DOI: 10.1002/cphc.201300605

Energy Relay from an Unconventional Yellow Dye to CdS/CdSe Quantum Dots for Enhanced Solar Cell PerformanceRemya Narayanan,[a] Amrita Das,[a] Melepurath Deepa,*[a] and Avanish Kumar Srivastava[b]

1. Introduction

Conventional quantum dot solar cells (QDSCs) rely on the prin-ciple of light harvesting by using photosensitive quantum dotsanchored to a wide band gap semiconductor.[1–6] Quantum dot(QD)-based solar cells are attractive as they offer a cost effec-tive route to efficient solar cells. QDs have high molar extinc-tion coefficients and offer size tunability through the control ofprocessing parameters, effectively allowing band gaps to betailored as desired.[7–12] However, insufficient loading of dots,narrow absorption spectra that span only a small portion ofthe visible range, rapid back-electron transfer rates, sluggishhole transfer rates, and anodic corrosion have a detrimentaleffect on solar cell efficiencies, and, as a consequence, thepower conversion efficiencies of QDSCs are limited to 1 to7 %.[13–15] Recently, Forster resonance energy transfer (FRET) hasemerged as a very powerful solution to these difficulties asthis method addresses the primary concern of poor spectralcoverage, minimizes excessive QD quenching by the electro-lyte, and bypasses the stringent design requirement of alignedenergy levels for the photoanode components. In a QDSC,FRET involves non-radiative energy transfer from a photoactivedonor species to acceptor molecules on excitation. The accept-or molecule must have a lower excitation energy than thedonor and the donor–acceptor separation must be short. Thedonor–acceptor architecture should satisfy certain pre-requi-sites for FRET to be feasible; these have been summarized in

detail in some of the early pioneering reports.[16, 17] The advant-age of the FRET mechanism over direct charge injection ina QDSC is the enhancement of photoconversion efficiencies inthe blue and red regions of the solar spectrum through use ofan ideal donor–acceptor pair with complementary absorptioncoverage.[18–20]

In a significant development, Hardin et al. demonstrateda 25 % increase in power conversion efficiency, relative to cellsbased on only acceptors or only donors, for a cell based onhighly luminescent organic chromophores dissolved in electro-lyte as donors and zinc phthalocyanine (ZnPc) molecules anch-ored to titania as acceptors.[21] In another study, the photocon-version efficiency was increased by 47 % due to FRET fromCdSe QDs to a symmetrical squaraine dye.[22] In a report ona cell with a photoanode configuration of TiO2/black dye,energy transfer from ZnPc molecules (as donors) dissolved inelectrolyte to black-dye-coated TiO2 nanowires resulted ina high external quantum efficiency (EQE) for red photons. Thequantum efficiency increased from approximately 6 % for theZnPc (acceptor)-only cell to approximately 10 % for the blackdye/ZnPc (donor–acceptor) system.[23] Similarly a panchromaticresponse due to FRET from a laser dye to ZnPc in a dye-sensi-tized solar cell (DSSC) resulted in a photoconversion efficiencyof 3.97 %; this constitutes a 35 % increase relative to the ac-ceptor-only cell.[24] Another FRET cell with a CdSe/CdS photo-anode and CoS as the counter electrode, showed an increasedefficiency of 0.05 %.[25] In yet another report on a solid-stateDSSC, containing poly(3-hexylthiophene) as both the hole-transporter and energy relay dye, and a ZnPc dye as the sensi-tizer,[26] the donor-only cell had a maximum EQE of 5.5 %; addi-tion of the acceptor to the same assembly resulted in an in-crease in the EQE maximum to approximately 11 %.

Based on the survey described above, it is evident that theremarkable manipulation of sequential interactive energy and

A new design for a quasi-solid-state Forster resonance energytransfer (FRET) enabled solar cell with unattached Luciferyellow (LY) dye molecules as donors and CdS/CdSe quantumdots (QDs) tethered to titania (TiO2) as acceptors is presented.The Forster radius is experimentally determined to be 5.29 nm.Sequential energy transfer from the LY dye to the QDs andelectron transfer from the QDs to TiO2 is followed by fluores-cence quenching and electron lifetime studies. Cells witha donor–acceptor architecture (TiO2/CdS/CdSe/ZnS-LY/S2�-

multi-walled carbon nanotubes) show a maximum incidentphoton-to-current conversion efficiency of 53 % at 530 nm.This is the highest efficiency among Ru-dye free FRET-enabledquantum dot solar cells (QDSCs), and is much higher than thedonor or acceptor-only cells. The FRET-enhanced solar cell per-formance over the majority of the visible spectrum paves theway to harnessing the untapped potential of the LY dye as anenergy relay fluorophore for the entire gamut of dye sensi-tized, organic, or hybrid solar cells.

[a] R. Narayanan, A. Das, Dr. M. DeepaDepartment of Chemistry, Indian Institute of Technology HyderabadOrdnance Factory Estate, Yedduaram-502205, Andhra Pradesh (India)E-mail : mdeepa@iith.ac.in

[b] Dr. A. K. SrivastavaNational Physical LaboratoryKrishnan road, New Delhi-110012 (India)

Supporting Information for this article is available on the WWW underhttp://dx.doi.org/10.1002/cphc.201300605.

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electron transfer dynamics between photoactive moietieswhich do not possess aligned conduction band offsets, offersviable opportunities for the efficient harvesting of blue/greenand red photons in both QDSCs and DSSCs. However, the un-derstanding of the mechanistic aspects of the synergy be-tween the donor and the acceptor molecules in these multi-component photoanode/electrolyte systems is at a nascentstage, as few photoanode architectures have so far been inves-tigated for FRET.[27, 28] For enhanced solar cell performance,there is an urgent need to design and implement new photoa-node configurations capable of FRET. In this article, we used anenergy relay donor dye, namely Lucifer yellow (LY), which todate has rarely been used in a QDSC, although it has beenused in biologically active assemblies. LY has a high molar ex-tinction coefficient (11194 m

�1 cm�1) and a broad spectralbandwidth (Dl= 110 nm) in the visible region. We used CdS/CdSe QDs as the acceptor. The highly luminescent donor (LY)chromophores were unattached to the acceptor and were dis-solved in the electrolyte. In this new design, the LY moleculesabsorb the high energy photons, undergo excitation and trans-fer their excitation to the TiO2 bound CdS/CdSe acceptor QDs.The QDs are disabled from accepting electrons by directcharge injection from LY by means of a wide band gap ZnS in-terlayer between the TiO2/CdS/CdSe layers and the electrolyte.We used a poly(3,4-ethylenedioxythiophene) (PEDOT) microfib-er-enriched sulfide gel as the electrolyte. Previously, PEDOTfilms with a conventional granular structure have been used asiodine/iodide-free hole transport layers in DSSCs.[29–31] ThePEDOT microfibers, synthesized herein have excellent electron-ic conductivity and the hole transport properties of the S2� gelimproved upon incorporating these polymer microfibers. Ournovel design for this FRET-enabled QDSC has several benefits.The covalently linked CdS/CdSe core-shell QDs have a strongand narrow absorption that is red-shifted relative to the ab-sorption maximum of the unattached LY dye molecules. Fur-thermore, it is not necessary for the conduction band of LY tolie above the conduction band of CdS/CdSe (with respect tothe vacuum level) ; this is essential in a non-FRET, co-sensitizedsystem. As LY is unattached, no pre-functionalization step is re-quired to attach it to the TiO2. As a consequence, a wider andstronger absorption can be obtained for the same thickness ofTiO2 film, which leads to higher photocurrents and largerpower conversion efficiencies. We present the synthesis, evalu-ation of the photo-physical properties, and calculation of theForster radius by using a spectroscopic ruler of a FRET-enabledsolar cell with the following configuration: TiO2/CdS/CdSe/LY-PEDOT-S2�gel/MWCNT, and correlate these with its photovolta-ic performance. We used electrophoretically deposited multi-walled carbon nanotubes as the counter electrode, which area cheap alternative to an expensive Pt layer. We provide mech-anistic insights in terms of the kinetics of energy and electrontransfer in this cell, which is capable of complementary lightabsorption. We also demonstrate the way in which this assem-bly overcomes the issues of fast electron recombination andproduces photocurrents which are considerably enhanced incomparison to cells with only donor or only acceptor compo-nents. Our new design and implementation of a FRET-enabled

inexpensive quasi solid-state QDSC unambiguously shows thatsolar radiation can be used more efficiently than expected.This can play a pivotal role in the development of advancedhigh performance solar cells.

2. Results and Discussion

HRTEM Analyses

The CdS layers are porous and therefore permit the percolationof the Cd(NO3)2 salt solution and its adsorption directly ontothe CdS QDs. On exposure to the Na2SeSO3 solution, CdSe QDsare formed in situ around the CdS QDs, mimicking the core/shell architectures of CdS/CdSe QDs. As no organic linkers orcapping agents were used, intimate nanoscale contact couldbe achieved between the two QDs. The low magnificationtransmission electron microscopy (TEM) image of the CdS/CdSe QDs shows the QDs to be aggregated and distinctivegrain boundaries are not perceptible (panel A in Figure 1 a).However, the high resolution TEM (HRTEM) image of the CdS/CdSe QDs shows the lattice fringes from the core (CdS) to beencapsulated by fringes from the shell (CdSe) (Figure 1 b). Theimage contrast in panel B clearly distinguishes the lighter CdScore from the darker CdSe shell. Panels C and D reveal the en-larged views of lattice scale images extracted from Figure 1 b,and the fringe separation was observed to be 0.33 nm in C.This is in agreement with an interplanar separation of 0.34 nmcorresponding to the <111> plane of the face-centered cubic(fcc) lattice of CdS (PDF: 65287). Similarly, the interplanar spac-ing in D was observed to be 0.30 nm, which matches the<200> orientation of the fcc structure of CdSe (PDF: 882346).We recorded several images of CdS/CdSe QDs from differentregions of the same sample, and the images were always com-posed of crisscross fringes indicating that the samples werecrystalline. However, the lattice scale image of CdS/CdSe QDswith a monolayer of ZnS and an overlayer of LY dye (Figure 1 c)reveals the co-existence of amorphous domains (encircled bysolid lines) and crystalline regions (encircled by dashed lines).As none of the images that we obtained for CdS/CdSe QDs re-vealed the presence of amorphous structures, we attributethese amorphous domains to the LY dye and the crystallinedomains to CdS/CdSe QDs. Therefore, the separation betweenthe amorphous and crystalline domains can be treated as theactual distance between the LY donor dye and the CdS/CdSeacceptor QDs. It is evident from the Figure 1 that the separa-tion never exceeds 5 nm, which is in fact close to the Forsterradius (determined spectroscopically later). As this value iswithin the distance over which FRET can occur (1–10 nm), FRETis possible from the LY dye to the CdS/CdSe QD system. The X-ray diffractogram and HRTEM image of pristine TiO2 are shownin Figures S1 and S2 (Supporting Information).

Energetics and Fluorescence Quenching

The CdS/CdSe QDs were deposited on a TiO2-coated transpar-ent conducting electrode using the chemical bath depositiontechnique. The LY dye (structure shown in Figure S3) was dis-

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solved in the aqueous sulfide ion electrolyte and functioned asthe donor or energy relay dye. The QDs tethered to the TiO2

electrode functioned as the acceptors. The conduction bandsof CdS and CdSe lie at �1.0 V and �1.2 V [vs normal hydrogenelectrode (NHE)] , respectively, and these positions are 0.589and 0.789 V more negative than the conduction band edge of

the LY dye donor [�0.411 V vs NHE, determined from thecyclic voltammogram (CV) of the pristine LY dye, Figure S4].The energy band offset between LY and the CdS/CdSe QDsrenders electron injection from the conduction band of the LYdye to the conduction band of CdS/CdSe QDs upon photo-ex-citation thermodynamically unfavorable (Figure 2). In addition,a monolayer of ZnS was deposited over the CdS/CdSe QDs,and because the conduction band of ZnS lies at �1.6 V (vsNHE), it acts as a barrier that prevents direct charge injectionfrom the donor (in the electrolyte) to the CdS/CdSe QDs or ti-tania electrode. As a consequence, the only available pathwayfor photo-excited electron deactivation in the LY dye isthrough FRET, in which the energy is non-radiatively trans-ferred to the CdS/CdSe QDs. Electrons in the valence bands ofCdS/CdSe QDs are promoted to their corresponding conduc-tion bands, transferred to the conduction band of TiO2 in a cas-cade fashion, and subsequently transferred to the current col-lector. The choice of LY as the donor was based on its extreme-ly large photoluminescence quantum efficiency and its strong,narrow, high energy absorption spectrum between 365 and475 nm and with lmax at 430 nm. The absorption and lumines-cence spectra of the LY dye are shown in Figure 3 a. The LYdye shows a band-edge emission peak at 533 nm upon excita-tion at 430 nm. For efficient energy transfer from the LY dye tothe CdS/CdSe QDs, the absorption of the QDs must overlapwith the emission of the LY dye. Figure 3 b reveals that thebroad absorption of the CdS/CdSe QDs spanning from 507 to586 nm is almost completely covered by the emission profileof the LY dye which lies between 475 and 650 nm. Sucha good overlap of the absorption and emission spectra of thedonor and acceptor makes FRET a viable phenomenon in thiscell architecture. FRET involves the transfer of donor excitationto neighboring acceptor molecules in a non-radiative fashion,so other evidence for FRET includes the quenching of the lumi-nescence of the donor in the presence of the acceptor anda reduction in the electron lifetime.

The strong emission of LY (with lem at 533 nm) wasquenched drastically upon introducing the TiO2/CdS/CdSe/ZnSfilm into the aqueous LY dye solution (Figure 3 c). The film wascomposed of three layers of CdS, one layer of CdSe, and onelayer of ZnS. The emission intensity of LY was quenched by ap-proximately 4.9 times its original value. On employing a QDfilm with more CdSe layers (CdS:3, CdSe:2, and ZnS:1), theemission intensity was quenched by approximately 100 times.We therefore used this composition for further studies. Otherindirect evidence for FRET was observed by monitoring theemission of ZnS/CdSe/CdS (donor QDs tethered to TiO2 film) inthe presence of the LY dye, by using a different excitationwavelength (lex = 370 nm). We observed that the pristine ZnS/CdSe/CdS QDs did not show any emission peak on excitationat 430 nm, which was the excitation wavelength of the LY dye.The emission spectra of a pristine TiO2/CdS/CdSe/ZnS film andthe same film in a LY solution, with lex = 370 nm, are shown inFigure 3 d. The band-edge emission peak for pristine CdSe/CdSQDs (acceptor in the absence of the donor) is observed at603 nm, and in the presence of the LY dye donor the intensityof this peak is amplified 1.5 fold, which indicates an interactive

Figure 1. a) HRTEM images of A) aggregated CdS/CdSe QDs, B) image con-trast to distinguish the core from the shell, and enlarged views of interplanarseparations of C) the CdS core QDs and D) the CdSe shell that prevail in (C).b) Lattice scale image of core/shell CdS/CdSe QDs. c) Lattice scale image ofthe CdS/CdSe QDs with an LY dye overlayer; the crystalline QDs and the dyeare encircled in dashed and solid ellipses, respectively.

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energy transfer from the LY dye to the CdS/CdSe QDs. Theemission spectrum of the TiO2/CdS/CdSe/ZnS-LY system (ob-tained at an excitation wavelength of 370 nm) comprises twocomponents, which can be deconvoluted. The emission peakat 530 nm corresponds to the emission of the LY dye, and thepeak at 603 nm corresponds to the emission of CdS/CdSe QDs;

the intensity of this signal is in-creased in the presence of theLY dye. A similar enhancementin the emission peak intensity ofthe acceptor (squaraine dye) wasachieved in the presence ofCdSe donor QDs, and the au-thors correlated the increase inthe emission of the acceptor toefficient energy transfer fromCdSe to the squaraine dye intheir FRET cell.[18] FRET inducesan increased charge separationin the acceptor, because the LYdye absorbs the photons towhich pristine CdS/CdSe QDs areinsensitive, and transfers its exci-tation to the CdS/CdSe QDs. Theemission enhancement observedfor CdS/CdSe QDs is due solelyto energy transfer.

Time-Resolved Emission Decay

In order to confirm that sequen-tial energy transfer from LY tothe CdS/CdSe QDs followed byelectron transfer to TiO2 will be

the dominant processes if this system is employed in a solarcell, fluorescence lifetimes for several assemblies were mea-sured. These decay profiles are shown in Figure 4. All the time-resolved fluorescence decay signals in this study were fitted toa bi-exponential function, except that of the LY dye, which ex-hibited a single exponential decay. In Equation (1), I is the nor-

Figure 2. a) Energy levels of a TiO2/CdS/CdSe/ZnS-LY/PEDOT/S2� assembly, illustrating possible modes for energy, electron, and hole transfer, b) Schematic ofa TiO2/CdS/CdSe/ZnS-LY/PEDOT/S2�-MWCNT cell, and c) (1) TiO2, (2) TiO2/CdS, (3) TiO2/CdS/CdSe/ZnS electrodes; and S2� gel, LY dye/S2� gel, and LY/PEDOT mi-crofibers/S2� gel.

Figure 3. a) Molar absorption coefficient (&) and fluorescence (*) spectra of the LY donor dye (1.5 mg mL�1,lex = 430 nm), b) overlap of the absorption (&) of the acceptor: TiO2/CdS/CdSe/ZnS electrode and the emission (*)of the donor (LY dye, at lex = 430 nm), c) fluorescence quenching of the LY dye (*) in the presence of the TiO2/CdS/CdSe/ZnS electrode with 3:1 (&) and 3:2 (~) layers of CdS:CdSe QDs (lex = 430 nm), and d) emission of thepristine TiO2/CdS/CdSe/ZnS electrode composed of 3:2 layers of CdS:CdSe QDs (~) and of the same electrode inthe presence of 1.5 mg mL�1 LY dye (*) (lex = 370 nm). In (d), the symbol (*) represents the raw data, whereas thesolid lines represent the deconvoluted fits. In all experiments ZnS was deposited as a monolayer.

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malized emission intensity, t is the time after LED excitation, A1,A2 are the amplitude coefficients, and t1, t2 are the decay timeconstants. The results of the fitting are collated in Table 1. The

average lifetime of the transient electron was determined byusing Equations (1) and (2).

I ¼ A1expð�t=t1Þ þ A2expð�t=t2Þ ð1Þ

< t >¼ SiðAitiÞ=SiAi ð2Þ

The pristine LY dye showed an excited electron lifetime of5.072 ns, obtained at an excitation wavelength of 450 nm. Thislifetime agrees well with the previously reported lifetime of5 ns for the same dye.[32] Upon incorporation of a TiO2/CdS/CdSe/ZnS film into the LY dye solution, the average lifetime ofthe photo-generated electron in the conduction band of theLY dye was reduced to 3.02 ns at the same excitation wave-length. The decrease in the lifetime is a clear indicator ofenergy transfer. As the rate of energy transfer is faster thanthat of charge transfer and band-edge recombination, we at-tributed the fast decay component of 0.189 ns to energy trans-fer from the LY dye to the CdS/CdSe QDs. The slow decay com-ponent of 3.765 ns is assigned to band-edge recombination inthe LY dye. The rate constant (Ket) of energy transfer, whichwas determined from the lifetime analysis, is 5.29 � 109 s�1. Asmentioned above, direct electron injection from LY to CdS/CdSe QDs is inhibited by the ZnS energy barrier layer and theunfavorable conduction band positions. Electron transfer fromthe LY dye to TiO2 is inhibited by the CdS/CdSe QDs which actas a physical barrier. As a consequence, the excited electrondecay mode is FRET. To confirm this electrons in CdS/CdSe QDsexcited by the absorption of indirect photons were injectedinto TiO2, and the average lifetime of the pristine TiO2/CdS/CdSe/ZnS electrode was measured at an excitation wavelengthof 370 nm and found to be 5.83 ns. The short-lived componentof 3.915 ns was attributed to direct electron injection fromCdS/CdSe QDs into TiO2 or fluorine-doped tin oxide (FTO) andthe long lived component of 15.60 ns was attributed to elec-tron–hole recombination in the QDs. The average electron life-time in the same assembly measured in the presence of the LYdye solution at the same excitation wavelength (370 nm) in-creased to 7.39 ns from 5.03 ns. This increase in the lifetime ofthe acceptor in the presence of the donor matches the in-crease in the emission of the acceptor in the presence of thedonor. Electron excitation in CdS/CdSe QDs by indirect photonabsorption of the donor LY dye is slower than excitation bydirect photon absorption (observed in the absence of the LYdye). In the presence of the donor, two processes occur; one iselectron transfer to TiO2 by direct photon absorption, and the

second is electron injection fromthe CdS/CdSe QDs to TiO2 afterexcitation energy transfer fromthe LY dye to the QDs. In previ-ous work, Etgar et al. also report-ed an increase in the lifetime ofthe acceptor squaraine dye inthe presence of CdSe QDs,which agrees with our observa-tion.[22] Fast and efficient energytransfer from the LY dye to theCdS/CdSe QDs results in a simul-

Figure 4. Time-resolved fluorescence decay traces of a) LY dye alone(1.5 mg mL�1, *) and in the presence of the TiO2/CdS/CdSe/ZnS electrode (&)with 3:2 layers of CdS:CdSe QDs (lex = 450 nm and lem = 530 nm). b) CdS/CdSe/ZnS electrode with 3:2 layers of CdS:CdSe QDs, measured at the exci-tation and emission wavelengths of the QDs (lex = 370 nm andlem = 600 nm), alone (*) and in the presence of the 1.5 mg mL�1 LY dye solu-tion (&). c) TiO2/CdS/CdSe/ZnS electrode with 3:2 layers of CdS:CdSe QDs(*), alone and in the presence of an aqueous solution of PEDOT microfibers(&, lex = 370 nm and lem = 603 nm).

Table 1. Kinetic parameters of emission decay analysis determined from biexponential fits.[a]

System composition lex [nm] lem [nm] c2 t1 [ns] t2 [ns] A1 A2 <t> [ns]

LY dye–TiO2/CdS/CdSe/ZnS 450 530 0.911 0.189 3.765 19.92 80.08 3.02TiO2/CdS/CdSe/ZnS–LY dye 370 603 1.034 0.311 19.293 62.66 37.34 7.39TiO2/CdS/CdSe/ZnS 370 603 1.000 3.915 15.603 83.33 16.67 5.83TiO2/CdS/CdSe/ZnS–PEDOT 370 603 1.067 0.262 5.426 7.94 92.06 5.03

[a] A is the relative amplitude of each lifetime, t1, and t2 are the components of the fluorescence lifetime andc2 denotes the fit quality. The LY dye and PEDOT were dissolved in water and the TiO2/CdS/CdSe/ZnS assemblywas a solid electrode.

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taneous decrease in the lifetime of the LY dye donor moleculesin the presence of the CdS/CdSe QD acceptors and an increasein the lifetime of the QD acceptors in the presence of the LYdye donors. As the excitation wavelengths that lead to theirdistinctive emission spectra are different for both the donorand the acceptor, it was possible to perform separate lifetimeanalyses.

In addition to using FRET to maximize the spectral coverageof the solar cell fabricated in this work, we modified the sulfideion electrolyte by dispersing hole-conducting PEDOT microfib-ers in the gel. Previously, Yanagida et al. successfully demon-strated the enhanced performance of a DSSC by replacing theiodine/triiodide electrolyte with a PEDOT-based hole transportpolymeric film.[31] To confirm that the PEDOT microfibers im-prove the hole-scavenging ability of the sulfide ion electrolytein a solar cell we measured the emission and decay profiles ofthe TiO2/CdS/CdSe/ZnS electrode in the presence of an aque-ous solution of PEDOT microfibers (lex = 370 nm and lem =

600 nm, Figures S5 and 4 c). One criterion for hole transfer isthat the conduction band level of the hole provider is morepositive (vs NHE) than that of the hole acceptor. The redox po-tential of the S2�/S couple lies at �0.47 V (vs NHE) and thework function of PEDOT is + 0.5 V.[33] As these values are morenegative than the valence band levels of CdS and CdSe QDs inthe quasi-core/shell structures (+ 1.4 and + 0.8 V vs NHE), holepropagation from the QDs to the PEDOT microfibers and S2� isfeasible. The band-edge emission of CdS/CdSe QDs at 603 nmis quenched by 1.3 times in the presence of the PEDOT micro-fibers, which indicates that holes are transported from the va-lence bands of the CdS/CdSe QDs to the PEDOT microfibers.The contribution of the PEDOT microfibers was determinedfrom lifetime data. Previously, Chakrapani et al. reported a de-creased electron lifetime of 3.30 ns for a CdSe/SiO2 electrode inthe presence of a 0.1 m Na2S electrolyte, compared to a valueof 4.58 ns obtained for the same electrode in the presence ofa NaOH solution, thus indicating that hole transfer occurredfrom CdSe to S2� and not to OH� .[34] Herein, the CdS/CdSe QDshad an excited state life time of 5.83 ns. This lifetime was re-duced to 5.03 ns in the aqueous solution of the PEDOT micro-fibers, which indicates that holes are transported from the va-lence bands of CdS/CdSe QDs more efficiently to the PEDOTmicrofibers than to S2� [Eqs. (3) and (4)]:

CdS=CdSeþ hn! CdS=CdSeðhþ þ e�Þ þ TiO2 !CdS=CdSeðhþÞþTiO2ðe�Þ

ð3Þ

CdS=CdSeðhþÞ þ PEDOTðNeutralÞ ! CdS=CdSeþ PEDOTþCðOxÞð4Þ

Forster Radius and Energy Transfer Rate

The Forster radius (Ro) is defined as the distance between thedonor–acceptor pair at which the energy transfer process hasan efficiency of 50 %, and this distance for efficient energyrelay typically lies in the range of 1 to 10 nm. Ro mainly de-

pends upon the photoluminescence quantum yield of thedonor (LY dye), and the extent of overlap between the emis-sion of the donor and the absorption of the acceptor (CdS/CdSe QDs). Ro was calculated from Equation (5):[35]

Ro6 ¼ 9000 lnð10Þk2QDJ=128 p5n4NA ð5Þ

where QD is the fluorescence quantum yield of the donor inthe absence of the acceptor, which was experimentally deter-mined as 0.3 by employing Rhodamine 6G as the standard(Figure S6). k2 is the dipole orientation factor, the magnitude ofwhich is assumed to be 2/3,[16] NA is Avogadro’s number, and nis the refractive index of the medium. All parameters except k2

in Equations (5), (6), and (7) were determined experimentally.We employed an aqueous solution of the LY dye and 0.1 m

Na2S as the medium, and we determined the refractive indexto be approximately 1.38 from reflection measurements. J, thespectral overlap integral, was calculated from Equation (6):

J ¼ 01Z

FDðlÞeAðlÞl4dl ð6Þ

FD is the donor fluorescence intensity normalized to unitarea (in arbitrary units) and eA is the extinction coefficient ofthe acceptor (66 815 m

�1 cm�1). The spectral integral (J) for theLY/CdS/CdSe system was determined to be approximately4.83 � 1015 and the resulting Forster radius (Ro) is 5.29 nm. Asour value of Ro falls within the limits of the acceptable distancefor effective FRET interaction, energy transfer from the LY dyeto the CdS/CdSe QDs is viable.

The FRET rate (KET) for energy transfer was calculated usingEquation (7):

K ETðrÞ ¼ 1=tDðRo=rÞ6 ð7Þ

where tD is the excited state lifetime of the donor (LY dye) inthe absence of the acceptor (CdS/CdSe QDs) and Ro is the For-ster radius. Higher FRET rates correspond to faster energytransfer from the LY dye to the CdS/CdSe QDs. The rate con-stant for the energy transfer from the LY dye to the CdS/CdSeQDs is 5.29 � 109 s�1. The efficiency of the energy transfer, Ecan be calculated from the fluorescence lifetime of the donorin the presence and absence of the acceptor by using Equa-tion (8):

E ¼ 1�tDA=tD ð8Þ

The LY dye has an energy transfer efficiency of approximate-ly 40 % to CdS/CdSe QDs. Buhbut and co-workers reported anenergy transfer efficiency of 44 % for the tunneling energyfrom a CdS/CdSe/ZnS QD assembly to a symmetrical squarainedye.[36] In another study, an energy transfer efficiency of 69 %was reported for the energy relay from CdSe QDs to a squar-aine dye.[21]

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Conduction by Means of PEDOT Microfibers

The SEM image of PEDOT microfibers is shown in Figure 5 a.The image shows that the microfibers are bundled together,with lengths varying between 1 and 1.5 mm. The elongated fi-brillar shapes are interconnected, which is advantageous be-cause it means that charges can propagate unhindered across

the electrolyte through the interlinked tubes after photo-exci-tation of the photoanode and reduction of PEDOT at the cath-ode. This reduced neutral polymer can diffuse through the sul-fide ion electrolyte and undergo oxidation at the anode, thusre-generating the electron in the valence band of the acceptor(or CdS/CdSe QDs). We observed a decrease in the electronlifetime in CdS/CdSe QDs in the presence of PEDOT microfib-ers, which indicates that PEDOT contributes to the improve-ment of the overall charge transport capability of the electro-

lyte. The electronic conductivity of PEDOT microfibers (mea-sured by I–V, Figure S7) was found to be 0.5 S cm�1, whichagrees well with the reported value of 0.1 S cm�1 for PEDOTdoped with poly(styrene sulfonate) or PSS.[37] PEDOT dopedwith PSS is known to be an excellent hole transport layer andis widely used in organic photovoltaic cells.[38, 39] In this case,the hole transport capability of PEDOT also galvanizes electronregeneration in the QDs. The work function of PEDOT thermo-dynamically favors for the acceptance of holes from the va-lence bands of CdS and CdSe QDs after excitation. We rule outthe possibility of hole transfer from LY to S2� or PEDOT, as thehole created upon photo-excitation of the donor ceases toexist after excitation transfer to the acceptor (CdS/CdSe QDs).

The CVs of an aqueous 0.1 m Na2S solution and the same so-lution with PEDOT microfibers dispersed therein, were record-ed between two Pt electrodes with an Ag/AgCl/KCl electrodeas reference (Figure 5 b). The CV plot of the pristine S2� electro-lyte shows a broad oxidation peak at + 0.59 V (or Ered =

�0.59 V vs Ag/Ag+) which corresponds to �0.393 V (vs NHE).This oxidation peak shifts to a more positive potential in thepresence of PEDOT microfibers and is observed at + 0.67 V (vsAg/Ag+) which corresponds to �0.473 V (Ered vs NHE). On theenergy scale, the redox potential of pristine S2� is 4.107 eV,and in the presence of PEDOT this level shifts to 4.027 eV. Thechange in the redox potential of S2� affects the open-circuitpotential of the solar cell. The open-circuit voltage of the cellis the difference between the energy of the conduction bandof TiO2 and the redox potential of the electrolyte. For cells con-taining PEDOT, such as TiO2/CdS/CdSe/ZnS-LY/PEDOT/S2�-MWCNT, the VOC was found to be 716 mV; for the same cellwithout PEDOT, the VOC was higher (721 mV). Although the po-sition of the conduction band of TiO2 remains unchanged, theshift in the redox potential of S2� to more negative potentials(vs NHE) in the presence of PEDOT is responsible for the ob-served slight reduction of VOC observed in the cell with PEDOTmicrofibers dispersed in the electrolyte.

The Nyqvist plots (Z’’ vs Z’) for a 0.1 m Na2S solution and fora 0.1 m Na2S/PEDOT microfiber solution recorded between twoPt electrodes are shown in Figure 5 c. The high frequency re-gions of the Z’’ vs Z’ curves reveal an incomplete arc in bothcases, and the point of intersection of the first arc with the realcomponent of impedance is 34 W for the S2� electrolyte and38 W for the PEDOT/S2� electrolyte. This value is a measure ofthe bulk resistance offered by the electrolyte. In the low-fre-quency domain, the behavior of the cell with the PEDOT/S2�

solution is resistive whereas the response is capacitive for thecell with the pristine S2� solution. The impedance data wasfitted to a circuit with two resistances and one capacitor, withsolution resistance (RS), charge transfer resistance (RCT), anddouble-layer capacitance (Cdl) as the circuit components. RCT

for charge transfer at the Pt/PEDOT/S2� solution interface wasfound to be 5.2 times smaller than the RCT of the Pt/S2� inter-face, indicating that the PEDOT microfibers improve thecharge conduction capability of the S2� solution. The benefitof this improved conduction ability was reflected in the in-creased photocurrent produced by the cell with PEDOT relativeto the cell without PEDOT; JSC was enhanced by 1.08 times on

Figure 5. a) SEM micrograph of PEDOT microfibers, b) CVs of a solution ofPEDOT microfibers/S2� (&) and an S2� solution (*), measured between twoPt electrodes, at a scan rate of 10 mV s�1, and c) Nyqvist plots of a solutionof PEDOT microfibers/S2� (*) and an S2� solution (&), measured betweentwo Pt electrodes, over a frequency range of 106 to 102 Hz and under an acsignal of 10 mV.

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incorporation of PEDOT into the sulfide electrolyte. The ab-sorption spectrum of PEDOT, which indicates that it is in theoxidized form in the as-synthesized sample, is shown in Fig-ure S8.

Solar Cell Performance

In order to quantify the effect of FRET on the solar cell perfor-mance metrics, quasi solid-state cells were constructed withthe following configuration: FTO/TiO2/CdS/CdSe/ZnS as thephotoanode, S2�-based polyacrylamide gel (with or withoutPEDOT microfibers) as the electrolyte with the LY dye dissolvedtherein, and functionalized multi-walled carbon nanotubes(MWCNTs) deposited on FTO-coated glass as the counter elec-trode. The gel was composed of 0.1 m Na2S and 1.5 mg mL�1 ofthe LY dye. The incident photon to current conversion efficien-cy (IPCE) performances of the cells are shown in Figure 6. Theacceptor-only (CdS/CdSe QDs) cell (TiO2/CdS/CdSe/ZnS-S2�-

MWCNT) showed a maximum IPCE of 30 % at 530 nm, whichmatches the absorption spectrum of the CdS/CdSe/ZnS QDfilm. The LY dye absorbs strongly at 430 nm (which was fixedas the excitation wavelength for fluorescence), but this donor-only cell (TiO2-LY/S2�-MWCNT) yielded a very poor maximumIPCE value of 0.009 % (at 530 nm, Figure S9). The reason for thedismal performance of the donor-only (LY dye) cell is likely to

be inefficient charge transfer to TiO2 upon excitation. However,using the donor–acceptor architecture in the following solarcell : TiO2/CdS/CdSe/ZnS-LY/S2�-MWCNT, resulted in a substantialincrease in the IPCE response over the entire spectral region of350–650 nm, with a maximum IPCE of 53 % at 530 nm. This in-crease confirms the effectiveness of FRET (in relation to energytransfer from the dye to the QDs) in greatly enhancing the per-formance of this cell. In addition to the electrons generated bythe direct absorption of photons by the CdS/CdSe QDs, moreelectron–hole pairs are produced in this FRET-enabled cell byindirect absorption of photons on energy transfer from the LYdye. Sarkar et al. reported an increased photocurrent in theentire spectral region for a ZnO/Dye/CdTe assembly relative tothe acceptor-only device due to additional photo-generatedcharge carriers from N719 dye.[40] Similarly Buhbut et al. report-ed a maximum IPCE of about 10 % in the blue region fora FRET-enabled cell, with CdS/CdSe/ZnS QDs as the donor anda squaraine dye as the acceptor.[36] The donor concentration inthe electrolyte was optimized on the basis of maximum IPCEfor cells based on the following assembly TiO2/CdS/CdSe/ZnS-LY/S2�-MWCNT. Maintaining all other components at fixed com-positions, the LY dye concentration in the electrolyte wasvaried successively from 0.5 to 2 mg mL�1 in small steps andthe IPCE of the cell was measured at each step (Figure 6 a).From the IPCE response, the optimum concentration of thedonor (LY dye) was fixed at 1.5 mg mL�1, as the IPCE was 53 %at this concentration, and decreased to 46 % at higher donorconcentrations. At high donor concentrations, the quantumyield of the donor decreases due to concentration quenching.The conduction bands of CdS and CdSe lie at 3.5 and 3.3 eVand the LUMO of the LY dye lies at approximately 4.09 eV. Asthe LUMO of the LY dye lies below the conduction bands ofthe QDs, the injection of excited electrons from the LUMO ofthe dye to the QDs is energetically impossible. The only plausi-ble mechanism, which can operate in this donor–acceptor celland cause an enhanced photovoltaic response, is energy trans-fer.

The current density–voltage (J–V) characteristics of quasi-solid-state cells with various electrolyte compositions weremeasured (Figure 7) and the cell parameters are summarizedin Table 2. The open-circuit voltage (Voc) is defined as the differ-

Figure 6. IPCE spectra of quasi solid-state solar cells. a) TiO2/CdS/CdSe/ZnS-LY/S2� cell with 0.5 (^), 1 (~), 1.5 (&) and 2 (*) mg mL�1 LY dye, b) donor–ac-ceptor iO2/CdS/CdSe/ZnS-LY/S2�cell with optimized 1.5 mg mL�1 LY dye con-centration (&), acceptor-only TiO2/CdS/CdSe/ZnS-S2� (*) cell. A poly(acryl-amide) gel containing 0.1 m Na2S was employed as the electrolyte anda MWCNT/FTO assembly was used as the counter electrode.

Table 2. Solar cell parameters.[a]

Electrode configuration VOC [mV] JSC [mA cm�2] FF Efficiency [%]

TiO2/CdS/CdSe/ZnS 657 7.63 30 1.5TiO2/CdS/CdSe/ZnS/LYDye

721 7.79 32 1.79

TiO2/CdS/CdSe/ZnS/PEDOT

617 8.07 32 1.6

TiO2/CdS/CdSe/ZnS/LY/PEDOT

716 8.2 33 1.94

[a] Experimental conditions: l>300 nm; 0.1 m Na2S aqueous poly(acryl-amide)-based gel electrolyte with or without 1.5 mg mL�1 LY dye andwith or without PEDOT; cell area: 1 cm2 ; under 1 sun illumination(100 mW cm�2) ; with the listed photoanodes; and all cells with MWCNT/FTO as the counter electrode.

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ence between the Fermi level of the photoanode and theredox potential of the electrolyte. The acceptor-only cell (TiO2/CdS/CdSe/ZnS-S2�-MWCNT) exhibits a VOC of 657 mV, whereasthe donor–acceptor cell (TiO2/CdS/CdSe/ZnS-LY/S2�-MWCNT)shows a VOC of 721 mV, approximately 1.09 times higher thanthat of the acceptor-only cell. This implies that either theredox potential of the electrolyte or the Fermi level of theworking electrode shifts in the presence of the LY dye. Shortcircuit current densities (Jsc) for the acceptor-only and thedonor–acceptor cells are 7.63 and 7.79 mA cm�2, respectively.Jsc increases by 1.02 times if the acceptor-only cell is replacedby a donor–acceptor cell. The overall power conversion effi-ciency (h) for the acceptor-only cell is 1.5 %, whereas the FRET-enabled donor–acceptor cell had an efficiency of 1.79 %. Theimproved efficiency also highlights the benefit of FRET.

Previously, Chakrapani et al. , showed that the oxidation po-tential of the S2� in the sulfide ion electrolyte varies in thepresence of Se metal.[34] The oxidation potential of the redoxelectrolyte was observed to decrease on increasing the con-centration of Se, which, the authors reasoned, was responsiblefor the decrease in the VOC, even though they observed a con-comitant increase in the current density and the overall currentconversion efficiency.[34] Similarly, in the presence of the PEDOTmicrofibers, the oxidation potential of the S2� electrolyte gelshifts relative to the blank S2�electrolyte (Figure 5). Therefore,the acceptor-only cell with the following configuration: TiO2/CdS/CdSe/ZnS-PEDOT/S2�-MWCNT, showed a slightly loweredVOC relative to the cell with the same configuration but devoidof PEDOT microfibers. The cell with PEDOT microfibers hasa VOC of 617 mV compared to the VOC of 657 mV for the cellwithout PEDOT. However, both JSC and h are higher for the cellwith PEDOT microfibers, and h is 1.6 %. For the FRET-enabledcell with PEDOT microfibers: TiO2/CdS/CdSe/ZnS-LY/PEDOT/S2�-MWCNT, the VOC was reduced by 1.01 times, but the overallpower conversion efficiency increased from 1.79 to 1.94 % incomparison with the same cell without PEDOT microfibers. Im-provements in efficiency of 6.66 and 8.4 % were registered forthe acceptor-only cell and the donor–acceptor cells withPEDOT microfibers relative to the same cells without PEDOTmicrofibers. The ability of PEDOT microfibers in combinationwith the S2� ions to effectively transport holes, is reflected inthe enhanced photocurrent and efficiency response. The as-sembly of cells in which the aqueous S2� electrolyte was re-placed with a PEDOT microfiber/water suspension or a drop-cast film on the photoanode was attempted, but no apprecia-ble photocurrents were obtained. This was possibly due to thepoor quality of the photoanode/hole transport layer interface,and, as a result, only the PEDOT microfiber/S2� combinationwas employed for the study of the hole/electron conductingability of PEDOT.

Electrochemical Impedance Spectroscopy

To confirm the role of PEDOT in the improvement of the holetransport ability of the S2� electrolyte, we recorded the electro-chemical impedance spectra (EIS) of the following two cells:TiO2/CdS/CdSe/ZnS-PEDOT/S2�-MWCNT and TiO2/CdS/CdSe/

ZnS-S2�-MWCNT (Figure 8 a). Both plots consist of one incom-plete and one distorted semicircle; the first semicircle can beassigned to a parallel combination of the resistance and capac-itance at the counter electrode/electrolyte interface, whereasthe second semicircle originates from similar circuit elementsoperative at the photoanode/electrolyte interface. As the spanof the two plots is similar, it is apparent that the charge trans-fer resistances at the two interfaces are unaffected by the pres-ence of PEDOT in the electrolyte. The Bode phase diagramscorresponding to the two cells are shown in the inset. Theelectron (in the conduction band of TiO2)–hole (in S2� orPEDOT) recombination time (td) was calculated from jZ j versuslog (frequency) plots, by using Equation (9):

td ¼ 1=2 pf ð9Þ

The middle frequency peak position shifts slightly to higherfrequency (inset of Figure 8 a), revealing a decrease in the elec-tron recombination time from 2.76 ms for the cell with S2� andPEDOT to 1.45 ms for the cell with S2� alone. The increase inthe electron–hole recombination time in the presence ofPEDOT indicates fast hole transport from the QDs to PEDOT,and, as a consequence, more efficient charge separation. Inorder to investigate the effect of the LY dye on recombinationtime, the Nyqvist plots for the cells with and without the

Figure 7. J–V characteristics of quasi solid-state solar cells with the followingphotoanode architectures: a) donor–acceptor TiO2/CdS/CdSe/ZnS-LY/S2� (*)and acceptor-only TiO2/CdS/CdSe/ZnS-S2� (&) cells, and b) donor–acceptor:TiO2/CdS/CdSe/ZnS-LY/PEDOT/S2� (&) and acceptor-only TiO2/CdS/CdSe/ZnS-PEDOT/S2� (~) cells. All experiments were performed under 1 sun illumina-tion and at l>300 nm; a poly(acrylamide) gel containing 0.1 m Na2S wasemployed as the electrolyte and a MWCNT/FTO assembly was used as thecounter electrode. Concentrations of PEDOT and LY dye in the gels were 2and 1.5 mg mL�1 respectively.

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donor dye were recorded. Both plots comprise two skewedsemicircles, but for the donor–acceptor cell, the impedance atlow frequencies is noticeably larger than that observed for theacceptor-only cell. From the corresponding Bode plots, theelectron–hole recombination time was calculated to be 2 ms inthe acceptor-only cell (TiO2/CdS/CdSe/ZnS-S2�-MWCNT) and0.96 ms for the donor–acceptor cell (TiO2/CdS/CdSe/ZnS-LY/S2�-MWCNT), which indicates that the donor inhibits electron–hole recombination.

3. Conclusions

FRET has been demonstrated in a solar cell based on a donor–acceptor architecture with unattached LY dye molecules asdonors and CdS/CdSe QDs anchored to titania as acceptors.On illumination facile excitation transfer occurs from the LYdye to CdS/CdSe QDs, and this is followed by rapid charge in-jection from the QDs to TiO2. The energy and electron transferprocesses have been mechanistically discerned by fluorescencequenching of the LY dye in the presence of CdS/CdSe/ZnSQDs, from the decreased electron lifetime of the dye in thepresence of the QDs, and also from the increase in the emis-sion and lifetime of the QDs in the presence of the LY dye. TheForster radius (or the CdS/CdSe/ZnS QD–LY dye separation)was estimated to be 5.29 nm and the energy and electrontransfer rates were determined to be 5.29 � 109 and 3.2 �

109 s�1, respectively. The synergy between the QDs and thedye molecules was evident from the remarkably enhancedIPCE of 53 % of the cell with a combination of tethered CdS/CdSe QDs and unattached LY dye molecules relative to thedismal performances of the acceptor-only (30 %) and donor-only (0.009 %) cells. The performance of the FRET-enabled cellwas further augmented by the incorporation of PEDOT micro-fibers into the sulfide ion electrolyte. The PEDOT microfibersimprove the hole transport ability of the S2� gel; this was evi-dent from the decreased lifetime of the CdS/CdSe QDs in thepresence of PEDOT. The overall power conversion efficiency ofthe quasi-solid-state FRET-enabled cell with the following con-figuration: TiO2/CdS/CdSe/ZnS-LY/PEDOT/S2�-MWCNT, in-creased by 1.94 % relative to the acceptor-only cell. The perfor-mance metrics, in particular the greatly enhanced IPCE in thevisible region spanning from 400 to 650 nm, for a cell contain-ing an unconventional luminescent donor, namely LY, open upopportunities to deploy this relatively lesser-used dye as an ef-ficient energy relay dye in a diverse range of solar cells.

Experimental Details

Chemicals

Cadmium acetate (Cd(CH3COO)2), sodium sulfide (Na2S), zinc nitrate(Zn(NO3)2, acrylamide (CH2CHCONH2), ammonium persulfate((NH4)2S2O8), bisacrylamide (C7H10N2O2), sodium sulfite (Na2SO3), andferric chloride (FeCl3) were procured from Merck. Multi-walledcarbon nanotubes (MWCNTs, 99.9 % pure), selenium powder, 3,4-(ethylenedioxythiophene) (EDOT monomer), titanium isopropoxide,and Lucifer yellow dipotassium dye were purchased from Aldrich.Ultrapure water with a resistivity of approximately 18.2 MW cm wasobtained by using a Millipore Direct-Q 3 UV system. Sulfuric acid,nitric acid, hydrochloric acid, n-hexane, ethanol, isopropanol, andmethanol were obtained from Merck. Inorganic transparent elec-trodes of SnO2 :F-coated glass (FTO, Pilkington, sheet resistance:14 W sq�1) were cleaned in a soap solution, 30 % HCl solution,double distilled water, and acetone prior to use.

Fabrication of the Photoanode

Titanium dioxide nanoparticles were prepared by a sol-gel methodby dissolving titanium isopropoxide (10 mL) in isopropanol(40 mL). This mixture was added dropwise to a solution containingwater (10 mL) and isopropanol (10 mL). The pH of the resultingturbid solution was maintained at pH 3 with hydrochloric acid andit transformed into a yellow colored translucent gel in 5–10 min.The gel was heated at 105 8C for 1 h and on evaporation of the sol-vents, a yellow solid was obtained. This was calcined at 500 8C for6 h in a furnace in air and a white TiO2 powder with an anatasecrystalline structure was obtained[41] (Figures S1 and S2).

Na2SO3 (0.5 g) and selenium metal powder (0.05 g) were dissolvedin ultrapure water (30 mL) and refluxed for 1 h at 80 8C. After 1 h,the black color of the selenium disappeared and a clear colorlesssolution was obtained; this indicated of the formation of sodiumselenosulfite. The solution was filtered and the filtrate was used toprepare the CdSe QD shell.

TiO2 films were deposited over FTO-coated glass substrates byusing the doctor blade technique from a dispersion of TiO2 nano-powder (3 g) in ethanol (15 mL). The resulting white opaque film

Figure 8. Nyqvist plots recorded under an ac amplitude of 10 mV and a zerodc bias for quasi solid-state cells a) with PEDOT: TiO2/CdS/CdSe/ZnS-PEDOT/S2� (*) and without PEDOT: TiO2/CdS/CdSe/ZnS-S2� (&), b) donor–acceptorsolar cells : TiO2/CdS/CdSe/ZnS-LY/S2� (&) and acceptor-only cells : TiO2/CdS/CdSe/ZnS-S2� (*). A 0.1 m Na2S based poly(acrylamide) gel served as theelectrolyte and a MWCNT/FTO assembly as the counter electrode. LY dyeand PEDOT microfiber concentrations in the gel were 1.5 and 2 mg mL�1, re-spectively. The inset shows the corresponding Bode plots (phase angle vsfrequency).

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was annealed at 450 8C for 3 h. CdS QDs were deposited by usingchemical bath deposition by immersing the TiO2/FTO substrate inan aqueous Cd(NO3)2 solution (0.5 m) for 5 min at room tempera-ture (ca. 25 8C) followed by an ultrapure water rinse and drying.This substrate was dipped in an aqueous Na2S solution (0.5 m) andrinsed in ultrapure water and dried. One layer of CdS coated overTiO2/FTO was obtained. The process was repeated twice and threelayers of CdS QDs were deposited over TiO2/FTO. The FTO/TiO2/CdS electrode was immersed in an aqueous solution of Cd(NO3)2

(0.5 m) for 5 min, rinsed in ultrapure water and dried. The resultingelectrode was immersed in an aqueous Na2SeSO3 (0.8 m) solutionat 50 8C for 1 h, rinsed and dried. Two iterations of this dip-rinse-drying of the cadmium and selenium precursors yielded two over-layers of CdSe QDs. The resulting FTO/TiO2/CdS/CdSe film wasdipped in a Zn(NO3)2 solution (0.5 m) for 5 min, rinsed in ultrapurewater, and dried, followed by an Na2S solution (0.5 m) dip and ul-trapure water rinse and drying steps. A ZnS overlayer was obtainedwhich served as a protective barrier and prevented the decomposi-tion of the CdS/CdSe QDs in the presence of the sodium sulfideelectrolyte. The FTO/TiO2/CdS/CdSe/ZnS electrode was used as thephotoanode in the solar cells.

Preparation of the MWCNT Electrode

MWCNTs (2.5 g) were dispersed in a mixture of HNO3 and H2SO4

(1:3 by volume) and this suspension was sonicated for 20 min,then refluxed at 80 8C for 6 h. The black residue was washed re-peatedly with ultrapure water until the supernatant liquid waspH 7. The black residue was dried in an oven at 60 8C for 48 h. Theresulting dried black solid was functionalized MWCNTs. A solutionof this solid (1 mg) in ultrapure water (3 mL) was sonicated for15 min. The homogeneous MWCNT dispersion was spin-coatedonto an FTO-coated glass substrate at 1000 rpm for 30 s, then thelayer was annealed at 100 8C for 1 h in a vacuum oven, yieldinga transparent black electrode.

Synthesis of PEDOT Microfibers

PEDOT microfibers were prepared by using a previously reportedsurfactant-assisted method.[42] A reverse micro-emulsion was firstsynthesized by dissolving dioctyl sulfosuccinate sodium salt (AOT,19.12 mmol) in n-hexane (70 mL). A solution of FeCl3 (10 mmol) inwater (1 mL) was added to the AOT solution. The resulting orangesolution was magnetically stirred at 25 8C for 5 min, then EDOT mo-nomer (3.52 mmol) was added. After 3 h of gentle stirring, a blueprecipitate of PEDOT microfibers was obtained. The deep blue pre-cipitate was filtered, washed with methanol and acetonitrile, anddried at 80 8C under vacuum for 12 h.

Synthesis of the Gel Electrolyte

A gel polymeric electrolyte was prepared according to a methodreported in the literature.[43]Acrylamide (0.1 g) was dissolved in ul-trapure water (0.9 mL) and bisacrylamide was incorporated as thelinker (1.5 wt. % with respect to the weight of the monomer). Afterdegassing this solution for 10 min, ammonium persulfate (0.4 wt. %with respect to the weight of the monomer) was added as the ini-tiator. The clear solution was converted into a hydrogel by heatingat 70 8C for 1 h. Water was expelled from the hydrogel by heatingand continued heating at 80 oC yielded a constant weight xerogel.In order to prepare the PEDOT microfiber-enriched gel, PEDOT mi-crofibers (2 mg), and ammonium persulfate were introduced intothe solution, and the solution was sonicated for 10 min prior tothe heating step. The xerogel (either with or without PEDOT micro-

fibers) was soaked in an aqueous solution of Na2S (0.1 m) for 12 hand the xerogel transformed into a transparent gel on uptake ofthe S2� solution. The electrolyte was a self-supporting, transparentgel; colorless if only S2� ions are present and blue if both PEDOTmicrofibers and S2� ions are present. The donor (LY dye) based gelpolymeric electrolyte was synthesized using the same procedure,except for a variation in the last step, in which the xerogel wassoaked in an aqueous solution of the yellow LY dye (1.5 mg mL�1)and Na2S (0.1 m) such that a transparent yellow (without PEDOTmicrofibers) or blue (with PEDOT microfibers) gel was obtained.The gel electrolytes were stored in vacuum desiccators. Quasisolid-state devices were fabricated by applying an acrylic spaceron the counter electrode film. A cavity of about 1 cm � 1 cm wascreated, then the free standing gel was cut to size using a scalpelblade and inserted into the cavity. The photoanode coating (activecoating facing inwards) was fixed to the electrolyte/counter elec-trode assembly using binder clips. The four edges of the cell weresealed using an epoxy sealant.

Instrumental Methods

HRTEM images of the QDs were obtained with an HRTEM FEITecnai G2 F30 STWIN with a FEG source operating at 300 kV. ForTEM, a thin layer of the sample was carefully extracted from thesubstrate into isopropanol using forceps, then transferred ontoa carbon-coated copper grid with a diameter of 3.05 mm, and thesolvent was evaporated. The optical absorption spectra were mea-sured in the visible region on a UV/Vis-NIR spectrophotometer (Shi-madzu UV-3600). Photoluminescence spectra of films were mea-sured on a Horiba Fluoromax-4 fluorescence spectrometer; a suita-ble filter was utilized during the measurement and a backgroundcorrection was also applied. The time-correlated single photoncounting (TCSPC) method was used for the determination of emis-sion lifetimes with a Horiba Jobin Yvon Data Station HUB function-ing in the TCSPC mode. NanoLED diodes emitting pulses at370 nm and 450 nm with a 1 MHz repetition rate and a pulse dura-tion of 1.3 ns were employed as excitation sources. Ludox solution(colloidal silica) was used as the prompt. A long pass 500 nm filterwas used in front of the emission monochromator in all measure-ments. Horiba Jobin Yvon DAS6 fluorescence decay analysis soft-ware was used to fit the model function (bi-exponential decays) tothe experimental data, with an appropriate correction for the in-strument response. The photoelectrochemical measurements werecarried out in the gel electrolyte based on Na2S (0.1 m) with orwithout LY (1.5 mg mL�1), and another set of photoelectrochemicalmeasurements was carried out in the presence and absence of thePEDOT enriched gel, with and without LY, and the MWCNT film ascounter electrode. Current versus potential (I-V) data were mea-sured using a Newport Oriel 3A solar simulator with a Keithleymodel 2420 digital source meter. A 450 W Xenon arc lamp wasused as the light source with a light intensity of 100 mW cm�2 andAir Mass (AM) 1.5 illumination; the spatial uniformity of irradiancewas confirmed by calibrating with a 2 cm � 2 cm Si reference celltraceable to NREL and re-affirmed with a Newport power meter.IPCE values were recorded using a 150 W Xenon arc lamp asa light source coupled with a Horiba monochromator equippedwith a 1200 groove nm�1 grating and a blaze wavelength of330 nm, over a wavelength span of 350–600 nm. Photocurrentswere measured under back illumination using a Keithley 2420 digi-tal source meter. The power of the incident beam was measuredby using a calibrated Si photodiode with a known response (Thor-Labs, FDS100) and also ratified with a radiant power meter fromNewport (842-PE), which included a thermopile sensor. IPCE valueswere extracted from Equation (10):

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IPCEð%Þ ¼ JSCðA cm�2Þ � 124 000=lðnmÞ � IðW cm�2Þ ð10Þ

where JSC is the short circuit current density, and I is the incidentpower. Impedance spectra were obtained for the cells by superim-posing an ac voltage of 10 mV over a zero dc bias (applied to thephotoanode) and Z’’ vs Z’ plots were obtained over a frequencyrange of 106 Hz to 10�2 Hz.

Acknowledgements

Financial support from Department of Science and Technology(DST/TM/SERI/2012/11(G)) is gratefully acknowledged.

Keywords: electron transfer · energy transfer ·photoluminescence · quantum dots · solar cells

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Received: June 27, 2013

Published online on November 21, 2013

� 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2013, 14, 4010 – 4021 4021

These are not the final page numbers! ��

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10 1011 1112 1213 1314 1415 1516 1617 1718 1819 1920 2021 2122 2223 2324 2425 2526 2627 2728 2829 2930 3031 3132 3233 3334 3435 3536 3637 3738 3839 3940 4041 4142 4243 4344 4445 4546 4647 4748 4849 4950 5051 5152 5253 5354 5455 5556 5657 57