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: Associate Professor - Doctor Pham Thu Nga, Institute of Materials Science-VAST Associate Professor - Doctor Nguyen Thi Thuc Hien, Duy Tan University Reviewers: Associate Professor – Doctor Nguyen The Binh, University of Science-VNU Associate Professor – Doctor Pham Van Hoi, Institute of Materials Science-VAST 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: 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 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 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 chapters, including 149 pages, 81 pictures and 14 tables The main results of the thesis are published in international journals, national scientific journal and reports at international and national conferences CHAPTER 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 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 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 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 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), 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 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 (a) (b) TiO2 Figure 2.10 Cross section image of MWCNT - TiO2 film of a counter electrode CHAPTER 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 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 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 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 ÷ 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 12 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 ZnSe ML shell, the diagram 13 also appears 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, 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 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, 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 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, 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 ML to 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, ML Figure 3.29 The time-resolved fluorescence decay curves of CdTeSe/ZnTe core/shell sample system with n = 0, 1, 2, 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 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, Figure 3.40 J-V characteristic curves of solar cells using sensitizer are CdSeTe/ZnTe nML core/shell QDs with n = 0, 1, Solar cells using sensitizer are CdTeSe core QDs and CdTeSe/ZnTe and 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 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 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 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 Cur Cur Cur 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 24 CONCLUSION From the scientific results, we have obtained the following conclusions: Optimal conditions for synthesis CdTeSe QDs crystalline zb structure, and CdTeSe/ZnSe, CdTeSe/ZnTe core/shell structures are: precursor molar ratio Cd:Te:Se = 10:1:1; ODE-OA environment; the growing temperature is 260 oC, grown for 10 minutes The shell growing temperature is lower than the core growing temperature of 30 oC The QDs have a cubic monocular crystal structure of zb-CdTeSe The shape of the QDs is slightly sphere, the average size is from nm to nm, depending on specific fabrication conditions and shell thickness The synthesis CdTeSe QDs have a maximum emission of 730 nm to 760 nm and high QY (~ 50%) When covered, their fluorescence spectrum is shifted towards longer wavelengths, ~ 900 nm Ratio of the intensity of two-phonon vibration spectral lines: at ~ 159 cm-1 - characteristics for CdTe-like LO phonon mode, and ~ 188 cm-1 - characteristic for CdSe-like LO phonon mode, which can be used to observe the change in the respective composition, in the ternary alloyed CdTeSe QDs With the covering of ZnSe and ZnTe shells, Raman spectroscopic analysis showed that when covered with ZnSe shell, the material formed a CdSe-rich shell while simultaneously forming a ZnTe shell and a CdTe-rich shell The time-resolved fluorescence decay curves of the QDs according to the two exponential functions: a part corresponding to very short time-off (size ns) and a part corresponding to a long time-out (tens of ns or more) The average lifetime of the excitons in CdTeSe QDs is about 53 ns When coated with a ZnSe 2ML shell, the lifetime of these QDs increases For the single CdTeSe/ZnSe 2ML QDs, a single QD curve fluorescence follows an exponential function, with a calculated lifetime of 110 ns The single fluorescence phenomenon of single QD core/shell was observed to be significantly reduced, almost unblinking during the observation period Successfully extracted and fabricated curcumin with the help of microwave technology and crystallized them in crystal form The study has allowed the identification of the crystalline phase to be produced as a curcuminoid mixture The detailed analysis of Raman spectra has allowed to identify the difference of curcumin extracted from natural turmeric with curcumin synthesized by chemical method The absorption and luminescence spectra of natural curcumin are a wide range typical for the transitions between the electronic energy states π and π * of carbonyl functional groups in curcumin Their strong absorption spectrum is located in the UV-Vis region, which allows curcumin to be used as a -sensitizer in solar cells Measurements of solar cell parameters and the calculation of the efficiency of essembly solar cells, with the use of sensitizer, are CdTeSe QDs, CdTeSe/ZnSe and CdTeSe/ZnTe core/shell QDs When using QDs coated with thin ZnSe shells, the PCE increased significantly compared to uncovered QDs solar cells With sensitizer is curcumin, the PCE is higher than that of solar cells of the same type published internationally by other authors during the recent same study period THE PUBLISHED PAPERS RELATED TO THESIS Le Xuan Hung, D B.Pascal, Pham Nam Thang, Nguyễn Thu Loan, D M.Willy, R D Amit, F Fu, U E.V Juan, Nguyen Thi Thuc Hien, Nguyen Quang Liem, C Laurent and Pham Thu Nga, Near-infrared emitting CdTeSe alloyed quantum dots: Raman scattering, photoluminescence and single-emitter optical properties, RSC Advances, 2017, 7, 47966-47974 Le Xuan Hung, Pham Nam Thang, Hoang Van Nong, Nguyen Hai Yen, Vu Đuc Chinh, Le Van Vu, Nguyen Thi Thuc Hien, Willy Daney de Marcillac, Phan Ngoc Hong, Nguyen Thu Loan, Catherine Schwob, Agnès Mtre, Nguyen Quang Liem, Paul Bénalloul, Laurent Coolen, Pham Thu Nga, Synthesis, structural and optical characterization of CdTeSe/ZnSeand CdTeSe/ZnTe core/shell ternary quantum dots for potential application in solar cells., Journal of Electronic Materials, 2016, 45, 4425-4431 Hoang Van Nong, Le Xuan Hung, Pham Nam Thang, Vu Duc Chinh, Le Van Vu, Phan Tien Dung, Tran Van Trung, Pham Thu Nga, Fabrication and vibration characterization of curcumin extracted from turmeric (Curcuma Longa) rhizomes of the northern Vietnam SpringerPlus, 2016, 5, 1147-1156 Pham Nam Thang, Le Xuan Hung, Nguyen Thi Minh Chau, Vu Thi Hong Hanh, Nguyen Ngoc Hai, Nguyen Thi Thuc Hien, Pham Thu Nga, Structural and Optical properties in Near Infrared of CdTeSe Colloidal Quantum Dots for Potential Application in Solar Cells, Vietnam Journal of Science and Technology, 2017, 55(4), 515-525 Le Xuan Hung, Pham Nam Thang, Nguyen Hai Yen, Nguyen Thi Thuc Hien, Pham Thu Nga, Raman spectroscopy and optical properties of the core/shell ternary alloyed quantum dots, The proceeding of international conference on spectroscopy & materials science ICS&M-2015, 2015, 138-144 Pham Thu Nga, Le Xuan Hung, Pham Nam Thang, Hoang Van Nong, Phan Tiến Dũng, A study on the Raman spectroscopy of the natural curcumin extracted from Vietnam turmeric, The proceeding of international conference on spectroscopy & materials science ICS&M-2015, 2015, 145-149 Le Xuan Hung, Pham Nam Thang, Hoang Van Nong, Nguyen Hai Yen, Dinh Hung Cuong, Nguyen Thi Thuc Hien, Pham Thu Nga, Fabrication and characterization of CdSeTe ternary alloy quantum dots and curcumin natural dye, Những tiến Quang học Quang phổ Ứng dụng, 2014, ISBN 1829-4271, 217-222 Lê Xuân Hùng, Hoàng Văn Nông, Lê Anh Thi, Phạm Thu Nga, Nguyễn Thị Thục Hiền, Phan Tiến Dũng, Chế tạo, tính chất quang, triển vọng ứng dụng nano tinh thể chấm lượng tử chất màu tự nhiên curcumin, Tuyển tập Những tiến vật lý kỹ thuật ứng dụng, 2014, ISBN:798-604-913-232-2, 503-508 Phạm Nam Thắng, Hoàng Văn Nông, Nguyễn Hải Yến, Đinh Hùng Cường, Lê Xuân Hùng, Nguyễn Ngọc Hải, Vũ Thị Hồng Hạnh, Khổng Cát Cương, Phạm Thu Nga, Preparation And Optical Properties Of The Ternary Alloy Quantum Dots For The Potential Application In Solar Cell, Những tiến Quang học Quang phổ Ứng dụng, 2014, ISBN 1829-4271, 436-441 10 Le Xuan Hung, Hoang Van Nong, Le Anh Thi, Pham Thu Nga, Nguyen Thi Thuc Hien, Phan Tien Dung, Synthesis, optical properties, application prospects of nano crystal quantum dots and curcumin natural dye exctracted from turmeric, International Conference on Spectroscopy & Application, 2013, 327-343 ... tiến Quang học Quang phổ Ứng dụng, 2014, ISBN 1829-4271, 217-222 Lê Xn Hùng, Hồng Văn Nơng, Lê Anh Thi, Phạm Thu Nga, Nguyễn Thị Thục Hiền, Phan Tiến Dũng, Chế tạo, tính chất quang, triển vọng ứng. .. Dũng, Chế tạo, tính chất quang, triển vọng ứng dụng nano tinh thể chấm lượng tử chất màu tự nhiên curcumin, Tuyển tập Những tiến vật lý kỹ thuật ứng dụng, 2014, ISBN:798-604-913-232-2, 503-508 9... strong fluorescence peak of the QDs Table 3.3 Fluorescence parameters of core/shell QDs CdTeSe/ ZnSe nML (với n =0, 1, 2, 4, ML) Sample CdTeSe CdTeSe/ZnSe 1ML CdTeSe/ ZnSe 2ML CdTeSe/ ZnSe 4ML CdTeSe/ ZnSe