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116

CHAPTER-6

We will make electricity so cheap that only the rich will burn candles.

Thomas A. Edison

Part of this chapter has been published in:

1. Proceedings of International Congress on Renewable Energy, 2011, 284- 291.

2. IEEE Xplore, DOI: 10.1109/ICPEN.2012.6492312, 2012, 1-5.

3. J. Renewable and Sustainable Energy, 5, 2013, 043115, 1-10.

4. Material Science Forum, 771, 2014, 133-141.

5. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 118,

2014, 938-943.

http://dx.doi.org/10.1109/ICPEN.2012.6492312

Chapter-6 Electrical Studies on Dye-Sensitized Solar Cells

117

6.1 Introduction

Electrochemical Impedance Spectroscopy (EIS) is a powerful tool for investigating

the dynamical properties of dye-sensitized solar cells. The various parameters, like,

diffusion-recombination impedance, electron life time, diffusion length, etc., could be

evaluated from EIS measurements and a possible mechanism occurring in DSSCs

could be ascertained. There have been several reports based on EIS measurements for

characterization of DSSCs, for instance by Wang et al. (2005), Hoshikawa et al.

(2006), Adachi et al. (2006). Wang et al. (2006) have carried out a detailed analysis of

electron transport and diffusion-recombination processes from EIS measurement.

They have also constructed J-V characteristics from the parameters obtained.

Excellent review articles exist in literature on dye-sensitized solar cells sensitized

with synthetic dyes, where the emphasis is been on variation of parameters,

mentioned above, as a function of biasing voltage [Peter (2007), Bisquert et al.

(2008)]. All these available studies have been done for DSSCs sensitized by organic

dyes (synthetic dyes to be precise). On the other hand, EIS studies on N-DSSCs have

not yet been reported in detail, as far as our knowledge goes. So, I have made an

attempt to study the transport properties of N-DSSCs using this method.

The objective of this work is to fabricate DSSC by using different natural dyes

adsorbed onto TiO2 film surface. Their characterization (structural, morphological,

thermal, optical etc.) has already been reported in previous chapters. Since this

chapter exclusively deals with electrical studies, therefore, I have divided the chapter

broadly in two sections-I and II. Section I deals with the Impedance studies of the

fabricated DSSCs using BM, MM and POM as sensitizers. Since Impedance studies

have been carried out in detail, therefore, a brief introduction of IS is considered

pertinent before results of DSSCs characterization are presented. At the outset, thus,

in the very next section, introduction to Impedance Spectroscopy has been given.

Section II deals with the studies on the J-V characteristics of the DSSCs and the cell

parameters, Isc, Voc, FF, Pmax and . Since stability is an important parameter of

DSSCs from application point of view, I have also carried out a preliminary study on

stability of cells using MM dye extracted both in ethanol and de-ionized water.


118

Section I

6.2 Elementary idea about Impedance Spectroscopy

In EIS technique a perturbing signal at different frequencies is applied across the

specimen via connecting leads and corresponding variation in current is observed. The

ratio of perturbing potential and resulting current or its inverse along with their phase

is then measured and accordingly impedance or admittance of the specimen is

determined. In a solar cell this could be carried out over a range of temperature, under

dark or illumination conditions and/or with variation of biasing voltage. The real and

imaginary impedances (or admittances) are then plotted against each other for range

of frequencies known as Cole-Cole or Nyquist plots [Baurele (1969), Macdonald

(1987)]. As discussed, impedance takes phase difference into account and thus in ac,

the resistance R is replaced by the impedance Z, which is the sum of resistance and

reactance. Impedance can be written as –

Z = Z+ iZ

where Z is the real part of Z and Z is the imaginary. Z is taken as a vector quantity

and plotted in a plane as shown in Fig. 6.1 with rectangular co-ordinates obviously,

Re (Z) = Z = Zcos Im (Z) = Z = Zsin

with phase angle = tan-1(Z/Z) and Z= (Z2 + Z2)1/2

Fig. 6.1: The impedance Z plotted as a planar vector.


119

From impedance data, other useful parameters (shown in Table 6.1) could be

evaluated and different schemes of plotting are thus adopted depending upon the

materials/devices under consideration in order to understand properties and

underlying mechanisms. It is, therefore, necessary to know relationships among

electrical parameters. Various electrical parameters and their inter relationships are

depicted in Table 6.1.

Table 6.1: Impedance representation

Elements Definition Real and imaginary part

Impedance Z () Z=Z + iZ

Admittance Y () =1/ Z () Y = Y + iY

Phase angle tan = Z/Z

Complex capacitance C*() = 1/i Z () C* = C + iC

Conductivity *() = L/A Z () *= + i

Complex dielectric constant

*() = L C*()/A * = + i

Complex electric modulus M*() = 1/() M* = M + iM

IS data are described by equivalent circuits composed of combinations of active and

passive elements, shown in Table 6.2 along with their impedances.

Table 6.2: Basic ac electrical elements

Elements Symbol Scheme Impedance

Resistance R R Capacitance C

1/iC

Inductor L iL

Constant phase element (CPE)

Qn

(i)-n/Qn


120

Therefore, experimental impedance data can be modeled with the impedance of an

equivalent circuit consisting of ideal circuit elements shown in Table 6.2. The

representative Cole-Cole and Bode (frequency versus phase angle or impedance) plots

of two RC circuits in series with R are shown in Fig. 6.2 and Fig. 6.3 represents

Nyquist and Bode plots with inductor in the circuit.

Fig. 6.2: (a) Cole-Cole (b) Bode plot and (c) Equivalent circuit model.

Fig. 6.3: (a) EIS spectrum (b) Bode plot and (c) Equivalent Inductive circuit model.

A typical EIS spectrum in case of DSSC may consist of two [Wang et al. (2006)] or

three [Lagemaat et al. (2000)] semicircles. The central arc in the mid-frequency range

is related to the charge transfer of TiO2/dye/electrolyte. The higher frequency arc, to

the left of the central arc, is related to electrochemical reaction at the Pt counter


121

electrode and the lower frequency arc represents Warburg diffusion process of I -/I3-

[Longo et al. (2003), Bernard et al. (2003)]. Thus, the total impedance of the DSSC is

the sum of the impedance at the Pt electrode, impedance due to diffusion of tri-iodide

in the electrolyte and characteristics of the impedance of the TiO2 electrode.

6.3 Theoretical Modeling of the Frequency Response

The DSSC contains three interfaces formed by TCO/TiO2, TiO2/electrolyte and

electrolyte/Pt-TCO. In the dark under forward bias, electrons are injected in the

conduction band of TiO2 and their motion is coupled to that of I-/I-3 ions in the

electrolyte [Wang et al. (2005)]. Illumination gives rise to the following processes (i)

sensitized electron injection (ii) recombination with the parent dye and (iii)

regeneration of the sensitizer.

DSSC is usually described by the infinite transmission line model, depicted in Fig. 6.4

[Santiago et al. (2007)].

Fig. 6.4: (a) General transmission line model of DSSC (b) simplified model under

illumination.


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6.3.1 Electron transfer at the platinum counter electrode

At higher frequency, a small deformation in the single arc occurs both in dark and

under illumination together with a small semicircle under open circuit bias as seen in

the Nyquist plot. This effect arises from the contribution of the counter electrode and

may be modeled by the parallel combination of the platinum charge transfer resistance

and the double layer capacitance.

1

1p

p

p

Z

i Cr

(6.1)

where, max

1

p pr C (6.2)

Therefore, the expression for the real and imaginary part of the impedance becomes,

'

2 2 21

p

p

p p

rZ

r C

(6.3)

and 2

"

2 2 21

p p

p

p p

r CZ

r C

(6.4)

6.3.2 Electron transport within the mesoscopic TiO2 film

The basic equations governing the model have been given by Bisquert [Bisquert

(2002)] and are described below.

The impedance function of the diffusion-reaction model is

1

1 12

2 2( ) / 1 /1 /

t rr d r

r

R RZ coth i

i

(6.5)

where Rt and Rr are transport and recombination resistances respectively. The value of

these resistances can be obtained from the impedance spectrum shown in Fig 6.4c.


123

Fig 6.4c: Nyquist plot of DSSCs for EIS analysis.

d = Dn/L2 = 1/RtCμ is the characteristic frequency of diffusion, Dn the electron

chemical diffusion co-efficient, r = 1/RrCμ is the rate constant for recombination,

is the angular frequency.

Under the condition of low recombination, Rt < Rr (or r < d), Eq. 6.5 reduces, at

low frequency, to

1

3 1 /

rt

r

RZ R

i

(6.6)

Thus, the real and imaginary part of the impedance become,

2

2 2

1'

3

r rt

r

RZ R

(6.7)

2 2" r r

r

RZ

(6.8)

The impedance is largely dominated by the reaction arc, with the characteristic

frequency r, the second term of Eq. 6.7.


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If the electron collecting efficiency is low, the condition Rt > Rr, applies leading to

Gerischer impedance [Gerischer (1951), Bisquert (2002), Boukamp et al. (2003)].

Ideally, it takes the form

1

2

1 /

t r

r

R RZ

i

(6.9)

with the real and imaginary parts,

11 2

2 2

1

2

1 1 122 2 2

1

( )'

21

r r

t r

r

r

R RZ

(6.10)

11 2

2 2

1

2

1 1 122 2 2

1

( )"

21

r r

t r

r

r

R RZ

(6.11)

Gerischer impedance shows a Warburg diffusion-like straight line at higher

frequencies making an angle of 45o with the X-axis, along with a semicircle at lower

frequencies in such cases.

The above mentioned form of Gerischer impedance in usual cases takes the form,

1

2

1 ( / )

t r

r

R RZ

i

(6.12)


125

This is known as modified Gerischer impedance. In this case, the angle with x-axis of

the straight line at higher frequency deviates from 45o. The appearance of Gerischer

impedance decreases the electron density in the TiO2 layer, thus, limiting the

efficiency of a cell [Bisquert (2002)].

6.3.3 Finite Warburg impedance of tri-iodide in Electrolyte

In practical case, the concentration of tri-iodide (I3-) is much lower than that of Iodide

(I-) and I- diffuses faster than I3- and hence I- contributes little to the overall diffusion

impedance. The diffusion of I3- is well described by Nernst diffusion impedance given

by,

2

1

2

1

1tanh

/

/

d D

iZ R

Di

D

(6.13)

where, RD is the dc resistance and D1 and represent the diffusion co-efficient of I3-

and the thickness of the film, respectively.

6.3.4 Impedance model for DSSC with inductive behavior

The combination of capacitor and resistance provides a spectrum that remains in the

first quadrant of the complex impedance plane, but it is not uncommon to find the

data cross to the fourth quadrant. One of the reasons for this is the inductance of the

lead which forms a tail at higher frequency and other may be due to Faradaic

impedance that forms an arc in the fourth quadrant at lower frequency [Mora-Serό et

al. (2006)].

This effect can be represented as a series RL branch complementing the RC circuit as

shown in Fig. 6.5.


126

Fig. 6.5: Impedance model with inductive behavior.

The total impedance of the circuit is given by

1 2

1 1 1j C

Z R R j L

(6.14)

with the real and imaginary impedance

2 2

1 2 1 2 1 1 1 2

2 2 2 2

1 2 1 1 2

( ) ( )'

( ) ( )

R R R R LCR LR L CR RZ

R R LCR L CR R

(6.15)

2

1 1 2 1 1 2 1 2

2 2 2 2

1 2 1 1 2

( ) ( )"

( ) ( )

R L R R LCR R R L CR RZ

R R LCR L CR R

(6.16)

Parameters under illumination, calculated using Eq. 6.15 and Eq. 6.16 for the

impedance in the fourth quadrant is shown in Table 6.4.

6.4 Results

6.4.1 Impedances of DSSCs obtained in Dark, with Bias and under

Illumination with different sensitizers

The impedance spectra of the cells sensitized with BM, POM and MM were obtained

by two-electrode method as discussed in chapter 4 (Section 4.7.1). The nomenclature

of the cells under different conditions has been given in Table 6.3 for easy reference

to be followed for further discussions.


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Table 6.3: Nomenclature of DSSCs under different conditions

DSSC sensitized with Cell Condition

BM

B1 Dark B2 Bias B3 illumination

MM

M1 Dark M2 Bias M3 illumination

POM

P1 Dark P2 Bias P3 illumination

Fig. 6.6 (a, b, c) shows the Nyquist plots of DSSCs sensitized by BM, POM and MM

dye in the dark and with the bias. The solid circles represent the experimental data and

the solid lines represent the fitted curves obtained using the Eqs. 6.3-6.12.

(a)


128

(b)

(c) Fig. 6.6: Nyquist plot of DSSCs sensitized with (a) BM, (b) POM and (c) MM dye in

dark and with bias.


129

The impedance curves under dark and under bias for the cells were fitted using Eqs.

6.3, 6.4, 6.7 and 6.8. In case of MM for the cell under dark (M1), however, a straight

line at higher frequency appears which signifies occurrence of Gerischer impedance

and hence, we have fitted the impedance curve for this cell with the Eq. 6.12 at higher

frequencies while at lower frequency, the best fitting was obtained using Eqs. 6.7 and

6.8. The departure from ideal Gerischer impedance was observed for M1 with a

depressed angle of 68o.

Fig. 6.7 shows the Nyquist plots of DSSCs sensitized by BM, POM and MM dye

under illumination. The fitting was carried out for all the cells (B3, P3 and M3) at

lower frequency using Eqs. 6.15 and 6.16 while at higher frequencies the curves were

fitted using Eqs. 6.7 and 6.8.

Fig. 6.7: Nyquist plot of DSSCs sensitized with BM, POM and MM under

illumination.


130

Table 6.4 shows the fitting parameters (Rt, Rr and Cμ) and the parameters, electron

lifetime, diffusion co-efficient and diffusion length calculated using the relation, τn

=RrCμ, Dn=d.L2 and Ln=L (Rr/Rt)1/2, respectively, for all the cells under various

conditions.

Table 6.4: EIS parameters of the fabricated cells in dark, with bias and under

illumination

Cell Condition Rt () Rr () Cμ x 10-5 (F)

n (ms) Dn x10-5 (cm2s-1)

Ln (μm)

B1 Dark 150 1850 1.43 26.52 16.78 21.06

B2 Bias 600 1550 1.28 19.84 4.68 9.63

B3 Light 150 175 10.05 17.6 2.37 6.47

M1 Dark 120 1710 1.45 24.79 9.53 22.64

M2 Bias 126 1158 1.37 15.86 5.08 18.18

M3 Light 150 200 5.98 11.96 2.05 6.92

P1 Dark 150 2000 1.44 22.83 16.58 22.97

P2 Bias 750 1800 1.22 21.96 6.90 9.42

P3 Light 150 300 2.65 7.96 1.03 8.48

It is noteworthy from Fig. 6.7 that at lower frequency, all the cells under illumination

form an arc in the fourth quadrant implying that the impedance spectra crosses the

real axis. In that case, the equivalent circuit shown in Fig. 6.5 was invoked to obtain

the electrochemical parameters for fabricated DSSCs. The fitting parameters have

been summarized below in Table 6.5. Apart from this, cells with BM and MM as

sensitizers have arc in fourth quadrant, at frequencies greater than 4 kHz. This could

be accounted for, by the lead resistance arising due to connecting leads.


131

Table 6.5: Parameters obtained for the cells under illumination at lower frequencies

using equivalent circuit of Fig. 6.5

Sensitizer R1() R2() C1(μF) L1(H)

BM 280 120 2 210

MM 300 125 4 300

POM 370 170 9 300

6.5 Discussions From Table 6.4 it is seen that, with bias and under illumination, the value of Rr

decreases for all the cells which is due to the increase in electron density with bias and

under illumination. The semicircle in the intermediate frequency regime is the

impedance due to the electron transfer from the conduction band of the mesoscopic

film to tri-iodide ions in the electrolyte through the outer circuit. From the Nyquist

plots, it is also found that the semicircle under light is much smaller than in dark and

under bias. This may be due to the difference in the local I3- concentration. In the dark

I3- is generated at the counter electrode which diffuses through the mesoporous TiO2

films, whereas under illumination I3- is formed by dye regeneration at the mesoporous

TiO2/electrolyte interface. The concentration of I3- under light is expected to

accelerate the recapture of conduction band electrons and shorten their lifetime within

the film. Hence, it is seen that the electron lifetime decreases under illumination for

all the three DSSCs. But electrons in DSSC sensitized with BM dye has higher

lifetime compared to the cells sensitized with MM and POM. This leads to higher

efficiency of DSSCs sensitized with BM as compared to MM and POM.

The presence of inductance appears as a negative capacitance and becomes more

negative towards lower frequency. The inductive effect produces a decrease of charge


132

accumulation ability of the solar cell and reduces the steady state performance as well

as the effective lifetime of the charge carriers. It is also observed that the change in

the value of the resistance connected in series with the inductor greatly changes the

shape of the arc (see Fig. 6.7). As is clear from Table 6.5, the cell sensitized with BM

has least resistance while the cell sensitized with POM has maximum resistance. It is

also to be noted that the cell with BM dye as sensitizer has least inductance. This

confirms that the cell sensitized with BM dye should have maximum efficiency

amongst all the cells studied.

Section II

6.6 Current-Voltage characteristics

The performance of the fabricated cells can be analyzed by plotting the current versus

voltage curve. The current established through the cell is obtained under illumination

by varying the voltage across the cells in small steps. The maximum power is then

obtained from the I-V curve. Using the Eqs. 6.17 and 6.18, the Fill factor (FF) and

efficiency () are obtained for the fabricated cells.

ocscVJ

PFF max (6.17)

in

ocsc

in P

VJFF

P

P )(max (6.18)

Cells were fabricated using TiO2 coated ITO plates which served as photoanode and

platinum coated ITO plates as counter electrodes. The process of TiO2 coating using

TiO2 sol. and platinum, using Hexachloroplatinic acid solution is described in

chapter 2. The cells were fabricated using three sensitizers, namely, BM, MM and

POM, the details of which is presented in chapter 3.


133

6.7 DSSCs fabricated using BM as sensitizer

Fig. 6.8 (a, b) shows the J-V and power curves of the DSSCs sensitized with BM dye.

The inset in Fig. 6.8 (a) shows the error bar for cell 1.

(a)

(b)

Fig. 6.8: (a) J-V characteristic (b) Power curve of BM sensitized DSSCs.


134

Table 6.6 summarizes the performance of these N-DSSCs in terms of Jsc, Voc, FF and

η of the DSSCs. The efficiency of the solar cells was found in the range of 1.73–1.86

± 0.07%. As evident from Table 6.6, the maximum efficiency of one of the cells is

found to be 1.86 ± 0.07%.

Table 6.6: Photovoltaic parameters of assembled DSSCs

Cell Jsc (mA/cm2) Voc (mV) FF (%) (η ± 0.07%)

1 2.328

485 33 1.86

2 2.177

480 34 1.76

3 1.922

470 38 1.73

4 2.154 490 35 1.84

6.8 DSSCs fabricated using MM as sensitizer

6.8.1 J-V characteristics

Fig. 6.9 (a, b) shows the J-V characteristics for the DSSCs using de-ionized water as

well as using ethanol as dye extracting mediums.

Table 6.7 shows the parameters in terms of Jsc, Voc, FF and for both sets of DSSCs.

The efficiencies obtained for DSSCs sensitized by extract in de-ionized water are

significantly higher (almost two to three times more) than that of the ones sensitized

by the extract in ethanol. The reason for this is the broad range of absorption of raw

MMD dye and TiO2 films adsorbed onto by MMD as compared to MME as discussed

in the chapter 5 (Optical studies of MM dye).

The maximum efficiency of DSSC as fabricated using dye extracted in de-ionized

water is 1.37 ± 0.05 %, while the maximum efficiency obtained using dye extracted in

ethanol is 0.72 ± 0.05%. Hence, de-ionized water was found to be suitable solvent for

the extraction of MM dye.


135

(a)

(b)

Fig. 6.9: J-V characteristics of DSSCs sensitized with (a) MMD (b) MME.


136

Table 6.7: Photovoltaic parameters of the DSSCs sensitized with MMD and MME

Cell Pmax Jsc (mA cm-2) Voc (mV) FF (%) η (±0.05%)

D 1 0.2756 1.506 430 43 1.37

D 2 0.2398 1.274 430 44 1.198

D 3 0.2228 1.250 420 43 1.113

E 4 0.1446 0.876 380 43 0.72

E 5 0.1136 0.747 380 40 0.56

E 6 0.0832 0.639 370 36 0.41

D - De-ionized water as extracting solvent, E - Ethanol as extracting solvent

6.8.2 Stability study

Fig. 6.10 (a, b) shows the J-V characteristic of D1 and E4 as a function of time and

the variations of Jsc, Voc, FF and of D1 and E4 have been depicted in Table 6.8. The

drop in Jsc and Voc is found to be 48.25% and 2.3% respectively for D1, and 71.0%

and 15.78% respectively for E4.

Fig. 6.11 shows the drop in efficiency of the cells plotted against time. It was seen

that the drop in efficiency of the MMD cell was 43% in 2 hrs, whereas it was 86% for

the MME cell. This study shows that the cell fabricated using dye extract in ethanol

degrades faster than the cell fabricated using dye extract in de-ionized water. The

underlying reason is that the dye is not properly soluble in ethanol and the functional

groups are not properly attached to the TiO2 surface as is clear from FTIR analysis.

The color of the photoanode sensitized with extract in ethanol was also found to have

faded after the exposure to incident light over a period of time. Thus, it is obvious that

the cell in MMD is more efficient and stable than the cell fabricated in MME.


137

(a)

(b)

Fig. 6.10: Stability study of (a) D1 fabricated using MMD; (b) E4 fabricated using

MME.


138

Fig. 6.11: Drop in efficiency of DSSC using MMD and MME with time.

Table 6.8: Stability test of cell D1 and E4 for 2 hrs time

Cell Time Pmax Jsc (mA cm-2) Voc (mV) FF (%)

D1

0 0.2756 1.506 430 43

30 mins 0.250 1.3693 430 42

1 hr 0.2109 1.215 430 40

2 hrs 0.1478 0.7793 420 40

E4

0 0.1446 0.876 380 43

30 mins 0.0411 0.380 370 29

1 hr 0.0264 0.266 340 29

2 hrs 0.0212 0.254 320 26


139

6.9 DSSCs fabricated using POM as sensitizer Fig 6.12 (a, b) shows the J-V characteristic and power curves of the fabricated

DSSCs. Table 6.9 presents the photoelectrochemical data obtained with the DSSCs

using POM dye, where the values of Isc and Voc are in the range 1.03-1.57 mA and

475-480 mV, respectively. The calculated efficiencies of the cells range from 0.9 ±

0.08% to 1.1 ± 0.08%.

Table 6.9: Photoelectrochemical parameters of cells sensitized with POM dye

Cell Jsc (mA/cm2) Voc (mV) FF (%) (±0.08%)

1 1.57 475 35 1.1

2 1.04 480 38 0.96

3 1.03 480 36 0.9

(a)


140

(b)

Fig. 6.12 (a) J-V characteristics and (b) power curve of POM DSSCs.

6.10 Conclusions

A transmission line model was used to describe the general electrochemical behavior

of dye-sensitized solar cells sensitized with BM, MM and POM dye. Under

illumination, the impedance spectra of DSSCs at very low frequency and at high

frequency are found to be below real axis. This behavior can be attributed to the

appearance of inductive response. Various EIS parameters were evaluated from

impedance data. The electron lifetime of BM sensitized cells was found to be higher

as compared to MM and POM under illumination. The resistance, capacitance and

inductance values of BM cells were also less as compared to MM and POM cells

thereby making it most efficient sensitizer among the sensitizers studied.

From the electrical studies of the DSSCs fabricated using three natural dyes, it was

found that the highest obtained efficiency under optimum condition for BM sensitized

DSSC is 1.86 ± 0.07%. The efficiencies obtained for DSSCs sensitized by MM


141

extract in de-ionized water are significantly higher than that of the ones sensitized by

MM extract in ethanol. The maximum efficiency of DSSC as fabricated using dye

extracted in de-ionized water is 1.37 ± 0.05%, while the maximum efficiency

obtained using dye extracted in ethanol is 0.72 ± 0.05%. Stability test shows that dye

extracted in de-ionized water is more stable than dye extracted in ethanol. The

maximum efficiency obtained for POM sensitized DSSC is 1.1 ± 0.08%. Hence, it is

clear that among these three dyes, BM turns to be the best sensitizer with the

maximum obtained efficiency.

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