Graphene nanosheets as counter electrode with phenoxazine dye for efficient dye sensitized solar cell

Organic Electronics 44 (2017) 32e41

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Graphene nanosheets as counter electrode with phenoxazine dye for efficient dye sensitized solar cell Iftikhar Ali Sahito a, b, Kyung Chul Sun c, Woosung Lee d, Jae Pil Kim e, Sung Hoon Jeong a, * a

Department of Organic and Nano Engineering, Hanyang University, Seoul, 133-791, Republic of Korea Department of Textile Engineering, Mehran University of Engineering and Technology, Jamshoro, Pakistan c Research Institute of Industrial Technology Convergence Technical Textile and Materials R&D Group, Korea Institute of Industrial Technology (KITECH), 143 Hanggaulro, Sangnokgu, Ansan-si, Gyeonggi-do, 426-910, Republic of Korea d ICT Textile & Apparel R&D Group, Korea Institute of Industrial Technology (KITECH), 143 Hanggaulro, Sangnokgu, Ansan-si, Gyeonggi-do, 426-910, Republic of Korea e Department of Materials Science and Engineering, Seoul National University, Seoul, 151-744, Republic of Korea b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 August 2016 Received in revised form 15 January 2017 Accepted 23 January 2017 Available online 26 January 2017

Efficient dye sensitized solar cells (DSSCs) are developed using phenoxazine (POZ) based organic dye (WS5) and graphene nanosheets (GNs) counter electrode (CE). Being organic, both these materials are used together to explore compatibility of organic materials in current DSSCs. Organic dye with POZ moiety is synthesized and graphene oxide nanosheets (GONs) are spin coated on FTO glass and thermally reduced afterwards. To increase the performance of WS5 through decreased dye aggregation, deoxycholic acid (DCA) is added to it. The results of adding DCA are observed and compared using UVeVis spectroscopy, external quantum efficiency (EQE), electrochemical impedance spectroscopy (EIS) and photovoltaic conversion efficiency (PCE). Prepared organic dye based DSSC cell results in a high PCE of 6.61%. The optimized WS5 dye and GNs CE, shows PCE of 5.77% and the GNs CE compared to Pt CE results in almost identical charge transfer resistance value at the CE/electrolyte interface. Low cost of this designed organic dye and GNs and the PCE results indicate that this combination may result in the reduction of cost of current DSSCs and the realization that expensive and rare inorganic materials can be replaced with organic ones in future. © 2017 Elsevier B.V. All rights reserved.

Keywords: Phenoxazine dye Graphene nanosheets DCA Charge transfer resistance DSSC

1. Introduction The current attention paid to dye sensitized solar cells (DSSCs) is extraordinary and is based on extremely important energy crises, to shift towards renewable energy sources rather than consuming fossil fuels. The increasing interest for DSSCs is also due to their ease of fabrication, low cost compared to other photovoltaics, reasonable conversion efficiency and their use in variety of applications [1e4]. However, very expensive organo-metallic dyes, as sensitizers, rare and expensive Platinum (Pt) metal, used at counter electrode (CE) in a typical DSSCs, limits its practical application, leaving it as an inorganic device, which may not be environmental friendly. Therefore, there is an acute need of replacing the rare and expensive materials of current DSSCs with promising organic materials that are abundant and cheap.

* Corresponding author. E-mail address: [email protected] (S.H. Jeong). http://dx.doi.org/10.1016/j.orgel.2017.01.035 1566-1199/© 2017 Elsevier B.V. All rights reserved.

In quest of finding alternatives to the expensive materials for DSSCs and polymer solar cell, organic dyes and carbonaceous materials are currently at high focus [5e8]. Various researchers [9e12] have developed and used organic sensitizers. As, the theoretical limit of the N719 dye is already achieved, therefore, organic dyes with much better photophysical properties, stronger lightharvesting ability and higher extinction coefficient [13,14] are focused to improve the current DSSCs. On the other hand various forms of carbon materials, including carbon nanotubes [15e17], activated carbon [18], graphene as nanosheets [19e22] and platelets [23], conducting polymers [24] and their composites [12,25] have been used and shown high efficiencies. However, in nearly all of the cases, the focus was either the organic dye or the carbonaceous CE and therefore, the PCEs have been high. Hence, we realized the use of both at the same time, for supplemented cost reduction of current DSSCs, making this cell environmental friendly. Amongst other carbon materials, graphene has been widely

I.A. Sahito et al. / Organic Electronics 44 (2017) 32e41

investigated, due to its superior electronic properties, ease of synthesis for bulk production, light weight and low cost [26e30]. Therefore, graphene nanosheets (GNs) were used as CE material for our proposed DSSC. In our previous work, an adequate PCE was shown by using POZbased organic dye [31]. To improve the performance of the dye, we have designed WS5, having methoxy phenyl ring in N position of POZ main structure and a hetero cyclic ring (furan) between POZ main structure and anchoring group. Substituting the phenyl ring would produce the steric hindrance in the dye molecules, which is aimed at reducing the charge recombination at the photoanode/ electrolyte interface. Moreover, addition of a hetero cyclic ring, as a bridge between main structure of POZ and the anchoring group, has showed high performance based on higher electron mobility between POZ structures and anchoring group. This bridge plays important role in POZ sensitizer, providing extended conjugation in the dye, which resulted in red-shifting the absorption spectra and increased the molar absorptivity of the dyes. Previously Yanping Hong et al. [32] have used asymmetric double donor-p-acceptor chains linked by a non-conjugated n-hexane chain and showed an efficiency of 6.06% by using the co sensitizer. They introduced two different donor groups to harvest a broader spectrum of light, however the optimization of co sensitizer was not mentioned clearly. Sekar Ramkumar and Sambandam Anandan [33] in another work also used a metal free complex of bibridged bianchoring ambipolar dyes by linking electron-donating triphenyl amine or phenoxazine and electron-accepting cyanoacetic acid through a cyanovinyl thiophene p-bridge. The adopted approach showed a better red shift however, a low cell efficiency of 1.78% was obtained. Haijun Tan and coworkers [34] used three different electron donors namely 10-phenyl-10H-phe-nothiazine, 10-phenyl-10H-phenoxazine and triphenylamine, which were separately appended onto the 7-position of the model dye (POZ
2), used with different POZ dyes and reached high cell efficiency of more than 7%, however, co sensitizer was not used. On the other hand Jiajia Li et al. [35] composed a phenoxazine unit and indolinum carboxyl acid derivative and obtained cell efficiency of 5.1%, however the use of co cosensitizer was not optimized, rather a high concentration of co sensitizer was used. Here, to enhance the performance of the organic dye, deoxycholic acid (DCA), as co sensitizer is studied. DCA was mixed in the 0.5 mM dye solution and its effect was observed using different photophysical, photovoltaic and electrochemical analysis. Further, optimized photoanodes, were used in conjunction with GNs to further lower the cost of DSSCs together with enhancing the use of organic materials at both sides of DSSC. The optimized cells were compared with the cells prepared with N719, the reference dye and Pt, the reference cathodic material, by using photovoltaic performance and electrochemical impedance spectroscopy (EIS).

33

powder was kindly provided by Asbury Carbons (USA) (particles size < 100 mm). Sulfuric acid (H2SO4), potassium permanganate (KMnO4), hydrogen peroxide (H2O2), hydrochloric acid (HCl), hydriodic acid (HI), and nitric acid (HNO3) were purchased from Sigma-Aldrich (USA). For the photovoltaic cell assembly, FTO glass (TEC 8, Pilkington Co.), N719 cis-diisothiocyanato-bis (2,20bipyridyl-4,40-dicarboxylato) ruthenium (II) bis(tetrabutylammonium), Solaronix Co.), Surlyn as spacer (60 mm, Dupont Co.), and chloroplatinic acid hexahydrate (Sigma-Aldrich Co.), were used as received. 1-Butyl-3-methylimidazoliumiodide (BMII), iodine (I2), Lithiumiodine (LiI), 4-tert-butylpyridine (TBP), Deoxycholic acid (DCA) and anhydrous acetonitrile were purchased for the composition of electrolytes by Aldrich Co. 2.2. Analytical instruments and measurements for organic dye 1H NMR and 13C NMR spectra were recorded on a Bruker Avance 300, 500 and 600 MHz using DMSO with the chemical shift against TMS. Mass data were measured using a JEOL JMS 600 W mass spectrometer. Cyclic voltammetry spectra were obtained using a three electrode cell with a 273 A potentiostat (Princeton applied research, Inc.). Measurements were performed using Ag wire (Ag/Agþ), glassy carbon and platinum wire as the reference, working and counter electrodes, respectively, in CH2Cl2 solution containing 0.1 M tetrabutylammonium tetrafluoroborate (TBATFB) as the supporting electrolyte. A standard ferrocene/ferrocenium (Fc/Fcþ) redox couple was employed to calibrate the oxidation peak. 2.3. Synthesis of dye 2.3.1. 10-(4-Methoxyphenyl)-10H-phenoxazine (1) Under nitrogen atmosphere, phenoxazine (1.5 g, 0.0082 mol), 4iodoanisole (2.88 g, 0.0123 mol), CuSn (1.49 g, 0.0082 mol), K2CO3 (3.4 g, 0.0246 mol) and 18-crown-6 (0.38 g, 0.00145 mol) were dissolved in dry 1, 2-dichlorobenznene (60 mL). The mixture was heated under refluxed for 48 h under nitrogen atmosphere. Then the reaction mixture was filtered and washed with dichloromethane (DCM). The filtrate was extracted with DCM, water and NH4OH. The organic phase was collected and dried over anhydrous MgSO4. After removing solvent, the residue was purified by column chromatography using DCM-hexane (1:3; v/v) to give 1, viscous pale yellow liquid (1.42 g, 81.6%). 1 H NMR (300 MHz, DMSO-d6): d ¼ 7.30 (d, J ¼ 8.6 Hz, 2H), 7.18 (d, J ¼ 8.6 Hz, 2H), 6.62e6.72 (m, 6H), 5.85 (d, J ¼ 9.2 Hz, 2H), 3.83 ppm (s, 3H). 13 C NMR (125 MHz, DMSO-d6): d ¼ 158.5, 142.6, 133.7, 130.9, 130, 123, 120.6, 115.9, 114.6, 112.5, 54.9 ppm; m/z (FAB) 289.1097 ((M þ), C19H15NO2 requires 289.1103).

2. Experimental 2.1. Materials and reagents for dye and graphene synthesis, cell fabrication Phenoxazine, N-bromosuccinimide and 4-ethoxyphenylboronic acid were purchased from TCI and used as received without further purification. 1-bromobutane, 5-formyl
2
furan-boronic acid, 5formyl
2
thiophene-boronicacid, tetrakis (triphenylphosphine) palladium (0), phosphorus oxychloride, 4-ethoxyphenylboronic acid, cyanoacetic acid and piperidine were purchased from Sigma Aldrich and used as received without further purification. All solvents (chloroform, tetrahydrofuran, dimethylformamide, dimethyl sulfoxide, dichloromethane, 1, 2-dichloroethane and acetonitrile) were obtained from Sigma Aldrich and used as received. Graphite

2.3.2. 3-Bromo-10-(4-Methoxyphenyl)-10H-phenoxazine (2) 1 (1.79 g, 0.0062 mol) and N-bromosuccinimide (1.106 g, 0.0062 mol) were dissolved in chloroform (30 mL), and the reaction was stirred for 1 h at ambient temperature. The reaction was quenched with water and extracted with water and DCM. The organic phase was collected and the solvent was removed by rotary evaporation. The residue was purified by column chromatography using DCM-hexane (1:3; v/v) to give 2, white solid (1.64 g, 72%). 1 H NMR (500 MHz, CDCl3): d ¼ 7.23 (d, J ¼ 9 Hz, 2H), 7.07 (d, J ¼ 8.5 Hz, 2H), 6.56e6.67 (m, 5H), 5.91 (d, J ¼ 7.5 Hz, 2H), 3.87 ppm (s, 3H). 13 C NMR (125 MHz, CDCl3): d ¼ 159.3, 144, 134.7, 131.7, 123.1, 121, 116.1, 115.2, 113.2, 55.4 ppm; m/z (FAB) 367.0210 ((M þ), C19H4BrNO2 requires 367.0208).

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I.A. Sahito et al. / Organic Electronics 44 (2017) 32e41

2.3.3. 5-(10-(4-Methoxyphenyl)-10H-phenoxazin-3-yl) furan
2
carbaldehyde (3) Under nitrogen atmosphere, a mixture of 2 (3.57 g, 0.0097 mol), 5-formyl
2
furan-boronic acid (6.51 g, 0.0464 mol), 2 M aqueous of K2CO3 (48 mL), Pd (PPh3)4 (2.24 g, 0.00194 mol) in dry THF (100 mL) was stirred for 30 min and refluxed at 80  C overnight. The reaction was extracted with DCM, water and brine. The organic phase was dried with anhydrous MgSO4, and then the solvent was removed in vacuum. The residue was purified by column chromatography using DCM-hexane (5:1; v/v) to give 3, orange solid (1.58 g, 43%). 1 H NMR (600 MHz, DMSO-d6): d ¼ 9.47 (s, 1H), 7.55 (s, 1H), 7.31 (d, J ¼ 8.7 Hz, 2H), 7.16e7.19 (m, 3H), 7.12 (d, J ¼ 8.4 Hz, 1H), 7.06 (s, 1H), 6.71 (d, J ¼ 7.6 Hz, 1H), 6.62e6.68 (m, 2H), 5.91 (d, J ¼ 8.4 Hz, 1H), 5.83 (d, J ¼ 7.5 Hz, 1H), 3.80 ppm (s, 3H). 13 C NMR (150 MHz, DMSO-d6): d ¼ 177.1, 159.2, 157.9, 151.1, 143.5, 142.9, 135.6, 133.4, 131.3, 129.7, 123.9, 121.9, 121.6, 120.9, 116.5, 115.3, 113.4, 113.3, 11.7, 107.5, 55.4 ppm; m/z (FAB) 383.1162 ((M þ), C24H17NO4 requires 383.1158). 2.3.4. (E)-3-(5-(10-(4-Methoxyphenyl)-10H-phenoxazin-3-yl) furan
2yl)
2
cyanoacrylic acid (WS5) WS5 as a red solid (0.35 g, 59%) was synthesized according to the procedure described above for the synthesis of POX25. 3 (0.5 g, 0.0013 mol), cyanoacetic acid (0.33 g, 0.0039 mol) and piperidine (0.496 mL, 0.0052 mol) were added to anhydrous CH3CN (50 mL). Eluent: DCM-methanol (10:1; v/v). 1 H NMR (500 MHz, DMSO-d6): d ¼ 7.99 (s, 1H), 7.49 (s, 1H), 7.36 (d, J ¼ 8.5 Hz, 2H), 7.20e7.27 (m, 4H), 7.17 (s, 1H), 6.77 (d, J ¼ 8.5 Hz, 1H), 6.67e6.79 (m, 2H), 5.93 (d, J ¼ 8.5 Hz, 1H), 5.89 (d, J ¼ 8.5 Hz, 1H), 3.85 ppm (s, 3H). 13 C NMR (125 MHz, DMSO-d6): d ¼ 163.4, 158.7, 158, 146.4, 143, 142.3, 136.8, 135.2, 132.7, 130.7, 129.1, 126.6, 123.3, 121.5, 120.9, 120.8, 116.1, 116, 114.7, 112.9, 112.7, 110.9, 108.2, 95.1, 54.8 ppm; m/z (FAB) 450.1214 ((M þ), C27H18N2O5 requires 450.1216). ATR-FTIR (cm
1): 2216 (cyano, C^N stretching band), 1682 (carbonyl, C]O stretching band). 2.4. Synthesis of graphene oxide nanosheets Graphene oxide nanosheets (GONs) were synthesized from graphite powder using the modified Hummer's method [36,37]. Briefly, 3 g of graphite powder was added to 69 mL of concentrated H2SO4 in an ice bath with continuous stirring for 30 min. KMnO4 (9 g) was added slowly at temperature no higher than 10  C. Afterwards, the mixture was allowed to react at 35  C for 4 h with vigorous stirring. To stop the reaction, the temperature was dropped to 10  C, with the use of ice, and 150 mL of DI water was added slowly. Later, 3 mL of H2O2 (30%) was added and the mixture was stirred for 30 min. The prepared dispersion was washed with 300 mL of 10% HCl and then 300 mL of DI water. Finally GONs cake was collected off the filtering media and 250 mL of water was added to the resulting product to form dispersion followed by bath sonication for 30 min. 2.5. Coating of graphene oxide nanosheets on FTO glass To prepare the graphene nanosheets (GNs) coated CE for DSSC, the as prepared GONs dispersion was further diluted to 2 wt % and was coated on the oxygen plasma treated one holed-FTO glass through spin coating at 2500 rpm for 15 s. The FTO glass was treated by oxygen plasma to improve the attachment of GONs film over it. For the reduction of GONs into GNs, the coated FTO glass was immersed in 40 mM hydriodic acid solution and kept for 25 min at 90  C, followed by oven drying at 70  C for 10 min and

then sintered at 300  C in the tube furnace with a gentle flow of N2 through the quartz tube, as the N2 doped GNs offer higher catalytic cites for the reduction of tri-iodide in electrolyte [38]. The N2 (>99.995) at 100 sccm was allowed to pass through the quartz tube and the temperature was elevated to 400  C. The samples were kept for 20 min before they were cooled to room temperature slowly. 2.6. Preparing photoanode and fabrication of solar cell The photoanode paste was prepared for a screen-printing process as given in the previous work [39]. The final composition of the paste comprised TiO2 nano powder (1 g), ethyl cellulose (0.5 g), terpineol (3.3 mL), and acetic acid (0.16 mL). After that, pre-washed FTO glass was coated by a doctor blade process and then heated at 70  C for 30 min for drying. After the printing, the TiO2 films were heated in four steps of 325  C, 375  C, 450  C, and 500  C for 5, 5, 15, and 15 min, respectively, using a high-temperature furnace (Lab house Co.). For the post treatment, the coated and sintered TiO2 films were immersed in 40 mM TiCl4 solution for 30 min at 70  C. After washing, the films were annealed at 500  C for 30 min. The TiO2 electrodes were immersed in EtOH/CH2Cl2 (7:3) solution containing 0.5 mM WS5 and N719 dyes separately, for 48 h at ambient temperature. After dye absorption, the photoanodes were washed using anhydrous ethanol and dried under nitrogen flow. The Pt CE were prepared by spin coating method, using 5 mM H2PtCl6 solution (in isopropanol) on one-holed FTO glass and heated at 400  C for 20 min. The dye-covered photo-electrode and Pt-electrode were assembled using ionomer surlyn with a hotpress at 80  C. After assembling, the electrolyte solution (composed of 0.6 M BMII, 0.05 M I2, 0.1 M LiI, and 0.5 M TBP in acetonitrile) was injected into the one holed FTO glass using a capillarity vacuum technique, and the hole was sealed with a coverglass using the same surlyn. A black mask aperture was placed on the photo-electrode for better analysis of the photovoltaic characteristics. 3. Characterization Optimized geometries, energy levels, and frontier molecular orbitals of the dyes' HOMOs and LUMOs were calculated at the B3LYP/6-31G (d,p) level. Photophysical properties of the cells were studied using UVevis absorbance spectrum obtained by using UV1650PC spectrophotometer (Shimadzu Co.) and Incident photonto-current conversion efficiency (IPCE) was measured as a function of wavelength from 350 to 730 nm using a specially designed IPCE system for dye-sensitized solar cells (PV measurements, Inc.). A 75 W xenon lamp was used as the light source for generating monochromatic beams. Calibration was performed using a silicon photodiode, which was calibrated based on the NIST-calibrated photodiode G425 standard. The IPCE values were measured under halogen bias light at a low chopping speed of 10 Hz. The active area of the dye-coated TiO2 film was ca. 0.24 cm2, which was measured by using software equipped camscope (ICS-305B, Sometech Co.). Photocurrent-voltage (I-V) measurements were performed using a K3400 power meter (McScience). A solar simulator with 160W Xenon arc lamp was used as light source (Spectral match; 0.75e1.25, Non-uniformity of irradiance; ±2%, Temporal instability; ±2%). The light intensity was calibrated with a KIER-calibrated Si solar cell (Mc science Co). Surface morphology of the GNs and Pt was investigated by Field Emission-Scanning Electron Microscope (FE-SEM) model, (JEOL JSM-6700F) at accelerating voltage of 15 kV and Transmission Electron Microscopy (FETEM, JEOL JEM-2010). The dimensional information of the GONs was obtained by atomic force microscope (AFM), XE-70, Park Systems in tapping mode. The Raman spectra were measured using a

I.A. Sahito et al. / Organic Electronics 44 (2017) 32e41

35

Fig. 1. Transmission Microscope image of single layer GO (a) TEM (b) AFM and (c) corresponding line diagram.

Raman microscopy system (NRS-3100, JASCO, Japan). Intensities of carbon and oxygen peaks functional group peaks, at the surface of fabric were measured through X-ray photoelectron spectroscopy (XPS) using Multilab ESCA 2000 system VG from Thermo scientific, USA. For this experiment mono-chromatized Al Ka radiation with energy step size of 0.05 eV was used. 4. Results and discussion 4.1. Graphene as counter electrode material To investigate the exfoliation of graphene oxide nanosheets, transmission electron microscopy and atomic force microscopy were used and the results are given in Fig. 1 (a) and (b) respectively. The single layers of GO can be easily seen in the TEM image, at high magnification. Also the corresponding line diagram of AFM image shows a height of 2 nm only, which is characteristic of a single layer of GONs.

The morphology of GNs and its counterpart, Pt, was studied using FE-SEM. The characteristic, wrinkled thin sheet of the GNs can be clearly seen in Fig. 2 (a) at low magnification, however, a rather rougher structure, which provides enough reduction of triiodide coupled with high electrical conductivity, was observed at high magnification (inset). On the other hand a typical highly porous structure of Pt can be observed in the inset of Fig. 2 (b). The electrical conductivity of the GNs were measured using a four point probe system and was found to be 12 U.sq
1, which was slightly higher than 8 U.sq
1 of the Pt coated FTO glass. Transformation of GO into GNs was confirmed by the Raman spectroscopy, which showed spectra of GONs and GNs having characteristic D and G peaks, as presented in Fig. 3 (a). In the spectra of GONs, the typical G band corresponding to the first-order scattering of the E2g mode is at 1593 cm
1 and D band at 1352 cm
1 for the disorder band caused by the graphite edges, indicating a reduction in the size of the in-plane sp2 domains, possibly due to the extensive oxidation [40]. After reduction, the G band shifts

Fig. 2. SEM images of (a) GNs and (b) Pt at low magnification (1000, scale bar 10 mm) and high magnification (inset, x 30000, scale bar 100 nm).

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I.A. Sahito et al. / Organic Electronics 44 (2017) 32e41

Fig. 3. (a) Raman spectra of GO and GNs (b) XPS survey spectra for GO and GNs.

slightly towards the D band, called the d-shift; therefore, G and D bands are found at 1575 cm
1 and 1343 cm
1 respectively. This shift can be attributed to the recovery of the hexagonal network of the carbon atoms with defects [41] indicating that the reduction process altered the structure of GO. More importantly, there is an increase in the ID/IG ratio of GONs to GNs from 0.95 to 1.1 suggesting the restoration of the carbonaceous network. A further investigation was carried using XPS to observe the intensity of the O1s/C1s peaks in the spectrum, as shown in Fig. 3 (b). A noticeable difference can be observed in the O1s and C1s peaks before and after reduction of GO. The reduced peak intensity of O1s and an increased peak intensity of C1s indicate a decrease in oxygen containing groups and partial restoration of the sp2 graphitic network, thereby increasing the intensity of carbon, suggesting the conversion of GONs into GNs.

4.1.1. Photophysical properties of the phenoxazine dyes based DSSCs Due to the p-p stacking in the organic dye molecule, which occurs because of the strong intermolecular interactions, the organic dye molecules tend to form stacks or the lamellar structure, resulting in dye aggregation and hence, decrease short circuit

current (JSC) due to inefficient electron injection [42]. Consequently, various researchers have either used co sensitizer or modified the structure of dye, to dissociate the p-p stacking, to lessen the dye aggregation and consequently achieve a higher JSC. Fig. 4 shows the absorption spectra of the WS5 on TiO2 coated FTO glass. FTO glass with a 2.5  2.5 cm active area of the TiO2 paste was sintered in the earlier mentioned sequence followed by keeping TiO2 coated samples in the dye solution for 48 h and then absorption spectra was measured. It is clear from the spectra that the adsorption of the dye molecules was higher with the addition of DCA, which was due to the reason that the dye molecules were dissociated with its addition [43] resulting in an even affinity of the dyes on TiO2 giving off a better absorbance spectra. The energy levels of the prepared dye are provided in Table 1 and Fig. 5 (b). The oxidation potential of the dye was measured using cyclic voltammetry (CV), the CV curve of the dye is shown in Fig. 5 (a). The HOMO level of the prepared dye correspond to the first oxidation potential vs. a normal hydrogen electrode (vs. NHE) calibrated by Fc/Fcþ (with 640 mV vs. NHE). The HOMO level of the dye was found to be 0.90 eV (vs. NHE) and was sufficiently positive compared to the redox potential of I=I
3 (0.4 eV vs. NHE), implying that the oxidized dyes can be effectively regenerated by the redox electrolyte [44]. The LUMO level of the dye was obtained by subtracting the zeroth-zeroth energy (E0
0) from Eox, in which the zeroth-zeroth energy of the dye is determined from the inflection point at the end of the visible absorption spectrum of the dye. LUMO level of the dye was
1.30 eV (vs. NHE), which is sufficiently negative compared to conduction band of TiO2 (
0.5 eV vs. NHE). WS5 also exhibited high molar extinction coefficient (lmax) in solution, as shown in Table 1. This is due to the planar structure of WS5, as shown in Table 2, caused by the introduction of the ethoxy phenyl ring, which inhibits the adsorption of the dye on the TiO2 surface. 4.1.2. Photovoltaic properties of the phenoxazine dyes based DSSCs To study the effect of adding DCA to WS5 on photovoltaic properties of the DSSC, the ratio between incident photons and current conversion, external quantum efficiency (EQE) % or the IPCE of the prepared cells was measured in the range between 350 and 730 nm wavelengths, as shown in Fig. 6.

EQE ¼ Fig. 4. Adsorption of the dye on TiO2 coated FTO glass with and without DCA.

Electrons out Photons in

(1)

I.A. Sahito et al. / Organic Electronics 44 (2017) 32e41

37

Table 1 Photophysical and electrochemical properties of WS5 dye. Dye

WS5

Absorptiona

Emissiona

Oxidation potential datac

lmax [nm]

ε [M
1c m
1] (at lmax)

labsb [nm] (on TiO2)

lmax [nm]

Eox [eV] (vs. NHE)

E0
0d [eV]

Eox
E0
0 [eV] (vs. NHE)

508

19250

473

652

0.90

2.20


1.30

Measured in 1  10
5 CH2Cl2 solutions at room temperature. b Measured on TiO2 film. c Measured in CH2Cl2 containing 0.1 M tetrabutylammonium tetrafluoroborate. (TBABF4) electrolyte (working electrode: glassy carbon; counter electrode: Pt; reference electrode: Ag/Agþ; calibrated with ferrocene/ferrocenium (Fc/Fcþ) as an internal reference and converted to NHE by addition of 630 mV). d E0
0 was determined from the intersections of absorption and emission spectra. a

Fig. 5. (a) CV curve of WS5 in CH2Cl2 (b) WS5 HOMO and LUMO energy levels.

IPCE of the cells showed that upon addition of the DCA, the IPCE enhanced significantly from 61% to 71% by using 0.1 mM of the DCA, which can be attributed to the fact that a separated monomer of the dye molecule can better inject the electrons to the TiO2, upon photoexcitation, rather than the aggregated dye, which may be attributed to the increase of EQE % [45]. Fig. 7 shows the overall photovoltaic performance, currentvoltage (I-V curve), of the DSSCs prepared with and without using DCA with the organic dye, evaluated under AM 1.5 illumination (1 sun, 100 mW cm
2). The obtained cell parameters of the short-circuit current (JSC), open-circuit voltage (VOC), fill factor (F.F) and PCE are summarized in Table 3. For the reference, cells were also prepared by using the same concentration of N719 dye. It was observed that the cells prepared with addition of DCA in

the dye, gave a JSC of 13.7 mA cm
2, which is much higher than 10.8 mA cm
2 of the cells prepared without adding DCA. As a matter of fact, upon co-adsorption with DCA, the TiO2 surface is protonated and conduction band edge shifts positively due to the proton adsorption resulting in a higher JSC value [42]. Due to a better and faster electron generation with the use of DCA, the interface of photoanode with the electrolyte and the cathode improved, resulting in enhancement of F.F which was found to be 75.20%. Furthermore, this phenomenon also enhanced the charge recombination at the dye/dye or dye/electrolyte interface, resulting in a lower VOC value of the cells compared to not adding the DCA, however, the decrease the VOC value was marginal. Consequently, the overall conversion efficiency was found to be 6.61%, which was higher than the cells prepared without using DCA.

Table 2 Structure of WS5 with electronic distribution in HOMO and LUMO levels. Structure of WS5 dye

HUMO

LUMO

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I.A. Sahito et al. / Organic Electronics 44 (2017) 32e41 Table 4 IV characteristics of the cells prepared with GNs coated FTO glass as Counter electrode.

Fig. 6. External quantum efficiency of photoanodes prepared at different DCA concentrations.

Dye

C. E

JSC [mA cm
2]

VOC [V]

FF

PCE %

N719 N719 þ DCA 0.1 mM WS5 þ DCA 0.1 mM

GNs

13.42 13.83 11.43

0.69 0.70 0.71

71.13 70.36 70.46

6.45 6.74 5.77

12.53 (mA cm
2), however, the WS5 does not lack behind much, with 11.43 (mA cm
2) with a 15 mm, single layer of TiO2. A very minor loss of JSC, using the organic dye can be certainly compensated by its low cost and ease of synthesis. On the other hand, VOC of the two cells is almost identical, indicating the compatibility of the electrolyte with the organic dye and that the interfacial charge transfer resistance at the TiO2 is same in both case. The FF of DSSC, apart from the porous structure of CE material, does also depend upon the attachment of material on the FTO glass and its conductivity [5]. As noticed, the FF of both the cells is slightly lower than the cells prepared earlier using Platinized FTO glass. It is due to mediocre attachment of GNs on the glass, which is not as good as Pt and a higher surface resistance of GNs, therefore, the FF reduces slightly. The PCE of the cells prepared N719 resulted in 6.45%, which increased a little with the addition of 0.1 mM DCA in N719, resulting in 6.73% PCE. Although, WS5 showed lesser PCE of 5.77%, as compared to its counterpart, however, again the low cost of these organic material, abundant availability, scalable and green production can prove them to be better choice in the future generations. 4.1.4. Electrochemical properties The internal kinetics of the DSSCs can be better understood using the electrochemical impedance spectroscopy (EIS) technique and therefore, the cells were measured for EIS under dark at forward bias of
0.60 V [31], in order to explain the correlation between the Voc of the cells and the dyes and their corresponding Nyquist and bode plots are shown in Fig. 9 (a) and (b) respectively. In the Nyquist plot, the major semicircle at the intermediate frequency represents the charge transfer impedance at the TiO2/ dye/electrolyte interface. The charge recombination resistance can be estimated by the radius of the major semicircle at the intermediate frequency, a shorter semicircle describes a better electron

Fig. 7. Polarization curves of DSSCs with and without DCA.

4.1.3. Photovoltaic properties of the graphene nanosheets counter electrode based DSSCs The cells prepared were measured for I-V and EIS under the same conditions as given earlier and were compared. For the reference, N719 was also prepared by adding 0.1 mM of DCA to maintain the identical conditions for both the dyes. The photovoltaic properties are listed in Table 4 and the polarization curves for both type of cells are depicted in Fig. 8. The short circuit current (JSC) is the most important parameter contributing to the PCE of a DSSC. The JSC of the cells prepared using N719 dye was measured as

Table 3 IV characteristics of the cells. Dye

C.E

JSC [mA cm
2]

VOC [V]

F.F [%]

PCE [%]

WS5 WS5 þ DCA N719

Pt

10.8 13.7 13.9

0.66 0.64 0.73

70.63 75.20 72.70

5.09 6.61 7.42

Fig. 8. Polarization curves of DSSCs with GNs coated FTO glass as counter electrode.

I.A. Sahito et al. / Organic Electronics 44 (2017) 32e41

39

Fig. 9. (a) Nyquist plots (b) bode plots of DSSCs prepared with and without DCA.

Fig. 10. Equivalent circuit of the full cell.

generation and an efficient electron transport, however, a greater electron recombination [39]. It can be seen clearly in the Nyquist plots that the impedance of the cells prepared using DCA, is lower than that of the cells not using DCA. It can be observed that in both the cases the impedance spectra started from the same point, indicating that CE is same in both cases; however, at the photoanode/electrolyte interface, the impedance increased much higher in the case of no DCA and continues until the end. These results are in agreement with the earlier results and showing that with the addition of co-adsorbent, the overall impedance of the cell can be reduced to increase the PCE %. However, as mentioned earlier, a smaller second semicircle in the EIS spectra corresponds to higher electron recombination. As can be seen in the Nyquist plot of the cells prepared with DCA, the smaller semicircle indicates a higher recombination rate. This results is in agreement with the VOC values of the cells given in Table 3. The rapid charge transport and the effective electron life time (te Þ are important characteristics of the photoanode/electrolyte interface in a DSSC. Bode plot serves to understand this transport behavior, as the electron life time is obtained from bode plot peaks in the middle-frequency range (0.1e100 Hz), by using the following

relation, in which ‘f’ is the frequency of peak.

te ¼

1

u

¼

1 2pf

(2)

As shown in Fig. 5 (b), bode plots of the cells shows a similar pattern to that in Nyquist plots. It can be seen that the frequency value (x-axis) of the cell with DCA, is much higher to that of no DCA. Using expression (2), the electron life time can be calculated by substituting the value of frequency (f). A higher frequency value in the case of DCA, implies a lower electron life time which results in a higher electron recombination at the photoanode/electrolyte interface. However, it shows a lesser overall resistance (y-axis) compared to the cell with no DCA.

4.1.5. Electrochemical properties To understand the internal kinetic of the interfacial charge transport in the DSSC, EIS technique was carried. The EIS spectra were simulated using EC-Lab software, according to the equivalent circuit shown in Fig. 10. The internal resistance and capacitance values of the cells are

Table 5 Internal impedance of GNs based cells. Dye-GO

RS [Ohm cm2]

R1 [Ohm cm2]

C1 [F cm
2]

R2 [Ohm cm2]

C2 [F cm
2]

R3 [Ohm cm2]

C3 [F cm
2]

N719 WS

1.051 1.157

0.529 0.545

1.39E-04 7.19E-05

3.716 5.188

9.81E-03 3.12E-03

1.315
0.955

0.192 260.1

40

I.A. Sahito et al. / Organic Electronics 44 (2017) 32e41

Fig. 11. (a) Nyquist and (b) bode plots of DSSCs prepared with organic sensitizer and GNs coated FTO glass as counter electrode.

listed in Table 5 and the corresponding Nyquist and bode plots are shown in Fig. 11 (a) and (b) respectively. As can be seen, both of the impedance spectra start from the same point, because of the same type of CE with same RS. Also, the first semicircle of the spectra is nearly same for both the types of cells, showing identical impedance for electrolyte R1 at the GNs CE. However, as the frequency decreases, the spectra of the cells start to differ at the second semicircle, showing change in interfacial charge resistance at TiO2/dye/electrolyte interface for both the cells R2. The N719, due to higher and faster photo-generation, gave lower charge transfer resistance of 3.7 U cm2 and a slightly higher value of 5.1 U cm2 was obtained using WS5 at this interface. The third semicircle, belonging to the diffusion of tri-iodide and influenced by second semicircle, was hidden in the second semicircle. Fig. 11(b) shows the bode plots of the two cells based on N719 and WS5 dyes with GNs cathode. As can be seen in the plot, cell based on N719 dye gave a peak frequency value of 2.68 Hz which is smaller than that of WS5 dye based cells giving 8.95 Hz. Corresponding electron life time for both types of cell were calculated using expression (2) and were found to be 59.4 ms and 17.7 ms for N719 and WS5 respectively. A higher electron life time suggests lower recombination [46], thus it can be said that the recombination rate in the case of WS5 dye is higher as compared to N719, which may the reason of slightly decreased JSC value. Nevertheless, due to their lower cost, easier modification and purification, high molar extinction coefficient, and environmental friendliness, a slightly decreased JSC can be defensible. 5. Conclusions High efficiency dye sensitized solar cell (DSSC) was successfully developed using designed and synthesized organic sensitizer (WS5) and the low cost, easily fabricated organic material of graphene nanosheets (GNs) for counter electrode (CE). The organic dye based DSSC showed high power conversion efficiency (PCE) of 6.61%, reaching 81% to its expensive N719 dye. On the other hand, when GNs based cell was compared to its counterpart, the expensive Platinum (Pt), the results were 6.74% and 7.42%, respectively which means GNs may overcome 91% of the PCE of Pt. Finally WS5 and GNs, both the organic materials were used together in one cell, as sensitizer and CE material respectively, resulting in a PCE value of

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