Effects of graphene counter electrode and CdSe quantum dots in TiO2

INTERNATIONAL JOURNAL OF ENERGY RESEARCH Int. J. Energy Res. 2014; 38:674–682 Published online 4 February 2014 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/er.3179

Effects of graphene counter electrode and CdSe quantum dots in TiO2 and ZnO on dye-sensitized solar cell performance A. Kathalingam1, Jin-Koo Rhee1,*,† and Sung-Hwan Han2 1

Millimeter-wave Innovation Technology Research Center (MINT), Dongguk University, Seoul 100-715, Korea Inorganic Nano-Materials Laboratory, Department of Chemistry, Hanyang University, Seoul, Korea

2

SUMMARY Fabrication and performance study of dye-sensitized solar cells using different counter electrodes and photoanodes is reported. Spin coated, E-beam coated platinum, and graphene electrodes were used as counter electrodes. Different combinations of TiO2 nanoparticle and ZnO nanorods (NRs) with CdSe quantum dots were prepared and used as photoanodes. The photoanodes comprising of both ZnO NRs and TiO2 nanoparticles have shown improved performances in short-circuit current density and open-circuit voltage comparing the devices fabricated using only ZnO NR or TiO2 nanoparticles. The inclusion of CdSe quantum dots has been found to increase the performance of dye-sensitized solar cell for all the photoanodes. In case of counter electrodes, the cells fabricated with graphene showed improved performance. Copyright © 2014 John Wiley & Sons, Ltd. KEY WORDS dye-sensitized solar cell; CdSe quantum dot; TiO2 nanoparticle; ZnO nanorod; graphene counter electrode Correspondence *Jin-Koo Rhee, Millimeter-wave Innovation Technology Research Center (MINT), Dongguk University, Seoul, 100-715, Korea. † E-mail: [email protected] Received 4 June 2013; Revised 13 December 2013; Accepted 18 January 2014

1. INTRODUCTION As the global demand of energy is increasing, the consumption of fossil fuels is also increasing day by day. It will reduce the stock of fossil fuels, and also it concerns more on environmental degradation. Thus, it urges to find new alternative ways to develop efficient, cost-effective, and nonpolluting devices to harvest solar light and fulfill the future energy demands. Although the conventional silicon solar cell modules have efficiency about 20%, they are more expensive as they require ultra-pure material and high-temperature processing [1]. In this respect, dyesensitized solar cells (DSSCs) are emerging as a viable alternative to the conventional solar cells due to their low cost and simple fabrication method [1–3]. The schematic representation of the DSSC is shown in Figure 1. A typical DSSC basically consists of four components: (i) a wide band gap (TiO2 or ZnO) semiconductor thin film electrode (photoanode); (ii) a sensitizer (dye) adsorbed onto the surface of the semiconductor thin film; (iii) electrolyte containing a redox couple, iodide/triiodide (I/I 3 ); and (iv) a counter electrode (CE) with a thin layer of catalytic materials such as platinum (Pt) [4,5]. When the dyeadsorbed semiconducting nanoparticle is illuminated, the

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dye absorbs the light and goes to excited state; subsequently, it undergoes a charge separation and injects an electron to the conduction band of semiconductor. The released electrons are injected into the porous oxide film and conducted through the load as shown in Figure 2. Simultaneously, the oxidized dye is reduced back to its original state by a redox mediator in the electrolyte and returns to the ground state [6,7]. Although the efficiency of DSSC (11%) is lesser than silicon solar cells, it has lot of scope to increase the efficiency by suitably modifying the photoanode, dye, and CEs. For the preparation of photoanode, nanocrystalline TiO2 is widely used because of its wide band gap, low cost, easy availability, and nontoxicity [8,9]. Many groups have attempted to improve the efficiency focusing on various aspects of photoanodes, dyes, and CEs. Zinc oxide (ZnO) has also been attracted as an alternative to TiO2 because of its band gap and electron affinity similar to TiO2, higher electron mobility, the availability of low temperature synthesis route, and the potential for controlling the morphology through simple processing from solution. It also has an additional advantage over TiO2 that is the ZnO can be produced in wide varieties of morphologies with a large surface area, which is an essential factor in maximizing

Copyright © 2014 John Wiley & Sons, Ltd.

Dye-sensitized solar cell

A. Kathalingam, J.-K. Rhee and S.-H. Han

Figure 1. Schematic of mesoporous TiO2 nanoparticle film based dye-sensitized solar cell.

Figure 2. Schematic energy diagram and electron transport in a dye-sensitized solar cell.

dye adsorption [10,11]. Poor transport is one of the limiting factors in the performance of DSSC. The generated free electrons suffer enhanced scattering in the disordered structure of nanoparticulate films, electron transport in that case is by trap mediated diffusion, which is a slow mechanism. Hence, the mobility of the electrons is reduced causing recombination at the grain boundaries between the nanoparticles. Instead of such nanoparticulate films, the use of aligned nanorods (NRs) film could provide direct electrical pathways ensuring the rapid collection of carriers generated throughout the device. Furthermore, the one-dimensional ZnO NR arrays can be easily prepared by a low-cost simple chemical method [12]. ZnO NRs grown perpendicular to the substrates are particularly interesting in electron transport as it has higher electron injection efficiency [13,14]. Int. J. Energy Res. 2014; 38:674–682 © 2014 John Wiley & Sons, Ltd. DOI: 10.1002/er

Hence, the replacement the nanoparticle film with an array of NRs offers an improved electron transport leading to higher photoconversion efficiencies. The efficient conduction of electrons from the point of photogeneration to the conducting substrate by the NRs grown vertically on the substrate greatly reduces the electron recombination losses of the photogenerated charge-carriers due to the fewer grain boundaries in charge transportation process. Moreover, electron transport in the crystalline rod is expected to be several orders of magnitude faster than percolation through a random polycrystalline network [15]. Although the nanowires exhibit significantly improved electron transport compared with porous particulate films, they have lower surface area than the porous films; this significantly reduces the dye adsorption causing poor efficiency [10]. Efficient light harvesting and electron 675


collection are the two critical factors should be taken care to improve efficiency of DSSC. By proper modification of the photoanode, these two factors can be improved largely. The photoanodes prepared incorporating vertical ZnO NRs with nanoporous TiO2 films can improve both light harvesting and electron collection efficiency of the DSSC, though the works on this direction is limited [16]. The use of quantum dots (QDs) such as CdSe, CdS, PbS, and PbSe is also a recent trend to increase the photoconversion efficiency of DSSC [17]. The performance of a DSSC depends on the efficiency of the sensitizer adsorbed on the TiO2 surface. Co-sensitization of using both organic and inorganic sensitizers is a new strategy to boost efficiency. The property of high extinction coefficient of QDs can increase the overall power conversion efficiency of solar cells [18]. QDs also have some favorable advantages as sensitizers in DSSC, including easy tuning of band gaps by changing the size of the QD. Among the various QDs, the CdSe QD has ideal band gap (1.7 eV) to match with solar spectrum [12]. Similarly, the CE is also an important part, which affects the efficiency of DSSC. It helps to regenerate the dye by the catalytic reaction with redox electrolyte. Thin platinum film is normally used as CEs because of its good conductivity and catalytic activity. Although it is widely used, the iodine present in the cell electrolyte (iodide/triiodide) can corrode and degrade the catalytic activity of platinum, causing reduction in the performance of DSSC. Moreover, the platinum is an expensive metal and in limited supply [19]. Thus, other inexpensive catalysts such as carbon nanotubes, graphene, and other conductive polymers are being investigated for the replacement of platinum [20–24]. A good CE should have highly conductive and excellent catalytic properties, because it is used to regenerate the dye by transferring the electrons from the external circuit to the electrolyte and by the reduction of iodide ions. There is a recent attraction on graphene as a replacement for platinum in DSSC, because it has high transparency and electrical conductivity, chemical and mechanical robustness, and materials abundance for transparent conductors [25–27]; it also has good catalytic ability like platinum [28]. In this work, an attempt has been made to incorporate the ZnO NRs and CdSe QDs with TiO2 nanoparticulate film for the use of DSSC by using different CEs.

2. EXPERIMENTAL PROCEDURE All the chemicals used in this work were purchased from the Sigma-Aldrich Chemicals, and they were used as received. Indium doped Tin Oxide (ITO)-coated glass plates of area 2 × 1.5 cm were used to prepare different photoanodes and CEs. 2.1. Preparation of photoanodes For the growth of vertically aligned ZnO NRs, a two step procedure was used [29]. In the first step, 10-mM solutions of zinc acetate dehydrate in 1-propanol was spin coated 676


onto ITO substrates to form seed layers for the growth vertical nanowires. Multiple spin coating cycles were carried out to increase the density of the seeds. The substrates were heated 1 min at 100 ºC for every cycle of spin coating in order to enhance adhesion. After this spin coating, the substrates were dried and annealed for 2 hrs at 300 ºC in order to form the nanocrystal seeds on the substrates. Prior to this seed growth, the ITO substrates were cleaned by sonication in acetone and isopropanol for 10 min each, then rinsed in deionized water and dried in a nitrogen flow. In the second step, vertical ZnO NRs were grown on the pre-formed nanocrystal seeds by immersing the seeded substrates in the precursor solution consisting a mixture of equimolar (25 mM) zinc nitrate hexahydrate Zn(NO3) 2·6H2O and hexamethylenetetramine for about 10 h at 70 º C. The substrates with vertically grown ZnO NRs were then removed from the solution, rinsed in deionized water, and dried using nitrogen gas flow. The TiO2 film photoanode was prepared by blending commercial TiO2 powder (Degussa, P25). One gram of TiO2 nanopowder was taken in 10 ml ethanol and sonicated for 30 min, and then 0.5 ml of titanium (IV) tetraisopropoxide was added into the suspension and mixed with magnetic stirrer to produce uniform suspension. This suspension was deposited on the substrate by a doctor-blade coating method with a glass rod and scotch tape as a frame and spacer. This prepared film was sintered at around 400 ºC for an hour in air. CdSe QD of size 3.3 nm dispersed in toluene procured from Sigma-Aldrich was used in this work. To prepare CdSe incorporated photoanodes, the CdSe QDs were mixed with TiO2 nanoparticles (TiO2 NPs) and coated using doctor-blade method; whereas in case of ZnO NRs, it was spin coated. For the preparation of CdSe QDs incorporated ZnO NR/TiO2 NP films, QDs were spin coated. 2.2. Preparation of counter electrodes Platinum and graphene coated CEs were used to fabricate DSSC. Two types of platinum films were used, such as spin-coated and E-beam evaporated platinum films. To prepare spin-coated platinum film, 5 mM chloroplatinic acid (H2PtCl6) in 2-propanol was spin-casted onto the pre-cleaned ITO. Then it was annealed at around 400 °C for 1 h. Third type of electrode was prepared by depositing graphene monolayer flakes on E-beam evaporated platinum. The graphene monolayer flakes dispersed in ethanol (1 mg/L) purchased from Graphene supermarket, USA, were used to drop cast on pre-deposited platinum layer. Several drop-casting cycles were performed by heating the substrate 1 min at 100 º C for every cycle. 2.3. Assembling of dye-sensitized solar cell The different photoanodes prepared were immersed in Eosin-Y dye solution for 24 h at room temperature in order to adsorb dyes on the surface of the photoanode films. Eosin-Y of 0.05 gm was mixed with 15 ml ethanol to prepare dye solution. After the set period, the substrates were Int. J. Energy Res. 2014; 38:674–682 © 2014 John Wiley & Sons, Ltd. DOI: 10.1002/er



removed and washed with fresh ethanol in order to remove the excess non-adsorbed dyes. DSSCs were assembled as sandwich-type keeping the conductive side of the CE facing the semiconducting film of photoanode. Surlyn sheet purchased from Idealprofit Company Ltd, Hong Kong was used as spacer between CE and photoanode. To fix both the electrodes with surlyn sheet, it was pressed under heat around 70 º C. The redox electrolyte solution, a mixture of 0.5 gm lithium iodide and 0.05 gm iodine in 10 ml acetonitrile was introduced into the interspaces between the working and the counter electrodes through two predrilled holes on the back of the counter electrode. The two holes were sealed up using a surlyn film, on which a thin glass slide was pressed under heat. The schematic of completely assembled cell is shown in Figure 3.

2.4. Characterization and measurement Prior to dye adsorption, the prepared photoanode films were examined for their morphology and compositional analysis using scanning electron microscopy and energy dispersive X-ray analysis techniques. Photocurrent– voltage measurements were performed using a 1-kW xenon lamp (Newport Corporation, Irvine, CA, USA) for the fabricated DSSC of an effective electrode area of 0.5 cm2, and the photocurrent was measured using a Keithley 2400 source meter (Keithley Instruments, Road Cleveland, Ohio, USA). The photovoltaic characteristics were analyzed by the following equations. FF ¼

ηð%Þ ¼

ðJ max x V max Þ ðJ sc x V oc Þ

ðJ sc x V oc x FFÞ x100 Pin

3. RESULT AND ANALYSIS Morphologies of different photoanodes are shown in Figures 4–6. The film of TiO2 NPs is shown in Figure 4; it shows a good and uniform coverage on the entire ITO substrate without any major hole or cracks. The inset in Figure 4 shows the close up view of the TiO2 film, clearly shows the spherical particles covering the whole substrate. The morphology of the ZnO NRs grown on the ITO substrate is shown in Figure 5; it shows uniformly distributed vertical NRs covering the whole substrate. The insets show the magnified view of the NRs. ZnO NRs are found in hexagonal shape, which is the expected growth habit of wurtzite ZnO structure. Length of the NRs is found to be greater than 1 μm and the average diameter about 50 nm. Morphology of the photoanode consisting of TiO2 NPs and CdSe QDs with vertical ZnO NRs is shown in Figure 6. It shows vertically grown ZnO NRs with TiO2 and CdSe particles filled in between the NRs. We can also see that the surface of the NRs has become rough, and the diameter increased because of the TiO2 NPs and CdSe QDs. Energy dispersive X-ray analysis of the film having TiO2 NPs, ZnO NRs, and CdSe QDs is shown in Figure 7; it shows the presence of all the expected elements. Photocurrent density (Jsc) and open-circuit voltage (Voc) characteristics of different cells are shown in the

(1)

(2)

The fill factor (FF), which is the maximum power delivered to an external load, is calculated from the short-circuit current density (Jsc) and open-circuit voltage (Voc). The overall conversion efficiency (η) is normalized by the intensity of incident light (Pin) [30], where Jmax and Vmax are the photocurrent and voltage for maximum power output (Pmax).

Figure 4. Scanning electron microscope image of TiO2 nanoparticle film, inset; magnified view of the TiO2 nanoparticles.

Figure 3. Schematic diagram of ZnO nanowire, TiO2 nanoparticle, and CdSe quantum dot incorporated dye-sensitized solar cell. Int. J. Energy Res. 2014; 38:674–682 © 2014 John Wiley & Sons, Ltd. DOI: 10.1002/er

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Figure 5. Scanning electron microscope image of ZnO nanorods film, insets; magnified views of ZnO nanorods.

Figure 7. Energy dispersive X-ray spectrum of ZnO nanowire, TiO2 nanoparticle, and CdSe quantum dot film.

Figure 6. Scanning electron microscope image of ZnO nanowire, TiO2 nanoparticle, and CdSe quantum dots incorporated film.

Figure 8. The measured Jsc, Voc, FF, and efficiency (η) of different cells fabricated are given in Table I. The device fabricated with vertically grown ZnO NRs only has shown Jsc 1.8719 and Voc 0.4613; it is low compared with other structures. Although the ZnO NRs provides recombination loss-free directed conduction of electrons (inset of Figure 8), this low Jsc and Voc is attributed to the low surface area of ZnO NRs. The photoanode made of using vertically grown ZnO NRs and TiO2 NPs has shown higher Jsc and Voc than the photoanodes produced using only with ZnO NRs or TiO2 NPs. This enhancement resulted in both Jsc and Voc could be attributed to the increase in the surface area and conductivity of the film. The addition of TiO2 NPs with ZnO NRs has increased the surface area without loss of conductivity so that more area for dye adsorption is available, and as from the directed path of ZnO NRs, the recombination loss is reduced [15]. Hence, this enhancement of ZnO NR/TiO2 NP device can be attributed to the improvements in both carrier collection and photogeneration due to ZnO NR 678

Figure 8. Current–voltage characteristics of different cells; (a) ZnO NW and spin coated Pt, (b) ZnO NW + CdSe QD and spin coated Pt, (c) TiO2 NP and spin coated Pt, (d) TiO2 NP + CdSe QD and spin coated Pt, (e) ZnO NW + TiO2 NP and spin coated Pt, (f) ZnO NW + TiO2 NP + CdSe QD and spin coated Pt, (g) ZnO NW + TiO2 NP + CdSe QD and E-beam coated Pt, and (h) ZnO NW + TiO2 NP + CdSe QD and E-beam coated Pt + graphene.

and porous TiO2 film, respectively. The QDs having high extinction coefficients and broad absorption spectra covering a wide part of the visible spectrum can also increase the conversion efficiency of solar cells [31,32]. In order to Int. J. Energy Res. 2014; 38:674–682 © 2014 John Wiley & Sons, Ltd. DOI: 10.1002/er


study the influence of CdSe QDs, we incorporated the CdSe QDs with TiO2 NPs and ZnO NRs and studied its influence. The inclusion of CdSe QDs has been found to show improved performance with the composite film of TiO2 NPs and ZnO NRs (Figure 8(g) and (h)). But at the same time, the use of CdSe QDs with either TiO2 NPs or ZnO NRs individually has shown reduced performance (Figure 8(b) and (d)). This increasing performance of CdSe QD with ZnO NR/TiO2 NP structure can be attributed to the effect of extra sensitization and carrier generation. However, the decrease of performance when using the CdSe QD either with TiO2 NPs or ZnO NRs is not clear. It is necessary to know the energy level scheme of the DSSC structure to understand the energy transfer process and to explain the effect of individual components. Figure 9 shows the charge transfer process of the ZnO/TiO2/QDs/ Dye system. The conduction band edges of both TiO2 and ZnO have been reported to be 0.5 eV versus normal hydrogen electrode (NHE) [33,34]; similarly, the conduction band edge of CdSe is close to 0.8 eV versus NHE [35], lowest unoccupied molecular orbital energy of Eosin-Y is 0.92 [36,37], and the redox potential of I/I 3 is 0.53 V versus NHE [4]. Although the conduction band edge of both TiO2 and ZnO is the same, the conduction band edge of TiO2 is a little bit higher than that of ZnO; therefore, it results in a transfer of the photoinduced electrons from TiO2 conduction band to ZnO conduction band [38]. When the Eosin-Y sensitizer absorbs a photon, it is transformed to excited state. It gives an electron to the lowest unoccupied molecular orbital of Eosin-Y, this electron is injected into the conduction band of CdSe QD. After that, the injected electron diffuses into Fluorine doped Tin oxide (FTO) and flows through the load via the external circuit and then reaches the Pt CE.


Usually, the valence band of photon absorber should be lesser than the highest occupied molecular orbital level of the dye for the efficient transfer of charges and regeneration of dye. But here, the valence band of CdSe is higher than the highest occupied molecular orbital level of dye. This may be the reason for the decrease of performance in case of CdSe QD inclusion with both TiO2 NPs and ZnO NRs. In spite of this, the devices made with the combination of TiO2 NPs and ZnO NRs (Figure 8(g) and (h)) have shown enhanced performance with increased Voc and Jsc. In general, the maximum attainable Voc of a DSSC consisting of a set of dye and absorber material combination is limited. It depends on the difference between absorber material’s Fermi level and electrolyte redox potential [39,40]. However, the Voc can be increased by improving the interface properties of the materials even in same set of absorber and dye combination. In fact, the efficiency limiting factors such as recombination of photogenerated electrons with oxidized species and the deficiencies in the charge collection and transport can be improved by proper interfacial modification. If there is an increase in the energy level difference between the Fermi level and the redox potential, Voc can be increased [41]. So, the increase of Voc may be due to the negative shift of ZnO/TiO2 conduction band or positive shift of I/I 3 redox energy level. Thus, the improvement of Voc may be due to shift of Fermi level upward or redox potential downward. The Fermi level can be moved upward by the surface modification or improving the electron mobility by suppressing the charge recombination. The TiO2 NPs encapsulated ZnO NRs could have acted as core-shell structure of ZnO NR/TiO2 NP structure, which can improve the electron mobility by suppressing the charge recombination. Hence, the TiO2 NPs encapsulated ZnO

Figure 9. Schematic of energy level diagram and electron transfer of ITO/ZnO/TiO2/CdSe dye-sensitized solar cell device, all the energy levels are referenced to normal hydrogen electrode scale. Int. J. Energy Res. 2014; 38:674–682 © 2014 John Wiley & Sons, Ltd. DOI: 10.1002/er

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Table I. Photovoltaic performance of different dye-sensitized solar cell studied under simulated AM1.5 radiation. Materials Sample a b c d e f g h

2

Anode

CE

Voc (V)

Jsc (mA/cm )

FF

Effiency(%)

ZnO NW ZnO NW + CdSe QD TiO2 NP TiO2 NP + CdSe QD ZnO NW + TiO2 NP ZnO NW + TiO2 NP + CdSe QD ZnO NW + TiO2 NP + CdSe QD ZnO NW + TiO2 NP + CdSe QD

Spin coated Pt Spin coated Pt Spin coated Pt Spin coated Pt Spin coated Pt Spin coated Pt E-beam coated Pt E-beam coated Pt + Graphene

0.4613 0.4605 0.4638 0.4291 0.4806 0.4909 0.5676 0.5854

1.8719 2.0360 4.1862 4.6094 4.7555 4.9235 5.9312 6.9347

0.2665 0.2825 0.4422 0.4293 0.3970 0.5172 0.5162 0.5103

0.2301 0.2703 0.8585 0.8491 0.9073 1.2500 1.7225 2.0892

CE, counter electrode; FF, fill factor.

NRs could have resulted the improvement in Jsc, Voc, and FF as the TiO2 nanoparticulate film encapsulation facilitates electron acceleration mechanisms in addition to increased dye absorption [42]. Irannejad et al. have shown improvement in Voc and FF by covering ZnO NRs with TiO2 shell [43]. Huiqin Zheng et al. have also demonstrated the suppression of recombination by depositing an oxide layer on TiO2 [44]. Crystalline, morphological, and surface modification can also alter the Voc [45]. Thermal annealing can enhance significantly the photovoltaic performance of DSSC by improving interface quality [46]. Thermal treatment improves the crystalline quality, contact between the nanoparticles, and reduction of internal defects, and also it can alter the band gap of the material. So, by annealing the recombination of photoexcited carriers can be reduced, and it results in higher power conversion efficiency increasing both Voc and Jsc. Yitan Li et al. have shown significant improvement both in Voc and Jsc in CdSe–TiO2 nanostructures based cells [47]. The cells in Figure 8(b) and (d) were annealed at 250 °C considering thermal instability of CdSe QD, but the other two films in Figure 8(g) and (h) were annealed first at 400 °C; then CdSe QDs was spin coated. This procedure of spin coating of CdSe QDs may not have produced the ZnO NR and TiO2 NP well covered with CdSe QDs. Moreover, the annealing of the ZnO NRs surrounded with TiO2 NPs might have also changed the band alignment because of structural and crystalline improvement. Hence, the poor coverage of CdSe QDs and thermal annealing induced interfacial changes of TiO2 NPs covered with ZnO NR might be the reason for the change Voc. The other possibilities to increase the Voc and Jsc are the concentration of the materials used in the CE and the presence of water molecules in the electrolyte [48,49]. In case of CEs, the electrode made of graphene with platinum has given enhanced Voc and Jsc compared with other platinum electrodes. Gentian Yue et al. have also reported the performance enhancement of graphene/platinum CE [50]. The E-beam coated platinum electrode has produced improved response than the spin coated and annealed platinum films as shown in Table I. This improved response of graphene can be attributed to the superior conducting and 680

catalytic property of graphene [44], whereas the improvement obtained in the case of E-beam coated film compared with spin coated films may be due to the flawless smooth and continuous films obtained with improved conductivity.

4. CONCLUSION Eosin-Y and CdSe QDs co-sensitized DSSC were fabricated using different combination of photoanodes and CEs. The TiO2 nanoparticle and ZnO NR mixed photoanodes have shown improved response than the films fabricated using either TiO2 nanoparticle or ZnO NR. CdSe inclusion has been found to increase both Jsc and Voc in ZnO NRs and TiO2 nanoparticle used films. The addition of graphene with platinum has shown improved response as CE. The DSSC fabricated using graphene as CE and CdSe QDs incorporated ZnO NRs and TiO2 NPs mixed film as photoanode has exhibited best performance. It has been proved that the graphene is a good catalytic and transparent conductor, which can advantageously be used for DSSC instead of expensive platinum.

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Int. J. Energy Res. 2014; 38:674–682 © 2014 John Wiley & Sons, Ltd. DOI: 10.1002/er