Nghiên cứu chế tạo và tính chất quang của các nano tinh thể bán dẫn cdtese và curcumin, định hướng ứng dụng trong quang điện (tt)

MINISTRY OF EDUCATION AND
TRAINING

VIETNAM ACADEMY
OF SCIENCE AND TECHNOLOGY

GRADUATE UNIVERSITY SCIENCE AND TECHNOLOGY

--------------o0o---------------

LE XUAN HUNG

STUDY SYNTHESIS AND OPTICAL PROPERTIES OF CdTeSe
NANOCRYSTALLINE AND CURCUMIN, ORIENTED APPLICATION IN
PHOTOVOLTAIC

Major: Optics
Code: 9440110

SUMMARY OF PHYSIC DOCTORAL THESIS

Ha Noi - 2018


This work was realized at: Graduate University of Science and Technology, Viet Nam Academy of Science
and Technology.

Supervisors:
1. Associate Professor - Doctor Pham Thu Nga, Institute of Materials Science-VAST.
2. Associate Professor - Doctor Nguyen Thi Thuc Hien, Duy Tan University.

Reviewers:
1. Associate Professor – Doctor Nguyen The Binh, University of Science-VNU
2. Associate Professor – Doctor Pham Van Hoi, Institute of Materials Science-VAST
3. Doctor Luong Huu Bac, Hanoi University of Science and Technology

The thesis will be approved at the Committee of Graduate University of Science and Technology, Viet
Nam Academy of Science and Technology,
.................................................................................................................
Date:
The thesis can be found at: .....................................................................
..................................................................................................................


1

INTRODUCTION
In globalization, energy demand is increasingly urgent, and the application of advanced
materials to the renewable energy industry has become a common trend of the world. The
development of photovoltaic devices can divide solar cells into three generations. The first generation
of solar cells is based on single Si crystal plates, with energy conversion efficiency (PCE) of ~ 25%.
The second generation of solar cells is based on thin-film technology, which has relatively low solar
energy conversion efficiency (~ 20%). The third generation of solar cells is a solar cell for high
conversion efficiency at a low cost, aiming to improve the limitations of the two generations. Some
examples of this type of solar cells are solar cells with dye (DSC), quantum dots (QDSC), solar cells
with colloidal quantum dots (CQDSC), solar cells with an organic dye, etc. ... In theory, the maximum
efficiency of a single-layer crystal Si is ~ 33%, due to the thermodynamic limit proposed by ShockleyQueisser. The conversion efficiency of QDSC can be up to 42% by the multiple exciton production
(MEG) effects of quantum dots (quantum dots-QD).
Based on the structure of the DSC, the QD was introduced as an alternative to dye by thanks
to its excellent optical and electrical properties. QDSC can be seen as an improvement, from dyesensitive solar cells (DSSC), as reported by O'Regan and Gratzel in 1991. To achieve higher PCE,
QDs sensitizer needs to the narrow bandgap (1.1-1.4 eV), the bottom of the conduction band is higher

than the bottom of the conduction band of TiO2 and high stability. Recently, the QD three-to-fourcomponent alloys is a promising choice, compared to the binary QD sensitizer, because their
photoelectric properties can be tuned. Their composition control without changing the particle size
and their bandgap is more narrow than that of the two-component system due to the "optical bowing"
effect. Today, in the testing of QD alloy as a sensitizer in QDSC, most aim at the CdTexSe1-x alloy
due to the absorption peak extending and shift to the near infrared region (NIR). The work of the
thesis is a new study, on the use of QD CdSeTe and CdTeSe / ZnSe alloys in solar cells. In Vietnam,
no group has mentioned the synthesis of ternary alloyed CdSeTe materials as we have in this thesis.
This is also the main content of the Nafosted project by our research group.
In terms of solar cells using dye (DSC), there have been some published works on the use of
a natural dye as a sensitizer for solar cells. This is one of the attempts to use "natural" resources to
serve for life. We also took advantage of this opportunity to study DSC, but PCE so far is still very
low. Recently, S. Suresh, et al. published solar cells using curcumin with an efficiency of 0.13%, S.J.
Yoon, et al. also showed PCE of about 0.11% when using only curcumin and up 0.91% when using
curcumin mixed with K2CO3. Very recently (6/2017), Khalil Ebrahim Jasim, et al. published solar
cells using natural curcumin dye to achieve 0.41% efficiency.
QDs are often synthesis in organic environments, so surface defects and ligand often appear
to reduce the luminescent quantum yield (QY) of the material. Therefore, QDs are often encased in
inorganic shells to passively surface, to improve QY. With the purpose of surface protection, CdTeSe
QDs are also coated with different shells, for example, covering the shell with a large band gap such
as CdS, ZnS. Besides, QD is also encased with buffer layer and CdS / ZnS shell to minimize lattice
defect, or cover the shell with ternary CdZnS alloys. In this thesis, we carried out the coverage of


2

CdTeSe QD with ZnSe and ZnTe shells, which are semiconductors, and have not been previously
published, for the purpose of applying the QDs in sensitized solar cells.
With natural dye, following the trend of using green energy for human service purposes, the
results of Zhou et al (2011) published using 20 different types of natural dye, to make sensitizers in
solar cells. In recent years, scientists have been interested in exploiting curcumin as a dye, intended

for use in solar cells, in the hope of creating solar cells in the simplest way, to obtain electricity from
the sun and natural sources. CdTeSe quantum dots and curcumin are considered sensitizers when
used in third-generation solar cells. This study and survey of synthesis and optical properties of
natural dye a sensitizer such as curcumin, and of a substitute sensitizer in new generation solar cells
such as CdTeSe quantum dots, aim to deepen our understanding of the materials applied in solar cell
assembly. In general, the research subjects are looking at the same type of sensitizer for solar cells.
In the context above, I have carried out the thesis research topic: Study on the synthesis and
optical properties of CdTeSe nanocrystallines and curcumin, for potential application in
photovoltaic
New scientific contributions of the thesis: i) Studied the fabrication of CdTeSe quantum dots
(QDs) in ODE-OA environment, at a suitable temperature (260oC) that we found at the same time
with the international publication (2016) on optimal temperature for a similar fabrication method.
The Raman scattering method was also used to investigate the change in the composition of CdTeSe
alloy QDs at different fabrication temperatures, but with the same growing duration of 10 min. ii)
Investigated the structure and optical properties of core CdTeSe QDs which were coated with ZnSe
or ZnTe shells. iii) The results of the single-dot survey for CdTeSe/ZnSe showed that the lifetime of
these dots was about 100 ns and that the "off" state was observed, but they only accounted for 20 %
of the time. iv) For the first time in Vietnam, we have studied the extraction of curcumin from turmeric
harvested from different regions, and systematically studied the properties of this dye, in crystal form
as well as liquid form. By the Raman method, it has been made possible to distinguish between
naturally-extracted and chemically-synthesized curcumin. v) Tested assembling solar cells using QDs
and curcumin. For solar cells that use curcumin as a light sensitizer, the conversion efficiency
achieved the internationally-published value on 6/2017 of 0.4 %.
Thesis layout: With the content above, the layout of the thesis - in addition to opening and
concluding - is divided into 4 chapters, including 149 pages, 81 pictures and 14 tables. The main
results of the thesis are published in 3 international journals, 1 national scientific journal and 6 reports
at international and national conferences.
CHAPTER 1
OVERVIEW OF SEMICONDUCTOR NANOCRYSTALS, CURCUMIN
NATURAL DYE AND SENSITIZER SOLAR CELLS

1.1. The semiconductor nanocrystals are quantum dots and ternary alloy quantum dots
The binary QD clearly shows quantization of energy levels and extends the bandgap when the
size of the QD decreases to a certain size in nm. With ternary alloyed QDs, optical properties in
addition to size dependence, they also depend on QD components. The non-linear dependence of
optical properties on the composition of some QDs is called the optical bowing effect.
1.2. Overview of natural curcumin colorants


3

Curcumin extracted from yellow turmeric consists of three main ingredients: curcumin
demethoxycurcumin (curcumin II), bisdemethoxycurcumin (curcumin III) and curcumin play a color
role for the compound and its yellow to bright orange color. Optical properties, as well as the physical
and chemical properties of curcumin, are specified in the chapter.
1.3. Structure, operating principle, and parameters affect the performance of solar cells
The structure of a sensitizer solar cell was introduced. The charge transport model, as well as
the parameters affecting PCE are designed to find the optimal conditions for assembling components.
CHAPTER 2.
METHODS OF SYNTHESIS MATERIALS AND EXPERIMENTAL TECHNIQUES
2.1. Synthesis of CdTeSe quantum dots and CdTeSe/ZnSe (ZnTe) core/shell structure
The whole process of manufacturing QD in ODE-OA media is summarized in figure 2.1
diagram.

Figure 2.1. Diagram of synthesis CdTeSe QD in ODE-OA media

The process of covering ZnSe or ZnTe shell for CdTeSe is similar and according to the
diagram of figure 2.3.

Figure 2.3. Diagram of synthesis QD core/shell in ODE-OA media

2.2. Extract curcumin from Vietnam yellow turmeric


4

The main stages in curcumin extraction process are shown in figure 2.6 and summarized as
follows:

Figure 2.6. Curcumin extract diagram from yellow turmeric.

2.3. The physical methods used in research
Principles of experimental techniques used in the thesis’s research are briefly shown, with the
following methods: TEM, SEM imaging, particle size determination by Image J software, X-ray
diffraction, absorption spectroscopy method, fluorescence spectroscopy method, the study of
vibrational characteristics of materials by Raman spectra, quantum efficiency measurement and
investigating the decay time curve and lifetime of the 1SeSh3/2 basic exciton.
2.4. Assembly solar cell components using quantum dots and curcumin color as a
sensitizer

Figure 2.8. Diagram of making solar cells using light sensitivity

A solar cell uses a sensitizer consisting of three main components: working electrode,
electrolyte and counter electrode. The working electrode, called photoelectrode or photoanode, is
made by depositing a layer of semiconductor crystalline nanomaterials of a size of 2-50nm (most
used is TiO2) on a conductive surface (ITO or FTO glass), then the absorption layer is dispersed into
this semiconductor material. Electrolytes are usually a liquid containing redox pairs filled between


5

the working electrode and the electrode to transmit the carrier particles. The counter electrode is
usually a layer of conductive glass coated with a catalyst (Pt, Au, Cu2S or MWCNT), to exchange
the charge between the counter electrode and electrolyte. The entire assembling process is given in
the diagram figure 2.8.
Research results on coating TiO2 film on the photoelectrode
Surface SEM images of TiO2 films prepared after heating at 450 oC for 30 minutes, with
different resolutions showed that TiO2 film surface is uniform, no flakes or cracks appear (figure
2.9a). Surface SEM images of TiO2 films (figure 2.9b) show that the TiO2 particles are bonded
together to form a porous structure, which helps to absorb dye or QDs.

(a)

(b)

(d)

(c)
8,93µm

16,5µm

Figure 2.9. TiO2 film surface image with magnifications of 35 times (a), 50000 times (b) and crosssection image of TiO2 film in 1time coating (c), 2 times coating (d) taken with SEM image

The SEM image of the cross-sectional surface of TiO2 film is coated with the Doctor-Blade
technique, which shows that at 1times the thickness of the film is 8,93 µm (figure 2.9c) and twice the
thickness of the film is about 16.5 µm (figure 2.9d). Thus, with the Doctor - Blade technique with 2
overlaps, the results show that our TiO2 films are suitable for making photoelectrode in solar cells.
Research results on MWCNT – TiO2 film on counter electrodes by SEM
SEM images show that the film thickness is about 20.4 µm, the link between the MWCNT –
TiO2 film and the FTO layer, as well as the glass, is good. The MWCNT is interlinked and linked to

TiO2 particles, the membrane is formed with high porosity which helps the electrolyte diffusion
process deep into the membrane.


6

(a)

(b)
TiO2

Figure 2.10. Cross section image of MWCNT - TiO2 film of a counter electrode

CHAPTER 3.
RESULTS AND DISCUSSION OF CORE AND CORE/SHELL STRUCTURED
CdTeSe
3.1. The CdTeSe quantum dots are fabricated according to the ratio of different initial
precursors
In order to evaluate the formation of ternary alloy CdTeSe QDs, we recorded Raman spectra
of samples with different initial molar precursors, according to the synthesis method of the diagram
of figure 2.1. Figure 3.2 is the Raman spectrum of CdTe QDs, and of the ternary QDs fabricated
according to two different initial precursor ratios. Raman spectra of QDs appear two wide bands at
150 ÷ 220 cm-1 and 300 ÷ 400 cm-1. QDs synthesized with molar ratio Cd:(Te:Se) = 1:(1,8:1,8) only
appear Raman peak at 159 cm-1, this peak is characteristic for long optical phonon vibration modes
(LO) of CdTe (CdTe-like). When QD was synthesized with molar ratio Cd:(Te:Se) = 10:(1:1), beside
the Raman peak at 159 cm-1, also appeared one shoulder at 188 cm-1; this peak is the characteristic
line for long optical phonon vibration mode (LO) of CdSe (CdSe-like).

Figure 3.2. Raman spectra of CdTeSe QD
with the different initial molar ratio

Figure 3.3. Absorption and fluorescence
spectra of samples with different initial molar ratio


7

The absorption and fluorescence spectra of samples with different molar precursors are given
in figure 3.3. From figure 3.3, we observe that the exciton absorption peak corresponds to the 1Sh3/2
→ 1Se basic absorption transfer. The fluorescence spectrum of the samples has a maximum of 680
nm and 668 nm, respectively, with a molar ratio of 1: (1,8: 1,8) and 10: (1: 1). The spectral width
(FWHM) of the samples is 57 nm and 50 nm respectively, narrower than the reports of QDs of the
same type in the infrared region. This result shows that the synthesized QDs are of good quality.
Thus, the molar ratio of the initial substances is 1:(1,8:1,8), the system will tend to produce
QDs that are very rich in CdTe. This can be explained as follows: in the same synthesis condition of
QDs, the reaction of Te and Cd is much faster than that of Se with Cd. Due to the difference in
reaction, CdTe's development speed is 2 times faster than CdSe. When the molar ratio of the initial
substances is 10:(1:1), during the reaction process there is always Cd residue, so the Se have the
opportunity to participate in the reaction to create the ternary alloy CdTeSe QDs
3.2. Effect of grown temperature on the properties of quantum dots
3.2.1. Morphology and crystal structure
Figure 3.4 shows the X-ray diffraction
spectrum of CdTeSe QDs manufactured
according to the diagram of figure 2.1, in ODEOA media, the ratio of initial precursor 10:(1:1),
grown at different temperatures, from 180 oC to
280 oC, for 10 minutes. The diagram of X-ray
diffraction shows that all diffraction peaks are
expanded more than the bulk material. This
indicates that fabricated QDs have nano size. The
maximum position of these peaks, located in the Figure 3.4. Schematic of X-ray diffraction of QDs

middle of the position of the peaks corresponding
synthesized at different temperatures
to the diffraction lines of the standard votes of the CdTe-zb and CdSe-zb crystal phases and the high
intensity. The appearance of peaks between the corresponding peaks of the two crystal phases CdTe
and CdSe proves that the ternary CdTeSe QDs have been formed.
From Raman spectra (figure 3.5a) corresponding to QDs grown at different temperatures, the
spectra appear two vibrational spectra located at 140 ÷ 220 cm-1 and 300 ÷ 400 cm-1. The spectrum
region at the 140 ÷ 220 cm-1 of the QDs is a double band, for which the samples are grown at a low
temperature, the peak at 159 cm-1 dominates, with high intensity. We also observed a second shoulder
at ~ 188 cm-1. When the grown temperature of the QDs increased gradually, from 200 °C to 240 °C,
the intensity of this shoulder increased gradually, forming a double peak. Raman spectra of the
samples were grown at high temperatures, it was observed that the splitting into two clear peaks, one
peak that corresponds to the wavenumber of 159 cm-1, and the second peak at 188 cm-1. Thus, for
samples CdTeSe QDs synthesis at higher temperatures, the intensity of the line at 188 cm-1 increases,
which is likely due to when the grown temperature of the QDs increases, the amount of Cd-Se will
be much formed in CdTeSe, leading to an increase in the intensity of this peak. When the temperature
increased to 260 oC and 280 oC, the intensity ratio of this peaks was almost unchanged, the intensity


8

ratio of LO2/LO1 did not increase (figure 3.5b). From here, we selected the optimal temperature to
grown CdTeSe QDs is 260 oC, and this temperature is also the optimal temperature for the synthesis
of CdTeSe QDs later.

Figure 3.5. Raman spectra of QDs were fabricated at different temperatures (a), and the intensity ratio of the
LO2 line (188 cm-1) with LO1 (159 cm-1) when fitting (b).

The TEM image of this sample at 260 oC (figure
3.6) shows that QDs shape has an irregular sphere, tend

to be slightly elongated, particles with the size of 6 ÷ 7
nm. The calculation results give an average size of 6.3
nm.
3.2.2. Absorption and fluorescence spectra
The absorption and fluorescence spectrum of the
sample depends strongly on the grown temperature, as
Figure 3.6. TEM images of
shown in figures 3.7 and 3.8. Observation of absorption
CdTeSe QDs synthesis at 260 oC
spectrum shows that: when the temperature increase,
general tendency, the absorbed band edge is shifted
towards longer wavelengths, from 650 nm to 830 nm when the grown temperature increases from
180 °C to 280 °C. Fluorescence spectra are a wide range where the maximum peak of the emission
band varies depending on the grown temperature, from ~ 630 nm (at 180 °C) to nearly 800 nm (at
280 °C). This emission range corresponds to 1Se - 1Sh exciton emission transition in alloyed CdTeSe
QDs. In general, the grown temperature increases, the maximum peak of the emission band changes
and redshift. The quantum yield of the fabricated samples is presented in Table 3.1.

Figure 3.7. Absorption spectra of QDs
fabricated at temperatures from 180 °C to 280 °C

Figure 3.8. Fluorescence spectra of QDs
fabricated at temperatures from 180 °C to 280 °C


9
Table 3.1. The fluorescent parameters of QD synthesized at different temperatures in ODE-OA media

Sample

max (nm)

FWHM (nm)

QY (%)

CdTeSe-180 oC
CdTeSe-200 oC
CdTeSe-220 oC
CdTeSe-240 oC
CdTeSe-260 oC
CdTeSe-280 oC

628
664
720
726
742
770

81
90
99
110
110
105

3,2
4,5
25,6

30,2
36,1
33,6

3.2.3. Raman scattering spectrum and fluorescence of CdTeSe quantum dots are measured
at temperatures ranging from 300K to 84K
a) Raman scattering spectra measured at different temperatures from 300K to 84K

Figure 3.9. (a) Raman dependence on the temperature of the ternary alloyed CdTeSe QDs. The
insets show the dependence of the frequency of the LO1 and LO2 lines on temperature. (b) A part of the
Raman spectrum in the range of 140 cm-1 to 220 cm-1 is normalized to observe changes in vibration modes
according to temperature.

Raman spectra of CdTeSe QDs at different temperatures, from room temperature 300K to
84K are presented in figure 3.9. The shape of the spectrum does not change when measured from
300K down to 84K. However, the maximum position and the intensity of the spectrum are change.
When the QDs sample temperature decrease, the position of the LO phonon lines is shifted towards
the longer wavenumbers. Specifically, the LO1 (CdTe-like) line displaced about 3.8 cm-1, the LO2
(CdSe-like) line is also displaced by the size of 4.3 cm-1 (figure 3.9b). These results were also
observed by Dzhagan and Mork authors but on CdSe. Explaining the increase in the intensity of the
vibration lines and the position of the vibration peak, when the temperature changes from 300K to
84K, we base on the Morse potential model.
b) Fluorescent spectra recorded at different temperatures from 300K to 84K


10

Figure 3.11 is the fluorescence
spectrum of CdTeSe sample synthesis at
260 oC for 10 minutes measured from 84K

to room temperature (300K). When the
sample measurement temperature is
reduced, the maximum position of the
emission band is shifted towards the
shorter wavelengths, given by the Vashni
formula, the wide spectrum (FWHM) is
also reduced (figure 3.12).

Figure 3.11. Fluorescence spectra measured at different
temperatures (from 84K to 300K) of CdTeSe QD

Figure 3.12. The dependence of the emission maximum (a) and FWHM (b) at the temperature of
CdTeSe QD

3.3. Effect of composition on the properties of CdTexSe1-x quantum dots
3.3.1. Crystal structure and morphology of CdTexSe1-x QDs.
The diffraction patterns (figure 3.13) show that the diffraction path consists of three diffraction
peaks between zb-CdTe and zb-CdSe lines, the diffraction peaks have a large wide. When the amount
of Te components increases, the position of the spectral peaks also changes and shifts towards the 2
angle near to the line of zb-CdTe. Show that these ternary QDs have formed alloys with zb structures.

Figure 3.13. X-ray diffraction diagram of CdTexSe1-x QDs
o

Figure 3.14. Raman spectra of CdTexSe1-x

fabricated at 260 C for 10 min with the composition Te

QDs fabricated at 260 oC for 10 minutes with

changed (x = 0.2; 0.4; 0.5; 0.6; 08). Diffraction lines for

the composition Te changed (x = 0.2; 0.4;

bulk materials for zb-CdSe and zb-CdSe are also given

0.5; 0.6; 08)


11

We used Raman spectra to evaluate the change in alloy composition (figure 3.14). When the
concentration of Te increases to x = 0.4, the peak characteristic for the LO mode of CdTe is more
clearly observed, this peak intensity increases, the position of CdSe peak observed at the frequency
~ 200 cm-1. When x = 0.5, the intensity of this line increases with the intensity of the same line of the
sample with x = 0.4, but the position of the peak represents the vibration mode of CdSe being shifted
at the frequency188 cm-1. When x = 0.6, the intensity of the two vibration lines is characteristic for
two vibration modes LO CdTe-like and CdSe-like with nearly equal intensity, and locate at frequency
positions 159 cm-1 and 188 cm-1. When the amount of Te increases to x = 0.8, the first peak intensity
at the wavenumber of 159 cm-1 increases sharply. This is the peak that represents the phonon vibration
mode of CdTe-like. This can be explained that the alloy CdTeSe QDs has a difference in lattice
constants (due to crystallization in the zb phase), which leads to the length of the bond be changed
and extended, causing the vibration frequency to be shifted to shorter the wavenumber, compared to
the vibration frequency of CdSe. Moreover, when
the content of Te is small (x ≤ 0.4), CdTeSe tends
to crystallize in the crystalline phase close to CdSe,
but CdSe has a stable crystal phase in a wz
structure, so CdSe-like vibration line located at 200
cm-1. When Te content is large (x ≥ 0.5), CdTeSe
crystallizes with zb structure, so CdSe-like

vibration line in 200 cm-1 will be shifted to position
188 cm-1, which characteristic for the LO vibration
line of zb-CdTeSe crystal.
From the TEM image of the samples that
changed the composition of Te (x) initially
introduced into the reaction (figure 3.15), the
prepared QDs have a rather spherical shape. The
average size of the QDs of the samples with
different Te components is from 5.1 nm to 5.4 nm

Figure 3.15. TEM images of CdTexSe1-x
QDs fabricated at 260 oC (10 min) with Te content
from 0.2 to 0.8

3.3.2. Optical properties of alloyed CdTexSe1-x QDs.
Absorption spectra show that the absorption edge changes when the alloy composition
changes and the longest absorption edge lies at about 800 nm. The absorption edge redshift longest
corresponding to component x = 0.5 (figure 3.18a). We also carry out the fitting of the obtained data
and compared with equation 1.22, we calculated the bowing optical value about b =  0.88 eV. This
value is close to the value published by I. Hernández-Calderón of 0.87 eV, and is in good agreement
with the publication that this coefficient varies from  0.59 eV to  0.91 eV.
With the fluorescence spectrum, when the content of Te (x) increases from x = 0.2 to x = 0.5,
the maximum position of the fluorescence band shifts toward longer wavelengths, from 731 nm to
756 nm, and then shift towards a shorter wavelength, down to 720 nm, if the concentration of Te is
increased. Thus, when the concentration of Te changes, the maximum position of fluorescence will
change but change nonlinear (also called bowing optical effect). The quantum efficiency of the
fabricated samples is quite high, there are more than 50% samples, and the sample with the best QY


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is the sample with the content x = 0.5 and 0.6 (table 3.2). Besides, the width of the spectrum decreases
when the concentration of Te increases (figure 3.18b).

Figure 3.16. Absorption spectra of CdTexSe1-x
QDs fabricated at 260 oC for 10 minutes with Te content
varying from 0.2 to 0.8

Figure 3.17. Fluorescence spectrum of
CdTexSe1-x QDs (x = 0.2; 0.4; 0.5; 0.6; 0.8) made at 260
o
C for 10 minutes under 532 nm excitation wavelength

Figure 3.18. Dependence of fluorescence maximum position, absorbing edge (a) and FWHM (b) into Te
component of CdTexSe1-x QDs fabricated at 260 oC for 10 minutes

Combining the above results, we found that the fabricated ternary QDs was homogeneous,
zinc-blend (zb) crystalline, with high luminescent efficiency. Samples have good luminescent
performance and locate on the infrared band with x = 0.5 or 0.6 components, suitable for use as a
sensitizer for the solar cell.
Table 3.2. Fluorescence parameters of QDs which Te composition changed

Sample
CdTe0,2Se0,8
CdTe0,4Se0,6
CdTe0,5Se0,5
CdTe0,6Se0,4
CdTe0,8Se0,2

max (nm)

731
742
756
731
720

FWHM (nm)
117
90
88
87
69

QY (%)

24,9
41,0
52,6
53,4
27,1

3.4. Effect of crust thickness on the properties of core/shell quantum structure
CdTeSe/ZnSe (ZnTe)
3.4.1. The QDs core/shell CdTeSe/ZnSe
With the core CdTeSe sample, X-ray diffraction diagram only appears three diffraction lines
that peak at the lattice surface distance of 3,616; 2,241 and 1,908 for the lattice side is (111); (220);
and (311) showed that QDs has a crystalline structure of zinc blend (zb) (19-191 and 15-770
corresponding to CdTe and CdSe). When the QDs are covered with a 2 ZnSe ML shell, the diagram

13

also appears 3 diffraction lines, but the position of the two peaks at the larger 2 angles shifted slightly
towards larger 2 values (figure 3.19). This shows that the ZnSe shell may have formed on the core
structure and does not change the core zb-CdTeSe structure. It may also be related to the Se ions
being attracted inside the core during the coating process.

Figure 3.19. Diffraction diagram of CdTeSe core
QDs and CdTeSe/ZnSe core/shell 2ML synthesised at
260 oC (10 minutes). Diffraction lines for bulk materials
for zb-CdSe and zb-CdSe are also given

Figure 3.20. Raman spectra of CdTeSe core QDs and
CdTeSe/ZnSe core/shell have different thickness

When uncovered, the Raman spectrum of the core sample showed only two vibration peaks
at 159 cm-1 and 188 cm-1, similar to prepared those by the Te component of 0.5 in the previous section.
When cover a thin 1ML shell, the spectrum occurs change: the characteristic line for the LO mode of
CdSe changes from 188 cm-1 to 200 cm-1, the intensity of the characteristic line for vibration of CdTe
decreased. Besides, there is a blurred line at 250 cm-1, this is the characteristic line for the LO ZnSe.
The results on the Raman spectra show that the ZnSe shell has been formed but in a small amount.
When the cover is thickened to 2, 4, 6 ML, the characteristic line for CdTe disappears, instead of the
intensity of ZnSe line at 250 cm-1 increases but not much.
The average size
(length) of CdTeSe core
QDs is about 6.3 nm and
increases to 8.3 nm when
covered by the ZnSe 2ML
shell. The shape of the
fabricated QDs is similar

to those observed in
Bailey R. E. et al.
The
absorption Figure 3.21. TEM image of core CdTeSe (a) and CdTeSe/ZnSe core/shell 2ML (b)
edge of CdTeSe core sample is about 820 nm, as seen in figure 3.22. When covering ZnSe shells, the
absorption edge shifts toward longer wavelengths, up to 905 nm with a 6ML thick shell.
The luminescence spectrum of the CdTeSe core is a wide emission band with a maximum
position of 760 nm (figure 3.23). When coated by ZnSe shell with increasing thickness, this maximum
position has a redshift towards the long wavelength, from 803 nm to 882 nm, as shown in table 3.3.
The quantum efficiency is increased by covering a thin layer of 1 ML when the thickness of the shell
still increased, the quantum efficiency begins to decrease.


14

Figure 3.22. Absorption spectra of CdTeSe core QDs Figure 3.23. Fluorescence spectra of CdTeSe core QDs and
and CdTeSe/ZnSe core/shell nML (with n = 1, 2, 4, 6)

CdTeSe/ZnSe core/shell nML (with n = 1, 2, 4, 6)

The redshift of the fluorescence spectrum when increasing the shell thickness is explained by
A.M. Smith published in Nature Nanotechnology. When grown a thin layer (1ML), the core is slightly
compressed due to the smaller lattice constant. Because of these simultaneous shifts of the core and
shell, there is a small difference in energy between the conduction bands of the core and the shell,
causing the electron wavefunctions to spread across the entire nanocrystal. Overgrowth of thicker
shells further increases the core conduction band energy and decreases the conduction band energy
in the shell. Thus the band offsets become staggered, shifting the electron almost entirely into the
shell material, resulting in a type-II alignment, leading to a strong fluorescence peak of the QDs.
Table 3.3. Fluorescence parameters of core/shell QDs CdTeSe/ZnSe nML (với n =0, 1, 2, 4, 6 ML)

Sample
CdTeSe
CdTeSe/ZnSe 1ML
CdTeSe/ZnSe 2ML
CdTeSe/ZnSe 4ML
CdTeSe/ZnSe 6ML

max (nm)
760
803
842
863
882

FWHM (nm)
116
130
141
153
153

QY (%)
44,9
56,7
28,4
7,7
2,7

3.4.2. The QDs core/shell CdTeSe/ZnTe.
The same as ZnSe shell QDs system, for the

ZnTeshell sample system, the two peaks at 159 cm-1
and 188 cm-1 have not been encapsulated for the
vibration mode, which represents the vibration mode of
CdTe and CdSe as in previous part (figure 3.25). On the
other hand, the ZnTe shell still formed with the CdTe
shell, as evidenced by the ZnTe line still appearing at
205 cm-1, despite the weak intensity. This result
demonstrates that the ZnTe shell has been formed but
in small amounts.

Figure 3.25. Raman spectra of core/shell QDs
CdTeSe/ZnTe nML (với n = 0, 1, 2, 4, 6)

The general trend observed on absorption spectra (figure 3.26) is the absorbance edge of QDs
as far away as the long wavelength when QDs is covered in thicker ZnTe shell. In these samples, the
absorption peak is not clear, because, for the ternary material, the energy band gap depends not only
on the size of the QDs but also on its composition.


15

Figure 3.26. Absorption spectra of CdTeSe core QDs
and CdTeSe/ZnTe core/shell nML (with n = 1, 2, 4, 6)

Figure 3.27. Fluorescence spectra of CdTeSe core QDs
and CdTeSe/ZnTe core/shell have different thickness

Fluorescence spectra of samples with increasing shell thickness are shifted towards long
wavelength (figure 3.27). The redshift of these spectra is relatively large from 763 nm to nearly 900
nm when the ZnTe shell increases to 6 ML. In the case of ZnTe shells, quantum efficiency is reduced,

which may be due to the coating process, which has created many electronic traps due to lattice
defects, which reduces the efficiency of electronic-hole recombination and thus reduce the
luminescent performance (table 3.4).
Table 3.4. Fluorescence parameters of core/shell QDs CdTeSe/ZnTe nML (với n =0, 1, 2, 4, 6 ML)

Tên mẫu
CdTeSe
CdTeSe/ZnTe 1ML
CdTeSe/ZnTe 2ML
CdTeSe/ZnTe 4ML
CdTeSe/ZnTe 6ML

max (nm)
763
785
812
829
900

FWHM (nm)
105
114
132
150
160

QY (%)
40,5
18,9
15,6

3,0
1,6

3.4.3. Radiative lifetime of excitons in CdTeSe/ZnSe, CdTeSe/ZnTe cores/shells and
fluorescence blinking of single dots.
3.4.3.1. The emission lifetime of the exciton in the QDs
Figure 3.28 is time-resolved fluorescence decay curves of the CdTeSe/ZnSe core/shells QDs
with the thickness of the shell changing from 1 ML to 6 ML, the fitting of the experimental curve
with the theory are carried out and the results are given in table 3.5. The core sample has a relatively
long lifetime when the thicker shell is covered, the lifetime decreases rapidly. This rapid decrease in
the fluorescence of light may be due to the fact that the thicker the cover, the greater the yield of
surface states leads to electronic loss.

Figure 3.28. The time-resolved fluorescence decay
curves of CdTeSe/ZnSe core/shell sample system with n
= 0, 1, 2, 4, 6 ML

Figure 3.29. The time-resolved fluorescence decay
curves of CdTeSe/ZnTe core/shell sample system with n
= 0, 1, 2, 4 ML


16
Table 3.5. The lifetime of the fitting of the time-resolved fluorescence decay curves in the core CdTeSe
QDs and the CdTeSe/ZnSe core/shell with varied shell thickness.

Sample

CdTeSe

1 (ns)
2 (ns)

4,9
53,5

CdTeSe/ZnSe
1ML
3,8
47

CdTeSe/ZnSe
2ML
3,1
6,17,6

CdTeSe/ZnSe CdTeSe/ZnSe
4ML
6ML
1,7
2,3
12,7
9,5

The fluorescence decay curve of ZnTe covered CdTeSe QDs with variable thickness given
in figure 3.29. When covering the ZnTe shell, we also observed a decrease in fluorescence intensity
over time, and divided into two segments. At the beginning of time fluorescence decreased very
quickly and after that it decreased slowly and more stable. The results match the experimental curve
with the theory that gives us the results in table 3.6.
Table 3.6. The lifetime of the fitting of the time-resolved fluorescence decay curves in the core CdTeSe

QDs and the CdTeSe/ZnSe core/shell with variable shell thickness.

Sample

CdTeSe

CdTeSe/ZnTe
1ML
1,0
16,2

CdTeSe/ZnTe
2ML
1,1
10,7

CdTeSe/ZnTe
4ML
0,9
7,2

2,8
1 (ns)
52,3
2 (ns)
3.4.3.2. Fluorescent blinking properties of CdTeSe/ZnSe 2ML single dots

We now use a microphotoluminescence setup to analyze the luminescence of individual
CdTeSe/ZnSe quantum dots. We investigated fluorescence blinking by observing the fluorescence of
QDs immediately after stopping excitation. The peaks at times multiples of 400 ns indicate that all

photons are emitted slightly after a laser pulse so that the delay between two photons is roughly a
multiple of 400 ns. The nearly-perfect absence of a peak at zero delays indicates that there is never
emission of two photons following the same laser pulse. This indicates that for these CdTeSe/ZnSe
nanocrystals we have obtained single-photon emission, most likely because multi-exciton emission
is quenched by Auger effect. The minor residual peak might be due to self-luminescence from the
substrate, possibly with a slight contribution from multi-exciton emission.

Figure 3.30. PL intensity autocorrelation function (arb.
units) of a typical individual CdTeSe quantum dot

Figure 3.31. Decay curve (norm.) of the same
quantum dot CdTeSe/ZnSe

Figure 3.31 plots the decay curve of the same quantum dot. This curve is remarkably close to a monoexponential, with an unusually long decay time of 110 ns. This observation, which was reproduced
for all single quantum dots observed with similar 110±15 ns decay times, is in contrast with ensemble
measurements, possibly because the latter is performed at much higher power which could excite


17

multi-excitonic or other nonradiative recombination pathways.
Under the single-QD observation
conditions, there is no (fast) multiexcitonic contribution and, during
the measurement (100 seconds),
there were very few fluctuations of
the decay time. This excellent
Figure 3.32. Intensity–time trace of a typical CdTeSe/ZnSe
stability
is
confirmed

by
quantum dot.
considering the intensity variations
of a typical quantum dot (Figure 3.32). Some non-fluorescent periods (“off”) are observed, but they
constitute only 20% of the total time (and less than 10% for
many other QDs). During the “on” periods, the emission remains remarkably stable
3.5. The optical properties of QDs have modified the surface

Figure 3.34. Fluorescence spectra of CdTeSe QDs with
variable Te composition are dispersed in water after
with MPA

Figure 3.35. Fluorescent spectrum of CdTeSe/ZnSe
core/shell QDs dispersed in water after surface
modification with MPA

Compared with the dispersion samples in toluene medium, samples after surface modification
by MPA, maximum radiation shift toward the shorter wavelength of about 25 to 30 nm. With
core/shell QDs, the radiation maximum has a very large shift toward the shorter wavelength, up to
nearly 60 nm.
3.6. Test results of essembled solar cells and use QDs as sensitizers
After the modified surface, we proceeded
to deposit the sensitizer to the photoelectrode of
the device. The sensitizer used is ternary alloy
CdTeSe QDs fabricated, then modified surface,
then dispersed in the water environment and
curcumin natural dye extracted from Vietnam
yellow turmeric. The electrode is then rinsed with
solvent to remove the unbound material before
assembling into a complete solar battery

component to measure the parameters.

Figure 3.36. Some pictures of solar cells


18

3.6.1. Effect of distance between two electrodes on the parameters of the solar cell
The J-V characteristic curve of the solar cell has a distance between two electrodes using
sensitizer, which is shown in figure 3.36 and the results of component parameters in Table 3.7.

Figure 3.37. The J-V characteristic curves of solar
cell use QDs as sensitizer with the distance between
the two electrodes changes

Figure 3.38. The J-V characteristic curves of solar
cells using sensitizer are that QDs have Te
components changed

Table 3.7. Typical parameters of solar cells with the distance between the two electrodes changed

Distance

Voc (V)

42 µm
70 µm
110 µm
140 µm

0,36
0,30
0,28
0,24

Jsc
(mA/cm2)
0,16
0,26
0,15
0,05

Vmax
Jmax
(V) (mA/cm2)
0,22
0,12
0,21
0,17
0,21
0,11
0,15
0,03

FF
(%)
45,8
40,9
43,0
41,7

PCE
(%)
0,026
0,036
0,024
0,005

The optimal distance between two electrodes is 70 µm, which corresponds to the highest PCE
and parameters.
3.6.2. Results of measurement of battery parameters when Te component of CdTeSe QDs
changes
Solar cells using sensitizer are the core QDs with Te components changed, the PCE of solar
cells is highest with 0.058% and 0.06% respectively (table 3.8). With the component Te of 0.5, the
fill factor and the open-circuit potential of this sample are quite low compared to a recent publication.
Table 3.8. Typical parameters of solar cells using QDs with Te components changed

Sensitizer
CdTe0,2Se0,8
CdTe0,4Se0,6
CdTe0,5Se0,5
CdTe0,6Se0,4
CdTe0,8Se0,2

Voc
Jsc
(V) (mA/cm2)
0.34
0.11
0.36

0.13
0.29
0.57
0.45
0.26
0.34
0.24

Vmax
(V)
0.22
0.23
0.16
0.30
0.22

Jmax
(mA/cm2)
0.09
0.09
0.36
0.20
0.16

FF
(%)
52.7
44.5
35.0
51.3

43.1

PCE
(%)
0.019
0.021
0.058
0.060
0.035

3.6.3. Light-sensitive solar cells are core/shell QDs
With solar cells use shell QDs types with a layer thickness of 1ML and 2 ML. The parameters
of a solar cell with different shell types and shell thicknesses are listed in table 3.9. The results showed
that the efficiency increased significantly (from 0.056% to 0.185%) when covering ZnSe shell with
a thickness of 1ML, but when the shell thickness increased to 2ML, the efficiency decreased to
0.147%.


19

Figure 3.39. J-V characteristic curves of solar
cells using sensitizer are CdSeTe/ZnSe nML
core/shell QDs with n = 0, 1, 2

Figure 3.40. J-V characteristic curves of solar
cells using sensitizer are CdSeTe/ZnTe nML
core/shell QDs with n = 0, 1, 2

Solar cells using sensitizer are CdTeSe core QDs and CdTeSe/ZnTe 1 and 2 ML core/shells
with PCE not equal to ZnSe cover samples. At the same time, when covering ZnTe shell for cores

with different thickness, the PCE decreases very quickly (table 3.9).
Table 3.9. Typical parameters of solar cells with different QDs cores/shells.
Sensitizer
CdTeSe
CdTeSe/ZnSe 1ML
CdTeSe/ZnSe 2ML
CdTeSe
CdTeSe/ZnTe 1ML
CdTeSe/ZnTe 2ML

Voc
Jsc
(V) (mA/cm2)
0.28
0.57
0.36
1.08
0.36
0.92
0.42
0.14
0.38
0.08
0.12
0.08

Vmax
Jmax
(V) (mA/cm2)
0.19

0.30
0.21
0.88
0.23
0.64
0.31
0.09
0.25
0.06
0.08
0.05

FF
35.4
47.5
44.4
47.8
47.0
43.3

PCE
(%)
0.056
0.185
0.147
0.027
0.015
0.004

CHAPTER 4.

RESEARCH RESULTS ON CURCUMIN NATURAL DYE
4.1. Study on the identification of the crystalline phase of curcumin
All
fabricated
curcumin
samples have the same maximum
diffraction pattern as shown in table
4.1. It can be seen that, on the XRD
diagram, some diffraction lines of
curcumin crystal phase overlap with
standard JCPDS (09-816) of this
substance. Position 2 of a number of
lines is compared to the lines of the
Figure 4.2. Powder XRD patterns of curcumin samples N1, N2, N3,
standard card 09-816. This change
N4 and N5 with different extraction conditions. Bulk diffraction
peaks for curcumin are indexed for identifcation purpose (JCPDS
was also observed by some other
card 9-816 and CCDC 82-8842)
authors when studying their fabricated
samples. It can be seen that these curcuminoid crystalline powder samples in addition to curcumin
also exist two types of crystals of type II and III. Therefore, as observed in the diagram, there are also
some lines not with standard card 09-816.


20

4.2. Study the vibration spectrum of curcumin molecule by Raman spectra.
Raman spectra of fresh turmeric and
natural curcumin samples extracted from

turmeric are presented in figure 4.4. It can be
seen that Raman spectra of samples in the
spectral region are observed, including many
narrow lines and narrowband groups. All
vibration lines observed in fresh turmeric also
appear in the Raman spectrum of all extracted
curcumin samples (N1 ÷ N5). This proves that
the quality of the fabricated samples is high and
Figure 4.3. Raman spectra of fresh turmeric, natural
of natural origins. Extraction by various
curcumin samples fabricated in this study (N1–N5) and the
methods used in this thesis does not change the
commercial curcumin samples (N8)
structure of curcumin. Raman spectra of the
synthetic curcumin product (N8) and the fabricated samples appear to be nearly identical lines except
for the line at 962 cm-1, 1248 cm-1 and the group of lines at the numbers. longer waves, about more
than 1600 cm-1.
In the spectrum range from 1550 cm-1 to 1650 cm-1, three samples N1, N12, N13 - samples
are extracted from turmeric and sample N6, although they are slightly different at the peak of 1625
cm-1 these four spectra are similar. The location of the vibration lines of these three samples shifts to
about 6 cm-1 toward long wavenumber due to differences in II and III curcumin content in the
compound. For sample N6, the vibration line 959 cm-1 shift to wavenumber about 21 cm-1 long
(Figure 4.5). Samples of N9, N10, and N11 on the market have a completely similar spectrum of N8,
so it can be said that these samples contain only curcumin I without other isomers.

Figure 4.4. (a) Raman spectra comparison of commercial curcumin samples being sold on the Vietnamese
market (N6, N8, N9, N10, N11) and fabricated (N1, N12, N13). (b) and a section of the spectra in the frequency
range from 1550 cm-1 to 1650 cm-1 is zoomed in for easy observation of differences at 959 cm-1 and 1625 cm-1 for
each different sample

21

Table 4.3. Experimental Raman (crystalline powder) spectral data of curcumin in frequency region 1700–900 cm-1
Peak Assignment









Cur N1

Cur N2

Cur N3

Cur N3-1

 C=O (II)
 C=O (III)
 C=O (I)  C=C

1637
1632

1627

1625

 C=C (I,II)Aromatic

1599 1599

1599

1599

1590

1579

1579

 C=O
Phenol C-O (I)

1523 1536
1428

1516
1435

1523

Phenol C-O (II, III)

1413 1413

 C=C (II,III)Aromatic

*





Cur N4

N6

 C=O

N8

N9

1413

1598

1524

1413

1625

1599 1599 1600

1599

1536 1531 1533
1428 1429 1427

1183
1166
1148
1118
963

1183
1166
1148
1118
971

1234
1196
1168

1120
976

1226

1183
1166

1148
1118
975

1236
1226
1183
1166
1148
1118
975

1529
1428

1413
1247

1226
1205 1205
1187 1181 1182
1161
1150 1149
1128
981 959 961





Cal.

Mangolim 2014

Kolev 2005

Kolev 2005

1626

1630

1601

1615

N10

1632 1625 1626

1248 1248 1247
1236
1229
1226

Cur

1636

Enol C-O ( I)

Enol C-O (II, III)

 Cur  Cur

1205
1182

1638
1639
1626
1600
1602
1591
1591
1430
1416
1415
1249
1234
1233

1587
1509
1431

1409

1230
1207
1184

1168

1149
961

1536
1420

1120
967

1216
1212
1196
1176
1169
1150
1107
966


22

4.3. Study the absorption and fluorescence properties of natural curcumin

Figure 4.5. Absorption spectra of curcumin—ethanol
solutions with different curcumin concentration from 1,
2.5, 5, 10 μg and 20 μg/mL. The inset is a linear relation
of the absorption intensity and curcumin concentration
in this concentration range

Figure 4.6. Normalized PL spectra of fabricated
curcumin samples in solids form and sample N6

The absorption spectrum of curcumin corresponds to the transition between π -π* electronic
energy states. When the solution is more diluted, the absorption intensity will decrease, and the
absorbance intensity decreases linearly according to the concentration of curcumin diluted in ethanol.
The main reason of reduced absorption in the solution when the curcumin concentration decreases is
the number of absorption centers decreases, while on the other hand it also the degradation of
curcumin in the water medium by a reaction at the keto-enol group.
The absorption spectrum of curcumin is in the wavelength range from 350 nm to 490 nm,
indicating strong absorption in the region of the solar spectrum, so curcumin dye can be absorbed
effectively strength of the solar spectrum.
The photoluminescence spectrum of curcumin is a wide emission band, and the peak is slightly
shifted, depending on the sample. The transfer characteristic (π* - π) of the carbonyl groups in
curcumin can affect the maximum fluorescence shift. Photoluminescence spectra of samples which
held in the dark for 6 months also showed no spectral changes and peak displacement occurred.

Figure 4.8. Normalized PL spectra of fabricated
curcumin samples and sample N6 after six months of
storage were re-measured

Figure 4.9. Normalized PL spectra of curcumin
N1 samples extracted from turmeric which is recrystallized
multiple times (N1-a, N1-b, N1-c) and samples left after six
months and recrystallized (N1 Recrystallization)


23

4.4. Results of solar cell parameters use curcumin as a light sensitizer

Figure 4.11. J-V characteristic curves of solar cells using sensitizer are is curcumin that varies with
concentration and time of immersion

Table 4.4. Typical parameters of solar cells using sensitizer are curcumin with varying concentrations
and time of immersion

Sensitizer

Voc (V)

Cur 1
Cur 2
Cur 3
Cur 4

0.21
0.28
0.40
0.47

Jsc
(mA/cm2)
0.72
0.92
1.52
1.66

Vmax

(V)
0.14
0.15
0.27
0.33

Jmax
(mA/cm2)
0.48
0.48
1.04
1.28

FF
44.4
28.0
46.2
54.3

PCE
(%)
0.067
0.072
0.281
0.422

The efficiency of solar cells using curcumin is a light-sensitive material that has obtained
certain results, from 0.067% to 0.42% depending on the concentration of curcumin dissolved in
ethanol as well as the time of immersion. The PCE for this material corresponds to a concentration
of curcumin-ethanol of 3mM and immersion for 24 h. This PCE is similar to that of K.E. Jasim et al.

published in the Journal of Energy and Power Engineering (6/2017), on the use of curcumin as a
sensitizer in solar cells, this result reached 0.41%. The Korean author, Hee-Je Kim et al in 2013,
published results of lower PCE: 0.36%, and 0.6% when mixed red-cabbage and curcumin at a ratio
of 70:1. Souad AM Al-Bat'hi obtained a PCE of 0.36%. Than Than Win and colleagues, reported on
PCE of 0.129% when using curcumin in 2012, S. Suresh and colleagues also published the results of
PCE of 0.13% in 2015. SJ Yoon et al also published that the solar cell used curcumin reached 0.11%
of PCE. Therefore, it can be said that PCE is low when using curcumin, except for the use of this
natural pigment is environmentally friendly and can meet the needs of individual power use.