High-Efficiency FTO-Free Counter Electrodes for Dye-Sensitized Solar

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High-Efficiency FTO-Free Counter Electrodes for Dye-Sensitized Solar Cells Based on Low-Pt-Doped Carbon Nanosheets Zico Alaia Akbar, Han-Ik Joh, Sungho Lee, and Sung-Yeon Jang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp510871c • Publication Date (Web): 09 Jan 2015 Downloaded from http://pubs.acs.org on January 14, 2015

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The Journal of Physical Chemistry

High-Efficiency FTO-Free Counter Electrodes for Dye-Sensitized Solar Cells Based on Low-Pt-Doped Carbon Nanosheets Zico Alaia Akbar,† Han-Ik Joh,‡ Sungho Lee,*,‡,§ Sung-Yeon Jang*,† †

‡

Department of Chemistry, Kookmin University, Seoul, 136-702, Korea

Carbon Convergence Materials Research Center, Korea Institute of Science and Technology, Wanju, Jeollabuk-do 565-905 Korea

§

Department of Nano Material engineering, University of Science and Technology, Daejeon 305350, Korea

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ACS Paragon Plus Environment


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ABSTRACT

Highly efficient fluorine doped tin oxide (FTO)-free counter electrodes (CEs) for dyesensitized solar cells (DSSCs) are developed by low amount doping of Pt on carbon nanosheet (CNS)-based charge-collecting electrodes. The low Pt-doped CNS-based CEs were prepared by simple deposition of Pt by a conventional method. The CNSs had a dual function as both highly conducting charge-collecting electrodes (sheet resistance of ~40 Ω/sq) and decent electrocatalytic CE layers for iodine reduction. Low Pt doping (using approximately 100 times less Pt than with conventional doping) on a CNS dramatically improved the electrocatalytic activity for the I3- reduction of the CNSs and the charge transfer resistance at CE/electrolyte interfaces, which was not possible using FTO. The performance of the low-Pt-doped CNS CEs was comparable to that of high (conventional)-Pt-doped FTO CEs. DSSCs using the low-Ptdoped CNS CEs showed a power conversion efficiency (PCE) of 7.56%, whereas those using high-Pt-doped CNS CEs showed a PCE of 8.05%. In contrast, DSSCs using low-Pt-doped FTO CEs showed a PCE of 4.50%, whereas those using high-Pt-doped FTO CEs showed a PCE of 8.21%. Pt/CNS is an intriguing CE material that can use 100 times less Pt than conventional Pt/FTO CEs, which suggests a useful strategy for reducing the fabrication cost of DSSCs.

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INTRODUCTION Dye-sensitized solar cells (DSSCs) are of great interest owing to their low cost, easy fabrication, and relatively high efficiency, and thus they are considered as next-generation photovoltaic devices. To fabricate DSSCs, dye-sensitized TiO2 nanoparticles are deposited on a transparent conductive oxide, fluorine-doped tin oxide (FTO) is used as the photoelectrode, and crystalline Pt-deposited FTO (Pt/FTO) is used as the conventional counter electrode (CE). The photoelectrode and CE are separated by a redox couple (for example I-/I3-)-containing electrolyte solution.1 The role of the CE is to transfer electrons to the redox media, which generates a sufficient number of I- species in the electrolyte, while simultaneously balancing the electron deficiency in the oxidized dye (dye regeneration process).2 Pt/FTO is considered a state-of-theart material for CEs because of its good catalytic activity and electrical conductivity. FTO has been considered a high-performance supporting electrode material because of its high electrical conductivity and rough surface, which provides sufficient Pt loading. Usually, Pt is deposited on FTO through vacuum deposition or pyrolysis of a Pt precursor. However, the use of Pt and FTO significantly increases the cost of materials, which has delayed the commercialization of DSSCs.3 There has been much effort to decrease the cost of CEs by either reducing or eliminating the use of Pt and/or developing alternative low-cost supporting electrodes.3 A range of carbon materials such as active carbon, carbon nanotubes, graphene,4-6 and their composites with conductive polymers7-8 have often been reported as alternative CE materials. These materials display reasonable electrocatalytic activity for the redox couples, and some studies have demonstrated activity comparable to that of the Pt/FTO CEs.5-6, 9-11 However, in those studies,

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FTO was still necessary in order to achieve high performance. Thus, the substitution of FTO with less expensive materials is one of the major challenges for reducing the cost of DSSCs. Low-cost carbon materials are abundant, and various synthetic methods have already been developed. These materials can potentially achieve good electrocatalytic activity and conductivity simultaneously, although both properties need to be improved much further.5, 12 The use of carbon materials as FTO-free CEs has frequently been demonstrated, and a few promising results have been published. Thick layers (20 µm) of activated carbon on graphite sheets,4 colloidal graphite particle layers (9 µm),12 vertically aligned carbon nanotubes,13 and graphene paper14 have been reported as FTO-free CEs. These carbon-based CEs have exhibited high electrical conductivity (surface sheet resistance values of 1.7 × 102–10-2 Ω/sq), and the power conversion efficiencies (PCEs) of the DSSCs have reached 2–6%. To further improve the performance of carbon-based FTO-free CEs, a balance between electrical conductivity and the electrocatalytic activity for the redox couple must be achieved. Recently, we developed highly conducting multilayer graphene-like carbon nanosheets (CNSs) using a polymeric carbon source (polyacrylonitrile, PAN). The conductivity of the CNS thin layers was as much as 1600 S cm-1, and the layers worked effectively as electrodes for organic field-effect transistors.15 This high conductivity demonstrates that these polymer-based CNSs have considerable potential as FTOfree CEs if further enhancement of the electrocatalytic activity can be achieved. Doping of Pt on carbon materials has been a very effective strategy for the enhancement of the electrocatalytic activity by reducing the overpotential for the I3- reduction.16-19 Our group recently synthesized Pt-doped graphene nanosheets in aqueous media, and the synthesized Pt/graphene nanohybrid on FTO showed performance comparable to that of Pt/FTO as CEs for DSSCs.20 This comparable CE performance of the Pt/graphene nanohybrid on FTO was achieved with a Pt


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loading of less than 10 wt% as compared to Pt/FTO. Even though the enhancement of electrocatalytic activity by the hybridization of Pt with carbon materials for DSSC CEs has been reported many times,19, 21-23 there are still few studies on the fabrication of efficient carbon-based FTO-free CEs with low amount Pt doping. In this study, we developed highly efficient FTO-free CEs for DSSCs by doping CNSs with low amounts of Pt. The CNSs were fabricated by carbonization of a precursor polymer film, oxidized PAN. The low-Pt-doped CNSs (Pt/CNS) were prepared by coating a conventional Pt precursor solution on CNS films followed by thermal treatment. The electrocatalytic properties and the performance of the Pt/CNS CEs were investigated as a function of Pt loading amount, and the values were compared to those of Pt/FTO CE counterparts. Although pristine CNSs were much inferior CE materials as compared to conventional Pt/FTO owing to their lower electrocatalytic activity, the minute loading of Pt on the CNSs significantly improved the electrocatalytic activity. The PCE of DSSCs using Pt/CNS CEs was as high as 8.05%, which is approximately two times higher than the PCE of DSSCs using pristine CNS CEs. This value is also comparable to that of optimized Pt/FTO-based DSSCs (8.21%). To the best of our knowledge, this is among the highest values reported for FTO-free CEs.13-14, 21, 24-27 The most intriguing observation was that the performance of the Pt/CNS CEs was maintained even when the amount of Pt doping was 100 times less than the conventional loading (PCE = 7.56%). However, the PCE of Pt/FTO CEs with the same amount of Pt doping was drastically degraded (PCE = 4.50%). Although the electrical conductivity (sheet resistance = 40 Ω/sq) and roughness (Rrms = 0.75 nm) of the CNSs were inferior to those of FTO (sheet resistance = 6-9 Ω/sq; Rrms = 23 nm)28 in terms of the charge-collecting electrodes, the performance of the Pt/CNS CEs was far superior to that of Pt/FTO when the amount of Pt doping was reduced. This intriguing result


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demonstrates a very efficient route for reducing the required Pt amount for the fabrication of CEs by using carbon materials to achieve nearly optimum performance, and this technique can lower the cost of DSSCs significantly.

EXPERIMENTAL SECTION Preparation of CNS CEs: Polyacrilonitrile (PAN)-based carbon nanosheets (CNSs) for use as a counter electrode were fabricated based on our previously reported method with some adjustments.15 A PAN (Aldrich, average MW = 150,000 g/mol) solution with a concentration of 8 wt% was prepared by dissolving PAN in N, N-dimethylformamide (DMF) (Fisher, HPLC grade). The PAN solution was spin coated onto bare quartz. The spun-coated PAN films were stabilized at 250 °C for 2 h under air, followed by carbonization at 1200 °C (unless otherwise described) in H2/Ar (10/90) atmosphere (flow rate: 2000 sccm). CNS films with different resistivity values were prepared by controlling the concentration of the PAN solution (5, 6 and 7 wt%), following the procedure described earlier. Fabrication of DSSCs: As the counter electrode, Pt/CNS CEs with various Pt amounts and Pt/FTO (Pilkington, TEC 8, 6-9 Ω/sq) CEs were prepared by the following procedure. The CNSs on quartz glass were drilled using a micro-drill and cleaned with ethanol for 10 min using an ultrasonicator. Pt precursor (H2PtCl6⋅H2O) solutions in DMF of various concentrations (5, 0.5, 0.05, and 0.0005 mM) were coated onto the cleaned CNS CEs, which are denoted as Pt/CNS-5, Pt/CNS-0.5, Pt/CNS-0.05, and Pt/CNS-0.0005. Following this, the Pt/CNS CEs were annealed at 400 °C for 20 min. Pristine CNS CEs were also fabricated for comparison. For the photoelectrode, a commercial paste consisting of 20-nm-sized TiO2 nanoparticles was used to produce a 9-µm-thick transparent TiO2 layer on a cleaned FTO glass by the doctor blade method.


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Adhesive tape (Scotch® Brand, 50 µm) was attached at two sides of FTO to control the thickness of the TiO2 layer prior to doctor blading. The transparent layer was then heated at 125 °C for 15 min. Following this, a commercial paste consisting of 500-nm-sized rutile TiO2 particles was coated onto the heated TiO2 transparent layer to form a 3-µm-thick scattering TiO2 layer. The photoelectrodes were then heated again at 125 °C for 15 min and then gradually annealed from 300 °C to 500 °C for 80 min using a programmable furnace. The annealed photoelectrodes were treated with 0.2 M TiCl4 solution for 30 min at 70 °C. After treatment with TiCl4, the photoelectrodes were annealed again for 30 min at 500 °C. N719 dye, Ru(dcbpy)2(NCS)2 (dcbpy = 2,2-bipyridyl-4,4-dicarboxylato), was used as a sensitizer. The photoelectrodes were immersed in a 0.3 mM dye solution containing a mixture of acetonitrile and tert-butyl alcohol (1:1) (v:v) for approximately 18 h and then dried at room temperature. The photoelectrodes and the prepared counter electrodes were then assembled together, separated by 60-µm-thick Surlyn® thermoplastic resin to form a sandwich-type configuration. Then an iodine-based organic electrolyte consisting of 0.65 M 1-methyl-3-butylimidazolium iodide (BMII), 0.1 M guanidiumthiocyanate

(GuSCN),

0.03

M

I2,

and

0.5

M

4-terbutylpyridine

in

acetonitrile/valeronitrile (85:15, vol%) was introduced into the cell via the drilled holes. Finally the holes were sealed with a thermal adhesive film and cover glass. Characterization: A Raman spectrometer (LabRAM HR UV-VIS-NIR, Horiba) with a laser excitation wavelength of 514.5 nm and a power of 16 mW was used to observe the graphitic structural development of the CNSs. To investigate the chemical bonds and relative amounts of the CNS elements, X-ray photoelectron spectroscopy (XPS) was employed (K-alpha, Thermo Scientific) using monochromated Al Kα (1486.6 eV) X-rays below a pressure of 3 × 10-7 Torr. The sheet resistance of each CNS CE was measured using four-point probes (FPP-RS8, Dasol


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Eng., Korea). An atomic force microscope (AFM, SPM Dimension 3100 with Nanoscope IIIA controller, Veeco) in tapping mode was employed to investigate the surface morphology and thickness of the CNSs. Cyclic voltammetry was conducted using Ag/Ag+ (AgNO3 10 mM), 0.1 M LiClO4 in acetonitrile as the reference electrode, and a Pt plate as the counter electrode. The active area was set to be ~1 cm2. The current density–voltage (J–V) characteristics of the DSSCs were extracted under 1 sun illumination (light intensity was adjusted using a Si solar cell equipped with a BK-5 filter for approximating AM 1.5G 100 mW·cm-2 light radiation intensity) using a Newport (USA) solar simulator (450 W Xe source) and a Keithley 2400 source meter. The active area of the DSSCs was set to be 0.175 cm2 using a black metal mask. The incidentphoton-to-current conversion efficiency (IPCE) spectra was measured as a function of wavelength from 300 nm to 800 nm using an IPCE Measurement system (McScience, K3100 IQX). Calibration was perform using calibrated silicon photodiode as a standard. The IPCE values were collected at a chopping frequency of 1 Hz. The electrochemical impedance spectroscopy (EIS) characteristics of the DSSCs were obtained using an impedance analyzer (IVIUM-Compactstat) under open-circuit conditions under AM 1.5G light illumination with a frequency ranging from 0.1 to 105 Hz. The EIS spectra obtained were then fitted using Z-Plot® software to gain each impedance parameter.

RESULTS AND DISCUSSION Figure 1 shows the fabrication methods for the CNS CEs and Pt/CNS CEs. The CNS CEs were fabricated by spin coating a solution of a polymeric carbon source, PAN, in dimethylformamide (DMF) on a quartz plate, followed by oxidation at 250 °C for 2 h under air atmosphere and carbonization at >800 °C under a H2/Ar gas mixture, as reported in our earlier papers.15 The


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resulting black shiny CNS CEs had smooth surfaces (root mean square roughness, RRMS, of 0.75 nm, as obtained from AFM, Figure S1) that were similar to those of the spin-coated precursor PAN films. The resulting CNSs were dominated by graphitic structures, as shown in the Raman spectra (Figure S2). The D peak at ~1350 cm-1, G peak at ~1600 cm-1, and a broad peak at ~2800 cm-1 (convolution of 2D, D+G, and 2D ) revealed sp2-bonded carbon networks with some amorphous portions (see the deconvoluted spectra in Figure S2). This structure resembled that of multilayered graphene, in which the calculated graphitic domain size from the intensity ratio of the D and G peaks was ~ 4 nm (Figure S2). The X-ray photoelectron spectroscopy (XPS) analysis revealed that the pristine CNSs were dominated by carbon networks, with small amounts of functional groups and heteroatoms (N and O) that originated from the oxidized PAN precursor (Figure S3). The Pt-loaded CEs (Pt/FTO CEs and Pt/CNS CEs) were prepared by a previously reported method with slight modification: 5 mM of a Pt precursor (H2PtCl6.H2O) solution in dimethylformamide (DMF) was spin cast onto FTO or CNSs, followed by thermal annealing at 400 °C for 20 min.29 Crystalline Pt nanoparticles were formed on the surfaces of the chargecollecting electrodes (FTO and CNS) (see the SEM images in Figure 2). The loading amount of Pt precursor was varied by controlling the concentrations of the Pt precursor solutions from 5 mM to 0.5 µM, as described in the experimental section. Since the Pt precursor was evenly coated on both the FTO and CNSs, the amount of Pt was considered to be reasonably proportional to the concentrations of the deposited Pt precursor solutions. Figure 2 shows SEM images of the Pt/CNS and Pt/FTO films. In the Pt/FTO samples, well-dispersed Pt nanoparticles were observed in the sample using the 5 mM Pt precursor solution (Figure 2a). However, Pt nanoparticles were not discernible in the sample using the much-diluted 0.05 mM precursor


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solution (Figure 2c). In the Pt/CNS films, Pt was found in a somewhat aggregated form (Figures 2b and d). We hypothesize that the high hydrophobicity of the CNS films induced incompatible surface energy with the Pt precursor solution in DMF, resulting in the Pt aggregation. When the concentration of the precursor solution was reduced, fewer Pt nanoparticles were observed; however, these nanoparticles were also in aggregated form. This may have reduced the total surface area of the Pt particles and thus reduced the catalytic activity. For further characterization of the Pt doping, X-ray photoelectron spectroscopy (XPS) analysis of the Pt-doped CEs was performed. The peaks at low energy levels of 70–75 eV confirmed the presence of Pt.30 As shown in Figure 3, there were two peaks, one at ~71 eV that corresponds to Pt4f7/2 and the other at ~75 eV that corresponds to Pt4f5/2 in both types of CEs. The intensity changes of Pt in the XPS spectra also indicate the proportionality between the Pt precursor load and the resulting amount of Pt on both charge-collecting electrodes. The peak intensities decreased nearly linearly as the Pt precursor concentration decreased in both the Pt/FTO and Pt/CNSs, and the Pt loading amounts of the resulting CEs were similar. The peak intensities of the samples using lower Pt precursor concentrations (0.05–0.0005 mM) were undetectable, which indicated that the Pt loading amount of those samples was indeed very little (

High-Efficiency FTO-Free Counter Electrodes for Dye-Sensitized Solar

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