Electrochimica Acta 91 (2013) Contents lists available at SciVerse ScienceDirect. Electrochimica Acta

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1 Electrochimica Acta 9 (0) 6 5 Contents lists available at SciVerse ScienceDirect Electrochimica Acta jou rn al h om epa ge: Quantum dot sensitized solar cells based on an optimized combination of ZnS, CdS and CdSe with CoS and CuS counter electrodes Nikolaos Balis a, Vassilios Dracopoulos b, Kyriakos Bourikas c, Panagiotis Lianos a,b, a Engineering Science Department, University of Patras, 6500 Patras, Greece b FORTH/ICE-HT, P.O. Box, 650 Patras, Greece c School of Science and Technology, Hellenic Open University, Tsamadou -5, 6 Patras, Greece a r t i c l e i n f o Article history: Received 0 October 0 Received in revised form 0 December 0 Accepted January 0 Available online xxx Keywords: Quantum dot sensitized solar cells Metal sulfide counter electrodes TiO CoS CuS a b s t r a c t Quantum dot sensitized solar cells have been constructed using photoanodes made of nanocrystalline titania and an optimized combination of ZnS, CdS and CdSe nanoparticles. Pt, CoS and CuS have been used as electrocatalysts on counter electrodes. Attachment of quantum dot sensitizers on mesoporous titania was made by successive ionic layer adsorption and reaction and by chemical bath deposition obeying a certain order, where the first layer was crucial in defining the quality and the quantity of the subsequent layers as well as of the ensuing solar conversion efficiency. Thus the first quantum dot layer consisted of 75% CdS and 5% ZnS and it was followed by a CdSe layer and by an additional ZnS layer on the top. The quantity of material deposition seems to be affected not only by the employed deposition method but also and mainly by the nature of the underlying layer. Optimized anode electrodes led to solar cells producing high current densities but did not much affect open-circuit voltage. The maximum solar conversion efficiency reached in this work was.7% and was obtained by using CuS electrocatalyst. Both CoS and CuS gave high currents and this was in line with the low charge transfer resistances recorded in their case. 0 Elsevier Ltd. All rights reserved.. Introduction Sensitized solar cells constitute one of the most popular fields of research, which is being active for several decades [,] but continues to attract an ever increasing number of adepts. In a sensitized solar cell, light is mainly absorbed by the sensitizer. Photogenerated electrons are then rapidly injected into the conduction band of the coupled semiconductor leading to electron hole separation. This is the most important asset of sensitized solar cells []. Dye sensitizers of metal oxide semiconductors in combination with liquid electrolytes have offered the highest photovoltaic efficiencies. However, inorganic semiconductor sensitizers, usually referred to as quantum dot (QD) sensitizers, are recently studied with great interest that might surpass their dye homologues. This interest stems from the several attractive features that QD sensitizers possess [ 7]: spectral tuning by controlling size due to the quantum confinement effect; high dipole moment that assists electron injection; high extinction coefficient, etc. In addition, QDs are easy to synthesize and, since their size is limited to a few nanometers, they can be easily accommodated within the Corresponding author at: Engineering Science Department, University of Patras, 6500 Patras, Greece. address: lianos@upatras.gr (P. Lianos). mesoporous structure of nanocrystalline oxide semiconductors, which is usually one order of magnitude larger. Several QDs have been studied as sensitizers of nanocrystalline titania (nc-tio ) and have been used in the construction of the socalled quantum dot sensitized solar cells (QDSSCs) [ 0]. The most popular QDs are metal sulfides and selenides. Previous works have shown that combinations of QDs lead to higher efficiencies than pure materials [,0 ]. There are a few reasons for this behavior. Combination of materials leads to passivation of structure defects, which are typical sites of electron hole recombination. Mixing of QDs also leads to charge carrier dispersion impeding charge recombination. Furthermore, combination of QDs by appropriate fitting of energy levels supports cascade charge transfer []. Combination then of a few different QDs is an important issue and for this reason in the present work, we searched for an optimized combination of ZnS, CdS, and CdSe leading to the construction of a QDSSC with promising characteristics. The main feature of this combination is the careful choice of the first layer of QDs directly deposited on nc-tio. We believe that this first layer is very crucial in defining the characteristics of the photoanode as a whole. For example, the quantity and the quality of the material forming the upper QD layer depends on the characteristics of the bottom QD layer. In the present work, the bottom QD layer was made through a combined deposition of 5%ZnS 75%CdS, which is more effective than plain CdS. ZnS CdS have been previously found [] to form solid /$ see front matter 0 Elsevier Ltd. All rights reserved.

2 N. Balis et al. / Electrochimica Acta 9 (0) solutions, without phase separation, allowing for spectral tuning and modifying the energy levels of the mixed semiconductor with respect to pure CdS. Such mixture also resulted in a more efficient solar cell. In addition, in the present work, nanostructured CoS and CuS have been used as electrocatalysts on the counter electrode. Nanocrystalline platinum is the most precious of all electrocatalysts but is not considered a good choice of a catalyst in QDSSCs employing polysulfide electrolytes [5], as in the present case. The construction of an efficient cathode is important not only for QDSSCs but for all types of electrochemical cells and for this reason it attracts a great interest. Alternative catalysts have been proposed by several researchers [5 8]. Metal sulfides are considered a good choice. However, their deposition on plain FTO electrodes does not always produce materials with sufficiently high specific surface or with structural stability. Combination of catalysts, especially, novel nanostructured materials [8] is equally important as it is in the case of photoanodes. The present work, supports the existing trends, opts for the employment of CoS and CuS as electrocatalysts and proposes some easy procedures for their deposition on transparent conductive electrodes.. Experimental.. Materials Unless otherwise indicated, reagents were obtained from Aldrich and were used as received. Commercial nanocrystalline titania Degussa P5 (specific surface area 50 m /g) was used in all cell constructions and Millipore water was used in all experiments. SnO :F transparent conductive electrodes (FTO, resistance 8 /square) were purchased from Pilkington... Construction of nc-tio films on FTO electrodes The following procedures were undertaken in order to construct photoanode electrodes. First, the nc-tio film was synthesized. It was deposited in two layers, a bottom compact layer and a top open structure. The bottom layer was synthesized by the sol gel method while the top layer was deposited by using a paste made of Degussa P5 nanoparticles. A densely packed nanocrystalline titania layer was first deposited on a patterned FTO electrode, cleaned by sonication in acetone, isopropanol and ethanol, according to the following procedure [9]:.5 g of the non-ionic surfactant Triton X-00 was mixed with 9 ml ethanol. Then. ml glacial acetic acid (AcOH) and.8 ml of titanium tetraisopropoxide were added under vigorous stirring. After a few minutes stirring, the film was deposited by dipping and then it was left to dry in air for a few minutes. Finally, it was calcined at 550 C. The temperature ramp was 0 C/min up to 550 C and the sample was left for about 0 min at that temperature. The procedure was repeated once more. Each layer gave a thin nanostructured film of about 70 nm thickness, as measured by its FE-SEM profile [9]. On the top of this compact nanostructured layer, commercial Degussa P5 was deposited. For this purpose, a paste of titania P5 was prepared according to Ref. [0]. The paste was applied by screen printing using a 90 mesh screen. It was finally calcined again at 550 C. The thickness of the top layer was about. m as measured by its FE-SEM profile [9]. The procedure was repeated once more so that the final nc-tio film was about 7.5 m thick. The geometrical size of the film was cm ( cm cm)... Formation of the CdS ZnS composite layer by the SILAR method CdS ZnS composite catalysts at various proportions were deposited on the nc-tio film by Successive Ionic Layer Adsorption and Reaction (SILAR method) [9,]. For this purpose, two aqueous solutions were used, one containing Cd(NO ) H O or Zn(NO ) 6H O or mixtures of both, and the second containing Na S 9H O. The total concentration of metal ions and the corresponding concentration of sulfur ions were 0. mol dm. The freshly prepared titania electrode was immersed for min in the metal salt solution, then copiously washed with triple-distilled water, then immersed for min in the Na S 9H O solution and finally washed again. This sequence corresponds to one SILAR cycle. 0 SILAR cycles were performed in all studied cases. Finally, the electrode with deposited CdS ZnS@nc-TiO film was first dried in a N stream and then it was put for a few minutes in an oven at 00 C... Formation of the CdSe layer by CBD CdSe was deposited by chemical bath deposition (CBD) either directly on a plain nc-tio film or on the above CdS ZnS@nc- TiO film. The procedure was similar to that employed by other researchers [,8] and involves the following steps: an aqueous solution of 0.08 mol dm Se powder was first prepared in the presence of 0. mol dm Na SO by continuous stirring and refluxing at 80 C. The procedure lasted about 5 h and was carried out overnight. The obtained solution, denoted in the following as sol A, actually aimed at the formation of sodium selenosulphate (Na SeSO ), which is a precursor for slow Se release. Two more aqueous solutions were then prepared, containing 0.08 mol dm CdSO 8/ H O (sol B) and 0. mol dm nitrilotriacetic acid trisodium salt (sol C), respectively. Sol B was mixed with an equal volume of sol C and the obtained mixture was stirred for a few minutes. The combination of sol B with sol C leads to the formation of a complex, which is used as precursor for slow release of Cd +. Finally, two parts of this last mixture were mixed with one part of sol A and the thus obtained final mixture was used for CBD. The final concentration of Se and Cd + ions was 0.08/ mol dm. The idea of the above procedure is to make a mixture of precursors, which slowly release selenium and cadmium ions so as to make them react after adsorption on the substrate. Thus the electrodes with substrate films were dipped in this final solution. Subsequently, they were put in a refrigerator at 5 C for h or left at ambient temperature for several hours. The choice of temperature or residence time is dictated by the substrate. It was observed that ZnS CdS@nc-TiO /FTO electrodes needed only h and low temperature to adsorb substantial quantities of CdSe and be deeply colored while more than h were required in the absence of the ZnS CdS layer. The procedure was accelerated if the CBD was done at ambient temperature..5. Formation of the top ZnS layer In several recent works [,,,], researchers have found that a layer of ZnS added on the top of CdSe greatly improves the behavior of solar cells. In line with these findings, we have also prepared samples with added top layer of ZnS, which was deposited by SILAR cycles as above, using 0. mol dm aqueous precursor solutions of the respective Zn + and S ions..6. Construction of the counter electrodes Some counter electrodes were functionalized with Pt electrocatalyst. For this purpose, a solution of 0.0 mol dm H PtCl 6 in isopropanol was spin-coated on properly cleaned FTO at 000 rpm. The obtained film was calcined at 50 C for 5 min. CoS was directly deposited on FTO by potentiodynamic electrodeposition [6]. FTO was used as working electrode, a Pt sheet as auxiliary electrode and a Ag/AgCl as reference electrode. The precursor solution contained mol dm CoCl 6H O and 0.5 mol dm thiourea.

3 8 N. Balis et al. / Electrochimica Acta 9 (0) 6 5 F(R) Wavelength / nm Fig.. Absorption spectra of various functional films: () plain FTO glass; () nc-tio /FTO; () ZnS CdS@nc-TiO /FTO; () CdSe@ZnS CdS@nc-TiO /FTO; (5) ZnS@CdSe@ZnS CdS@nc-TiO /FTO; and (6) CdSe@nc-TiO /FTO. A few drops of diluted NH OH may be added [6] to bring ph close to 6.0. The distance between working and auxiliary electrode was cm. The procedure consisted of 5 cycles of voltage cycling between. and +0. V at a rate of 5 mv s. Afterwards, the electrode with film was dried, first in a N stream and then in an oven at 00 C. CoS CuS mixtures were also deposited by the same procedure and by utilizing mixtures of CoCl 6H O with CuCl H O (Merck) at a total salt concentration of mol dm. Finally, CuS was also deposited on FTO electrodes by a SILAR procedure, by modifying the method presented in Ref. []. Precursor solutions contained 0.5 mol dm Cu(NO ) xh O in methanol and mol dm Na S 9H O in a : water:methanol mixture. A well cleaned FTO electrode was immersed for 5 min in the metal salt solution, then copiously washed with triple-distilled water and dried in an air stream, then immersed for 5 min in the Na S 9H O solution and finally washed and dried again. This sequence again corresponds to one SILAR cycle. 0 SILAR cycles were performed. Finally, the electrode with deposited CuS film was first dried in a N stream and then it was put for a few minutes in an oven at 00 C..7. Cell assembly The photoanode and the counter electrode were assembled by using a thermoplastic foil (surlyn type). After assembly, the distance between the two electrodes was around 60 m. The space in-between was filled with an aqueous polysulfide electrolyte containing mol dm Na S and mol dm S. IV curves were traced by illuminating the cell through a mask of 0. cm (0. cm cm)..8. Methods Illumination of the samples was made with a PECCELL PEC- L0 Solar Simulator set at 00 mw cm. J V characteristic curves were recorded under ambient conditions with a Keithley 60 source meter, which was controlled by Keithley computer software (LabTracer). IPCE values were measured by using a home-made apparatus employing a 00 W Oriel xenon lamp and a Jobin-Yvon monochromator. Radiation intensity for < 00 nm was measured with a PMA 00 Radiant Power meter (Solar Light Co.), calibrated for the near UV spectral range and for 00 nm with an Oriel Radiant Power meter (7060) calibrated for the visible range. Field emission scanning electron microscope (FE-SEM) images were recorded with a LEO SUPRA 5VP. Diffuse reflectance spectra (DRS) were recorded with a Perkin Elmer Lambda 5 UV/vis spectrophotometer equipped with an integration sphere accessory. Electrochemical impedance spectra on symmetrical cells of the type counter-electrode/electrolyte/counter-electrode were obtained using an Autolab PGSTAT8N potentiostat, equipped with a frequency response analyzer (FRA), at 0 V bias in the dark and recorded over a frequency range of 00 khz 00 mhz. The distance between the two electrodes was cm, their active size.5 cm.5 cm and the aqueous electrolyte was the same as the one filling the above solar cell, i.e. it contained mol dm Na S and mol dm S. XRD patterns were recorded with a D8 ADVANCE (Bruker AXS) diffractometer working with parallel beam optics (Gobel mirror) in grazing incidence mode. Powder was scratched from the electrode and was placed on a zero backround holder. The grazing angle was (source: CuKa, power:.6 kw).. Results and discussion.. Structural and spectroscopic characteristics of the photoanodes As it has been described in Section, each photoanode electrode was made of a double layer of nc-tio deposited on FTO glass. The bottom layer was a thin compact layer (about 50 nm) and the top layer was a relatively thick open structure (about 7 m). Thicknesses were estimated by using the Scanning Electron Microscope profiles of the films. The bottom layer facilitates stable attachment on the FTO electrode and enhances electric conductivity whereas at the same time prevents the contact of the electrolyte with the electrode. This is a standard configuration having been used by us and by others in previous works [9,]. The top open structure facilitates deep electrolyte penetration and deep photosensitizer deposition. Images of the nanostructure of both layers have been presented in a previous publication [9]. The area of each titania film was cm ( cm x cm) but JV curves were recorded by employing a mask so that the active area was 0. cm. Semiconductor QDs were deposited on freshly prepared titania films by combining the SILAR method and CBD, as explained in Section. The deposition sequence was the following. First, we deposited ZnS CdS mixtures by the SILAR method, then CdSe by CBD and finally on the top, ZnS by additional SILAR cycles. It is of interest to note that the bottom seeds of ZnS CdS induce rapid CdSe deposition. If CdSe is deposited directly on titania, i.e. without any ZnS CdS or pure CdS seeds, it is found that after h of CBD the quantity of CdSe is still very small, much smaller than that obtained after h of deposition on seeded substrates. Indeed, as seen in Fig., which depicts absorption spectra of progressively deposited materials, if we make the reasonable assumption that absorbance and quantity of material grow in parallel, seeded films adsorbed relatively large quantities of CdSe and substantial quantity of ZnS on the top. Non-seeded substrates adsorbed only a very small quantity of CdSe (curve 6 of Fig. ). The first photosensitizer layer was composed of a mixture of ZnS and CdS. It has been previously found [] that the combination of these two semiconductors produces solid solutions, which allow band gap tuning in a substantial range of the visible spectrum. This is true both for powdered samples and for thin films synthesized in situ on nanocrystalline titania []. Indeed, as can be seen in Fig., the absorption onset of the sensitized film varied up to 500 nm by changing the ratio of Cd and Zn in the SILAR deposited composite photosensitizer. Composite ZnS CdS photosensitizer is more efficient than pure CdS or ZnS [].

4 N. Balis et al. / Electrochimica Acta 9 (0) F(R) wavelength / nm Fig.. Absorption spectra of CdS ZnS composite semiconductor nanoparticles deposited on nanocrystalline TiO by the SILAR method at various combinations: () 00% Zn; () 75%Zn 5%Cd; () 50%Zn 50%Cd; () 5%Zn 75%Cd; and (5) 00% Cd. Spectra were recorded against nanocrystalline TiO film as reference. 5 coating. In the high magnification image of Fig. a, one can distinguish the big blocks of FTO covered with Pt nanoparticles [5] (cf also image of sputtered Pt, shown in Ref. [6]). In the second case, a CoS film was obtained by potentiodynamic electrodeposition. The image of Fig. b reveals a rough nanostructure similar to that obtained in a previous work [6] by the same technique. Lin et al. [6] found that the number of deposition cycles affects film structure. 5 cycles have presently been applied in order to obtain the rough nanostructure of Fig. b. Finally, Fig. c shows the image of a CuS film deposited by SILAR, which also presents a rough nanostructure but very different from that of CoS. It seems that what defines the CuS nanostructure is not the deposition technique but the material itself. Indeed, plain CuS and CoS CuS combinations have also been synthesized by potentiodynamic electrodeposition where the aspect presented by Fig. c prevailed, thus showing that the material and not the technique defines this aspect. Both CoS and CuS were studied by XRD to determine their crystallinity. Only CuS gave a detectable signal. The corresponding XRD spectrum is shown in Fig.. The size of CuS nanoparticles, as determined by the Scherrer formula, was approximately 9 nm... Electrochemical characterization of the counter electrodes.. Structural characteristics of the counter electrodes Three main types of counter electrodes have been studied. Their synthesis is detailed in Section. The FE-SEM images of the corresponding electrocatalytic films are shown in Fig.. In the first case, Pt nanoparticles were deposited on an FTO electrode by spin The three counter electrodes described in Section. were characterized by electrochemical impedance spectroscopy (EIS) using symmetric cells and the same electrolyte as the one filling the solar cells. Fig. 5 shows Nyquist plots for the three counter electrodes. All presented a single semicircle. By proper fitting of the diagrams with simple electrical equivalent circuits, the following parameters Fig.. FE-SEM images of various electrocatalysts on FTO electrodes: (a) Pt nanoparticles; (b) CoS; and (c) CuS. The corresponding scale bar is: (a) 0 nm; (b) m; and (c) 00 nm.

5 50 N. Balis et al. / Electrochimica Acta 9 (0) 6 5 Intensity / a.u. CuS Θ degree Fig.. XRD spectrum of CuS nanoparticles deposited on an FTO electrode by the SILAR method. Fig. 6. Current density vs. potential curves for solar cells made with various photoanodes: () CdSe@CdS@nc-TiO /FTO; () CdSe@CdS(75%) ZnS(5%)@nc- TiO /FTO; ( 5) ZnS@CdSe@CdS(75%) ZnS(5%)@nc-TiO /FTO. Curves ( ) correspond to cells using Pt/FTO counter electrodes curve () CoS/FTO and (5) CuS/FTO... Current electric potential characteristics of solar cells made with the above photoanodes and counter electrodes Fig. 5. EIS diagrams for three symmetric cells made of FTO electrodes with the following electrocatalysts: () Pt nanoparticles; () CoS; and () CuS. All symmetric cells contained mol dm Na S and mol dm S aqueous electrolyte. The active area of each electrode was.5 cm.5 cm and the distance between the electrodes was cm. The frequency range was 00 khz 00 mhz. Insert: Magnification of the horizontal axis revealing curve, which corresponds to the shorter charge transfer resistance. could be easily determined and they are listed in Table. The second column shows the values of the contact resistance R s, which also includes the sheet resistance of the electrodes themselves. R s values were relatively small. Their variations reflect the quality of the ohmic contact with the electrodes, which was the best in the case of metal (Pt) nanoparticles and worse (but not very different) in the case of metal sulfides. Charge-transfer resistance R ct at the electrode/electrolyte interface greatly varied from one electrode to the other. The highest resistance was recorded with Pt/FTO while it was an order of magnitude smaller in the case of CoS and still one order of magnitude smaller in the case of CuS. These data show in a rather dramatic manner and in accordance with literature [7,5,6,,6,7] that Pt is not a good choice when used with polysulfide electrolytes while CuS is the material offering the smallest charge-transfer resistance and it is expected to offer the best solar cell functionality, as it is verified in the next section. Table Data derived by EIS analysis of three symmetric cells made of FTO electrodes with various electrocatalysts. Electrocatalyst R s (Ohm cm ) R ct (Ohm cm ) Pt 560 CoS 8 8 CuS 9 6 The purpose of the present work is to show that the combination of various semiconductor quantum dots and the use of metal sulfides as electrocatalyst on the counter electrode can offer solar cells with appreciable efficiency. Indeed, the JE plots of Fig. 6 and the data of Table show that an efficiency of around.7% is easily obtained by an optimized combination of these materials. One striking result is the importance of the presence of Zn in the bottom layer of CdS. The efficiency of the cell almost tripled when we added Zn in the precursor solution used for the bottom SILAR deposition of QDs on nc-tio. Improvement of efficiency was previously observed by studying photoactivated fuel cells employing a photoanode made of ZnS CdS QDs deposited on nanocrystalline titania []. In these cells, the electrolyte was NaOH containing ethanol as fuel. The cell run by photocatalytic oxidation of ethanol, sensitized by the QD sensitizers. It was then observed that a mixture of 5% ZnS with 75% CdS makes a more efficient photocatalyst than pure CdS []. This result was attributed both to the suppression of electron hole recombination and to an increase of the oxidative power of the photocatalyst in the presence of ZnS. We believe that similar reasoning holds true in the present case. The addition of a limited percentage of ZnS (i.e. 5%) does not greatly affect the light absorbance range of the ZnS CdS mixture. Thus in Fig., it is seen that the presence of 5% Zn had a minor effect on the blue shift of the absorption spectrum (cf. curves and 5 of Fig. ). When 5% ZnS was combined with 75% CdS, it offered a bottom QD layer, which created the seeds that attracted a substantial amount of CdSe and finally of ZnS on top, as seen in Fig. (curves and 5). On the contrary, absence of this bottom layer resulted in a meager quantity of CdSe on nc-tio (curve 6). The mixture of semiconductors apparently assists electron hole separation while the high oxidative power of the ZnS valence band facilitates rapid hole transfer to the surrounding species, which happen to have favorably located energy levels for hole transfer. Indeed, as seen in Fig. 7 showing some characteristic energy levels, with the exception of titania, ZnS possesses the most oxidative valence band thus enhancing the oxidative power of the ZnS CdS mixture. The positioning of energy levels in Fig. 7 is expected in bulk. When the nanoparticles come in contact, Fermi level alignment [] will replace energy levels, particularly in the case of CdSe, which moves to more negative potentials, thus allowing for cascade charge transfer []. Equally impressive was the efficiency increase when an additional ZnS layer was put on the top. The beneficial role of the top ZnS layer has been observed and reported in several previous works [,,,]. The top ZnS layer passivates surface states of the underlying semiconductors while it prevents electron leakage from

6 N. Balis et al. / Electrochimica Acta 9 (0) Table Short circuit current density (J sc), open circuit potential (E oc), fill factor (FF) and efficiency data of cells made of a few different photoanodes and counter electrodes. Photoanode Electrocatalyst J sc (ma cm ) E oc (V) FF (%) CdSe@CdS@nc-TiO /FTO Pt/FTO CdSe@CdS(75%) ZnS(5%)@nc-TiO /FTO Pt/FTO ZnS@CdSe@CdS(75%) ZnS(5%)@nc-TiO /FTO Pt/FTO // CoS/FTO // CuS/FTO titania to the electrolyte. Indeed, the high-lying conduction band of ZnS (cf. Fig. 7) is expected to block electron leakage. This has been electrochemically demonstrated by Guijarro et al. []. Nevertheless, the same authors consider passivation of surface states as the most important reason of efficiency increase []. An equally important finding of the present work was that all three quantum dot layers, i.e. ZnS@CdSe@ZnS CdS, are necessary in order to achieve the recorded maximum efficiency. Thus in the absence of CdSe or in the absence of ZnS CdS the efficiency dropped below %. Low efficiency in the absence of the ZnS CdS layer is among other reasons also due to the meager CdSe layer formation (curve 6 of Fig. ). All QD species applied on the photoanode electrode function as sensitizers of titania. This is seen by comparing the absorption spectrum of the combined layers (curve 5 of Fig. ) with the corresponding IPCE% spectrum shown in Fig. 8. The two curves are equivalent and show that all QD species are involved in the sensitization process. Fig. 5 and Table show that the maximum efficiency reached in the present work, i.e..7% was obtained with CuS on the counter electrode. Metal sulfide electrocatalysts, i.e. CoS, CuS and their mixtures gave higher current densities than Pt. On the contrary open-circuit voltage values were practically not affected by the electrocatalyst. The major problem encountered in the present work was with the value of the fill factor (FF). It remained below IPCE / % wavelength / nm Fig. 8. IPCE% spectrum () and absorption spectrum () for a cell made with a ZnS@CdSe@ZnS CdS@nc-TiO /FTO photoanode and a CuS/FTO counter electrode. 0. and this limited the overall efficiency, even though, the current densities presently recorded were high. The search for a higher FF is an open question and has occupied many other researchers. It is believed that higher FFs will be obtained with even better electrocatalysts and more functional counter electrodes. Thus higher FFs have been obtained with PbS/Pb [5], a Cu S graphene composite [5,0,8] and carbon nanostructures [6,7]. All these different possibilities are on stage and they are currently studied in our laboratories. 5 Absorbance. Conclusions QDSSCs have been constructed and optimized by combining nc-tio with ZnS, CdS and CdSe QDs on the anode electrode. Pt, CoS or CuS were used as electrocatalysts on counter electrodes in combination with a polysulfide electrolyte. The maximum solar conversion efficiency of.7% was obtained with a CuS counter electrode. The most important finding of this work is the importance of the first QD layer deposited on the mesoporous titania film, which affected the quantity and the quality of the subsequent QD layers and the ensuing cell efficiency. Thus the first layer consisted of 75%CdS 5%ZnS, deposited by the SILAR method. High current densities were obtained with all cells having optimized anode electrodes. Among them, the highest currents were obtained with metal sulfide electrocatalysts. Electrochemical impedance spectroscopy showed that the lowest charge transfer resistance was recorded with metal sulfides and this directly reflected on the electric current values. Finally, it must be stressed that the methods of metal sulfide electrocatalyst deposition in the present work were easy one-step procedures that provide well shaped nanostructured materials. Fig. 7. Approximate energy levels for a few semiconductors and the polysulfide redox couple. The positioning of the levels refers to values of isolated nanocrystallites. When the nanoparticles come in contact, Fermi level alignment [] is expected to approximately locate CdSe to a higher level (more negative potentials, red frame). This will allow cascade charge transfer []. Values are adapted to data from Refs. [,8,9] and correspond to ph 7.0. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Acknowledgments This research has been co-financed by the European Union (European Social Fund ESF) and Greek national funds through the Operational Program Education and Lifelong Learning of the National Strategic Reference Framework (NSRF) Research

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