A transparent and Pt-free all-carbon nanocomposite counter electrode catalyst for efficient dye sensitized solar cells

Solar Energy 193 (2019) 568–575

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A transparent and Pt-free all-carbon nanocomposite counter electrode catalyst for efficient dye sensitized solar cells

T

M. Gurulakshmia, A. Meenakshammaa, K. Susmithaa, N. Charanadharb, V.V.S.S. Srikanthb, ⁎ S. Narendra Babuc, Y.P. Venkata Subbaiaha, Katta Venkateswarlud, M. Raghavendera, a

Department of Physics, Yogi Vemana University, Kadapa 516005, India School of Engineering Sciences and Technology, University of Hyderabad, Hyderabad 500046, India c Department of Physics, Osmania University, Hyderabad 500007, India d Department of Chemistry, Yogi Vemana University, Kadapa 516005, India b

A R T I C LE I N FO

A B S T R A C T

Keywords: Cost effective composite counter electrode Transparent DSSC module Reduced graphene oxide SWCNH Dye sensitized solar cell

Recently, substantial focus has made in the development of cost effective Pt-free dye sensitized solar cells (DSSCs). In this article we report a potential, Pt-free, carbon based nanocomposite as counter electrode catalyst for dye sensitized solar cells. Graphene oxide (GO) was made into reduced graphene oxide (SSrGO) by irradiating with light (Xe source). Suspension of SSrGO in DMF was spray coated onto fluorine doped tin oxide (FTO) coated glass substrate, followed by spray coat of single walled carbon nanohorns (SWCNH) suspension in DMF. SSrGO and SWCNH coated FTO glass substrate was investigated as an effective Pt-free composite counter electrode (CE) for DSSC and also showed the comparable catalytic activity with Pt. The test cell reveals power conversion efficiency (PCE) of 8.27%, whereas the DSSC module exhibited a PCE of 5.18%. By using the fabricated DSSC module, an electric motor (15 mW) was operated both in indoor and outdoor light conditions. The procedures followed in this work pave a way for the easy fabrication of composite CE towards transparent DSSC modules and further the integration of the DSSC module with the standard Si solar cell is of great potential for various real-time application.

1. Introduction The Si based photovoltaic technology is well-established and successfully demonstrated in various practical applications. However, its environment pollution owing to hazardous by-products (Yassin et al., 2005) and expensive processing of crystalline Si has diverted the attention of many researchers towards the development of 3rd generation solar cells (Imalka et al., 2013) such as dye sensitized solar cell (DSSC), which is environmentally benign, cost-effective, fabrication-wise easy (O'Regan and Grätzel, 1991), and proven to exhibit 14% power conversion efficiency (PCE) (Mathew et al., 2014). DSSCs were successfully demonstrated in various emerging applications such as portable electronic gadgets and large area smart windows (http://gcell.com/casestudies/wireless-solar-keyboard; http://gcell.com/gcell-products/ custom-solar-cell). Moreover, simple and cost-effective methods, such as spray coating, spin coating and roll-to-roll fabrication, are used in the case of highly efficient DSSCs. These features prompted the researchers to pay a greater attention on the development of cost effective DSSC solar modules (Ahn et al., 2007; Ramasamy et al., 2007). Further, the

⁎

DSSC technology offers color options, transparency, flexibility, low weight, etc., and more importantly high efficiency values under diffused light and even at low illumination conditions (Susmitha et al., 2017; Cornaro and Andreotti, 2013) and hence facilitates its use in both indoor and outdoor applications (De Rossi et al., 2015; Yoon et al., 2011). A typical DSSC consists of dye anchored photo electrode (PE), counter electrode (CE), and an electrolyte (either in solid or liquid form) with redox couples. The CE plays a vital role in immediate dye regeneration through efficient catalytic activity and faster charge transfer. To achieve efficient and faster catalytic activity, the CE should exhibit high exchange current density, high specific surface area and low charge transfer resistance value. Generally, 15–20 nm thick Pt layer deposited onto a conductive FTO substrate is used as the CE. However, due to its high cost, limited availability, and degrading nature when it interacts with the I3− I − based liquid electrolyte over a period of time (Agresti et al., 2015), etc., the scientific community is in search of alternative materials to replace Pt. Many materials have been explored to replace the expensive Pt with carbon-based materials, conducting

Corresponding author. E-mail address: toraghavender@rediffmail.com (M. Raghavender).

https://doi.org/10.1016/j.solener.2019.09.081 Received 14 August 2019; Received in revised form 13 September 2019; Accepted 25 September 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.

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simulator (Peccell Inc., Japan, Model PECL01) for 4 h. Due to the irradiation, the GO was converted into solar simulator reduced graphene oxide (SSrGO). The localized energy from the focused solar radiation incident on the GO is found to be sufficient to not only reduce the oxygen containing species attached to GO to form SSrGO but also to exfoliate the GO layers. Later, the resultant suspension was centrifuged and dried at 150 °C to collect SSrGO. To understand the crystal structure of the samples, X-ray diffraction (XRD) was carried out with Rigaku diffractometer (Rigaku, Japan) in the 2θ range of 5–50°. Cu Kα xray source (λ = 1.54 Å) was used for the XRD measurements. Further, to understand the phase formation, Raman spectroscopic measurements were carried out in a backscattering geometry using Raman Spectrometer (Lab Ram, HORIBA JOBIN YVON) in the spectral region 450–3500 cm−1 using 633 nm LASER wavelength light. The spectral resolution of all the measurements is 1 cm−1. X-ray photoelectron spectroscopic (XPS) studies was carried out, for elemental analysis, at a scan rate of 1 eV/sec and 0.05 eV using ESCA SHIMADZU machine. Optical transmission studies were carried out, for transmittance measurement in the wavelength 400–800 nm region, using the UV–Vis-NIR Spectrophotometer (Carry 5000, Agilent Technologies).

polymers, polymer-carbon composites and metal chalcogenides. Importantly 2D nano materials such as graphene oxide (GO), molybdenum disulphide, boron nitride, etc., have attracted a great attention as lowcost CEs (in DSSCs) due to their quantum confinement yielding exotic electronic behavior and surface effects which could be very much essential in the energy conversion and storage applications (Coleman et al., 2011; Wang and Hu, 2012; Hou et al., 2013). Of all 2D materials, graphene has been widely explored due to its low cost, high electrical conductivity, large specific surface area, and good electrochemical stability. The use of graphene in the form of GO and reduced graphene oxide (rGO) as CEs in DSSCs has been on rise from the past few years. In the case of the rGO based CEs, the method of conversion of GO into rGO is a major concern owing to the use of harsh chemicals and unwanted defects in the resulting rGO and hence researchers switched to the chemical free reduction of GO to rGO (Yeh et al., 2014). Reports in this perspective are evidenced (Cote et al., 2009; Gilje et al., 2010) including those on photo-reduction of GO to rGO (Zhou et al., 2010; Strong et al., 2012; El-Kady et al., 2012; Cote et al., 2009). In one of our previous works, we have shown that natural sunlight could be used to reduce the GO to rGO, which was then used as a CE in DSSC to efficiently convert solar energy into electrical energy (Charanadhar et al., 2016). Recently, not only single carbon allotrope such as single wall carbon nanohorns (SWCNHs) (Costa et al., 2013) but also novel composites (Luo et al., 2018) constituted by carbon allotropes have been shown as exemplary CEs in DSSCs. On the similar lines, in this work, we elucidate the use of all-carbon (SWCNHs and a unique reduced-GO constituted) composite CE in DSSC, which exhibited a PCE of 8.27% at 1 Sun condition. The uniqueness of reduced-GO used in this work is in its preparation. It is prepared by reducing GO with the aid of light emanating from a solar simulator and abbreviated as SSrGO. The advantage of composites counter electrodes evidenced a relatively high catalytic activity and rapid reduction rate towards highly efficient DSSC performance (Chen and Shao, 2016; Velten et al., 2012). In the present work, SWCNH/SSrGO composite film was developed by using a simple spray coating onto cleaned FTO coated glass substrate, which served as a composite CE along with typical photo electrode for in small and large area transparent DSSCs. In this work the fabricated large area DSSC modules were also tested in real time application.

2.1.1. Counter electrode preparation Initially, fluorine doped tin oxide (FTO) coated glass substrates (Great cell solar, TEC7, 7 Ω/square) were cleaned in a mild detergent liquid solution and chemically treated with acetone, isopropanol, DI water and absolute ethanol in ultrasonicator to remove contaminants and residual organic species, and were placed on a pre-heated hot plate at 180 °C. GO, SSrGO and SWCNH (Carbonium, Italy) were individually dispersed in dimethylformamide (DMF) and ultrasonicated to obtain spray solutions. Later, the dispersion solutions were sprayed onto preheated FTO glass substrates to obtain desired CEs. The spray coating process using a micro-tip needle spray gun (dia of 0.3 mm) under the N2 gas ambience is depicted in Fig. 1(b). The solution spray dispersion was controlled and care was taken for uniform distribution over the FTO glass substrates. The deposited amount is ~1 mL/cm2 and the obtained CEs were further annealed at 180 °C for 30 min. Optical transmittance of ~65–90% was measured for GO, SSrGO, SWCNH and SWCNH/ SSrGO CEs (Fig. 1(c)). For photovoltaic performance comparison, the standard Pt based CEs were also prepared.

2. Experimental 2.2. Preparation of photo electrode and DSSC fabrication 2.1. Synthesis and characterization of GO and SSrGO 2.2.1. Photo electrode preparation FTO substrates were cleaned as mentioned earlier. Titanium dioxide (TiO2) compact layer was coated over the cleaned FTO glass substrate by spin coating method using 15–20 nm sized Ti-Nanoxide solution (Solaronix, T-L/SC) with 4000 rpm for 20 sec, and later heated to 180 °C for 10 min. Subsequently, TiO2 paste (Greatcell Solar Materials, 18 NR-

Initially, GO was synthesized from graphite flakes (300 mesh, Alfa Aesar) using a modified Hummer’s method (Hummers and Offeman, 1958) and then its suspension in DI water was prepared. The GO suspension was then irradiated with 100 mW/cm2 radiation (as shown in Fig. 1(a) and video-S1-synthesis (Supporting information)) using a solar

Fig. 1. (a) Reduction of GO by irradiating 100 mW/cm2 light, (b) preparation of CEs by spray deposition, (c) transmittance spectra of spray coated CEs. 569

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T) was screen printed to form 10–12 µm thick film followed by sintering at 500 °C for 30 min to obtain transparent TiO2 film. The compact layer treatment as mentioned above was repeated again and heated at ~180 °C. While cooling the substrates, at around 100 °C they were immersed in red dye (N719) solution (0.3 mM in absolute ethanol) under the dark conditions for 16 h. Finally, the electrodes were rinsed gently with absolute ethanol to remove any unanchored N719 dye molecules. 2.2.2. DSSC fabrication The DSSC test cells were fabricated by assembling the photo electrodes with the developed CEs by placing a thermal adhesive film (Solaronix, polymer melt film) that acts as a spacer in between the two electrodes followed by pressing gently over a hot plate heated to 110 °C. Few drops of acetonitrile (AcN) solvent based redox coupled modified high performance electrolyte solution (Solaronix, HI-30) was injected through a hole at rare side of CEs using vacuum back technique and finally the hole at CEs side was closed with the cover glass and adhesive film.

Fig. 2. (a) XRD patterns and (b) Raman spectra of graphite flakes, GO, SSrGO and SWCNH, respectively.

2.3. Characterization techniques

1345 cm−1 and the G band around 1582 cm−1. The G band corresponds to the graphite carbon structure. Further, G band is associated to the first order scattering of E2g phonon of sp2 C atoms at the Brillouin zone center. The D band is linked to typical defects endorsed to structural edge effects. It appears as the breathing mode of rings or k-point photons of A1g symmetry. The ratio of ID/IG together with the I2D/IG ratio quantifies the quality of the functional groups or disorder/defects in the sample. The ratio of intensities of D and G bands was found as ID/ IG = 0.90 for GO and ID/IG = 0.88 for SSrGO, and are in good agreement with the reported data (Amare et al., 2014). As the both GO and SSrGO showed similar ID/IG intensity ratios, I2D/IG were further measured to understand the quantity of defects in samples. The I2D/IG ratio for SSrGO is found to (0.79), which is higher than that of GO (0.45). The higher I2D/IG ratio in the case of SSrGO indicates smaller average size of sp2 domains and the restoration of carbon–carbon bonds after the removal the oxygenated functional groups (Kong et al., 2012). In other words, the SSrGO produced in the present study is not defective as a consequence of the reduction process. Moreover, the emergence of broad 2D peak at 2687 cm−1 in the case of SSrGO shows that there is a good recovery of the graphitic nature after reduction of GO. Raman spectrum of SWCNH has depicted the standard bands. ID/IG for SWCNH was 0.86, and the G-band with high intense energy modes is observed at 1350 cm−1 in addition to the D-band at 1580 cm−1 with low intensity.

To evaluate the electrocatalytic performance of the CEs, CyclicVoltammetry (C-V) was carried out in the three-electrode configuration in I3− I − redox solution. C-V studies are performed with an electrochemical workstation with standard Pt wire as the counter electrode, Ag/Ag+ electrode (0.1 M LiClO4 with Acetonitrile) as reference electrode in the high pure solution of acetonitrile that contains 10 mM of LiI, 0.1 M of LiClO4 and 1 mM of I2. The electrodes for C-V are prepared by spray coating of GO, SSrGO, SWCNH and SWCNH/SSrGO dispersed DMF solutions, following of immersing the CEs in the electrolyte solution for 5 min before starting the experiments and before the C-V scan, pure N2 gas is purged for obtaining accurate results. The Tafel polarization (at scan rate of 50 mV s−1) and electrochemical impedance spectroscopy (EIS) measurements were carried out for symmetrical dummy test cells composed of same catalysts on both electrodes (two similar counter electrodes) using an Compactstat.h workstation (10 mV amplitude and frequency range 1 Hz–1 MHz). Photocurrent density–Voltage (J-V) measurements of the DSSCs were conducted with a solar simulator (PEC-L01, Peccell Inc.) equipped with a spectral filter (AM 1.5) and a source meter (2401 N Keithley). The light of the solar simulator was adjusted to one sun intensity of 100 mW/cm2, a mask of 0.16 cm2 is used and consider it as an active area of the test cells. 3. Results and discussion 3.1. Structural and phase analysis

3.2. XPS analysis Fig. 2(a) shows the XRD patterns of graphite flakes, GO, SSrGO and SWCNH. The XRD pattern of graphite flakes exhibited high intensity peak at 2θ ~ 26.5° corresponding to (0 0 2) plane confirming the high degree crystalline structure for graphite flakes. In the XRD pattern of GO, a broad diffraction peak at 2θ = 10.40°, and a small peak at 2θ = 42.50° was noticed. The high intensity peak at 2θ = 10.40° corresponds to the (0 0 1) plane of GO with inter layer spacing of 0.83 nm and the weak peak at 42.5° shows the presence of a turbostratic band of disordered carbon materials. The increase in inter layer spacing was due to the oxygen containing functional groups between graphene layers (Paranthaman et al., 2018) After irradiation treatment using solar simulator, a new (0 0 2) peak at 2θ = 25.2° with lower intensity was noticed, which confirms the reduction of GO (Abdul et al., 2017). The XRD pattern of SWCNH depicts standard diffraction peaks. Fig. 2(b) shows the Raman spectra for GO, SSrGO and SWCNH. These spectra could further support the structural changes and defects in the reduction of GO into SSrGO. In the present study, both GO and SSrGO showed two strong representative peaks indexing to D band at

XPS analysis was performed on samples to understand the exact mechanism of how GO was reduced, when it’s suspension in DI water is irradiated with the light of solar simulator. The XPS survey spectrum (Supporting information, Fig. S1) clearly indicates the presence of oxygen along with carbon in both GO and SSrGO and no other impurities are detected. The characteristic BE peaks at ~286.0 and ~534.0 eV can be assigned to C1s and O1s. The carbon and oxygen composition and corresponding C/O atomic ratios for GO and SSrGO were calculated from the areas under these peaks (Ganguly et al., 2011). The C/O ratios were found to be 0.54 and 1.03 (Table S1 of the Supporting information) for GO and SSrGO, respectively, indicating that the oxygen deficiency in SSrGO due to irradiation. Fig. 3 shows the high resolution core level spectra of C1s and O1s resolved into components corresponding to the oxygen functional groups. In the Fig. 3(a), the presence of C1s doublet peak confirms the oxidation of graphite to form GO. The deconvoluted spectrum of C1s showed carbonyl (> C]) and epoxide (CeOeC) functional groups’ domination, as indicated by 570

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Fig. 3. High resolution core level XPS C1s spectra and O1s spectra for GO (a & b), and for SSrGO (c & d).

sluggish catalytic activity and on the other hand higher degree of reduction favors intense catalytic activity. C-V characteristic performance of GO, SSrGO, SWCNH and SWCNH/SSrGO counter electrodes in comparison to that of standard Pt CE are shown in Fig. 4(a). C-V data clearly demonstrates the typical redox peaks (annotated in the figure), which correspond to the oxidation (positive peak) and the reduction (negative peak) of I3− I − species present in electrolyte solution for GO, SSrGO, SWCNH and SWCNH/SSrGO, and Pt CEs (Imoto et al., 2003). Fig. 4(a) clearly depicts a better redox current density value indicating better catalytic activity for SSrGO and SWCNH/SSrGO CEs compared to GO and SWCNH, since the current density value (at Red1) is directly proportional to the capability of electrode to reduce the triiodide species. The second reduction peak at redox potential (around 0.46 V) was changed to positive value for SSrGO and SWCNH/SSrGO leading to improved Voc in DSSC cells (Dürr et al., 2006). The potential difference between anodic and cathodic current peaks gives the EPP which is inversely proportional to intrinsic kinetic redox capability. In the present work, EPP for SSrGO is 0.55 V and SWCNH/SSrGO is 0.36 V, which are lower compared to that of GO (0.80 V), SWCNH (0.58 V). This indicates the higher reduction kinetic capability of I − I3− for SSrGO, SWCNH/SSrGO and hence an improved electrocatalytic activity is witnessed in case of SWCNH/SSrGO and SSrGO than GO, SWCNH.

the peaks centered at 287.5 and 287.2 eV respectively. Further, sufficient amount of graphenaceous nature can be seen in the structure due to the presence of the sp2 peak at 285.3 eV. The low intensity wide peaks at 286.6 and 289.0 eV in the GO’s C1s spectrum indicates the presence of hydroxyl and carboxyl groups in low quantities. The O1s spectrum shown in Fig. 3(b) also confirms the presence of the oxygen functional groups due to the presence of oxygen doubly bonded to aromatic peak (I1) at 532.3 eV, oxygen singly bonded to aliphatic carbon peak (I2) at 533.0 eV and high intense peak (I3) indicating the component corresponding to oxygen strongly bonded to aromatic carbon atoms in the GO structure. The peak (I4) at 534.5 eV indicates the component due to chemisorption of water molecules. The XPS investigations of SSrGO shows increase in hydroxyl group (C-OH) with the peak intensity (286.6 eV) comparable to the sp2 component (285.5 eV) indicating the increase of local defects due to irradiation and might be due to the local reactions with the eOH+ and O− radicals present in the DI water, since there is no much change in the epoxide group (CeOeC) component (287.0 eV) as observed in C1s spectrum of GO and SSrGO. 3.3. Electrochemical characterization 3.3.1. Cyclic-Voltammetry (C-V) studies Cyclic -Voltammetry studies were used to investigate the redox kinetics of I − I3− at the interface of electrode/electrolyte. It was found high degree of reduction in case of SSrGO and SWCNH/SSrGO composite counter electrodes, since a lower degree of reduction causes

3.3.2. Tafel polarization studies Tafel measurements were carried out on symmetric dummy cells fabricated with GO, SSrGO, SWCNH and SWCNH/SSrGO CEs to evaluate the interfacial charge transfer parameters at electrolyte/electrode. 571

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Fig. 4. (a) Cyclic-Voltammetry plots of different CEs, and (b) Tafel polarization curves, (c) EIS plots, (d) Bode-phase curves for GO, SSrGO, SWCNH and SWCNH/ SSrGO based symmetrical dummy cells.

SWCNH/SSrGO and an increase in the rate of electron transfer process at SWCNH/SSrGO - electrolyte interface (Zhou et al., 2009; Liu et al., 2011). Fig. 4(d) presents the Bode phase plots for GO, SSrGO, SWCNH and SWCNH/SSrGO based symmetrical dummy test cells. The characteristic peak frequency (fmax) for SWCNH/SSrGO is 100 Hz and SSrGO is 177 Hz and is positioned at lower frequencies side. The electron life time (τe) was calculated using the τe = 1 2 π fmax equation and was found for SWCNH/SSrGO is 1.5 ms, which could lower recombination with the electrolyte and helps in enhanced performance for DSSCs.

Fig. 4(b) shows the Tafel polarization measurement curves for GO, SSrGO, SWCNH and SWCNH/SSrGO CEs. In the Tafel zone of the curves, the exchange current density (Jo) of different CEs are evaluated by extrapolating cathodic as well as anodic curves on the cross point at 0 V. It can be seen from Fig. 4(b) that the SWCNH/SSrGO has larger slope in its Tafel curve, reflecting higher exchange current density (Jo) value of 10.5 mA/cm2 compared to Jo value of SSrGO (4.8 mA/cm2), SWCNH (1.4 mA/cm2) and GO (0.7 mA/cm2), reveals better catalytic performance at the interface of the electrolyte/electrode interface for SWCNH/SSrGO and SWCNH. These observations are complement well with the C-V results.

3.4. Photovoltaic performance of DSSC test cells and large area DSSC module

3.3.3. Impedance analysis Electrochemical impedance spectroscopy studies are made using the symmetric dummy shift to Tafel analysis test cells, as mentioned in previous section of Tafel analysis. Fig. 4(c) presents Nyquist plots at diverse impedance regions of symmetric dummy test cells with GO, SSrGO, SWCNH and SWCNH/SSrGO CEs. Fig. 4(c) gives the information about different electrochemical parameters such as ohmic series resistance (Rs) and the charge transfer resistance (Rct) value at CE/ electrolyte interface (Yen et al., 2011; Jang et al., 2012; Beliatis et al., 2014). The impedance characteristic parameters are extracted by fitting an equivalent circuit with the software (IVIUMSOFT) and the corresponding parameters for different CEs are tabulated in Table 1. The Rct value plays a vital role in estimating electrochemical catalytic activity of CE. The SWCNH/SSrGO based CE showed Rct of 13.2 Ω-cm2, indicating the higher catalytic ability. This shows better conductivity for

The power conversion efficiency (PCE) of DSSC test cells, fabricated using GO, SSrGO, SWCNH, SWCNH/SSrGO and Pt CEs with acetonitrile (AcN) solvent based high performance electrolyte, was measured and reported. Fig. 5 depicts the photo current density – voltage (J-V) performance of fabricated with N719 dye sensitized solar test cells of 0.16 cm2 active area for GO, SSrGO, SWCNH, SWCNH/SSrGO and Pt CEs. The efficiency (η), fill factor (FF), open-circuit voltage (Voc) and short-circuit current density (Jsc) of different CE based cells are tabulated in Table 2. Test cells with SWCNH/SSrGO performed high power conversion efficiency of 8.27% with Jsc of 17.22 mA cm−2, Voc of 0.73 V, and FF of 0.65. The DSSC with SSrGO CE exhibits a Jsc of 17.15 mA cm−2, Voc of 0.73 V, and FF of 0.62, yielding to photovoltaic conversion efficiency (η) of 7.86%. Whereas SWCNH CEs revealed Jsc = 15.94 mA cm−2, Voc = 0.72 V, and FF of 0.63 to achieve PCE of 7.74%. In contrast, the cell made with GO produces slightly lower efficiency of η = 6.71% with Jsc = 14.53 mA cm−2, Voc = 0.70 V and FF = 0.62. Compared to DSSC made with GO, the DSSC made with SSrGO, SWCNH and SWCNH/SSrGO showed improved Jsc, resulting in enhanced performance. As stated in C-V studies, the second reduction peak for SWCNH/SSrGO (0.35 V) and SSrGO (0.40 V) and for Pt – (0.44 V) plays a key role in achieving higher I − I3− conversion and the

Table 1 EIS parameters of developed devices with AcN solvent electrolyte. Device

GO

SSrGO

SWCNH

SWCNH/SSrGO

Rs (Ω-cm2) Rct (Ω-cm2)

26.8 45.4

20.1 10.4

16.1 14.2

15.0 13.2

572

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Fig. 5. Current density - Voltage curves of fabricated test devices. Table 2 Photovoltaic performance of DSSC test cells with HI-30 electrolyte. Parameter

GO

SSrGO

SWCNH

SWCNH/SSrGO

Pt

Voc (V) Jsc (mA/cm2) Fill Factor Efficiency η (%)

0.70 14.53 0.62 6.71

0.73 17.15 0.62 7.86

0.72 15.94 0.63 7.74

0.73 17.22 0.65 8.27

0.72 18.55 0.62 8.36

positive shift in iodide/tri-iodide redox energy level leads to higher Voc for SWCNH/SSrGO and Pt compared other CEs. In addition, Voc value normally depends on the number of electrons which are injected to TiO2 conduction band. In the present study, the higher Voc of SWCNH/ SSrGO based DSSC can be attributed to more number of electrons injected to TiO2 conduction band. The device fabricated with Pt CE showed η = 8.36% having Jsc = 18.55 mA cm−2, Voc = 0.72 V, and FF = 0.62. Mira Tul Zubaida Butt et al. studied the DSSC performance using N719 dye and Pt as counter electrode with HI-30 electrolyte by faintly change in fabrication procedure compared to present work (Butt et al., 2018). Overall performance of DSSC made with SWCNH/SSrGO are close to Pt based test cells. The bare Pt and GO based DSSC performance is tabulated in Table 2 for comparison. Diffusion impedance of CEs is endorsed to its compact surface morphology, is considerably hampers to diffusion nature of electrolyte throughout CEs. Significant impedance values of CEs might affect devices efficiency. Sheet resistance, electrochemical property / catalysis performance, which is normally calculated by inverse of Rct value and optical nature or light reflection. Inverse value of Rct for SWCNH/ SSrGO 0.07, and for SSrGO (0.09) is higher when compared to GO (0.02) which offer a increased catalyst reduction rate at CE vicinity side representing superior number of negative charge ions are gathered, and is in results of higher Voc value. The more improvement in Jsc may be due to the combined effect of smaller Rct and raise in catalytic sites due to the superior conductivity in the presence of SSrGO. A smaller Rct signifies a higher catalytic reaction and reflected result in improved Jsc values. Having considered the higher performance of SWCNH/SSrGO based test cells, an attempt was made to demonstrate the scalability of SWCNH/SSrGO for large area, transparent DSSC module. Fig. 6 shows the photographs of various steps involved in the fabrication of large area and transparent SWCNH/SSrGO based DSSC module (Casaluci et al., 2016). Initially, the FTO based glass substrates are thoroughly cleaned as mentioned in the experimental section. The 18 NR-T TiO2 paste is deposited using screen print technique to achieve graphene pattern design as shown in Fig. 6(a) (the total active area is 12.60 cm2). The TiO2 coated substrate is placed in a dust free environment for 30 min to settle the coated paste as uniform TiO2 film and then heated

Fig. 6. Photographs of large area SWCNH/SSrGO based DSSC module fabrication process and its testing with tiny electric motor. (a) Graphene like structured TiO2 coated on FTO glass substrate as photo electrode (PE), (b) N719 dye anchored PE, (c) Hole drilled FTO glass for CE (d) Spray coated SWCNH/SSrGO based transparent counter electrode, (e) Polymer melt film, (f) DSSC Module assembling using PE and CE, (g, h) working function with tiny electric motor under indoor and outdoor light conditions.

to 500 °C for 30 min. While cooling, the substrates (at 110 °C) are immersed in N719 dye (0.3 mM) in Absolute ethanol. The counter electrodes were prepared using spray coat technique as discussed by the use of SSrGO and SWCNH dispersed DMF solutions. Initially, SSrGO dispersed solution is sprayed on FTO glass plate and followed by SWCNH solution is sprayed as discussed in the experimental section and large area, transparent CE photograph was depicted in Fig. 6(d). Surlyn polymer film is used to combine the photo and counter electrodes and heated the assembly to 110 °C and gently pressed. The Acetonitrile based electrolyte solution is introduced through the predrilled hole which is at the SWCNH/SSrGO CE rear side and is sealed with cover glass and polymer melt film. The SWCNH/SSrGO based DSSC module device performance was 573

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Fig. 7. Photographs of (a) standard Si solar cell, (b) integration of DSSC module with Si solar cell.

measured. The fabricated SWCNH/SSrGO based DSSC module showed an efficiency of 5.18% with Jsc = 37 mA/cm2, Voc = 0.70 V and FF = 0.25. The solar simulator spatial uniformity is ≤ ± 5%. A 15 mW electric motor was successfully operated both in indoor as well as outdoor light conditions using the fabricated DSSC module (Video-S2Indoor, Video-S3-Outdoor, Supporting information). 3.5. Integration of standard Si solar cell with fabricated DSSC module Further an integrating standard Si solar cell with the developed DSSC module was made to obtain higher open circuit voltage. Fig. 7(a) shows the photographs of standard Si solar cell and its integration (Fig. 7(b)) with DSSC module (placing the DSSC module over the silicon solar cell). The integrated device showed an enhanced Voc value of 2.34 V compared to Voc of standard Si solar cell (1.69 V). The idea of proposed integration of developed low cost, transparent DSSC module with standard Si solar cell has great potential in enhancing the open circuit voltages of real time solar power plants and hence higher efficiencies can be achieved. 4. Conclusions In summary, we successfully demonstrated the reduction of graphene oxide into reduced graphene oxide using artificial sun light (Xe light source). Cost effective, transparent all-carbon composite SWCNH/ SSrGO counter electrode exhibited best electrochemical performance. The test cells with SWCNH/SSrGO CEs shown PCE of 8.27% and its module exhibited 5.18% efficiency. A tiny electric motor was operated under indoor as well as outdoor light conditions. Further, integration of standard Si solar cell with developed DSSC module was performed to achieve higher open circuit voltage than the Si solar cell. The methodologies adopted in the present study provides a roadmap for the development of transparent, cost effective DSSC module with best efficiencies for various applications. Acknowledgements M. Gurulakshmi is thankful to DST-MHRD, India for providing DSTINSPIRE JRF and financial support through IF 160564. M. Raghavender thanks SERB, DST, India (Grant No. EMR/2016/007049) for providing partial financial support to conduct a part of this work. M. Raghavender also thankful to Dr. L. Giribabu and Dr. I. Nanaji, IICT-Hyderabad for characterization. MR also thank to DST-FIST, India through SR/FST/ PSI-182/2012(C). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2019.09.081. 574

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