Influence of Poly (N-vinylcarbazole) as a Photoanode Component in

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Influence of Poly(N-vinylcarbazole) as a Photoanode Component in Enhancing the Performance of a Dye Sensitized Solar Cell Alagar Ramar, Ramiah Saraswathi, Muniyandi Rajkumar, and Shen-Ming Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06582 • Publication Date (Web): 25 Sep 2015 Downloaded from http://pubs.acs.org on September 28, 2015

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The Journal of Physical Chemistry

Influence of Poly(N-vinylcarbazole) as a Photoanode Component in Enhancing the Performance of a Dye Sensitized Solar Cell Alagar Ramara, Ramiah Saraswathi a* Muniyandi Rajkumarb, and Shen-Ming Chenb

a

Department of Materials Science, School of Chemistry, Madurai Kamaraj University, Madurai-625 021, Tamil Nadu, India.

b

Electroanalysis and Bioelectrochemistry Laboratory, Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, No.1, Section 3, Chung-Hsiao East Road, Taipei 106, Taiwan (ROC).

* Corresponding author. Phone: +91-452-2458247. Fax: +91-452-2459181. Email address: [email protected] (R. Saraswathi)

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ABSTRACT Increasing the dye adsorption and minimization of charge recombination are pivotal in improving the efficiency of a dye sensitized solar cell (DSSC). In the present investigation, these two effects have been accomplished through the introduction of poly(N-vinylcarbazole) (PVK) as an auxiliary component of the TiO2 photoanode in a N719 dye-based DSSC. TiO2/PVK hybrid nanocomposites were prepared by simple blending of TiO2 with small quantities of PVK (0.3 to 1.3 wt%) in the presence of PEG20000, acetylacetone, Triton X-100. XRD data revealed that the crystalline properties of TiO2 were not altered by the presence of PVK. The porosity and surface roughness of TiO2 were enhanced by the addition of PVK. Spin coated TiO2-PVK nanocomposite films showed highly enhanced dye adsorption in comparison to pristine TIO2 film which had been attributed to an increase in the basicity of the TiO2 surface. The N719 dye sensitized TiO2/PVK photoanodes were assessed for their performance in a DSSC assembled with the I−/I3− redox electrolyte and sputtered Pt as counter electrode. The quantity of PVK in the nanocomposite was optimized by evaluating the photovoltaic performance. A maximum power conversion efficiency of 7.10% was achievable with a 0.9 wt% PVK loaded TiO2 which was 58% greater than that of the pristine TiO2 cell (4.48%). Impedance data showed a very low charge transfer resistance of 3.79 Ω and a high electron life time of 31.41 ms implying an effective reduction in the back electron transfer.

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1. INTRODUCTION Dye sensitized solar cell (DSSC) is a promising alternative to conventional inorganic solar cells due to its ease of fabrication, low cost and high efficiency.1 A typical DSSC mainly comprises a dye adsorbed semiconducting metal oxide, usually TiO2 coated on an optically transparent conducting oxide substrate as photoanode, an electrolyte mostly acetonitrile containing LiI and I2 and a counter electrode, usually Pt.2 The dramatic increase in the efficiency of the DSSC to 7.1% achieved by O'Regan and Grätzel1 in 1991 had led to intensive research with numerous types of mesoporous TiO2, several dyes as well as electrolytes in order to improve the efficiency further.2,3 At present, a record efficiency of 12.3% has been achieved for a DSSC made of nanocrystalline TiO2 sensitized with a chemically synthesized zinc porphyrin dye in a Co(II/III)tris(bipyridyl)-based redox electrolyte.4

Among all the components in a DSSC, the photoanode plays a very important role in determining the power conversion efficiency, since it is involved in both the charge generation and charge separation processes.5 Extensive efforts have been made to improve the performance of photoanodes. The primary strategy adopted to increase the power conversion efficiency of a DSSC is to enhance the dye adsorption by increasing the specific surface area of the nanostructured TiO2 anode.6,7 Increase in dye adsorption has also been realized by the incorporation of high specific surface area nanomaterials such as carbon nanotubes and graphene into TiO2.8,9 Also adsorption of certain acidic dyes like N3, N719 and N749 had been accomplished through an increase in the basicity of TiO2 by the addition of small quantities of BaCO3 10, Al2O3 11 and HfO2 12.



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Another recommended strategy to improve the power conversion efficiency of a DSSC relies on arresting the back electron transfer from the conduction band of TiO2 to the oxidized dye or the redox couple in the electrolyte.2 Intensive studies have revealed that the back electron transfer can be retarded by the surface modification of TiO2 with certain metal oxides such as La2O3 13, SiO2

11

and Ga2O3 14 whose conduction band edges are higher than

that of TiO2. The thin coating of metal oxide refrains the direct contact between TiO2 and electrolyte reducing the recombination.15 The back electron transfer can also be suppressed by introducing an extra thin layer of some organic materials on the TiO2 photoanode as coadsorbents. A variety of co-adsorbents such as tris(dodecyloxy)benzoic acid, poly(ethylene glycol) based oligomers, citric acid and 4-guanidinobutyric acid have been reported to aid in the minimization of back electron transfer through the so-called surface passivation of recombination sites (trap states) in TiO2 leading to an enhancement in the photocurrent and overall performance. 16-19

Organic conducting polymers are being used in DSSCs either as hole transport materials20 or as counter electrodes2 but recently there have been some studies reported on their use as photoanode components. Polyaniline/ZnO nanohybrid21, polypyrrole/ZnO nanocomposite22 and PEDOT-PSS/TiO2 23 have been the photoanode materials investigated so far and these conjugated polymers were used for different functions.

For example, the

high electron density of polyaniline/ZnO nanohybrid was reported to lead to a more effective

charge

separation

and

faster

interfacial

charge

transfer

across

the

ZnO/polyaniline/dye layer. The increase in efficiency in polypyrrole-ZnO nanocomposite was mainly attributed to the morphological effects. The PEDOT-PSS was used as a binding layer between the optically transparent substrate and TiO2 to block the substrate from coming into direct contact with the redox electrolyte reducing the recombination.


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In the present work it is proposed to use poly(N-vinylcarbazole) (PVK) as an effective component of the TiO2 photoanode and study its role in the performance of DSSCs. PVK is one of the most studied photoconducting polymer materials in organic electronics. It possesses good thermal stability and processablity.24 PVK has been used as a photoactive material in polymer solar cells and used as a hole transport material in organic solar cells and DSSCs.25-27 It has also been suggested as a good transparent window befitting its use as a photoanode.28,29 In this study, we report that PVK, as auxiliary component of TiO2 can be expected to improve the efficiency of DSSC by playing a dual role i.e. towards achieving both improved dye adsorption and minimizing back electron transfer. The lone electron pairs on the nitrogen atom in PVK can facilitate the anchoring of the acidic N719 dye molecules on the TiO2 surface. In other words the basicity of the TiO2 can be increased by the addition of electron-rich PVK which will lead to enhanced dye adsorption. Secondly, the conduction band edge (LUMO level) of PVK (−2.3 eV) is higher than that of TiO2 (−4.2 eV) and this will create an energy barrier for the back electron transport of the photoinjected electrons in TiO2 to come in contact with the oxidized dye molecules or redox mediator in the electrolyte.

2. EXPERIMENTAL 2.1 Preparation of Photoanodes and Fabrication of DSSC TiO2/PVK hybrid nanocomposites were prepared by blending a variable amount of PVK (Mw ~ 42,000, Sigma-Aldrich) with 6 g TiO2 (P25, Sigma-Aldrich), 0.66 g of PEG20,000 (Average Mn 20,000, Sigma-Aldrich), 200 µL acetylacetone (Sigma-Aldrich) and 200 µL Triton X-100 (Sigma-Aldrich) using 12 mL deionized water in an agate mortar for 1 h. The amount of PVK in the nanocomposite was varied as 0.3, 0.5, 0.7, 0.9, 1.1 and 1.3 wt%. The resulting nanocomposites are designated as TiO2/(PVK)x, where x denotes the wt% of PVK 5 ACS Paragon Plus Environment


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used in the preparation. For control experiments, a colloidal paste of TiO2 was prepared as described above without the addition of PVK.

The anodes were prepared by spin coating of about 50 µL of TiO2 or TiO2/(PVK)x colloidal paste on a pre-cleaned indium tin oxide (ITO) (Merck Display Technologies Ltd, Taiwan) substrate. The coated films were dried at room temperature for 30 minutes and annealed at 450 °C. This procedure was repeated once more to improve the thickness of the resulting films. The measured thickness of the TiO2 and the nanocomposite films are nearly the same (17 ± 0.1 µm). The films were dye sensitized by soaking in 5 x 10-4 M solution of N719 dye (Sigma-Aldrich) in anhydrous ethanol (Sigma-Aldrich) at room temperature. After 24 h of standing, the anodes were rinsed with anhydrous ethanol and dried in air. Sputter-coated thin Pt films on ITO were used as cathodes. The DSSCs were assembled by hot-pressing the dye-sensitized anodes and Pt cathode with a 60 µm Surlyn film (Solaronix) spacer at 100 ºC. The liquid electrolyte consisting of 0.3 M 4-tert-butylpyridine (SigmaAldrich), 0.5 M LiI (Sigma-Aldrich), and 0.05 M I2 (Sigma-Aldrich) in acetonitrile was then injected between the electrodes (active area 0.25 cm2) through a capillary effect. In order to ascertain the reproducibility of the data, three identical DSSCs were assembled for each kind of photoanode and tested under identical conditions.

2.2 Characterization The thickness of the coated films on the ITO substrate was measured using a MITUTOYO stylus profilometer (SJ 301, Japan). Thermogravimetric analysis (TGA) was performed using a thermogravimetric analyzer (TAQ500) at a heating rate of 10 °C/min under nitrogen atmosphere. FT-IR spectra were recorded using 8400S Shimadzu FT-IR spectrophotometer in the region of 4000 cm−1 – 400 cm−1 with a spectral resolution of 2 cm−1 using dry KBr at room temperature. The field emission scanning electron microscope 6 ACS Paragon Plus Environment

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(FESEM) images were obtained using FESEM (Jeol-JSM7600F, 30 kV). The surface topography was imaged using an atomic force microscope (APE Research Model A100 SGS). The transmission electron microscopy (TEM) images were obtained using Hitachi H7000 model equipment with a CCD camera attachment, operating at 100 kV. The X-ray diffraction (XRD) patterns of the samples were obtained using an X-ray diffractometer (XPERT-PRO PANalytical) with Cu Kα radiation (λ = 1.540 Å). The accelerating voltage and applied current were 45 kV and 30 mA respectively. The absorption spectra were obtained using a UV-visible spectrophotometer (Jasco V-630, Japan).

The electrochemical impedance spectroscopy (EIS) data were obtained with an impedance analyzer (Zahner, Germany) in a two-electrode configuration under illumination with (AM 1.5G) 100 mW/cm2. The TiO2 and TiO2/(PVK)x nanocomposite coated films on ITO were used as working electrodes and sputtered Pt electrode was used as the counter electrode. The applied bias voltage was set at the open-circuit voltage (Voc) of the DSSCs. The measurements were carried out at an AC amplitude of 10 mV in the frequency range from 100 mHz to 100 kHz. The EIS data were analyzed with the built-in Thales software of the impedance analyzer. The photovoltaic data were obtained with the help of a digital source meter (Keithley Instruments Inc., Model 2400) and a solar simulator (HONG-MING TECH) under one-sun illumination (AM 1.5G, 100 mW/cm2). Cyclic voltammograms were recorded using an electrochemical workstation (Model 680, CH Instruments, USA) with a conventional three electrode system. Dye coated TiO2 or TiO2/(PVK)x nanocomposite film was used as the working electrode. A large Pt wire and Ag/AgCl were used as counter and reference electrodes respectively. Acetonitrile containing 0.5 M LiClO4 served as the electrolyte.



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2.3 Estimation of Amount of Adsorbed Dye The amount of N719 dye adsorbed by TiO2 and TiO2/(PVK)x nanocomposite films was determined by desorbing the dye in 3 mL of 1x10–3 M KOH in ethanol-water (1:1) for 2 h and measuring its UV-visible absorption spectrum.30 The UV-visible spectrum of a known concentration of N719 dye in anhydrous ethanol was used as a reference to determine the amount of desorbed dye. The amount of dye adsorbed was calculated using the BeerLambert law by taking the absorbance value at 502 nm. The molar extinction coefficient (ε) of the N719 dye was calculated to be 11,520 Μ−1cm−1 by a standard calibration method.

3. RESULTS AND DISCUSSION 3.1 Characterization of TiO2 and TiO2/(PVK)x nanocomposite The preparation of photoanodes involves annealing of TiO2 and TiO2/(PVK)x samples at 450 °C which is necessary to remove the organic materials used during the electrode preparation and to make coherent films with good adhesion on the ITO substrate. Also annealing will increase the crystallinity of TiO2 and promote chemical interaction between the particles for better electrical connection.31 TiO2 possesses a good thermal stability upto 850 °C.32 On the other hand, conducting polymers are known to have a poor thermal stability. The temperature for the onset of decomposition of PVK has been reported to be about 435 °C.33 Therefore it becomes necessary to ascertain the thermal stability of the organic polymer in the TiO2/(PVK)0.9 nanocomposite. Figure 1A shows a comparison of the TGA curves of TiO2, and the representative TiO2/(PVK)0.9 nanocomposite and the inset shows the TGA curve of PVK. The TGA curve of PVK shows an initial weight loss of 5% upto 240 °C. Thereafter, the rate of weight loss becomes rather fast and a weight loss of 52% has occurred upto 450 °C. About 93% of the polymer has degraded upto 478 °C with a total degradation occurring at 570 °C. In the TGA curve of TiO2, the initial weight loss below 150 °C is primarily due to evaporation of physically adsorbed water from TiO2 pores. There is a 8 ACS Paragon Plus Environment

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sharp weight loss between 150 to 300 °C which is ascribed to the removal of the acetylacetone.34 The weight loss between 250 to 350 °C can be attributed to the decomposition of Triton X-100.35 At about 400 °C there is a retention of 86.2 wt% of TiO2 which remains constant thereafter until the investigated temperature of 800 °C. The thermogram of TiO2/(PVK)0.9 nanocomposite follows a profile similar to that of TiO2. Based on a comparison of the two TGA curves, it can be inferred that the weight loss at 450 °C is almost the same for both TiO2 (13.8%) and TiO2/(PVK)0.9 nanocomposite (13.6%). Therefore, it is apparent that the presence of 0.9 wt % of PVK in the nanocomposite does not affect the thermal properties of TiO2.

In order to further ascertain that the PVK remains intact in the nanocomposite after annealing at 450 °C, FT-IR spectra of annealed TiO2 and TiO2/(PVK)0.9 were obtained. For comparison the FT-IR spectrum of PVK was also recorded and the spectra are shown in the supporting information (Figure S2). The FT-IR spectrum of TiO2 shows its characteristic absorption between 470−800 cm−1 corresponding to vibration of Ti-O-Ti bonds.36 The characteristic bands of PVK can be found in the FT-IR spectrum of PVK.37 The bands at 1450 and 1483 cm−1 correspond to ring vibration of N-vinylcarbazole moiety. The C-H inplane deformation of aromatic ring and vinylidene group can be found at 1218 cm−1 and 1328 cm−1 respectively. The FT-IR spectrum of the TiO2/(PVK)0.9 nanocomposite reflects the characteristic bands of its individual components. The PVK bands viz. 418 and 719 cm−1 overlap with the broad band of TiO2 between 470-800 cm−1. The presence of several vibrational bands corresponding to PVK in the annealed sample of TiO2/(PVK)0.9 nanocomposite provides a clear evidence that the PVK remains stable during the annealing process.



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Figure 1. (A) TGA curves of TiO2 and TiO2/(PVK)0.9 nanocomposite under N2 atmosphere. Inset shows TGA of PVK. and (B) XRD patterns of PVK, TiO2 and TiO2/(PVK)0.9 nanocomposite films. The peaks marked with an asterisk (*) are those of ITO.

The influence of PVK on the crystalline properties of TiO2 can be understood from a comparison of the XRD patterns of PVK, TiO2 and TiO2/(PVK)0.9 nanocomposite shown in Figure 1B. The TiO2 sample used in this study is the commercial P25 TiO2 which is a mixture of 86% anatase and 14% rutile forms.38 Therefore the XRD pattern can be expected to have characteristic peaks of both anatase and rutile phases. The diffraction peaks at 25.06, 37.60, 47.94, 53.92, 55.06 and 62.63° correspond to signals produced by crystal planes, (101), (004), (200), (105), (211) and (204) of the body centered tetragonal crystalline structure of anatase form of TiO2 [JCPDS No.21-1272]. The diffraction peak at 27.27° corresponds to the (110) plane of the rutile phase of TiO2.39 The XRD pattern of PVK shows a broad peak centered at 21° indicating the amorphous nature of the polymer and is in accordance with the earlier reported value.40 It can be observed that the shape and position of the characteristic diffraction peaks for TiO2/(PVK)0.9 nanocomposite are nearly the same as those of pristine TiO2. This perhaps implies that the crystalline properties of TiO2 are not altered by the presence of PVK in the nanocomposite. The XRD peak of PVK is not


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distinctly observable in the XRD pattern of TiO2/(PVK)0.9 which may be due to the presence of very small amount of PVK (0.9 wt%) in the nanocomposite. The average crystallite size values of TiO2 and TiO2/(PVK)0.9 nanocomposite were calculated using the Debye-Scherrer equation41 taking into account the three prominent crystal planes (101), (004) and (200) and are found to be 25.57 and 28.43 nm respectively.

The surface morphologies of the photoanode materials have been obtained. The FESEM images of TiO2 and TiO2/(PVK)0.9 nanocomposite are presented in Figures 2A and B respectively. TiO2 appears as densely-packed spherical nanoparticles. The morphology of TiO2/(PVK)0.9 is somewhat similar to that of TiO2. The presence of voids suggests a highly porous structure. Figures 2C and 2D show the 2D AFM images of TiO2 and TiO2/(PVK)0.9 nanocomposite films while those of 3D and roughness plots are given in the supporting information (Figure S1). The AFM image of TiO2 shows small spherical nanoclusters while that of the TiO2/(PVK)0.9 nanocomposite indicates a distribution of the spherical TiO2 nanoclusters on the PVK domains. In other words, the nanocomposite film consists of numerous interconnected TiO2/PVK particles fused together to form an extended semiconducting network. A comparison of the root mean square surface roughness data of TiO2 (0.59 nm) and TiO2/(PVK)0.9 nanocomposite (1.54 nm) films shows a nearly three time increase in the latter. Such an increase in the surface roughness in the TiO2/(PVK)0.9 nanocomposite can be expected to aid in the dye adsorption.


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A

B

C

D

Figure 2 FESEM and AFM images of TiO2 (A, C) and TiO2/(PVK)0.9 nanocomposite (B, D) films. Figure 3 shows the TEM images of PVK, TiO2 and TiO2/(PVK)0.9 nanocomposite. The TEM image of PVK shows the presence of both small and large spherical clusters in the size range of 7-50 nm. The TiO2 nanostructure is observed as highly crystalline particles in the size range 15-40 nm. The TEM image of TiO2/(PVK)0.9 nanocomposite indicates a very good miscibility between the TiO2 nanocrystals and the spherical nanoclusters of PVK and the particle size varies between 33 and 75 nm.

A

20 nm

B

C

20 nm

20 nm

Figure 3. TEM images of (A) PVK, (B) TiO2 and (C) TiO2/(PVK)0.9 nanocomposite.


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3.2. Absorption Spectroscopic Investigation of Dye Adsorption UV-visible spectroscopy was used to study the effects of addition of PVK to TiO2 on the dye adsorption by the films. The absorption spectra of the dye sensitized films of TiO2 and TiO2/(PVK)x nanocomposites in the visible region are presented in Figure 4A. The spectrum of the dye sensitized TiO2 film shows a characteristic absorption maximum at 486 nm, pertaining to the metal - ligand charge transfer in the N719 dye.30 The absorption spectra of the dye sensitized TiO2/(PVK)x nanocomposite films coated on ITO substrates show an increase in the absorbance value along with a red-shift in the absorption maximum on increasing the PVK loading in the nanocomposite. The increased intensity clearly reveals that the concentration of adsorbed dye molecules at the TiO2 surface of TiO2/(PVK)x nanocomposites has increased with PVK loading. The electron-rich PVK can be considered as a Lewis base and its addition to TiO2 can thus facilitate the anchoring of the N719 dye molecule with carboxylic acid groups (Figure 5). Secondly, the red shift of the absorption maximum value from 486 nm for TiO2 to 521 nm with increasing loading of PVK in the nanocomposite may be ascribed to the aggregation of the N719 dye molecules adsorbed on the TiO2 surface.42,43 The N719 dye contains two carboxylic groups which are trans to each other. When the dye molecule anchors on the TiO2 surface through only one carboxylic group, aggregation might occur through hydrogen bonding between the free carboxylic groups of two different dye molecules.44,45



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Figure 4. UV-visible absorption spectra of (A) N719 dye adsorbed TiO2 and TiO2/(PVK)x nanocomposite films coated on ITO substrates, (B) N719 dye desorbed from TiO2 and TiO2/(PVK)x nanocomposite films and (C) Relation between the estimated concentration of adsorbed dye and the PVK loading in TiO2.

Figure 5. Schematic representation of anchoring of N719 dye on PVK modified TiO2 surface


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The absorption spectra of an anhydrous ethanolic solution of desorbed N719 dye from TiO2 and the TiO2/(PVK)x nanocomposite films are given in Figure 4B. The concentrations of the adsorbed dye by the various TiO2 films are estimated based on the absorbance value of the peak at 502 nm and are shown in Table 1. The results show an impressive improvement in the concentration of adsorbed dye in TiO2/(PVK)1.3 film by about four times with respect to the TiO2 film. A plot of the dye concentration against the wt% of PVK is shown in Figure 4C which gives an exponential relation between the two. These results confirm that PVK loading increases the basicity of TiO2 resulting in enhanced dye adsorption.

3.3 Electrochemical Impedance Analysis Electrochemical impedance spectroscopy (EIS) has been widely used to investigate the kinetics and energetics of charge transport and recombination in DSSCs.46 The Nyquist plots of the assembled DSSCs with TiO2 and TiO2/(PVK)x nanocomposite films under illumination are shown in Figure 5A. All the impedance plots show two semicircles in the investigated frequency range of 100 mHz to 100 kHz. The semicircle in the high frequency region corresponds to the charge transfer resistance at the counter electrode (Rct1) while the intermediate frequency response can be attributed to the charge transfer resistance (Rct2) at the TiO2/dye/electrolyte interface.47 The impedance data can be fitted to an equivalent circuit shown as inset of Figure 5A. The derived EIS parameters are given in Table 1. The relevant parameters to be considered are the charge transfer resistance at the photoanode (Rct2) and the electron life time (τe). The Rct2 value of the TiO2 cell is 36.86 Ω. This value is considerably reduced upon PVK loading. The lowest Rct2 value of 3.79 Ω is observed for the TiO2/(PVK)0.9 nanocomposite photoanode based DSSC. This means that the fastest electron (hole) generation and transport as well as the lowest hole-electron recombination occurs in



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this cell. However, when the PVK content is increased beyond 0.9 wt%, the Rct2 value begins to increase (Table 1). This may be attributed to the aggregation of dye molecules in TiO2/(PVK)1.1 and TiO2/(PVK)1.3 nanocomposites which causes the suppression of electron injection into TiO2 and thereby increases the charge transfer resistance (Rct2). The electron lifetime values in the DSSCs based on various photoanodes are determined from the Bode plots constructed from the impedance data. Figure 5B shows the Bode plots of the TiO2/(PVK)x nanocomposite photoanodes in comparison to TiO2. The Bode plots are found to be shifted to lower frequencies for all the TiO2/(PVK)x films with respect to that of the pure TiO2 film. The electron lifetime (τe) can be obtained using Equation 1.48

τe = 1/2πfmax

(1)

where fmax is the peak frequency.

The τe values of TiO2 and TiO2/(PVK)x nanocomposites based DSSCs are given in Table 1. The τe for all the TiO2/(PVK)x nanocomposites are found to be higher than that observed at the TiO2 photoanode. These results imply a very efficient control of the back electron transfer at the TiO2/(PVK)x photoanodes in comparison to that of the TiO2 cells. Among the six nanocomposite photoanodes investigated, the highest τe value of 31.41 ms has been obtained for the cell with the TiO2/(PVK)0.9 nanocomposite as photoanode. A longer electron lifetime enables good charge collection efficiency at the ITO and also it implies a reduced back electron transfer. Thus the impedance data helped to identify the TiO2/(PVK)0.9 as the most efficient photoanode material with the lowest Rct2 value (3.79 Ω) and highest τe value (31.41 ms).


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Figure 5 (A) Nyquist and (B) Bode plots of DSSCs based on TiO2 and TiO2/(PVK)x nanocomposites (Inset of (A) is the equivalent circuit diagram used for fitting the EIS data)

Table 1 Amount of dye adsorbed by the nanocomposite films and electrical parameters derived from fitting the impedance data of the DSSCs assembled with TiO2/(PVK)x photoanodes, to the equivalent circuit shown as inset of Fig. 5 (A).

Dye adsorbed

Rct2

fmax

τe

(nmol/cm2)

(Ω)

(Hz)

(ms)

TiO2

125

36.86

18.62

8.55

TiO2/(PVK)0.3

142

26.58

7.14

21.66

TiO2/(PVK)0.5

179

13.93

7.14

22.15

TiO2/(PVK)0.7

245

3.91

7.14

22.60

TiO2/(PVK)0.9

301

3.79

4.98

31.41

TiO2/(PVK)1.1

383

9.35

7.14

22.27

TiO2/(PVK)1.3

472

16.08

7.14

21.85

Cell



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3.4. Photocurrent-Voltage Characteristics The photovoltaic characterization of DSSCs was obtained by assembling three identical cells for each of TiO2 and the six TiO2/(PVK)x nanocomposites based photoanodes. The J-V curves and the photovoltaic parameters obtained from the J-V curves for the best cells are shown in Figure 6 and Table 2 respectively. The primary observation is that the photovoltaic performance of the DSSCs based on the TiO2/(PVK)x nanocomposite films is strongly dependent on the amount of PVK present in the TiO2 photoanodes. The trend in the change of photovoltaic parameters with respect to the PVK loading in the nanocomposites has been shown in Figure 7. The pure TiO2 based DSSC shows an open circuit voltage (Voc) of 761 mV, short-circuit current density (Jsc) of 10.28 mA/cm2, fill factor (FF) of 0.57 and efficiency (η) of 4.48%. Even an addition of a small amount of 0.3 wt% of PVK increases the Jsc value from 10.28 to13.74 mA/cm2 and η is also increased from 4.48 to 5.59 % without any change in the fill factor. Further increase in loading of PVK results in a favourable change in the photovoltaic parameters compared to the pristine TiO2 cell. Among the various TiO2/(PVK)x nanocomposites, the TiO2/(PVK)0.9 shows the best photovoltaic performance in terms of the Jsc (18.45 mA/cm2) and η (7.10%) which are 79% and 58% higher than the values obtained for the pristine TiO2 cell. The enhanced Jsc can be ascribed to the efficient charge generation due to better dye adsorption, effective photoelectron injection into the conduction band of TiO2 and also minimization of back electron transfer through the imposed energy barrier upon modification of the TiO2 by PVK (Figure 8). The fill factor for all the DSSCs remains almost the same.


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Figure 6. J-V curves of DSSCs fabricated with TiO2 and TiO2/(PVK)x nanocomposites.

Table 2 Comparison of the photovoltaic parameters of the DSSCs based on TiO2 and TiO2/(PVK)x nanocomposite measured at 100 mW/cm2.

Photoanode

Voc (mV)

Jsc (mA/cm2)

FF

η (%)

TiO2

761

10.28

0.57

4.48

TiO2/(PVK)0.3

710

13.74

0.57

5.59

TiO2/(PVK)0.5

701

16.09

0.53

6.43

TiO2/(PVK)0.7

701

17.91

0.53

6.67

TiO2/(PVK)0.9

677

18.45

0.56

7.10

TiO2/(PVK)1.1

665

18.33

0.54

6.65

TiO2/(PVK)1.3

647

16.84

0.52

5.77



20

B

A

780

Jsc (mA/cm2)

Voc (mV)

740 700 660 620 -0.1

0.2

0.5 0.8 PVK (wt%)

1.1

16

12

8 -0.1

1.4

0.7

0.2

0.5 0.8 PVK (wt%)

0.2

0.5 0.8 PVK (wt%)

1.1

1.4

7.5

D

C 6.5

η (%)

0.6 FF

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5.5

0.5

4.5 0.4 -0.1

0.2

0.5

0.8

1.1

3.5 -0.1

1.4

PVK (wt%)

1.1

1.4

Figure 7. The effects of PVK loading on (A) Voc (B) Jsc (C) FF and (D) η in the TiO2/(PVK)x nanocomposites based DSSCs. Each measurement is repeated with three identical DSSCs assembled with fresh photoanodes. The relative standard deviation values are 2.1% for Voc, 6.4% for Jsc, 4.6% for FF and 5.6% for η.


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Figure 8. Schematic representation of energy level diagram and electron transfer processes in a DSSC with TiO2/(PVK)x nanocomposites. The dotted arrows correspond to back electron transfer.

However, the Voc values of the DSSCs with the nanocomposite photoanodes are found to decrease with increased loading of PVK. Voc is given by the difference of the Fermi level of the electrons in the TiO2 and the redox potential of the electrolyte.49 It is very likely that the Fermi level of the electrons in TiO2 has been shifted positively upon loading with PVK. It has been established that acidic dyes like N719, N712 and N3 upon adsorption, transfer most of the protons to the TiO2 surface charging it positively and causing the Fermi level to move down.50,51 The addition of PVK to TiO2 eases the proton transfer from the N719 to the TiO2 surface. The increase in PVK loading makes the proton transfer even more effective as evidenced from the increased dye adsorption. So one can expect that the Fermi level of electrons in TiO2 is gradually moved down upon PVK loading resulting in lower Voc.



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The changes in Voc and the trapping states induced by the presence of PVK were investigated by cyclic voltammetry.16,17 The cyclic voltammograms of dye adsorbed TiO2 and TiO2/(PVK)x nanocomposite electrodes were measured in acetonitrile containing 0.5 M LiClO4 electrolyte. Figure 9 shows the cyclic voltammograms and energy level diagrams obtained by plotting the total injected charge (Q) against the applied potential (V vs Ag/AgCl) for TiO2/N719 and TiO2/(PVK)0.9/N719 electrodes. The cyclic voltammetric data for the other nanocomposites are given in the supporting information (Figure S3). The capacitive currents of the electrodes reveal a gradual anodic shift in the onset potentials with increased loading of PVK (Table S1). For example, the onset potential of TiO2 (−205 mV) is shifted to −72 mV for TiO2/(PVK)0.9 electrode indicating a positive shift in the conduction band of the TiO2 upon loading with 0.9 wt% PVK. These positive shifts in the conduction band clearly account for the decrease in Voc values of the DSSCs with the nanocomposite photoanodes.

Figure 9. (A) Cyclic voltammogram of TiO2 and TiO2/(PVK)0.9 electrode in 0.5 M LiClO4/acetonitrile as supporting electrolyte at a scan rate of 50 mV/s. (B) Energy levels at the TiO2/electrolyte interface.


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The results demonstrate that the modification of TiO2 photoanode by a small amount of PVK (< 1%) is very favourable in that the efficiency of the DSSC can be enhanced to a maximum of 58% with respect to the pristine TiO2 photoanode based cell. The presence of PVK increases the dye adsorption onto TiO2, thereby leading to an enhanced photocurrent. The impedance data obtained under illumination clearly indicate that the TiO2/PVK nanocomposite effectively reduces the back electron transfer and thereby the overall cell performance is improved.

4. CONCLUSION For the first time, the effects on the modification of TiO2 photoanode by a photoconducting polymer, PVK was investigated. The polymer was added in very small amounts (< 1.3 wt% PVK). The polymer is found to be very effective in improving the dye adsorption properties of TiO2. The estimated concentration of adsorbed dye was found to vary exponentially upon increase in PVK loading from 0.3 to 1.3 wt%. EIS data for the TiO2/(PVK)x nanocomposites showed favourable results in terms of reduced charge transfer resistance (Rct2) and increased electron lifetime (τe) with respect to the TiO2-based cell. The increased dye adsorption and also electron life time have led to an enhancement in both photocurrent and efficiency values. Under optimized condition, the DSSC based on the TiO2/(PVK)0.9 nanocomposite as photoanode showed the best photovoltaic performance with a Voc of 677 mV, Jsc of 18.45 mA/cm2, FF of 0.56 and η of 7.10%.

ACKNOWLEDGEMENTS Financial support of this work by India-Taiwan Programme in Science and Technology, New Delhi, India is gratefully acknowledged. The authors also acknowledge



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the Department of Science and Technology, Government of India for the funding a major research project (SR/S2/CMP-58/2006).

Supporting Information: The AFM images (3D and roughness plots) of TiO2 and TiO2/(PVK)0.9 nanocomposite films are in Figure S1. FT-IR spectra of TiO2, PVK and TiO2/(PVK)0.9 nanocomposites are in Figure S2. Cyclic voltammograms and onset potentials of TiO2 and TiO2/(PVK)x nanocomposite electrodes are in Figure S3 and Table S1 respectively. This information is available free of charge via the Internet at http://pubs.acs.org.


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Table of Contents (TOC) Image

ITO Pt Electrolyte

ITO TiO2

PVK

N719


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Influence of Poly (N-vinylcarbazole) as a Photoanode Component in

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