Journal of Alloys and Compounds 613 (2014) 364–369

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Journal of Alloys and Compounds

journal homepage: www.elsevier .com/locate / ja lcom

Study of resonance energy transfer between MEH-PPV and CuFeS2

nanoparticle and their application in energy harvesting device

http://dx.doi.org/10.1016/j.jallcom.2014.06.0070925-8388/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +91 9475237259; fax: +91 3324138917.E-mail address: partha@phys.jdvu.ac.in (P.P. Ray).

Animesh Layek a, Somnath Middya a, Arka Dey a, Mrinmay Das a, Joydeep Datta a, Chandan Banerjee b,Partha Pratim Ray a,⇑a Department of Physics, Jadavpur University, Kolkata 700 032, Indiab CEGESS, Indian Institute of Engineering Science and Technology, Shibpur, Howrah 711103, West Bengal, India

a r t i c l e i n f o

Article history:Received 23 April 2014Received in revised form 30 May 2014Accepted 2 June 2014Available online 13 June 2014

Keywords:Nanostructured materialsSemiconductorsOptical propertiesPhotovoltaics

a b s t r a c t

In this work wide band gap semiconducting chalcopyrite CuFeS2 has been synthesized by hydrothermaltechnique. The optical and electro-chemical studies approved the semiconductor nature of thesynthesized material with direct band gap 4.04 eV and room temperature electrical conductivity(r) = 4.7 � 10�8 S cm�1. We have explored the possible applicability of this material in conjunction withpolymer MEH-PPV to harvest green energy. The steady-state and time dependent luminescence analysisof MEH-PPV:CuFeS2 composite demonstrates the possibility of successful charge transportation. The typeof energy quenching has been investigated by Stern–Volmer analysis. Fluorescence Resonance EnergyTransfer (FRET) calculation confirms the static resonance energy transfer between donor (MEH-PPV)and acceptor (CuFeS2) nanoparticles. Finally the energy harvesting device with configuration ITO/MEH-PPV:CuFeS2/Al has been successfully fabricated and characterized.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Compounds of the chalcopyrite type are of considerable interestas they are closely related to the well known semiconductors witha wide range of energy gap. Among all the pronounced chalcopy-rite, CuFeS2 plays a crucial role in determining their eventual usefor any particular application. The iron chalcopyrite is extensivelystudied system among the chalcopyrite family containing transi-tion metal ions and its structure consists of Cu1+ and Fe3+ ions. Itwas reported earlier that CuFeS2 is a semiconductor having unu-sual optical [1], electrical [2] and magnetic [3] properties. The bandgap energy of CuFeS2 is reported very low (0.5–0.6 eV) with anom-alous conductivity [4]. The high mobility of Cu and the existence ofmixed valence states in copper sulfides [5,6] offer chemical andstructural freedom for the easy diffusion of other metal ion/ionsin them. CuFeS2 has been quite beneficial to tune the Fermi energyover a wide range during their chemical growth. Depending uponthe precursor and chemical route, morphology can also be alteredto tune the optical properties. In this report we have illustrated theeasiest approach to synthesize CuFeS2 with wide band gap energy.The wide band gap and temperature dependent electrical conduc-tivity of the synthesized materials encouraged us to incorporate it

with organic donor polymer and to understand the different mech-anism going on within organic–inorganic composite. Moreover it isan approach for the synthesized material towards application of inenergy harvesting solar cell.

The pioneering work on organic–inorganic hybrid nanocompos-ites consists of conjugated polymer poly(2-methoxy-5-(20-ethyl)-hexyloxy-p-phenylenevinylene) (MEH-PPV) and CdSe nanocrystalswith a power conversion efficiency (PCE) of 0.2% [7]. The perfor-mance of organic–inorganic hybrid solar cells has greatly beenimproved with recent advances in chemical synthesis of CPs andNCs and also by optimization of device engineering [8–13]. More-over, there are no such exploration on the fabrication and charac-terization of the hybrid solar cell using CuFeS2 nanoparticles. Forthe successful fabrication of CuFeS2 based active electronic devicesit is important to ensure the resonance energy transfer betweenCuFeS2 and other organic donor material.

In this paper, we have demonstrated the hydrothermal synthe-sis of CuFeS2 nanocomposite and checked its applicability inorganic–inorganic hybrid solar cell with MEH-PPV organic poly-mer. The optical and electrical characterization determined itsoptical band gap and electrical conductivity. The semiconductornature of the material was confirmed from the temperature depen-dent electrical conductivity measurement. The emission spectra ofthe organic–inorganic composite approved the acceptor likebehavior of the synthesized material. The steady state energy

A. Layek et al. / Journal of Alloys and Compounds 613 (2014) 364–369 365

quenching has been investigated by Stern–Volmer plot. The suc-cessful energy transfer between MEH-PPV (donor) and CuFeS2

(acceptor) has been confirmed by FRET analysis. The performanceof the hybrid solar cell is analyzed from the current density vs.voltage (J vs. V) characteristics of the device with configurationITO/MEH-PPV:CuFeS2/Al. The photoinduced charge transportmechanism is illustrated by investigating the band position ofthe donor and acceptor material (Scheme 1).

2. Materials and methods

In typical synthesis of CuFeS2 nanomaterial, 10 ml of 0.1 M aqueous solution ofFeCl3 was prepared and was mixed with 10 ml of 0.1 M CuCl2 aqueous solution in aglass beaker and the mixture was stirred vigorously. 100 ml of 0.1 N Na2S solutionwas pre-prepared. Under continuous stirring, Na2S solution was added drop wisewith the mixture until a dark brown precipitate (ppt) was obtained. PVP [poly(vinyl-pyrrolidone)] was added drop wise as surfactant with the above mixtureduring chemical growth of material. All the reaction was carried out in openatmosphere. The ppt was transferred into a linear Teflon autoclave and was heatedovernight at temperature 140 �C. The baked ppt was collected and washed withethanol and distilled water repeatedly and sequentially by centrifuge technique.The synthesized material was heated within a vacuum oven at 100 �C and collectedin powder form for characterization.

The structural, optical and electrical properties of the material were character-ized by Bruker D8 powder X-ray Diffractometer, JEOL transmission electron micro-scope, Shimadzu 2401PC UV–vis spectrophotometer, Perkin Elmer LS-55spectrofluorimeter and Keithley 2400 sourcemeter.

3. Device fabrication and characterization

Scheme 1 shows a schematic diagram of the device, consistingof an organic–inorganic based active layer on ITO coated glass sub-strate and a top contact electrode of aluminium. The spin coatingtechnique was used for depositing MEH-PPV: CuFeS2 compositelayer on pre-cleaned (by acetone, ethanol and distilled watersequentially) ITO coated glass substrate. The MEH-PPV: CuFeS2

composite was prepared in chloroform medium at weight ratio1:1. Thereafter the film was dried under vacuum desiccator. Finallythe top aluminium electrode was deposited on the active thin layerby thermal evaporation technique with the help of vacuum coatingunit (12A4D HHV, Vacuum coating unit). The photoinduced cur-rent density vs. voltage characteristic of the device was measuredwith the help of Keithley 2400 sourcemeter, interfaced with PC.

4. Results and discussions

Fig. 1(a) represents the powder X-ray diffraction (PXRD) spectraof the synthesized material. The responsible Bragg’s diffractionpeaks approved CuFeS2 with phase purity [14], supported by the

Scheme 1. Structure of MEH-PPV:CuFeS2 solar cell and cha

JCPDS Card No: 01-0842. The lattice strain of the system was esti-mated as 3.88 � 10�4 by Williamson–Hall (W–H) plot technique. Inthis approach D = Kk/Br cosh had been considered, where Br isfull-width at half maximum (FWHM) in radian. Fig. 1(b) showsthe W–H plot (Br cosh vs. sinh) and strain l is determined fromthe slope of fitted straight-line by considering the equation:

Br cos h ¼ kkDþ l sin h

Fig. 2 exhibits Transmission Electron Microscopy (TEM) image ofthe material at 20 nm scale which depicts that the particles arecubic in shape and nano scale in size. The TEM image and SelectedArea Diffraction Pattern (SAED) pattern (inset of Fig. 2) of synthe-sized material illustrates that the particles are highly crystallinein nature.

The room temperature conductance of the material was esti-mated as 4.72 � 10�8 S cm�1, from the current–voltage character-istic which was measured by two probe technique of sandwichlike structure. The temperature variant electrical conductivitywas measured by recording the current–voltage characteristic attemperature 303 K, 318 K, 333 K, 348 K, and 363 K. Fig. 3represents the temperature variant electrical conductivity plot.The nonlinear behavior of variation in conductivity with tempera-ture assigned its semiconductor nature.

The optical characterization of the material was performed byrecording UV–vis absorption spectra of the nanoparticle inchloroform medium (inset Fig. 4(a)). The absorption spectra ofthe synthesized material exhibit energy absorption in visibleregion. With the help of the Tauc’s equation the direct optical bandgap of the composite was estimated as 4.31 eV, which depicts thatthe synthesized material is belonging to wide band gap semicon-ductor family. Fig. 4(a) represents the corresponding Tauc’s plotfor direct transition. Where a is the absorption coefficient, h isPlanck’s constant, and c is the frequency of incident photon. In thisregard the absorption coefficient (a) was calculated from UV–visabsorption data by using the Beer–Lamberts’ equation. Such highvalue of band gap of CuFeS2 has not been recognized till date.The electro-chemical study was performed for better approxima-tion of the band position by investigating the onset state of oxida-tion and reduction of the material. The electrochemicalmeasurements were carried out using a Gamry reference 750potentiostat in a three-electrode configuration. The workingelectrode was a photocatalyst powder-modified glassy carbonelectrode (GCE) (3 mm diameter), the reference electrode was anAg/AgCl (in 3 M NaCl) electrode, and the counter electrode was aplatinum wire. Fig. 4(b) shows the cyclic voltametry curve of

rge separation mechanism with energy band diagram.

Fig. 1. (a) X-ray diffraction spectra of synthesized material and (b) Williamson–Hall plot of CuFeS2.

Fig. 2. TEM image of synthesized material at 20 nm scale. Inset exhibits corre-sponding SAED pattern.

Fig. 3. Temperature vs. electrical conductivity curve at temperature 303 K, 318 K,333 K, 348 K and 363 K.

366 A. Layek et al. / Journal of Alloys and Compounds 613 (2014) 364–369

dispersed CuFeS2 nanoparticles in chloroform medium. The volta-mogram exhibits an oxidation peak (i.e., the ionization potential)at approximately +2.83 V and a reduction peak (i.e., the electronaffinity) at about�1.21 V. In order to find the electrochemical bandgap energy (DE), the conduction (EC.B) and valence band (EV.B)energy positions were estimated at �2.93 eV and �6.97 eV withthe help of equations: EV.B = � (Eox + 4.14) eV and EC.B =� (Ered + 4.14) eV. Where Eox and Ered are the onset potentials ofthe oxidation and reduction relative to an Ag reference electrode.The value 4.14 eV was considered for the difference between thevacuum level potential of the normal hydrogen electrode and thepotential of the Ag/AgNO3 electrode. Calculating the differenceDE = EC.B � EV.B [15], the electrochemical energy band gap ofCuFeS2 was evaluated as 4.04 eV.

The HOMO and LUMO level of MEH-PPV occurs at �5.15 eV and�2.7 eV with band gap energy 2.45 eV (reported elsewhere)[16,17]. Eventually the position of conduction energy level of thenanomaterial is (�2.93 eV) energetically lower than the MEH-PPV,so there is every possibility of transportation of the photo-excitedcarrier electron for its better stability. The optical absorption andPhotoluminescence spectra of MEH-PPV:CuFeS2 composite wererecorded sequentially by varying weight ratio. We have started with100 lL solution of MEH-PPV and 100 lL solution of CuFeS2 both inchloroform medium. The first scan was recorded for donor solution.Subsequently, the donor solution was iteratively titrated by additionof CuFeS2 solution sequentially by maintaining the weight ratio.Fig. 5(a) illustrates the significant change in absorption spectra ofthe composite with increasing concentration of CuFeS2. Absorptionpeak position is found to be monotonically blue shifted (Fig. 5(b))with increase in concentration of acceptor materials. The systematicblue shift in the absorption peak exhibits interaction between donorand acceptor materials.

Fig. 6(a) shows the photoluminescence (PL) spectra of donor–acceptor composites with assigned weight ratio which depictsenergy quenching. These spectra exhibits that the addition ofacceptor material with increasing weight concentration decreasesthe emission intensity of steady state of donor. This fact promisesthe energy quenching due to electron transfer from donor(MEH-PPV) to acceptor (CuFeS2). To get insight of the state ofquenching the fluorescence intensity data were analyzed accordingto Stern–Volmer equation. The linearity of the Stern–Volmer plot(Fig. 6(b)) with intercept nearly equal to unity signifies the staticquenching. To get further confirmation we have also studied andexamined the fluorescence data with modified Stern–Volmerequation. From the plot of F0/(F0 � F) against 1/[Q] (Fig. 6(c)) thevalue of 1/fa and 1/Ksv were determined as 0.38 and 1.18 � 104

Fig. 4. (a) Tauc’s plot and direct band gap of CuFeS2, Inset (a): UV–vis absorption spectra of CuFeS2 and (b) cyclic voltametry of CuFeS2.

Fig. 5. (a) UV–vis absorption spectra of MEH-PPV/CuFeS2 composites with different weight ratio and (b) variation of absorption peaks with various concentration curve.

A. Layek et al. / Journal of Alloys and Compounds 613 (2014) 364–369 367

respectively, where F0 and F are the fluorescence intensities ofMEHPPV in the absence and presence of CuFeS2 respectively, Kvs.

is the Stern–Volmer quenching constant, fa is the fraction of the ini-tial fluorescence, which is accessible to quencher and [Q] is theconcentration of the quencher. Hence, fa and Ksv was evaluatedas 2.62 and 8.47 � 10�5 respectively, which interprets the staticquenching phenomena.

To verify the possibility of dynamic quenching, time resolvedfluorescence measurement was carried out (with the help of PerkinElmer LS-55 spectrofluorimeter), which showed that there was nosignificant change in lifetime of MEHPPV after addition of acceptoras shown in Fig. 6(d). Lifetime of donor (s1) and donor–acceptorcomposite (s2) signify that decay of MEHPPV is biexponential innature. The lifetimes (s1 and s2), relative amplitudes of emissionintensities (A1 and A2) and the various decay analysis of theMEH-PPV:CuFeS2 system are listed in Table 1. Thus, the possibilityof dynamic quenching was ruled out and confirms that the solemechanism is static quenching. The static quenching has occurreddue to the formation of a stable complex between the twocompounds after returning from the excited state.

The formation mechanism of stable ground state complex maybe explained on the basis of energy transfer between the acceptorand donor. For possibility of successful energy transfer the distancebetween the acceptor and donor must be less than 8 nm [18,19].There is a considerable overlap between the absorption spectrumof acceptor and the fluorescence spectrum of donor which pledgedthe basis of fluorescence resonance energy transfer (FRET) (asshown in Fig. 6(e)). Thus Föster’s theory of non-radiative energy

transfer can be used to calculate the distance between donor andacceptor. According to this theory, the efficiency of energy transfer(E) can be calculated by:

E ¼ 1� FF0

� �¼ R6

0

ðR60 þ rÞ

where r is the distance between the donor and the acceptor and R0

is the critical energy transfer distance, at which 50% of the excita-tion energy is transferred to the acceptor [20]. R0 was calculatedfrom the equation:

R60 ¼ 8:79� 10�25 K2 m�4 Ju

where K2 is the orientation factor related to the geometry of thedonor–acceptor dipole, m is the refractive index of medium, u isthe fluorescence quantum yield of the donor, and J expresses thedegree of spectral overlap between the donor emission and theacceptor absorption. The value of J is given by the equation:

J ¼X

FðkÞeðkÞk4Dkh i. X

FðkÞDkh i

where F(k) is the fluorescence intensity of the fluorescent donor atwavelength k, e(k) is the molar absorption coefficient of the accep-tor at k. Under the experimental condition, K2 = 2/3, m = 1.445 (forchloroform) and u = 0.15 [21,22] for MEHPPV. By using above datathe value of J was found to be 2.34 � 10�15 L M�1 cm3, E = 0.23,R0 = 0.72 � 10�7 cm and r = 1.756 � 10�7 cm.

Fig. 6. (a) Photoluminescence spectra of MEH-PPV composite with increasing concentration of CuFeS2; (b) Stern–Volmer plot; (c) modified Stern–Volmer plot; (d) lifetime bi-exponential curve of MEH-PPV and MEH-PPV/CuFeS2 composite; (e) overlap between the absorption of CuFeS2 and the fluorescence spectrum of MEH-PPV.

Table 1Lifetime data of the donor and donor–acceptor composites.

Sample Weight ratio Lifetimes (ns) Amplitude (A1) Amplitude (A2) Avg. lifetime s0 (ns)

s1 s2

MEH-PPV:CuFeS2 1:0 0.41 1.65 49.44 50.56 1.03MEH-PPV:CuFeS2 1:1 0.21 0.85 92.86 7.14 0.53

Fig. 7. Current density vs. voltage characteristic of ITO/MEH-PPV:CuFeS2/Al basedsolar cell under dark and photo condition.

368 A. Layek et al. / Journal of Alloys and Compounds 613 (2014) 364–369

Under the present circumstances, the conditions are fulfilled asr is less than 8 nm and 0.5 R0 < r < 1.5 R0, which indicates that theenergy transfer from MEHPPV to inorganic nanomaterial occurswith high probability. Thus we propose that the observedquenching of photoluminescence that occurs in composite(MEH-PPV:CuFeS2) heterojunction is due to photo induced electrontransfer from donor MEH-PPV to acceptor CuFeS2. In MEH-PPV/CuFeS2 system, photoexcited electron transfer from MEH-PPV toCuFeS2 can also be illustrated on the basis of electron affinity andionization potential (Scheme 1) of MEH-PPV (E.A. = �2.7 eV andIP = �5.1 eV), and the position of valence band and conductionband of CuFeS2 (EVB = �6.97 eV and ECB = �2.93 eV).

In consequence to optical study we have investigated thesteady-state and time dependent luminescence properties of thesynthesized material by introducing MEH-PPV polymer as donormaterial. The PL energy quenching analysis elucidated that thesynthesized nanomaterial would play a dynamic role in Hybridsolar cell with donor polymer (MEH-PPV).

The current density vs. voltage characteristic of the device wasmeasured by Keithley 2400 sourcemeter interfaced with PC. Fig. 7

A. Layek et al. / Journal of Alloys and Compounds 613 (2014) 364–369 369

exhibits the current density vs. voltage characteristics under darkand photo condition. The effective cell area was taken as7.065 � 10�2 cm2. Under 80 mW cm�2 photo irradiance the shortcircuit current density and open circuit voltage for the devicewas measured as 1.75 mA cm�2 and 367 mV respectively. Thepower conversion efficiency was estimated as 0.3% with fill-factor0.38.

In this scenario, a photon is absorbed by MEH-PPV and pair ofexcited-state charge carriers is created. Depending upon the elec-tron affinity photo generated electron is transferred to the LUMOlevel of donor polymer. Due to the relative conduction band edgethis excited electron follows the radiative pathway and penetratesinside the conduction band of acceptor. The electric field developedin between Aluminium and ITO assures the charge transfer path-way, which results in photo induced current. The open circuit volt-age that is developed inside the cell is because of the energydifference between HOMO level of MEH-PPV and conductionenergy level of CuFeS2 acceptor (Scheme 1).

5. Conclusions

In this paper we have demonstrated the hydrothermal synthesisof semiconductor CuFeS2 nanomaterial. The photoluminescenceand lifetime analysis of the MEH-PPV:CuFeS2 composite firmlyestablished the successful energy quenching and resonance energytransfer from MEH-PPV (donor) to CuFeS2 (acceptor). The photoin-duced charge transport phenomenon was explained with the helpof band diagram. We have successfully applied CuFeS2 to fabricatethe hybrid solar cell. The efficiency of the device can further beimproved by optimizing the indulging parameters and morphologyof the synthesized CuFeS2.

Acknowledgements

The authors appreciate the University Grants Commission(UGC), Govt. of India for the financial support under project

39-508/2010(SR). The authors wish to acknowledge MadhusudanNandy of the Department of Chemistry, Jadavpur University forhis valuable advice and enormous technical assistance. The sup-port of FIST program of DST, Government of India is alsoacknowledged.

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