Photovoltaic and Impedance Properties of Hierarchical TiO 2 Nanowire Based Quantum Dot Sensitized Solar Cell

Growth and characterization of TiO 2 nanowire (NW) assemblies on FTO glass using a typical hydrothermal synthesis have been reported. CdS quantumdots (QDs) have been deposited onTiO 2 nanowires by successive ion layer adsorption and reaction (SILAR) method. FESEM image exhibits the flower-like hierarchical TiO 2 bunch of nanowires. HRTEM image confirms the size of CdSQDs between 5 and 6 nm. XRD and absorption studies revealed proper growth of CdS quantum dots on TiO 2 nanowires. At AM 1.5 illumination intensity, the solar cell, with the configuration FTO/TiO 2 -NW/CdS-QDs/Pt-FTO, displays a short circuit current (Jsc) of 1.295mA and an open circuit voltage (Voc) of 0.38V. The Voc and Jsc showed linear behavior at higher illumination intensities. The peak in power-voltage characteristics at various illuminations showed a shift towards higher Voc values. Capacitance-voltage (C-V), conductance-voltage (G-V), and series resistance-voltage (Rs-V) measurements of the cell in the frequency ranging from 5 kHz to 5MHz showed decreasing trend of capacitancewith increase of frequencywhereas increase in conductance and decrease in resistance have been noticed with increase of frequency. All the results including the individual behavior of the plots of capacitance, conductance, and series resistance as a function of bias voltage have been discussed.


Introduction
To address the current energy crisis and remedy to the pollution from conventional energy resources, the unlimited solar energy provides us a solution for an economically feasible and clean source of energy.In the recent past, for the improvement of solar cells, substantial emphasis on research has been devoted towards the application of quantum dots (QDs), also known as semiconductor nanocrystals (NCs), because of their interesting optoelectronic properties [1][2][3][4][5][6].QDs find interesting applications as their band gap can easily be tailored by adjusting their size to suit the solar spectrum [7,8].High extinction coefficients and large intrinsic dipole moments of QDs contribute to speedy charge separation [9].Besides, QDs offer novel prospects to create hot electrons and multiple electron-hole pairs with a distinct photon via impact ionization [10,11].Due to these exciting features, relatively narrow band gap semiconductor QDs are found to be perfect candidates towards the optimization of solar cell to obtain enhanced performance.A distinctive approach to fabricate quantum dots sensitized solar cells (QDSSCs) is to choose semiconductor QDs as light absorbing candidates to sensitize wide band gap metal oxide nanostructured films such as SnO 2 [12], ZnO [13], and TiO 2 [14,15].Quantum dots (QDs) from inorganic semiconductors such as CdS [16], CdSe [17], CdTe [18], PbS [19], and Bi 2 S 3 [20], which absorb light in the visible region, have been discovered to assist as sensitizers in place of dye molecules for photovoltaic cells.CdS, with broadly tunable band gap, is considered to be one of the potential photovoltaic materials.The sensitization of wide band gap semiconductors such as TiO 2 by the QDs is a strategic configuration for collecting the visible light used in photoelectrochemical applications.
One-dimensional nanostructures that include wires, rods, belts, and tubes have demonstrated increasing interest due to their attractive properties and exceptional uses [21].As compared to other low dimensional systems, the distinctive feature of nanowires is that they possess two quantum confined directions while still leaving one unconfined direction for electrical conduction.Hence, they find application where electrical conduction rather than tunneling is required [22,23].Titanium oxide exists in three crystallographic forms known as anatase, rutile, and brookite.Extensive studies have been carried out on one-dimensional (1D) anatase TiO 2 nanostructures such as nanotubes, nanorods [24], nanofibers, and nanowires [25], which offer better 1D electron transport path during the solar energy conversion process.The challenging task of increasing the current energy conversion efficiency of a solar cell is modification of 1D TiO 2 structure at the nanoregime.It is anticipated that split nanostructures can result in enlarged specific surface area and improved electron conductivity for enhanced photocurrent harvest efficiency [26].Several reports exist on branched nanostructures synthesized by vapor or wet chemical methods [27,28], but reports on the synthesis of 1D TiO 2 flower-like hierarchical hyperbranched nanostructures (aggregates of nanowires) and their sensitized composite structures are fewer.
In the present work, QDSSC is fabricated using flowerlike hierarchical TiO 2 hyperbranched nanostructure (clusters of nanowires) which was synthesized through hydrothermal process.CdS QDs are acquired by successive ion layer adsorption and reaction (SILAR) method in order to sensitize TiO 2 hyperbranched nanowires based microspheres films for the solar cell.Morphological, structural, and optical characterizations have been carried out and finally the photovoltaic and impedance properties of the solar cell with the configuration FTO/TiO 2 -NW/CdS-QD/Pt-FTO have been discussed.

Experimental Details
2.1.Synthesis of TiO 2 Nanowires.Flower-like hierarchical TiO 2 nanowire assemblies were grown on FTO substrates.In a typical synthesis, the substrate was ultrasonically cleaned sequentially in acetone, isopropyl alcohol (IPA), and deionized (DI) water for 15 min each and was finally dried under N 2 flow.Separately, 1 mL of a titanium isopropoxide solution was added dropwise to a 1 : 1 mixture of DI water and concentrated (35%) hydrochloric acid to obtain a clear transparent solution.The substrate was placed at an angle in a 60 mL Teflon liner and the precursor solution was added to it.The Teflon liner was loaded in an autoclave and was placed in an oven at 150 ∘ C for 15 hrs.After synthesis, the autoclave was cooled to room temperature and the substrates were taken out, rinsed extensively with deionized water, and left to dry in ambient air.TiO 2 coated substrates were finally treated with diluted TiCl 4 solution at 60 ∘ C, for 15 min.

Synthesis of CdS Quantum Dots.
The quantum dots are deposited on TiO 2 coated FTO film by SILAR technique.This film was dipped into a 0.2 M cadmium nitrate (Cd(NO 3 ) 2 ) ethanol solution (cadmium cationic precursor) for 5 min, rinsed with ethanol, heated for 5 min, cooled to room temperature, and then dipped for another 5 min into a 0.2 M sodium sulfide (Na 2 S) water solution (sulphur anionic precursor) and rinsed again with water, heated for 5 min, and cooled to room temperature.The CdS adsorbed TiO 2 film is dried with N 2 air stream.The two-step dipping procedure is called one SILAR cycle and the process was repeated for 6 cycles.

Preparation of Electrolyte Solution.
Polysulfide electrolytes were prepared by mixing suitable quantities of Na 2 S, S, and KCl powders in water/methanol solution taken in the ratio 3/7.

2.4.
Fabrication of QDSSC Cell.The QD-adsorbed TiO 2 film was used as the working electrode and platinum coated FTO glass as counter electrode.The electrodes were assembled into a sealed cell with a cellotape spacer and binder clips with an active area equal to the area of 0.28 cm 2 .A suitable electrolyte solution was introduced to this sealed cell.The electrolyte was injected from the edges into the open cell, and the cell was tested immediately.

Characterization Details. XRD analysis was done on
TiO 2 /FTO films using multipurpose X-ray diffractometer (Bruker, D8 Discover) with Cuk source radiation.Surface morphology of the films was carried out by using JEOL (JSM-7600F) Field Emission Electron Microscope (FESEM) as well as JEOL-2100F Transmission Electron Microscope (HRTEM).Optical absorption studies were made at room temperature by using UV-Vis-NIR Spectrophotometer (JASCO V-670) in the wavelength range of 200-800 nm.The current-voltage and capacitance-voltage characteristics were investigated using a Semiconductor Characterization System SC-4200 from Keithley.The films were illuminated by a Class-BBA Solar Simulator and TM-206 Solar Power Meter was used for measurement of light intensity.

Morphological Studies of the As-Prepared TiO 2 Nanowire
Clusters and CdS QDs. Figure 1(a) shows FESEM image of a hierarchical structure which is flower-like clusters of TiO 2 nanowires on FTO substrate.The average diameter of the nanowire was 25 nm, exhibiting directly aligned onedimensional nanostructure on the FTO substrate.This image approves the growth of a nanowire structure.This TiO 2 architecture may extend a one-dimensional electron pathway of photo injected electrons along the photoanode, and such a design would be more expedient to overpower the charge recombination between the photo injected electron in the photoanode and the cation in the electrolyte [29,30].
The surface morphology of the SILAR grown CdS QDs on TiO 2 nanowire clusters studied by the high resolution transmission electron microscopy (HRTEM) is shown in Figure 1(b).The existence of lattice planes is evident in the HRTEM image which indicates the crystallinity in CdS quantum dots [31].The image also depicts substantial dispersion of CdS QDs and most of the dots are observed to have the elliptical profile with diameter ranging from 5 to 6 nm.Two encircled quantum dots with clear lattice planes can be seen in the figure.

XRD Analysis of the
where  is the X-ray wavelength and  is the full width at half maximum by using the (110) diffraction peak at 2 = 27.43 and was about 20-25 nm.The rutile phases match very well with the reports by Wyckoff [34], Baur and Khan [35], and Meagher and Lager [36].CdS QDs coated on TiO 2 nanowire clusters with different SILAR cycles 0, 2, 4, and 6. Figure 3   nanowire clusters film sensitized with CdS nanocrystals takes the lead for harvesting the visible light.

Absorption Studies.
Furthermore, from the UV-Vis analysis and taking into account the absorption edge at 422 nm, the QDs radius was estimated by using the hyperbolic band model (HBM) given by [40] where  bulk denotes bulk band gap,  nano stands for band gap of nanomaterial, and  * is effective mass of electron in bulk CdS ( * = 0.19  ).The calculated QDs radius was found to be 2.52 nm.Therefore, the particle size was evaluated as twice the radius; that is, 2 = 5.04 nm.

Photovoltaic
where  is the diode ideality factor,  is Boltzmann's constant,  is the electric charge, and   is the reverse saturation current density.The photocurrent-voltage (I-V) characteristics of the solar cell with the configuration FTO/TiO 2 -NW/CdS-QD/Pt-FTO are shown in Figure 4.It is evident that the photocurrent and photovoltage values exhibit increasing behavior with increasing illumination intensities.We observe an enhanced current density values which match well with the previous reports of increase in electrical conduction by the use of QDs [44,45].The plot of short circuit current density  sc versus open circuit voltage  oc is illustrated in Figure 5(a).Increase of  sc increases the value of  oc exponentially.This exponential behavior of  oc with  sc obeys the relationship given in (3) [42,43].By using (3), the graph of  oc - sc is fitted to determine the ideality factor and is found to be 3 [46].Under room temperature, the ideality factor is considered to be approximately 1 for an ideal p-n junction diode.The possible explanations for higher ideality factors may be due to various imperfections and also due to local nonlinear shunts anywhere in the cell area that are accountable for the ideality factor value larger than 1 [47].The value of short circuit current density  sc of the solar cell can be determined using J-V curves under light illumination.In Figure 5(b), it can be seen that there exists a nonlinear relationship between current and light intensity in the low intensity region.This occurs due to the large shunt resistance in the device.The linear relation is completely restored after the light intensity is increased to 40 mW/cm 2 which indicates that the photofilling effect has saturated the nonradiative recombination center.
PV measurements at various intensities provide better understanding to the photoconduction performance as well as photosensing aspects of the cell.Figure 6 shows the plot of electric power versus voltage for FTO/TiO 2 -NW/CdS-QD/Pt-FTO cell.The electric power increases with increasing of the bias voltage and reaches its maximum power and then decreases till it reaches zero value with the further increase of applied voltage.The maximum power is expressed as follows: J sc (mA/cm 2 ) V oc -J sc Fit using V oc = (nkT/q)ln(J sc /J o + 1) where   and   stand for the maximum current and maximum voltage, respectively, at each illumination intensity.The maximum power value designates how much the QDSSC can supply its maximum power to an external load.From Figure 6, it can also be seen that the maximum power point is moved to the higher voltages with the increasing incident light as follows: 0.14 V, 2 W at 20 mW/cm 2 and 0.25 V, 54.3 W at 120 mW/cm 2 , respectively.study the capacitance-voltage characteristics in a widespread frequency range.The capacitance measured by impedance spectroscopy studies at the solid/electrolyte interface of the cell is actually the total capacitance obtained from the electronic states (  ), the space charge ( sc ) on the semiconductor side, Helmholtz layer (  ), and the surface adsorbed ionic species ( ad ) on the electrolyte side, where   ,  sc ,   , and  ad are in parallel with (  | sc -  | ad ), respectively, as reported by Wang et al. [48].Gerischer and Seraphin have designated this as a simple three-layer model [49,50].

Impedance Properties
It is elaborated by them to differentiate within the electric double layer: (i) the space charge region in the electrolyte (also known as Gouy layer (C G ) or diffused part of the ionic layer) with thickness of 1-10 nm, (ii) an intermediate region called Helmholtz layer (C H ) with thickness of 0.4-0.6 nm, and (iii) the space charge layer in the semiconductor ( sc ) with thickness of 10-100 nm.These capacitances add to the total capacitance of the interface in a way and they are in series 1/ = 1/ sc + 1/  + 1/  .Since   and   are very small because of the lower thickness of the Gouy and Helmholtz layers, their contribution to the total capacitance is negligible and hence neglected.It can be viewed that, with the increase of bias voltage from −2.0 V to +2.0 V, the capacitance also shows an intermittent increasing trend and thereafter it reaches a maximum followed by a decrease in its values, showing an increasing behavior after the bias voltage 1.3 V for 5 kHz and 10 kHz (almost similar stabilized behavior) frequencies.However, by increasing the frequency from 50 kHz to 5 MHz the device capacitance represents a decreasing trend towards zero.increase of bias voltage from −2 V to +2 V.However, from each of the plots, it can be seen that a marginal increase of capacitance is noticed with the increase of bias voltage from −2 V to −1 V, saturated in the bias range from −1 V to +1 V, and thereafter shows a minimal decrease in the bias range from +1 to +2 V.The plot exhibits neither a severe fall nor a negative capacitance trend in the whole range from  −2 V to +2 V and even at higher frequencies as described by most of the impedance spectroscopy reports with DSSCs or QDSSCs [51,52].The basis of negative capacitance is usually ascribed to the injection of electron from front FTO electrode into TiO 2 [51].The nonexistence of negative capacitance in this range for these types of films could possibly be due to the surface alteration of the FTO layer for the formation of the hierarchical structure of TiO 2 as elucidated in the morphological section earlier.

Conductance-Voltage Characteristics (G-V).
Figure 8 shows the room temperature measured plots of conductance as a function of bias voltage from −2 V to +2 V for FTO/TiO 2 -NW/CdS-QD/Pt-FTO solar cell at different frequencies.This is an established technique on the losses of conductance because of the exchange of majority carriers between the interface states and majority carrier band of the semiconductor when a small ac signal is applied (25 mV in the present study) to the semiconductor devices [53].It is evident from Figure 8 that the conductance decreases and reaches a minimum of around −1.0 bias voltage and thereafter shows a saturation trend up to +1.0 V and then shows an increasing behavior up to the bias voltage +2 V. Furthermore, there is a gradual increase in conductance values with the increase of applied frequency from 5 kHz to 1 MHz.However, there is a noticeable increase in its values with the frequency increase of 3 MHz and 5 MHz.

Series Resistance-Voltage Characteristics (R s -V).
For a device, the noise ratio in terms of frequency can be determined using significant parameter known as series resistance     [54].Hence, it is important to evaluate the values of   at different voltages in a wide frequency range.From the investigated C-V-f measurements, the voltage and frequency dependent series resistance of the device can be evaluated as [55] where  MA and  MA denote the measured values of conductance and capacitance, respectively.Figure 9 shows the   -V characteristics of FTO/TiO 2 -NW/CdS-QD/Pt-FTO solar cell at different frequencies.It can be seen from Figure 9 that the series resistance increases and reaches a maximum of around −1.0 bias voltage and thereafter shows a saturation trend around +1.0 V and then shows a decreasing behavior up to the bias voltage +2.0 V. Furthermore, there is a gradual decrease in resistance values with the increase of applied frequency from 1 MHz.However, there is noticeable decrease in its values with the increase of 3 MHz and 5 MHz frequencies.This is interpreted as follows.At low frequencies, the interface states can track the ac signal and harvest an excess capacitance which depends on the frequency [56].At higher frequencies, the interface states cannot track the ac signal.In this situation, the input of interface state capacitance to the total capacitance is very minimal.

Conclusions
We have successfully synthesized flower-like hierarchical TiO 2 nanowire clusters on FTO substrates.CdS QDs were deposited on these nanowire clusters using SILAR technique.
FESEM image revealed that the average diameter of the nanowire was 25 nm.HRTEM image confirmed the size of CdS QDs in the range of 5-6 nm.XRD analysis, HRTEM, and UV-Vis absorption studies corroborated the effective implantation of CdS QDs into the TiO 2 matrix.The size of the CdS QDs was computed by HBM and found to be 5.04 nm.
The fabricated solar cell with the configuration FTO/TiO 2 -NW/CdS-QD/Pt-FTO measured a  sc of 1.295 mA/cm 2 and  oc of 0.38 V at AM 1.5 light intensity.The maximum power value of the device moved to the higher voltages with increasing incident light as follows: 0.14 V, 2 W at 20 mW/cm 2 and 0.25 V, 54.3 W at 120 mW/cm 2 , respectively.Impedance spectroscopic investigation of the cell with C-V, G-V, and R s -V plots exhibited frequency dependence which was varied between 5 kHz and 5 MHz in the bias voltage range from −2 V to +2 V. Capacitance showed decreasing trend with the increase of frequency whereas increase in conductance and decrease in series resistance with the increase in frequency have been recorded.The nonexistence of negative capacitance for these types of films could possibly be due to the surface alteration of the FTO layer for the formation of the hierarchical structure of TiO 2 .
As-Prepared TiO 2 Nanowire Clusters and CdS QDs Embedded on TiO 2 Nanowire Clusters.Typical XRD pattern of the as-prepared TiO 2 nanowire clusters sample is shown in Figure2(a).XRD shows that the film on fluorine doped tin oxide (FTO) substrate has been indexed to rutile phase of TiO 2 .The diffraction peaks are relatively sharp and agree well with those of the tetragonal TiO 2 with rutile structure (JCPDS 01-076-0318)[32] while Figure2(a) also exhibits the characteristic peaks indexed to FTO (highlighted with stars) and similar observations are reported by Lee and Lu[33] as well.All the diffraction peaks also agree well with those of tetragonal TiO 2 with rutile structure in Crystallography Open Database (COD number 96-900-9084, P/42 m n m, a = b = 0.45937 nm, and c = 0.29581 nm)[34].The average crystallite size of the TiO 2 nanowires was also calculated by Scherer's formula given by
[38]bits the fact that TiO 2 nanowire clusters film largely absorbs UV light, having an absorption band at 370 nm; this can be attributed to the wide band gap of rutile (3.35 eV) which is greater than the bulk rutile (3.0 eV)[38].It can be seen that with SILAR cycles 2 and 4 CdS-QDs sensitization, CdS-QDs/TiO 2 -NWs film exhibits marginal red shifted absorption.However, with SILAR cycle 6 CdS-QDs sensitization, the CdS-QDs/TiO TiO 2 -CdS: 2 cycles TiO 2 -CdS: 4 cycles TiO 2 -CdS: 6 cycles [39]Ws film shows significant red shifted absorption band at 422 nm (2.94 eV) in the visible region which enables the absorption of ample visible light with increased intensity.The calculated band gaps are higher than the bulk CdS (2.38 eV) indicating that the size of the CdS coated on the TiO 2 nanowire clusters film is within the scale of QDs[39].Therefore, it is established that TiO 2 [41][42][43]dS-QDs sensitized TiO 2 nanowires photoelectrode was fabricated to study the photoelectrochemical properties in solar cells.For a solar cell, the open circuit voltage is given by[41][42][43]