Grafting and co-grafting of dyes on Cd-doped ZnS ...

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 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 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 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 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 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 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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