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Electron injection from the excited dye to semiconductor
Electrons need to be injected into semiconductor conduction band after its activation. Effective transitions which can easily transfer into the semiconductor are needed. The first requirement for this is that the position of LUMO or of another orbital which is the excited electron destination (for example LUMO+1) of dye must be near anchoring group (Le Bahers et al., 2014). Computational studies can investigate how the dye attaches to semiconductor surface, behavior of electron excitation and electron distribution in dye attached semiconductor system (Le Bahers et al., 2011). Figure 2.10 shows an example of computational calculation result that give information about how the N749 dye is attached onto (101) TiO2 anatase surface. Furthermore, it describes the electron injection process.
There are other factors that influence the effectiveness of the electron injection process, such as the heterogeneity of the semiconductor surface, the binding mode and strength of the dye on the semiconductor surface and also the possible interaction between dyes (Asbury et al., 2000; Hagfeldt et al., 2010). Additives that are dissolved in the electrolyte and have the ability to shift semiconductor conduction band energy will also influence the electron injection (Koops et al., 2009).
Electron transport in Electrolyte
Charge transport in electrolyte occurs through redox-couple species diffusion. The redox shuttle is reduced at the counter-electrode of the device whereas the dye adsorbed on the semiconductor surface is place where the redox-couple is oxidized by the oxidized-dye. The properties of the electrolyte, as medium transport, are the primary factor that influenced electron transport. Electrolyte viscosity, concentration and distance between the two electrodes are the parameters that influence electron diffusion (Boschloo and Hagfeldt, 2009; Hagfeldt et al., 2010). For cobalt-based DSSC, it is important to have larger size of semiconductor pores to facilitate the diffusion of redox species (Hamann, 2012).
Basic Principle in Cell Characterization
The basic principles of the main characterization techniques used here for the cell performance and functioning investigations are described in this section. They include current-voltage (I-V) curve measurement, Incident-photon-to-electron conversion efficiency (IPCE) and Impedance Spectroscopy (IS).
Gaussian and Plane Waves Method (GPW)
Gaussian and Plane Wave (GPW) method is an hybrid method that combine an atomcentered Gaussian-type basis set to describe the wave functions, but uses an auxiliary plane wave basis to describe the density (VandeVondele et al., 2005). The implementation of two representations of the electron density allows for solving the total energy and Kohn-Sham matrix using computational resources that has a linear scaling with system size due to an efficient treatment of the electrostatic interactions.
In the two previous sections, it has been mentioned that it is necessary to replace the core electrons with the pseudopotential to avoid computational inefficient due to an expansion of an atomic all-electron density or wave function in plane waves. In GPW method, the r (a.u) u (r) pseudopotential Goedecker, Teter and Hutte (GTH) (Goedecker et al., 1996) is implemented to replace the core electrons. The GTH pseudopotential consists of (1) a local part that include a long-ranged (LR) and a short-ranged (SR) term and (2) a non-local part with the Gaussiantype projectors. The utilization of this pseudopotential in plane wave method requires relative high cut-off values. On the contrary, in GPW method it is less computationally effort since the kinetic energy and the short range pseudopotential terms are integrals over Gaussians functions which can be calculated analytically (VandeVondele et al., 2005; Krack and Parrinello, 2004).
Synthesis of particles for the photoanodes
The section below explains the preparation of TiO2 anatase particles, brookite nanoparticles and ZnO nanorods.
TiO2_A nanoparticles were prepared by using modifications of previously reported methods (Ito et al., 2008). 15.3 mL of titanium iso-propoxide (TIP) was poured in to 50 mL Erlenmeyer under argon. The TIP was obtained from Aldrich. 2.857 mL of glacial acetic acid (Merck) was then added into the TIP to modify it. The mixture was stirred for 15 minutes at room temperature. 72.5 mL water demineralization was poured in a 100 mL round-flask. The modified TIP was then poured into the water as soon as possible under vigorously stirring to produce a white precipitate. The stirring process was continued for 1 hour to complete the hydrolysis reaction. 1 mL HNO3 68 % (Merck) was added into the mixture, then the temperature was increased gradually from room temperature to 80°C for 40 minutes under reflux conditions. The mixture was left at this temperature for 75 minutes to get a good dispersion. After that, 2 mL of water was added into the dispersion to adjust the volume at 92.5 mL. It was then allowed to cool at room temperature. The synthesis was continued by putting the dispersion into a teflon-lined hydrothermal reactor. The hydrothermal reaction was done in an oven at 225°C for 12 hours. After cooling a white precipitate was formed in the reactor. After addition of 0.6 mL of HNO3 68%, the mixture was re-dispersed by ultrasonication for 5 minutes, twice. To make the dispersion separation easier, it need to be concentrated. It has been done by removing 50 gram of water using rotary evaporator at 45°C and 70 mbar. The nanoparticles were then washed in water to remove nitric acid. They were centrifugated at 12000 rpm for 15 minutes. The supernatant was removed and the particles were redissolved in water. This step was done three times then the particles were treated in the same manner three times in ethanol to eliminate water. The resulting powder was dried for 1- 3 days at 40°C in an oven. They were used without any further purification.
Synthesis of brookite TiO2 nanopartciles (TiO2_B1 and TiO2_B2)
The brookite particles B1 and B2 were synthesized in collaboration with the Laboratoire de Chimie de la Matière Condensée de Paris, Collège de France, Paris, France. The brookite particles B1 were synthesized by adding pure TiCl4 (Aldrich) in a 3 M HCl (Merck) solution to form a colorless solution with a Ti concentration of 0.15 M (Pottier et al., 2001a). The solution was heated and aged at 95°C during 3 days and was then after peptized to eliminate the rutile phase. The brookite particles B2 were synthesized by co-hydrolysis of the aqueous precursors TiCl3 and TiCl4 with a total Ti concentration of 0.04 M (Pottier et al., 2001b) The pH of an equimolar solution of Ti3+ and Ti4+ was adjusted to 4.5 and the suspension was aged one week at 60°C.
Synthesis ZnO nanoparticles (ZnO_NR)
ZnO_NR were prepared in collaboration with the Laboratoire des Sciences des Procédés et des Matériaux, Université Paris 13, Villetaneuse, France. Zinc acetate dihydrate (Zn(OAc)2.2H2O), and an appropriate volume of distilled water were successively added to diethylene glycol DEG (O(CH2CH2OH)2) at 161oC. The size and morphology of the particles were controlled by adjusting the zinc concentration (z), z = 0.5 M, and the hydrolysis ratio, defined as h=nH2O/nZn2, h = 5. The mixture was then heated under reflux at a specified temperature for 1 hour. After hydrolysis, the white precipitate of zinc oxide nanoparticles was centrifuged, washed several times with ethanol and acetone, and dried at 60°C.
Porous layer preparation by doctor blading
The photoelectrodes were prepared by spreading the paste on FTO glass substrate. The FTO glass substrates (TEC15, Pilkington) were cleaned with soap and rinsed with distilled water. They were then treated in acetone in an ultrasonic bath in acetone for 5 min and in ethanol for 5 min. The substrates were dried and placed in furnace at 450°C for 30 min.
In the case of TiO2 layers, after cooling down, the substrates were immersed in a 40 mmol.L-1 aqueous solution of TiCl4 at 70°C for 30 min and rinsed with water and ethanol. The substrates were then dried and p at 500°C for 30 min. This process was repeated once. A layer of oxide paste was spread on the FTO glass substrates by the doctor blading technique (Figure 3.3 and 3.4), relaxed under an ethanol atmosphere for at least 5 minutes and dried at 125°C for 5 min. The step was repeated several times in order to achieve the desired film thickness. No scattering layer was used for all the investigated cells. The films were then annealed at 500°C for 15 min, for TiO2 layer and 410°C for 30 min, for ZnO layer. In the case of TiO2 layers, a final TiCl4 treatment was finally done by immersing the TiO2 films in a 40 mmol.L-1 TiCl4 solution at 70°C for 30 min and annealing again at 500°C.
BET (Brunauer, Emmett and Teller)
The BET specific internal surface area measurements were done to see the effect of graphene on the specific surface area of the TiO2 composite (see chapter IV). It was determined from the adsorption isotherms of Kr at the boiling point of liquid nitrogen (∼77 K) using a Micromeritics ASAP 2010 apparatus (Pauporte and Rathousky, 2009; Pauporté and Rathouský, 2007).
FTIR (Fourier Transform Infra-Red) spectroscopy
FTIR analysis was carried out to confirm the convert of the graphene-oxide into reduced graphene and to control thekind of bound that happened in the composite of TiO2/graphene (see Chapter IV). The FTIR curves were measured with a Tensor 27 apparatus from Bruker. The investigated samples were mixed with dry KBr, pressed as a pellet, and measured in a transmission mode.
Micro-Raman spectroscopy was done to verify the graphene existence in TiO2_Gr composite. The micro-Raman spectra of the films were measured using a Horiba Jobin-Yvon LabRam IR system with backscattering geometry, using a 514.5 nm Ar+ laser as an excitation source.
Solar cell performance characterizations
The I−V curves were recorded by a Keithley 2400 digital sourcemeter, using a 0.01 V·s−1 voltage sweep rate. The solar cells were illuminated with a solar simulator (Abet Technology Sun 2000) filtered to mimic air mass AM 1.5G conditions. The power density was calibrated to 100 mW·cm−2 by the use of a reference silicon solar cell. The illuminated surface was delimited by a black mask. The incident-photon-to-electron conversion-efficiency (IPCE) spectra were measured at a short circuit with a homemade Jobin-Yvon system. The spectra measurement of electrical impedance spectra (IS) were measured in the dark (except when mentioned), over a large potential range, by a Solartron FRA1255 frequency response analyzer coupled with a PAR273 EGG potentiostat. The AC signal was 10 mV and the frequency range was 100 kHz to 0.05 Hz. The reproducibility of the impedance data was checked on several cells prepared from different batches. The spectra were fitted and analyzed using the Zview modeling software (Scribner). The impedance spectroscopy results were corrected for IR-Drop over the sum of all series resistances. The real potential (Vcor) applied was determined by the subtraction of the voltage drop (VDrop) from the applied potential (Vapplied). The voltage drop was calculated by the integration of the sum of all series resistances, Rseries= Rs + Rce with Rs being the series resistance, and Rce the resistance due to the counter electrode (the diffusion resistance was negligible) over the current passed.
Table of contents :
Table of Contents
List of Figures
List of Tables
List of Appendix
List of the Important Abbreviations and Symbols
Chapter I: Introduction
I.1.1. Solar Energy as an Alternative of Renewable Energy
I.1.2. Dye-sensitized solar cells: towards iodine free devices
I.3. Research Approach
1.3.1. TiO2 Brookite Based DSSC
1.3.2. TiO2/graphene Based DSS
1.3.3. ZnO_NR Based DSSC..
1.4. Thesis Organization
Chapter II: Literature Review
II. 1. Fundamental of DSSC
II. 1.1. DSSC Component
II. 1.2. Electron Transport in DSSC
II.1.3 Basic Principle in Cell Characterization
II.2 Theoretical Background in Computational Methods
II.2.1 Quantum Chemistry
II.2.2 Density Functional Theory
II.2.3 Models in Computational Chemistry
Chapter III: Research Methods
III.1. Synthesis of particles for the photoanodes
III. 2. Cell Preparation
III.4. Computational Details
Chapter IV: TiO2-Based DSSC I: Comparison of Anatase and Brookite-Based Dye- Sensitized Solar Cells
IV. 1. Introduction
IV.2 Characteristics of the particles
IV. 3. Cell Performance and Impedance Study
IV.3.1. I-V Measurement Result
IV.3.2 Analysis of the open circuit voltage
IV.3.3. Analysis of charge transport and recombination in the photoelectrodes .
IV.4 Computational Study: Comparison of adsorption of Iodine (I2) on Anatase (101) and Brookite (210) planes
IV. 5 SUMMARY
Chapter V: TiO2-Based DSSC II: The Effect of Graphene Incorporation in TiO2/graphene for Photoelectrode
V. 1. Introduction
V.2 Composite TiO2/Graphene (TiO2_Gr) Preparation
V.3 The cell performance of TiO2/Gr in iodine based DSSCs: The investigation of graphene’s role in TiO2 photoelectrode
V.4 The cell performance of TiO2/Gr in cobalt based DSSC
V. 5 SUMMARY
Chapter VI: ZnO Nanorod Based Electron Transport Layer for DSSC
VI. 1. Introduction
VI. 2. Particle Characterization ZnO_NR
VI. 3. Utilization ZnO_NR for the comparison of the performance of an organic and an inorganic sensitizer
IV. 4. Impedance Study D149- and TG6-sensitized ZnO_NR cell
IV. 5. Computational investigation of the electronic and optical properties in D149 and TG6 dyes
VI. 6. Use of photoelectrode ZnO_NR for iodine free DSSC
VI. 7. Computational investigation of the electronic and optical properties of SD4, JM131 and JM164 dyes
Chapter VII: General Conclusions and Suggestions for Future Works
List of Publications, Seminars and Workshop