DSpace at VNU: The Dynamic Resistance of CdS CdSe ZnS Co-Sensitized TiO2 Solar Cells

GENERAL AND APPLIED PHYSICSSolar Cells Tung Ha Thanh&Lam Quang Vinh&Huynh Thanh Dat Received: 25 May 2014 / # Sociedade Brasileira de Física 2014 Abstract Quantum dots' sensitized solar

GENERAL AND APPLIED PHYSICS

Solar Cells

Tung Ha Thanh&Lam Quang Vinh&Huynh Thanh Dat

Received: 25 May 2014 /

# Sociedade Brasileira de Física 2014

Abstract Quantum dots' sensitized solar cells (QDSSCs)

can create the high-performance and low-cost

photovolta-ic in the future In this study, we synthesized the film of

TiO2/CdS/CdSe/ZnS photoanodes by successive ionic

lay-er adsorption reaction (SILAR) method The absorption

spectra, photoluminescent spectra and electrochemical

im-pedance spectra (EIS) of the film TiO2/CdS/CdSe/ZnS

photoanodes show that the structure of energy levels in

the conduction band (CB) of photoanode materials CdS,

CdSe, and ZnS quantum dots (QDs) can absorb a great

number of photons in each region and inject stimulated

electrons quickly into the conduction band (CB) of TiO2

Furthermore, we also studied the influence of the SILAR

cycles on the dynamic resistance, the lifetime of electrons

in QDSSCs through Nyquist and Bode

Keywords Counter electrode Quantum dots Solar cells

1 Introduction

One of the main reasons for the growing interest in

quantum dots is their use in cheap solar cells, which have

the possibility to increase the thermodynamic conversion

efficiency above the Shockley–Queisser limit [1] The

thermodynamic limit of the light to electric power con-version efficiency, also known as Shockley–Queisser

lim-it, originates from the fact that photons with energies below the band-gap energy are not absorbed, while pho-tons with energies above the band-gap energy release the additional energy (Ephoton-Egap) mostly as heat Third-generation solar cells aim toward conversion efficiencies beyond the Shockley–Queisser limit through advanced photovoltaic (PV) concepts such as multijunction cells, optical up- and downconverters, multiple carrier genera-tion by impact ionizagenera-tion Their development has been based on different p–n junctions and the use of quantum dots (QDs) to replace dyes Performance above 40 % has been obtained [2] In recent years, researchers have dis-covered the QDs which can create the high performance

of solar cells [3] QDs can be changed in particle size, leading to a change in absorption spectrum [4] Controlling QDs size, we can change their absorption spectrum Furthermore, in association with biological molecules, QDs can transfer charge faster while reducing losses and helping passivated surface (reduced defect states) of them In 1990, Vogel and his colleagues have used CdS QDs with Pt cathode [5] However, this is a new direction in quantum dots sensitized solar cells (QDSSCs) research Since then, there have been a large number of studies such as different QDs replacement, TiO2 semiconductor materials, electrolyte, and counter electrodes to enhance photovoltaic performance [6–8] Lee and his colleagues studied CdSe and CdTe QDs using

Pt counterelectrode with an efficiency of under 1 % [9] One year later (2008), they went on investigating CdS and CdSe QDs and improved the performance efficiency to 1.2 %, with the use of polysulfide electrolyte [8] Meanwhile, Lopez-Luke et al., Mora-Sero et al., Shen

et al., and Tachibana et al [10–13] synthesized CdS and CdSe QDs with Pt counterelectrode, but in different

T Ha Thanh (*)

Faculty of Physics, Dong Thap University, Cao Lãnh, Dong Thap

Province, Vietnam

e-mail: httung@dthu.edu.vn

L Quang Vinh

University of Science, Vietnam National University —HCM City,

Hanoi, Vietnam

H Thanh Dat

Vietnam National University —HCM City, Hanoi, Vietnam

DOI 10.1007/s13538-014-0266-y

electrolyte systems (Na2S, NaOH + Na2S + S) and

ob-tained a better performance efficiency of 2.2 % From

2009 to 2012, various QDSSCs were studied Cheng

et al [14] examined CdS and CdSe co-sensitized TiO2

nanowires and nanorods by using Na2S + Na2SO3

elec-trolyte, and obtained a high efficiency of 2.41 %

Although there has been much research in point as

men-tioned above, no study has been conducted about the

mechanism, processes (combined processes, electron

transport processes in semiconductor films and at

junc-tions, and corrosion of the electrode anode by electrolyte)

or about the resistances on QDSSCs performance

In this paper, we present our investigation of the

pho-tovoltaic based on CdS/CdSe/ZnS photoanodes by SILAR

method [15, 16] The absorption spectra of CdS/CdSe/

ZnS photoanode greatly extended to the visible region,

while the photoluminescent spectra quickly extinguished

The reason is that the complex structure of CdS, CdSe,

and ZnS QDs is CBTiO2<CBCdS<CBCdSe<CBZnS

(ener-gies relative to vacuum level of TiO2, CdS, CdSe, and

ZnS are −4.2, −3.6, −3.49, and −3.0 eV [17–19]) So,

photoanodes can absorb a large amount of photons in

each region and put stimulated electrons quickly into

the conduction band of TiO2 Furthermore, we also

studied the influence of the number of SILAR deaths, annealing temperature on the diffusion process, and the existence of electrons in QDSSCs through Nyquist, bode spectra

2 Experiment 2.1 Investigation on sensitized TiO2films The films were coated with TiO2 layers by silk-screen printing, and they were then annealed at 500 °C for

30 min Their sizes ranged from 10 to 30 nm (Transmission Electron Microscopy image in Fig 1a) The thickness of TiO2films was around 4 μm measured

by Stylus spectra Then, the films were dipped in 40-mmol TiCl4 solution for 30 min at 70 °C and sintered

at 500 °C for 30 min The specific surface area of the mesoporous TiO2was examined by using N2 adsorption and desorption isotherms before and after the calcula-tion The surface area is 120.6 m2 g−1 (measured by BET devices) This result indicates that the synthesized material had a wider mesoporous structure

c

b

a

d

1000

800

600

400

200

400 0 1 2

500 600 700 800

Fig 1 a X-Ray Diffraction of the

TiO 2 /CdS/CdSe/ZnS in different

electrolytes, b, c TEM images of

TiO 2 film and TiO 2 /CdS/CdSe/

ZnS, and d UV –Vis absorption

spectra of the TiO 2 films

sensi-tized by CdS/CdSe/ZnS QDs

show the light absorption

behav-ior of photoanodes changed with

the SILAR cycles of CdS, CdSe,

and ZnS

Braz J Phys

2.2 Investigation on TiO2/CdS/CdSe/ZnS films

TiO2/CdS/CdSe/ZnS films were synthesized by SILAR

method as follows: firstly, the TiO2 film was dipped in

0.5 M Cd2+-ethanol solution for 1 min and rinsed with

ethanol Then, it was dipped for 1 min in 0.5 M S2−

-methanol solution and rinsed with -methanol after being

dried in the air (a cycle SILAR) The number of CdS

QDs was increased by repeating the assembly cycles

from 1 to 5 cycles Secondly, TiO2/CdS was dipped into

1 M Cd2+-ethanol solution for 1 min at room temperature

and rinsed with ethanol Then, it was dipped for 1 min in

0.5 M Se2−-aqueous solution and rinsed with pure water

after being dried in the air (a cycle SILAR) The number

of CdSe QDs was increased by repeating the assembly

cycles from one to five For the ZnS passivation layer,

TiO2/CdS/CdSe films were dipped into 0.1 M Zn2+

-so-lution and 0.1 M S2−-solutions for 1 min and rinsed with

pure water between two dips (a total of 2 cycles)

Finally, they were annealed in a vacuum environment

with different temperatures to avoid oxidation TiO2/

CdS/CdSe/ZnS thickness was measured by the Stylus

spectra The average thicknesses of CdS (3 cycles),

CdSe (3 cycles), and ZnS (2 cycles) were 351.9, 56.1,

and 257.8 nm, respectively The coating of F− ions was

performed by dipping the TiO2 photoelectrode into a

1 M NH4F aqueous solution for 2 min, rinsed with

deionized water [20] Two layers of F−ions were coated:

the first was coated before the deposition of CdS QDs,

the second after the deposition of three layers of QDs,

and the same for CdSe

2.3 Fabrication of QDSSCs

The structure of QDSSCs was designed by the Surlyn

between photoanodes and counterelectrodes at 170 °C

The electrolyte was filled from a hole made on the

coun-ter electrode The active area of QDSSCs was 0.38 cm2

The polysulfide electrolyte was 0.5 M Na2S, 0.2 M S, and

0.2 M KCl in Milli-Q ultrapure water/methanol (7:3 by

volume)

2.4 Characterizations

The morphology of the investigated samples was

ob-served by means of TEM The crystal structure was

ana-lyzed with an X-ray diffractometer (Philips, PANalytical

X’pert, CuKα radiation) The absorption properties of the

samples were investigated with a diffuse reflectance UV–

Vis spectrometer (JASCO V-670) Photocurrent–voltage

measurements were performed on a Keithley 2400

SourceMeter via a simulated Air mass 1.5 standard (AM 1.5) sunlight with an output power of 100 mW/cm2 pro-duced by a solar simulator (Solarena, Sweden)

3 Results and Discussions QDSSCs used these QDs to replace dye in DSSCs So,

we studied the stability of the photoanodes in different electrolyte for the examined photovoltaic Figure 1a

shows the XRD of TiO2/CdS/CdSe/ZnS photoanode in

0 1 2 3 4 5 6

Voltage (V)

2 )

TiO2/CdSe at 1 hour TiO

2 /CdSe at 10 hour TiO

2 /CdSe at 18 hour TiO2/CdSe at 20 hour TiO

2 /CdSe at 24 hour

a

b

c

Fig 2 a –c The J–V curves of the QDSSCs with different photoanodes under one sun illumination

different electrolytes It is clear that the photoanode was

oxidized with I−/I3−electrolyte Two new peaks appeared

at 34 and 52° positions after the photoanode was

im-mersed in I−/I3− electrolyte Moreover, there were the

quenched diffraction peaks of the photoanode at 30, 48,

and 54.5° Meanwhile, the sample immersed in S2−/Sn −

electrolytes did not change the structure Therefore, in

this study, we decided to select the S2−/Sn − electrolyte

to produce QDSSCs Detailed morphological features

and crystallinity of the pure TiO2 and TiO2/CdS/CdSe/

ZnS photoanodes were investigated with a TEM image

Figures1b, cshow a TEM image of pure TiO2and TiO2/

CdS/CdSe/ZnS photoanodes conducted with the SILAR

cycle numbers of CdS, CdSe, and ZnS at 3, 3, and 2,

respectively We can see that QDs covered the surface of

TiO2 nanoparticles It shows that the mean diameter of

QDs is from 2 to 5 nm The results from TEM

demon-strate that the SILAR method is an efficient TiO2

strat-egy for obtaining the covering QDs on the TiO2surface

We know that the optical TiO2/CdS/CdSe/ZnS

photoanode was important for conducted photovoltaic

Figure 1d shows the UV–Vis absorption spectra of photoanodes measured after each cycle of SILAR As expected, the absorbance increased with the increasing cycles of CdS and CdSe However, only the absorption spectra with SILAR cycles of TiO2/CdS(3)/CdSe(3)/ ZnS(2) photoanode show the best deposition Under 550-nm wavelength region, an increase in absorption was due to more CdS loaded on TiO2/CdS/CdSe/ZnS film From 550 to 629 nm, a higher deposition degree of CdSe on TiO2/CdS/CdSe/ZnS electrode resulted in the shift of the absorption peak toward the red region The sizes of QDs were consistent with the sizes measured from the TEM images A higher absorption was thus obtained because the absorption spectrum of ZnS

c o m p l e m e n t e d t h o s e o f C d S e a n d C d S Q D s Furthermore, ZnS acted as a passivation layer to protect CdS and CdSe QDs from photocorrosion [21]

We conducted a set of QDSSCs based on TiO2/CdS, TiO2/CdSe, and TiO2/CdS/CdSe/ZnS photoanodes with polysulfide electrolyte Figures 2a, b present the J–V curves of QDSSCs based on TiO2/CdS and TiO2/CdSe photoanodes with different deposition times (an active area

of 0.38 cm2) at AM 1.5 (100 mW/cm2) The best power conversion efficiencies of QDSSCs based on TiO2/CdS and TiO2/CdSe photoanodes were obtained with the depo-sition times at 3 and 20 h, respectively Lower power conversion efficiencies were obtained from the QDSSCs with deposition times less than 3 h (for CdS) and 20 h (for CdSe) or more than 3 h (for CdS) and 20 h (for CdSe) The QDSSCs based on TiO2/CdS (TiO2/CdSe) show an open-circuit voltage (Voc) of 0.294 V (0.33 V), a short-circuit current density (Jsc) of 2.23 mA/cm2 (5.47 mA/cm2), fill factor (FF) of 0.34 (0.31), and an energy conversion effi-ciency (η) of 0.22 % (0.575 %) These are in line with those reported in [22–24]

The TiO2/CdS/CdSe/ZnS co-sensitized solar cells demonstrated a better performance (1.52 %) than the

Table 2 Photovoltaic

perfor-mance parameters of QDSSCs

based on TiO 2 /CdS/CdSe/ZnS

photoanodes

Solar cells J SC (mA/cm2) V OC (V) Fill factor

FF

Efficiency

η (%) TiO 2 /CdS(1)/CdSe(3)/ZnS(2) 2.18 0.29 0.35 0.22 TiO 2 /CdS(2)/CdSe(3)/ZnS(2) 4.28 0.54 0.37 0.86 TiO 2 /CdS(3)/CdSe(3)/ZnS(2) 4.79 0.76 0.41 1.52 TiO 2 /CdS(4)/CdSe(3)/ZnS(2) 5.73 0.39 0.31 0.68 TiO 2 /CdS(5)/CdSe(3)/ZnS(2) 3.05 0.45 0.32 0.45 TiO 2 /CdS(3)/CdSe(1)/ZnS(2) 6.05 0.356 0.256 0.55 TiO 2 /CdS(3)/CdSe(2)/ZnS(2) 4.21 0.55 0.38 0.88 TiO 2 /CdS(3)/CdSe(4)/ZnS(2) 3.30 0.48 0.31 0.50 TiO 2 /CdS(3)/CdSe(5)/ZnS(2) 2.08 0.33 0.27 0.18 TiO2/CdS(3)/CdSe(3)/ZnS(1) 7.03 0.39 0.26 0.73

Table 1 Photovoltaic performance parameters of QDSSCs based on

TiO 2 /CdS and TiO 2 /CdSe photoanodes

Solar cells J SC (mA/cm2) V OC (V) Fill factor

FF

Efficiency

η (%) TiO 2 /CdS at 1 h 0.763 0.276 0.255 0.054

TiO 2 /CdS at 2 h 1.87 0.38 0.242 0.17

TiO 2 /CdS at 3 h 2.23 0.294 0.34 0.22

TiO 2 /CdS at 5 h 1.7 0.22 0.3 0.12

TiO 2 /CdSe at 1 h 0.256 0.31 0.25 0.02

TiO 2 /CdSe at 10 h 0.59 0.32 0.24 0.046

TiO 2 /CdSe at 18 h 2.08 0.33 0.27 0.184

TiO 2 /CdSe at 20 h 5.47 0.33 0.31 0.575

TiO 2 /CdSe at 24 h 2.13 0.29 0.24 0.15

Braz J Phys

TiO2/CdS (0.22 %) and TiO2/CdSe QDSSC (0.575 %)

(shown in Tables 1 and 2) [25] This suggests that the

charge injection from CdSe conduction level to TiO2

conduction level may not be effective, due to the

quasi-Fermi levels of CdSe being lower than that of TiO2[26]

However, the quasi-Fermi level of CdS quantum dots

was higher than that of the TiO2 layer [27], and it is

expected to improve the charge injection from CdSe to

TiO2 Moreover, a ZnS coating formed a potential barrier

between QDs and the electrolyte, which blocked the

electrons in the CB from QDs to the electrolyte and

reduced the defect states in QDs [28] So, the electron

density in the conduction band of QDs increased and the

enhanced JSC was obtained And thus, a high

perfor-mance was obtained In addition, with the increasing

electron density in the conduction band of QDs, the

quasi-Fermi level correspondingly increased and

conse-quently, VOC= (EFn–EFo)/(−q) increased (Fig 3) With

the combination of CdS, CdSe, and ZnS, the CdS

Fermi energy level was higher than that of TiO2, and

beneficial effects were conferred to the coupled QDSSCs

system It is evident that the parameters of the coupled

QDSSCs were influenced by CdS/CdSe/ZnS

co-sensitization cycles [29] This is because CdS, CdSe,

and ZnS QDs led to the quasi-Fermi level alignment

and it resulted in a cascade energy level structure in the

order of CBTiO2<CBCdS<CBCdSe<CBZnS That is, the

introduction of a CdS layer between TiO2 and CdSe

elevated the conduction band edge of CdSe, making a

higher driving force for the injection of stimulated

elec-trons out of the CdSe layer [25] Moreover, the

photo-current density might be enhanced with QDs loaded by

means of increasing coating cycles [28]

To further investigate the dynamic resistance of

QDSSCs, the electrochemical impedance spectra (EIS)

under illuminated conditions for QDSSCs with different

CdS and CdSe SILAR cycles were carried out to

re-search the charge transfer process [29] Figures 4a–c

show the Nyquist plots of TiO2/CdS and TiO2/CdSe

0 300 600 900

1200

(5)

(4)

(3) (2)

(Ohm)

(Ohm)

Z'

(1)_TiO2/CdS(1)/CdSe(3)/ZnS(2) (2)_TiO2/CdS(2)/CdSe(3)/ZnS(2) (3)_TiO2/CdS(3)/CdSe(3)/ZnS(2) (4)_TiO2/CdS(4)/CdSe(3)/ZnS(2) (5)_TiO2/CdS(5)/CdSe(3)/ZnS(2)

(1)

0 1000 2000

Z'

(5)

(4)

(3)

(2)

TiO2/CdS(3)/CdSe(1)/ZnS(2) TiO2/CdS(3)/CdSe(2)/ZnS(2) TiO2/CdS(3)/CdSe(3)/ZnS(2) TiO2/CdS(3)/CdSe(4)/ZnS(2) TiO2/CdS(3)/CdSe(5)/ZnS(2)

(1)

a

b

c

d

Fig 4 a, b Nyquist and Bode impedance plots of EIS spectra measured under the illuminated conditions for QDSSCs with CdS SILAR cycles from one to five layers and c, d CdSe SILAR cycles from one to five layers

Fig 3 The energy level alignment of QDSSCs [ 30 ]

QDSSCs when SILAR cycles of CdS changed from one

to five The EIS illustrated two semicircles at high

fre-quency and low frefre-quency The small semicircle was due

to the resistance against movement of charge at Pt/

electrolyte (Rct1) and FTO/TiO2 interface Meanwhile,

the large semicircle was due to a resistance against the

electron diffusion in the TiO2 and the charge

recombi-nation resistance at the TiO2/QDs/electrolyte interface

(Rct2) and against the inner diffusion in an electrolyte

(Zw) With Fit & Simulator software, we fitted for EIS of

all samples and the values ofRs,Rct1, andRct2 are listed

in Tables3 and 4.Rsis a set of resistance to the charge

transfer at Ag/FTO/TiO2 front contact and Ag/FTO/Pt

back contact; Rs values are obtained at about 38.1 Ω

for the best photoanodes The result shows that the

applied technique is significant

We can find in Fig.4a–cthat the radius of the

semicir-cles increased when SILAR cysemicir-cles of CdS or CdSe were

under three layers or over three layers Compared with the

other photoanodes, the TiO2/CdS(3)/CdSe(3)/ZnS(2)

photoanode exhibited a smaller Rct1 and Rct2 (9.21 and

83.5Ω) and larger lifetime (3.2 ms) (shown in Tables 3

and4) Also, it shows a fast electron transfer at TiO2/QDs/

electrolyte interface and reduction in recombination as a

ZnS coating protected CdS/CdSe QDs [31] With

increas-ing SILAR cycles of CdS and CdSe over three layers, the

dynamic resistance increased This is because the amount

of CdS and CdSe loaded more on TiO2/CdS/CdSe/ZnS,

which indicated the increasing recombination in

photoanodes The results show that the photogenerated

electrons were captured by the defect states in QDs So

the electron transfer was more diffusive hindrance, which increased charge recombination and back transport reaction

Figures 4b–d indicate the Bode plots of the TiO2/ CdS and TiO2/CdSe QDSSCs when SILAR cycles of CdS changed from one to five We find that the fre-quency peak of the charge transport process isshifted to the lowest frequency region corresponding to TiO2/ CdS(3)/CdSe(3)/ZnS(2) photoanode Therefore, it indi-cates that the lifetime of the electron in the TiO2/ CdS(3)/CdSe(3)/ZnS(2) film increases, which favors the electron transfer with less diffusive hindrance Hodes and co-workers found that the lifetime of elec-tron transfer into TiO2 reached 10−12 s (very fast) and that of electron recombination got 10−6 s [32] It is shorter than the lifetime of electrons in the conduction band of QDs (calculated in this paper about 3.2 ms) So the charge transfer in the conduction band increased

4 Conclusions

We successfully fabricated the QDSSCs based on the TiO2/ CdS, TiO2/CdSe, and TiO2/CdS/CdSe/ZnS photoanode The TiO2/CdS/CdSe/ZnS co-sensitized solar cells demonstrated a better performance (1.52 %) than those of the TiO2/CdS (0.22 %) and TiO2/CdSe QDSSC (0.575 %) This is because

a ZnS coating formed a potential barrier between QDs and the electrolyte, which blocked the electrons in the conduction band from QDs to the electrolyte and reduced the defect states

in QDs The dimension of the semicircles increased when

Table 3 The resistance and

life-time obtained from the EIS

mea-surements of TiO 2 /CdS/CdSe/

ZnS for different CdS SILAR

cycles

1 TiO2/CdS(1 layer)/CdSe(3)/ZnS(2) 21.9 351 1,570 4.9

2 TiO 2 /CdS(2 layers)/CdSe(3)/ZnS(2) 33.9 333 158 3.2

3 TiO 2 /CdS(3 layers)/CdSe(3)/ZnS(2) 38.1 83.5 9.21 3.2

4 TiO 2 /CdS(4 layers)/CdSe(3)/ZnS(2) 35.4 1930 2330 1.6

5 TiO 2 /CdS(5 layers)/CdSe(3)/ZnS(2) 26 16,100 59,100 1.8

Table 4 The resistance and

life-time obtained from the EIS

mea-surements of TiO 2 /CdS/CdSe/

ZnS for different CdSe SILAR

cycles

1 TiO 2 /CdS(3)/CdSe(1 layer)/ZnS(2) 28.6 1,670 106 5.9

2 TiO 2 /CdS(3)/CdSe(2 layers)/ZnS(2) 67 1,190 268 4.9

3 TiO 2 /CdS(3)/CdSe(3 layers)/ZnS(2) 38.1 83.5 9.21 3.2

4 TiO 2 /CdS(3)/CdSe(4 layers)/ZnS(2) 24.8 385 179 1.7

5 TiO2/CdS(3)/CdSe(5 layers)/ZnS(2) 21.3 415 243 1.8

Braz J Phys

SILAR cycles of CdS and CdSe were under three layers or

over three layers Compared with the other photoanodes, the

TiO2/CdS(3)/CdSe(3)/ZnS(2) photoanode exhibited a smaller

Rct1andRct2(9.21 and 83.5Ω) and larger lifetime (3.2 ms)

The results show a fast electron transfer at TiO2

/QDs/electro-lyte interface and a reduction in recombination when a ZnS

coating protected CdS/CdSe QDs [26]

Acknowledgments This work was supported by Vietnam National

University by the name of the project: B 2012-18-5TD, the University

of Science of Ho Chi Minh City and Dong Thap University.

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