Nanoscale RSC - Guangyu Liguangyuli.weebly.com/uploads/4/0/8/2/40824981/li_au-czts... · 2019-08-03 · Powder X-ray diffraction (XRD) was performed to investigate the phase and composition

Nanoscale RSC

PAPER

This journal is © The Royal Society of Chemistry 2013 J. Name., 2013, 00, 1-3 | 1

Synthesis of Hexagonal Au/Cu2ZnSnS4 core-shell

Nanoplate Heterostructures and Their Application in

Organic-Inorganic Hybrid Photovoltaic Cells

Guangyu Li, Chenchen Yuan, Yawei Liu, Shangfeng Yang, Guoshun Jiang, Weifeng Liua and Changfei Zhub

Au/CZTS core-shell nanoplate heterostructures with hexagonal shape was successfully

synthesized by a one-pot multi-step colloid process. A mechanism based on an Au-Cu alloy

bridge and a preferential absorption of molecules and ions in solution to different crystal faces

was proposed to explain the formation of such hexagonal core-shell nanoplate structure. Further,

such nanoplates were incorporated into the active layer consisting of poly(3-hexylthiophene)

(P3HT) and phenyl-C61-butyricacid methyl ester (PCBM) in polymer solar cells (PSCs),

resulting in a 8.8% enhancement of short-circuit photocurrent density (Jsc) and a 26.4%

enhancement of power conversion efficiency (PCE) as the weight percent of doped Au/CZTS

core-shell nanoplates is 5 wt% in the active layer of corresponding PSCs. The obvious

enhancement can also be attributed to the localized surface plasmon resonance (LSPR) effect of

Au core and the light harvest and excitions generation of CZTS shell. The enhancement may

also benefits from the core-shell heterostructures, in which the CZTS shell can prevent the

quenching of excitons at the surface of Au nanoparticles.

Introduction

Polymer solar cells (PSCs) based on a bulk heterojunction (BHJ)

of conductive polymer and fullerene derivate have high potential

for large-scale application due to the high throughput, low

weight and the possibility to use flexible substrates and to

employ low-cost production processes such printing

technologies. Among the reported types of current BHJ-PSCs

devices, poly(3-hexylthiophene):phenyl-C61-butyricacid methyl

ester (P3HT:PCBM) combination as electron-donor/acceptor

system has been demonstrated to be the most popular and

efficient architecture of PSCs, whose power conversion

efficiency (PCE) being reported is still limited and reaches about

5%. [1,2] In fact, the prime restrictions are a low absorption

coefficient and limited absorption spectra. Further, the exciton

diffusion length in organic PV materials is less than 20 nm, and

hence, the excition readily recombine before being separated at

the junction interface because of potential drops. Moreover,

owing to the low carrier mobility, the optimized thickness for

organic conjugated active layers should be less than 100 nm,

which is not sufficient for strong absorption of the incident light.

[3,4]

During the past few years, much attention have been paid to

localized surface plasmon resonance (LSPR) of metal

nanoparticles for effective light harvest. As far as we know, we

believe that the metal nanoparticles embedded in the active layer

of PSCs is better for light absorption than that embedded in other

layers, because the magnified electromagnetic field near the

particle surface is strongest, and embedding metal nanoparticles

directly in active layer will benefit energy transfer from that to

organic conjugate active polymers. In addition, incorporated NPs

in the active layer would increase the optical path and improve

the light absorption within the absorbing layer via their scattering

effect of incident light in the intrinsic absorbing layer. Therefore,

embedding metal nanoparticles in active layer of PSCs will

achieve better light absorption. [5] A 10-20% increase in the

power conversion efficiency (PCE) was observed when bare Au

and Ag NPs were incorporated into organic photovoltaic cells.

[6,7]

Despite of the aforementioned successes on utilizing LSPR

effect induced by bare metal NPs in PSCs for effective light

trapping, two critical issues should be considered, including the

fading of the plasmon-enhanced efficiency with time as a result

of the interaction of metallic NPs with their dielectric

environment and the increasing recombination rate of light

generated charge carriers at the surface of such metallic NPs

[8,9]. As a practical solution for these problems, coating metallic

NPs with one or more dielectric shells to form a so called core-

shell structure has been demonstrated to be beneficial to reduce

defects or to optimize the interfaces between NPs and the

dielectric environments, thus inhibiting the loss of localized

surface plasmon [10,11]. For instance, Q. Wang et al.

incorporated Au/SiO2 core-shell NPs into the active layer of the

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polymer solar cell to improve its performance. [12,13] Besides,

Y.Yang et al. demonstrated an enhancement of PCE of PSCs by

incorporating Au/SiO2 core-shell nanorods. [14,15] However,

SiO2¬ shells, which is photovoltaically inactive, act just as the

inert spacer, making no contribution to neither light absorption

nor generation of charge carriers.

Recently, much attention have been paid to hybrid PV cells

that are based on the BHJ structure and comprise both organic

and inorganic PV materials recently, since these hybrid PV cells

have the unique properties of inorganic semiconductors as well

as preserve the favorable properties of organic materials.

Generally, inorganic semiconductors have a high absorption

coefficient, good carrier mobility, and excellent

photoconductivity. [16,17] Among these inorganic compounds,

Cu2ZnSnS4 (CZTS), a quaternary I2–II–IV–VI4 kesterite

semiconductor, has been widely studied as a photovoltaic

material due to its high optical absorption coefficient (>104 cm-

1), optimal direct band energy of ~1.5eV, good chemical,

thermal and radiant stabilities, as well as being composed of non-

toxic and abundant elements.

Although, to the best of our knowledge, CZTS NPs has not

been reported to be applied in BHJ-PSCs yet, it is reasonable to

expect that incorporating Au/CZTS core–shell nanoparticles into

P3HT:PCBM layer may maximize the enhancement of PCE of

BHJ-PSCs. Since such coupled heterostructure can not only lead

to enhancement of light absorption induced by the Au core as

well as prevent the recombination of light generated charge

carriers on the surface of the Au cores with an inert shell, but also

lead to further light harvest and more exitions generated induced

by the CZTS shell, compared with Au/SiO2 core-shell NPs. It is

worth to mention that the absorption and exition generation of

CZTS shell can also be boosted by the interaction with Au core.

[18,19,20] Moreover, incorporating these nanoparticles may

result in the improvement of chemical, thermal and radiant

stabilities of PSCs.[21,22]

However, unlike binary noble metal/semiconductor system,

few reports concern the synthesis of Au/CZTS heterostructures

with controllable morphologies, due to the large lattice

mismatches and the complication of quaternary system. In this

work, Au/CZTS core-shell nanoplate heterostructures with

hexagonal shape had been prepared with a one-pot multi-step

colloid process for the first time, and the formation mechanism

of these core-shell hexagonal nanoplates was discussed in detail.

Further, such nanoplates were incorporated into the active layer

of P3HT:PCBM BHJ-PSCs, resulting in an obvious efficiency

enhancement. The role of such core-shell heterostructures was

discussed on the basis of comparative studies of incorporating

bare Au and CZTS NPs.

Experimental Section

Growth of Au NPs

In typical modification experiments, Au NPs were prepared

following standard literature protocols.[23]

Growth of Au-core/CZTS-shell nanoplate

Firstly, 0.227 g CuCl2•2H2O, 0.169 g SnCl2•2H2O, 0.125 g

ZnCl2, 15 mL octadecene and 3 mL oleylamine are mixed at

room temperature in a three-neck round bottom flasks connected

to a Schlenk line. The flask is evacuated to rough vacuum

condition at 80◦C in the oil-bath for 20 min, followed by Ar

bubbling for 10 min. The mixture is then heated to 120◦C under

Ar atmosphere. Mixtures of 3 mL octadecene and 1 mL

oleylamine containing 40 mg of HAuCl4•xH2O are added to the

flask under vigorous stirring. The reaction is kept for 20 min for

Au seeds formation. Secondly, the temperature is increased to

200◦C and then 0.128 g sublimed sulfur dissolved in the solution

of 4 mL oleylamine are added to the flask for Cu2ZnSnS4 shell

growth. The reaction is kept at 200◦C for 1 h under vigorous

stirring. Subsequently, the system is cooled down in room

temperature for 10 min, and then 40 ml isopropanol is added to

quench the reaction. The products are finally separated by

centrifugation, washed with absolute ethanol, and then dried

under vacuum at 60◦C.

Fabrication of Hybrid Photovoltaic Device

Our detailed fabrication procedure of the P3HT:PCBM

BHJPSCs has been reported previously [24]. Briefly, the ITO

coated glass substrate (8Ω, purchased from Shenzhen Nan Bo

Group, China) was cleaned by sonication in detergent, deionized

water, acetone and isopropanol for 15 min each. After dried, it

was treated with UV-ozone for 12 min. prior to spin-coating. A

thin film (∼35 nm) of Baytron P (PEDOT:PSS, obtained from

SCM Industrial Chemical Co. Ltd.) was first spin-coated at 4500

rpm for 60 s and then annealed at 120°C for 30 min. For the Au

NPs-incorporated devices, Au/CZTS (Au, CZTS) NPs were

added into Baytron P solution with an optimized doping

concentration of ∼5 wt%. The P3HT:PCBM (from

Luminescence Technology Corp and Nichem Fine Technology

Co. Ltd. respectively) blend solution (1:0.8 w/w) was prepared

by stirring at 40°C until both were completely dissolved. The

blend films were spin-coated at 850 rpm for 60 s to form a 90 nm

thick active layer. After all of the solution process were carried

out in air, the device was transferred into a vacuum chamber

(∼10−5 Torr) to deposit an Al electrode (∼100 nm) and a shadow

mask was used to define the device active area (2×5mm2).

Finally, the device was annealed at 135°C for 10 min in a

nitrogen atmosphere.

Characterization

The chemical composition and the crystallographic orientation

of Au-core/CZTS-shell nanoplate and CZTS nanoparticles were

investigated using X-ray diffraction (XRD TTR III), with a

diffraction angle 2θ ranging from 10◦ to 70◦. X-ray photoelectron

spectra (XPS) of Au-core/CZTS-shell nanoplate were obtained

using a XPS (Thermo Scientific ESCALAB 250). Transmission

electron microscopy (TEM), high resolution TEM (HR-TEM)

images were taken using a FETEM (JEOL 2100F TEM) at 200

kV. The element mapping of the nanoplate were characterized

with Mo grid by ARM-TEM (JEOL JEM-ARM200F). The

optical property was characterized by UV–vis spectrophotometer

(SHIMADZU SolidSpec 3700). The photocurrent response was

obtained under simulated AM 1.5 irradiation (100 mW cm−2)

with a xenon-lamp-based solar simulator (Oriel Sol 3A, USA)

Results and Discussion

Composition

Powder X-ray diffraction (XRD) was performed to investigate

the phase and composition of the Au-core/CZTS-shell hexagonal

nanoplate (Fig. 1). The diffraction peaks appeared at 2θ=28.49,

47.30, 56.03 can be attributed to the (112), (220)/(204) and (132)

planes of a tetragonal structure respectively and matched well

with those of kesterite CZTS (JCPDS No. 34-1246). The peaks

found at 2θ=38.27 and 44.60, are assigned to the (111) and (200)

lattice planes of fcc Au (JCPDS No. 04-0784). No secondary

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phases or impurity peaks were detected. The size of the CZTS

crystals along the [221] axis, correspond with (112) planes, can

be estimated as around 11 nm using the Scherrer equation.

Fig. 1 XRD pattern taken from the powder of Au/CZTS core-

shell nanoplates.

The oxidation states of surface elements of Au-core/CZTS-

shell hexagonal nanoplate were investigated by X-ray

photoelectron spectroscopy (XPS). The XPS survey spectrum

identified the presence of Cu, Zn, Sn, S, C and O. The absence

of Au peak suggest that all the Au core should be completely

embedded in CZTS shell without exposure. Cu(I) state was

identified from the peak at 931.6 and 951.4 eV with a peak

splitting of 19.8eV (Fig. 2), as well as from characteristic LMM

Auger electrons peak at 917.1 eV (Fig. 6a). The characteristic Zn

2p peaks located at 1021.2 indicate the presence of Zn(II) (Fig.

2). Sn(IV) state was confirmed with 3d peaks which appeared at

486.4 and 494.8 eV with its characteristic peak separation of 8.4

eV (Fig. 2). The peaks at 162.6 and 161.6 eV could be assigned

to the binding energies of S 2p1/2 and S 2p3/2, respectively (Fig.

2), which are separated by a spin-orbit splitting of 1.0 eV. These

results are consistent with the reported data of CZTS in literature

[25], further ensuring the structural homogeneity.

Fig. 2 High-resolution XPS spectra of Cu 2p, Zn 2p, Sn 3d and

S 2p regions taken of Au/CZTS core-shell nanoplates.

Morphology

The morphology and structure of the as-synthesized Au-

core/CZTS-shell hexagonal nanoplates were investigated by

transmission electron microscopy (TEM). As shown in Fig. 3a,

the Au-core/CZTS-shell nanoplates were relatively

monodisperse with well-defined hexagonal facets. The

difference in the atomic masses of Au and CZTS resulted in clear

contrast, revealing the core/shell structure. The diameter of

embedded Au sphere core was around 11 nm on average and the

outer diameter of CTZS hexagonal nanoplate was between

25~30 nm. The thickness of Au-core/CZTS-shell hexagonal

nanoplates was around 11 nm (Fig. 3b), matches well with the

size of the CZTS crystals along the [221] axis estimated by the

Scherrer equation.

Fig. 3 (a) TEM image, (b) Side view TEM image and (c)

HRTEM images of Au/CZTS core-shell hexagonal nanoplates;

(d) FFT pattern of CZTS shell.

The element mapping taken from as-prepared nanoplate (Fig.

4) indicates the co-existence and the distribution of Au, Cu, Zn,

Sn and S, matches well with the core-shell nanoplate

heterostructure, further ensuring the structural homogeneity.

The lattice fringes of Au-core/CZTS-shell hexagonal

nanoplates were observed by high-resolution TEM images (Fig.

3c). The measured inter-plane distance of 0.235 nm can be

ascribed to the (111) plane of fcc Au, which is found as major

planes in sphere Au nanoparticles in common. [26] The observed

inter-plane distance of 0.19 nm agrees with (220) and (204)

planes of kesterite CZTS with an angle of 60◦, which suggests

that the normal direction of the nanoplates is [221] direction and

the surface of the nanoplates is correspond with (112) planes of

kesterite CZTS. These results can be further confirmed by FFT

pattern of CZTS shell (Fig. 3d), which also suggest the high

degree of crystallinity with clear spot on of the

pattern.Interestingly, HRTEM images also show the Moiré

patterns on the Au core area, which can be explained by the

interference between the CZTS layers above and under the Au

core with lattice distortion induced by the embedded Au core.

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Fig. 4 Element mapping of an Au/CZTS core-shell nanoplate.

Formation Mechanism

The Au cores were formed with the typical nucleation growth

process in which the initially formed gold atoms self-nucleate to

form a fixed number of seeds during the first stage of the reaction,

and the particles then continue to grow by diffusion-driven

deposition of gold atoms onto these existing seeds, while the

reducing equivalents in the process were provided by the amine

of oleylamine via β-elimination. [23]

After the growth of Au core of 10 min, the oleylamine

solution containing Cu2+, Zn2+ and Sn2+ was heated from 120◦C

to 180◦C. It is reasonable to propose that there is an

underpotential deposition (UPD) process of cupper on the

surface of gold nanoparticles [27], since the standard electrode

potential of Cu2+/Cu0 (+0.34 eV vs NHE) is lower than that of

Au3+/Au0 (+1.52 eV vs NHE), and the bond energy of Cu-Cu

(201.7±0.4 kJ mol-1) is less than that of Au-Cu (235.6±9.2 kJ

mol-1). Thus a layer of Cu0 was grown on the surface of Au

nanoparticles in the Frank-van der Merwe mode. [28] Actually,

the newly formed Cu atoms on the surface of Au nanoparticles

will diffuse into Au nanoparticles under such a high temperature

of 180◦C (453K) forming an Au-Cu layer on the surface of Au

nanoparticles [29], which may be an intermediate in the growth

of CZTS shell on the surface of Au core. [30,31]

The proposed formation mechanism can be demonstrated by

identifying the presence of Au-Cu alloy with X-ray diffraction

(XRD) pattern of Au nanoparticles centrifugal separated from

the reaction which was quenched at 200◦C without injecting S

source (Fig. 5). The peak at 38.32, 44.66 and 64.72◦ 2θ can be

attributed to fcc Au (JCPDS No. 04-0784). The obvious shoulder

peak right next to the main peak of fcc Au can be explained by

the presence of a thin Au-Cu alloy layer. The large-angle shift of

the peak is due to the smaller atomic radii of Cu (0.128 nm)

compared with Au (0.144 nm).

Fig. 5 XRD pattern of Au nanoparticles centrifugal separated

from the reaction which was quenched at 180◦C without

injecting S source.

Other evidence comes from the Auger Electron Spectroscopy

(AES) investigation of the core/shell nanoplates. The Cu LMM

AES spectrum collected from the surface of the core/shell

nanoplates show a characteristic peak at 917.1 eV, well matched

with Cu+ in the CZTS shell (Fig. 6a). And no Cu0 signal was

observed since the Auger electron collected only from the very

surface less than a few nanometers. In the contrast, an

asymmetric and broad Auger kinetic peak was observed after the

sample was sputtered off around 8 nm depth by Ar ion gun for

50 s. The broad peak was deconvoluted into two symmetrical

peaks centered at 917.1 and 918.2 eV corresponding to Cu+ and

Cu0 respectively (Fig. 6b). These results proved the existence of

the Au-Cu alloy layer during the formation of Au-core/CZTS-

shell hexagonal nanoplates. [31]

Fig. 6 (a) Cu LMM AES spectrum of the surface and at depths

of about 8 nm of Au/CZTS core/shell nanoplates; (b) The

deconvolution result of the Cu LMM AES spectrum at depths of

about 8 nm of Au/CZTS core/shell nanoplates.

When the S source was injected to the precursor solution, the

CZTS nanocrystals was formed via a widely reported process

that Cu0 act as the main nucleation centers for the binary copper

sulfide, transitioning through ternary and subsequently

progressing to yield the quaternary form by the incorporation of

Zn and Sn. [32,33]

From the perspective of crystal structure, kesterite

Cu2ZnSnS4 can be regarded as alternative stacking of positively

charged (112) planes and negatively charged (1̅1̅2̅) planes along

the [221] axis [Fig. 7a]. For the solution phase crystal growth,

various shapes of nanoparticles are formed due to the different

growth rates along different crystal axes caused by the

preferential absorption of molecules and ions in solution to

different crystal faces [34]. Generally, oleylamine molecules will

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be preferentially absorbed onto the polar and positively charged

plane (112), which limit the growth of (112) plane during the

synthetic process.[35] On the other hand, the d-spacing of {110},

{102} and {012} facets are relatively equivalent in kesterite

structure, which suggest that the six planes including (110),

(1̅1̅0), (102), (1̅02̅), (012) and (01̅2̅) will have nearly consistent

growth rate according to the law of Bravais. Besides, the

included angles of these six planes are approaching 60◦ with a

viewing direction along the [221] axis (towards (112) plane) [Fig.

7b], matches well with the hexagonal nanoplate [Fig. 3a]. In

addition, it is worth to mention that a relatively low concentration

and the sharply drop in temperature induced by the injection of

S source may enlarge the difference of growth rates among

different lattice planes. The absorption of oleylamine on the

surface of Hexagonal Au-core/Cu2ZnSnS4-shell nanoplates was

demonstrated by FTIR (Fig. 8).

Fig.7 (a) Positively charged (112) planes of kesterite Cu2ZnSnS4

(Cu-Zn-Sn metallic planes) along the [221] axis; (b) Six planes

including(110), (1̅1̅0), (102), (1̅02̅), (012) and (01̅2̅) of kesterite

Cu2ZnSnS4 with a viewing direction along the [221] axis

(towards (112) plane).

Fig.8 FTIR spectra of oleylamine and asprepared Au/CZTS

core/shell nanoplates dispersed in cyclohexane.

Based on the analysis above, we proposed a formation

mechanism of Hexagonal Au-core/Cu2ZnSnS4-shell nanoplate

heterostructures in several discrete steps as shown in Scheme 1.

Firstly, Au nanoparticles was formed via a typical nucleation

growth for 10 min using oleylamine as the reducing agent,

followed by the formation of a thin Au-Cu alloy layer on the

surface of Au nanoparticles while the reaction system was

heating to 200◦C. After the injection of S source, the CZTS

nanocrystals was formed on the surface of Au nanoparticles

using the Au-Cu alloy layer as an intermediate, then was grown

mainly via (110), (1̅1̅0), (102), (1̅02̅), (012) and (01̅2̅)planes,

due to the limited growth of {112} and the nearly equivalent

growth rate of the six planes mentioned above, which eventually

lead to the formation of hexagonal Au-core/Cu2ZnSnS4-shell

nanoplates.

Light Absorption Properties

The absorption of the samples are investigated by UV-vis

Spectroscopy. Fig. 9 demonstrates that Au-core/CZTS-shell

nanoplates significantly enhance the absorption in the range of

600-1000 nm, particularly in NIR region, compared with CZTS

nanoparticles. This is due to extensive perturbation of energy

states by the plasmonic field of the interface of Au core and

CZTS shell. [18,36]

The absorption spectrum of pure Au nanoparticles shows an

intense peak at ~520 nm due to the LSPR effect. As a comparison,

an obvious redshift of Au SPR energy from 520 nm to ~620 nm,

obtained by the Au-core/CZTS-shell nanoplates heterostructures,

is induced by the effect of introducing a high dielectric constant

of low band gap CZTS to the composite material.[37]

In these experiments, the size of CZTS nanoparticles is

between 20 and 30 nm, which is close to the size of Au-

core/CZTS-shell nanoplates. All of these samples were dispersed

in toluene with the same concentration.

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Fig. 9 UV-vis spectra of Au/CZTS core/shell nanoplates and

CZTS nanoparticles.

Hybrid Photovoltaic Cells

Hybrid photovoltaic cells were fabricated by incorporating Au

NPs, CZTS NPs and Au/CZTS core-shell nanoplates into the

active layer of P3HT:PCBM BHJ-PSCs separately, with a

concentration of 5 wt%. The absorption spectra for these films

was shown in Fig. 10. All curves displayed an absorption peak

centered at near 500 nm with three pronounced vibronic

absorption peaks, showing good accord with the typical spectral

response of P3HT:PCBM composites. Since P3HT is the strong

absorber and the weight ratios of NPs were relatively low, those

NPs hardly changed shapes of each absorption curve. The little

blue-shifted peaks of CZTS NPs incorporated hybrid films

compared to that of the purely organic film are due to the

confinement effect by CZTS. [38]

Fig. 10 UV-vis spectra of P3HT:PCBM films with and without

bare Au NPs, CZTS NPs and Au/CZTS core-shell nanoplates

incorporation.

However, the absorption intensity of NPs incorporated

P3HT:PCBM layers are obviously stronger than that of pristine

P3HT:PCBM layer, among which the layer incorporated with

Au/CZTS core-shell nanoplates show largest absorption

enhancement compared with layers incorporated with merely Au

and CZTS NPs.

Meanwhile, an obvious enhancement of absorption can be

observed in the short wave range of visible region in both layers

incorporated with CZTS NPs and Au/CZTS core-shell

nanoplates, which shows good accord with the relatively high

absorption coefficient of CZTS in the same region (Fig. 9).

The optimized PSC performance with and without incorporation

in the active layers was shown in Table 1, and the corresponding

current density–voltage (J–V) characteristics under the AM 1.5

illumination were shown in Fig. 11. As shown in Table 1, the

JSC values of devices with different nanoparticles incorporated

show various enhancement compared with that of reference

devices. The largest JSC improvement belongs to the device with

Au/CZTS core-shell nanoplates as excepted, namely from 8.03

mA/cm2 to 8.74 mA/cm2, corresponding to a noticeable

enhancement in PCE from 2.08% to 2.63%.

It is well-known that the FF is closely related to electrical

property of the photovoltaic device. Among our devices, the FF

of the device with bare Au NPs is relatively lower than that of

reference device, while the FF of the devices with CZTS NPs and

Au/CZTS core-shell nanoplates are slightly enhanced from that

of reference device, which demonstrates that the electrical

property of PSCs can be relatively improved by coupling Au core

with CZTS shell.

Fig. 11 J-V curves of the P3HT:PCBM BHJ-PSC devices after

thermal annealing with and without bare Au NPs, CZTS NPs and

Au/CZTS core-shell nanoplates incorporation. The

measurements were carried out under AM 1.5 illumination at an

irradiation intensity of 100 mW•cm-2.

Based on the core-shell heterostructure of the Au/CZTS

nanoplate, we conclude three main factors that influence the

efficiency of PSC.

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The first one is the light harvest and excitions generation of

CZTS shell, which is boosted by the interaction with Au core.

Upon the absorption of light, the electrons in Au core are excited

to a higher energy state due to the LSPR effect. Due to the

comparable energy difference between these excited electrons

from Au core with those of conduction band of CZTS shell, there

is apparently a strong electronic interaction (coupling) between

these two states. Such electron confinements and interactions in

the nano-region and particularly at the nano-materials interface

is expected to induce electron transfer or associated field effect

to each other hence enhancing the local density of states (LDOS)

for a higher quantity of excitons formation. [18,19,20]

The second important factor should be that the coated CZTS

shell will prevent the excited electrons on Au nanoparticle

surface transferring to active materials in PSCs. As shown in

Scheme 2a, we believe that excited electrons in the outer orbit of

Au atoms on the surface of Au nanoparticles may directly

transfer to P3HT or PCBM in the vicinity of the nanoparticles,

thus this process quenches the excitons in active layer. However,

in the P3HT:PCBM:Au/CZTS core-shell NPs blend system,

direct exciton quenching would be blocked by the CZTS shell as

in Scheme 2b, but the energy induced by the LSPR effect at Au

nanoparticle surface can go through the thin CZTS shell and

transfer to P3HT:PCBM active material layer. In addition, there

are several reports [3,39,40,41] supporting this point of view that

incorporated bare metallic NPs in active layer should cause the

exciton quenching.

The third factor is energy levels introduced by CZTS shell as

shown schematically in Scheme 2b. As a result, the energy

barrier for separation of carriers between P3HT and PCBM was

lowered by introducing CZTS shell, which results in the

improved short-circuit current. [42,43]

Besides, the organic ligands capping the NPs, namely

oleylamine in this study, which plays a key role in preventing

aggregation and improving dispersity, can cause poor charge

transport through the inorganic NPs-organic material interface,

limiting effective current flow and instigate internal resistance in

the matrices. The defect which restrict the performance of the

hybrid cells can be considerably eliminated by exchanging these

surfactants with short alkyl molecules (e.g. ethanethiol and

pyridine) or removing them. [44,45,46]

Conclusions

In summary, Au/CZTS core-shell nanoplates have been

synthesized via a one-pot multi-step process. The preformed Au

NPs were employed as “seeds” and served as primary substrate

centers for growing CZTS shells. A mechanism was proposed to

explain the formation of such hexagonal core-shell nanoplate

structure, including the Au-Cu alloy intermediate bridge and the

preferential absorption of molecules and ions in solution to

different crystal faces, supported by various characterizations. It

was also demonstrated that a PCE enhancement of 26.4% in the

BHJ-PSCs has been achieved by incorporating Au/CZTS core-

shell nanoplates into the active layer of solar cells. The as-

prepared hybrid nanostructure not only brings both advantages

of LSPR effect of Au core and the light harvest and excitions

generation of CZTS shell, but also prevent the quenching of

excitons at the surface of bare Au nanoparticles by the core-shell

structure. We believe that this study provides a new insight into

the formation of noble metal /I2–II–IV–VI4 core-shell structure

and a new approach of using metal nanoparticles and inorganic

semiconductor materials to enhance the performance of polymer

solar cells.

Acknowledgements

This work was supported by National Basic Research Program

of China (973 Program)-2012CB922001, the Fundamental

Research Funds for the Central Universities (WK2060140005).

Notes and references

a CAS Key Laboratory of Materials for Energy Conversion, Department of

Materials Science and Engineering, University of Science and Technology

of China, Hefei 230026, China.

Email: [email protected]

Paper Nanoscale

8 | J. Name., [year], [vol], 00-00 This journal is © The Royal Society of Chemistry 2014

b CAS Key Laboratory of Materials for Energy Conversion, Department of

Materials Science and Engineering, University of Science and Technology

of China, Hefei 230026, China.

Email: [email protected]

† Electronic Supplementary Information (ESI) available: [details of any

supplementary information available should be included here]. See

DOI: 10.1039/b000000x/

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