoparticles was observed using a JEOL 2000FX transmission electron microscope
(TEM) operated with an accelerating voltage of 300 kV. TEM samples were prepared by drop-casting dilute
nanoparticle-toluene dispersion onto carbon-coated copper TEM grids (300 mesh, Agar Scientific UK). The morphology of thin films were observed using a LEO Gemini 1525 field emission gun scanning electron microscope
(FEGSEM) with an accelerating voltage of 5 kV. The films were made by spray-coating nanoparticles on glass
substrates and then the glass substrates were mounted on aluminium stubs (Agar Scientific UK) by double-sided
carbon conductive adhesive tape (Agar Scientific UK). The morphology of nanoparticles and thin films were
observed using a JEOL 2000FX transmission electron microscope (TEM) and a LEO Gemini 1525 field emission
gun scanning electron microscope (FEGSEM). X-ray diffraction (XRD) studies of the particles were performed
on a Bruker D2 diffractometer with monochromated Cu Kα source. Ultraviolet-visible (UV-Vis) spectra of the
sample were measured using a Perkin Elmer, LambdaBIo10 spectrometer. Monochromatic incident photon to
current efficiency (IPCE) spectra of the as-prepared thin films were recorded in a three-electrode photoelectrochemical cell. The electrolyte was 0.1 mol dm −3 EuCl3 with 0.1 mol dm −3 KCl. A saturated Ag/AgCl electrode and

a platinum mesh electrode were used as the reference electrode and counter electrode respectively. A μ-Autolab
potentiostat coupled to a PTi OC-4000 optical chopper and a SR830 lock-in amplifier at a reference frequency of
about 0.05 Hz was used to record the photocurrent density at a monochromatic wavelength λ and a potential of

- 0.5 V _vs._ saturated Ag/AgCl reference electrode. The incident light power intensity was monitored by a Thorlabs
PM100A light power meter.

Solar Cell Fabrication and Characterization. To test the performance of Au@CZTS and CZTS nanoparticles as photocathode material, Au@CZTS and CZTS thin films were incorporated in to devices fabricated
using a commercial DSSC kit (Solaronix). Titania thin films (Solaronix) were immersed in methanol containing
0.3 mmol dm −3 ruthenizer 535-bisTBA (N719, Solaronix) for 8 hr. The dye-stained titania thin films were used as
photoanodes and assembled with various counter electrodes, _i.e._, CZTS nanoparticles thin films on FTO glass,
Au@CZTS thin films on FTO glass and platinized FTO, using a 100 μm thick Surlyn® thermoplastic sealing gasket film (Solaronix, Meltonix 1170–100). An iodide/tri-iodide electrolyte (AN-50, Solaronix) was then injected
into the gap between the two electrodes. Finally a black mask was attached to allow a 0.25 cm 2 active area. The
steady state current density-voltage (J–V) curves were recorded using a Keithley 2400-LV source meter under
simulated solar light illumination with an intensity of 100 mW cm −2 (1 sun), generated by a certified AAA solar
simulator (Newport, LCS-100TM, 94011A) with an AM 1.5 G filter. Electrochemical impedance spectroscopy
studies were performed on the as-prepared cells under AM 1.5 G illumination and a DC voltage bias of −600 mV
_vs._ the open circuit voltage, an AC modulation of 0.01 V was applied.

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Acknowledgements
The authors acknowledge Engineering and Physical Sciences Research Council (EPSRC) funding (EP/
G060940/1). X.Z. appreciates Stephen and Anna Hui Scholarship, Imperial College London, for sponsoring her
PhD project. Dr. Centeno’s research is supported by a FRGS Grant (FRGS/2/2013/SG02/ UTM/02/3) from the
Malaysian Ministry of Education.

Author Contributions
X.W., D.J.R. and F.X. elaborated the concept and designed experiments. X.Z. carried out nanomaterial synthesis
and characterization. X.W. performed the photoelectrochemical cell tests and characterisation. A.C. carried out
the electromagnetic modelling work. All authors participated in scientific discussions and paper drafting. F.X.
supervised the overall project.

Additional Information
Supplementary information [accompanies this paper at http://www.nature.com/srep](http://www.nature.com/srep)

Competing financial interests: The authors declare no competing financial interests.

How to cite this article : Zhang, X. _et al._ Significant Broadband Photocurrent Enhancement by Au-CZTS CoreShell Nanostructured Photocathodes. _Sci. Rep._ 6, 23364; doi: 10.1038/srep23364 (2016).

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