Enhanced photovoltage in DSSCs: synergistic combination of a silver modified TiO₂ photoanode and a low cost counter electrode

Abstract

In this study, we have tailored both the active electrode with silver modified TiO₂ (Ag–TiO₂) as well as the counter electrode (CE) with Pt–reduced graphene oxide (Pt@RGO) nanocomposites to realize efficient and low cost devices. The synergistic combination of both modified electrodes leads to an improved light to electrical energy conversion with an overall efficiency of 8%. An increase in the photovoltage (V_OC) of ∼16% (0.74 to 0.86 V) is achieved using Ag–TiO₂ in comparison to the bare TiO₂. This can be attributed to the shifting of the quasi-Fermi level of the TiO₂ photoanode close to the conduction band in the presence of Ag nanoparticles (NPs) due to the formation of the Schottky barrier. On the other hand, the facile synthesis of Pt NPs on RGO nanosheets by a photo-reduction method without using chemical reducing or stabilizing agents demonstrates a higher efficiency than Pt as a CE due to the cooperation of the catalytic activity of Pt and the high electron conductivity of the RGO as a stable supporting material having more interfacial active sites. The quantity of Pt in the Pt@RGO nanocomposites is 10 times lower than in the Pt CE which reduces the cost and makes it viable for large scale commercial utilization.

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1. Introduction

Solar energy is the most promising resource to meet future energy demands due to the depletion of fossil fuels. Among various technologies, dye-sensitized solar cells (DSSCs) have attracted widespread attention as a solar energy conversion technology because of abundantly available low cost materials, ease of fabrication and relatively high photon to current conversion efficiency.¹ DSSCs consist of a nanocrystalline porous semiconductor electrodes-sensitized with light absorbing dye as a photoanode, I⁻/I₃⁻ or Co²⁺/Co³⁺ redox couples as electrolytes and platinized conducting glass as counter electrode (CE).^1a Extensive efforts have been made to improve the efficiency of DSSC by engineering all the components. For instance, nanocrystalline TiO₂ films with various morphologies² and dopants,³ synthesizing panchromatic dyes to absorb visible and near-infrared light effectively,⁴ synthesizing alternative redox electrolytes⁵ and substituting platinized counter electrodes with low cost materials⁶ have all been explored to enhance the photocurrent and photovoltage, in order to improve the efficiency of DSSCs.

Among all the factors, the photoanode is regarded as sovereign for improving the efficiency. In order to modulate the photoanode, thickness of the nanocrystalline titanium dioxide layer has been extensively studied to improve both the photocurrent and photovoltage.⁷ Generally lowering the thickness of the active layer reduces the recombination at the interface between electrode and electrolyte which in turn increases the efficiency. However, the dye loading also decreases with decreasing thickness resulting in lower photocurrents. To combat this problem, surface modification of semiconductor nanoparticles (NPs) with plasmonic metal nanostructures such as gold, silver and copper have been investigated recently.⁸ Solar cells employing hybrid materials have revolutionized efficient solar energy conversion applications through cost-effective fabrication techniques.⁹ Hupp et al. reported the enhancement of photocurrent in DSSC by the localized surface plasmon resonance (SPR) of silver NPs which increases the effective absorption cross section of dye molecules.^8a,10 The enhancement of photocurrents in DSSC by using silver doped semiconductor nanostructures are well documented in the literature.¹¹ But significant improvement of the photovoltage by surface modification with silver is reported sparsely in the literature. Recently, Kamat et al.¹² have reported significant photovoltage boosting of DSSC with thiolated gold nanocluster anchored on TiO₂ surface along with squaraine as a sensitizing dye. Gold nanoclusters increase the quasi-Fermi level of TiO₂ close to the conduction band which enhances the open circuit photovoltage. In the present study, we aimed to raise the photovoltage of DSSC by silver modification on TiO₂ surface as well as modification of CE with low cost materials.

Another aspect in order to realize the cost effective DSSCs is the development of alternative counter electrode materials. In DSSC, the most prevailing CE is a platinum (Pt) thin film due to its exceptionally high electrochemical activity.¹³ However, the high cost and corrosion of Pt in presence of the iodine electrolyte restricts it from large scale applications and long term use.¹⁴ In this regard, carbon based materials such as carbon nanotube, graphene and reduced graphene oxide (RGO) as CE have induced current research interests as a substituent of Pt due to low cost, high charge carrier mobility¹⁵ and reasonable performances.¹⁶ However, the energy conversion efficiency is lower in case of graphene CE compared to that of Pt electrodes and also requires large quantities of graphene to achieve an acceptable catalytic activity.^16a,17 Thus hybridization of small amount of Pt with graphene or reduced graphene oxide can meet the requirements of low cost, long term stability as well as high catalytic activity. Significant efforts have been made to fabricate Pt NPs on reduced graphene oxide film or substrate by using chemical reducing agents,¹⁸ photo-reduction,¹⁹ pulsed laser ablation method,²⁰ dry plasma reduction,^6a thermal decomposition²¹ and hydrogen plasma²² etc. However, in most cases, the energy conversion efficiency has not reached the expected level as well as the complex fabrication process limits its large scale application. The modification of both the active and counter electrode can provide both high efficiency and low cost requirements which indeed remain unexplored in the contemporary literature.

In the present study, we have modified both the active and counter electrodes to realize an energy efficient device using simple fabrication processes. We have synthesized and characterized small silver NPs embedded on the TiO₂ surface (Ag–TiO₂) as a photoanode material which exhibits significant enhancement in the photovoltage. On the other hand, the CE has been modified with the Pt NPs supported on RGO (Pt@RGO) using a facile photo-reduction method without any reducing or stabilizing agents where Pt loading is very low compared to that of the standard Pt CE. This reduces the cost and also increases the catalytic activity to regenerate the electrolyte more efficiently. The combination of both factors enhances the efficiency of the device as high as 8%. The open circuit photovoltage decay measurement in the presence of Ag–TiO₂ photoanode unravels slow back electron transfer from the active layer to the electrolyte or oxidized dye molecules. The present study will be helpful in designing an easy fabrication process of low cost, efficient and large scale production of future solar devices.

2. Experimental section

2.1. Reagents

TiO₂ (P25) was purchased from Evonik. Platinum chloride (H₂PtCl₆), silver perchlorate (AgClO₄), platinum(II) acetylacetonate (Pt(C₅H₇O₂)₂), di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) (N719), lithium iodide (LiI), iodine (I₂) and 4-tert-butylpyridine (TBP) were purchased from Sigma-Aldrich. Ultrapure water (Millipore System, 18.2 MΩ cm) and ethanol (≥99% for HPLC, purchased from Sigma-Aldrich) were used as solvents. Analytical-grade chemicals were used for synthesis without further purifications. Fluorine-doped tin oxide (FTO) conducting glass substrates, acquired from Sigma-Aldrich were cleaned by successive sonication with soap water, acetone, ethanol and deionized (DI) water for 15 min, each with adequate drying prior to their use.

2.2. Synthesis of silver modified TiO₂

The Ag–TiO₂ samples were obtained by radiolytic reduction of Ag⁺ in the TiO₂ suspension following our previous report.²³ An ethanolic solution of AgClO₄ (2 × 10⁻³ M) and TiO₂ (Evonik P25) suspension is sonicated for 3 min, then degassed with nitrogen and irradiated (under stirring) with a ⁶⁰Co panoramic gamma source (dose rate, 2.3 kGy h⁻¹). The silver ions were reduced by the solvated electrons and the alcohol radicals induced by solvent radiolysis under 1 h 20 min irradiation to reduce all the silver ions. The Ag–TiO₂ samples were centrifuged and dried at 60 °C.

2.3. Synthesis of Pt NPs supported on reduced graphene oxide

The graphene oxide (GO) was synthesized from graphite powder by the modified Hummers method as reported in our previous publication.²⁴ The GO (1 mg mL⁻¹) was then dispersed in 2-propanol and exfoliated in an ultrasonic bath for 20 min. Then 1 mM of platinum(II) acetylacetonate was added to the GO dispersion followed by deoxygenation under a N₂ flow. Then the mixture was irradiated under UV light for 6 h under a N₂ atmosphere.²⁵ Then as synthesized Pt–reduced graphene oxide nanohybrid was centrifuged and dried at 60 °C. The reduced GO was synthesized following the same procedure without adding the platinum salt.

2.4. Fabrication of DSSCs

For the fabrication of DSSCs, platinum, Pt@RGO and RGO deposited on FTO substrates were used as counter electrodes. The platinum (Pt) were deposited on the FTO substrates by thermal decomposition of 5 mM platinum chloride (in isopropanol) at 385 °C for 30 min. The N719 coated TiO₂ and Ag–TiO₂ were used as the photo anodes and the two electrodes were placed on top of each other with a single layer of 60 μm thick Surlyn (Solaronix) as a spacer between the two electrodes. A liquid electrolyte composed of 0.5 M lithium iodide (LiI), 0.05 M iodine (I₂) and 0.5 M 4-tert-butylpyridine (TBP) in acetonitrile was used as the hole conductor and filled in the inter electrode space by using capillary force, through two small holes (diameter = 1 mm) predrilled on the counter electrode. Finally, the two holes were sealed by using another piece of Surlyn to prevent the leakage of electrolyte from the cell. In all our experiments, the active area of the DSSCs was fixed at 0.49 cm².

2.5. Characterization methods

Field Emission Scanning Electron Microscopy (FESEM, QUANTA FEG 250) was used to investigate the surface morphology of the samples. Transmission electron microscopy (TEM) was carried out using an FEI (Technai S-Twin) instrument with acceleration voltage of 200 kV. A drop of sample was placed on a carbon-coated copper grid and particle sizes were determined from micrographs recorded at a high magnification of 100 000×. The Pt content in Pt@RGO nanocomposite was determined by inductively coupled plasma-mass spectrometry (ICP-MS). The solvent was first evaporated and samples were mineralized using 5 mL of aqua regia and injected. After 20 times dilution with ultrapure water, via a peristaltic pump at 0.1 mL min⁻¹ flow rate, nebulization of samples was performed by means of a micro concentric nebulizer (Micromist). A 7500 ce ICP-MS (Agilent) was used as the elemental detector. Detection of Pt was performed by selecting abundant isotopes free of interferences, i.e., ¹⁹⁴Pt and ¹⁹⁵Pt. X-ray diffraction (XRD) was used to characterize crystal phase by a PANalytical XPERTPRO diffractometer equipped with Cu Kα radiation (at 40 mA and 40 kV) at a scanning rate of 0.02° s⁻¹ in the 2θ range from 20° to 80°. Raman scattering experiments were carried out by a Micro Raman setup (Horiba LabRAM) with the excitation wavelength of 532 nm at room temperature. Thermal gravimetric analysis (TGA) of GO, RGO and Pt@RGO solid powder was performed under nitrogen atmosphere with a heating rate of 10 °C min⁻¹ from 30 °C to 900 °C by using a Perkin-Elmer TGA-50H. The current density–voltage characteristics of the cells were recorded by a keithley multimeter under irradiance of 100 mW cm⁻² (AM 1.5 simulated illuminations, Photo Emission Tech). The fill factor (FF) and power conversion efficiency (η) of the solar cells are determined from eqn (1) and (2),

FF = V_MJ_M/V_OCJ_SC (1)

η = V_OCJ_SCFF/P_in (2)

where V_M and J_M are the voltage and current density at the maximum power output, respectively. J_SC and V_OC are the short-circuit photocurrent density and open-circuit photovoltage, and P_in is the intensity of the incident light (100 mW cm⁻²). The incident photon-to-current conversion efficiency (IPCE) was evaluated by measuring short-circuit photocurrent (J_SC) at different wavelengths (λ) using a homemade setup with a Bentham monochromator and dual light (tungsten and xenon) sources. The IPCE was calculated using eqn (3),

IPCE% = [1240 × J_SC (A cm⁻²)]/[λ (nm) × P (W cm⁻²)] × 100 (3)

where, P is the incident light power.

Photovoltage decay measurements were carried out after illuminating the cells under 1 Sun condition. The photovoltage decays after switching off the irradiation were monitored by an oscilloscope (Owon) through computer interface. The decays were fitted with exponential decay functions using origin software. Electrochemical impedance spectroscopy (EIS) were performed on an electrochemical workstation CHI650E (CH instruments) with a frequency range from 100 kHz to 0.1 Hz in the open circuit condition. All impedance measurements were carried out under a bias illumination of 100 mW cm⁻². The obtained spectra were fitted using the CHI650E software in terms of appropriate equivalent circuits.

3. Results and discussion

The formation of silver modified TiO₂ (Ag–TiO₂) by radiolysis and detailed characterization are reported in our previous publication.²³ The silver ions were reduced by the free radicals such as solvated electrons and alcohol radicals generated due to high energy radiation. The silver nanoparticles (NPs) (2% metal loading) are homogeneously distributed on P25 TiO₂ NPs as shown in a Transmission Electron Microscopic image (Fig. 1a). The average size of the silver NPs is 2 nm. The elemental distribution of the Ag–TiO₂ was further characterized by energy dispersive spectroscopic (EDS) mapping. The EDS elemental mapping clearly confirmed the presence and homogeneous distribution of Ag, Ti, and O elements in the Ag–TiO₂ as shown in Fig. 1b. The EDS spectrum indicates the presence of 1.61% Ag in the material (Fig. 1c).

Fig. 1 Characterization of Ag–TiO₂ nanocomposites. (a) TEM image of silver modified TiO₂. (b) SEM image and Ag, Ti, O elemental mapping images of Ag–TiO₂ nanocomposites. (c) EDAX spectrum of Ag–TiO₂ nanocomposites.

In order to lower the Pt content in CE which in turn decreases the cost, small amount of Pt has been hybridized with RGO (Pt@RGO). The simultaneous photo-reduction of GO and Pt salt under UV light irradiation without any chemical reducing or stabilizing agents results in the successful formation of the nanohybrid. In hydrogen-donating solvents, Pt(acac)₂ undergoes efficient photodegradation to yield colloidal platinum. The photoreduction mechanism of Pt(acac)₂ in alcohol solution has been established in our previous publication.²⁵ Fig. 2a and b illustrate the scanning electron microscopic (SEM) images of RGO and Pt NPs on the RGO, respectively. The homogeneous distribution of Pt NPs all over the RGO nanosheet is clearly evident from the TEM image as shown in Fig. 2c. The average particle size of Pt NPs is estimated by fitting the experimental TEM data over 200 particles and it is found to be 2.6 nm as shown in the inset of Fig. 2c. From high resolution TEM (HR-TEM) image of Pt@RGO, the inter-planar distance between the fringes of Pt NPs is found to be about 0.234 nm which is consistent with (111) planes (Fig. 2d).²⁰ The crystal planes of the RGO nanosheets are also evident from the HR-TEM image. The Pt content in Pt@RGO nanocomposite has been estimated from the ICP-MS and it is found to be 20.7%.

Fig. 2 Electron microscopic images of the Pt@RGO nanocomposites. SEM images of (a) RGO nanosheets and (b) Pt@RGO nanocomposites. (c) TEM and (d) HRTEM images of Pt@RGO nanocomposites. Inset of (c) shows the size distribution of the Pt NPs on RGO nanosheets.

The X-ray diffraction patterns (XRD) of GO, RGO and Pt@RGO are presented in Fig. 3a. The GO sample shows characteristic diffraction peaks at 25.9° and 42.3° which can be assigned to the (002) and (100) crystal planes, respectively. In case of RGO and Pt@RGO, the peak at 25.9° moved slightly to a higher angle which indicates the partial reduction of GO to the conjugated crystalline graphene network (sp²).²⁶ The peak at 42.3° corresponding to the (100) planes diminishes after the deposition of Pt on RGO nanosheets due to the surface coverage by Pt NPs.²⁷ In addition, the peaks of the Pt@RGO nanocomposites at 39.8°, 46.5° and 67.5° can be assigned to the (111), (200) and (220) crystal planes of face centered-cubic (fcc) Pt NPs, respectively. The results confirm the successful formation of Pt NPs over the RGO nanosheets. To further characterize the nanocomposites, Raman spectra of GO, RGO and Pt@RGO have been collected as shown in Fig. 3b. The Raman spectra show two major peaks, a G band at 1590 cm⁻¹ and a D band at 1352 cm⁻¹. The G band corresponds to the in plane bond stretching motion of pairs of C sp² atoms (E_2g mode), while D band corresponds to the disordered structures, such as bond angle disorders and asymmetric lattices in the graphite.^6a,18 The intensity ratio of D band to G band (D/G) increases after reduction of GO to RGO and further increases after the formation of the nanocomposite. The increase in D/G intensity ratio can be attributed to (a) an increase in the quantity of amorphous carbon, (b) a higher density of defects on the structure, or (c) a reduction in the crystallite size or domains.²⁸ In the present case, the increase in the intensity ratio may be due to the increase in the density of defects on the graphene structures after deposition of Pt NPs. In case of Pt@RGO, a strong D′ peak at 1615 cm⁻¹ is observed, which can be assigned to the nonzero phonon density of the states at energies above the energy of the G band. These phonons are usually Raman-inactive but become active due to phonon confinement caused by the defects.²⁹ Moreover, thermogravimetric analysis has been carried out to assess the thermal stability of the nanocomposite as shown in Fig. 3c. For the GO sample, a weight loss of ∼40% occurs between 180 to 230 °C due to a pyrolysis of the labile oxygen-containing functional groups, yielding CO, CO₂ and steam.³⁰ The weight loss in the similar temperature range for the RGO and Pt@RGO decreases to ∼33% and 27%, respectively. This observation indicates the decrease of functional groups in the RGO and Pt@RGO samples after reduction. The complete decomposition of the GO and RGO samples occurs around 500 °C, while the carbonaceous material present in the Pt@RGO decomposes around 400 °C. The lower decomposition temperature can be attributed to an increase in the interlayer distance of the nanosheets due to the presence of Pt NPs. A residue of ∼20% remains at high temperature in case of Pt@RGO due to the presence of Pt NPs which is well consistent with the ICP-MS data.

Fig. 3 Characterization of the Pt@RGO nanocomposites. (a) X-ray diffraction patterns (b) Raman spectra and (c) thermogravimetric analysis profiles of Pt@RGO (red), RGO (blue) and GO (black).

In order to demonstrate practical applications utilizing Ag–TiO₂ at active electrode and Pt@RGO at counter electrode, a series of DSSCs were fabricated. Fig. 4a shows the photocurrent–voltage characteristics of DSSCs fabricated with different active and counter electrodes. The corresponding values of the photovoltaic parameters, such as the short circuit photocurrent density (J_SC), open circuit voltage (V_OC), fill factor (FF), power conversion efficiency (η) for the best cells along with the average efficiency values are presented in Table 1. An increase of around 16% (from 0.72 to 0.84 V) in the photovoltage (V_OC) is observed in case of Ag–TiO₂ at photoanode compared to that of the bare TiO₂ along with Pt at the counter electrode. During steady state illumination, V_OC is the difference in potential between the quasi-Fermi level of the TiO₂ film and the rest potential of the counter electrode, which remains in equilibrium with the redox couple. Semiconductor and metal NPs undergo electron equilibration under light illumination. In the Ag–TiO₂ nanocomposites, due to the larger work function of Ag (4.5–4.7 eV)³¹ compared to the electron affinity of TiO₂ (1.59 eV),³² a Schottky barrier exists at their interface.³³ The possible electron transfer pathways and formation of Schottky barrier are shown in Scheme 1. Under light illumination, the electrons from excited N719 dyes are injected to the Ag NPs on TiO₂ surface which results in accumulation of electrons in Ag NPs. As a result, the Fermi level of the Ag NPs shifted closer to the conduction band (CB) of TiO₂ in the Ag–TiO₂ nanocomposites which is responsible for the increase in V_OC. To the best of our knowledge, such a significant enhancement in the V_OC in presence of Ag NPs has not been reported earlier. Time resolved microwave conductivity (TRMC) studies have shown that electrons can be injected from Ag NPs to the CB of TiO₂ (ref. 23) in order to establish charge equilibrium in the system. In contrast, the electrons at the CB of the TiO₂ cannot reverse their path to recombine with the oxidized dye or electrolyte due to the presence of Schottky barrier which results in enhanced efficiency of the device. In addition to the enhancement in V_OC, photocurrent also increases (14.5 to 15.1 mA cm⁻²) in case of Ag–TiO₂ nanocomposite which is well documented in the literature.^10,11b This is due to an increase in effective absorption cross section of the dye (N719) and a modest increase in the framework surface area.

Fig. 4 Device performance. (a) J–V characteristics, (b) incident photon to current conversion efficiency (IPCE) spectra and (c) open circuit voltage decay profiles of different DSSCs.

Table 1 Solar cell performance using different active and counter electrodes^a

Photoanode	Counter electrode	Best values				Average value η (%)
		JSC (mA cm^−2)	VOC (V)	Fill factor (FF)	η (%)
a Short-circuit photocurrent densities (JSC cm^−2), open-circuit voltage (VOC) and efficiency (η).
TiO2	Pt–RGO	14.3	0.74	59.5	6.37	5.64
	Pt	14.5	0.72	59.1	6.21	5.58
	RGO	10.5	0.69	26.8	1.95	1.50
Ag–TiO2	Pt–RGO	15.4	0.86	60.8	8.02	7.22
	Pt	15.1	0.84	61.4	7.73	6.80

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Scheme 1 Schematic presentation of the dye-sensitized solar cell consists of Ag–TiO₂ photoanode and Pt@RGO counter electrode. Lower panel shows the energy level diagram.

After the successful demonstration of Ag–TiO₂ as a photoanode, the counter electrode has been modified with Pt@RGO to reduce the Pt loading as well as optimization of the catalytic activity. The amount of Pt in the Pt@RGO nanocomposite (2 μg in each CE) is 10 times smaller than that of the standard Pt counter electrode (20 μg in each CE) which lowers the cost and makes it viable for large scale commercial utilization. Although the amount of Pt decreases, the catalytic activity of Pt@RGO is comparable or slightly higher than the Pt only. The DSSCs with Ag–TiO₂ photoanode along with Pt@RGO and Pt counter electrode show energy conversion efficiency of 8.02% and 7.73%, respectively. The effect of CE on DSSC performance is dictated by the electrical conductivity and electrocatalytic activity in the reduction of triiodide to iodide.³⁴ In case of Pt@RGO, the catalytic activity of Pt and high electron conductivity of the stable support material RGO are synergistically combined owing to more interfacial active sites. The combination of Pt@RGO with bare TiO₂ as photoanode also exhibits slightly higher efficiency (η = 6.37%) than the Pt alone as CE (η = 6.21%). However, using control RGO as CE with TiO₂ as photoanode showed lower efficiency of 1.95%. In order to investigate the light harvesting by the N719 dye, the incident photon to current conversion efficiency (IPCE) of the cells were monitored as shown in Fig. 4b. The spectra collected from different cells show good agreement between the wavelength of photocurrent maximum and N719 absorption maximum (λ_max = 520 nm), signifying that the N719 sensitizers on the photoanode surface are indeed responsible for photocurrent generation. The trend in the photocurrent responses is consistent with the short circuit currents of the corresponding cells as shown in Fig. 4a. J_SC value of a cell can be estimated from the integrated area under the IPCE curve.³⁵ The J_SC values calculated from IPCE curves are found to be less compared to that obtained from the I–V measurements. The difference in the J_SC values is due to the difference in the light intensity or power (mW cm⁻²) used for the two measurements. The I–V characteristics were measured under 1 sun condition (100 mW cm⁻², AM 1.5 simulated illuminations from AAA certified solar simulator), whereas the IPCE measurements were performed under monochromatic light illumination from dual light (tungsten and xenon) sources using a monochromator. Thus the light intensity or power falling on DSSC at a given wavelength during IPCE measurement is much lower than that of the I–V measurements. The observation is consistent with the reported literature.³⁵

In order to investigate the lifetime of accumulated charges at the photoanode, the decay of open circuit voltage has been monitored for different cells in the dark following a brief period of illumination. Upon visible light illumination, the excited sensitizers inject electrons into TiO₂ electrode. The accumulated electrons in the TiO₂ drives the Fermi level to the more negative potential which reflects in the photovoltage enhancement. The forward electron injection from sensitizer to TiO₂ is terminated upon stopping the illumination, thus discharging of electrons occurs through the back electron transfer or recombination with the oxidized electrolytes. The open circuit voltage decay reflects the timescales for the recombination processes. Fig. 4c shows the decay profiles of different DSSCs fabricated using Ag–TiO₂ and TiO₂ as photoanode, Pt@RGO and Pt as CE. The fitted timescales are presented in Table 2. The Ag–TiO₂ photoanode in combination with Pt@RGO CE exhibit a remarkably slow rate of voltage decay (15.68 s) compared to the bare TiO₂ photoanode (3.74 s). This is due to the storage of electrons in Ag NPs²³ which prevents recombination with the oxidized electrolytes. This observation is consistent with the enhanced photovoltage in the presence of Ag–TiO₂ as a photoanode. The Pt@RGO CE in combination with TiO₂ photoanode shows slower decay with a lifetime of 3.74 s compare to the Pt CE (1.91 s). This can be attributed to the higher catalytic activity of Pt@RGO than Pt only at the CE in regenerating the oxidized electrolyte which in turn reduces the recombination processes. The stability of the DSSCs fabricated using Ag–TiO₂ photoanode and Pt@RGO CE, the photocurrent–voltage characteristics were monitored for 240 h. Fig. 5a depicts the change in short circuit current and open circuit voltage with time. Initially the J_SC decreases but after few days it is stable for a long period. The V_OC is very stable during the experimental time window which can be attributed to the stability of the Ag–TiO₂ photoanode. It is reported that the Ag NPs are easily damaged under oxidizing conditions.³⁶ However, our observation suggests that the Ag modified TiO₂ photoanode are stable which is consistent with the reported literature.^10,11 The fill factor and overall efficiency of the DSSCs are also consistent with time as shown in Fig. 5b.

Table 2 Dynamics of photovoltage transients of DSSCs fabricated using different active and counter electrodes^a

Active electrode	Counter electrode	τ1 (s)	τ2 (s)	τ3 (s)	τavg (s)
a Numbers in the parenthesis indicate relative weightages.
TiO2	Pt–RGO	0.09 (48.5%)	1.28 (16.9%)	10.08 (34.6%)	3.74
	Pt	0.08 (32.4%)	0.39 (19.9%)	3.78 (47.7%)	1.91
Ag–TiO2	Pt–RGO	0.26 (25.7%)	1.67 (19.2%)	27.77 (55.1%)	15.68
	Pt	0.16 (30.2%)	1.17 (24.3%)	9.99 (45.5%)	4.87

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Fig. 5 Stability of the device. Plot of (a) J_SC and V_OC, (b) fill factor and efficiency of the DSSC fabricated using Ag–TiO₂ photoanode and Pt@RGO CE with time.

In order to investigate the electronic and ionic processes in DSSCs, electrochemical impedance spectroscopy (EIS) were performed. As shown in Fig. 6a, the Nyquist plot features two semicircles, one in the high frequency region and the other one in the intermediate frequency region. The semicircle in the high frequency region can be attributed to the charge transfer at counter electrode while the semicircle in the intermediate frequency region is associated with the electron transport in the mesoscopic TiO₂ film and back reaction at the TiO₂/electrolyte interface. The Nyquist plots are fitted using the equivalent circuit shown in Fig. 6b and the parameters are summarized in Table 3. The R₂ value represents the interfacial recombination resistance at TiO₂/electrolyte interface. The R₂ value for Ag–TiO₂ photoanode (72.1 Ω) is greater than that of the bare TiO₂ photoanode (12.6 Ω) which can be attributed to the less charge recombination from Ag–TiO₂ photoanode to the triiodide ions in the electrolyte.^11a The observation is consistent with the slower photovoltage decay of the DSSC with Ag–TiO₂ photoanode as shown in Fig. 4c. This confirms that Ag NPs act as electron–hole separator due to the formation of Schottky barrier at the Ag–TiO₂ interface which prevents recombination with the oxidized electrolytes.³³ The R₁ value represents the charge transfer resistance at the CE interface i.e. the catalytic activity of the electrode for reducing triiodide ions. The R₁ value for Pt CE (1.9 Ω) is less than that of the Pt@RGO CE (11.0 Ω). Thus the Pt CE prepared by drop casting method exhibits slightly higher catalytic activity than that of the Pt@RGO CE as observed from EIS spectra^21a which can be attributed to the 10 times lower Pt content in Pt@RGO CE than that of the Pt CE. However, the device performance of the Pt@RGO CE is comparable or slightly higher than Pt CE as shown in Fig. 4a which decreases the cost of CE significantly.

Fig. 6 EIS of the device. (a) Nyquist plots of different cells. Inset shows the full region. (b) Equivalent circuit model that was used to fit the EIS spectra.

Table 3 EIS parameters of DSSCs fabricated using different active and counter electrodes

Active electrode	Counter electrode	Rs (Ω)	R1 (Ω)	R2 (Ω)
Ag–TiO2	Pt–RGO	15.5	11.0	72.1
TiO2	Pt	20.3	1.9	12.6

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4. Conclusion

In summary, we have successfully utilized Ag modified TiO₂ (Ag–TiO₂) as photoanode material in dye sensitized solar cell. On the other hand, we have demonstrated a facile synthesis of Pt NPs supported on RGO (Pt@RGO) surface by simple photo-reduction method without using chemical reducing or stabilizing agents. The quantity of Pt in the Pt@RGO nanocomposite is 10 times lower than that of the Pt counter electrode which reduces the cost and makes it viable for large scale commercial utilization. The combination of Ag–TiO₂ as photoanode and Pt@RGO as counter electrode leads to a light to electrical energy conversion with an overall efficiency of 8%. An enhancement of around 16% in the photovoltage (V_OC) is observed in case of Ag–TiO₂ at photoanode compared to that of the bare TiO₂, which can be attributed to the shifting of the quasi-Fermi level of the TiO₂ photoanode close to the conduction band in the presence of Ag NPs due to the formation of Schottky barrier at Ag–TiO₂ interface. Moreover, Pt@RGO is found to be more efficient that Pt only as CE due to the catalytic activity of Pt and high electron conductivity of the stable support material RGO. These advantages are synergistically combined owing to the large number of active interfacial sites. The open circuit voltage decay profiles show remarkably slow rate of electron recombination from Ag–TiO₂ photoanode to the oxidized electrolyte. The present study holds great promise for low cost, large scale fabrication of efficient solar devices.

Acknowledgements

P. K. thanks CSIR (India) for fellowship. We thank DST (India) for financial grants DST-TM-SERI-2k11-103 and SB-S1-PC-011-2013. We also thank DAE (India) for financial grant, 2013-37P-73-BRNS. PL and BL thank the DFG Research Training Group GrK1952/1 NanoMet, "Metrology for Complex Nanosystems" and the NTH-School “Contacts in Nanosystems”.

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