Morphological impact of ZnO nanoparticle on MEHPPV:ZnO based hybrid solar cell

Morphological impact of ZnO nanoparticle on MEHPPV:ZnObased hybrid solar cell

Somnath Middya • Animesh Layek •

Arka Dey • Partha Pratim Ray

Received: 24 May 2013 / Accepted: 16 August 2013 / Published online: 28 August 2013

� Springer Science+Business Media New York 2013

Abstract In this study, we have synthesized semicon-

ducting ZnO nanoparticles (NP) of different morphology in

solvothermal route, by using different capping reagents and

applied it as an acceptor in poly(2-methoxy-5(2-ethyl-

hexyloxy)-phenylene-vinylene (MEHPPV):ZnO NP based

hybrid photovoltaic devices. Material properties of ZnO

NP of different morphology and the compositions were

studied with the help of X-ray diffraction, scanning elec-

tron microscopy, fourier transform infrared and thermo

gravimetric analysis. The direct band gap energies of

flower, sphere and rod-like ZnO NP were calculated as

3.68, 3.15 and 3.25 eV by using Tauc’s plot with the help

of UV–Vis absorption. We have studied different material

properties of MEHPPV:ZnO composite to check their

applicability as an active layer of hybrid solar cell. Photo-

luminescence (PL) spectra of the composite materials were

recorded to check PL energy quenching which is quite

noticeable in case of sphere-like nanoparticle. Finally the

hybrid solar cell of the structure ITO/PEDOT:PSS/ME-

HPPV:ZnO/Al has been fabricated and studied. From the

current density (J)–voltage (V) characteristic, we have

found that the solar cell fabricated with ZnO sphere-like

structure gives better result.

1 Introduction

In last few years there has been a tremendous effort to

develop inorganic semiconducting nanoparticles (NP) with

their application in organic–inorganic hybrid solar cells to

reduce the cost by optimizing the active material. Inorganic

NP incorporation within polymer matrix has been the

subject of considerable research because of their potential

for optical, electrical, and photovoltaic application [1, 2].

Recent investigations of promising materials for organic

light emitting diodes (OLEDs) and solar cell have dem-

onstrated that introduction of n-type inorganic NP into

conjugated polymers produce stable and high performance

devices [3]. Several inorganic NP were successfully

applied to develop solar cell [4]. Among the inorganic NP,

ZnO is a non toxic n-type wide bandgap semiconducting

material, which was applied efficiently in thin film tran-

sistors [5], light emitting diodes [6], optical sensors [7] and

in solar cells [8]. Since the band gap energy of ZnO NP is

wide, exciton binding energy is very high (60 meV) [9] and

optical transparency is also high [10], it was selected as one

of the promising material in fabrication of organic inor-

ganic hybrid solar cells [11]. The large excitonic binding

energy of ZnO leads to extreme stability of excitons at high

temperature [12] and enables the devices to work at low

threshold voltage. Another important property of ZnO NP

is the morphology dependant tunability of electron mobil-

ity [13]. So the nanomaterial with a suitable band gap for a

particular device application can be prepared by changing

their morphology. The absorption depends on the effective

surface area of the absorbing materials, which effectively

modifies the direct band gap. According to Burstein–Moss

effect, depending upon the morphological growth of

semiconducting NP and doping concentration [14, 15] the

absorption edge is pushed to higher energies because all

states close to the conduction band become populated. The

higher band gap material is superior in transition of exci-

tons above room temperature.

There had been a tremendous effort to synthesize

ZnO NP of different morphology. Flower-like, prism like,

S. Middya � A. Layek � A. Dey � P. P. Ray (&)

Department of Physics, Jadavpur University, Kolkata 700 032,

India

e-mail: [email protected]

123

J Mater Sci: Mater Electron (2013) 24:4621–4629

DOI 10.1007/s10854-013-1453-2

rod-like, snow-flakes like ZnO NP had been reported by

Zhang et. al. [16]. Gao et al. [17] prepared flower-like ZnO

by thermolysis technique. Yang et al. presented the syn-

thesis of flower-like, disk like and dumbbell like ZnO

structures with the assistance of capping molecules such as

citric acid and polyvinyl alcohol [18]. In this work we have

described the synthesis of flower-like, rod-like and sphere-

like ZnO NP by using solvothermal technique with various

capping reagents.

The main objective of this work is to study the effect of

incorporation of different nanostructured ZnO NP within

conducting unsaturated poly[2-methoxy-5-(2-ethyl-hexyl-

oxy)-1,4-phenylene-vinylene] (MEH-PPV) matrix. MEH-

PPV polymer has a high hole mobility but low electron

mobility [19] with HOMO level at -5.1 eV and LUMO

level at -2.7 eV. Imbalance in intrinsic carrier mobility

within MEH-PPV severely limits the performance of

polymer based solar cells. To overcome this imbalance,

ZnO can be selected as one of the semiconducting electron

acceptor with band gap 3.37 eV. The formation of exciton

and the charge transformation process from donor (MEH-

PPV) to acceptor (ZnO NP) has been schematically shown

in Fig. 1.

In this research work flower-like, sphere-like and rod-

like ZnO NP with different band gap and higher thermal

stability, were synthesized by solvothermal technique with

the help of poly (vinyl-pyrrolidone) (PVP), cetyl-trimethyl-

ammoniumbromide (CTAB) and tetra-octyl-ammonium-

bromide (TOAB) capping reagent. These ZnO NP with

different morphology have been incorporated within MEH-

PPV to prepare the active layer of hybrid solar cell. We

have characterized and studied the variation of absorption

of the polymer-NP composite material with effective sur-

face area of ZnO NP, due to the change in morphology.

The organic–inorganic hybrid solar cell, having structure

ITO/PEDOT:PSS/MEHPPV:ZnO(NP)/Al was fabricated

with synthesized nanoparticle of different morphology.

2 Experimental reagents

Dehydrated zinc-acetate (99 % pure), sodium hydroxide

pellet, TOAB, CTAB, PVP and Methanol of AR grade

were purchased from Merck, whereas MEH-PPV was

purchased from Sigma Aldrich. All these reagents were

used as precursor without further purification.

3 Experimental details

3.1 Synthesis of ZnO nanoparticles of different

morphology

ZnO NP of different morphology were synthesized by

using the procedure proposed by Bahnemann et al. [20] and

have also been adopted by various other groups [21]. In a

typical synthesis of surfactant-coated ZnO NP, in 20 ml

methanol, 1 mM (0.05 g) Zn (ac) 2:2H2O was dissolved by

vigorous stirring at about 50 �C and diluted to 200 ml

followed by cooling to room temperature. The solution was

then divided to three aliquot volumes, each of which was

taken in separate flask and were marked as (a), (b) and (c).

Specific surfactant TOAB, CTAB, PVP were added

respectively into the above pre-marked solutions under

vigorous stirring [22]. 0.02 M, 15 ml NaOH solution in

methanol was added drop-wise to each of the mixture

within few minutes under stirring. A white ppt was formed

in each case, which was found to be stable at ambient

conditions. Each ppt was then transferred to linear Teflon

autoclave and was heated at 160 �C for 2 days and was

cooled to room temperature. Thereafter the solutions were

washed with distilled water and ethanol repeatedly to

extract ZnO NP by centrifuged technique and were dried at

about 120 �C under vacuum heater.

In analytical measurements, X-ray diffraction (XRD) pat-

tern of ZnO NP were done with the help of Bruker D8 X-ray

diffractometer with CuKa radiation (k = 0.15418 nm). The

different ZnO nanoparticle morphological structures were

depicted by using scanning electron microscopy (SEM).

Thermogravimetric analysis of the materials was done with

the help of DTG-60 Thermogravimetric and differential

thermal analyzer of Shimadzu. Fourier transform infrared

(FTIR) spectra of ZnO were recorded with the help of FTIR-

8400S Spectrophotometer of Shimadzu. With FTIR Spec-

troscopic data, the occurrence of functional groups were

investigated. Optical absorption spectra were taken by spec-

trophotometer 2401PC from Shimadzu, whereas in fluores-

cence studies, the emission spectra were recorded on a VarianFig. 1 Schematics diagram of exciton dissociation and charge

transfer

4622 J Mater Sci: Mater Electron (2013) 24:4621–4629

123

Fluorescence Spectrophotometer of Carrier Eclipse. The

average particle size of each sample (lies between and

100 nm) was determined from XRD-spectra using scherrer’s

broadening equation. The direct band gap energies of syn-

thesized NP were measured with the help of Tauc’s plots from

UV–Vis absorption spectrum.

3.2 Fabrication and characterization of active materials

(MEHPPV:ZnO nanocomposite) and solar cell

To study the optical and electrical properties of the active

material, the solutions of MEH-PPV and ZnO NP of dif-

ferent morphology marked as (a), (b) and (c) were prepared

separately in chloroform with a concentration of 1.5, 0.3,

0.3 and 0.3 mg/mL respectively and were ultrasonicated

for 10 min at room temperature after stirring by magnetic

stirrer for 2 h each at open atmosphere. Once the solutions

were ready, MEH-PPV and ZnO nanoparticle were mixed

with appropriate wt. ratio 2:0.1 to get the desired composite

and was ultrasonicated for 30 min.

Prior to device fabrication, the ITO coated glass sub-

strates were cleaned with soap solution, acetone, ethanol

and distilled water by ultrasonication technique and were

dried under a vacuum oven. Before preparing the active

layer, a thin transparent buffer layer of PEDOT:PSS was

spin coated (at spin rate 1400 rpm for 2 min) on the

cleaned ITO coated glass substrate and was dried at 70 �C

under vacuum oven. As the substrate was ready, pre-pre-

pared active composite solutions of MEHPPV:ZnO (NP of

different morphology) were spin coated on the PE-

DOT:PSS film by spinning at the rate of 1200 rpm for

2 min. The desired film was obtained and dried under

vacuum desiccator at room temperature for 24 h. For the

top contact, aluminum electrode was deposited by thermal

evaporation with shadow mask. As the devices of structure

ITO/PEDOT:PSS/MEHPPV:ZnO/Al became ready, one

need to characterize it. The current density (J)–voltage

(V) characterizations of the respective cells were measured

with the help of Keithley 2400 source meter under the

incident illumination of white light (80 mWcm-2).

4 Results and discussions

4.1 SEM images and morphology

Figure 2a–c represent the SEM images of the synthesized

materials premarked as (a), (b) and (c) sequentially. The

images illustrate that morphology of the material (a),

(b) and (c) are flower-, sphere- and rod-like respectively

with their dimension in nano range. The architectural

growth of the nanoparticle eventually depends upon the

solubility of the precursor in the solvents with different

saturated vapor pressure and also upon the initial nucle-

ation of the crystals [23, 24]. The water soluble surfactant

(like TOAB, CTAB and PVP) can form a coordinate bond

with the Zn ions on growing particles [25]. The lower

vapor pressures set aside for rapid growth of the ZnO

nuclei. The chain length of the surfactant TOAB, CTAB

and PVP also effected the crystal growth accordingly. The

flower-like architecture was formed due to the coagulations

of ZnO flakes, which are formed due to uncontrolled

growth of materials. The ZnO nanorods have their different

orientations with hexagonal cross-section.

4.2 X-ray diffraction analysis

As synthesized ZnO NP of different morphology

[(a) flower-like, (b) sphere-like and (c) rod-like] were

characterized by powder XRD spectra, recorded with the

help of Bruker D8 X-ray diffractometer with copper (Cu)

as a target material. Figure 3a–c show the XRD spectra of

the flower-, sphere- and rod-like nanomaterials respec-

tively. The responsible Braggs diffractions were taken

place for the (hkl) planes (100), (002) and (101) were

identified at angle 2h = 32�, 34.6� and 36.4� respectively

Fig. 2 a SEM image of ZnO flower-like nanoparticle. b SEM image of ZnO sphere-like nanoparticle. c SEM image of ZnO rod-like nanoparticle

J Mater Sci: Mater Electron (2013) 24:4621–4629 4623

123

for each morphology, which indicate about the phase

identity. The XRD data approved ZnO, which were sup-

ported by the JCPDS Card No: 36-1451 [16]. Although

there was no significant change in diffraction pattern yet

change is found in relative intensity. For the (002) plane

the variation of intensity is quite noticeable. It may be due

to the orientation of responsible plane during crystal

growth. Using XRD data and applying Debye–Scherrer’s

broadening equation, the average particle size of the

(a) flower-like ZnO, (b) sphere-like ZnO and (c) rod-like

ZnO NP are calculated as 86, 26 and 57 nm respectively.

4.3 FTIR analysis

Figure 4a–c represent the fourier transformation infrared

spectroscopy (FTIR) of the synthesized ZnO NP of dif-

ferent morphology [(a) flower-, (b) sphere- and (c) rod-like

sequentially]. The spectra show strong peak around

3400–3430 cm-1 for O–H stretching and a small peak

around 2925–2920 cm-1 corresponding to C–H stretching

for each samples. Peaks around 1650–1500 and

1400–1370 cm-1 indicated the formation of ZnO [26] and

symmetric stretching of carboxylate group (COO–) which

probably comes from the un-reacted acetates. Peaks at

around 560 cm-1 show the distinct stretching vibration of

Zn–O. These functional groups exhibited solubility of the

NP in solvent.

(a)

(b)

(c)

Fig. 3 a XRD spectra of ZnO flower-like nanoparticle. b XRD

spectra of ZnO sphere-like nanoparticle. c XRD spectra of ZnO rod-

like nanoparticle

Fig. 4 Fourier transform Infrared spectra of a flower-like ZnO (black

in colour), b sphere-like ZnO (red in colour) and c rod-like ZnO

nanoparticles (blue in colour) (Color figure online)


123

4.4 UV–Vis absorption spectra and direct band gap

The absorption spectra of the ZnO NP (of (a) flower-like,

(b) sphere-like and (c) rod-like) in chloroform medium

were recorded in the wavelength range of 300–600 nm.

From these spectra, given in inset of Fig. 5a–c respectively

clearly illustrated that the (b) sphere-like nanoparticle

absorbs comparably larger part of the incident radiations in

comparison with (a) flower-like and (c) rod-like ZnO NP.

Considering the absorption data, with the help of Tauc’s

equation direct band gap of the samples were measured as

3.68, 3.15 and 3.25 eV respectively (plots are given in

Fig. 5a–c) for flower-, sphere- and rod-like morphology.

The band gap of ZnO nanoparticle is 3.3 eV, as reported

earlier in the literature by Berger et al. 1997 [27] and

3.4 eV, reported by Samuel et.al. [28]. Since the band gap

of flower-like structure is comparably high, its absorbance

is low. But the absorption is quite high in case of sphere-

like nano morphology, due to its lower band gap. For

photovoltaic application in higher temperature (from room

temperature), inorganic semiconductor with wide band gap

has more significant role. The direct band gap energy was

significantly changed in case of sphere and rod-like struc-

ture of ZnO, depending upon their morphological changes.

It may be due to the increased populations of all states

close to the conduction band. Hence one can say that

sphere-like structure is more suitable in high temperature

application of hybrid solar cells with organic polymer.

4.5 Thermo-gravimetric analysis

DTG-60 thermogravimetric and differential thermal ana-

lyzer of Shimadzu was used to study the thermal stability

of ZnO NP. Approximately 5 mg of the samples were

heated to 600 �C at the rate of 10 �C/min. From Fig. 6, it is

observed that TGA curves for the nano ZnO of different

morphology show no drastic loss in weight. This indicates

the thermal stability and high purity of nano ZnO. The

TGA curves corresponding to the (a) flower-like,

(b) sphere-like and (c) rod-like ZnO samples show a very

small decrease in weight percentage at around 190–250 �C.

As reported by Morishige et al. [29] the change at around

200 �C may be seen due to the decomposition of the

condensation dehydration of the hydroxyls. As the tem-

perature was increased, the decomposition starts gradually

and results in weight lost. The rate of decomposition is

quite high in case of ZnO (a) flower-like NP (represented

as (a) in graph), and at temperature 600 �C it is decom-

posed to 65 % of its initial weight. Whereas in case of

(c) rod-like (represented as (c) in graph) and (c) sphere-like

ZnO (represented as (b) in graph), weight was degraded to

75 and 72 % respectively. From the curves it is clearly

demonstrated that the (c) rod-like and (b) sphere-like ZnO

Fig. 5 a Tauc’s plot of ZnO flower-like nanoparticles. b Tauc’s plot

of ZnO sphere-like nanoparticles. c Tauc’s plot of ZnO rod-like

nanoparticles


123

NP are thermally more stable in comparison with flower-

like ZnO one.

4.6 UV–Vis absorption and band gap of MEHPPV

To optimize the photo sensing behavior of MEH-PPV,

UV–Vis absorption spectra in the range of 300–700 nm

was recorded with the help of spectrophotometer (given in

Fig. 7). From these spectra the most excitation wavelength

558 nm for MEH-PPV was observed and hence corre-

sponding band gap energy was calculated as 2.22 eV. [30].

Fig. 6 Thermogravimetric (TG) plots of a flower-like ZnO, b sphere-

like ZnO and c rod-like ZnO nanomaterials

Fig. 7 UV–Vis absorption spectra of MEH-PPV polymer in chloro-

form medium

(a)

(b)

(c)

Fig. 8 a UV–Vis absorption spectra of MEH-PPV polymer, ZnO

flower-like NP and MEHPPV:ZnO composite in chloroform medium.

b UV–Vis absorption spectra of MEH-PPV polymer, ZnO sphere-like

NP and MEHPPV:ZnO composite in chloroform medium. c UV-Vis

absorption spectra of MEH-PPV polymer, ZnO rod-like NP and

MEHPPV:ZnO composite in chloroform medium


123

Lee et.al. [31] reported the band gap energy of MEH-PPV

as 2.1 eV by optimizing HOMO and LUMO energy levels.

4.7 UV–Vis absorption analysis for the organic–

inorganic composites

Figure 8a–c represent the UV–Vis absorption spectra of

MEH-PPV, ZnO and MEHPPV:ZnO composite with dif-

ferent morphology in chloroform medium. From these

spectra it is clear that the absorption of MEH-PPV

increased due to incorporation of inorganic ZnO nanopar-

ticle. The spectrum of the composite represents the

superposed spectra of donor (MEH-PPV) and acceptor

(ZnO) material. Red shift of the resultant spectrum is

observed due to incorporation of inorganic nano material

within MEH-PPV.

4.8 Photo-luminescence and energy quenching

Figure 9 represents the photoluminescence spectra of

donor polymer MEH-PPV and MEH-PPV with ZnO

nanoparticle in chloroform medium, as recorded in the

wavelength range of 450–750 nm. In this figure, the photo-

luminescence (PL) peaks of MEH-PPV in chloroform

medium are observed around 583 and 638 nm. The PL

intensities are significantly reduced from the value of

MEH-PPV during formation of composite with ZnO

nanoparticle. In these spectra the static energy quenching

of MEH-PPV with incorporation of ZnO nanoparticle was

observed distinctly. The PL quenching is the evidence for

exciton dissociation. As the photogenerated excitons are

dissociated, the probability for recombination should be

significantly reduced. This is the ultrafast electron transfer

phenomena from donor to acceptor and it is expected to

increase the exciton dissociation efficiency in photovoltaic

devices [32]. In this composite, ZnO semiconducting

nanoparticle acts as acceptor material.

4.9 Current–voltage characteristic of solar cells

The J–V characteristics of the devices ITO/PEDOT:PSS/

MEHPPV:ZnO flower-like NP/Al, ITO/PEDOT:PSS/ME-

HPPV:ZnO sphere-like NP/Al and ITO/PEDOT:PSS/ME-

HPPV:ZnO rod-like NP/Al were measured with the help of

Keithley 2400 source meter, interfaced with PC. Figure 10

represents the J–V characteristic curves of each device. The

effective surface area of the active device that was fabri-

cated by masking is taken as 7.065 9 10-2 cm2. From

these J–V characteristic curves, the open circuit voltage

(VOC) and short circuit current density (JSC) were measured

as given in Table 1. The corresponding efficiency (g) as

well as fill factor (FF) of the devices was measured as 0.40,

0.55, 0.45 % and 0.51, 0.51, 0.58. In this investigation,

these characteristic curves demonstrate that the Jsc of the

device is improved for the ZnO sphere-like nano acceptor

materials. In organic–inorganic hybrid solar cell several

factors affect the performance of solar cell. Depending

upon the surface morphology and sphere-like symmetry of

ZnO nanoparticle, absorption ability of the conjugated

materials with MEH-PPV improved, which could increase

the generation rate of excitons. Moreover, the bulk resis-

tance that was developed due to the thickness of the film,

somehow controlled by morphology, was assume to be

reduced, as the sphere-like ZnO was incorporated within

MEH-PPV. ZnO rod-like structure was supposed to give

Fig. 9 Photo-luminescence spectra of MEH-PPV and MEHPPV:ZnO

composites with different morphology in chloroform medium

Fig. 10 Current density–voltage characteristics of a ITO/PE-

DOT:PSS/MEHPPV:ZnO (flower-like)/Al device, b ITO/PE-

DOT:PSS/MEHPPV:ZnO (sphere-like)/Al device, and c ITO/

PEDOT:PSS/MEHPPV:ZnO (rod-like)/Al device


123

better transport property for their architecture. But from the

J–V characteristic, the measured Jsc approved the contra-

dictory one, which rose due to the random direction of

alignment of nano rods. Performance of solar cell mostly

depends on the successive interfacing and energy quench-

ing between donor-acceptor materials, which mostly rela-

ted with the band gap energy and the absorption power of

the nano-inorganic materials. The energy quenching is

crudely related with the position of energy band spectra of

donor and acceptor materials. There is another possible

way to improve VOC by modifying the band gap of the

inorganic acceptor material, specifically by changing the

position of LUMO energy level, which can only be pos-

sible by controlling the growth of crystal during synthesis.

Since according to Shockley and Queisser [33], VOC is

directly related with the energy difference between LUMO

of acceptor and HOMO of donor material. The band gap

energy of flower-like morphology was increased in com-

parison to rod-like structure, as obtained from Tauc’s

equation. This increment in band gap energy demonstrated

with Burstein–Moss theory. Depending upon the morpho-

logical growth of semiconducting NP [34, 35], the

absorption edge is pushed to higher energies because all

states close to the conduction band become populated.

Hence the difference between LUMO energy level of

flower-like ZnO and HOMO energy level of MEH-PPV

approved the increment of Voc in comparison to the device

fabricated with rod-like nanoparticle in conjunction with

MEH-PPV. The formation of successive heterojunction

between organic donor and inorganic acceptor influences

upon the current density, depending upon the homogeneity

of the composite materials. This homogeneity is mostly

impacted by the dispersity of the inorganic nano particles

with their morphological identity and the occurrence of

functional groups. In this study the sphere-like morphology

was highly dispersed in chloroform with no detectable

agitation, in comparison to the rest synthesized materials,

which has been confirmed by the sharp and intense FTIR

spectra. Thus it had highest possibility to fabricate suc-

cessive hetero-junction with donor polymer in solution

phase as well as in thin film, as it had performed. PL

spectra exhibited the successive static energy quenching,

which approved the better charge transportation for ME-

HPPV:ZnO sphere-like composite [36–38]. It is important

to note that the higher series resistance reduces the FF. The

improved FF for the device fabricated with rod-like ZnO

NP exhibited the lower resistance of the film in comparison

to the device fabricated with flower-like ZnO. In this

experimental study, VOC as well as efficiency has been

improved for the device fabricated with sphere-like ZnO

nano semiconducting acceptor because of its lower bulk

resistance that may be occurred depending upon the mor-

phology. Thus there is every possibility to improve VOC.

Whereas, the VOC and the efficiency of the device, fabri-

cated with flower-like ZnO, improved in comparison with

the device, fabricated with rod-like ZnO nano material, due

to its higher absorption with lower band gap.

5 Conclusions

ZnO nano semiconducting material of different morphol-

ogy (flower-like, sphere-like and rod-like) were synthe-

sized successively in solvothermal technique by using

different capping reagents. Among these the sphere-like

one is more superior architecture with nano dimension and

better absorbing property. We have investigated the opti-

cal, thermal and electrical properties of all these synthe-

sized materials with conscience. The organic–inorganic

based solar cell, configured with MEH-PPV:ZnO sphere-

like morphology gives the highest efficiency with

improved Jsc and Voc. This is only because the formation of

successive junctions in between organic–inorganic com-

posite, which occur depending upon the structural sym-

metry with small particle size of sphere-like ZnO and its

high absorption with lower band gap energy.

Acknowledgments This work was supported by Department of

Science and Technology, Govt. of West Bengal under project No.

280(Sanc.)/ST/P/S&T/4G-10/2009.

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123

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Morphological impact of ZnO nanoparticle on MEHPPV:ZnO based hybrid solar cell

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