CdHgTe Quantum Dots Sensitized Solar Cell with Using of Titanium Dioxide Nanotubes

doi:10.4236/jpee.2013.15011

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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)

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.

Email: [email protected]

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

(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

(a) (b)

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

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

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

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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