An efficient quasi solid state dye sensitized solar cell based on polyethylene glycol/graphene nanosheet gel electrolytes

Abstract

A poly(ethylene glycol)/graphene nanosheet quasi solid gel electrolyte was synthesized using an in situ polymerization technique for dye-sensitized solar cells (DSSCs). Fabrication of the DSSCs was carried out by sandwiching the poly(ethylene glycol)/graphene nanosheet (PEG/graphene nanosheet) gel electrolyte in between the dye sensitized TiO₂ nanoflower photoanode and platinum based counter electrode using a spacer of thickness 25 μm. However, the graphene nanosheets form a network with PEG matrix channels in the gel electrolyte which enhances charge transportation. The PEG/graphene nanosheet gel electrolyte was characterized using Fourier transform infrared spectroscopy, cyclic voltammetry, scanning electron microscopy, transmission electron microscopy, thermal gravimetric analysis, atomic force microscopy and electrochemical impedance spectroscopy. Electrochemical impedance spectroscopy results demonstrate the reduction of charge transfer resistance (R_ct) with the incorporation of graphene nanosheets which promotes charge transportation through the gel electrolyte. The reduction of (R_ct) enhances the device efficiency which was observed in the current density vs. voltage (J–V) measurements and thereby the incident photon to converted electron (IPCE) curves. The maximum photovoltaic conversion efficiency of 5.16% was achieved.

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Introduction

The first demonstration of dye-sensitized solar cells (DSSCs) was done by O'Regan and Grätzel in 1991.¹ After that DSSCs have been recognized as the most promising next generation solar cells due to their low fabrication cost and decent power conversion efficiency. The efficiency of DSSCs depends on different properties such as light harvesting, quantum yield, and charge collection efficiency at the electrodes.^2,3 The liquid electrolytes employed in DSSCs use the iodide/triiodide (I⁻/I₃⁻) redox couple and involve the regeneration of dye and completion of the external electrical circuit. However, liquid electrolyte based DSSCs face different types of challenges such as leakage, desorption of the attached dye, stability and durability associated with the different solvent used.⁴ Therefore, several approaches have been investigated such as the use of organic and inorganic hole conductors, room temperature molten ionic salts, and p-type semiconductors to overcome the problems associated with liquid electrolyte. DSSCs demand alternative materials to the liquid electrolyte because of its abnormal behavior towards long term stability, liquid leakage, electrode corrosion, dye degradation and volatility.

The quasi-solid state gel polymer electrolyte containing a solution of the I⁻/I₃⁻ redox couple is an alternative way towards stability improvement.⁵ Jihuai Wu et al. reported a novel polyblend electrolyte consisting of KI and I₂ dissolved in a blending polymer of polyvinyl pyrrolidone (PVP) and polyethylene glycol (PEG) for DSSCs.⁶ Akhtar et al. studied a novel composite solid based electrolyte which is prepared by using TiO₂ nanotubes as filler in polyethylene glycol (PEG) and effectively used for the fabrication of solid-state DSSCs device.⁷ The liquid like property with viscous nature of gel polymer electrolyte can eradicate the contact interface problem at high temperature. However charge transfer kinetics via gel polymer containing redox couple is limited at room temperature. Therefore, further improvement of ionic mobility will enhance the cell performance in DSSCs. Many efforts so far have been made to improve the cell efficiency using different nanofillers which reduce the charge transfer resistance of the polymer gel electrolyte.⁸ The addition of carbon nanomaterials as nanofillers improves different properties such as stability, ionic conductivity, electrochemical and mechanical behaviour of polymer gel electrolytes.⁹ Among them graphene nanosheet have drawn special attention in the field of solar cell due to its higher electron mobility, large specific area, etc.^10,11 The polymer gel electrolyte shows some hindrance in diffusion of triiodide and recombination reaction due to viscous nature, thereby affecting performance of DSSCs. The excellent conductivity of graphene nanosheet in polymer gel electrolyte helps in accelerating the ions in DSSCs.

It is required to prepare the photoanode with different structural change, strong light scattering, efficient electron transport and quick electrolyte diffusion.¹² So far commercially available titania nanoparticles have been extensively studied as photoanode for DSSCs. Photoanode optimization with structural change of TiO₂ nanoparticles are important for enhancement of the device performance. Many efforts have been made towards designing of 1D porous hollow spheres and their property changes such as large surface area, photocatalytic behavior and excellent loading capacity etc.¹³ Design of photoanode with the multistacked arrangement of layer by layer distinct hyperbranched TiO₂ architectures shows significant improvement in device parameter J_sc and V_oc attaining maximum efficiency of 11.01% at film thickness of 34 μm.¹⁴ There are few reports on hydrogel electrolyte based TiO₂ nanoflower photoanode. However, with the 55% dye adsorption of the photoanode based on TiO₂ coated urchin like SnO₂ microspheres showed conversion efficiency of 6.05%.¹⁵ High electron transport and recombination kinetics in DSSCs based on an urchin like Zn/ZnO heterostructure has been demonstrated by Wang et al.¹⁶ The strong scattering effect enhances the light harvesting competence of the photoanode which extends the path length of light within the photoanode film which may provide extended opportunity for the photons to be absorbed by sensitizers (i.e., dye molecules).

In this paper, we have synthesized highly effective quasi solid PEG/graphene nanosheet gel electrolytes using in situ polymerization technique. The obtained quasi solid gel electrolytes were further applied for fabrication of DSSCs using hydrothermally synthesized TiO₂ nanoflower as photoanode. The prepared PEG/graphene nanosheet gel electrolytes were characterized using Raman spectroscopy, cyclic voltammetry, electrochemical impedance spectroscopy etc. The graphene nanosheet highly charge transportation nature which further interconnects channels of PEG matrix thereby enhances DSSCs efficiency. Finally, current density vs. voltage (J–V) and incident photon to converted electron (IPCE) curve shows the effect of graphene nanosheet concentrations on efficiency enhancement and their device parameters.

Experimental section

Materials used

All chemicals graphite flakes, polyethylene glycol (PEG), hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃), acetonitrile, ethanol, cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylate) ruthenium(II) bis-tetrabutylammonium (N-719 were purchased from Sigma-Aldrich) and used without further purification.

Synthesis of TiO₂ nanoflower

In TiO₂ nanoflowers synthesis, 25 mL of ultrapure distilled water was mixed with same amount of concentrated hydrochloric acid to reach a total volume of 50 mL (1 : 1 v/v). The mixture was stirred at room temperature for 30 min then 1.5 mL of titanium tetrachloride (TiCl₄) was added. After addition of TiCl₄ stirring was continued for another 15 min. Then the whole mixture was transferred to the autoclave and two pieces of fluorinated tin oxide (FTO) substrates were kept which was ultrasonically cleaned with deionized water, acetone and ethanol. The hydrothermal synthesis was carried out in an autoclave at 180 °C for 10–12 h. After synthesis, the autoclave was allowed to cool down to room temperature. Finally, FTO substrate was taken out, rinsed extensively with ultrapure deionized water and allowed to dry in vacuum oven.

Synthesis of graphene nanosheet

The graphene nanosheet was synthesized by implementing the well-known modified Hummer's method.¹⁷ In brief, (0.05 g) graphite flakes were mixed with concentrated (5 mL) H₂SO₄ and (15 mL) HNO₃ placed in an ice bath under stirring for 24 h. After that, the mixture was subsequently treated ultrasonically for 8 h with ultrasonication bath instrument. The prepared mixture was dispersed in water and purified by filtration. Then resulting black color product was calcinated at 700 °C for 2 h in a horizontal furnace and graphene nanosheet was obtained.

Synthesis of polyethylene glycol (PEG)/graphene nanosheet hydrogel electrolyte

PEG/graphene nanosheet hydrogel were synthesized through two step procedure. At the very beginning PEG (10%) was added into the different concentrations of graphene nanosheet suspension gradually and kept under stirring at 80 °C until the PEG completely soluble. Then the mixture was subjected to bath ultrasonication for 1 h to obtain the PEG/graphene nanosheet hydrogel. A mixed solution of 0.5 M LiI, 0.05 M I₂, 0.5 M tert-butylpyridine, 0.6 M 1-propyl-3-methylimidazolium iodide and acetonitrile was absorbed inside PEG/graphene nanosheet gel as electrolyte.

The assembling of DSSCs

FTO glass sheets (Sigma-Aldrich, sheet resistance: 15 Ω per sq.) were first cleaned in a detergent solution using an ultrasonicator bath for 10 min, then rinsed with acetone, water, ethanol and finally dried under N₂ atmosphere. The TiO₂ nanoflower photoanode with a film thickness of 23 μm and an area of 0.25 cm² was prepared. Subsequently, the TiO₂ nanoflowers photoanode was sensitized by soaking in 0.3 mM N-719 mixed solvent of acetonitrile/ethanol (volume ratio 1 : 1) solution for 24 h. The quasi solid PEG/graphene nanosheet hydrogel allowed to soak in a mixed solution of 0.5 M LiI, 0.05 M I₂, 0.5 M tertbutylpyridine, 0.6 M 1-propyl-3-methylimidazolium iodide and acetonitrile for 96 h to reach the absorption saturation. Platinum deposited FTO coated glass slide using thermal vacuum coating unit was used as a counter electrode. Finally, fabrication of DSSCs was carried out by placing gel electrolyte between TiO₂ photoelectrode and Pt counter electrode and sealed with a 25 μm thick solaronix thermal polymer spacer. The fabricated devices were kept at 70 °C for 10 min for better penetration of gel electrolyte into the dye adsorbed TiO₂ layer.^18–20

Measurements

Fourier transform infrared (FTIR) spectra of the samples were recorded in Nicolet Impact-410 IR spectrometer using KBr pellet at room temperature in the range of 400–4000 cm⁻¹. The morphologies of the polymer gel electrolyte were studied using a Jeol-JSM-6390L V scanning electron microscope. Freeze-drying method was used for SEM analysis. Transmission electron microscopy (TEM) analysis was carried out with (JEOL JEM 2100) at an acceleration voltage of 200 kV. In the TEM measurement, the samples were prepared on the carbon coated copper grid and dried in a vacuum oven at room temperature. The impedance spectroscopy was performed on HIOKI IM 3570 electrochemical impedance workstation at constant temperature of 25 °C with ac signal amplitude of 90 to 264 V in the frequency range from 4 Hz to 5 MHz. Cyclic voltammetry (CV) of DSSCs measurement was performed under illumination on a PGSTATE 302 N with two electrode system on FTO/gel electrolyte/Pt coated glass. The electrochemical characteristics of the composite sample were investigated by cyclic voltammetric scanning at a scan rate 50 mV s⁻¹. Thermo gravimetric analysis of composites was studied in a Shimadzu TA50 thermal analyzer. A pre weighted amount of the composite samples was loaded in a platinum pan and heating was done under nitrogen atmosphere at a heating rate of 5 °C min⁻¹ in the range of 25–600 °C. The photocurrent voltage (J–V) characteristic curves of the gel electrolyte based DSSC were measured under irradiation of a simulated 100 mW cm⁻² xenon arc lamp in ambient atmosphere. The fill factor (FF) and light-to-electric conversion efficiency (η) were calculated using the following equations:where J_sc is the short-circuit current density (mA cm⁻²), V_oc is the open-circuit voltage (V), P_in is the incident light power, and J_max (mA cm⁻²) and V_max (V) are the current density and voltage in the J–V curves, respectively.

Results and discussion

The UV-Vis spectra of PEG/graphene nanosheet content as shown in Fig. 1 exhibits two characteristic peaks, an intense peak at 262 nm, corresponding to π–π* transitions of aromatic C–C bonds, and a shoulder at 303 nm, which can be attributed to n–π* transitions of C=O bonds. However, increase in graphene nanosheet concentration within PEG matrix shows red shift which indicates the restoration of electronic conjugation within the graphene nanosheets.^21,22

Fig. 1 UV-Vis spectra of (a) PEG/0.25% graphene nanosheet, (b) PEG/0.50% graphene nanosheet, (c) PEG/0.75% graphene nanosheet, (d) PEG/1% graphene nanosheet and (e) PEG/1.25% graphene nanosheet.

Fig. 2 shows the FT-IR spectra of graphene nanosheet and PEG/graphene nanosheet at different concentration graphene nanosheet. All the different absorption peaks at 3400 (O–H stretching vibrations), 1720 (stretching vibrations from C=O), 1600 (C–OH stretching vibrations), 1220 (skeletal vibration of un-oxidized graphitic domains) and 1060 (C–O stretching vibrations) cm⁻¹ indicates formation of graphene nanosheet.²³ Next, the spectrum of PEG/graphene nanosheet by changing graphene nanosheet concentration, shows the broad peak at 3409 cm⁻¹ is attributed to O–H bonds vibration, whereas the strong peaks detected at 2878, 1468 and 1348 cm⁻¹ are related to C–H bonds.²⁴ The new absorption peaks were observed in PEG/graphene nanosheet at 2878, 1459 and 1348 cm⁻¹ which indicates that the graphene nanosheet has been successfully incorporated into the PEG.²⁵

Fig. 2 FTIR spectra of (a) graphene nanosheet, (b) PEG/0.25% graphene nanosheet, (c) PEG/0.50% graphene nanosheet, (d) PEG/0.75% graphene nanosheet and (e) PEG/1.25% graphene nanosheet.

SEM micrographs reveal the surface morphology of TiO₂ nanoflower and interconnection of PEG/graphene nanosheet based gel electrolyte were shown in Fig. 3. The morphology of TiO₂ nanoflower film over FTO glass at different magnification clearly shows the successful synthesis of nanoflower. Furthermore, PEG/graphene nanosheet micrograph demonstrates the interaction among PEG and graphene nanosheet which lead to the formation of PEG/graphene nanosheet. However, micrograph taken at various concentrations of graphene nanosheet does not reveal much morphological changes in the PEG matrix. In order to examine the detailed structure of graphene nanosheet and PEG/graphene nanosheet TEM analysis was performed. Fig. 4(a–d) shows the TEM images of the graphene nanosheet and PEG/graphene with high resolution which clearly gives an impression of single nanosheet with folding behavior as general feature of single layer graphene nanosheet. The SAED pattern shows graphitic laminar structure which can be resolute from the ordered region of graphene nanosheet.^26,27 The nanosheet are nearly transparent and display a very stable nature under the electron beam. The obtained graphene nanosheet diffraction pattern confirms the crystalline behavior. Therefore, TEM images shows embedment of graphene nanosheet within the PEG matrix. Atomic force microscopy (AFM) is presently the notable methods letting definitive evidence of single-layer structures of graphene nanosheet.²⁸ Fig. 5 shows atomic force microscopy of single layered graphene nanosheet. To get the average thickness which is nearly 2.85 nm and lateral distance of 898 nm AFM was performed. From the image we have observed typically flat graphene sheet and successful preparation of graphene nanosheet.

Fig. 3 SEM images of (a and b) TiO₂ nanoflower, (c) cross sectional image of TiO₂ nanoflower, (d) PEG/0.25% graphene nanosheet, (e) PEG/0.50% graphene nanosheet, (f) PEG/0.75% graphene nanosheet and (g) PEG/1.25% graphene nanosheet.

Fig. 4 TEM images of (a and b) graphene nanosheet, (c) SAED pattern of graphene nanosheet and (d) PEG/graphene nanosheet.

Fig. 5 AFM image of graphene nanosheet.

In order to get the thermal stability of prepared samples thermogravimetric analysis (TGA) of graphene, PEG and PEG/graphene was carried out under N₂ flow using Shimadzu TA50 and their mass degradation were recorded as a function of temperature. Fig. 6 shows TGA graph of graphene nanosheet, PEG and PEG/graphene nanosheet. Graphene nanosheet shows slight decrease in mass during initial stage of heating from room temperature to 120–150 °C and significant decrease was observed after reaching 150 °C which extend further upto 700 °C. This major mass reduction at 150 °C was caused by pyrolysis of the oxygen-containing functional groups, generating CO, CO₂ and stream.²⁹ The synthesized PEG without graphene nanosheet shows weight loss at 200 °C possibly due to evaporation of solvent. Further degradation was observed at 400 °C.³⁰ The incorporation of graphene nanosheet within PEG matrix showed a significant improvement in thermal stability. Thus, the enhancement of thermal stability in the final composites with incorporation of graphene nanosheet, which facilitates the strong interaction as well as formation of crosslink network among polymer matrix.³¹

Fig. 6 TGA graph of (a) graphene nanosheet, (b) PEG and (c) PEG/1.25% graphene nanosheet.

The electrochemical impedance spectroscopy (EIS) was performed to investigate the interfacial charge transfer resistance among TiO₂/dye/electrolyte interface and the counter electrode of DSSCs.^32,33 EIS experiment was carried out under constant illumination of light (AM1.5G 100 mW cm⁻²). Fig. 7 shows the Nyquist plot for DSSCs using quasi solid gel-state electrolyte of PEG/Graphene nanosheet. However, parameters were calculated by fitting curves method (software ZSimpwin) using their equivalent circuits and further summarized in Table 1. There were three semicircles observed in the whole frequency ranges (1 mHz to 1 MHz), first in the lower frequency range, second one in the mid frequency region and finally at high frequency range. Under illumination and biased condition the first semicircle appeared at higher frequency region corresponds to electron ejection at the counter electrode/electrolyte interface which refers as (R_ct1) i.e. charge transfer resistance. The second semicircle at mid frequency region corresponds to the charge-transfer process (R_ct2) at TiO₂/dye/gel electrolyte and further transport in TiO₂ photoanode film. The third extended line at low-frequency fitted graph governed by the Warburg diffusion process of I⁻/I₃⁻redox couple or charge Nernstian diffusion in the electrolyte. Another parameter the high frequency intercept on abscissa represents series resistance (R_s) is associated with the electrolytes and electric contacts in the DSSCs.³⁴ As shown in Table 1, PEG/1% graphene nanosheet shows lower values of (R_ct1) and (R_ct1) which contribute towards catalytic activity as well as photo-conversion efficiency. However, as we keep on increasing concentration beyond 1% graphene nanosheet, decrease in photo-conversion efficiency was observed. That might be due to agglomeration of graphene nanosheet which hamper in charge transportation through polymer matrix. Therefore, R_ct plays a crucial role during electron transportation within the electrode to the electrolyte interfaces, which is favorable to improve the photovoltaic performance of DSSCs.^35,36

Fig. 7 EIS of (a) PEG/0.25% graphene nanosheet, (b) PEG/0.50% graphene nanosheet, (c) PEG/0.75% graphene nanosheet and (d) PEG/1% graphene nanosheet and (e) PEG/1.25% graphene nanosheet.

Table 1 EIS parameters of DSSCs from Nyquist plot using equivalent circuit

Samples	Rs	Rct1	Rct2	Wd
PEG/0.25% graphene nanosheet	21.3	36	22.4	17.4
PEG/0.50% graphene nanosheet	17	32.4	20	16
PEG/0.75% graphene nanosheet	14	27	18	14
PEG/1% graphene nanosheet	10	18.6	15	12
PEG/1.25% graphene nanosheet	13.7	23.8	16.8	12

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We have performed cyclic voltammetry (CV) to study the effect of graphene nanosheet on redox couple (I₃⁻/I⁻) within PEG/graphene nanosheet gel electrolyte. Fig. 8 shows the CV curves obtained for PEG/graphene nanosheet with various concentrations of graphene nanosheet. CV was carried out using three electrode systems over a scan rate 50 mV s⁻¹ vs. silver (Ag) as reference electrode. The redox wave was observed due to the oxidation (anodic peak) and reduction (cathodic peak) of I₃⁻ with the following electron transfer reaction in DSSCs.

I₃⁻ + 2e⁻ ↔ 3I⁻ (3)

3I₂⁻ + 2e⁻ ↔ 2I₃⁻ (4)

Fig. 8 Cyclic voltammetry of (a) PEG/0.25% graphene nanosheet, (b) PEG/0.50% graphene nanosheet, (c) PEG/0.75% graphene nanosheet (d) PEG/1% graphene nanosheet and (e) PEG/1.25% graphene nanosheet.

This contributes toward efficiency of DSSCs. CV graph demonstrate an increase in the current density with increase of graphene nanosheet content by reducing the charge transportation resistance. PEG/1% graphene nanosheet gel electrolyte has shown maximum current density which is further reflected in photo conversion efficiency. As the graphene nanosheet loading increases beyond (1%), current density slowly started shrinking due to the agglomeration of graphene nanosheet in PEG matrix. Thus, current density peak facilitates us to assess the catalytic activity, towards I₃⁻/I⁻ reduction. The graphene nanosheet provides large surface area and better charge transport through PEG matrix.

In order to analyze the defect mediated peaks Raman spectroscopy was performed. Fig. 9 shows the Raman spectra for the graphene and PEG/graphene hydrogel. In the Raman spectrum first order dominant vibrational modes, D band 1352.34 cm⁻¹ (A_1g symmetry mode of sp³carbon) structure indicate about the order or disorder in the system and G band 1594.17 cm⁻¹ (vibrational mode of sp² carbon) gives an idea about doubly degenerate E_2g mode of the Brillouin zone center. The second order vibrational 2D₁ band arises at 2689 cm⁻¹ and 2D₂ band at 2929.12 cm⁻¹from zone boundary is very sensitive to the stacking order of the graphene with number of layers, and shows often a doublet with increasing number of graphene layers. An overtone weak band 2D′ is observed at 3194.49 cm⁻¹.³⁷ The shape of 2D band and intensity ratio between the two bands (I_G/I_2D) decide about the number of layers and inclusive stacking behavior of graphene.³⁸ The presence of defect due to different sample preparation environment can be determined by measuring the intensity ratio (I_D/I_G) of the D and G bands. With the increase in number of graphene layers, intensity ratio of G and 2D increases. The graphene spectra display D-band which is less intense as compared to the G-band. From the intensity ratio of the G-band to the D-band the in plane crystallite size, L_a can be calculated using the formula, L_a = 4.4(I_G/I_D) and we have obtained L_a value 4.75.^39,40 Thus, it is believable that the sample contains highly disordered and randomly arranged graphene sheets. The incorporation of graphene into the PEG, spectra confirms shift in the D and G band of graphene towards lower frequency. These shifting towards lower frequency confirm to the stacking behavior of graphene nanosheet in PEG/graphene nanosheet. The observed peak at 2890 and 2938 cm⁻¹ are due to symmetric and anti-symmetric stretching vibrations of methylene group of PEG respectively.⁴¹ Two band appeared at 1280 and 1230 cm⁻¹ correspond to CH₂ twisting vibrations.⁴² Whereas, band observed at 1148 cm⁻¹ due to the C–O stretching mode and the skeletal band vibration of PEG observed at 846 cm⁻¹. Therefore, Raman study of PEG/graphene nanosheet confirms successful incorporation of graphene nanosheet in PEG matrix.

Fig. 9 Raman spectra of (a) graphene nanosheet and (b) PEG/1.25% graphene nanosheet.

Fig. 10 shows the current density–voltage (J–V) curves of the DSSCs under solar illumination of 100 mW cm⁻² (AM 1.5). The obtained photovoltaic parameters of DSSCs were expressed in Table 2. The optimum value of photo-conversion efficiency (η) 5.18% was achieved using PEG/1% graphene nanosheet gel electrolyte. The higher current density of the PEG/1% graphene nanosheet gel electrolyte as it was observed in cyclic voltammetry (CV) contributes immensely towards conversion efficiency. Open circuit voltage (V_oc) gives an idea of potential difference between the quasi Fermi levels of the charges mainly electron in TiO₂ photoanode (under illumination) and the redox potential of the redox couple in the electrolyte.

where E_redox redox potential of the redox couple, E_C the conduction band edge of TiO₂, kT is the thermal energy, n_c is the free electron density in the conduction band of TiO₂, and N_C is the density of the accessible state in the conduction band. Therefore, the negative shift of the conduction band edge (E_C) and higher electron density in the conduction band (n_c) of TiO₂ increases the V_oc.⁴³ As we increase graphene concentration V_oc of the devices increase upto (0.74 V) in case of PEG/1% graphene nanosheet. This increase in V_oc of the gel electrolytes related with the catalytic activity of I⁻/I₃⁻ redox couple which decreases the over potential among interfaces. The incorporation of graphene nanosheet into the PEG gel electrolyte reduces the charge recombination rate by faster charge transport within DSSCs electrodes. Therefore, it is reasonable that the photovoltaic parameters have shown improved results of V_oc or J_sc and finally conversion efficiency. The significant value of J_sc (10.27 mA cm⁻²) was achieved by optimized device structure using PEG/1% graphene nanosheet gel electrolyte. This enhancement of J_sc was attributed to the enhancement of the charge collection and transport of electrons through different interconnecting channels of gel electrolytes. The maximum conversion efficiency (η) 5.16% was achieved with PEG/1% graphene nanosheet. Finally, in order to examine the photo-electrochemical properties of the nanocomposites, the incident photon-to-collected electron (IPCE) of the fabricated devices was examined.

IPCE (%) = 100 × 1240 × J_sc/(P × λ) (6)

where J_sc is the short circuit current (mA cm⁻²), P is the incident light intensity (W cm⁻²), and λ is the wavelength (nm). Fig. 11 shows the IPCE spectra of PEG/graphene nanosheet at various content of graphene nanosheet. The significant increase in J_sc of PEG/1% graphene nanosheet reflects the parallel-connection of the devices which verifies the results of the UV-Vis and IPCE spectra. Therefore, the fabricated device can capture an approximately visible region spectrum of the sun light. However, J–V curve shows, increase of graphene nanosheet concentration beyond (1%) hampers device conversion efficiency which might be due agglomeration problem in the polymer matrixes.

Fig. 10 Current density–voltage curves of (a) PEG/0.25% graphene nanosheet, (b) PEG/0.50% graphene nanosheet, (c) PEG/0.75% graphene nanosheet, (d) PEG/1% graphene nanosheet and (e) PEG/1.25% graphene nanosheet.

Table 2 Device performance of gel state electrolyte based DSSCs at various concentration of graphene nanosheet

Samples	Jsc (mA cm^−2)	Voc (V)	FF	Efficiency% (η)
PEG/0.25% graphene nanosheet	7.90	0.69	0.66	3.59
PEG/0.50% graphene nanosheet	8.60	0.70	0.67	4.03
PEG/0.75% graphene nanosheet	9.35	0.72	0.67	4.51
PEG/1% graphene nanosheet	10.27	0.74	0.68	5.16
PEG/1.25% graphene nanosheet	9.89	0.72	0.67	4.81

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Fig. 11 IPCE spectra of (a) PEG/0.25% graphene nanosheet, (b) PEG/0.50% graphene nanosheet, (c) PEG/075% graphene nanosheet, (d) PEG/1% graphene nanosheet and (e) PEG/1.25% graphene nanosheet.

Conclusion

Efficient quasi solid PEG/graphene nanosheet gel electrolyte and TiO₂ nanoflower has been synthesized using in situ polymerization as well as hydrothermal methods respectively. The synthesized PEG/graphene nanosheet and TiO₂ were used for fabrication of DSSCs. Raman and FTIR spectra confirm successful incorporation of graphene nanosheet into the PEG gel polymer matrix. Morphological analysis reveals uniform distribution of graphene nanosheet in the PEG matrix. AFM study shows lateral dimension and thickness of graphene nanosheet. The electrochemical impedance spectroscopy and cyclic voltammetry study demonstrate graphene nanosheet role in reducing the charge transfer resistance (R_ct) as well as improvement of current density in PEG gel electrolyte. The increase in J_sc and IPCE substantiates their relation of device performance with PEG/1% graphene nanosheet gel electrolyte. The enhancement in photo-conversion efficiency (η) with the incorporation of graphene nanosheet in PEG gel electrolyte was observed and their device parameters were summarized in Table 2. The maximum efficiency (η) of 5.16% was achieved with PEG/1% graphene nanosheet. However, beyond 1% graphene nanosheet concentration the efficiency and device parameter decreases due to agglomeration of graphene nanosheet in the PEG matrix. Further attempts will be made towards improvement of efficiency and device parameters.

Acknowledgements

This work is financially supported by the Department of Electronics and Information Technology (DeitY), Govt. of India (Grant No. 1(11)/2012-EMCD).

References

1. B. O'Regan and M. Grätzel, Nature, 1991, 353, 737–740.
2. J. Wang and T. Chen, J. Power Sources, 2014, 267, 136–139.
3. M. Grätzel, J. Photochem. Photobiol., A, 2004, 164, 3–14.
4. Z. Yu, N. Vlachopoulos, M. Gorlov and L. Kloo, Dalton Trans., 2011, 40, 10289–10303.
5. W. Kubo, K. Murakoshi, T. Kitamura, S. Yoshida, M. Haruki, K. Hanabusa, H. Shirai, Y. Wada and S. Yanagida, J. Phys. Chem. B, 2001, 105, 12809–12815.
6. J. Wu, P. Li, S. Hao, H. Yang and Z. Lan, Electrochim. Acta, 2007, 52, 5334–5340.
7. M. S. Akhtar, J. M. Chun and O. B. Yang, Electrochem. Commun., 2007, 9, 2833–2837.
8. M. S. Kang, K. S. Ahn and J. W. Lee, J. Power Sources, 2008, 180, 896–901.
9. M. S. Akhtar, S. J. Kwon, F. J. Stadler and O. B. Yang, Nanoscale, 2013, 5, 5403–5411.
10. G. H. Chen, W. G. Wenig, D. J. Wu, C. L. Wu, J. R. Lu, P. P. Wang and X. F. Chen, Carbon, 2004, 42, 753–759.
11. L. C. Hontoria, A. J. P. Lopez, J. D. G. Lopez, M. J. C. Rojas and M. R. M. Aranda, Carbon, 1995, 33, 1585–1592.
12. Q. F. Zhang, K. Park, J. T. Xi, D. Myers and G. Z. Cao, Adv. Energy Mater., 2011, 1, 988–1001.
13. J. H. Pan, X. Z. Wang, Q. Huang, C. Shen, Z. Y. Koh, Q. Wang, A. Engel and D. W. Bahnemann, Adv. Funct. Mater., 2014, 24, 95–104.
14. W. Q. Wu, Y. F. Xu, H. S. Rao, C. Y. Su and D. B. Kuang, J. Am. Chem. Soc., 2014, 136, 6437–6445.
15. A. Thapa, J. Zai, H. Elbohy, P. Poudel, N. Adhikari, X. Qian and Q. Qiao, Nano Res., 2014, 7, 1154–1163.
16. F. Wang, Y. Chen, C. Liu, Q. Ma, T. Zhaoa and M. Wang, RSC Adv., 2014, 4, 34531–34538.
17. S. W. Hummers and E. R. Offeman, J. Am. Chem. Soc., 1958, 80, 1339.
18. T. Miyamoto and K. Shibayama, J. Appl. Phys., 1973, 44, 5372–5376.
19. Q. Li, J. Wua, Z. Tanga, Y. Xiaoa, M. Huanga and J. Lin, Electrochim. Acta, 2010, 55, 2777–2781.
20. L. Y. Lin, C. H. Tsai, T. W. Huang, L. Hsieh, S. H. Liu, H. W. Lin, K. T. Wong, C. C. Wu, S. H. Chou, S. H. Chen and A. I. Tsai, J. Org. Chem., 2010, 75, 4778–4785.
21. J. I. Paredes, S. Villar-Rodil, A. Martınez-Alonso and J. M. D. Tascon, Langmuir, 2008, 24, 10560–10564.
22. D. Li, M. B. Muller, S. Gilje, R. B. Kaner and G. G. Wallace, Nat. Nanotechnol., 2008, 3, 101–105.
23. X. Yuxi, H. Bai, G. Lu, C. Li and G. Shi, J. Am. Chem. Soc., 2008, 130, 5856–5857.
24. G. Socrates, Infrared and Raman Characteristic Group Frequencies: Tables and Charts, John Wiley & Sons, New York, 3rd edn, 1994.
25. M. I. A. Umar, C. C. Yapa, R. Awang, M. M. Salleh and M. Yahaya, Appl. Surf. Sci., 2014, 313, 883–887.
26. C. J. Meyer, K. A. Geim, I. M. Katsnelson, S. K. Novoselov, J. T. Booth and S. Roth, Nature, 2007, 446, 60–63.
27. H. L. Guo, X. F. Wang, Q. Y. Qian, F. B. Wang and X. H. Xia, ACS Nano, 2009, 9, 2653–2659.
28. Z. J. Fan, W. Kai, J. Yan, T. Wei, L. J. Zhi, J. Feng, Y. M. Ren, L. P. Song and F. Wei, ACS Nano, 2011, 5, 191–198.
29. A. Lerf, H. He, M. Forster and J. Klinowski, J. Phys. Chem. B, 1998, 102, 4477–4482.
30. H. Chen, B. M. Müller, J. K. Gilmore, G. G. Wallace and D. Li, Adv. Mater., 2008, 20, 3557–3561.
31. S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A. Kleinhammes, Y. Y. Jia, Y. Wu, S. T. Nguyen and R. S. Ruoff, Carbon, 2007, 45, 1558–1565.
32. M. Miyashita, K. Sunahara, T. Nishikawa, Y. Uemura, N. Koumura, K. Hara, A. Mori, T. Abe, E. Suzuki and S. Mori, J. Am. Chem. Soc., 2008, 130, 17874–17881.
33. J. H. Chen, C. H. Tsai, S. A. Wang, Y. Y. Lin, T. W. Huang, S. F. Chiu, C. C. Wu and K. T. Wong, J. Org. Chem., 2011, 76, 8977–8985.
34. Y. Lai, C. Lin, J. Chen, C. Wang, K. Huang, K. Liu, K. Lin, J. Lin and K. Ho, Sol. Energy Mater. Sol. Cells, 2010, 94, 668–674.
35. K. Lee, S. W. Park, M. J. Ko, K. Kim and N. G. Park, Nat. Mater., 2009, 8, 665–671.
36. R. Kern, R. Sastrawan, J. Ferber, R. Stangl and J. Luther, Electrochim. Acta, 2002, 47, 4213–4225.
37. A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. S. Novoselov, S. Roth and A. K. Geim, Phys. Rev. Lett., 2006, 97, 187401–187404.
38. S. Stankovich, D. A. Dikin, R. D. Piner, A. K. Kohlhaas, A. Kleinhammes, Y. Y. Jia, Y. Wu, S. T. Nguyen and R. S. Ruoff, Carbon, 2007, 45, 1558–1565.
39. M. A. Pimenta, G. Dresselhaus, M. S. Dresselhaus, L. A. Cancado, A. Jorio and R. Saito, Phys. Chem. Chem. Phys., 2007, 9, 1276–1290.
40. A. C. Ferrari, Solid State Commun., 2007, 143, 47–57.
41. Y. Jin, M. Sun, D. Mu, X. Ren, Q. Wang and L. Wen, Electrochim. Acta, 2012, 78, 459–465.
42. N. B. Colthup, L. H. Daly and S. E. Wiberley, Introduction to Infrared and Raman Spectroscopy, Academic Press, Boston, 3rd edn, 1990.
43. C. Wu, L. Jia, S. Guo, S. Han, B. Chi, J. Pu and L. Jian, ACS Appl. Mater. Interfaces, 2013, 5, 7886–7892.

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