Aqueous-solution-processed PPV-CdxHg1-xTe hybrid solar cells with a significant near-infrared contribution

Article abstract of DOI:10.1039/c2jm33958g

Aqueous poly(p-phenylene-vinylene) (PPV):Cd_xHg_1-xTe nanocrystal (NC) hybrid solar cells are prepared with heat-introduced aggregation and growth of the NCs. The morphology of the active layer is easily controlled by tuning the compos

Full text of DOI:10.1039/c2jm33958g

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PAPER
Aqueous-solution-processed PPV–Cd Hg Te hybrid solar cells with a
x
1ꢀx
significant near-infrared contribution†
Haotong Wei, Hao Zhang, Haizhu Sun, Weili Yu, Yi Liu, Zhaolai Chen, Liying Cui, Wenjing Tian
and Bai Yang*
Received 19th June 2012, Accepted 5th July 2012
DOI: 10.1039/c2jm33958g
Aqueous poly(p-phenylene-vinylene) (PPV):Cd
x
Hg1ꢀxTe nanocrystal (NC) hybrid solar cells are
prepared with heat-introduced aggregation and growth of the NCs. The morphology of the active layer
is easily controlled by tuning the composition of the donor and acceptor and the charge carrier mobility
is largely improved after annealing for 1 h. Through passivating the surface traps on the NCs with the
UV-ozone radiation, the Voc can be improved. The photovoltaic devices present a wide response range
in the solar spectrum from UV to 1200 nm. The near-infrared absorption is attributed to the
Cd
favors the stability of the NCs due to the strong ionic bond of HgTe. The power conversion efficiency
PCE) of such photovoltaic devices exhibits about 1.5% under AM 1.5G illumination with a high short-
x
Hg1ꢀxTe NCs and the absorption extends to near-infrared as the Cd/Hg ratios decrease, which
(
ꢀ
2
circuit current density (Jsc) of 12.84 mA cm . 11.4% near-infrared contribution is observed from the
external quantum efficiency (EQE) measurement with only simple device structures.
Introduction
environmental pollution, aqueous solution-processed materials
27–32
are welcome in the construction of solar cells.
However, the
Because they can be solution-processed, p-conjugated polymers
have been considered as competitive candidates to produce large
fabrication of aqueous solution-processed HSC is still challenging
due to the limitation of too many surface traps of the aqueous
NCs, leading from the complex aqueous environment in the NCs
synthesis. What’s more, most aqueous solution-processed HSC
are composed of NCs with UV–visible absorption, while near-
infrared NCs are rarely applied in device construction.
1
–9
area, low cost, and flexible solar cells. The power conversion
efficiency (PCE) of polymer solar cells (PSC) has approached to
10–12
% since the bulk heterojunction structure was employed.
9
Meanwhile, the low charge carrier mobility of the polymer
materials greatly stimulates the construction of organic–inorganic
13–17
hybrid solar cells (HSC)
As a conventional material in construction of thin film solar
cells, bulk Cd Hg Te crystal has a changeable band gap, from
because inorganic materials are
x
1ꢀx
to 1.6 eV, potentially suitable for fabrication of HSC with proper
capable of overcoming the electron transport limitations associ-
13,16,18,19
ated with the organic materials.
0
In particular, the recent
x
conjugated polymers. In this paper, aqueous Cd Hg1ꢀxTe NCs
advances in colloidal synthesis of high quality nanocrystals (NCs)
20–22
allow the band gaps of inorganic materials to be tailored,
with tunable band gap were synthesized using mercapto-
compounds as the capping ligands. Moreover, the Cd/Hg ratio in
thus
opening the door for further enhancing the performance of HSC.
With respect to the light harvesting, employing NCs with near-
infrared absorption is another effective method to increase the
photocurrent, because half of the Sun’s power reaching the
Cd
x
Hg1ꢀxTe NCs could be altered in a broad range, which made
Hg1ꢀxTe tunable from visible to near-
the absorption of Cd
x
infrared region. We reported the fabrication of the HSC from
aqueous solution-processed materials, employing the water-
soluble poly (p-phenylene-vinylene) (PPV) precursor, which were
converted to the PPV (donor) after annealing under nitrogen, and
aqueous Cd Hg Te NCs (acceptor) with near-infrared absorp-
23
surface of the Earth belongs to the infrared. In this aspect, Pb-
containing NCs, such as PbS and PbSe, are mostly investi-
23–26
gated.
It should be mentioned that the near-infrared NCs
x
1ꢀx
have been synthesized in organic media, and the device fabrica-
tion is carried out in organic solvents. To exploit the merit of the
solution-processed nature of HSC as well as avoiding
tion. Efficient HSC with the PCE of 1.5% was fabricated through
the controllable heat-introduced aggregation and growth of the
NCs, which involved a near-infrared contribution up to 11.4%.
State Key Laboratory of Supramolecular Structure and Materials, College
of Chemistry, Jilin University, Changchun 130012, People’s Republic of
China. E-mail: byangchem@jlu.edu.cn; Fax: +86-431-85193423; Tel:
Experimental
Materials
+
86-431-85168478
0
Tellurium powder (200 mesh, 99.8%), R, R -dichloro-p-xylene
† Electronic supplementary information (ESI) available: The absorption
of the NCs with different Cd/Hg ratios. See DOI: 10.1039/c2jm33958g
(98%), tetrahydrothiophene (99%) and 3-mercaptopropionic
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 17827–17832 | 17827

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ꢁ
acid (MPA) were all purchased from Aldrich Chemical Corp.
CdCl (99+ %), HgCl (99+ %) were commercially available. All
The blend film was annealed at 300 C in the glove box, followed
2
by the evaporation of Al electrode with a 4 mm mask.
2
2
of the solvents were analytical grade and used as received.
Characterization
The preparation of the PPV
UV–vis spectra were acquired on a Shimadzu 3600 UV–vis–NIR
spectrophotometer. Fluorescence spectra were acquired on a
Shimadzu RF-5301 PC spectrofluorimeter and the excitation
wavelength was 365 nm. Atomic force microscopy (AFM)
images were recorded in tapping mode with a Digital Instru-
ments NanoScope IIIa under ambient conditions. Transmission
electron microscopy (TEM) images were recorded on a JEOL-
The synthesis method of the PPV precursor could be found
33
elsewhere. Briefly, 0.4 M NaOH 10 ml was added to 10 ml of
0
.4 M p-xylylenebis (tetrahydrothiophenium chloride) methanol
ꢁ
solution. This solution was cooled to 0–5 C in an ice bath. The
reaction proceeded for 1 h and then was terminated by the
addition of 0.4 M HCl aqueous solution to neutralize the reac-
tion solution. The PPV precursor aqueous solution was dialyzed
against deionized water for one week. The PPV precursor can be
converted to conjugated PPV after annealing under nitrogen,
and the synthesis route is presented in Scheme 1.
2
010 electron microscope operating at 200 kV. The film thick-
nesses were measured on an Ambios Tech. XP-2 profilometer.
EQE was measured under illumination of monochromatic light
from the xenon lamp using a monochromator (Jobin Yvon,
TRIAX 320) and detected by a computer-controlled Stanford
SR830 lock-in amplifier with a Stanford SR540 chopper. The
current density–voltage (J–V) characterization of PV devices
under white-light illumination from an SCIENCETECH 500–W
The synthesis of aqueous Cd
x
Hg1ꢀxTe NCs
Cd Hg Te NCs covered with 3-mercaptopropionic acid were
x
1ꢀx
ꢀ
2
34
solar simulator (AM 1.5G, 100 mW cm ) was carried out on
computer-controlled Keithley 2400 Source Meter measurement
system in a glove box filled with nitrogen atmosphere (<1 ppm
H O and <1 ppm O ). All the other measurements were per-
prepared according to the previous method. Freshly prepared
NaHTe solution was added to a N -saturated aqueous solution
2
of CdCl
2
and HgCl
2
at pH 9 in the presence of MPA as a
ꢀ
stabilizing agent. The ratio of (Cd + Hg) : MPA : HTe was
2
2
formed under ambient atmosphere at room temperature.
1
3
: 2.9 : 0.5 and the concentration of metal ion (Cd + Hg) was
.75 ꢂ 10ꢀ M. The precursor solution was then placed in an
3
ꢁ
oven at 40 C for 30 min. For the CdTe NCs fabrication, the
precursor solution was suffered refluxing for 5 h instead of oven.
The absorption of the NCs solution can be found in Fig. S1.†
Results and discussion
The PPV precursor converts to PPV with a conjugated structure
during annealing under nitrogen and the formation process of
the PPV can be found in Scheme 1, while the surface ligands of
the NCs are also removed and the electron transport path forms
as the NCs aggregate and grow. The process can be illustrated by
the scheme in Fig. 1(a). Before the annealing process, the NCs
disperse in the polymers and less interlinking exists. During the
annealing process, the aggregation and growth of the NCs take
place. The growth of the NCs results in interlinking of the NCs,
and an effective path is generated for the charge carrier
Devices fabrication
ITO-coated glasses first underwent ultrasonic treatment in
chloroform, acetone, isopropyl, then were rinsed by deionized
2
water before drying in N flow, followed by the oxygen plasma
treatment for 5 min; then spin-coating of the PEDOT:PSS layer
ꢁ
(
45 nm) at 3000 rpm for 60 s followed by annealing at 250 C for
3
0 min under N in order to avoid dissolution by the active layer.
2
Before blending the donor and acceptor, Cd Hg Te NCs
x
1ꢀx
ꢀ1
underwent ligand exchange with 0.2 mg ml 2-mercaptoethyl-
amine water solution (2-MAW), and the precipitate was
obtained by centrifugation at 6000 r per min for 5 min. The
precipitated NCs were redissolved in 2-MAW and further cen-
trifugated by the addition of isopropanol to remove the super-
fluous salts and ligands, and this process was repeated two times.
The precipitated NCs were redissolved in deionized water with a
ꢀ
1
concentration of 84 mg ml . The photoactive layer was formed
by spin-coating (at 600 r per min), with an aqueous solution
ꢀ1
ꢀ
1
containing 1 mg ml PPV precursor and 42 mg ml
ꢀ1
N Cs.
Fig. 1 (a) The scheme describes the annealing process of the photoactive
layer, and the interpenetrating network structure forms after annealing.
(
b) The absorption spectra of the blend films with different
Cd0.75Hg0.25Te NC contents. (c) The PL emission quenching process as
the Cd0.75Hg0.25Te NC content increases.
Scheme 1 The synthetic route for the conjugated PPV.
7828 | J. Mater. Chem., 2012, 22, 17827–17832
1
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transport. Fig. 1(b) presents the absorption of the blend films
after annealing as the weight percent of the Cd0.75Hg0.25Te NCs
(
3 : 1 Cd/Hg ratio) changes and the PPV film seems to absorb
little light above 600 nm, while the absorption of the NCs after
annealing covers from UV to 1200 nm. The broad absorption of
the NCs illustrates a broad size distribution caused by the
inhomogeneous growth of the NCs because of the limitation of
the PPV chains. The photons absorbed by the PPV form the
excitons, and exciton dissociation is able to take place at the
heterojunction, as a result, the electron is transferred from PPV
to NCs. This transferred process is demonstrated by the photo-
luminescence (PL) quenching of PPV described in Fig. 1(c). The
PL emission of the PPV film with strong p–p interacting chains is
located at 546 nm, while the PL emission of the weak interactions
without p–p scaffold can be also detected at 516 nm. The PL of
the PPV is gradually quenched by the NCs as the NC content
increases and the relative PL emission intensity of the chains with
strong p–p interactions is decreased compared to that of weak
interacting chains because the interactions is largely relaxed by
Fig. 3 AFM (5 mm ꢂ 5 mm) topography (a–c) and the corresponding
phase images (d–f) of the PPV:Cd Hg Te NCs active layer with
0.25
0
.75
various compositions: (a and d) 30 wt% NCs, (b and e) 60 wt% NCs, (c
and f) 97 wt% NCs. The active layer is suffered from annealing for 60 min
2
under N .
ensures the charge separation and clear transport at the hetero-
junction and apparent phase separation occurs. Generally, too
rough an active layer surface may lead to poor contact with the
metal cathode. However, the RMS roughness of the blend films is
only 4.62 nm after the larger NCs formed.
34
the NCs. The energy structure of the HSC is described in Fig. 2.
The highest occupied molecular orbital (HOMO) level of the
PPV is ꢀ5.1 eV, and the lowest unoccupied molecular orbital
(
LUMO) level of the PPV is about ꢀ2.8 eV. While the conduc-
tion band of the NCs is ꢀ4.0 eV, and the valence band of the NCs
is ꢀ5.3 eV relative to the vacuum level. This energy band favors
The internal morphology of the blend films are explored by
using TEM. From Fig. 4 we can see that the size of the NCs is
inhomogeneous and the average size is about 40 nm, what’s more,
this growth process mainly takes place during the first 10 min for
34
the electron transport from PPV to the Cd0.75Hg0.25Te NCs.
Since the nanoscopic morphology of the blend film directly
determines the charge separation and transport, tapping mode
AFM is employed to study the film morphology. Because of the
excellent membrane forming properties of PPV, the blend film
with 30 wt% NCs shows a smooth surface with a root-mean-
square (RMS) roughness of 1.15 nm as presented in Fig. 3(a) and
the NCs are separately distributed in the blend film. As the
weight percent of the NCs reach 60 wt%, the RMS roughness of
the blend films (1.43 nm) doesn’t change much while the two
phase structure presents apparent as shown in Fig. 3(e). The large
aggregation region of the NCs appears though no inter-
penetrating network forms. However, the photocurrent is often
supplied with a path in the form of network. Fig. 3(c) and (f)
show that the interpenetrating network structure is established as
the content of the NCs reaches 97 wt%. The network structure
35
the little change of the morphology after annealing for 10 min.
However, we observe that the electron-only carrier mobility
increases a lot after annealing for 1 hour compared to that after
11,13
only 30 min as shown in Fig. 4(d).
The electron mobility of the
ꢀ7
2
ꢀ1 ꢀ1
blend film heated for 10 min is 3.90 ꢂ 10 cm V
of the blend film is improved after annealing for 30 min (5.18 ꢂ
s , and that
ꢀ7
2
cm V s ). Both of the values are too low for an effective
ꢀ1 ꢀ1
1
0
photovoltaic device. However, after annealing for 1 hour, the
ꢀ4
2
electron mobility of the hybrid film increases to 1.47 ꢂ 10 cm
ꢀ1 ꢀ1
V
s . Therefore, 30 min is not sufficient to remove most ligands
Fig. 4 (a–c) TEM images of the PPV:Cd0.75Hg0.25Te blend films with
0.5
different annealing time (10 min, 30 min and 60 min). (d) J vs. V plots
for the blend film with different annealing time and the insert is the
corresponding amplification image. The solid line is a fit of the data
points. The films thicknesses are about 80 nm.
Fig. 2 The energy structure of the HSC.
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J. Mater. Chem., 2012, 22, 17827–17832 | 17829

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covering the surface of the NCs. X-ray photoelectron spectros-
copy (XPS) analysis for the S2p is used to calculate the ligand
content on the NCs’ surface after annealing as shown in Fig. 5(a).
As the annealing time increases, the ligands are gradually
removed. Through the S/M (Cd + Hg) atom ratios, the weight
percents of the ligands can be calculated and the corresponding
calculation results are plotted in Fig. 5(b). From Fig. 5(b) we can
see that the contents of the ligands after 30 min annealing of the
NCs films (68.0 wt%) is similar to that of 10 min (73.1 wt%).
However, there are only 29.4 wt% ligands left after 60 min
annealing which ensures the interpenetrating network structure
and the enhancement of the electron mobility. Hole-only charge
carrier mobility was measured according to a similar method
Fig. 6 Hole mobility of the PPV:Cd0.75Hg0.25Te hybrid device.
using
thiophene):poly(styrenesulfonate) (PEDOT:PSS)/PPV:Cd0.75
Hg0.25Te/MoO /Au by using the space-charge-limited-current
SCLC) model at low voltage which is described by:
a
diode configuration of ITO/poly(ethylenedioxy-
-
3
(
2
3
J ¼ 93
0 r
3 mV =8L
ꢀ
12
ꢀ1
F m ), 3
r
where 3
0
is the permittivity of free space (8.85 ꢂ 10
is the dielectric constant of the PPV (assumed to 3), m is the hole
mobility of the PPV, V is the applied voltage, and L is the film
13,36,37
thickness.
By fitting the results to a space charge limited
0
.5
form, J versus Vappl is plotted in the Fig. 6. The thickness of the
blend film was about 80 nm, which was determined by the
Ambios Tech. XP-2 profilometer. The hole mobility of the PPV
was about 1.12 ꢂ 10ꢀ cm V
4
2
ꢀ1 ꢀ1
s , which was similar to the
electron mobility of the Cd0.75Hg0.25Te. The electron mobility is
measured according to a similar method with an electron-only
device structure of ITO/Al/PPV:Cd0.75Hg0.25Te/LiF/Al. The
balanced carrier mobility results in the effective performance of
the HSC.
Fig. 7(a) depicts the current density–voltage (J–V) curves of
the PPV:Cd0.75Hg0.25Te HSC devices measured under AM 1.5 G
ꢀ
2
illumination (100 mW cm ). As the annealing time increases,
both of the short-circuit current density (Jsc) and the open-circuit
voltage (Voc) are enhanced, and this is also consistent with the
electron mobility increase. The device performance with an
annealing time of 30 min is better than that of 10 min. However,
efficient HSC is obtained after annealing for 60 min with J of
sc
ꢀ
2
2.84 mA cm , Voc of 0.34 V, fill factor (FF) of 34.3% and PCE
1
of 1.5%. Sufficient annealing time allows the surface ligands to be
removed as much as possible, and efficient interpenetrating
network is formed as the NCs grow, which are beneficial to the
Fig. 7 (a) Device performance of the PPV:Cd0.75Hg0.25Te HSC annealed
ꢁ
at 300
C for different time. (b) Devices performance of the
Fig. 5 (a) XPS spectra of S2p in the Cd0.75Hg0.25Te NCs films with
different annealing time. (b) The weight contents of the ligands in the
NCs calculated with the S atom in the ligands employing metal elements
as a reference.
PPV:Cd Hg1ꢀxTe HSC with different Cd/Hg ratios. For the CdTe NCs
x
and Cd0.91Hg0.09Te NCs, the HSC performs better if UV-ozone is used to
treat the blend films. (c) The external quantum efficiency measurement of
the PPV:Cd0.75Hg0.25Te HSC device with 60 min annealing.
1
7830 | J. Mater. Chem., 2012, 22, 17827–17832
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Table 1 Current–voltage characteristics of the photovoltaic devices with
different Cd/Hg ratio. For the CdTe and Cd0.91Hg0.09Te NCs, UV-ozone
radiation is used to reduce the surface traps
1.5% under AM 1.5G illumination. 11.4% near-infrared contri-
bution is observed from the EQE measurement with only simple
devices structure. All the present findings prove that the aqueous
PPV:Cd
x
Hg1ꢀxTe HSC is promising and attractive for organic–
Cd/Hg
ratio
UV-ozone
time (min)
J
sc
(mA cm
a
ꢀ2
)
V
oc (V)
FF (%)
PCE (%)
inorganic optoelectronic devices for the clean, environment-
friendly, renewable energy application.
3
5
1
1
1
1
: 1
: 1
0 : 1
0 : 1
: 0
0
0
0
10
0
10
12.84
9.05
6.81
6.37
3.04
3.62
0.34
0.32
0.12
0.26
0.12
0.34
34.3
35.0
31.4
30.2
28.4
33.2
1.50
1.01
0.26
0.50
0.10
0.41
Acknowledgements
This project was supported by the National Basic Research
Development Program of China (2012CB933802), the NSFC
: 0
a
The corresponding NCs compositions with different Cd/Hg ratios
3 : 1, 5 : 1, 10 : 1, and 1 : 0) are described as Cd0.75Hg0.25
(
50973039, 91123031, 20921003 and 20804008)
(
,
Cd0.83Hg0.17Te, Cd0.91Hg0.09Te and CdTe respectively.
Notes and references
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PCE enhancement. The low Voc of the devices is caused by the
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3
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H. Chen, J. H. Hou, S. Q. Zhang, Y. Y. Liang, G. W. Yang, Y. Yang,
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x
Hg1ꢀxTe HSC devices with different Cd/Hg ratios.
ꢀ2
ꢀ2
The Jsc is 3.04 mA cm for the CdTe, 6.37 mA cm for the
2
Cd0.91Hg0.09Te (10 : 1), 9.05 mA cm for the Cd0.83Hg0.17Te
ꢀ
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2
(
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.12 V due to the surface traps caused by oxidation, and the Voc
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for 10 min. However, there is no effect for Cd0.83Hg0.17Te NCs
and Cd0.75Hg0.25Te NCs which may be due to the strong
oxidation resistance resulting from the strong ionic bond of the
HgTe compared to the CdTe. The corresponding external
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presented in Fig. 7(c), and we can see that the
PPV:Cd0.75Hg0.25Te HSC has a broad response range of the
spectra from 300 to 1000 nm as expected from the absorption
spectra. The maximum value reaches 50% at 370 nm and the
contribution of near-infrared (800–1000 nm) to the photocurrent
reaches 11.4% which is assigned to the absorption of the
Cd0.75Hg0.25Te NCs. The devices performance of NCs with
different Cd/Hg ratios are also summarized in Table 1, from
which we can see that the Jsc of the device is enhanced as the
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Conclusions
In summary, we have reported the fabrication and character-
ization of the aqueous PPV:Cd
Hg1ꢀxTe HSC with broad
x
coverage of the solar spectrum. By employing the method of
controllable heat-introduced aggregation and growth of the
NCs, the blend film morphology can be easily controlled and the
charge carrier mobility can be easily improved by increasing
the annealing time. The largely improved charge carrier mobility
is also derived from the elimination of the surface ligands of the
NCs and the formation of the interpenetrating network struc-
ture. The coverage range of the absorption spectra can be easily
tuned by changing the Cd/Hg ratio and the surface traps of the
aqueous NCs can be reduced to some extent by UV-ozone
radiation. The PCE of such photovoltaic devices exhibit about
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