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Electrodeposited cobalt-copper sulfide counter electrodes for highly efficient quantum dot sensitized solar cells
Accepted Manuscript Title: Electrodeposited Cobalt-Copper Sulfide Counter Electrodes for Highly Efficient Quantum Dot Sensitized Solar Cells Author: Lida Givalou Maria Antoniadou Dorothea Perganti Maria Giannouri Chaido-Stefania Karagianni Athanassios G. Kontos Polycarpos Falaras PII: DOI: Reference:
S0013-4686(16)31282-8 http://dx.doi.org/doi:10.1016/j.electacta.2016.05.191 EA 27415
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
21-2-2016 27-5-2016 28-5-2016
Please cite this article as: Lida Givalou, Maria Antoniadou, Dorothea Perganti, Maria Giannouri, Chaido-Stefania Karagianni, Athanassios G.Kontos, Polycarpos Falaras, Electrodeposited Cobalt-Copper Sulfide Counter Electrodes for Highly Efficient Quantum Dot Sensitized Solar Cells, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.05.191 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrodeposited Cobalt-Copper Sulfide Counter Electrodes for Highly Efficient Quantum Dot Sensitized Solar Cells
Lida Givalou1,2, Maria Antoniadou1, Dorothea Perganti1,2, Maria Giannouri1, Chaido-Stefania Karagianni2, Athanassios G. Kontos1 and Polycarpos Falaras1*
1. Institute of Nanoscience and Nanotechnology, NCSR “Demokritos”, Agia Paraskevi Attikis, Athens 15310, Greece. 2. School of Chemical Engineering, National Technical University of Athens, 9 Iroon Polytechniou St., 15780 Zografou, Athens, Greece.
*Corresponding author: Dr. Polycarpos Falaras phone: +30 2106503644 fax: +30 2106511766 e-mail: [email protected]
Abstract A series of novel composite Cobalt-Copper Sulfide (CoS-CuS) counter electrodes were developed for Quantum Dot Sensitized Solar Cells (QDSSCs) based on coreshell cadmium sulfide-cadmium selenide quantum dots. The new electrodes were prepared by one-step electrodeposition from an aqueous solution containing cobalt chloride (CoCl2·6H2O), copper chloride (CuCl2·2H2O) and thiourea. The composite electrodes present rough morphology consisting of CuS agglomerates on a CoS continuous nanowire network, allowing easy electron transfer across the CEelectrolyte interface. The corresponding QDSSCs demonstrate power conversion efficiencies (PCE) exceeding 5%, under one sun illumination. By increasing the mesoporous layer thickness, adding a scattering layer and adjusting the QDs deposition conditions, a PCE as high as 7.49% was obtained. These very promising results further justify and account for the innovative character of the composite CoSCuS counter electrodes.
Keywords: composite CoS-CuS; counter electrodes; CdS/CdSe quantum dots; nanocrystalline TiO2; Quantum dot solar cells.
1. Introduction The replacement of transition metal complexes and organic dyes in third generation photovoltaics by low cost light harvesting inorganic materials has occupied the scientific interest during the recent decades. In this context, the use of quantum dots as photosensitizers, contributes to electrical the development of efficient Quantum Dot Sensitized Solar Cells (QDSSCs) [1]. Semiconductor quantum dots (QDs) are attractive light absorbers for solar cells because of their unique optical and properties. They exhibit significantly differing optoelectronic properties with respect to their bulk analogues, mainly due to the quantum confinement effect of the electron cloud in the 3D spatial dimensions of the semiconducting nanocrystals. The quantum confinement makes the energy levels discrete and this widens up the band gap [2]. Thus, the main advantages of using quantum dots lay on the ability to tune the bandgap according to their size (which allows the selection of a wider variety of materials for the cell) and their high molar extinction coefficients permit the generation of high photocurrents from thin film sensitized photoelectrodes. A fascinating challenge of the QDSSCs is the utilization of hot electron injection quantum effect that is the generation of multiple electron/hole pairs per single photon. This could in principle, lead to the construction of single junction QDSSCs devices, which surpass the Shockley-Queisser 33% limit in the efficiency [3]. After several years of research, the power conversion efficiency of quantum dot sensitized solar cells has been improved to 4-6% [4-13] while higher values (5-7%) were obtained with cells based on colloidal systems [14-23], with the record of 9.28% [24]. However, besides important progress in QDSSCs, significant problems remain unsolved including relatively low open circuit potential (Voc) and fill factor (FF) values. These problems are mainly attributed to the fast charge
recombination at the TiO2/QD/electrolyte interface and high charge-transfer resistance at the counter electrode (CE) [25]. The operation mechanism of the QDSSCs is based on the bandgap excitation of the QDs and the injection of the excited electrons into the TiO2 mesoporous layer; the QDs are subsequently reduced by the redox species present in a liquid electrolyte. The counter electrode serves an important and indispensable role in collecting electrons from the external circuit and reducing back the oxidized redox species. Since polysulfide (S2-/Sx2-) redox mediators are used in QDSSCs, the replacement of the commonly employed platinum (Pt) counter electrode (CE) is required, since Pt is easily corroded by sulfur species adsorbed on its surface. Thus, CuS and CoS have also been extensively used as counter electrodes in QDSSCs [26-29] since they are inexpensive and present high electrocatalytic activity for the redox reaction of polysulfides, (Sx2-+2e-↔Sx-12-+S2-). CuS CEs generally present high Voc values but show low electrocatalytic activity, due to corrosion by sulfur and poor adhesion to the FTO substrate, which increases its resistance and affects its stability adversely. On the contrary, CoS based counter electrodes have shown up to now some of the highest FF and efficiencies in QDSSs, due to their remarkable electrocatalytic activity; nevertheless, they suffer from relatively low open circuit voltages [25]. The idea of optimizing CuS counter electrode by mixing it with CoS has therefore logically been raised in an attempt to combine the benefits of the two sulfides. Thus, QDSCs using CuS/CoS counter electrodes prepared following a complex multistep method involving several synthetic, deposition and heating steps, provided efficiency as high as 4.1% [8]. Such a complex procedure raises significant stability issues (related to air-drying at 100oC) that probably affect the device long term operation and life time.
In the present study, we investigate the synthesis of composite CoS-CuS counter electrodes, prepared by electrochemical deposition on FTO glass substrates. The composites electrodes were prepared utilizing mixtures of CoCl2·6H2O:CuCl2·2H2O and thiourea as the sulfur source. The electrocatalytic activity of the films was optimized by varying the precursors concentration and the electrodeposition time. Our one-step approach lacks complexity and avoids additional need of post thermal treatment. The CEs were tested in QDSSCs, demonstrating high power conversion efficiency, exceeding 5% under one sun illumination and leaving good perspectives for significantly higher solar cell performances. 2. Experimental 2.1 Preparation of the QD sensitized TiO2 photoanodes The titania film electrodes were deposited on patterned FTO electrode and consisted of two layers: a compact layer at the bottom and a mesoporous at the upper part. Details of their preparation are given in the supporting information S1. CdS-ZnS quantum dots were deposited on the nc-TiO2 films by Successive Ionic Layer Adsorption and Reaction (SILAR method) [30], as described in S2. The CdSe QDs were deposited on the above CdS-ZnS core layers, using a modification of the Chemical Bath Deposition (CBD) technique, described in the literature [31] (see S3). Finally, a passivation ZnS layer was added on the top of the CdSe layer using 2 SILAR cycles [32] (see S4). 2.2 Electrodeposition of the counter electrodes Initially, CoS loaded FTO electrodes were prepared by the electrochemical deposition method [33]. The deposition bath, was an aqueous solution (80 ml) containing (from 5 to 50mM) cobalt (II) chloride hexahydrate (CoCl2·6H2O) and 150mM thiourea (CH4N2S). The deposition was performed at room temperature in a one-compartment-
glass cell using a three-electrode configuration. The substrate was a 4cm2 FTO conductive glass cleaned prior to its use in ultrasounds, using soap, acetone and ethanol. A Pt foil served as the counter electrode and an Ag/AgCl/KCl 3M electrode was used as the reference. The electrodeposition was performed by applying a potential of -0.8 V vs. Ag/AgCl for 30 min. When an electrolytic bath containing 50mM cobalt (II) chloride hexahydrate was employed, 0.043 mg/cm2 CoS loading was obtained on the film, which led to optimum combination of electrocatalytic activity and visible light transmittance. In order to further improve the counter electrodes, an amount of CuCl2·2H2O was added to the aqueous solutions of CoCl2·6H2O, keeping the thiourea content constant (150mM). Thus, the CoS-CuS composites were electrochemically deposited by varying the (CoCl2·6H2O):(CuCl2·2H2O) metal precursors concentration (C) [from 50:5mM, to 100:10mM and to 150:15 mM] and modifying the deposition time (t) (from 30 min to 60 min). Table 1 presents the different combinations of C and t employed for the preparation of the CoS-CuS composite films. Detailed characterization of the composite films was done for the 2C-1t samples, which presented the best performance in QDSSCs. For comparison, CuS loaded FTO electrodes were similarly prepared by electrodeposition, using 5mM of CuCl2·2H2O as the Cu precursor keeping the thiourea concentration 150mM. Cu2S/brass electrodes (the most common counter electrode used in Quantum Dot Sensitized Solar Cells), were also prepared and examined in QDSSCs. The brass foil substrates were immersed into 37% HCl at 80°C for 10 min, then rinsed with water and dried in air. In turn, the etched brass foils were dipped into 2M S and 2M Na2S aqueous solution, to form black Cu2S counter electrodes.
2.3 Cell assembly The photoanode and the counter electrode were assembled in a sandwich-like cell made by Teflon. The two electrodes were kept in a distance of about 2 cm. This configuration, where the electrodes are kept far apart, has been proved to be favorable for QDSSCs by increasing the effective electrocatalyst/electrolyte interface [34]. The space inside the cell was filled with 4ml of the aqueous polysulfide electrolyte containing 2.0 mol L-1 Na2S·9H2O, 2.0 mol L-1 S and 0.2 mol L-1 KCl. The solution was prepared by continuous stirring of the mixture at 70-80 oC for about 20 min. 2.4 Characterization Techniques A PHILIPS Quanta Inspect Scanning Electron Microscope with energy dispersive Xray analysis (SEM-EDX) was used to investigate the morphology and the chemical stoichiometry of the photoelectrodes and the counter electrodes. Atomic Force Microscope (AFM) used to determine the roughness of the composite CoS-CuS electrode with a Digital Instruments Nanoscope III atomic force microscope, operating in the tapping mode. The adhesion of the CoS-CuS films on the FTO substrate was assessed using the ARDGO Model P-A-T apparatus, following the ASTM D-3359 standard. Raman analysis was performed in backscattering configuration on a Renishaw in Via Reflex microscope using a diode laser (λ=514.4 nm) as excitation source. The laser beam was focused onto the samples by means of a 50× objective, while the laser power density was kept at low levels to avoid local heating of the samples. Core-level X-ray photoemission (XPS) spectra were collected with a PHOIBOS 100 (SPECS) hemispherical analyzer at a pass energy of 7 eV with a Mg X-ray source (1253.6 eV). The take-off angle was set at 37° relative to the sample surface. The binding energy scale was calibrated using the position of both the Au 4f7/2 and Ag 3d5/2 peaks at 84 and 368.3 eV, respectively measured on clean
gold and silver foils. UV-Visible diffuse reflectance spectra were recorded in a Hitachi 3010 spectrophotometer equipped with an integrating sphere and reflectance data were transformed into Kubelka-Munk units. Electrochemical impedance spectra (EIS) were recorded on symmetrical dummy cells, (two identical counter electrodes that enclose the electrolyte), using the Autolab PGSTAT-30 potentiostat (Ecochemie), equipped with a frequency response analyzer (FRA). Spectra were recorded in the dark over a frequency range of 100 kHz to 10 mHz and were fitted with the FRA software provided by the Autolab in terms of appropriate equivalent circuits. Linear sweep voltammograms of the dummy cells were also obtained at a scan rate of 20 mVs-1. Current density–voltage (j–V) measurements of the QDSSCs were performed by illuminating the QDSSCs using solar simulated light (1 sun, 100 mW cm− 2) emitted from a 300 W Xe source and filtered at AM 1.5G using appropriate optical filters (Oriel). The active area was set at 0.152 cm2, using a black mask in front of the cells in order to avoid any light peeping inside the cell. The j–V characteristics were recorded by linear sweep voltammetry (LSV) on the Autolab potentiostat working in a two-electrode mode, at a scan rate of 20 mVs-1. 3. Results and Discussion 3.1Characterization of the photoelectrode The prepared sensitized photoelectrodes were first characterized by scanning electron microscopy (SEM). A characteristic top view image of the photoanodes is presented in Fig.1a. The surface consists of aggregated nanoparticles, which are predominantly spherical in shape with diameters from 65 to 110 nm, whereas the QDs are not distinguished from the titania nanoparticles under the examined SEM image
magnifications. These well packed spheres comprise a dense layer of 9 μm thickness, as it can be calculated from the cross section image shown as inset at Fig.1a. To assess the adhesion of the CoS-CuS films on the FTO substrate, the ASTM D3359 standard was used. The method consists in applying and removing pressuresensitive tape over cuts made in the coating. The performed analysis (Figure S.0 in Supporting Information) has shown that the surface of cross-cut area from which flacking has occurred present classification 3, confirming that the adhesion of the composite coatings is at a generally adequate level. The prepared ZnS/CdSe/CdS-ZnS/TiO2 photoelectrodes were further structurally characterized by Raman spectroscopy as shown in Fig.1b. The spectra present a strong band at 143 cm-1 attributed to the TiO2 predominant anatase phase. The narrow band at 207 cm-1 corresponds to the first-order LO CdSe phonon and presents an asymmetric broadening at the low-frequency side which can be well accounted by the contribution of a surface optical Raman mode at 197 cm−1. A CdS LO mode appears at 300 cm-1. Additionally, a CdSxSe1-x mode was observed at 279 cm-1, attributed to the mixed CdSxSe1-x interfacial layer formed between CdSe and CdS QDs. At higher frequencies, weak and broad modes emerge (e.x. 410 cm-1 - 2nd order CdSe mode) [35]. The QD bandgaps were estimated by UV-vis absorption spectroscopic data shown in Fig.1c. Titanium dioxide is a wide bandgap semiconductor with energy gap equal to 3.2 eV that it is unable to absorb any visible light on its own. Sensitization of TiO2 with CdS 75%-ZnS 25% quantum dots permits the extension of the absorbance up to 500 nm corresponding to 2.48 eV. This value approaches that observed in the bulk CdS, which means that the size of largest CdS QDs is nearly twice the CdS exciton Borr radius, that is about 6 nm for CdS [35]. Subsequent further sensitization with
CdSe extends the absorption up to 650nm corresponding to 1.91 eV film and allows an almost complete absorption of the visible light. In this case, the absorption threshold energy of CdSe is considerably higher than that of the bulk material and permits the estimation of the size of the largest CdSe QDs on the films in between 4.5 to 5 nm [36]. Furthermore, the core cell structure permits the efficient separation of the photoexcited carriers at the interface of the two sulfides and allows their easy injection to the TiO2 conduction band. Ahead, by the presence of the CdS core QDs we skip the difficulties of the direct deposition of CdSe on TiO2 [5]. In summary, the structural and optical characteristics of the sensitized films are very promising to support its efficient operation as photoanodes in solar cells. 3.2 Characterization of the counter metal sulfide electrodes 3.2.1 Morphological Characterization In order to understand the properties of the synthesized composite CoS-CuS counter electrodes (which, to the best of our knowledge, are studied for the first time), the surface morphology of the electrochemically CuS and CoS counter electrodes prepared separately on FTO substrate was first examined by scanning electron microscopy SEM and their stoichiometry with energy dispersive X-ray (EDX) spectroscopy. Fig.2 displays top surface SEM images of the CoS, CuS, and CoS-CuS electrodes developed on the FTO-glass substrates. Regarding the CoS electrode, one of the most complicated metal sulphide systems because of different phases and chemical compositions [37], Fig.2 a,b reveal a particular surface morphology comprised of crystalline rods. The atomic ratio between Co and S is estimated to 1:2 indicative of predominant formation of CoS 2 structure.
From the SEM images at low magnifications in Fig.2c, it is shown that CuS is evaluated as sponge like microspheres randomly distributed across the FTO glass substrate. The mean diameter of the hemispheres is around 3 μm. High magnification images show the formation of large particles in nanoplate form. The presence of Cu and S was confirmed from the EDX analysis with atomic ratio of 1.5:1, suggesting mixed copper valence. The surface morphology of the composite CoS-CuS (Fig.2 e,f) electrochemically synthesized counter electrodes takes the form of an extensive porous structure. The morphology of the CuS electrodes with the clustering in large spheres collapses, resulting in their segmentation/agglomeration in numerous small dense aggregates supporting a grid of nanowires. In fact, the initial morphology of CoS is severely influenced during the formation of the composite film. The crystalline rods have been altered by a honeycomb porous network comprised of nanowires with a mean diameter of 20 nm. Detailed EDX analysis on the white nanoflake area (A) in Fig. 2e shows that the ratio between Cu:Co:S is 1:2:3.5, while on the gray honeycomb type area (B) the Cu content is severely reduced and the corresponding atomic ratio becomes 0.5:2:3.5, verifying the CoS nature of the nanowires. It is remarkable that the composite films present such unique porous morphology and their composition shows considerable excess of CoS. This comes in accordance to the literature, which reports versatile morphologies and elemental compositions of sulfide films depending on the ratio and the type of copper, cobalt and sulfur sources as well as on the applied deposition method [38]. It is obvious that in the mixed product, the atomic ratio between Cu:S and Co:S differs from the corresponding values in the single CuS and CoS films. The above discrepancy is due to the different
concentrations of the salts during the electrodeposition and also to the various discharge potential of the ions on the FTO during the electrophoretic deposition [38]. Subsequently, the surface roughness of the CoS-CuS 2C-1t counter electrodes was investigated by atomic force microscopy (AFM). AFM images at 20x20 μm area (Fig.3) show rough surface morphology which resembles that observed by SEM. From the analysis of the AFM images the average as well as the root mean square roughness of the surface is estimated to be 500 and 415 nm, respectively, verifying the rough characteristics of the films appropriate for efficient electrocatalytic reactions. 3.2.2 Structural characterization The various sulfide electrodes were structurally characterized by Raman spectroscopy. Characteristic spectra are shown in Fig. 4. Copper sulfide presents a broad weak feature at 265 cm-1 as well as a relatively narrow asymmetric Raman band with peak maximum at 473.5±0.5 cm-1. The observed Raman spectra are attributed to mixed CuS and Cu2S, since the two copper sulfides have similar Raman profiles with main frequency bands which peak very closely, at frequencies of 474 and 472 cm-1, correspondingly [39]. The CoS peak shows a broad Raman feature which covers the range of 240-370 cm-1 and is deconvoluted to two broad lines which peak at 285 and 340 cm-1 as well as a second band which covers the 400-520 cm-1 range also well fitted by two lines at 445 and 485 cm-1. The low frequency Raman feature resembles that observed in the amorphous CoS2 film and is tentatively attributed to Co−S vibrations, while the high frequency one can be correspondingly assigned to S-S vibrations [40]. The composite copper and cobalt sulfide counter electrodes show several Raman vibration peaks which at first glance, appear as superposition of the CoS and the CuS
ones. Thus, there is the relatively narrow 473.5 cm-1 peak which is directly related to the CuS as well as a low frequency broad feature (composed again by two lines at 265 and 315 cm-1) and a high frequency broad shoulder (at 430 cm-1) which very much resemble the corresponding ones from the CoS bare films, slightly red shifted in frequency. The missing of the high frequency mode of the pure CoS film from the spectrum of the composite film and the considerable shifting of the other modes is justified by the substantial alternations in the structural and morphological characteristics of CoS. To complete the structural characterization (micro-Raman results), we investigated the surface chemical composition (binding states of cobalt, copper and sulfur) of the CoS-CuS composites by performing XPS analysis. Fig. 5a shows the Co 2p XPS spectrum exhibiting the well separated spin-orbit components Co 2p3/2 and Co 2p1/2. In particular, the Co 2p3/2 peak is characterized by a dominant contribution at 781 eV assigned to CoS formation as well as a broad peak at higher binding energies (about 787 eV) that can also be originated from a shake-up satellite of CoS, both indicating that that the Co ion was in the form of the Co2+ state [41]. In addition, very little amount of the metallic Co at 778.5 eV exists. It should be noted that the shakeup satellite peak could also be contributed to the presence of CoO species that are being formed upon sample exposure to the atmosphere [42]. In fact, cobalt ion has a very strong affinity to oxygen and it is difficult to exclude it impurities from the resultant materials [43]. Concerning the Cu 2p XPS spectrum (Fig. 5b), two main peaks located at 932.8 and 952.7 eV were observed, assigned to Cu 2p3/2 and Cu 2p1/2, respectively. In addition, there are weak satellite peaks at around 943 eV, suggesting the presence of the paramagnetic chemical state of Cu2+ [44,45]. The corresponding XPS spectrum of
S 2p for the CoS-CuS composites (shown in Fig. 5c) presents the S 2p3/2 and S 2p1/2 peaks centered at 162.2 and 163.3 eV, respectively, indicating that the S species exist as S2- in the deposits [46,42]. The XPS results support the findings from the Raman and EDS analyses and indicate most definitely that CoS-CuS composites were successfully deposited onto the FTO surface via a facile electrodeposition method. 3.2.3 Electrochemical Characterization After their incorporation in symmetrical dummy cells, the fabricated counter electrodes were characterized via Tafel polarization measurements in order to evaluate their catalytic activity towards the employed (S2-/Sx2-) redox couple [47]. Thus, identical electrodes from each one of the categories: CuS, CoS and CoS-CuS, were used to enclose the S/Na2S 2M electrolyte in between them. Fig. 6 presents the obtained Tafel-polarization curves (log (j) vs Applied potential) of the CoS (5mM), CoS (50mM) and CoS-CuS 2C-1t – based symmetrical dummy cells. Through the Tafel curves, the exchange current density (jo) can be estimated by the intercepts of the tangent lines to the anodic and cathodic branches, in the Tafel zone. From the analysis between the CoS-based cells, the ones with the highest concentration of CoCl2 (50mM) hold slightly higher jo values. Above all, the CoSCuS 2C-1t symmetrical dummy cells present the largest jo (4.7 mA cm-2 is estimated from the intercept in Fig. 6), showing the best electrocatalytic performance as they promote faster reduction of the polysulfide species [47,48]. Symmetrical dummy cells of the structure CuS/electrolyte/CuS were also constructed. The corresponding Tafel curves (not shown) present high instability and were not reproducible, due to the fast corrosion of the CuS electrodes in the polysulfide electrolyte, in agreement with the literature [28].
The above results were confirmed by electrochemical impedance spectroscopy (EIS), which was also applied to the symmetric dummy cells under dark conditions at zero bias. The interfaces of the symmetric cells were simulated by an equivalent electrical circuit (Fig. 7a), which consists of a series resistance (RS,S) at high frequency and a semicircle at the middle frequency region corresponding to the electronic transport resistance (RCT,S) and the double layer capacitance (Qdl) at the counter electrode/electrolyte interface [49]. The RS,S is the sum of the ohmic resistances in the circuit at which the first arc starts (includes the resistances of the two FTO substrates and of the CE) and is not strongly affected by the kind of the counter electrode, as it is also confirmed by the corresponding data depicted in Fig. 7b. On the other side, it is evident from the results that the RCT,S values, which account for the electrocatalytic activity of the counter electrodes, present important changes for the different films. The CoS electrode (5 mM) presents a high electronic resistance RCT,S at the counter electrode/electrolyte interface, equal to 124 Ohm cm2, which is decreased by about 60% to 49 Ohm cm2 for higher CoS concentration (50 mM). The RCT,S of the CuS electrode has an average value 10 Ohm cm2 showing its great electrocatalytic activity comparing to CoS electrode. This value is not indicative because of the fast corrosion of the CuS electrode towards the polysulfide electrolyte. The best performance is attained for the CoS-CuS 2C-1t loaded electrodes, which show the minimum RCT,S, equal to 3.04 Ohm cm2. Such low RCT,S resistance is one of the best in the literature [8, 25, 28, 47, 48] and can be applied in equation (1), in order to estimate the exchange current density [33].
Jo
RT nFR CT,S
(1),
where R is the gas constant, T is the absolute temperature, n is the number of electrons exchanged in the reaction (in our case n=2) and F is the Faraday constant. From eq. 1, a jo value of 4.2 mA cm-2 is extracted which is also in relative agreement with that obtained by the Tafel plots. Such high jo values, combined with the large electrode spacing of our QDSSC design, secure easy electronic transport from the electrolyte to the counter electrode. This is indispensable for efficient counter electrodes, which are able to attain high photocurrents at 1 sun. 3.3 Photovoltaic Performance of the QDSSCs Quantum-dot solar cells of the type: FTO/TiO2/core-shell QDs//electrolyte//CE were constructed by assembling the ZnS/CdSe/CdS-ZnS/TiO2 photoelectrodes, the newly prepared counter electrodes and the S2-/Sx2- redox couple. The cells were evaluated under one sun illumination conditions (AM 1.5G, 100 mW cm− 2). The obtained j-V curves are shown in Fig. 8 and the corresponding parameters are summarized in Table 2. In a first step, we compared cells having either single sulfide CE (the references CuS and CoS and the optimized CoS prepared with 50 mM) or the composite CoS-CuS (1C-1t) CEs. For these cells (Fig. 8a), the highest conversion efficiency (3.14%) was achieved with CoS-CuS composite counter electrode, which also demonstrates the best Voc values. Subsequently, the CoS-CuS composite CEs were further improved by optimizing the duration of the electrodeposition and the concentration of salts (according to the Table 1 given in the experimental part). Τhe characteristic parameters and the performance of the above solar cells are presented in Fig. 8b and Table 2. It is apparent that the CoS-CuS counter electrodes work effectively even with low concentrations of the salts (1C-1t), while the increase of the deposition time enhances the power conversion efficiency of the cell (from 3.14 to 5.03 %), mainly due to the
higher attained FF values (Fig. 8b). It is also evident that with the increase of the CoS:CuS concentration of the salts in the electrolytic solution from 50:5 to 100:10, all the characteristic parameters of the solar cell are improved leading to increase of the overall efficiency by 60%. Thus, the highest power conversion efficiency was achieved with the concentration ratio of CoS-CuS equal to 100:10mM, while further increase of salt concentration (150:15mM) leads to lower values of all the cell characteristics. The optimum concentration of 100 mM CoS and 10 mM CuS in terms of maximizing the corresponding QDSSCs efficiency is justified by the easier regeneration of the S2-/Sx2- redox couple at the corresponding interface, verified by the LSV results. Comparison of Photocurrent Density vs time for QDSCs based on the CuS and CoS-CuS counter electrodes confirmed that those based on the composite CoS-CuS CEs not only work more efficiently than those corresponding to CuS but also present significantly higher stability (Fig. S.3, Supporting Information). In fact, the photocurrent density of the cells employed the CuS counter electrodes drops from 13.97mAcm-2 to 0.7mAcm-2 during the first 4 hours, while those using the mixed CoS-CuS counter electrodes were stable for several hours. Finally, regarding the deposition time, it is important to note that the increase from 30 to 60 minutes slightly affects the electrical parameters of the solar cells. Finally, the optimized CoS-CuS counter electrodes were compared with the performance of the Cu2S counter electrode (widely used in the literature), prepared by chemical reaction on a sheet of brass. The results are also included in Table 2. The Cu2S counter electrode shows higher Voc value (0.59V) than the composite films. On the contrary, the CoS-CuS counter electrodes present considerably larger jsc and much higher FF, resulting to over 25% relative increase in efficiency.
The above FF values obtained from the j-V characteristic diagrams comply well with the
obtained
electrochemical
characterization
results
from
the
various
photoelectrodes. Thus, the CoS-CuS counter electrodes which show the best catalytic activity presenting the highest exchange current densities (jo) and the lowest charge transfer resistance (RCT,S) achieved the highest photocurrents (jsc=16.09 mA cm-2) and fill factors FF (0.58) when used in QDSSCs. The outstanding electrocatalytic activity of the composite counter electrodes cells are particularly favored from their extended nanoporous structure generated by CuS agglomerates supporting a network of CoS nanowires whereas moreover, they preserve sufficiently high open circuit voltages. Consequently, the power conversion efficiencies obtained with solar cells shaving the CoS-CuS electrodes excels over those from the cells based on standard Cu2S and is among the largest values ever reported for this type of QDSSCs. It is important to note that this work mainly focuses on the development and characterization of the composite counter electrode, leaving apart all other cell components. To valorize further our materials, quantum dot solar cells are being developed, incorporating the novel CEs in combination with more appropriate photoanodes. Work is now in progress to optimize the multilayered titania substrate (including an additional scattering layer) and fine tune the QDs deposition procedure (control of both size and deposition rate). The first results (summarized in Figure S.2) are very promising and account for the innovative character of our composite CoSCuS electrodes. In fact, the efficiency goes to 5.41 % when the thickness of the mesoporous layer was increased to 18.3 µm and a scattering layer (of 11 µm) was added. In addition, using these electrodes and fine tuning the CdSe QDs deposition procedure (control of both size and deposition rate), power conversion efficiencies as high as 7.5% were obtained (Figure S.2, Supporting Information). It is important to
notice that the optimized QDSCs using the CoS-CuS CEs present high stability and the jSC values remain constant (and above 16 mA cm-2) for more than 70 hours (Fig. S.4, Supporting Information). The above behavior was further supported by Electrochemical Impedance Spectroscopy (Nyquist plots in Figure S.5, Supporting Information), confirming reduced recombination at the photoelectrode /electrolyte interface (in agreement with increased Voc values). 4. Conclusions CoS, CuS and mixed CoS-CuS counter electrodes (CEs) were prepared by one-step electrochemical deposition on FTO conducting glass and were employed as counter electrodes in the fabrication of TiO2 sensitized solar cells with CdS-ZnS/CdSe/ZnS core/shell quantum dots. The CoS-CuS composite counter electrodes show optimum electrocatalytic activity as shown by the electrochemical analysis and extended nanoporous and rough structure, which is engendered by the establishment of a composite network of CuS aggregates supporting a grid of CoS nanowires. Consequently, the cells based on composite CEs present the highest photocurrents and fill factor values as well as the lowest charge transfer resistances at the CEelectrolyte interface amongst all different prepared QDSSCs. Their supreme efficiencies obtained are above 5%, which are among the highest values reported for QDSSCs of this type and excel over those for cells based on standard Cu2S, by 25%. Current research in the preparation stage shows that the use of CoS-CuS counter electrodes in combination with optimized photoelectrodes further increase significantly the cell performance.
Acknowledgements The authors greatly acknowledge precious help and assistance from Dr. D. Tsoutsou (recording XPS spectra) and D.F. Tsoukleris (performing scratch tests) as well as financial support from FP7 European Union (Marie Curie Initial Training Network DESTINY/316494). Research has also been co-financed by the European Union (European Social Fund - ESF) and national funds through the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF) - Research Funded Project: EXCELLENCE-AdMatDSC / 1847. L.G. acknowledges scholarship from Special Account for Research (EPP) of the NTUA.
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Fig.1. Top view SEM image of the ZnS/CdSe/CdS-ZnS/TiO2 photoanode surface and cross section image as an inset (a); Raman spectrum of ZnS/CdSe/CdS-ZnS/TiO2. Bands marked as follows: sensitized photoelectrode TiO2 (in black), CdSe (in red), CdS (in blue), CdSSe (in magenta) (b); Absorption spectra (Kubelka-Munk units) of bare TiO2 photoanode, TiO2 film with CdS-ZnS layer and TiO2 film with ZnS/CdSe/CdS-ZnS layer (dashed curves are used to estimate the corresponding band gaps) (c).
Fig.2. SEM images of the CoS (a,b), CuS (c,d), and CoS-CuS 2C-1t (e,f) electrodes at two different magnifications. In Fig. 2e, typical areas rich of CuS (A) and CoS (B) are marked where from EDX data have been acquired.
Fig.3. 3D AFM image at 20 μm x 20 μm from the surface of the CoS-CuS 2C-1t electrode.
Fig.4. Raman spectra of the CuS, CoS and CoS-CuS 2C-1t films. Fitted components (dashed lines) and simulated curve (solid line) are shown for the composite film.
Fig.5. High resolution XPS spectra of Co 2p (a); Cu 2p (b); and S 2p (c); for the CoSCuS composite CEs.
Fig.6. Tafel-polarization curves on symmetrical dummy cells using the CoS and CoSCuS -2C-1t films (scan rate: 20 mV s-1). Dotted line is the tangent of the current in the Tafel zone extrapolated to estimate the exchange current density.
(a)
Fig.7. (a) Equivalent circuit employed for the symmetrical dummy cells; (b) Nyquist plots of the CoS- and CoS-CuS 2C-1t symmetrical dummy cells obtained at 0 V under dark conditions.
Fig.8. j-V curves of QDSSCs with different counter electrodes. (a) Comparison of CoS and CuS cells with the composite CoS-CuS (1C-1t), (b) Comparison of CoSCuS cells prepared with different concentration and deposition time.
Table 1. Composite CoS-CuS counter electrodes prepared by varying the (CoCl2·6H2O):(CuCl2·2H2O) metal precursor concentration (C) and the electrodeposition time (t). The concentration of thiourea was kept constant at 150 mM. Sample
(CoCl2·6H2O):(CuCl2·2H2O)
Deposition time
CoS-CuS
C [C1(mM):C2(mM)]
t (min)
1C-1t
50:5
30
1C-2t
50:5
60
2C-1t
100:10
30
2C-2t
100:10
60
3C-1t
150:15
30
Table 2. Characteristic parameters of QDSSCs prepared with the composite CoSCuS counter electrodes. The corresponding performances using cobalt sulfide, copper sulfide and Cu2S-Brass CEs are also reported, for comparison. Voc (V)
FF
η (%)
12.13
0.45
0.47
2.54
CoS(50mM)
13.87
0.43
0.51
3.03
CuS(5mM)
13.97
0.49
0.42
2.85
CoS-CuS 1C-1t
13.56
0.49
0.47
3.14
CoS-CuS 1C-2t
13.51
0.49
0.54
3.57
CoS-CuS 2C-1t
16.09
0.54
0.58
5.03
CoS-CuS 2C-2t
15.27
0.58
0.51
4.55
CoS-CuS 3C-1t
14.46
0.50
0.43
3.13
Cu2S-Brass
15.12
0.59
0.45
4.00
Counter Electrode (on FTO) CoS(5mM)
jsc (mA cm-2)
S0013-4686(16)31282-8 http://dx.doi.org/doi:10.1016/j.electacta.2016.05.191 EA 27415
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Electrochimica Acta
Received date: Revised date: Accepted date:
21-2-2016 27-5-2016 28-5-2016
Please cite this article as: Lida Givalou, Maria Antoniadou, Dorothea Perganti, Maria Giannouri, Chaido-Stefania Karagianni, Athanassios G.Kontos, Polycarpos Falaras, Electrodeposited Cobalt-Copper Sulfide Counter Electrodes for Highly Efficient Quantum Dot Sensitized Solar Cells, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.05.191 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrodeposited Cobalt-Copper Sulfide Counter Electrodes for Highly Efficient Quantum Dot Sensitized Solar Cells
Lida Givalou1,2, Maria Antoniadou1, Dorothea Perganti1,2, Maria Giannouri1, Chaido-Stefania Karagianni2, Athanassios G. Kontos1 and Polycarpos Falaras1*
1. Institute of Nanoscience and Nanotechnology, NCSR “Demokritos”, Agia Paraskevi Attikis, Athens 15310, Greece. 2. School of Chemical Engineering, National Technical University of Athens, 9 Iroon Polytechniou St., 15780 Zografou, Athens, Greece.
*Corresponding author: Dr. Polycarpos Falaras phone: +30 2106503644 fax: +30 2106511766 e-mail: [email protected]
Abstract A series of novel composite Cobalt-Copper Sulfide (CoS-CuS) counter electrodes were developed for Quantum Dot Sensitized Solar Cells (QDSSCs) based on coreshell cadmium sulfide-cadmium selenide quantum dots. The new electrodes were prepared by one-step electrodeposition from an aqueous solution containing cobalt chloride (CoCl2·6H2O), copper chloride (CuCl2·2H2O) and thiourea. The composite electrodes present rough morphology consisting of CuS agglomerates on a CoS continuous nanowire network, allowing easy electron transfer across the CEelectrolyte interface. The corresponding QDSSCs demonstrate power conversion efficiencies (PCE) exceeding 5%, under one sun illumination. By increasing the mesoporous layer thickness, adding a scattering layer and adjusting the QDs deposition conditions, a PCE as high as 7.49% was obtained. These very promising results further justify and account for the innovative character of the composite CoSCuS counter electrodes.
Keywords: composite CoS-CuS; counter electrodes; CdS/CdSe quantum dots; nanocrystalline TiO2; Quantum dot solar cells.
1. Introduction The replacement of transition metal complexes and organic dyes in third generation photovoltaics by low cost light harvesting inorganic materials has occupied the scientific interest during the recent decades. In this context, the use of quantum dots as photosensitizers, contributes to electrical the development of efficient Quantum Dot Sensitized Solar Cells (QDSSCs) [1]. Semiconductor quantum dots (QDs) are attractive light absorbers for solar cells because of their unique optical and properties. They exhibit significantly differing optoelectronic properties with respect to their bulk analogues, mainly due to the quantum confinement effect of the electron cloud in the 3D spatial dimensions of the semiconducting nanocrystals. The quantum confinement makes the energy levels discrete and this widens up the band gap [2]. Thus, the main advantages of using quantum dots lay on the ability to tune the bandgap according to their size (which allows the selection of a wider variety of materials for the cell) and their high molar extinction coefficients permit the generation of high photocurrents from thin film sensitized photoelectrodes. A fascinating challenge of the QDSSCs is the utilization of hot electron injection quantum effect that is the generation of multiple electron/hole pairs per single photon. This could in principle, lead to the construction of single junction QDSSCs devices, which surpass the Shockley-Queisser 33% limit in the efficiency [3]. After several years of research, the power conversion efficiency of quantum dot sensitized solar cells has been improved to 4-6% [4-13] while higher values (5-7%) were obtained with cells based on colloidal systems [14-23], with the record of 9.28% [24]. However, besides important progress in QDSSCs, significant problems remain unsolved including relatively low open circuit potential (Voc) and fill factor (FF) values. These problems are mainly attributed to the fast charge
recombination at the TiO2/QD/electrolyte interface and high charge-transfer resistance at the counter electrode (CE) [25]. The operation mechanism of the QDSSCs is based on the bandgap excitation of the QDs and the injection of the excited electrons into the TiO2 mesoporous layer; the QDs are subsequently reduced by the redox species present in a liquid electrolyte. The counter electrode serves an important and indispensable role in collecting electrons from the external circuit and reducing back the oxidized redox species. Since polysulfide (S2-/Sx2-) redox mediators are used in QDSSCs, the replacement of the commonly employed platinum (Pt) counter electrode (CE) is required, since Pt is easily corroded by sulfur species adsorbed on its surface. Thus, CuS and CoS have also been extensively used as counter electrodes in QDSSCs [26-29] since they are inexpensive and present high electrocatalytic activity for the redox reaction of polysulfides, (Sx2-+2e-↔Sx-12-+S2-). CuS CEs generally present high Voc values but show low electrocatalytic activity, due to corrosion by sulfur and poor adhesion to the FTO substrate, which increases its resistance and affects its stability adversely. On the contrary, CoS based counter electrodes have shown up to now some of the highest FF and efficiencies in QDSSs, due to their remarkable electrocatalytic activity; nevertheless, they suffer from relatively low open circuit voltages [25]. The idea of optimizing CuS counter electrode by mixing it with CoS has therefore logically been raised in an attempt to combine the benefits of the two sulfides. Thus, QDSCs using CuS/CoS counter electrodes prepared following a complex multistep method involving several synthetic, deposition and heating steps, provided efficiency as high as 4.1% [8]. Such a complex procedure raises significant stability issues (related to air-drying at 100oC) that probably affect the device long term operation and life time.
In the present study, we investigate the synthesis of composite CoS-CuS counter electrodes, prepared by electrochemical deposition on FTO glass substrates. The composites electrodes were prepared utilizing mixtures of CoCl2·6H2O:CuCl2·2H2O and thiourea as the sulfur source. The electrocatalytic activity of the films was optimized by varying the precursors concentration and the electrodeposition time. Our one-step approach lacks complexity and avoids additional need of post thermal treatment. The CEs were tested in QDSSCs, demonstrating high power conversion efficiency, exceeding 5% under one sun illumination and leaving good perspectives for significantly higher solar cell performances. 2. Experimental 2.1 Preparation of the QD sensitized TiO2 photoanodes The titania film electrodes were deposited on patterned FTO electrode and consisted of two layers: a compact layer at the bottom and a mesoporous at the upper part. Details of their preparation are given in the supporting information S1. CdS-ZnS quantum dots were deposited on the nc-TiO2 films by Successive Ionic Layer Adsorption and Reaction (SILAR method) [30], as described in S2. The CdSe QDs were deposited on the above CdS-ZnS core layers, using a modification of the Chemical Bath Deposition (CBD) technique, described in the literature [31] (see S3). Finally, a passivation ZnS layer was added on the top of the CdSe layer using 2 SILAR cycles [32] (see S4). 2.2 Electrodeposition of the counter electrodes Initially, CoS loaded FTO electrodes were prepared by the electrochemical deposition method [33]. The deposition bath, was an aqueous solution (80 ml) containing (from 5 to 50mM) cobalt (II) chloride hexahydrate (CoCl2·6H2O) and 150mM thiourea (CH4N2S). The deposition was performed at room temperature in a one-compartment-
glass cell using a three-electrode configuration. The substrate was a 4cm2 FTO conductive glass cleaned prior to its use in ultrasounds, using soap, acetone and ethanol. A Pt foil served as the counter electrode and an Ag/AgCl/KCl 3M electrode was used as the reference. The electrodeposition was performed by applying a potential of -0.8 V vs. Ag/AgCl for 30 min. When an electrolytic bath containing 50mM cobalt (II) chloride hexahydrate was employed, 0.043 mg/cm2 CoS loading was obtained on the film, which led to optimum combination of electrocatalytic activity and visible light transmittance. In order to further improve the counter electrodes, an amount of CuCl2·2H2O was added to the aqueous solutions of CoCl2·6H2O, keeping the thiourea content constant (150mM). Thus, the CoS-CuS composites were electrochemically deposited by varying the (CoCl2·6H2O):(CuCl2·2H2O) metal precursors concentration (C) [from 50:5mM, to 100:10mM and to 150:15 mM] and modifying the deposition time (t) (from 30 min to 60 min). Table 1 presents the different combinations of C and t employed for the preparation of the CoS-CuS composite films. Detailed characterization of the composite films was done for the 2C-1t samples, which presented the best performance in QDSSCs. For comparison, CuS loaded FTO electrodes were similarly prepared by electrodeposition, using 5mM of CuCl2·2H2O as the Cu precursor keeping the thiourea concentration 150mM. Cu2S/brass electrodes (the most common counter electrode used in Quantum Dot Sensitized Solar Cells), were also prepared and examined in QDSSCs. The brass foil substrates were immersed into 37% HCl at 80°C for 10 min, then rinsed with water and dried in air. In turn, the etched brass foils were dipped into 2M S and 2M Na2S aqueous solution, to form black Cu2S counter electrodes.
2.3 Cell assembly The photoanode and the counter electrode were assembled in a sandwich-like cell made by Teflon. The two electrodes were kept in a distance of about 2 cm. This configuration, where the electrodes are kept far apart, has been proved to be favorable for QDSSCs by increasing the effective electrocatalyst/electrolyte interface [34]. The space inside the cell was filled with 4ml of the aqueous polysulfide electrolyte containing 2.0 mol L-1 Na2S·9H2O, 2.0 mol L-1 S and 0.2 mol L-1 KCl. The solution was prepared by continuous stirring of the mixture at 70-80 oC for about 20 min. 2.4 Characterization Techniques A PHILIPS Quanta Inspect Scanning Electron Microscope with energy dispersive Xray analysis (SEM-EDX) was used to investigate the morphology and the chemical stoichiometry of the photoelectrodes and the counter electrodes. Atomic Force Microscope (AFM) used to determine the roughness of the composite CoS-CuS electrode with a Digital Instruments Nanoscope III atomic force microscope, operating in the tapping mode. The adhesion of the CoS-CuS films on the FTO substrate was assessed using the ARDGO Model P-A-T apparatus, following the ASTM D-3359 standard. Raman analysis was performed in backscattering configuration on a Renishaw in Via Reflex microscope using a diode laser (λ=514.4 nm) as excitation source. The laser beam was focused onto the samples by means of a 50× objective, while the laser power density was kept at low levels to avoid local heating of the samples. Core-level X-ray photoemission (XPS) spectra were collected with a PHOIBOS 100 (SPECS) hemispherical analyzer at a pass energy of 7 eV with a Mg X-ray source (1253.6 eV). The take-off angle was set at 37° relative to the sample surface. The binding energy scale was calibrated using the position of both the Au 4f7/2 and Ag 3d5/2 peaks at 84 and 368.3 eV, respectively measured on clean
gold and silver foils. UV-Visible diffuse reflectance spectra were recorded in a Hitachi 3010 spectrophotometer equipped with an integrating sphere and reflectance data were transformed into Kubelka-Munk units. Electrochemical impedance spectra (EIS) were recorded on symmetrical dummy cells, (two identical counter electrodes that enclose the electrolyte), using the Autolab PGSTAT-30 potentiostat (Ecochemie), equipped with a frequency response analyzer (FRA). Spectra were recorded in the dark over a frequency range of 100 kHz to 10 mHz and were fitted with the FRA software provided by the Autolab in terms of appropriate equivalent circuits. Linear sweep voltammograms of the dummy cells were also obtained at a scan rate of 20 mVs-1. Current density–voltage (j–V) measurements of the QDSSCs were performed by illuminating the QDSSCs using solar simulated light (1 sun, 100 mW cm− 2) emitted from a 300 W Xe source and filtered at AM 1.5G using appropriate optical filters (Oriel). The active area was set at 0.152 cm2, using a black mask in front of the cells in order to avoid any light peeping inside the cell. The j–V characteristics were recorded by linear sweep voltammetry (LSV) on the Autolab potentiostat working in a two-electrode mode, at a scan rate of 20 mVs-1. 3. Results and Discussion 3.1Characterization of the photoelectrode The prepared sensitized photoelectrodes were first characterized by scanning electron microscopy (SEM). A characteristic top view image of the photoanodes is presented in Fig.1a. The surface consists of aggregated nanoparticles, which are predominantly spherical in shape with diameters from 65 to 110 nm, whereas the QDs are not distinguished from the titania nanoparticles under the examined SEM image
magnifications. These well packed spheres comprise a dense layer of 9 μm thickness, as it can be calculated from the cross section image shown as inset at Fig.1a. To assess the adhesion of the CoS-CuS films on the FTO substrate, the ASTM D3359 standard was used. The method consists in applying and removing pressuresensitive tape over cuts made in the coating. The performed analysis (Figure S.0 in Supporting Information) has shown that the surface of cross-cut area from which flacking has occurred present classification 3, confirming that the adhesion of the composite coatings is at a generally adequate level. The prepared ZnS/CdSe/CdS-ZnS/TiO2 photoelectrodes were further structurally characterized by Raman spectroscopy as shown in Fig.1b. The spectra present a strong band at 143 cm-1 attributed to the TiO2 predominant anatase phase. The narrow band at 207 cm-1 corresponds to the first-order LO CdSe phonon and presents an asymmetric broadening at the low-frequency side which can be well accounted by the contribution of a surface optical Raman mode at 197 cm−1. A CdS LO mode appears at 300 cm-1. Additionally, a CdSxSe1-x mode was observed at 279 cm-1, attributed to the mixed CdSxSe1-x interfacial layer formed between CdSe and CdS QDs. At higher frequencies, weak and broad modes emerge (e.x. 410 cm-1 - 2nd order CdSe mode) [35]. The QD bandgaps were estimated by UV-vis absorption spectroscopic data shown in Fig.1c. Titanium dioxide is a wide bandgap semiconductor with energy gap equal to 3.2 eV that it is unable to absorb any visible light on its own. Sensitization of TiO2 with CdS 75%-ZnS 25% quantum dots permits the extension of the absorbance up to 500 nm corresponding to 2.48 eV. This value approaches that observed in the bulk CdS, which means that the size of largest CdS QDs is nearly twice the CdS exciton Borr radius, that is about 6 nm for CdS [35]. Subsequent further sensitization with
CdSe extends the absorption up to 650nm corresponding to 1.91 eV film and allows an almost complete absorption of the visible light. In this case, the absorption threshold energy of CdSe is considerably higher than that of the bulk material and permits the estimation of the size of the largest CdSe QDs on the films in between 4.5 to 5 nm [36]. Furthermore, the core cell structure permits the efficient separation of the photoexcited carriers at the interface of the two sulfides and allows their easy injection to the TiO2 conduction band. Ahead, by the presence of the CdS core QDs we skip the difficulties of the direct deposition of CdSe on TiO2 [5]. In summary, the structural and optical characteristics of the sensitized films are very promising to support its efficient operation as photoanodes in solar cells. 3.2 Characterization of the counter metal sulfide electrodes 3.2.1 Morphological Characterization In order to understand the properties of the synthesized composite CoS-CuS counter electrodes (which, to the best of our knowledge, are studied for the first time), the surface morphology of the electrochemically CuS and CoS counter electrodes prepared separately on FTO substrate was first examined by scanning electron microscopy SEM and their stoichiometry with energy dispersive X-ray (EDX) spectroscopy. Fig.2 displays top surface SEM images of the CoS, CuS, and CoS-CuS electrodes developed on the FTO-glass substrates. Regarding the CoS electrode, one of the most complicated metal sulphide systems because of different phases and chemical compositions [37], Fig.2 a,b reveal a particular surface morphology comprised of crystalline rods. The atomic ratio between Co and S is estimated to 1:2 indicative of predominant formation of CoS 2 structure.
From the SEM images at low magnifications in Fig.2c, it is shown that CuS is evaluated as sponge like microspheres randomly distributed across the FTO glass substrate. The mean diameter of the hemispheres is around 3 μm. High magnification images show the formation of large particles in nanoplate form. The presence of Cu and S was confirmed from the EDX analysis with atomic ratio of 1.5:1, suggesting mixed copper valence. The surface morphology of the composite CoS-CuS (Fig.2 e,f) electrochemically synthesized counter electrodes takes the form of an extensive porous structure. The morphology of the CuS electrodes with the clustering in large spheres collapses, resulting in their segmentation/agglomeration in numerous small dense aggregates supporting a grid of nanowires. In fact, the initial morphology of CoS is severely influenced during the formation of the composite film. The crystalline rods have been altered by a honeycomb porous network comprised of nanowires with a mean diameter of 20 nm. Detailed EDX analysis on the white nanoflake area (A) in Fig. 2e shows that the ratio between Cu:Co:S is 1:2:3.5, while on the gray honeycomb type area (B) the Cu content is severely reduced and the corresponding atomic ratio becomes 0.5:2:3.5, verifying the CoS nature of the nanowires. It is remarkable that the composite films present such unique porous morphology and their composition shows considerable excess of CoS. This comes in accordance to the literature, which reports versatile morphologies and elemental compositions of sulfide films depending on the ratio and the type of copper, cobalt and sulfur sources as well as on the applied deposition method [38]. It is obvious that in the mixed product, the atomic ratio between Cu:S and Co:S differs from the corresponding values in the single CuS and CoS films. The above discrepancy is due to the different
concentrations of the salts during the electrodeposition and also to the various discharge potential of the ions on the FTO during the electrophoretic deposition [38]. Subsequently, the surface roughness of the CoS-CuS 2C-1t counter electrodes was investigated by atomic force microscopy (AFM). AFM images at 20x20 μm area (Fig.3) show rough surface morphology which resembles that observed by SEM. From the analysis of the AFM images the average as well as the root mean square roughness of the surface is estimated to be 500 and 415 nm, respectively, verifying the rough characteristics of the films appropriate for efficient electrocatalytic reactions. 3.2.2 Structural characterization The various sulfide electrodes were structurally characterized by Raman spectroscopy. Characteristic spectra are shown in Fig. 4. Copper sulfide presents a broad weak feature at 265 cm-1 as well as a relatively narrow asymmetric Raman band with peak maximum at 473.5±0.5 cm-1. The observed Raman spectra are attributed to mixed CuS and Cu2S, since the two copper sulfides have similar Raman profiles with main frequency bands which peak very closely, at frequencies of 474 and 472 cm-1, correspondingly [39]. The CoS peak shows a broad Raman feature which covers the range of 240-370 cm-1 and is deconvoluted to two broad lines which peak at 285 and 340 cm-1 as well as a second band which covers the 400-520 cm-1 range also well fitted by two lines at 445 and 485 cm-1. The low frequency Raman feature resembles that observed in the amorphous CoS2 film and is tentatively attributed to Co−S vibrations, while the high frequency one can be correspondingly assigned to S-S vibrations [40]. The composite copper and cobalt sulfide counter electrodes show several Raman vibration peaks which at first glance, appear as superposition of the CoS and the CuS
ones. Thus, there is the relatively narrow 473.5 cm-1 peak which is directly related to the CuS as well as a low frequency broad feature (composed again by two lines at 265 and 315 cm-1) and a high frequency broad shoulder (at 430 cm-1) which very much resemble the corresponding ones from the CoS bare films, slightly red shifted in frequency. The missing of the high frequency mode of the pure CoS film from the spectrum of the composite film and the considerable shifting of the other modes is justified by the substantial alternations in the structural and morphological characteristics of CoS. To complete the structural characterization (micro-Raman results), we investigated the surface chemical composition (binding states of cobalt, copper and sulfur) of the CoS-CuS composites by performing XPS analysis. Fig. 5a shows the Co 2p XPS spectrum exhibiting the well separated spin-orbit components Co 2p3/2 and Co 2p1/2. In particular, the Co 2p3/2 peak is characterized by a dominant contribution at 781 eV assigned to CoS formation as well as a broad peak at higher binding energies (about 787 eV) that can also be originated from a shake-up satellite of CoS, both indicating that that the Co ion was in the form of the Co2+ state [41]. In addition, very little amount of the metallic Co at 778.5 eV exists. It should be noted that the shakeup satellite peak could also be contributed to the presence of CoO species that are being formed upon sample exposure to the atmosphere [42]. In fact, cobalt ion has a very strong affinity to oxygen and it is difficult to exclude it impurities from the resultant materials [43]. Concerning the Cu 2p XPS spectrum (Fig. 5b), two main peaks located at 932.8 and 952.7 eV were observed, assigned to Cu 2p3/2 and Cu 2p1/2, respectively. In addition, there are weak satellite peaks at around 943 eV, suggesting the presence of the paramagnetic chemical state of Cu2+ [44,45]. The corresponding XPS spectrum of
S 2p for the CoS-CuS composites (shown in Fig. 5c) presents the S 2p3/2 and S 2p1/2 peaks centered at 162.2 and 163.3 eV, respectively, indicating that the S species exist as S2- in the deposits [46,42]. The XPS results support the findings from the Raman and EDS analyses and indicate most definitely that CoS-CuS composites were successfully deposited onto the FTO surface via a facile electrodeposition method. 3.2.3 Electrochemical Characterization After their incorporation in symmetrical dummy cells, the fabricated counter electrodes were characterized via Tafel polarization measurements in order to evaluate their catalytic activity towards the employed (S2-/Sx2-) redox couple [47]. Thus, identical electrodes from each one of the categories: CuS, CoS and CoS-CuS, were used to enclose the S/Na2S 2M electrolyte in between them. Fig. 6 presents the obtained Tafel-polarization curves (log (j) vs Applied potential) of the CoS (5mM), CoS (50mM) and CoS-CuS 2C-1t – based symmetrical dummy cells. Through the Tafel curves, the exchange current density (jo) can be estimated by the intercepts of the tangent lines to the anodic and cathodic branches, in the Tafel zone. From the analysis between the CoS-based cells, the ones with the highest concentration of CoCl2 (50mM) hold slightly higher jo values. Above all, the CoSCuS 2C-1t symmetrical dummy cells present the largest jo (4.7 mA cm-2 is estimated from the intercept in Fig. 6), showing the best electrocatalytic performance as they promote faster reduction of the polysulfide species [47,48]. Symmetrical dummy cells of the structure CuS/electrolyte/CuS were also constructed. The corresponding Tafel curves (not shown) present high instability and were not reproducible, due to the fast corrosion of the CuS electrodes in the polysulfide electrolyte, in agreement with the literature [28].
The above results were confirmed by electrochemical impedance spectroscopy (EIS), which was also applied to the symmetric dummy cells under dark conditions at zero bias. The interfaces of the symmetric cells were simulated by an equivalent electrical circuit (Fig. 7a), which consists of a series resistance (RS,S) at high frequency and a semicircle at the middle frequency region corresponding to the electronic transport resistance (RCT,S) and the double layer capacitance (Qdl) at the counter electrode/electrolyte interface [49]. The RS,S is the sum of the ohmic resistances in the circuit at which the first arc starts (includes the resistances of the two FTO substrates and of the CE) and is not strongly affected by the kind of the counter electrode, as it is also confirmed by the corresponding data depicted in Fig. 7b. On the other side, it is evident from the results that the RCT,S values, which account for the electrocatalytic activity of the counter electrodes, present important changes for the different films. The CoS electrode (5 mM) presents a high electronic resistance RCT,S at the counter electrode/electrolyte interface, equal to 124 Ohm cm2, which is decreased by about 60% to 49 Ohm cm2 for higher CoS concentration (50 mM). The RCT,S of the CuS electrode has an average value 10 Ohm cm2 showing its great electrocatalytic activity comparing to CoS electrode. This value is not indicative because of the fast corrosion of the CuS electrode towards the polysulfide electrolyte. The best performance is attained for the CoS-CuS 2C-1t loaded electrodes, which show the minimum RCT,S, equal to 3.04 Ohm cm2. Such low RCT,S resistance is one of the best in the literature [8, 25, 28, 47, 48] and can be applied in equation (1), in order to estimate the exchange current density [33].
Jo
RT nFR CT,S
(1),
where R is the gas constant, T is the absolute temperature, n is the number of electrons exchanged in the reaction (in our case n=2) and F is the Faraday constant. From eq. 1, a jo value of 4.2 mA cm-2 is extracted which is also in relative agreement with that obtained by the Tafel plots. Such high jo values, combined with the large electrode spacing of our QDSSC design, secure easy electronic transport from the electrolyte to the counter electrode. This is indispensable for efficient counter electrodes, which are able to attain high photocurrents at 1 sun. 3.3 Photovoltaic Performance of the QDSSCs Quantum-dot solar cells of the type: FTO/TiO2/core-shell QDs//electrolyte//CE were constructed by assembling the ZnS/CdSe/CdS-ZnS/TiO2 photoelectrodes, the newly prepared counter electrodes and the S2-/Sx2- redox couple. The cells were evaluated under one sun illumination conditions (AM 1.5G, 100 mW cm− 2). The obtained j-V curves are shown in Fig. 8 and the corresponding parameters are summarized in Table 2. In a first step, we compared cells having either single sulfide CE (the references CuS and CoS and the optimized CoS prepared with 50 mM) or the composite CoS-CuS (1C-1t) CEs. For these cells (Fig. 8a), the highest conversion efficiency (3.14%) was achieved with CoS-CuS composite counter electrode, which also demonstrates the best Voc values. Subsequently, the CoS-CuS composite CEs were further improved by optimizing the duration of the electrodeposition and the concentration of salts (according to the Table 1 given in the experimental part). Τhe characteristic parameters and the performance of the above solar cells are presented in Fig. 8b and Table 2. It is apparent that the CoS-CuS counter electrodes work effectively even with low concentrations of the salts (1C-1t), while the increase of the deposition time enhances the power conversion efficiency of the cell (from 3.14 to 5.03 %), mainly due to the
higher attained FF values (Fig. 8b). It is also evident that with the increase of the CoS:CuS concentration of the salts in the electrolytic solution from 50:5 to 100:10, all the characteristic parameters of the solar cell are improved leading to increase of the overall efficiency by 60%. Thus, the highest power conversion efficiency was achieved with the concentration ratio of CoS-CuS equal to 100:10mM, while further increase of salt concentration (150:15mM) leads to lower values of all the cell characteristics. The optimum concentration of 100 mM CoS and 10 mM CuS in terms of maximizing the corresponding QDSSCs efficiency is justified by the easier regeneration of the S2-/Sx2- redox couple at the corresponding interface, verified by the LSV results. Comparison of Photocurrent Density vs time for QDSCs based on the CuS and CoS-CuS counter electrodes confirmed that those based on the composite CoS-CuS CEs not only work more efficiently than those corresponding to CuS but also present significantly higher stability (Fig. S.3, Supporting Information). In fact, the photocurrent density of the cells employed the CuS counter electrodes drops from 13.97mAcm-2 to 0.7mAcm-2 during the first 4 hours, while those using the mixed CoS-CuS counter electrodes were stable for several hours. Finally, regarding the deposition time, it is important to note that the increase from 30 to 60 minutes slightly affects the electrical parameters of the solar cells. Finally, the optimized CoS-CuS counter electrodes were compared with the performance of the Cu2S counter electrode (widely used in the literature), prepared by chemical reaction on a sheet of brass. The results are also included in Table 2. The Cu2S counter electrode shows higher Voc value (0.59V) than the composite films. On the contrary, the CoS-CuS counter electrodes present considerably larger jsc and much higher FF, resulting to over 25% relative increase in efficiency.
The above FF values obtained from the j-V characteristic diagrams comply well with the
obtained
electrochemical
characterization
results
from
the
various
photoelectrodes. Thus, the CoS-CuS counter electrodes which show the best catalytic activity presenting the highest exchange current densities (jo) and the lowest charge transfer resistance (RCT,S) achieved the highest photocurrents (jsc=16.09 mA cm-2) and fill factors FF (0.58) when used in QDSSCs. The outstanding electrocatalytic activity of the composite counter electrodes cells are particularly favored from their extended nanoporous structure generated by CuS agglomerates supporting a network of CoS nanowires whereas moreover, they preserve sufficiently high open circuit voltages. Consequently, the power conversion efficiencies obtained with solar cells shaving the CoS-CuS electrodes excels over those from the cells based on standard Cu2S and is among the largest values ever reported for this type of QDSSCs. It is important to note that this work mainly focuses on the development and characterization of the composite counter electrode, leaving apart all other cell components. To valorize further our materials, quantum dot solar cells are being developed, incorporating the novel CEs in combination with more appropriate photoanodes. Work is now in progress to optimize the multilayered titania substrate (including an additional scattering layer) and fine tune the QDs deposition procedure (control of both size and deposition rate). The first results (summarized in Figure S.2) are very promising and account for the innovative character of our composite CoSCuS electrodes. In fact, the efficiency goes to 5.41 % when the thickness of the mesoporous layer was increased to 18.3 µm and a scattering layer (of 11 µm) was added. In addition, using these electrodes and fine tuning the CdSe QDs deposition procedure (control of both size and deposition rate), power conversion efficiencies as high as 7.5% were obtained (Figure S.2, Supporting Information). It is important to
notice that the optimized QDSCs using the CoS-CuS CEs present high stability and the jSC values remain constant (and above 16 mA cm-2) for more than 70 hours (Fig. S.4, Supporting Information). The above behavior was further supported by Electrochemical Impedance Spectroscopy (Nyquist plots in Figure S.5, Supporting Information), confirming reduced recombination at the photoelectrode /electrolyte interface (in agreement with increased Voc values). 4. Conclusions CoS, CuS and mixed CoS-CuS counter electrodes (CEs) were prepared by one-step electrochemical deposition on FTO conducting glass and were employed as counter electrodes in the fabrication of TiO2 sensitized solar cells with CdS-ZnS/CdSe/ZnS core/shell quantum dots. The CoS-CuS composite counter electrodes show optimum electrocatalytic activity as shown by the electrochemical analysis and extended nanoporous and rough structure, which is engendered by the establishment of a composite network of CuS aggregates supporting a grid of CoS nanowires. Consequently, the cells based on composite CEs present the highest photocurrents and fill factor values as well as the lowest charge transfer resistances at the CEelectrolyte interface amongst all different prepared QDSSCs. Their supreme efficiencies obtained are above 5%, which are among the highest values reported for QDSSCs of this type and excel over those for cells based on standard Cu2S, by 25%. Current research in the preparation stage shows that the use of CoS-CuS counter electrodes in combination with optimized photoelectrodes further increase significantly the cell performance.
Acknowledgements The authors greatly acknowledge precious help and assistance from Dr. D. Tsoutsou (recording XPS spectra) and D.F. Tsoukleris (performing scratch tests) as well as financial support from FP7 European Union (Marie Curie Initial Training Network DESTINY/316494). Research has also been co-financed by the European Union (European Social Fund - ESF) and national funds through the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF) - Research Funded Project: EXCELLENCE-AdMatDSC / 1847. L.G. acknowledges scholarship from Special Account for Research (EPP) of the NTUA.
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Fig.1. Top view SEM image of the ZnS/CdSe/CdS-ZnS/TiO2 photoanode surface and cross section image as an inset (a); Raman spectrum of ZnS/CdSe/CdS-ZnS/TiO2. Bands marked as follows: sensitized photoelectrode TiO2 (in black), CdSe (in red), CdS (in blue), CdSSe (in magenta) (b); Absorption spectra (Kubelka-Munk units) of bare TiO2 photoanode, TiO2 film with CdS-ZnS layer and TiO2 film with ZnS/CdSe/CdS-ZnS layer (dashed curves are used to estimate the corresponding band gaps) (c).
Fig.2. SEM images of the CoS (a,b), CuS (c,d), and CoS-CuS 2C-1t (e,f) electrodes at two different magnifications. In Fig. 2e, typical areas rich of CuS (A) and CoS (B) are marked where from EDX data have been acquired.
Fig.3. 3D AFM image at 20 μm x 20 μm from the surface of the CoS-CuS 2C-1t electrode.
Fig.4. Raman spectra of the CuS, CoS and CoS-CuS 2C-1t films. Fitted components (dashed lines) and simulated curve (solid line) are shown for the composite film.
Fig.5. High resolution XPS spectra of Co 2p (a); Cu 2p (b); and S 2p (c); for the CoSCuS composite CEs.
Fig.6. Tafel-polarization curves on symmetrical dummy cells using the CoS and CoSCuS -2C-1t films (scan rate: 20 mV s-1). Dotted line is the tangent of the current in the Tafel zone extrapolated to estimate the exchange current density.
(a)
Fig.7. (a) Equivalent circuit employed for the symmetrical dummy cells; (b) Nyquist plots of the CoS- and CoS-CuS 2C-1t symmetrical dummy cells obtained at 0 V under dark conditions.
Fig.8. j-V curves of QDSSCs with different counter electrodes. (a) Comparison of CoS and CuS cells with the composite CoS-CuS (1C-1t), (b) Comparison of CoSCuS cells prepared with different concentration and deposition time.
Table 1. Composite CoS-CuS counter electrodes prepared by varying the (CoCl2·6H2O):(CuCl2·2H2O) metal precursor concentration (C) and the electrodeposition time (t). The concentration of thiourea was kept constant at 150 mM. Sample
(CoCl2·6H2O):(CuCl2·2H2O)
Deposition time
CoS-CuS
C [C1(mM):C2(mM)]
t (min)
1C-1t
50:5
30
1C-2t
50:5
60
2C-1t
100:10
30
2C-2t
100:10
60
3C-1t
150:15
30
Table 2. Characteristic parameters of QDSSCs prepared with the composite CoSCuS counter electrodes. The corresponding performances using cobalt sulfide, copper sulfide and Cu2S-Brass CEs are also reported, for comparison. Voc (V)
FF
η (%)
12.13
0.45
0.47
2.54
CoS(50mM)
13.87
0.43
0.51
3.03
CuS(5mM)
13.97
0.49
0.42
2.85
CoS-CuS 1C-1t
13.56
0.49
0.47
3.14
CoS-CuS 1C-2t
13.51
0.49
0.54
3.57
CoS-CuS 2C-1t
16.09
0.54
0.58
5.03
CoS-CuS 2C-2t
15.27
0.58
0.51
4.55
CoS-CuS 3C-1t
14.46
0.50
0.43
3.13
Cu2S-Brass
15.12
0.59
0.45
4.00
Counter Electrode (on FTO) CoS(5mM)
jsc (mA cm-2)