Journal of Power and Energy Engineering, 2013, 1, 67-72
http://dx.doi.org/10.4236/jpee.2013.15011 Published Online October 2013 (http://www.scirp.org/journal/jpee)
67
CdHgTe Quantum Dots Sensitized Solar Cell with Using of
Titanium Dioxide Nanotubes
M. Y. Feteha, M. Ameen
Department of Materials Science, Institute of Graduate Studies and Research, Alexandria University, Alexandria, Egypt.
Received September 2013
ABSTRACT
The sensitization of TiO2 nanotubes with CdHgTe quantum dots (QDs) was applied by using the direct dispersion tech-
nique. The CdHgTe-QD s were fabricated with differen t Hg% ratio in organic medium for contro lling their particle size.
While TiO2 nanotubes (NTs) were fabricated by anodization technique. The QDs and NTs were characterized using
SEM, TEM and UV-VIS spectrophotometer. In this work, the photovoltaic parameters of the quantum dots sensitized
solar cell (QDSSC) depend mainly on the Hg% ratio in the QDs. The most efficient QDSSC was obtained at 25% of Hg
ratio with Jsc of 4 mA/cm2, Voc of 0.63 V, FF of 0.32 and efficiency of 0.81% .
Keywords: Solar Cells; Quantum Dots; Nano-Tubes; TiO2; Sensitization
1. Introduction
Quantum dots photovoltaic cells combine low-cost solu-
tion process-ability with quantum size-effect tunability to
match absorption with the solar spectrum [1]. Relative to
the dyes used in solar cells, QDs offer several advantages
such as photo-stability, greater molar extinction coeffi-
cients, and size-dependent optical properties [2]. They also
have the potential to increase the maximum attainable
thermodynamic conversion efficiency of solar photon con-
version up to 66% by utilizing hot photo-gener ated car ri-
ers to produce higher photo-voltages or higher photocur-
rents via multi-exciton generation capability. While the
charge carriers are confined within an infinitesimal vo-
lume, thereby increasing their interactions and enhancing
the probability for multiple exciton generation [3,4]. The
calculations of the dependence of ideal solar cell conver-
sion efficiency on band gap show that CdTe is an excel-
lent match to our sun [5,6].
In order to control the CdTe QDs band gap, mercury
was added to the preparation medium [2]. The key chal-
lenge with such cells, the quantum dot sensitized solar
cells (QDSSCs), is the relatively low absorption cross
section of the QD s [6]. Titania nanotubes (TNTs) present
the key solution for carrier’s recombination and low ab-
sorption issues via the highly ordered nano-structured
matrix which provides continuous electron pathways to
facilitate a good electrons collection with high surface
area [7].
TiO2 nanotubes arrays and particulate films were mod-
ified with CdS quantum dots with an aim to tune the re-
sponse of the photo-electrochemical cell in the visible
region via successive ionic layer adsorption and reaction
(SILAR) [8].
The QDSSCs were fab rica ted by inco rpo rati n g CdHgTe
nanocrystals (NCs) and CdTe quantum dots, which pre-
pared separately from aqueous mixtures of NaHTe,
Cd(NO3)2, and 3-mercaptopropionic acid in the presence
and absence of HgCl2 respectively, th en the sensitization
of a modified TiO2 nanoparticles was carried out to ob-
tain an energy conversion efficiencies of 1.0% and 2.2%
[2].
In this work, the sensitization of TiO2 nanotubes-pre-
pared with anodization method—with CdHgTe-QDs-pre-
pared with organic method—was applied by direct dis-
persion. The aim of the present work is to enhance the
QDSSCs efficiency by tuning the energy gab of the
CdHgTe-QDs with changing H g% ratio. The effect of Hg
ratio in the QDs on the photovoltaic parameters of the
QDSSC was investigated.
2. Experimental Work
2.1. Materials
Ti foil (99.7% B.D.H., England), Ethylene Glycol (Nice
chemicals, India), Ammonium fluoride (oxford, India),
Acetic Acid (99.9%, fluka, Germany), De-ionized water
(DI), Cadmium acetate (MP Biomedicals), Tellurium
powder (99%, Aldrich), Octadecene (Acros), Trioctyl-
phosphine (TOP) (99%Acros), Oleic acid (99%, Aldrich),
Mercury Chloride, Isopropanol (Aldrich) and n-hexane

CdHgTe Quantum Dots Sensitized Solar Cell with Using of Titanium Dioxide Nanotubes
68
(Aldrich) were used for fabricating the CdHgTe QDs-
TiO2 nanotubes solar cells.
2.2. Preparation of TiO2 Nanotubes
Titania nanotubes were prepared by the electrochemical
anodizing of Ti foil which was ultrasonically degreased
with a mixture of Acetone, Methanol and Ethanol then
rinsed with DI water and dried with nitrogen. The anodi-
zation process was carried out in an electrochemical cell
with two electrodes model where the titanium is the
anode and platinum wire is the counter electrode (the
cathode). The anodization electrolyte was a mixture of
Ethylene Glycol, Ammonium fluoride (0.5%wt), Acetic
acid (for pH adjustment) and DI water (2% volume). All
experiments were carried out at room temperature (25˚C)
and the anodizing voltage was 40 V and applied for 3
hours. Once the process completed, the anodized sample
removed from the cell and washed with a large amount
of water then dried with nitrogen. The Ti electrode was
annealed in air at 500˚C for 3 hours in order to improve
Anatase phase. The morphology (i.e. length, diameter,
and the shape) of the TiO2 nanotubes was investigated by
the scanning electron microscope (SEM: Joel Jsm 6360
LA, Japan).
2.3. Preparation of CdTe and CdHgTe QDs
Tellurium powder (0.127 gm) was added to triocytl-
phosphine (TOP, 2 ml) and Octadecene (8 ml) with stir-
ring at 60˚C for 30 min. till giving the greenish gray col-
or with a complete dissolving. In a three neck rounded
flask, Cadmium acet ate (0.186 gm) was mixed with Oleic
acid (2 ml) and Octadecene (5 ml) and then rising the
temperature up to 80˚C. The tellurium precursor was
added to the above cadmium solution at 80˚C. To synthe-
sis CdHgTe QDs from organic medium, mercury chloride
was added to above cadmium solution. The CdTe and
CdHgTe QDs were separated in Isopropanol via centri-
fuge at 5000 rpm. These QDs were dissolved again in
n-hexane to prepare the sensitization solution. UV-visible
characterization of the CdHgTe QDs solution was carried
out using Thermo-Evolution 600 spectrophotometer and
the HRTEM images of the QDs were obtained using
JEOL (JEM-2100 LaB6) TEM.
2.4. Preparation of Quantum Dot Sensitized
Solar Cell (QDSSC)
The prepared nanotubes electrodes were immersed into
the QDs/N-Hexane suspension. Then left for 3 days with
sonication every 12 hours in airtight tubes to ensure a
good QDs di ffusion through the TNTs layer.
The QD sensitized solar cells with a structure of ITO/
liquid electrolyte/CdHgTe-QDs/TiO2 (NTs)/Ti were as-
sembled as shown in Figure 1. The sensitized Ti-TNTs
Figure 1. QDSSC assembly.
is the working electrode and a carbon activated indium
tin oxide (ITO, R = 14 - 16 Ω) is the other electrode. The
two electrodes were spaced by a polymeric spacer with
50 µm thickness. The Redox electrolyte was injected
through the cell’s sides.
All cells were illuminated by using Xenon lamp with
an intensity of 100 m W/cm2 (AM1.5). The lamp was
calibrated with Solarex standard solar cell and the J-V
measurements were obtained by using Keithley 2635A
source-meter.
3. Results and Discussion
3.1. Characterization of TiO2-NTs
Images of scanning electron microscope (SEM) for the
anodized titanium show a highly ordered, smooth, and
dense packed Titania nanotubes formed at 40 V for 3 h in
Ethylene glycol/ammonium fluoride electrolyte. The ob-
tained nanotubes having an average outer diameter of 75
nm and a wall thickness in the range of 10 - 14 nm while
the tubular layer thickness was around 24 µm (as shown
in Figure 2) which provides a naturally n-type semicon-
ductor with a relatively high surface area with a 3.2 eV
band gap (after annealing) referred to the anatase crystal
structure [9].
The arrangement of the highly ordered Titania nano-
tubes array perpendicular to the surface permits a facile
charge transfer along the length of the nanotubes to the
conductive substrate [10]. The prepared nanotubes may
resolve the problem of relatively low absorption cross
section of the QDs by increasing the area of contacts in
addition with the appropriate layer thickness which ex-
pected to increase the solar cell performance.
3.2. Characterization of CdHgTe QDs
3.2.1. UV-Vis. Spectroscopy
Figure 3 presents the UV-visible spectra of CdHgTe

CdHgTe Quantum Dots Sensitized Solar Cell with Using of Titanium Dioxide Nanotubes
69
(a) (b)
(c) (d)
Figure 2. SEM images for a sample anodized at 40 V for 3 h in Ethylene Glycol: (a) and (b)Top views;(c) Side view; and (d)
Side view at higher magnification.
Figure 3. UV-visible absorption spectra of CdHgTe QDs
prepared with different ratios of mercury. The obtained
curves are similar to those reported in the previous work
[12].
QDs prepared using organic medium at different ratios of
Hg. For CdTe sample, the absorption peak is around 460
nm indicating the formation of CdTe QDs [11]. It was
noted that as the relative concentration of mercury in-
creases, the ex citonic absorpti on peak of CdTe QDs shifts
to the longer wavelengths from 460 nm (for 0% of Hg )
to 855 nm(for 100% Hg) due to the quantum confine-
ment effect as the QDs grow to larger size and this is a
good evidence for the formation of CdHgTe QDs [12].
The Hg 2+ ions substitute Cd2+ ions at the surface of the
nanocrystals forming a CdHgTe allo y in the near-surface
region, possibly with a concentration gradient decreasing
towards the dot interior [11].
Using the absorption spectrum, the direct optical band
gap energy of the QDs was calculated by simply p lotting
(αhν)2 versus ( hν), obtained from the following relation
[11]:
αhν = A (hν − Eg)1/2 (1)
Where α and A are the absorption coefficient and a
constant respectively. The direct optical band energy gap
of the CdHgTe QDs was calculated to be 2.3, 1.62, 1.5,
1.35 and 1.3 eV for 0, 10, 25, 50, and 100% ratio of Hg,
respectively. The decrease in the optical band gap with
increasing the Hg percentage is regarded to the formation

CdHgTe Quantum Dots Sensitized Solar Cell with Using of Titanium Dioxide Nanotubes
70
of a layer of HgTe on the QDs surface [12].
3.2.2. High Resolution Transmission Electron
Microscope (HRTEM)
Figure 4 shows HRTEM images of CdHgTe QDs pre-
pared using organic medium with different ratios of Hg.
The existence of the lattice planes in the HRTEM images
indicates that the QDs are highly crystalline as shown in
the white circles mentioned in Fi gu re 4. It was noted that
the increase in Hg% ratio from 0 to 10, 25, 50 and 100
leads to an increase in the size of QDs from 2.8 to 8.6, 9,
9.5 and 10.8 nm respectively. This is due to the forma-
tion of a HgTe layer on the surface [12].
3.3. Characterization of TiO2-NTs
(TNTs)/CdHgTe Quan t um Dots Solar Cells
(QDSSCs)
The SEM images (Figur e 5) showed fully covered TNTs
with the sensitizer at the top and sides of the NT while
there is no indication about the quantity of the QDs that
decorate the TNTs from the inner. Figure 6 shows the
characteristic J-V curves for the QDSSCs prepared by
using TNTs and CdHgTe QDs with different Hg% ratios.
It is clear that the addition of mercury to the QDs core
shell shifts the absorption of the QDs toward the red re-
gion as shown in Figure 3 providing a wider absorption
toward IR region and hence increases the short circuit
current and the open circuit voltage as indicated in Table
1. As seen from the Figure 6 that Jsc increased from 0.4
mA/c m 2 corresponding to 0% Hg to 3.1 and 4 m A/cm2
for 10% Hg and 25% Hg respectively. The most efficient
cell (regarded to 0.81% efficiency) with the highest cur-
rent density value (4 m A/cm2) was obtained when using
25% Hg corr esponding to a band gap of 1.5 eV. While a
noticeable decrease in the values of Voc and Jsc were oc-
curred when changing the energy band gap away from
Figure 4. H RTEM images of colloidal CdHgTe solution prepared using organic method. (a) 0% of Hg, (b) 10% of Hg at low
magnification, (c) 10% of Hg at higher magnification, (d) 25% of Hg, (e) 50% Hg, and (f) 100% Hg.

CdHgTe Quantum Dots Sensitized Solar Cell with Using of Titanium Dioxide Nanotubes
71
Figure 5. SEM images of Titania nanotubes after sensitization: a) Top view for a sample sensitized with CdHgTe QDs, b) Top
view of a sample sensitized with CdTe QDs.
Figure 6. J-V characteristic curves of TiO2-NTs/CdHgTe-QDs sensitized solar cell with different Hg % ratios.
Table 1. Photovoltaic parameters of the prepared QDSSCs.
CdHgTe (0% Hg) CdHgTe(10% Hg) CdHgTe (25% Hg) CdHgTe (50% Hg) CdHgTe (100% Hg)
Particle size (nm) 3.5 8.5 9 9.5 10.5
Eg(eV) 2.3 1.62 1.5 1.35 1.3
Jsc(mA/cm2) 0.4 3.1 4 3.6 1.7
Voc(V) 0.15 0.61 0.63 0.43 0.41
FF 0.18 0.23 0.32 0.34 0.2
Efficiency% 0.012 0.435 0.81 0.53 0.14
this value (i.e. 1.5 eV). This result up to 1.5 eV is due to
the excited state of the QD lies well above the TiO2 con-
duction band level and photo-excited electrons injection
into TiO2 would be energetically favorable[1]. While, by
increasing the band gap (>1.5 eV), the hole level exhibits
a large discontinuity with the TiO2 valence band, pro-
viding a very large barrier to the undesired passage of
majority holes from the p-type QD layer into the n-type
TiO2 electrode [1].
Additionally, Voc and Jsc values were dramatically drop-
ping by increasing the Hg% ratio from 25% to 100%.
This behavior can be explained by another cause which is
regarded to the distorted QDs structure with an access of
mercury concentrations. When the Hg% ratio increased,
Hg2+ ions should be substituted at the surface of the na-
nocrystals, and in theory, if complete surface exchange
occurs, a core/single-monolayer-shell structure will result
[12]. The substitution reaction then completes either due
to an exhaustion of Hg ions in solution or because of the
formation of a complete “locking” layer of HgTe on the
surface. And finally the outer layer forms an almost per-
fectly passivated quantum dot [12].
From another point of view, the obtained J-V curves
suffering from a bad fill factor due to the high series re-
sistance of cell and moreover the spacer issue which is a
strong factor that may reduce the cell efficiency. On the
other hand, the performance of the prepared solar cell in
this work can be enhanced in further work using a better
sensitization technique, better sealing conditions and by
the usage of a linker.

CdHgTe Quantum Dots Sensitized Solar Cell with Using of Titanium Dioxide Nanotubes
72
4. Conclusions
The direct dispersion technique was used for fabricating
CdHgTe-QDs sensitized TiO2-NTs solar cells with dif-
ferent Hg% ratios in QDs. The obtained highly ordered
and dense packed Titania nanotubes were having an av-
erage outer diameter of 75 nm and a layer thickness of 24
µm. In addition, the direct energy gap of the highly crys-
talline CdHgTe QDs was calculated to be 2.3, 1.62, 1.5,
1.35 and 1.3 eV for 0%, 10 %, 25%, 50%, and 100% ratio
of Hg respectively.
The increase in Voc and Jsc values corresponding to a
change in Hg% ratio from 0% to 25% was due to the
absorption enhancement in IR region while the decrease
in Voc and Jsc values corresponding to a change in Hg
ratio from 25% to 100% was due to either the discontinu-
ity in the energy level or the formation of locking layer
of HgTe on the QDs surface. The most efficient QDSSC
was obtained at 25% of Hg ratio with Jsc of 4 mA/cm2,
Voc of 0.63 V, F F of 0.32 and e fficiency of 0.81%.
The performance of the QDSSC mentioned in this
work can be enhanced using a better sensitization tech-
nique, better sealing conditions and by the usage of a
linker.
5. Acknowledgements
The authors are very grateful to the research team of the
US-Egypt project chaired by Prof. Moataz Soliman for
the funding support and the research student: Wessam
Kamal for her help in preparing the materials.
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