الفهرس 
Theoretical Review and Calculation Methods
Results and discussionsOnly 14 pages are availabe for public view
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Abstract
The demand for clean and environmentally friendly energy sources continues to increase due to the depletion of conventional energy sources. One of the promising candidates for these new energy sources is solar energy, which is clean, renewable, and limitless. Dye-sensitized solar cells have gained much more attention as one of the most promising energy harvesting devices. This work aims to fabricate stable and low-cost DSSCs based on photoelectrodes of ZnO and TiO2 prepared by low- cost and simple thermal techniques. The synthesized metal oxides’ physical properties were examined to explore their potential use as photoanodes in DSSCs. One of our goals is to enhance photoanodes by combining them with rGO to give them excellent electrical conductivity, increased dye adsorption, and a broad interfacial contact area between the electrolyte and the dye.
The thesis consists of five chapters:
Chapter 1 provides a general introduction to the subject and a literature review of the materials used in the fabrication of the DSSC photoanodes, as well as the aim of the work.
Chapter 2 describes the theoretical review and calculation methods used for analyzing the results of X-ray diffraction (XRD), kinetic thermal decomposition, and dye-sensitized solar cells.
Chapter 3: This section includes the materials used in this work, the experimental methods for preparing the investigated materials, and the fabrication of DSSCs. It also contains the techniques used for the sample’s characterization: including XRD analysis, scanning electron microscopy (SEM), UV-vis absorbance, photoluminescence spectroscopy (PL), BET, and thermal gravimetric analysis (TGA).
Chapter 4 shows the results and the discussion of the characterization data used in the investigations of the samples.
Chapter 5 includes the results and discussion of the thermal decomposition kinetics of zinc carbonate basic, zinc acetate dihydrate, and the DSSCs.
The main results and conclusions obtained are:
(1) The zinc oxide (ZnOa) from the thermal decomposition of zinc acetate dihydrate is produced at a lower temperature than the ZnOc from the zinc carbonate base. This means that the production of ZnO from the zinc acetate dihydrate is more economical than that from the zinc carbonate basic.
(2) rGO has an amorphous nature, ZnO exhibits a Wurtzite structure, and TiO2 has a tetragonal structure, and the composites showed mixed phases of their constituents.
(3) All the pure and composite samples have a nanoscale structure, in which the size follows the following order: TiO2 > ZnOc > rGO@TiO2 > rGO@ ZnOc > ZnOa
> rGO@ZnOa.The decrease in the particle size of the pure metal oxides due to the addition of rGO is explained based on the attraction between the rGO with an electron-rich material with the metal ion in the composites.
(4) As the particle size increases, the BET-surface area increase.
(5) The rGO has a fiber-like morphological structure, whereas ZnO and TiO2 have a highly porous structure with quasi-spherical forms of varying nanoparticle sizes. The porosity of the composite samples is greater than that of the metal oxide.
(6) This means more dye adsorption in the composites than in the pure metal oxides.
(7) The optical band gap energies (Eg) are 3.31, 3.27, 3.22, and 3.20 eV for ZnOa, ZnOc, rGO@ZmOa, and rGO@ ZnOc, respectively, and 3.10 as well as 2.95 eV for TiO2 and rGO@TiO2, respectively.
(8) (Isothermal and nonisothermal techniques study the air atmosphere’s decomposition kinetics of zinc carbonate basic at four different heating rates. The average value of activation energy needed for the decomposition reaction to form ZnOc is 185.0, 173.0, 175.7, 180.9, and 181.8 kJ mol-1 obtained using the isoconversional methods of FWO, KAS, Kissinger, Starink, and Friedman, respectively. All the isoconversional methods studied show similar trends in slight changes in the conversion function’s activation energy. The results obtained based on the different calculation procedures used were similar, implying that their average value could be taken. The mostprobable degradation mechanism and the thermodynamic parameters for forming ZnOc also better determined using iterative methods. They are -42.87 Jmol-1K-1, 168.4 kJ mol-1 ,
192.5 kJ mol-1, for, ∆S#, ∆H# and ∆G# , respectively. The positive values of
∆H≠ 𝑎𝑛𝑑 ∆𝐺≠ mean that the decomposition of zinc carbonate basic is a non- spontaneous process connected with introducing heat.
(9) The thermal decomposition of zinc acetate dihydrate proceeded in two separate steps. The first step is due to the dehydration process. The second step is due to the conversion of the dehydrated salt to ZnOa. The kinetics of the thermal decomposition were done for each step using isothermal and nonisothermal techniques at four different heating rates. The average value of activation energy needed for the dehydration reaction is 103.6, 102.6, 102.1, 108.1, and 102.8 kJ mol-1 obtained using the isoconversional methods of FWO, Kissinger, Strink, Friedman, and iterative method, respectively. Moreover, the average activation energy value needed for dehydrated salt’s thermal decomposition to ZnOa is 238.3,238.3, 236.8, 245.3, and 238.5 kJ mol-1. For FWO, Kissinger, Strink, Friedman, and iterative method, respectively. All the isoconversional methods studied show similar trends in slight changes in the conversion function’s activation energy. The results obtained based on the different calculation
procedures used were similar, which implies that their average value could be taken. The most probable degradation mechanism and the thermodynamic parameters for forming ZnOa were also better determined using iterative methods. They are -29.97 Jmol-1K-1, 99.67 kJ mol-1 , 111.0 kJ mol-1, for, ∆S#,
∆H# and ∆G# , respectively. The positive values of ∆H≠ 𝑎𝑛𝑑 ∆𝐺≠ mean that the decomposition of dehydrated zinc acetate is a non-spontaneous process connected with the introduction of heat.
(10) The photocurrent–photovoltage (I–V) characteristics of the DSSCs showed that the DSSCs’ efficiency improved with the presence of rGO in zinc oxide or titanium dioxide photoanodes. The DSSC equipped with the rGO@ZnO/rGO@TiO2 photoanode has the highest Isc, Voc, and . Meanwhile, the pure TiO2 photoanode device had the lowest Isc, Voc, and . The rGO@ZnO/rGO@TiO2 solar cell’s stellar performance may be due to its large specific surface area, which allowed more dye molecules to be absorbed into the rGO@TiO2 composite’s surface, increasing its light-collection capabilities. In addition, the rGO@ZnOa layer deposited on the rGO@TiO2 with a lower electronic affinity (higher minimum energy level in the conduction band) creates an energy barrier that can prevent the injected electrons from recombining back into dye molecules or electrolyte species
(11) The impedance spectra of the DSSCs generally comprise three parts, including two circles. The high-frequency semicircle (R1) represents charge transfer at the counter electrode/electrolyte interface, while the low-frequency semicircle (R2) represents electron transport at the dye-sensitized metal oxide film/electrolyte interface. The third part at the high frequencies (Rs) represents the resistance between the glass and the photoanode. The R2 values of DSSCs based on photoanode films exhibit 22.4, 24.0, 28.5, 57.9, 68.1, 80.61, and 112.2 ohm for
rGO@ZnOa /rGO@TiO2, rGO@ZnOa, rGO@ ZnOc, rGO@TiO2, ZnOa, ZnOc, and TiO2, respectively. The results demonstrate that the rGO creates an effective pathway that can improve electron transport/ injection while decreasing charge recombination in the DSSC due to the formation of rGO-based Schottky junctions in the photoanode.
(12) The impedance data showed a DROP in R1 and Rs in the same sequence as for R2. The decrease in R1 indicated that the charge flowed faster across counter- electrolyte interfaces to regenerate oxidized dye. Regeneration can occur soon after the charge has transferred to the oxidized dye. The DROP in Rs value might be due to the introduction of rGO into the metal oxides, which increased the electrical conductivity of the photoanode.