Novel high pressure exfoliated graphene-based semi-transparent

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Novel high pressure exfoliated graphene-based semitransparent stable DSSCs for building integrated photovoltaic Sivasankar Nemala, Purnendu Kartikay, Karanveer S. Aneja, Parag Bhargava, H. L. Mallika Bohm, Sivasambu Bohm, and Sudhanshu Mallick ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00254 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 19, 2018

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ACS Applied Energy Materials

Novel high pressure exfoliated graphene-based semi-transparent stable DSSCs for building integrated photovoltaic Siva Sankar Nemalaa, Purnendu Kartikaya, Karanveer S. Anejaa, Parag Bhargavaa, H. L. Mallika Bohmb, Sivasambu Bohma,b* and Sudhanshu Mallicka* a) Particulate Materials Laboratory, Department of Metallurgical Engineering & Materials Science, Indian Institute of Technology-Bombay, Mumbai, India-400076 b) Talga Technologies Ltd, 15-17 Cambridge Science Park, Milton Road, Cambridge CB4 2HY, UK Corresponding author e-mail address: *

[email protected] (S. Mallick), Telephone number: +91 22 2576 7641

*

[email protected](S. Bohm), Telephone number: +44 -1223 420416

ABSTRACT. Integrating dye-sensitized solar cell (DSSC) with building’s architecture is required for its commercialization. Coupling semi-transparent designer DSSC with window provides the dual benefit of providing daylighting and power generation. To achieve this, we report low-cost novel high-pressure exfoliation technique for graphene and utilizing it as a transparent counter electrode for fabrication of semi-transparent DSSC. By adopting environmental friendly and economic exfoliated graphene instead of conventional platinum, the overall device cost comes down. The electrocatalytic behavior of fabricated transparent

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graphene counter electrode was assessed using cyclic voltammetry, Tafel plot in symmetry cell configuration and impedance spectroscopy. We have fabricated DSSC with >70% transmittance in the visible spectrum, which gives promising power conversion efficiency of 3.19%. The fabricated cells were stable for more than 500 hours under constant illumination with no significant efficiency drop. Also, we have fabricated designer semi-transparent DSSCs using various symbols.

KEYWORDS: Solution-processed graphene; Electrochemical Exfoliation; Eco-friendly; Building integrated PV; Transparent; Stable counter electrode; DSSC Solar cell.

Introduction Dye-sensitized solar cells (DSSCs) have attracted enormous research interest in the recent past majorly owing to the low cost and abundant raw material, in addition to simple fabrication procedure. Over the past decade, significant efforts have been made to make DSSC more stable and economical1,2. Stability, efficiency, and low cost are the three parameters required for successful commercialization of DSSC. Andreas Hinsch et al. (2011) demonstrated a prototype of 6000 cm2 dye solar cell modules3. Furthermore, DSSC technology can easily be readily adapted for various common appliances like solar calculator, charger, bags, etc. Technically, DSSC is a photo-electrochemical cell which consists of a semiconductor photoanode (usually TiO2 or ZnO or SnO2) on which dye is adsorbed, a counter electrode typically platinum and iodide/triiodide based electrolyte. The detailed working principle of DSSC can be found elsewhere4. DSSCs are fabricated on FTO or ITO coated glass substrate. Therefore this technology is an ideal candidate for glazing products and can be integrated with the window glass, fenestration and vehicles window. Utilizing DSSCs in building integrated photovoltaic not only provides power, it also enhances aesthetical appearance. The primary requirements to use DSSCs in BIPV especially windows


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are transparency and colour. Transparency of photoanode of DSSC can be controlled by varying the size of TiO2 nanoparticles and the thickness of the film. The colour of the cell is mainly due to dye and liquid electrolyte (primarily yellow in the case of triiodide); hence the colour can be tuned by the desired selection of dye, which can be varied from red to black to purple etc. Coloured pigments of fruit and vegetables can also be used to obtain the desired color5. The primary challenge is the preparation of the transparent counter electrode (CE) with desired electrocatalytic activity. There have been few reports on transparent CE based on metal selenide6 and platinum nanoparticles7, but former being carcinogenic and synthesis time and cost associated with the later eliminates their utility for large-scale production and commercialization. Herein, we report novel high pressure exfoliated few-layer graphene (Ex. Gr) based transparent counter electrode for DSSC. The exfoliated graphene-based counter electrode has been used extensively in past by DSSC research community due to interesting properties of graphene. Graphene (sp2-bonded carbon)consists of layers of carbon in arranged in 2D honeycomb hexagonal lattice8. It has a high specific surface area (~2630 m2g−1)9, good electrical conductivity due to high electron mobility (~200,000 cm2V-1s-1)10 and excellent thermal conductivity (~5000 Wm−1K−1)9,11. Apart from all these remarkable properties graphene also shows exceptional optical transmittance ∼97.7%12 and has transparency over the entire solar spectrum, which provides a conduit for the transparent CEs. Graphite exfoliation for graphene production can be achieved by various techniques such as mechanical exfoliation13, thermal exfoliation14, electrochemical exfoliation15, liquid phase exfoliation16,17, microfluidization18,19.Solutionprocessed exfoliation technique for production of layered graphene provides a low-cost method to prepare inks, which can be directly utilized for printed electronics, transistors, sensors, photovoltaic devices etc19. We have adapted the novel high-pressure technique to exfoliate graphene. High-pressure exfoliation of graphene has not been reported in the past.



For this method we have used water as a solvent, making the process eco-friendly and economical. In this article, we report bulk production of graphene using our novel solution processed high-pressure exfoliation technique and fabrication semi-transparent DSSC with beautiful designs.

Materials and Method Materials: Natural electrochemical expanded graphite (10 µm, Talga Advance Materials GmbH) and surfactants were used described in Reference 20. Lithium Iodide (LiI, 99.9%), Iodine (I2, 99.99%), 4-terta butyl pyridine (4-tbp, 96%), Acetonitrile (99.8%), Lithium perchlorate (LiClO4, 99.99%) were obtained from Sigma Aldrich. Fluorine doped tin oxide on glass (FTO, sheet resistance 7 Ω/sq.), dye (N719) and surlyn spacer were procured from Dyesol. Ethanol was bought from ChangshuYangyuan chemicals, China. Synthesis method: In the typical high-pressure exfoliation method, high-quality graphite (Talga Advance Materials GmbH) was intercalated with salt ions (e,g, tetrabutylammonium sulfate or TBA) by soaking the salt it in a solution containing water and surfactants (this procedure and formulations are under patent). Then the mixture was subjected to bath sonicated for 30 min to get the homogeneous solution. Later this solution was left to soak for 10 days to get the expanded graphite solution by intercalation. The further expanded graphite solution was then sprayed (Graco Magnum 262805 X7 HiBoy Cart Airless Paint Sprayer) into a container at a high pressure of 2000 psi. This cycle was repeated three times. The solution was then subjected to bath sonication (30 min) and centrifugation (10,000 rpm for 30 min). The supernatant (containing the few-layer graphene) was separated for further processing.


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Preparation of counter electrode Initially, FTO substrates of size 2 cm x 2.5 cm were cut and cleaned using soap solution. Then the substrates were bath sonicated with DI water and ethanol sequentially for 15 min. Finally, to remove the existence of any residual organic species the cleaned FTO substrates were heated to 450 °C for 30 minutes. The as-prepared few-layer graphene solution was coated on pre-cleaned FTO substrates by dip coating process in a controlled manner. FTO substrates were dipped/withdrawal in the suspension with a speed of 50 mm min-1 and 10 min dipping time was maintained. Figure 1 (a) shows the few-layer graphene suspension and Figure 1 (b)& (c) show the dip-coating arrangement. After dipping process, the graphenecoated FTO substrates left over for 30 min in the ambient air then heated at 400°C for 1 hr under an inert atmosphere. After cooling down, the graphene-coated FTO substrates (were characterized and used for DSSC fabrication.

Figure 1: (a) Few-layer graphene suspension; (b) and (c) dip coating arrangement of graphene suspension for CE preparation. Photoanode preparation and DSSC assembly The TiO2 slurry was prepared as per our previous work21 was coated on the pre-cleaned FTO substrates by doctor blade technique, and then sintered at 450 °C for 1 h. The thickness of the



sintered TiO2 film was found to be 6 µm (after single coating) and the active area was 0.25 cm2. The sintered TiO2 films were immersed in a dye solution (0.3 mM of N719) for 24 hr in a dark place at room temperature to adsorb the dye molecules. This dye-sensitized TiO2 film (photoanode) was then rinsed with anhydrous ethanol to remove excess amount of dye and dried in moisture free air. Photoanodes were combined with transparent exfoliated graphene coated CEs by employing a surlyn spacer in between them. A drop of iodide/triiodide (I3-/I-) redox-electrolyte consisted of 0.5 M of LiI, 0.05 M of I2, 0.5 M of 4-tert butyl pyridine and 0.5 M of 1-butyl-3-methylimidazolium iodide (ionic liquid) in acetonitrile solution was injected into the gap between the electrodes17,22. Characterizations: Structural and crystallographic analysis of Ex. Gr was carried out by Xray diffractometer (PANanlytical X-ray diffractometer) with Cu Kα source at 1.54 Å, selective area diffraction using transmission electron microscopy (JEOL–JEM 2100F. The microstructure of exfoliated graphene coated films was studied using scanning electron microscopy (JEOL–JSM 7600F) and high-resolution transmission electron microscopy (JEOL–JEM 2100F). Raman analysis of exfoliated graphene was carried out using (HORIBA HR800-using argon laser, an excitation wavelength of 514.5 nm) to identify the vibrational modes corresponding to graphene and its defects. The transmission and absorption studies of exfoliated graphene-based CE and DSSCs were evaluated using UV–Vis spectrophotometer (Jasco, V–650, attached with an integrating sphere). Cyclic–voltammetry (CV) measurements were done using potentiostat Autolab PGSTAT302N electrochemical workstation system (Metrohm) and NEWPORT solar simulator, respectively. The CV curves of various CEswere recorded in a three-electrode system (Reference electrode–Ag/AgCl, auxiliary electrode–Pt and working electrode–exfoliated graphene/Pt/FTO) to understand the electrocatalytic behavior towards the reduction of I3- in the redox couple. The electrolyte solution contains0.01 M LiI, 0.001 M I2, and 0.1 M LiClO4 in acetonitrile and as the supporting


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electrolyte. The scan range for CV analysis was -0.6 V to 1.2 V at a scan rate of 100 mVs-1. Additionally, using the same symmetrical cell Tafel polarization measurements were performed (at the scan rate of 100 mVs-1) to verify the electrocatalytic activity of the prepared CE by knowing its exchange current density. The current density-voltage (J–V) measurements were performed using the solar simulator, which has a Xenon arc–lamp of 150 W and capable of one sun (AM 1.5, 100 mWcm-2) output. The voltage sweep during the J–V measurement was controlled by Keithley–2420 source meter.

Results and Discussion

Figure 2: (a) XRD and (b) Raman spectra of natural and exfoliated graphene Figure 2 (a) shows the XRD patterns of as prepared few-layer graphene (Ex. Gr) and natural graphite. It is observed from the graph, at 26.7° of 2θ value; a strong peak is appeared with high intensity and at 54.8 ° of 2θ value; a low-intensity. These peaks are corresponding to (002) and (004) planes respectively, confirming natural graphite consists of highly organized crystalline nature23,24. Whereas a larger peak broadening is observed around (002) peak with a little shift towards lower 2θ value for exfoliated graphene, which confirms the exfoliation of natural graphite into exfoliated graphene17,25.



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Further, Raman spectroscopy was carried out to investigate the quality and the probable presence of defects in prepared exfoliated graphene and is compared with natural graphite. Figure 2 (b) shows the Raman spectra of natural graphite and the exfoliated graphene, which consists of various bands namely D band, G band and 2D band related to various vibrational modes. The ‘G’ band comes from in–plane vibrational mode of sp2 carbon atoms associated to the E2g phonon. The ‘D’ band associated with the breathing mode of A1g symmetry of sp2 hybridized carbon rings. This ‘D’ band corresponds to the defects present in the graphene sheet. The ‘2D’ band arises due to second order Raman scattering process which signals at the double the frequency of ‘D’ band26,27. In case of natural graphite, this 2D band is asymmetric, after exfoliation process this band looks symmetric and slightly shifted towards the low wavenumber side (redshift), the ‘G’ band’s full width at half maxima is higher for Ex. Gr. These observations confirming the exfoliation of graphite successfully yielded the Ex. Gr through the high-pressure exfoliation process. The intensity ratio of D and G bands is evaluated as 0.72 for exfoliated graphene which is much higher than that of the natural graphite (0.06), which clearly features the exfoliation of graphite17,28. Various parameters of Raman spectra of Ex. Grin comparison with Natural graphite is tabulated in Table 1. Table 1: Raman spectra parameters of natural and exfoliated graphene. Sample

D band (cm-1) G band (cm-1) 2 D band (cm-1) I2D/IG ID/IG

NG

1350

1576

2717

0.42

0.06

Exfoliated graphene

1348

1575

2696

0.57

0.72


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Figure 3: (a) Exfoliated graphene solution and (b) Absorbance spectra of exfoliated graphene. It is essential to find out the concentration of dispersed exfoliated graphene in the prepared suspension. Using Beer-Lambert principle; (equation 1) the concentration can be evaluated by considering the optical absorption of as prepared exfoliated graphene suspension. Aλ = αλl c ------------------ (1) Where c is the concentration of the exfoliated graphene (gL-1), l is the distance that light passes through the suspension (m), and Aλ and αλ are the absorbance (a.u.) and optical absorption coefficient of the material(Lg-1m-1) at a particular wavelength λ (nm), respectively. The suspension of prepared Ex. Gr in water and its O.D curve are presented in Figure 3 (a) and (b) respectively. The suspension prepared is diluted to 10 times of its volume, to prevent the scattering losses and the corresponding corrections are made during calibration29. The single peak in the UV spectrum (Figure 3 (b)) is significant due to van Hove singularity and is in the UV-Visible region, arises from the linear dispersion of Dirac electrons30. The concentration of graphene in the dispersion prepared is calculated as 0.81 mg ml-1 using λ 660 nm - 2460 Lg-1m-1 31.



Figure 4: (a) FEG-SEM of exfoliated few layers graphene; (b) FEG-TEM of exfoliated few layers graphene; (c) SAED pattern of exfoliated few layers graphene and (d) HR-TEM image of exfoliated graphene showing few layers. The microstructures and morphological information of exfoliated graphene using electron microscopic studies are shown in Figure 4. Figure 4(a) shows the FEGSEM image of exfoliated graphene; it can be seen the sheets are randomly crumpled with a plane, the smooth and thin-layered structure having the lateral dimensions of few hundred nanometers28. This confirms the exfoliation of the natural graphite (highly well-ordered thick-layered structure) into exfoliated graphene by the high-pressure force. The morphological investigation by TEM study revealed the presence of graphene sheets as shown in Figure 4(b). The hexagonal lattice of exfoliated graphene can be observed by the selective area electron diffraction (SAED) pattern with (0-110) and (1-210) reflections as shown in Figure


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4(c). Further, to confirming the presence of few layers (around 3-5 layers) of graphene (Figure 4 (d)) in exfoliated graphene, HRTEM analysis was recorded. These results are well agreement with XRD results.

Figure 5: (a) Transmittance of bare and dip coated graphene FTO; (b) FE-SEM of exfoliated graphene coated FTO. The analyzed data for the FTO-1 (bare) and FTO-2 (Ex. Gr coated) samples were presented as wavelength Vs % of Transmittance as shown in Figure 5 (a). The figure indicates that in the wavelength range of 350–800 nm, the % of Transmittance is about 75–80% and 60-70% for FTO-1 and FTO-2 respectively. The corresponding photographic images for FTO-1 and FTO-2 are also shown in Figure 5(a), wherein Figure5 (b), the SEM micrographs of FTO-2 were presented. From the SEM images, it is seen that the graphene sheets are interconnected with each other.



Figure 6: Cyclic voltammograms of the Pt electrode, bare FTO electrode, and exfoliated graphene electrode The electrocatalytic behavior of prepared exfoliated graphene-based CE and standard Pt counter electrode was investigated using cyclic voltammetry (CV) with three electrode system. For comparison, CV with Bare was also performed. In the exfoliated graphene coated FTO, Pt-coated FTO, and bare FTO as working electrode; Ag/AgCl as a reference electrode and Pt as an auxiliary electrode. The cyclic voltammograms for std. Pt, bare FTO, and the exfoliated graphene-based electrode, at the scan rate of 100 mV s-1 in an electrolyte solution contains 0.01 M LiI, 0.001 M I2 and 0.1 M LiClO4 in acetonitrile are shown in Figure 6. The redox peaks in two parts can be explicitly observed from the CV of the exfoliated graphenebased electrode, which confirms its electro-catalytic nature towards I3-/I- redox reaction. The cathodic and anodic peaks play a significant role in the device performance where the cathodic peak is associated with the reduction reaction of I3-/I- redox couple regenerates the dye molecules in DSSC. Further, the electro-catalytic nature of different CEs is based on two parameters namely: higher cathodic peak current and peak-to-peak separation (Epp)32,33. For


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bare FTO, no oxidation and reduction peaks were observed confirming the absence of electrocatalytic activity towards the I3-/I- redox reaction. Based on Randles–Sevcik theory, which defines the effect of scan rate on the cathodic peak current Ipc, the relationship between bonding sites, the diffusion constant of iodide and the adsorption quantity of the reacting ion (I3-) in a CE are estimated using the following equations 2 and 334. Ipc1 = 268,000 n1.5 A C Dn0.5 v0.5 ----------------- (2) Ipc1 = [(nF)2v ψ] / RT----------------- (3) Where Ipc1 is the cathodic peak current, Dn is the diffusion coefficient, A is the electrode area, n is the number of electrons of reduction reaction, C represents the concentration of I3 and v is scan rate. F is the Faraday constant, R is the universal gas constant, T is the absolute temperature and ψ is adsorption quantity of the reacting ion (I3-). Table 2: Electrochemical parameters for various CE Jox1 (mAcm-2)

Jred1 (mAcm-2)

-0.179

1.508

-0.168

1.284

CEs

Eox1 (V)

Ered1 (V)

Pt

0.396

Ex. Gr

0.424

EPP (V)

Dn(cm2 s-1)

ψ(mol cm-2)

-1.399

0.575

3.4x10-5

2.53x10-10

-1.151

0.586

2.3x10-5

1.26x10-10

From the Table 2, it is seen that the lower value of cathodic peak current density (Jred1) and the equivalent value of Epp for exfoliated graphene with that of the Pt electrode, reveals the better electro-catalytic behavior exfoliated graphene towards I3-/I- redox reaction. This is attributed to the more availability of electro-catalytic sites for the favor of redox reaction in the exfoliated graphene and the sites arise from the defects that are present in the graphene sheets (as shown in Figure 2(b))17. The higher electrocatalytic activity of exfoliated



graphene-based CE is due to the fact that the lesser number of I3- ions are adsorbed on its surface when compared with the Pt CE (revealed from the lower value of adsorption quantity ψ).

Figure 7: Tafel polarization plot for exfoliated graphene, Pt and bare FTO counter electrodes using symmetrical cells. Using symmetry cell configuration, interfacial charge transfer properties of prepared CEs can be evaluated by Tafel polarization plots. Figure 7 shows the plot of logarithmic scale of current density as a function of voltage. Usually, Tafel polarizations consist of three different zones, such as a polarization zone at low overpotential (

Novel high pressure exfoliated graphene-based semi-transparent

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