147 Chapter V

CChhaapptteerr VV

E I S CHARACTERIZATION

Objective of this chapter

To study the impedance characteristics of DSSCs

5.1. Introduction

The steady-state J–V characteristic of DSSCs provide the performance

parameters of the cell, such as the short-circuit current (Jsc), open-circuit potential

(Voc), fill factor (FF) and photocurrent efficiency (η). Nevertheless, more detailed

properties cannot be extracted from steady-state measurements, and so dynamic

techniques should be considered. These techniques allow the interpretation of the

charge-transfer kinetics, mainly characterized by diffusion coefficients and lifetime

of the different charge carriers. One of the most powerful characterization techniques

of DSSCs involving transient probing is electrochemical impedance spectroscopy

(EIS).

EIS is a dynamic technique that has many advantages, not only because it is

user-friendly but also for the reason that of its sensitivity and ability to separate

different complex processes occurring in DSSCs (Wang, et al., 2005; Bisquert, 2002;

Fabregat-Santiago, et al., 2002). Despite being a relatively easy method to apply to

the study of electrochemical systems, the results may sometimes be difficult to

interpret. The use of equivalent electrical analogs to fit the EIS experimental data is a

useful tool in this respect, as it helps to identify and interpret the characteristic

parameters of the system, such as, for instance, the internal resistances of DSSCs.

5.2. Electrochemical impedance spectroscopy

EIS is a powerful technique for the characterization of electrochemical

systems. It is one of the most useful experimental techniques as it permits a

148 Chapter V

simultaneous characterization of the different processes taking place in the cell

(Bisquert and Mora-Ser, 2010; Walker, et al., 2006). In EIS measurements the

potential applied to the solar cell is perturbed by a small amplitude sinusoidal

modulation and the resulting sinusoidal current response is measured as a function of

the modulation frequency (Oekermann, et al., 2005).

The meaning of electrical impedance can be understood starting from the

concept of resistance. The electrical resistance is the ability of a circuit element to

resist the flow of electrical current. The well known Ohm's law defines resistance (R)

in terms of the ratio between voltage (V) and current (I). This relationship is limited to

only one circuit element, an ideal resistor. But usually the systems under study contain

circuit elements that exhibit a much more complex behaviour. The simple concept of

resistance needs to be replaced by a more general parameter: the impedance, which

includes not only the relative amplitudes of the voltage and the current, but also the

relative phases. Like resistance, impedance is a measure of the ability of a circuit to

resist the flow of electrical current (Villanueva-Cab, et al., 2010).

The excitation signal can be written as

V(ω) = V0 cos ωt

where V(ω) is the ac potential applied to the system, V0 is the amplitude signal and ω

is the angular frequency (ω = 2πf rad s-1

). The current response will be shifted with

respect to the applied potential

I(ω) = I0 cos (ωt+ )

where I(ω) is the ac electrical current response signal, I0 the amplitude and the

phase shift. The phase factor contains the current lag with respect to the voltage. An

expression analogous to Ohm’s law allows for calculating the impedance of the

system as

149 Chapter V

tcosI

tcosV

I

VZ

0

0

Usually, it is convenient to use complex exponentials to express the

impedance. Complex numbers allow for a simpler representation of the relative

magnitude and phase of the input and output signal. Besides, it is a more powerful

representation for circuit analysis purposes. Taking into account Euler’s relationship,

xsinjxcose jx

it is possible to express the potential as

tj0 eVV

and the current response can be described as

tj0 eII

Since Z(ω)=V(ω)/I(ω) the exponential exp (jωt) terms cancel out, so that

j0

j

0

0eZe

I

VZ

The impedance is, therefore, expressed in terms of a magnitude Z0 (ratio of

the voltage amplitude to the current amplitude) and a phase shift . Using Equation

5.4, it is possible to separate the real part and the imaginary part of the impedance

Z0e-jφ

= Z0 cos (φ) - Z0 j sin (φ)

Real Imaginary

By varying the frequency of the applied signal, one can get the impedance of

the system as a function of frequency. The recorded data can either be represented as

magnitude and phase vs. frequency (Bode plot) or on a complex plane (Nyquist plot).

150 Chapter V

In a Nyquist plot the real part of the impedance (Z') is plotted on the X axis and the

imaginary part (Z´´) is plotted on the Y-axis.

EIS data are commonly analyzed in terms of an equivalent circuit model.

Most of the circuit elements in the model are common electrical elements such as

resistors, capacitors and inductors. These elements can be combined in series and in

parallel to give complex equivalent circuits. The most common cell model for

electrochemical systems is the Randles Circuit (Fig.5.1). This circuit models a cell

where polarization is due to a combination of kinetic and diffusion processes. It

includes a series resistance (Rs), a double layer capacitor (Crec), a charge transfer

resistance (Rrec), capacitance (Cct) at electrolyte/CE interface, electron transport

resistance (Rct) at electrolyte/CE interface and Warburg diffusion impedance (Zd).

Fig.5.1: EIS equivalent circuit diagram

The Nyquist plot for this circuit is shown in Fig.5.2. The double layer

capacitance and the charge transfer resistance in parallel define the time constant

(τrec= RC) or electron life time of the system (O’Regan and Durrant, 2009). The

Nyquist plot for a RC combination is always a semicircle as the one indicated in

Fig.5.2. The series resistance is expressed by the real intercept at high frequencies of

this semicircle. Finally, the Warburg diffusion impedance appears as a straight line

with a slope of 45° (Fig.5.2).

151 Chapter V

Fig.5.2: Transmission line model with Nyquist plot

A dye-sensitized solar cell is a very complex system and its impedance

response will be related to the response of the different components of the device.

Generally, a transmission line model is used to describe the system, as shown in

Fig.5.2. Different circuit elements are attributed to the different processes taking

place in the cell. The processes occurring in the mesoporous oxide film are usually

modeled by a diffusion-recombination transmission line. Its application to DSSCs

was first proposed by Bisquert (Peter, 2009). This transmission line is composed of a

network of resistive and capacitive elements, which describe the transport and

interfacial transfer of electrons that take place in the oxide. Fig.5.3 shows the model

Bode and frequency–phase plots of DSSC.

The mesoporous oxide film has two main contributions in the impedance

spectrum. One feature is the intermediate frequency arc, which accounts for the

parallel connection of the charge transfer resistance Rrec and the capacitance of the

152 Chapter V

film Crec. The other feature is a Warburg-like diffusion element, a 45º phase shift at

high frequencies related to the electron transport resistance Rt in the mesoporous

layer. The diffusion impedance of redox species in the electrolyte (Zd) is usually

modeled using a finite-length Warburg element of the type used for thin layer

electrochemical cells. Generally, Zd in DSSC is small and it is difficult to identify it

in the overall impedance solar cell response. However, it is clearly visible when

electrolytes based on ionic liquids are used. In that case, a semicircle at low

frequencies can be distinguished in the spectra.

Fig.5.3: Impedance Vs frequency and phase Vs frequency plots

A cathodic impedance due to the counter electrode is also present in the

spectra and it is expressed as a parallel combination of the electron transfer resistance

Rct (characterizing electron transfer to and from the redox system) and a capacitance,

Cct. In the Nyquist spectra, this feature is represented by a semicircle at high

frequencies. Finally, Rs accounts for the series resistance of the conducting glass plus

any other element that might be considered to be in series with the rest of the circuit.

153 Chapter V

The other useful electrochemical parameters of DSSCs are keff (rate of

recombination of the electrons in the photoanode film), τrec (effective electron life

time), Ln (electron diffusion length), Deff (effective diffusion coefficient),

µ (electron mobility), σ (electron conductivity), ηcc (electron collection efficiency)

and concentration of electrons in the TiO2/dye/electrolyte interface. These

parameters are calculated from the following expressions (Adachi, et al., 2006).

The lifetime of electrons in DSSC is a particularly important quantity. The

measured lifetime is a strong function of the Fermi level or open-circuit photo

voltage, Voc. The lifetime is a kinetic quantity that contains information not only on

the rate constants of charge transfer but also on the distribution of electronic states

and electronic transitions that intervene in the operation of the DSSC.

recrecrec CRs

----- Eqn.(5.1)

CctRcts d ----- Eqn.(5.2)

Where τd is the electron transport time.

keff is the effective rate constant for recombination of charge carriers at

photoanode and electrolyte interface. keff is estimated from the peak frequency ωmax

of the central arc, which describes the rate of recombination of electron in the film.

rec

1-eff

1)s(k

----- Eqn.(5.3)

Deff is the effective diffusion coefficient of electrons is calculated as,

rec

FLscm

22

effD ----- Eqn.(5.4)

154 Chapter V

The diffusion length Ln is the distance that an electron travels through the film before

it is recombined to the acceptor

21

effrecn DmL ----- Eqn.(5.5)

Conductivity of the photoanode film

eff

B

effD

TK

eDsVcm 9.382 ----- Eqn.(5.6)

Electron mobility

rec

F

RP1S

LcmmS

----- Eqn.(5.7)

Electron collection efficiency

rec

d

cc 1 ----- Eqn.(5.8)

Concentration of charge carriers describes the charge transport resistance and its

recombination rate along the entire thickness of working electrode and is calculated

as

effFrec kLRscmConc. ----- Eqn.(5.9)

where, LF - the film thickness of the photoanode (15 µm); S-active area of the

DSSCs; P-porosity of the photoanode film (0.6); (Rct x Crec); e – Charge of an

electron; KB - Boltzmann constant; T – Room temperature

The electrochemical impedance spectra were fitted using the Zsimpwin

software by means of equivalent circuit shown in Fig.5.1.

155 Chapter V

5.3. Experimental

Electron impedance spectra of DSSCs were recorded with Gamry-300

impedance analyzer. The applied bias voltage and ac amplitude were set at open-

circuit voltage of the DSSCs at 10 mV between the ITO/CE at the ITO/TiO2/dye

working electrode, respectively, and the frequency range explored was 1 mHz to

105 Hz. The impedance spectra were analyzed by an equivalent circuit model

interpreting the characteristics of the DSSCs through Zsimpwin software

(www.gamry.com). Fig.5.4 shows the Gamry impedance analyzer.

Fig.5.4: Gamry impedance analyzer

(Impedance analyzer (EIS) – EIS 300 specifications)

Overview

The EIS 300 Electrochemical Impedance Spectroscopy allows us to measure

impedances spanning 15 orders of magnitude over a frequency range of 10 µHz to 1

MHz. Data analysis in the Echem Analyst is easy. Spectra can be fit to standard cell

models or custom models created with graphical Model Editor. Gamry EIS 300 data

files ZSimpWin and Equivalent Circuit.

Data Acquisition Features

Continuous AC amplitude and DC offset correction for the most accurate

potential and current measurement.

156 Chapter V

Lissajous (I vs. E) real-time display at each frequency.

Bode or Nyquist real-time display of spectrum as it develops.

Realtime monitoring of signal-to-noise ratio for deciding when a

measurement is done for the best data in the shortest time.

Data Analysis Features

Impedance or Admittance plots.

Graphic Model Editor for equivalent circuit models with the ability to define

custom components.

Choice of Levenberg-Marquardt or Simplex algorithms in fitting the data to

equivalent circuits.

Kramers-Kronig Transform to evaluate the quality of the EIS data.

A unique Auto-Fit routine takes the guesswork out of estimating initial

values for your model elements.

System Requirements

Gamry PCI4/Series G, Reference, or Interface Family Instrument

Microsoft® Windows™ XP SP3/Vista/7/8 (32-bit or 64-bit)

157 Chapter V

5.4 Results and discussion

Electron transport properties of DSSCs were investigated using

electrochemical impedance spectroscopy (EIS). In general, the Nyquist plot exhibited

three semicircles, which are attributed to the redox reaction at the counter electrode

in the high-frequency region, the electron transfer at the TiO2/dye/electrolyte

interface in the middle-frequency region and carrier transport by ions within the

electrolytes in the low-frequency region. The model equivalent circuit diagram

(shown in Fig. 5.1) consisted of a series of resistance (Rs, starting point of the first

semicircle in Nyquist plot), electron transport resistance at the counter

electrode/electrolyte (Rct, first semicircle in Nyquist plot), charge transfer resistance

at the photoanode/electrolyte (Rrec, second semicircle in Nyquist plot), the constant

phase elements of capacitance Crec (Rrec), Cct (Rct) and Warburg diffusion impedance

Zd. The charge transfer kinetics has been explained on the basis of the diffusion–

recombination model proposed by Bisquert, 2002 and has been frequently employed

to analyze similar structures (Bisquert, 2002; Tan and Wu, 2006 ; Wu et al., 2008).

The internal impedances Rs, Rrec, Rct, Zd were determined from EIS analysis and

listed in Tables 5.1, 5.3 and 5.5. The other useful electrochemical parameters of

DSSCs, keff (rate of recombination of the electrons in the film), τeff (effective electron

life time), Ln (electron diffusion length), Deff (effective diffusion coefficient), µ

(electron mobility), σ (electron conductivity), ηcc (electron collection efficiency) and

concentration of electrons in the TiO2/dye/electrolyte interface were calculated using

Eqns. (5.1)-(5.9) and presented in Tables 5.1, 5.3 and 5.5.

5.5 EIS characterization of DSSCs made with core-shell photoanode

Fig. 5.5 shows the Nyquist plots of DSSCs made with different types

photoanodes and electrolytes. From the Nyquist plots it could be seen that first and

third semicircles are weak when compared the second semicircle. From the Fig. 5.5,

158 Chapter V

comparison of the middle semicircles of Nyquist plot of the DSSCs made with

different electrolytes indicated the decrease of the diameter in the order

DSSC-1>DSSC-3>DSSC-2>DSSC-4, suggested that DSSC-4 contributed to the

lowest charge transfer resistance at the TiO2/dye/electrolyte interface. When

comparing the Nyquist plots of DSSC-4 and DSSC-5, the Rrec value is increased in

DSSC-5 due to uncoated ZrO2 photoanode. Similarly when comparing the Nyquist

plots of DSSC-4 and DSSC-6, the Rrec value is increased in DSSC-6 due to uncoated

blocking layer HfO2. The DSSC-7 showed the highest value of Rrec. In this study,

higher charge collection efficiency is achieved in DSSC-4 based on TNPWs/ZrO2

core-shell with HfO2 blocking layer structure photoanode through quasi-solid state

electrolyte as compared to the other cells.

From the Table 5.1 it could be observed that the DSSC-4 made with novel

photoanode strucuture illustrated higher values of Crec, τrec, Ln, Deff, µ, σ and ηcc as

comparing other DSSCs. In addition, keff and concentration of electrons in the

TiO2/dye/electrolyte interface are lower values as compared to the other DSSCs.

Control of interfacial electron injection and recombination at the

photoanode/dye/electrolyte interface is pivotal for the best performance of DSSCs.

Modification at the metal oxide/dye interface has received increased attention among

researchers to improve the injection efficiency and suppress the recombination as the

interfacial energetic can be controlled by this strategy. Introducing shell layer on the

surface of photoanode materials exerts larger dipole moment that changes the

interface energetic in an energetically more favourable way for electron transfer

(Watson and Meyer, 2004). Recombination rate was diminished because the

insertion of ZrO2 could shield electron back flow through TNPWs to the electrolyte.

In the present investigation, the surface modification of photoanode with very thin

layer as well as larger band gap shell material improved the photocurrent.

159 Chapter V

Hence, core-shell material provided faster electron injection, and suppressed

recombination at photoanode/dye/electrolyte interface. In our examination on the

performance of core–shell structure-based DSSCs hints that the essential

improvement in electron injection efficiency and thereby overall performance can be

obtained by proper designing of the second metal oxide with suitable band gap,

optimal thickness of shell layer and IEP.

In addition, one-dimensional (1D) nanostructure, the grain boundaries effect

could be restricted (Fujihara et al., 2007, Baxter and Aydil, 2006). Moreover, the

loading of dye can be much higher in 1D nano structured material than the

nanoparticles, for instance, the TNPWs has allowed for larger adsorption of dye.

These results were indicated that the combining TNWs with TNPs having relatively

high thermal stability and low aggregation property. The enhanced charge collection

efficiency of the TNPWs/ZrO2 core-shell DSSCs is achieved by optimizing device

architectures, which enhanced light absorption and facilitated electron transport by

determining and designing appropriate dimensions of TNPWs/ZrO2, by optimizing

the cation concentration in the electrolyte solution for promoting electron injection

yield from sensitizing D149 dye molecules to core-shell TNPWs/ZrO2 electrodes, or

by synthesizing thermally stable TNPWs with a stable high surface area. In the core-

shell TNPWs/ZrO2 photoanode, the lower Rrec suggested that each TNWs made more

difficult for an electron to jump outside the nanostructure than to stay within the

structure during diffusion; this explained the high charge collectin collection

efficiency and thus high short-circuit photocurrent.

160 Chapter V

Fig.5.5: Nyquist plots of DSSCs 1-7

The core-shell TNPWs/ZrO2 photoanode exhibited different electrochemical

behaviours when they are employed in dye-sensitized solar cells. In this photo-

electrochemical system, charge separation, transport and recombination strongly

depend on the nanostructure and core-shell structure of the photoanode. From EIS

data it could be concluded that for the cell containing core-shell TNPWs/ZrO2,

formation of a space charge layer at the surface of the electrode effectively promoted

the separation of photo generated charge carriers and prevented recombination of the

electron with the hole carriers. The uninterrupted long pathway with fewer

boundaries is beneficial for transport of electron toward the conducting substrate.

Simultaneously, the decrease of trap sites in the band gap and the surface state can

effective enhance the charge-diffusion coefficient and suppression of the interfacial

charge transfer (Qi et al., 2010). In addition, the holes transport to electrolyte via the

different channel, which could prevented the charge recombination at the core-shell

161 Chapter V

TNPWs/ZrO2/redox electrolyte interfaces and led to the electrons diffusion and

transport become easy. Therefore, the DSSCs based on the core-shell TNPWs/ZrO2

film with QSSE has the lower values for Rrec, Rct and the longer electron lifetime,

consequently led to an enhanced charge collection efficiency of the cell. Electron

transport in core-shell TNPWs/ZrO2 photoanode is tortuous and often confront with

dead end (Hagfeldt et al., 2010), which enormously increased the electron transport

route.

As revealed from Table 5.1, the presence of core-shell TNPWs/ZrO2 layer

increased the electron lifetime and made the electron transfer more easily. In other

words, the charge transfer resistance in core-shell TNPWs/ZrO2 interfaces and

electron recombination are decreased, led to a positive influence on the improvement

of solar cell performance. Hence with EIS investigation, it is observed that the

enhanced electron transport properties, reduced charge recombination and increased

electron life-time which are imperative for higher solar cell performance in DSSC-4.

Therefore, DSSCs fabricated with core-shell TNPWs/ZrO2, which provided direct

and enhanced charge transport while minimizing charge recombination resulted in

enhanced the charge collection efficiency. Use of core-shell nanostructure to lower

the charge recombination is based on a hypothesis that a coating layer might build up

an energy barrier at the semiconductor/electrolyte interface and, thus, retarded the

reaction between the photogenerated electrons and the redox species in electrolyte.

In core-shell nanostructure, an energy barrier is established at the

semiconductor/electrolyte interface, it required that the conduction band potential of

the shell material is more negative than that of the core material. As such, the back

electron transport is retarded and the interfacial recombination could be reduced

(Kanmani and Ramachadran, 2012). The De and τrec values of the DSSC-5 with

Hafnium oxide layer are shown to be higher than those of DSSC-7 built with TNPWs

162 Chapter V

bare photoanode. The higher De means that the TNPWs film becomes more

conductive owing to the increased electron density in the conduction band by

introducing Hafnium oxide barrier.

Table 5.1: EIS parameters of DSSCs fabricated using core-shell photoanode structure

DSSCs Rrec

Ω

Rct

Ω Crec

µF Cct

µF

Zd

Ω-s

0.5

keff s

-1

τrec

ms τd

ms Ln

µm

Deff

×10-6

cm2/s

µ×10-4

cm2/V

/s

σ

×10-6

S/cm

ηcc

%

Conc.

Ωcms

-1

DSSC-1 130 98 180 12 2.4 42.73 23.40 17.64 17.27 128 49.62 7.21 24 5.55

DSSC-2 44 23 720 32 0.4 31.57 31.68 16.56 20.75 136 52.85 21.31 47 1.39

DSSC-3 80 52 350 19 1.4 35.71 28.00 18.20 18.61 124 48.09 11.72 35 2.86

DSSC-4 40 18 800 35 0.2 31.25 32.00 14.40 22.36 156 60.78 23.44 55 1.25

DSSC-5 96 67 280 15 2.0 37.20 26.88 18.76 17.96 120 46.66 9.77 30 3.57

DSSC-6 48 28 640 28 0.5 32.55 30.72 17.92 19.64 126 48.84 19.53 41 1.56

DSSC-7 140 108 160 11 2.6 44.64 22.40 17.28 17.08 130 50.65 6.69 22 6.25

The increased τrec value in DSSC-5 by introducing Hafnium oxide might be

due to the Hafnium oxide layer formed on the TNPWs surface could effectively

retarded the electron recombination process between photoanode and electrolyte.

Hence, the application of HfO2 as an efficient blocking layer material to modify the

core-shell TNPWs/ZrO2 electrodes in DSSCs. It is due to the more conductive core-

shell TNPWs/ZrO2 electrodes film (that is, the more accumulation of the electrons in

the conduction band of the TiO2 film) and the reduced recombination rate at the

interface between the TNPWs/ZrO2 and dye/electrolyte. In the present investigation,

DSSC-4 with quasi-solid state electrolyte have relatively high ambient ionic

conductivity, intimate interfacial contact with core-shell TNPWs/ZrO2, and

remarkable stability which led to higher efficiency as compared to the other

electrolytes.

163 Chapter V

The cell performance confirms that suitable coating of ZrO2 shell layer with

TNPWs core film can suppress the charge recombination process effectively and

improve the overall conversion efficiency. Fig. 5.5, which suggests that the

improvement in ηcc and τrec result from a suppression of the charge recombination by

ZrO2 shell coating. The role and the mechanism of the wide-band semiconductor as

shell coating on TiO2 have been discussed in many researches (Chen et al., 2001 and

Kruger et al., 2000). The present study shows that the thin ZrO2 shell coating can

suppress the charge recombination. Since, ZrO2 has a conduction band more negative

than that of TiO2, ZrO2 shell coating on TNPWs surface can form an energy barrier

on the surface of photoanode, which can suppress the injected electrons from

recombination with oxidized dye or I3-, thus reduces the charge recombination

process and increases the conversion efficiency (Chen et al., 2001; Kruger et al.,

2000; Burnside et al., 1999; Lenzmann et al., 2001; Vilan et al., 2000). Generally, it

can be found that reasonable thickness of the shell layer is beneficial to suppress the

recombination process and achieve relative high conversion efficiency.

Fig.5.5 shows EIS of the cells of DSSC-4 and 5 with and without ZrO2 shell

coating. The result also confirms that DSSC with ZrO2 shell coating has less electron

recombination at the photoanode/dye/electrolyte interface and longer electron

lifetime compared with the uncoated DSSC-5. Thus the application of ZrO2 shell

coating on TNPWs composite photoanode can promote the interfacial electron

transfer and suppress the electron recombination effectively.

From the frequency - phase plots in the lower frequency regime, it can be seen

that the characteristic frequency peak of DSSC-4 shifted to lower frequency as

compared to other DSSCs. The peak shifted to a lower frequency indicates an

increased lifetime of the DSSC-4. The longer lifetime implied a lower recombination

rate and enhanced charge collection efficiency. Therefore the interfacial charge

recombination of TNPs between the photo-injected electron and electrolyte materials

164 Chapter V

reduced after the TNWs decoration/shell layer on the core TNPWs. This

consequently leads to a significantly enhanced energy conversion efficiency of the

cell. The electron life time was calculated to be 32 msfor the DSSC-4, which is

highest among other DSSCs. The characteristic frequency peak of DSSC-4 in higher

frequency regime is shifted to lower frequency region when comparing to other

DSSCs. This result could be attributed to the higher electron transport rate, faster

electron diffusion coefficient and large surface area of core-shell structure

photoanode in the DSSC-4. These results indicate that the DSSC-4 built with core-

shell photoanode through QSSE has fast electrons transport and lower recombination

rate at TiO2/dye/electrolyte interface and the same property leading to higher charge

collection efficiency.

Fig. 5.7 shows the Bode plots of DSSCs 1-7. Fig. 5.7 indicates that the

DSSC-4 has lower impedance at electrolyte/counter electrode interface and

photoanode/dye/electrolyte interface as compared to other DSSC. This might be to

higher electrons transfer & lower recombination of charge carriers at

photoanode/dye/electrolyte interface and faster electron transport at redox couple

electrolyte/counter electrolyte interface. Hence in the present investigation, it could

be concluded that, in the core-shell DSSCs, the electrons need to diffuse several

micrometers into the TNPWs layer (core) surrounded by electron acceptor (shell) at a

distance of only several nanometers. The mesoporous structure of the TNPWs layer

provided a large surface area, allowing absorption of enough D149 dye molecules to

achieve significant charge collection efficiency. Core-shell photoanode consisting of

a mesoporous TNPWs covered with a shell of ZrO2 slows the recombination

processes by the formation of an energy barrier at the TiO2 surface.

165 Chapter V

Fig.5.6: Frequency Vs phase plots of DSSCs 1-7

Fig.5.7: Bode plots of DSSCs 1-7

166 Chapter V

Table 5.2: Frequency and phase of DSSCs fabricated using core-shell photoanode structure

DSSCs f1

(KHz)

f2 (Hz)

PEAK 1 PEAK 2

υ1 (Hz)

θ1 (dec)

υ2 (KHz)

θ2 (dec)

DSSC-1 132 6.92 12.0 25.6 575 64.5

DSSC-2 229 4.79 8.3 26.6 479 42.6

DSSC-3 158 5.75 10.0 26.0 479 55.9

DSSC-4 229 4.79 8.3 27.8 479 38.3

DSSC-5 158 5.75 10.0 25.5 575 59.4

DSSC-6 191 5.75 8.3 25.7 479 45.9

DSSC-7 132 6.92 12.0 25.5 575 65.6

The conduction band potential of the shell should be more negative than that

of TiO2 in order to generate an energy barrier for the reaction of the electrons present

in TiO2 with the oxidized dye or the redox mediator in solution. An energy barrier

forms not only at the electrode/electrolyte interface but also between the individual

TiO2 nanoparticles. The TiO2 nanoparticles are connected directly to each other

allowing electron transport through TiO2. The shell having more negative conduction

band potential acted as an energy barrier that slowed recombination reactions. Photo

excitation of dye molecules anchored to ultra thin outer shell of insulators or

semiconductors on n-type semiconductor crystallites resulted in electron transfer to

the inner core material.

167 Chapter V

5.6 EIS characterization of DSSCs made with Al@TNPWs doping photoanode

Electron transport properties of DSSCs were investigated by EIS. Fig. 5.8

shows the Nyquist plots of DSSCs fabricated with different types photoanodes and

electrolytes. The internal impedances Rs, Rrec, Rct, Zd were determined from the

Nyquist plot and listed in Table 5.3. The other useful electrochemical parameters of

DSSCs, keff (rate of recombination of the electrons in the film), τrec (effective electron

life time), τd (electron transport time), Ln (electron diffusion length), Deff (effective

diffusion coefficient), µ (electron mobility), σ (electron conductivity), ηcc (electron

collection efficiency) and concentration of electrons in the TiO2/dye/electrolyte

interface was calculated and presented in Table 5.3.

From the Nyquist plots of DSSCs, it could be seen that first and third

semicircles are weak as compared the second semicircle. From the Fig. 5.8,

comparison of the middle semicircles of Nyquist plots of the DSSCs made with

different electrolytes indicates the decreased of the diameter in the order

DSSC-8>DSSC-10>DSSC-9>DSSC-11. This observation suggested that DSSC-11

contributed to the lowest charge transfer resistance at the photoanode/dye/electrolyte

interface as a result of the quasi-solid state electrolyte. When comparing the Nyquist

plots of DSSC-11 and DSSC-12, the Rrec value is increased in DSSC-12 because of

undoped Al photoanode. Likewise, when comparing the Nyquist plots of DSSC-11

and DSSC-13, the Rrec value is increased in DSSC-13 due to the absence of HfO2

blocking layer in the photoanode. The DSSC-7 showed the highest value of Rrec

among the other DSSCs as a consequence of sluggish electron transport. In the EIS

study, higher ηcc was achieved in DSSC-11 fabricated based on Al-doped TNPWs

with HfO2 blocking layer structure photoanode through quasi solid state electrolyte

as compared to the other cells. The DSSC-11 illustrated higher values of Crec, τrec, Ln,

Deff, µ, σ and ηcc as compared to other DSSCs. In addition, keff and concentration of

electrons in the photoanode/dye/electrolyte interface were lower values as compared

168 Chapter V

to the other DSSCs. Introducing cations as dopant on the surface of photoanode

materials exerts larger dipole moment that changes the interface energetic in an

actively more favourable way for electron transfer (Warson and Meyer, 2004).

Consequently, Al-doped TNPWs photoanode provided faster electron injection at the

photoanode/dye/electrolyte interface. Recombination rate was diminished because of

HfO2 layer could shield electron back flow through TNPWs to the electrolyte. In our

examination on the performance of Al-doped TNPWs photoanode structure-based

DSSCs hints that the essential improvement in electron injection efficiency and

thereby overall performance could be obtained by proper designing of dopant with

suitable band gap,optimal thickness of blacking layer.

The improved photoelectrochemical performance of DSSCs is attributed to

two main factors:(1) increased light harvesting efficiency caused by the large amount

of sensitizing dye D149 adsorbed on the large surface area of Al-doped TNPWs

composite and (2) increased electrical conductivity due to Al ions doped into the

TiO2 lattice at the divalent Ti2+

site, allowing electrons to move easily into the Al-

doped TiO2 conduction band. The optical response of material is largely determined

by its underlying electronic structure. The electronic properties of a material are

closely related to its chemical composition, its atomic arrangement, and its physical

dimension. The chemical composition of TiO2 can be altered by doping. It is

desirable to support the integrity of the crystal structure of the photocatalytic host

material and to produce favourable changes in electronic structure. As the Fermi

levels of Al are lower than that of TiO2, photo-excited electrons can be transferred

from conduction band (CB) of TiO2 to Al band, while photo-generated valence band

holes stay on the TiO2. These activities greatly reduce the possibility of electron-hole

recombination, resulting in proficient ηcc (Patel et al., 2008; Wang and Caruso,

2011).

169 Chapter V

The presence of dopant metal can act as an electron sink and significantly

decreased the electron transport time and improved mobility of photogenerated

electrons (Carneiro et al., 2009). In DSSC-11, the lower Rrec suggested that each

TNW made more difficult for an electron to jump outside the nanostructure than to

stay within the structure during diffusion; this explained the higher values of ηcc and

Deff. The De and τrec values of DSSC-13 with Hafnium oxide layer are shown to be

higher than those of DSSC-7 built with bare electrode. The higher De illustrated that

the TNPWs film becomes more conductive owing to the increased electron density in

the conduction band by introducing Hafnium oxide barrier. In addition, the elevated

τrec value of the by introducing Hafnium oxide also proved that the Hafnium oxide

layer formed on the TNPWs surface could effectively retarded the electron

recombination process between photoanode and electrolyte. Hence, the Al-doped

TNPWs with blocking layer photoanode with QSSE exhibited enhanced

photoelectrochemical behaviours in DSSC-11. In this photoelectrochemical system,

charge separation, transport and recombination strongly depend on the nanostructure

of the photoanode and nature of electrolytes.

Therefore, the DSSCs based on the Al-doped TNPWs film has the lower

values for Rrec, Rct and the longer electron lifetime, led to enhanced ηcc of the cell. In

the present investigation, D149 dye is used as the efficient sensitizer for DSSCs. The

Al-doped TNPWs photoanode demonstrated a successful photoelectrode within

DSSCs which keep the desired specific surface area for dye-molecule adsorption and

enough light-harvesting from prolonged light travelling. In addition, QSSE had

relatively high ambient ionic conductivity, intimate interfacial contact with Al doped

TNPWs-HfO2, and remarkable stability which led to higher efficiency as compared

to the other electrolytes. The ηcc improvement can result from retardation of

interfacial charge recombination and/or from possible upward shift of the CB edge

position of TiO2 due to development of a dipole at the interface of TiO2/metal oxide

170 Chapter V

(Law et al., 2006; Butler and Ginley, 1978; Diamant et al., 2004). The CB edge for

all composite photoanodes, by extension is the same leaving retardation of charge

recombination as the origin of the observed ηcc increase.

According to Fig.5.8, in DSSC-11, Al2O3 presented the highest blocking

ability for interfacial charge recombination because of its lowest Rrec, which also

resulted in the highest electron lifetime as determined from the EIS measurements.

The blocking ability of Al is most likely due to its higher CB edge position. It is

notable that the mechanism of charge transfer determining Rrec is via direct transfer

from the CB of TiO2 (Fabregat-Santiago et al., 2005). However, electrons may

recombine not only from the CB but also from localized intra band states as well as

surface states in the band gap (Fabregat-Santiago, 2005; Bisquert et al., 2004).

Therefore, it is also interesting to probe the region of lower Fermi levels (Zaban et

al., 2003). Thus, the EIS technique is employed as it allows us to monitor the kinetics

of recombination in the domain of low photo-voltages (Bisquert, et al., 2004; Zaban

et al., 2003].

Fig.5.9 shows the frequency – phase plots of DSSCs 7-13. The characteristic

frequency peak of DSSC-11 in higher frequency regime shifted to lower frequency

region as compared to DSSCs. This result could be attributed to the higher electron

transport rate, electron diffusion coefficient and large surface area of doping

structure photoanode in the DSSC-11. These results indicated that the doping

photoanode DSSC-11 had fast electrons transport and lower recombination rate at

photoanode/dye/electrolyte interface and the same property leading to higher

photocurrent efficiency.

171 Chapter V

Fig.5.8: Nyquist plots of DSSCs 7-13

Table 5.3: EIS parameters of DSSCs fabricated using doping photoanode structure

DSSCs Rrec

Ω

Rct

Ω Crec

µF Cct

µF

Zd

Ω-s

0.5

keff s

-1

τrec

ms τd

ms Ln

µm

Deff

×10-6

cm2/s

µ×10-4

cm2/V

/s

σ ×10

-6

S/cm

ηcc

%

Conc.

Ωcms

-1

DSSC-8 86 57 320 18 1.6 36.34 27.52 18.24 18.42 123 47.99 10.90 33 3.13

DSSC-9 60 34 490 26 0.7 34.01 29.40 16.66 19.92 135 52.54 15.63 43 2.04

DSSC-10 72 45 390 22 1.1 35.61 28.08 17.55 18.97 128 49.87 13.02 37 2.56

DSSC-11 56 31 530 29 0.6 33.69 29.68 16.43 20.16 137 53.27 16.74 44 1.89

DSSC-12 106 75 250 14 2.2 37.74 26.50 18.75 17.83 120 46.68 8.84 29 4.00

DSSC-13 62 36 470 26 0.8 34.32 29.14 16.92 19.69 133 51.73 15.12 41 2.13

DSSC-7 140 108 160 11 2.6 44.64 22.40 17.28 17.08 130 50.65 6.69 22 6.25

172 Chapter V

Fig.5.9: Frequency Vs phase plots of DSSCs 7-13

Fig.5.10: Bode plots of DSSCs 7-13

173 Chapter V

Table 5.4: Frequency and phase of DSSCs fabricated using doping photoanode structure

DSSCs f1

(KHz)

f2 (Hz)

PEAK 1 PEAK 2

υ1 (Hz)

θ1 (dec)

υ2 (KHz)

θ2 (dec)

DSSC-8 158 5.75 10.0 26.0 479 57.2

DSSC-9 158 5.75 8.3 26.7 479 49.3

DSSC-10 110 6.92 14.5 25.7 575 67.4

DSSC-11 158 5.75 10.0 26.9 398 47.8

DSSC-12 158 5.75 10.0 25.5 575 60.9

DSSC-13 158 5.75 10.0 26.7 479 50.2

DSSC-7 132 6.92 12.0 25.5 575 65.6

Fig.5.10 shows the Bode plots of DSSCs 7-13. The Bode plot of DSSC-11

explicated that in the lower frequency region the impedance of the cell is lower than

the other DSSCs. These results indicated that the DSSC-11 had lower impedance at

electrolyte/counter electrode interface and photoanode/dye/electrolyte interface. This

is might be to higher electrons transfer & lower recombination of charge carriers at

photoanode/dye/electrolyte interface and faster electron transport at redox couple

electrolyte/counter electrolyte interface.

Hence in the present investigation, it could be concluded that the Al doped

TNPWs had better ηcc observed in DSSC-11 built with QSSE. The electronic

properties of a material are closely related to its chemical composition (chemical

nature of the bonds between the atoms or ions), its atomic arrangement, and its

physical dimension (confinement of carriers) for nanometer-sized materials. The

chemical composition of TiO2 can be altered by doping. It is desirable to maintain

the integrity of the crystal structure of the photocatalytic host material and to produce

favourable changes in electronic structure. It appears easier to substitute the Al cation

174 Chapter V

in TiO2 strucutre and it is more difficult to replace the O2-

anion with other anions

due to differences in charge states and ionic radii. The small size of the nanoparticle

is beneficial for the modification of the chemical composition of TiO2 due to the

higher tolerance of the structural distortion than that of bulk materials induced by the

inherent lattice strain in nanomaterials (Burda, et al., 2003; Chen, et al., 2003). In the

present investigation, Al doped TiO2 showed enhanced photocurrent due to the

compatible energy level between Al and TiO2. The presence of Al metal ion dopants

in the TiO2 matrix significantly influenced the charge carrier recombination rates and

interfacial electron-transfer rates.

In the present investigation, QSSE had relatively high ambient ionic

conductivity, intimate interfacial contact with Al doped TNPWs-HfO2, and

remarkable stability which led to higher efficiency as compared to the other

electrolytes. This may be attributed to the high dielectric constant and low viscosity

of the organic solvent. The stronger polarity of solvent with high dielectric constant

is beneficial to the enhancement of ionic dissociation in the QSSE, leading to the

higher conductivity (Wang Miao et al., 2006). Furthermore, the corresponding Jsc

value listed in Table 1 also have the same trend as the change of ionic conductivity

data. This indicated that the organic solvent have great effects on the performance of

QSSE solar cells. Fig. 5.11 shows time-dependent change in the PCE of DSSCs

made with different electrolytes through aging test. It was observed that the PCE of

the cell based on liquid electrolyte decreased immediately with increasing time and

after 30 days was close to zero. On the other hand, the PCE of the cell based on

quasi-solid state electrolyte containing PEG polymer decreased much slowly and

after 100 days still remained 5.4%, which kept 66% of its initial conversion

efficiency, because the addition of the polymer into the electrolyte changed the state

175 Chapter V

of electrolyte from liquid state to quasi-solid state and made the organic solvent

sealed in the polymer network, which suppressed the evaporation of solvent and

hence improved the stability of the cell. The potential problems caused by LE and

ILE are leakage and volatilization of organic solvent, considered as some of the

critical factors limiting the long-term performance and practical use of DSSC-8 and

DSSC-9. The lesser stability of DSSC-10 might be due to the high temperature

deposition process of SSE would certainly degrade the sensitized dye on the surface

of nanocrystalline TiO2 (Li et al., 2006). Thus quasi-solid state electrolyte with

higher ionic conductivity of 16.74×10−6

S/cm

was obtained by solidifying the organic

solvent-based liquid electrolyte with PEG polymer. It was applied in the dye

sensitized solar cells and acquired PCE of 7.26%. Furthermore, the stability of the

cell based QSSE is much better than that of the cell based on the other types of

electrolytes, and even after 100 days the PCE still remains 66%.

Fig. 5.11. Stability of DSSCs with different electrolytes

176 Chapter V

5.7 EIS characterization of DSSCs made with bi-layer photoanode

Nyquist plots of DSSCs made using bi-layer photoanode is shown in

Fig.5.12. From the Fig.5.12, comparing the middle semicircles of Nyquist plot of the

DSSCs made with bi-layer photoanode structure indicates the decrease of diameter in

the order DSSC-14>DSSC-16>DSSC-15>DSSC-17, suggesting that DSSC-17

contributed to the lowest charge transfer resistance at the photoanode/dye/electrolyte

interface. When comparing the Nyquist plots of DSSC-17 and DSSC-7, the Rrec

value increased in DSSC-7 due to uncoated Nb2O5 photoanode. When comparing the

Nyquist plots of DSSC-17 and 18, the Rrec value increased for DSSC-18 due to bare

Nb2O5 photoanode layer. The DSSC-7 shows the highest value of Rrec. In this study,

higher ηcc is achieved in DSSC-17 based on D149 sensitized TNPWs-Nb2O5 bi-layer

structure photoanode through quasi-solid state electrolyte as comparing to other

cells. The DSSC-17 made with novel photoanode illustrated higher values of Crec,

τrec, Ln, Deff, µ, σ and ηcc when comparing other DSSCs. In addition, keff and

concentration of electrons in the TiO2/dye/electrolyte interface were lower values

compared to other DSSCs.

The bi-layer TNPWs-Nb2O5 photoanode exhibited better electrochemical

behaviours when they are employed in dye-sensitized solar cells. In this photo

electrochemical system, charge separation, transport and recombination strongly

depend on the bi-layer structure of the photoanode. From EIS data it could be

concluded that for the cell containing TNPWs-Nb2O5 bi-layer, formation of a space

charge layer at the surface of the electrode effectively promoted the separation of

photo generated charge carriers and prevented recombination of the electron with the

hole carriers. In addition, the holes transported to electrolyte via the different

channel, which could prevented the charge recombination at the bi-layer TNPWs-

Nb2O5/redox electrolyte interfaces and led to the electrons diffusion and transport

177 Chapter V

become easy. Therefore, the DSSCs based on the composite core-shell TNPWs-

Nb2O5 film had the lower values for Rrec, Rct and the longer electron lifetime,

consequently leading to an enhanced conversion efficiency of the cell.

As revealed from Table 5.5, the presence of bi-layer TNPWs-Nb2O5 layer

increases the electron lifetime and makes the electron transfer more easily. In other

words, the charge transfer resistance in bi-layer TNPWs-Nb2O5 interfaces and

electron recombination are decreased, leading to a positive influence on the

improvement of solar cell performance. Hence with EIS investigation, it was showed

the enhanced electron transport properties, reduced charge recombination and

increased electron life-time which are imperative for higher solar cell performance

could be achieved in DSSC-17. Therefore, DSSCs fabricated with bi-layer

TNPWs-Nb2O5, which provided direct and enhanced charge transport while

minimizing charge recombination resulted in enhanced efficiency in bi-layer

TNPWs-Nb2O5 based DSSCs compared to other DSSCs. Use of bi-layer

nanostructure the Nb2O5 coating layer may build up an energy barrier at the

semiconductor/electrolyte interface and, thus, retard the reaction between the

photogenerated electrons and the redox species in electrolyte.

Instead of using only nanocrystalline TiO2 particle films, it has been

proposed that a bi-layer structure composed of light scattering layer and

nanocrystalline semi-transparent layer can improve photocurrent density

substantially due to the fact that the confinement of incident light by light scattering

particles can gain more photons (Ito, et al., 2006; Hore, et al., 2006). When light

collides with the large TiO2 particle having submicrometer size, the light scattered

strongly, this increased the path length of the incident light in the nanocrystalline

TiO2 films. Eventually, the scattering effect by introduction of the large TiO2

particles is expected to enhance the photocurrent density and thereby overall

178 Chapter V

conversion efficiency. The scattering effect is known to be dependent on size

(Vargas, 2000), refractive index (Hore, et al., 2006) and position

(Wang, et al., 2004), of the scattering particles.

The Deff and τrec values of the DSSC-17 shown to be higher than DSSC-7.

The higher Deff revealed that the TNPWs film becomes more conductive owing to

the increased electron density in the conduction band by introducing Tantalum oxide

barrier. The increased τrec value of the photoelectrode by introducing Hafnium oxide

also proved that the blocking layer formed on the TNPWs surface can effectively

retard the electron recombination process between nanocrystalline TiO2 and

electrolyte. The enhancement of the electron injection might be attributed to the shift

of conduction band edge of TiO2. Previously, it has been reported that the acidic

property of Nb2O5 shifted the TiO2 flat band energy potential (Efb) toward positive

values and increased the driving force for electron injection (determined as the

difference between Efb and the LUMO state of the dye), thereby enhancing the

electron injection efficiency. Here, in EIS spectra, the Nyquist plot revealed a large

semicircle in the frequency range between 1 mHz to 105 Hz. The size of the arc

depended on the concentration of I3− and on the rate of back-electron transfer at the

TiO2/electrolyte interface. The impedance spectrum was analyzed using an

equivalent circuit for the TiO2/electrolyte interface in the conductive state. The

recombination resistance (Rrec) is obtained by the curve of the large semicircle.

The EIS is sensitive to the interfacial electron flow in the electrode/electrolyte

interface. The EIS curves show the reduction peaks attributed to the

charging/discharging at the interface and the peak was shifted positively upon the

Nb2O5 coating. This result confirms the positive shift of the conduction band edge in

Nb2O5-coated TiO2 electrodes. The much larger τrec in the bi-layer DSSC-17 as

compared to DSSC-18 may be explained by an internal radial electric field that is

179 Chapter V

developed within the walls of the bi-layer. This electric field drives the electrons

away from the interface, preventing recombination with the electrolyte

(Xu et al., 2010).

The primary reason for longer lifetimes in DSSC-17 is due to the intrinsic

material properties. It has been reported that the conduction band of TiO2 is

composed of d-orbital electrons that are less susceptible to recombination than the

electrons in Nb2O5 (Bandaranyake et al., 2004). Among the different electrodes, the

bi-layer TNPWs−Nb2O5 electrode exhibited the slowest recombination rate, lower

than the bare Nb2O5 (DSSC-18). This could be due to the energy barrier formed by

the Nb2O5 coating, forcing the electrons to flow toward the current collecting surface

and preventing the electrons from flowing in the opposite direction. Introduction of

Nb2O5 may have also passivated the reactive low-energy surface states. These factors

lead to further lowering of the recombination rates, resulting in enhanced ηcc for

DSSC-17 based on the bi-layer TNPWs−Nb2O5 electrodes.

The appliance of Nb2O5 is an efficient layer material on the bi-layer

TNPWs-Nb2O5 electrodes in DSSCs. The diffusion coefficient (Deff) and lifetime

(τrec) of the DSSC-17 were increased by the Nb2O5 energy barrier. It is due to the

more conductive bi-layer TNPWs-Nb2O5 electrodes film (that is, the more

accumulation of the electrons in the conduction band of the TiO2 film) and the

reduced recombination rate at the interface between the TNPWs-Nb2O5 and

dye/electrolyte. In the present investigation, DSSC-17 with quasi-solid state

electrolyte had relatively high ambient ionic conductivity, intimate interfacial contact

with core-shell TNPWs-Nb2O5, and remarkable stability which lead to higher

efficiency.

180 Chapter V

Fig.5.12: Nyquist plots of DSSCs 14-18

Table 5.5: EIS parameters of DSSCs fabricated using bi-layer photoanode structure

DSSCs Rrec

Ω

Rct

Ω Crec

µF Cct

µF

Zd

Ω-s

0.5

keff s

-1

τrec

ms τd

ms Ln

µm

Deff

×10-6

cm2/s

µ×10-4

cm2/V

/s

σ ×10

-6

S/cm

ηcc

%

Conc.

Ωcms

-1

DSSC-14 118 86 220 13 2.2 38.52 25.96 18.92 17.57 119 46.26 7.95 27 4.55

DSSC-15 74 47 380 22 1.2 35.56 28.12 17.86 18.82 126 49.01 12.67 36 2.63

DSSC-16 90 62 300 17 1.8 37.03 27.00 18.60 18.07 121 47.06 10.42 31 3.33

DSSC-17 68 42 420 23 1.0 35.01 28.56 17.64 19.09 128 49.62 13.79 38 2.38

DSSC-18 160 126 140 10 2.8 44.64 22.40 17.64 16.90 128 49.62 5.86 21 7.14

DSSC-7 140 108 160 11 2.6 44.64 22.40 17.28 17.08 130 50.65 6.69 22 6.25

181 Chapter V

Fig.5.13 shows the frequency-phase plots of the DSSCs made up of bi-layer

photoanode structure. The frequency peak at the high frequency region can be

ascribed to the charge transfer at the interface of the electrolyte/counter electrode,

and the other low frequency region to the accumulation/transport of the injected

electrons with bi-layer photoanode and the charge transfer at the interfaces of

electrolyte, respectively. From the frequency - phase plots in the lower frequency

regime, it could be seen that the frequency peak of DSSC-17 is shifted to lower

frequency as compared to other DSSCs. The peak shifted to a lower frequency

indicated an increased lifetime of the DSSC-17. The longer lifetime implied a lower

recombination rate and enhanced electron-collection efficiency. Therefore the

interfacial charge recombination of TNPs between the photo-injected electron and

electrolyte materials was reduced after the TNWs decoration and shell layer on the

core TNPWs. This consequently led to a significantly enhanced energy conversion

efficiency of the cell. The electron life time is calculated to be 28.56 ms for the

DSSC-17.

The characteristic frequency peak of DSSC-17 in higher frequency is regime

shifted to lower frequency region when compared to DSSCs. This result could be

attributed to the higher electron transport rate, electron diffusion coefficient and large

surface area of doping structure photoanode in the DSSC-17. These results indicated

that the bi-layer photoanode DSSC-17 has fast electrons transport and lower

recombination rate at TiO2/dye/electrolyte interface and the same property leading to

higher photocurrent efficiency.

182 Chapter V

Fig.5.13: Frequency Vs phase plots of DSSCs 14-18

Fig.5.14: Bode plots of DSSCs 14-18

183 Chapter V

Table 5.6: Frequency and phase of DSSCs fabricated using bi-layer photoanode structure

DSSCs f1

(KHz)

f2 (Hz)

PEAK 1 PEAK 2

υ1 (Hz)

θ1 (dec)

υ2 (KHz)

θ2 (dec)

DSSC-14 132 6.92 12 25.5 575 62.7

DSSC-15 158 5.75 10 26.2 479 54.4

DSSC-16 132 5.75 10 25.6 479 58.4

DSSC-17 158 5.75 10 26.2 479 52.7

DSSC-18 158 5.75 10 26.3 479 53.8

DSSC-7 132 6.92 12 25.5 575 65.6

Thus, the peak shift to a lower frequency represents longer electron

recombination lifetime. For DSSC-17 with a bi-layer TNPWs−Nb2O5 photoanode, a

longer electron lifetime of 28.56 ms is obtained compared to 22.40 ms for DSSC-18

based on a bare Nb2O5 photoanode. The longer electron recombination lifetime of the

bi-layer TNPWs−Nb2O5 photoanode compared to the bare Nb2O5 photoanode is in

agreement with the trend observed in the transient photo-voltage measurement. It

needs to be noted that the absolute values for recombination lifetimes obtained by

EIS differ from those reported by photo-voltage decay measurements. The primary

reason for this could be that, in photo-voltage decay measurements, the cell was

allowed to decay through low carrier density states, resulting in longer electron

lifetimes. On the other hand, EIS measurements were performed at open-circuit

voltage, when electron density is the highest. The results indicate that the bi-layer

photoanode is superior to the bare Nb2O5 photoanode with significantly larger Ln,

Deff, and τrec but smaller keff. Besides the expected increase in electron lifetime, a

larger effective diffusion coefficient was also observed in the case of the bi-layer

184 Chapter V

photoanode. The reason for large Deff could be due to the passivation of the low

energy Nb2O5 surface trap states. These results explain the improved photovoltaic

performance of the bi-layer TNPWs−Nb2O5 photoanode as compared to the bare

Nb2O5 photoanode.

Fig.5.14 shows the Bode plots of DSSCs 14-18. The Bode plots of DSSCs

explicated that in the lower frequency region the impedance of the cell is higher than

the higher frequency region. These results indicated that the DSSC-17 has lower

impedance at electrolyte/counter electrode interface and photoanode/dye/electrolyte

interface when compared to other DSSC. This is due to higher electrons transfer &

lower recombination of charge carriers at photoanode/dye/electrolyte interface and

faster electron transport at redox couple electrolyte/counter electrolyte interface. In

DSSCs transport of charge carriers is limited by trapping-detrapping processes in

which both energy and morphological disorder play a role. Furthermore, in these

devices, the electron transport is strongly coupled with charge transfer to external

electron acceptors (recombination). This coupling determines the collection

efficiency of the device, which is the parameter to optimize for efficient production

of energy.

From the EIS results of DSSCs, it could be concluded that in order to get

better photocurrent efficiency, the photo anode materials must have (i) mesoporosity

and large surface area allow anchoring a large amount of dye molecules which

enhances the absorption cross-section, (ii) larger density of states in metal oxides

than the molecular orbital of dye allows faster injection of electrons from the dye to

the metal oxide. At the same time, the detrimental factors are (i) inefficient electrical

transport through nanocrystalline network and (ii) charge recombination within the

materials and also with the electrolyte. Inefficient electrical transport in

nanostructured metal oxide semiconductors arise from the trapping and detrapping of

185 Chapter V

electrons at the surface atomic states in the electronic band. The surface atoms are a

large fraction in nanostructured materials, so as the trap density. Trapping and

detrapping reduced the kinetic energy of the flowing electrons, which ultimately

results in inferior cell performance.

Various morphologies and approaches, such as doping, bi-layer, core/shell

structure, have been explored in the present study to enhance the electrical transport

properties. One dimensional morphology showed great promise in enhancing the

electrical transport properties. In the present investigation, one-dimensional TNWs

synthesized through hydrothermal method increased the electron life time and

allowed enhanced charge collection efficiency in DSSCs. Doping of Al in TNPWs is

found to be beneficial in increasing the photocurrent efficiency.

In the present investigation, the best performing DSSC-4 fabricated with the

organic dye D149 sensitized TNPWs/ZrO2 core-shell photoanode, quasi-solid state

electrolyte and cobalt sulfide counter electrode. Most likely the dye molecules form a

monolayer on the surface of shell layer of ZrO2 photoanode and aggregation seems to

be insignificant. Efficient sensitization and better absorption properties of D149

sensitizer are the key factors for its higher performance with respect to the indigo

carmine dye tested.

Taking into account that one of the most interesting properties of TiO2 is its

flexibility in synthesis and morphology, the influence of the semiconductor

morphology on the performance of TiO2 based solar cells has been studied. A

comparative study of D149 sensitized TiO2 electrodes based on core-shell, doping

and bi-layer have been carried out. A significant conclusion of this study is that the

performance of the one-dimensional nanostructures studied here is not limited by a

low light harvesting as could have been expected as a consequence of the relatively

low surface area. Nanowire based solar cell performance seems to be limited by poor

186 Chapter V

injection from the dye to the nanowires and/or strong recombination. Nanoparticles

deposited on the surface of the nanowires decreased significantly electron

recombination and lead to an improvement of the photovoltage and fill factor of the

cells and of the overall performance of the devices. This fact strongly suggests that

surface modification may significantly improve the performance of the

electrodeposited nanowire studied.

Comparative experiments reported on metal-free dyes emphasize the

importance of improving the photovoltaic performance by suitable molecular

engineering. The unambiguous enhancement of photopower-conversion efficiency

was examined by extending the length of the alkyl chain on the sensitizer with a

hierarchical photoelectrode composed of aggregated TiO2 nanoparticles. Although

the efficiency of DSSCs cannot compete with metal based sensitizers presently, we

hope that these investigations will shed light on the development of organic

sensitizers and can be used in the TiO2 nanostructure optimization for the proposed

solar cell applications. This investigation strongly suggested that the performance of

DSSCs based on organic dye sensitizer D149 could be improved. In the present

investigation the higher photocurrent efficiencies of D149 sensitizer suggested that

the appropriate HOMO and LUMO energy level of the dye with photoanode and

electrolyte energy levels (Ooyama and Harima, 2009).

Organic dyes have been widely employed as sensitizers for TiO2

nanomaterial to improve its optical properties, i.e., in dye-sensitized nanocrystalline

solar cells (DSSCs). The D149 organic dye linked to TiO2 nanoparticle surfaces via

functional groups by various interactions between the dyes and the TiO2 nanoparticle

substrate: (a)covalent attachment by directly linking groups of interest or via linking

agents, (b) electrostatic interactions via ion exchange, ion-pairing, or donor-acceptor

interactions, (c) hydrogen bonding, (d) van der Waals forces, etc. Carboxylic and

187 Chapter V

phosphonic acid derivatives react with the hydroxyl groups to form esters, while

amide linkages are obtained via the reaction of amine derivatives and dicyclohexyl

carbodiimide on TiO2. The most common and successful functional groups are based

on carboxylic acids. The interfacial charge separation between the adsorbed dyes and

TiO2 nanomaterials involves one of three mechanisms, which differ by the nature of

the donor that transfers the electron to the semiconductor: (1) excited state; (2)

reduced state; or (3) molecule-to-particle charge-transfer complex (Li, et al., 2009).

Ultrafast electron transfer from metal-to-ligand charge transfer (MLCT)

excited states to anatase TiO2 is the most common category in dye-sensitized TiO2.

The mechanism of the dye sensitization of TiO2 nanoparticles normally involves the

excitation of the dye and the charge transfer from the dye to TiO2 nanoparticles. The

low-lying MLCT and ligand-centered (ð-ð*) excited states of these complexes are

fairly long-lived, allowing them to participate in electron transfer processes. As an

efficient photosensitizer, the dye has to meet several requirements. First, the dye

should have high absorption efficiency and a wide spectral range of coverage of light

absorption in the visible, near-IR, and IR regions. Second, the excited states of the

dye should have a long lifetime and a high quantum yield. Third, the dye should have

matched electronic structures for the ground and excited states with TiO2

nanoparticles to ensure the efficient charge transfer between them; that is, the energy

level of the excited state should be well matched to the lower bound of the

conduction band of TiO2 to minimize energetic losses during the electron-transfer

reaction (Li, et al., 2009).

Quasi-solid state electrolyte is an alternative to liquid electrolytes in order to

improve the stability of electrolytes and thus that of their DSSCs. Quasi-solid state

electrolyte, especially in the gel state, have been a research hotspot due to their high

ionic conductivity, excellent thermal stability and long-term stability of the DSSCs

188 Chapter V

based on them. The concentration of iodide salts significantly influences the ionic

conductivity of quasi-solid state electrolyte and the performance of the DSSCs

because of viscosity and bond formation between the cation and atoms of polymer

chains. Higher polymer concentration leads to a decrease in ionic conductivity due to

smaller polymer cages and for polymer blend quasi-solid state electrolyte, the ionic

conductivity decreases as the concentration of the lower-conductivity and higher-

viscosity polymer increases. In the present investigation, quasi-solid state

electrolytes exhibited highest transport of charge carriers when compared to ionic

liquid electrolyte.

One way to reduce the cost of the DSSC is to use less expensive counter

electrode materials. The electron transfer rate of redox couples has to depend

strongly on the electrode material. From the present analysis it could be concluded

that the dependence of the surface area of cobalt sulfide electrodes on the catalytic

activity for reduction of redox couple at the counter electrodes is higher when

compared to copper sulfide electrode. The catalytic activity sites in cobalt material

are located on the crystal edges and the activity of cobalt is therefore expected to

increase with increasing surface area of the counter electrodes.

For DSSC to have a real impact in the commercial domain, mastering the

factors that control the electron injection and transport at the materials interfaces is

vital. The development of new dye materials, transport materials, electrolytes, hole

transport materials has countenanced interfaces to be formed efficiently and in many

cases elegantly. The electrons are injected across the dye/metal oxide interface into

metal oxide, a process that is controlled by the energetics between dye and metal

oxide, electronic states and chemical nature of metal oxides, and process condition.

Although we have not given any direct correlation between chemical property of the

metal oxides and device performance in this work.

189 Chapter V

1D nanomorphologies render controlled (surface focused) unidirectional

electron pathway in the metal oxides. It will be particularly important to explore in-

depth and reveal all the factors that are barriers for 1D nanostructured materials and

their interfaces with dye and electrolyte from sterling performance. Larger area of

interface is necessary for more exciton separation, which could be achieved in 1D

nanowires even for a limited film thickness nonetheless we cannot rule out the

possibility that increasing the interface area to a certain extent could also lead to high

interfacial recombination. Thus it is important to carefully form the interface with

right combination of donor and acceptor materials, and control the interface to yield

complete dissociation of electron and hole under optimized operating condition.

DSSCs nowadays pay attention, not only to obtain higher solar-current

conversion efficiency, and also by their potential for being cost effective, and more

stable for longer period of time. Caution must be exercised when more materials are

involved in DSSCs as there will be trouble for scaling up the cell design and

controlling the manufacturing the cost. Researchers therefore currently concentrate

on maximizing the performance of DSSCs by developing multifunctional engineered

materials-based interfaces as well as reducing the complexity in the cell design to

make DSSCs entirely a commercially viable.

Low cost is a very important benefit of producing dye sensitized solar cells

compared to the widely used conventional Si-solar cell. Moreover, enhanced dye-

sensitized solar cell efficiency would provide enormous economical advantages.

Recently, nano-sized TiO2 powders have been used as a working electrode for dye-

sensitized solar cells (DSSC) due to a higher efficiency than any other metaloxide

semiconductor. However, as reported, the best TiO2 solar cell efficiency could hardly

reach 10% . Therefore, photo-generated charge recombination should be prevented

for enhanced efficiency because solely enlarging the oxide electrode surface area is

not sufficient. A very thin metal oxide layer has often been applied, forming a

190 Chapter V

passive layer and preventing injected dye electron recombination between TiO2 and

electrolyte.

In recent years, research has focused on minimising charge recombination

processes. One approach involves the formation of a shell around the core

semiconductor by coating the semiconductor with a metal oxide that has a higher

conduction band than the semiconductor itself. This coating allows the photo injected

electrons generated upon photo-excitation of the dye by incident light to tunnel

through the shell to the semiconductor core and, subsequently, into the conducting

oxide layer simultaneously creating an energy barrier for charge recombination.

When the electrodes were constructed into DSSCs and had their photovoltaic

performance tested under the full visible light spectrum the efficiencies were found

to vary substantially. The pure TiO2 electrode used in this study had a relatively low

performance due to the opaque nature of the cells. The ZrO2 coating on the TiO2

electrodes produced a consistent trend in efficiencies. All ZrO2-coated TiO2

electrodes performed to a higher efficiency to that of the uncoated TiO2 electrodes. In

this work, the effect of a ZrO2 shell on the TiO2 core semiconductor on the efficiency

of DSSC has been ascertained. It has been found that the thin ZrO2 shell performed

the most efficiently out of the modified electrodes. The efficiency of thin ZrO2-

coated TiO2 electrode was remarkably better than the uncoated TiO2 exibiting

efficiencies of 8.03% and 3.98% respectively.

There are several electron transfer models such as diffusion, hopping,

tunneling, and a trap-detrap mechanism. The photo-generated electron can more

easily flow through surface defects toward TCO rather than bulk because the trap-

detrap transfer mechanism and diffusion can be more effectively operated at the

surface. In the present investigation, TEM image shows the Al dopes TNPWs had

less surface defect results in more compact dye molecule absorption for equal

soaking time. Densely absorbed and aggregated dye molecules inhibit tri-iodide ions

191 Chapter V

reaching the TiO2 surface due to steric hindrance. Therefore, direct recombination of

excited and transferred dye electrons with tri-iodide ion was greatly decreased by

doping Al on TiO2 semiconductor. When electrons recombine at the TiO2/electrolyte

interface, not only would a decrease of Voc result from the reduction of ions but also

a photocurrent degradation occurs due to less electrons transferred to the TCO

electrode. It was found that dye-sensitized solar cell characteristics fabricated with

AL doped TiO2 were remarkably better than those with undoped TiO2. Al-doping

results in a higher overall performance in comparison with DSSCs containing un-

doped DSSCs.

In the present investigation, after introducing Nb2O5 blocking layer, the

increase of interfaces would result in the increase of trap density, especially related

with Nb2O5 new phases. It is some difficult for electrons to jump across the potential

barrier to reach ITO phases because the potential of the Nb2O5 blocking layer.

However, the electron injection can occur through electron tunnelling because of the

µm-sized thickness of the Nb2O5 blocking layer. According to the Eqn. (5.5) the Ln

(diffusion length) in DSSC-17 with Nb2O5 blocking layer becomes higher than that

of non-blocking DSSC-7. The longer Ln is preferable for the improvement of

performance of DSSCs. Since the thickness of blocking layer should play a key role

in obtaining desirable value of Deff, we need to control and optimize the thickness of

blocking layer to get both larger Voc and diffusion length (Ln).

Thin Nb2O5 films as a novel potential scattering layer between ITO and TiO2

nanocrystaline film have been employed in ionic liquid electrolyte cells, improving

Voc and finally giving a better conversion efficiency of the dye-sensitized TiO2 solar

cells using hydrophobic dye D149. The main reason for the improvement in

performance of the DSSCs may be due to an electronically controlled barrier formed

between nano-TiO2 film and electrolyte, which can suppress the leakage of the

electron at this interface effectively, resulting especially in the increment of open

192 Chapter V

circuit voltage of QSSE-based DSSCs. Impedance spectroscopy reveals that after the

construction of the potential blocking layer, it can greatly increase shunt resistance

and decrease the series resistance to favorable extent, leading to the unidirectional

charge transport at the interfaces of ITO–photoanode/dye/ electrolyte. In addition,

such kind of the structured TNPWs- Nb2O5 bi-layer electrode can be more effective

on quasi solid state electrolyte investigated in this work partly because of

minimization of the void between ITO and nano-TiO2. Electron leakage suppression

at interfaces in DSSC is so important that we should pay attention to this during the

manipulation of unidirectional electron transporting in order to get higher

performance in DSSCs.

D149 is an aromatic heterocyclic compound have been extensively used in

designing new metal-free sensitizers. Assimilation of these moieties into the dye

molecular framework often extends the p-conjugation (towards the red side of the

visible spectra) as well as the electron-donating ability and thus helps to improve the

efficiency. The IPCE covers the entire visible region of the spectrum with 90% at the

526 nm region, resulting in a high Jsc value of 19 mA/cm2 being observed. The D149

organic dye sensitizer based on a D-A configuration in which indoline is used as an

electron donor, benzothiadiazole as an auxiliary acceptor, with thiophene moiety a

linker and cyanoacrylic acid as an anchoring unit cum acceptor. The additional

acceptor benzothiadiazole unit adjacent to the donors in a p-conjugation has several

advantages, such as improving the distribution of donor electrons to enhance the

photostability of indoline compounds, red shifting the absorption spectra into the

NIR region and weakening the deprotonation effect on a TiO2 film, beneficial to light

harvesting, as well as optimizing the match in energy levels to get the maximum

photon-to electricity conversion.

A quasi-solid state electrolyte is an effective material that can circumvent the

problems in dealing with the liquid component, such as the complicated sealing of

193 Chapter V

DSSCs. The quasi-solid state electrolyte can be formed by adding polymer matrix

and ionic liquid electrolyte. One of the questions accompanying the quasi-solid state

electrolyte is how to fill the pores inside the mesoporous photoanode. If a part of the

photoanode leaves empty, the empty part becomes the source for the electron–hole

recombination and the parasitic current. An effective method to fill the pores

completely is to employ in situ polymerization.

The quasi-solid state, or gel state, is a particular state of matter, neither liquid

nor solid, or conversely both liquid and solid. Generally, a quasi-solid-state

electrolyte is defined as a system which consists of a polymer network (polymer

host) swollen with liquid electrolytes. Owing to its unique hybrid network structure,

quasi-solid-state electrolytes always possess, simultaneously, both the cohesive

property of a solid and the diffusive transport property of a liquid. Namely, quasi-

solid-state electrolyte show better long-term stability than liquid electrolytes do and

have the merits of liquid electrolytes including high ionic conductivity and excellent

interfacial contact property. Quasi-solid state electrolytes are usually prepared by

incorporating a large amount of a liquid electrolyte into organic monomer or polymer

matrix, forming a stable gel with a network structure via a physical or chemical

method. The quasi-solid-state electrolyte formed via physical cross-linking is called

“entanglement network”, which is thermo-reversible (thermoplastic). In the present

investigation, QSSE exhibited better performance in the charge transport in the

DSSCs.

In the present work, CoS have been used as electrocatalysts on the counter

electrode. Nanocrystalline platinum is the most precious of all electrocatalysts but is

not considered a good choice of a catalyst in DSSCs due to over cost. Metal sulfides

are considered a good choice. The present work, supports the existing trends, opts for

the employment of CoS as electrocatalysts and proposes some easy procedures for

their deposition on transparent conductive electrodes. The values of the contact

194 Chapter V

resistance Rs, which also includes the sheet resistance of the electrodes themselves.

Rs values were relatively small. Their variations reflect the quality of the ohmic

contact with the electrodes, which was the best in the case of cobalt sulfide. Charge-

transfer resistance Rct at the electrode/electrolyte interface greatly varied from one

electrode to the other. The EIS results showed CuS is the material offering the

smallest charge-transfer resistance and it is expected to offer the best solar cell

functionality.

EIS, describing electron transport and charge recombination in nanocrystalline

titania films in terms of continuity equations for the conduction band and the trap

states, which allows an interpretation of spectra of DSSCs operated in the open

circuit under illumination. In the present investigation reliable values of the

parameters related to electron transport and recombination, such as keff, Rrec, Rct and

the effective diffusion coefficient of electrons, Deff were determined simultaneously

from the working state of solar cells when careful analysis of the spectra of EIS

measured under the open circuit voltage under the various experimental conditions

was done based on the information obtained by classification and clarification of

characteristics of the impedance equation.

(1) Reliable values of the parameters relating to electron transport in dye-

sensitized solar cells can be determined from the spectra measured by

electrochemical impedance spectroscopy when careful analysis of the measured

spectra is done based on the guidelines from the classification and clarification of the

characteristics of the impedance equations of the photoanode. Reliable parameter

values provide the fundamental information for improving the cell efficiency.

(2) A high electron diffusion coefficient, Deff, a low recombination rate

constant, keff, a much higher recombination resistance, Rrec, than the electron

diffusion resistance, Rct, and a high electron density, ns, are the requisites for highly

efficient dye-sensitized solar cells.

195 Chapter V

5.8 Conclusions arrived from this chapter

DSSCs constructed using novel photoanode structures, namely core-shell

(TNPWs/ZrO2), doping (Al@TNPWs) and bi-layer (TNPWs-Nb2O5) with

D149 sensitizer, different types of electrolytes (LE, ILE, SSE & QSSE) and

cobalt sulfide counter electrode. Out of these structures, core-shell

photoanode structure DSSC-4 exhibited superior ηcc is the best one.

In DSSC-4, the values of τrec,Ln, Deff, µ, σ and ηcc are higher than other

DSSCs. Stablity of DSSCs 4, 11 and 17 fabricated with QSSE is higher than

DSSCs made with other types of electrolytes.

The surface modification namely core-shell (TNPWs/ZrO2), doping

(Al@TNPWs) and bi-layer (TNPWs-Nb2O5) can possibly (i) build an energy

barrier at the semiconductor/electrolyte interface to retard the electron

transfer to the electrolyte and (ii) reduce the density of surface traps, which

are considered to be the recombination centers.

Scattering of the incident light within the photoanode due to core-shell

(TNPWs/ZrO2), doping (Al@TNPWs) and bi-layer (TNPWs-Nb2O5)

strucutrues also helps in enhancing the light harvest by making more light

available for absorption by dye molecules adsorbed on the film.

To improve PCE of DSSCs, the multilayer microstructure of the

semiconductor nanocrystalline TiO2 films is optimized. Core-shell

(TNPWs/ZrO2), doping (Al@TNPWs) and bi-layer (TNPWs-Nb2O5) layers

of larger pore-size films and light-scattering particles with the desirable

thickness are being employed to high efficiency DSSCs.

D149 sensitizer, quasi-solid state electrolyte and cobalt sulfide counter

electrode were demonstrated better results of the photo-current efficiency.

HfO2 is an efficient blocking layer in the photoanode structure.

EIS technique precisely measured internal mechanism of the DSSCs.