grafting and co-grafting of dyes on cd-doped zns
TRANSCRIPT
Grafting and co-grafting of dyes on Cd-doped ZnS nanocrystalsand their application on dye-sensitized solar cells
UZMA JABEEN1,2, SYED MUJTABA SHAH2, MUHAMMAD AAMIR3,* and IQBAL AHMAD4
1 Faculty of Basic Sciences, Sardar Bahadur Khan Women’s University, Quetta 87300, Pakistan2 Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan3 Materials Laboratory, Department of Chemistry, Mirpur University of Science and Technology (MUST),
Mirpur 10250, Pakistan4 Department of Chemistry, Allama Iqbal Open University, Islamabad 44000, Pakistan
*Author for correspondence ([email protected])
MS received 18 June 2021; accepted 23 August 2021
Abstract. Herein, we report that an efficient nanohybrid material consists of Cd-doped ZnS nanocrystals (NCs),
merocyanine and fullerene (C60-pyrrolidine tris acid). Cd-ZnS NCs serve as a substrate for supramolecular complexation
between merocyanine 540 and fullerenes. The impact of grafting and co-grafting of dyes on the NCs surface was checked
by Fourier transform infrared (FTIR), photoluminescence and ultraviolet–visible (UV–Vis) spectroscopic studies. Cd-
doped ZnS NCs were synthesized by wet chemical approach and described by powdered X-ray diffraction, UV–Vis
spectroscopy, field emission scanning electron microscopy and transmission electron microscopy. The grafted and co-
grafted NCs were then used as an active blend in hybrid solar cells. The hybrid solar cell of grafted material blend (Cd-
ZnS-MC540) shows the maximum short circuit current density 4.60 mA cm–2 and power conversion efficiency of 0.83%.
The open circuit voltage, fill factors and cell conversion efficiency of all photovoltaic devices based on co-grafted Cd-
doped ZnS NCs and P3HT decreases with the increase in concentration of donor and acceptor species. We note that by co-
grafting, dye/dye interaction is replaced by dye/fullerene interaction but unfortunately co-grafting may have led to the
formation of big clusters. Hence, the lack of morphological homogeneity may be held responsible for the weak perfor-
mance of the solar cells.
Keywords. Zinc sulphide; merocyanine; grafting; co-grafting; dye-sensitized solar cell; short-circuit current density.
1. Introduction
Substantial effort has been employed in past few decades
to fabricate efficient renewable energy conversion devices
[1]. Recently, a great attention has been paid on organic
polymers and nanocrystals (NCs)-based devices like thin
film transistors, lasers, light-emitting diodes and photo-
voltaic devices due to their low cost, flexibility and
simple processability [2–4]. Among photovoltaics, dye-
sensitized solar cells (DSSCs) have gained enormous
attention owing to cost effectiveness, easy processability
and low toxicity [5–7]. In the field of photonics and
artificial photosynthesis, the efficient electron transfers,
long-lived charge separated states, and strong quantum
yield by decelerating charge recombination and enhanced
charge separation are the critical challenges [8,9]. In this
perspective, transition metal polyperidyl complexes and
organic dyes have been employed as a photosensitizer
with considerable efficiency [10–12]. Organic dyes along
with D-p-A structures, riveted to the semiconducting
surface via carboxylic functionalities, have been estab-
lished as efficient photosensitizers [13,14].
The fullerenes like C60 and its derivatives have been
widely explored as an electron acceptor to fabricate efficient
light harvesting devices with better charge separation
[15,16]. Fullerenes are carrier acceptors owing to their
photophysical and electrochemical characteristics, as they
can accept up to six electrons in the three lowest unoccupied
molecular orbitals and efficiently transfer electron to gen-
erate a long-lived charge separated state along with strong
quantum yield [17,18]. Provoked by the beyond funda-
mental features of fullerenes, donor–acceptor charge
transfer complex systems have shown distinctive inter and
intramolecular interactions after photoexcitation [19].
MC540, an anionic cyanine dye, is a heterocyclic chro-
mophore which is employed as a photosensitizer in
chemotherapy, DSSCs and in photo-electrochemical
devices [20].
Among various semiconducting nanostructures, zinc
oxide (ZnO) nanorods have fascinated scientific community
Bull. Mater. Sci. (2021) 44:291 � Indian Academy of Scienceshttps://doi.org/10.1007/s12034-021-02575-3Sadhana(0123456789().,-volV)FT3](0123456789().,-volV)
due to the surface functionalization with organic dyes,
which can be synthesized by means of appropriate length/
width characteristic fraction and correspond to best scaf-
folds to accumulate organic colour barrier on their surface.
The porphyrin molecules have been successfully grafted
onto the surface of zinc oxide [21–23]. Furthermore, zinc
oxide, as an electron acceptor can favour the charge sepa-
ration characteristics of the organic binary complex [24].
Likewise, zinc sulphide (ZnS) and cadmium sulphide (CdS)
have also been found promising semiconductors for pho-
tovoltaic applications [25].
In this study, we selected merocyanine 540 as a sensitizer
or donor species for Cd-ZnS NCs. In this context, Cd-ZnS
NCs were grafted with merocyanine 540 and fullerene
derivatives with a view to facilitate charge injection. The
sensitizer or donor species, bear the IUPAC name
sodium;3-[(2Z)-2-[(E)-4-(1,3-dibutyl-4,6-dioxo-2-sulfanyli-
dene-1,3-diazinan-5-ylidene) but-2-enylidene]-1,3-benzox-
azol-3-yl] propane-1-sulfonate, (merocyanine 540) were
abbreviated as MC540 in this study. Whereas the acceptor
species are derivatives of fullerene (C60-PTA) bearing
three –COOH anchoring groups. Chemical structures of
these compounds are shown in figure 1a and b.
Cd-ZnS NCs were served as a substrate for supramolec-
ular complexation between merocyanine 540 and fullerenes.
These combinations of co-grafted donors and acceptors
should allow a well-ordered pattern (self-assembly) on
semiconductor nanoparticles, where donor and acceptor
species may be sufficiently close to each other to undergo
Van der Waal’s type interactions and as a result enhanced
the charge transfer.
2. Experimental
2.1 Chemicals
Analytical grade zinc acetate (Zn(CH3COO)2�2H2O), cad-
mium acetate (Cd(CH3COO)2�4H2O) and sodium sulphide,
2-butanol, merocyanin 540, fullerene (C60-PTA) were
purchased from Sigma Aldrich and were used without fur-
ther purification.
2.2 Synthesis of Cd-doped ZnS NCs
Cd-doped ZnS NCs were synthesized by wet chemical
approach. Briefly, 1.0 M zinc acetate dehydrate solution
was prepared in 50 ml of ethanol/deionized water (50:50)
mixture. Likewise, 1.0 M sodium sulphide and 0.5 M cad-
mium acetate solutions were prepared in ethanol-deionized
water mixture, separately. To synthesize Cd-ZnS NCs, zinc
acetate solution (1.0 M) was mixed with cadmium acetate
solution (0.5 M) under stirring for 60 min at 80�C. Subse-
quently, sodium sulphide solution (1.0 M) was added
dropwise in the above solution under stirring at 80�C until
yellow precipitates of Cd-doped ZnS NCs appeared. The as-
obtained precipitates were collected by centrifugation and
washed several times with ethanol and deionized water.
Finally, the Cd-doped ZnS NCs were dried at 110�C and
characterized for further use.
2.3 Synthesis of Cd-ZnS-MC540-C60-PTA nanohybridself-assembly
Functionalization of Cd-ZnS NCs was accomplished in two
steps. In the first step, the Cd-ZnS NCs were grafted with
merocyanine 540 to attain Cd-ZnS-MC540 nanohybrids
assemblies, which were further grafted with fullerene (C60-
pyrrolidine tris acid) to produce co-grafted nanohybrids
indicated as ZnS:Cd-MC540-C60-PTA.
Briefly, the synthesis of Cd-ZnS-MC540 nanohybrid
assembly was achieved by mixing 5.0 ml of MC540 solu-
tion of concentration varying in the range of 4 9 10–6 to
12 9 10–6 M with 5.0 ml of 0.1 mg ml–1 solution of Cd-
ZnS NCs in butanol. The resultant mixture was stirred for
16 h at room temperature. For the synthesis of co-grafted
Figure 1. The chemical structures of donor (a) MC540 and accepter (b) C60-PTA used for grafting and co-grafting
of Cd-ZnS NCs.
291 Page 2 of 9 Bull. Mater. Sci. (2021) 44:291
Cd-ZnS-MC540-MC-C60-PTA nanohybrid assembly, C60-
PTA molecules of 1:1 ratio of MC:C60-PTA was added in
the butanol solution of Cd-ZnS-MC540 under stirring for
16 h at room temperature. Afterward, 1.0 ml of the co-
grafted Cd-ZnS-MC540-MC-C60-PTA nanohybrid assem-
bly solution was centrifuged at 4500 rpm for 4 min. The
obtained centrifuged sample was further washed with
butanol to remove un-absorbed dye molecules. The sche-
matic diagram for the synthesis of Cd-ZnS-MC540-C60-
PTA nanohybrid self-assembly is given in figure 2.
2.4 Device fabrication
The indium tin oxide (ITO)-coated glass substrates were
cleaned repetitively with cleaner powder, distilled water,
acetone, and isopropyl alcohol in an ultrasonic bath for
20 min each and dried in air. Then, the patterned substrates
were heated at about 90�C for 15 min. Moreover, the pro-
cess of plasma cleaning was done on ITO substrates for
25 min. Consequently, ZnO gel was spin coated on ITO
substrate at 3700 rpm for 15 s and annealed at 130�C for
30 min to obtain a 30 nm thick zinc oxide film. The mixture
of co-grafted Cd-ZnS-MC540-C60-PTA NCs and organic
polymer, P3HT in the ratio of 70:30 by weight in
dichlorobenzene was stirred for 36 h at room temperature.
The active layer of above-mentioned mixture solution was
deposited onto the ZnO layer by spin-coating at 2000 rpm.
The obtained film was annealed at 90�C for 10 min. The
active layer thickness was found varying in the range of
140–180 nm. Lastly, physical vapour deposition system
was employed for depositing MoO3 (5–6 nm) film and Ag
(70–80 nm) on all the cells at a pressure of 1 9 106 mbar.
In addition, both top and bottom electrodes were wiped with
chloroform for good contacts. It was found that 0.06 cm2
was the active area of solar cell. The cell configuration is
displayed in figure 1b.
3. Results and discussion
3.1 Characterization of Cd-doped ZnS NCs
The structure of as-synthesized Cd-ZnO NCs were charac-
terized by powder XRD as shown figure 3a. The as-pre-
pared Cd-ZnO NCs has shown diffraction peaks at an angle
2h of 28.8�, 33.5�, 47.5�, 56.5�, 69.2� and 76.5� with cor-
responding lattice plans [111], [200], [220], [311], [400]
and [331], respectively. The cubic zinc blend phase of the
synthesized NCs is well matched with reference code
00-001-0792. It is observed that no additional peak for
cadmium dopant and other impurity phase appeared in
powder XRD diffractogram of Cd-doped ZnS sample.
However, the intensity of metal-doped ZnS NCs could be
associated with decrease in the crystalline property with
metal ion doping [26].
Optical properties of the Cd-ZnS NCs were studied by
using UV–visible (UV–Vis) absorption spectroscopy.
Figure 3b shows the absorption band at 340 nm in n-butanol
at room temperature. The absorption peak of Cd-doped ZnS
(Cd-ZnS) is slightly blue shifted compared to the bulk ZnS
(345 nm), which could be related to the quantum confine-
ment effect [27].
Morphology of the as-prepared Cd-ZnS NCs was
determined by field emission scanning electron micro-
scopy as shown in figure 3c. Field emission scanning
electron microscopy micrographs shows the regular size
and regular geometry in Cd-doped sample and image
shows agglomeration as no surfactant has been used in the
synthesis procedure. Transmission electron microscopy
was used to further explore the morphology of as-prepared
NCs (figure 3d). The synthesized Cd-ZnS NCs were found
spherical in shape with average particle size of 8–10 nm
for Cd-doped sample, as shown in the inset image
(figure 3d) [28].
3.2 Grafted Cd-ZnS-MC540 nanohybrid
The grafting of the as-prepared Cd-doped ZnO NCs with
merocyanine (MC540) was synthesized by immersing the
merocyanine (MC540) solution into the Cd:ZnO NCs in
butanol. The as-prepared grafted Cd-ZnS-MC540 nanohy-
brid was characterized by Fourier transform infrared
(FTIR), UV–Vis absorption and photoluminescence (PL)
spectrophotometry. Figure 4a shows the FTIR spectra of
free merocyanine (MC540) and the corresponding grafted
nanohybrid Cd-ZnS-MC540. The results were well matched
with literature [29]. The typical FTIR peaks of MC540
become boarder than the Cd-ZnO-MC540 nanohybrid,
which indicates the successful grafting of Cd-ZnS with an
anionic dye onto the surface of NCs surface via sulphonic
group.
Optical absorption spectra of pure MC540 and the grafted
sample Cd-ZnS-MC is shown in figure 4b. It can be
observed that the pure MC540 shows an absorption peak
Zn(CH3COO)2
Cd(CH3COO)2
Na2S
Cd-ZnS NCs
Mercyanine (MC540)
C60-PTA
Cd-ZnS-MC540 NCs Cd-ZnS-MC540-C60-PTA
Figure 2. Schematic diagram of the synthesis of Cd-ZnS-MC540-C60-PTA nanohybrid self-assembly.
Bull. Mater. Sci. (2021) 44:291 Page 3 of 9 291
edge at 541 nm. Upon grafting MC540 on to the Cd-ZnS to
produce CD-ZnS-MC, a broadening in the absorption band
has been observed with absorption band edge shifted to
760 nm. This red-shifted broad absorption attributed to
charge-transfer bands has been noted for donor–acceptor
systems.
The effect of increasing concentration of MC540 on the
optical properties of nanohybrid assemblies (Cd-ZnS-MC)
was also studied. It was observed that the absorption
intensity increases with the increase in concentration of
MC540 grafting up to 8 9 10–6 M and it started to decrease
as the concentration of MC540 further increases. This may
be explained as that in solution at low concentration of the
MC540 (up to 8 9 10–6 M), the monomeric form dominates
but thereafter aggregates formation begin.
The absorption band positions and broadening in n – p*
band of the reference MC540 and nanohybrid assemblies, as
a function of concentration of dye also depends upon the
MC540 concentration. With increasing concentration, there
is a blue shift of the absorption band, which changes into
red shift when the concentration exceeds a certain limit.
This may be interpreted in terms of different types of
aggregates at different concentrations on the surface of Cd-
ZnS NCs. The Cd-ZnS NCs were dispersed in butanol and
scatter light. The grafting of dyes at low concentration
increases their solubility (to some extent) and thus reduces
the scattering. However, when the concentration exceeds a
certain limit, excessive aggregation of the dye results once
again in big clusters formation.
PL spectra at excitation wavelength of 450 nm is shown
in figure 4c. The PL bands are slightly blue shifted with the
increase in concentration of MC540 compared to pure
MC540. The emission intensity first increases with
increasing dye concentrations (up to 8 9 10–6 M) than
decreases at higher concentrations, which indicates the
aggregates formation and it became more and more
prominent with further increase in concentration. It is also
observed that, at all concentrations of the nanohybrids PL
intensity is lower than the pure MC540. This may be due to
aggregation of dye or by transferring electrons from excited
Figure 3. (a) Powder XRD spectrum of Cd-ZnS NCs. Panel (b) represents the UV–vis absorption spectrum, (c) presents the field
emission scanning electron microscopy image and (d) the transmission electron microscopy image of as-prepared Cd-ZnSNCs.
291 Page 4 of 9 Bull. Mater. Sci. (2021) 44:291
state of merocyanine to the conduction band of Cd-ZnS
NCs.
3.3 Co-grafted Cd-ZnS-MC540-C60-PTA nanohybrid
FTIR spectra of the free merocyanine (MC540), free full-
erene (C60-PTA) and the corresponding grafted nanohybrid
sample (Cd-ZnS-MC540) and co-grafted (Cd-ZnS-MC-
C60-PTA) is presented in figure 5. It can be observed that
before grafting, MC540 shows specific FTIR signals of
sulphonic groups covering the range from 800 to
1400 cm–1. Fullerene displays a strong peak characteristic
of C=O stretching of the carboxylic acid group positioned at
1729 cm–1 [30]. In the grafted and co-grafted samples, the
peaks characteristic of sulphonic group and carboxylic acid
functionality become boarder, which proves the interaction
of donor and acceptor species with Cd-ZnS NCs.
To study the mutual interaction of donor and acceptor
species and with the Cd-ZnS NCs, we have taken the
absorbance spectra of Cd-ZnS-MC540 and ZnS-MC-C60-
PTA in butanol as represented in figure 6a. Optical
absorption spectrum of the grafted sample (Cd-ZnS-
MC540) shows a broad absorption band. After adding C60-
PTA, an interesting change was observed. The absorbance
maxima of the intense transition band were observed to be
slightly blue shifted when co-grafted on Cd-ZnS NCs,
which indicates electronic coupling between merocyanine
and acceptor at the surface of Cd-ZnS NCs.
Furthermore, the effect of increasing merocyanine and
fullerene concentrations in co-grafted Cs-ZnS-MC540-
C60-PTA was also explored by using UV–Vis absorption
spectrophotometry. It can be observed that the transition
bands appeared in the first two samples (4 9 10–6 and
6 9 10–6 M), which gets blue shifted step by step for
each additional increase in concentration. This blue shift
shows successful co-grafting of merocyanine and C60-
PTA molecules onto the surface of Cd-ZnS NCs. This
brings the donor and acceptors close to each other for
electron coupling. Hence, at the highest concentration the
transition band gets blue shifted by only 2 nm. This
behaviour may be elucidated that there is sufficient space
for MC540 and C60-PTA molecules to get grafted on the
Figure 4. (a) FTIR spectra of the pure merocyanine and Cd-ZnS-
MC540 nanohybrid. (b) The UV–Vis absorption spectra of MC540
and Cd-ZnS-MC nanohybrid with various grafting concentrations
of MC540 and (c) PL emission spectra of MC540 and Cd-ZnS-MC
nanohybrid with various grafting concentrations of MC540.
Figure 5. FTIR spectra of MC 540, C60-PTA, grafted sample
(Cd-ZnS-MC540) and co-grafted sample (Cd-ZnS-MC-C60-PTA).
The spectra range in the region (600–4000 cm-1), where major
changes upon grafting and co-grafting takes place.
Bull. Mater. Sci. (2021) 44:291 Page 5 of 9 291
surface of metal-doped ZnS NCs and acquired good
interaction between donor and acceptor species. However,
at high concentrations, the movements of molecules are
restricted because of limited surface area of NCs. Thus,
some of the dye molecules may not have an interaction
with fullerenes in the direct locality. Absorption spectra
also reveals that the optical density increases at low
concentration of donor and acceptor species and tend to
decrease at high concentration. This again boosts the idea
of dye aggregation at high concentration. Furthermore, we
find that the intense transition band of the co-grafted
samples are narrower than the corresponding grafted
samples at all concentrations. This supports the idea that
MC540 and C60-PTA are interacting significantly. C60-
PTA being a strong acceptor reorganizes all the accu-
mulated dye on the surface, subsequently the dye/dye
interaction is replaced by the donor/acceptor interaction.
PL emission intensities (at excitation of 450 nm) of the
MC540, grafted sample (ZnS-MC) and co-grafted sample
(ZnS-MC-C60-PTA) are shown in figure 6b. PL intensities
of the co-grafted samples are insignificantly lesser than
those of the grafted samples, the reason suggesting the
interaction of C60-PTA with MC540. When C60-PTA
shows a strong effect, the emission from donor dye is nearly
totally quenched. Hence, the enormously weak emission
from the co-grafted samples may be associated with the
small part of the grafted dye, which in some way outflows
the effect of acceptor.
3.4 Current–voltage measurements
The Cd-ZnS-MC540 NCs were applied in the P3HT-based
DSSCs. To fabricate DSSCs, Cd-ZnS-Mc540 NCs were
blended with P3HT and the efficiency of fabricated device
was determined and compared with controlled device. The
controlled device consists of Cd-ZnS NCs and organic
polymer. The device configuration is presented in figure 7a.
Whereas, figure 7b presents the energy level alignment of
all the device components. I–V measurements of all devices
with respect to the reference are presented in figure 7c and
table (1). Whereas hybrid photovoltaic devices were con-
structed by photosensitizing the NCs with MC540 at several
concentrations ranging from 2 9 10–6 to 8 9 10–6 M. It can
be observed that the reference device (P3HT-Cd-ZnS)
exhibits an excellent diode behaviour in dark. Where,
controlled device (reference device) has shown short circuit
current (Jsc) of 3.69 mA cm–2, open circuit voltage (Voc) of
0.43 V, fill factor (FF) of 43.7% and PCE of 0.69%. The
device containing grafted Cd-ZnS NCs with MC540 con-
centration of 2 9 10–6 M shows the Jsc of 3.86 mA cm–2,
Voc of 0.43 V, FF of 49.1% and PCE of 0.82%. This
increase in photovoltaic performance is due to the grafting
of dye molecule, which increases the solar light absorption
of sample.
However, a significant rise in the value of Jsc was noted
in the grafted sample with concentration of merocyanine
dye (6 9 10–6 M). The value of Jsc touched to
4.60 mA cm–2 and an efficiency was elevated by 0.13% in
comparison to reference device and PCE reached to 0.83%.
When the concentration of mercyanine was further
increased to 8 9 10–6 M, then PCE and Jsc were decreased.
The decrease in Jsc and PCE below and above the optimal
concentration of merocyanine (6 9 10–6 M, in this study)
might be credited to the aggregation of MC540 on the Cd-
ZnS NCs and consequent self-quenching occurrences
among merocyanine molecules. In addition, rise in internal
resistant and weak electrode may be responsible for poor
productivity.
To explore co-grafting effect on the photovoltaic per-
formance, we have used P3HT-Cd-ZnS-MC540 6 9 10–6
M and P3HT-Cd-ZnS-MC540 8 9 10–6 M samples for
Figure 6. (a) UV–Visible absorption spectra of Cd-ZnS-MC540-C60-PTA samples at various concentrations in butanol. (b) Emission
spectra of reference MC540, Cd-ZnS-MC540 and Cd-ZnS-MC-C60-PTA samples at a concentration of 4 9 10-6 M in butanol.
291 Page 6 of 9 Bull. Mater. Sci. (2021) 44:291
Figure 7. (a) Device configuration and (b) alignment of energy level of device components. (c) Current–voltage measurements of the
reference and merocyanine photosensitized photovoltaic devices. P3HT-Cd-ZnS under dark, P3HT-Cd-ZnS under light, P3HT-Cd-
ZnS-MC540 2 9 10-6 M, P3HT-Cd-ZnS-MC540 4 9 10-6 M, P3HT-Cd-ZnS-MC540 6 9 10-6 M, P3HT-Cd-ZnS-MC540
8 9 10-6 M. (d) Current–voltage measurements of the reference and dye sensitized photovoltaic devices as a function of
concentration of donor and acceptor in ratio of 1:1. P3HT-Cd-ZnS dark, P3HT-Cd-ZnS light, P3HT-Cd-ZnS-MC-C60PTA
6 9 10-6 M, P3HT-Cd-ZnS-MC-C60PTA 8 9 10-6 M.
Table 1. Performance of the photovoltaic devices constructed out of Cd-ZnS, grafted Cd-ZnS-MC540 and co-grafted Cd-ZnS-MC540-
C60-PTA NCs and organic polymer blend at 6 9 10–6 and 8 9 10–6 M in ratio of 1:1 compared with the reference device under the
conditions of AM 1.5 (75 mW cm–2).
Device composition Jsc (mA cm–2) Voc (V) Fill factor (%) Efficiency (%)
Reference 3.69 0.43 43.7 0.69
Cd-ZnS-MC540 (2 3 10–6 M) 3.86 0.43 49.1 0.82
Cd-ZnS-MC540 (4 3 10–6 M) 4.06 0.41 46.7 0.78
Cd-ZnS-MC540 (6 3 10–6 M) 4.60 0.39 46.1 0.83
Cd-ZnS-MC540 (8 3 10–6 M) 2.96 0.41 34.2 0.41
Cd-ZnS-MC60PTA (6 3 10–6 M) (1:1) 4.98 0.35 0.447 0.78
Cd-ZnS-MC60PTA (8 3 10–6 M) (1:1) 2.24 0.33 0.396 0.29
Cd-ZnS-MC540 = P3HT-Cd-ZnS-MC540
Cd-ZnS-MC60PTA = P3HT-Cd-ZnS-MC540-C60PTA (1:1)
Bull. Mater. Sci. (2021) 44:291 Page 7 of 9 291
co-grafting with PTA (1:1 ratio). The active blend is
composed of organic polymer, merocyanine 540, an
electron acceptor (C60-PTA) and Cd-ZnS NCs. The
photovoltaic device with P3HT-Cd-ZnS-MC540-PTA
(6 9 10–6 M) has shown highly improved photovoltaic
parameters with PCE of 0.78% (figure 7d). However,
P3HT-Cd-ZnS-MC540-PTA (8 9 10–6 M) does not show
any improvement in the photovoltaic performance of the
device, as merocyanine is known as self-aggregating dye
and aggregates at high concentration. The decrease in
short circuit current density and power conversion effi-
ciency below and above the optimal merocyanine con-
centration (6 9 10–6 M, in the present case) could be
ascribed to the accumulation of merocyanine on the ZnS
NCs and consequent self-quenching phenomena among
MC540 molecules. Additionally, an increase in internal
resistant and poor semiconductor/electrode might also be
responsible for poor efficiency. However, Cd-doped ZnS
produced more efficiency with merocyanine dye because
of small bandgap and ease of electron transfer from
donor to acceptor. These results indicate that by co-
grafting, dye/dye interaction is replaced by merocya-
nine/fullerene interaction but unfortunately co-grafting
may have led to the formation of big clusters. Hence it
may be held responsible for the low photovoltaic per-
formances [29,31].
4. Conclusions
This study reported the synthesis of Cd-doped zinc sul-
phide NCs by a co-precipitation approach and character-
ized by various techniques. Afterwards, grafting of
organic dye (MC540) was performed at the surfaces of
as-prepared Cd-ZnS NCs. The grafted Cd-ZnS NCs were
characterized by FTIR, UV–Vis absorption and PL
spectroscopies. Finally, co-grafting was performed with
C60PTA. This work demonstrated the board absorption
band in higher wavelength credited to charge-transfer
bands for donor–acceptor systems. Highly photoactive
donor organic dye (MC540) was shown to form charge
transfer complex with Cd-ZnS NCs. The complex for-
mation was characterized by absorption, PL and FTIR
spectrophotometry. Bulk heterojunction hybrid solar cells
were fabricated using merocyanine-sensitized Cd-doped
ZnS nanoparticles and P3HT blend. The fabricated devi-
ces exhibited enhanced PCE than the reference device at
low concentration. This trend continued and the efficiency
increased with the increase in the number of dye mole-
cules on the surface until the dye concentration reached
6 9 10–6 M. Thereafter the trend was reversed and at the
highest concentration the efficiency was found to be
much lower than that of the reference device. The co-
grafted Cd-doped ZnS nanoparticles at merocya-
nine/fullerene ratio of 1:1 (when used as a component of
the active blend) showed a good photovoltaic
performance (in terms of efficiency) between the fullerene
and the dye-grafted Cd-ZnS NCs.
Acknowledgements
We are highly thankful to the Higher Education Commis-
sion, Pakistan, under project No-20/2329/NRPU/R&D/
HEC/12 for financial support and Quaid-e-Azam University,
Islamabad, for providing laboratory and space facilities.
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