الفهرس 
The Need for Renewable EnergiesOnly 14 pages are availabe for public view
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Abstract
Solar energy is the greatest renewable energy resource in the world.
Fortunately, the supply of energy from the sun to the earth is gigantic, namely 3x1024 J per year or about 104 times more than what mankind consumes currently.
Solar cell is a device used to convert solar energy into electricity directly by photovoltaic effect. In 1954, the first solar cell made of silicon was developed. Solar cells can be classified into different types such as, silicon-based solar cells, thin film cells, DSSCs, SSDSSCs and so on. The cost of the Si-based solar cell production is still high and the technology is not commercially available. As a result, the low cost DSSC was studied to get low cost and considerable efficiency devices. A DSSC has many advantages such as inexpensive material cost, easy fabrication and relatively high efficiency. In 1991 Gratzel and O’Regan announced the development of DSSC with 7 % efficiency using nanometer size TiO2.
DSSCs consist basically of four main components: photoanode, sensitizer, mediator (electrolyte) and counter electrode. When light with appropriate wavelength incident on the cell; the dye absorbed it and an
electron is excited from the highest unoccupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The electron is then injected into conduction band of TiO2 where it percolates its way to the transparent conducting oxide (TCO) glass then, the electron flows through the external circuit, to the counter electrode, CE. At CE the electron reduces the electrolyte iodide / triiodide (I-3/3I-) and the reduced species (3I-) acting as charge shuttle. Upon arrival at the dyed photoanode, the 3I- is oxidized back to I-3, reducing the previously oxidized dye and regenerating it for another cycle. A voltage should be established by the solar cell to be able to do work.The open-circuit voltage of the cell is due to the difference between the Fermi level of the nanostructure semiconductor and the electrochemical potential of the liquid electrolyte.
Eventhough, liquid electrolyte-based DSSCs show promising efficiency; there are some drawbacks which limit their efficiency and application. These drawbacks are divided into two categories, namely, recombination processes and liquid electrolyte related problems. Accordingly, solid state DSSCs (SSDSSCs) are considered as a solution for liquid electrolyte problems.
SSDSSCs are solar cells where liquid electrolyte is replaced by organic or inorganic hole transport materials (HTMs). They also have a homogenous structure in contrast to the sandwiched design of liquid state-based DSSCs.
One of the most used solid state electrolytes is copper iodide (CuI) which has large band gap and three crystalline phases between room temperature and its melting point (600 °C). These phases are known as α-, β- and γ- phases. The low temperature phase is the γ-phase (formed below 370 ◦C) with zinc blende crystal structure and behaves as a p-type semiconductor.
In this work we prepared CuI nanoparticles with different particle sizes and employed them as the solid state electrolyte for fabricated of SSDSSCs.
This thesis contains three chapters:
Chapter I
This chapter contains an ”introduction” about the basic of DSSCs, SSDSSCs and ended with literature review.
Chapter II
This chapter contains the experimental work and the used characterization techniques.The first section of the experimental work involves preparation of CuI nanoparticles with different particle sizes by employing different preparation conditions. Whereas, the second section is devoted to the procedure of preparation all components of solid state dye sensitized solar cells (SSDSSCs) and its assembly.
Characterization techniques
X-ray diffraction (XRD) technique
Transmission electron microscope (SEM) technique
UV-Visible spectrophotometer.
Scanning electron microscope
Electrical measurement (J-V) and (V-t) Curves
Chapter III
The first part of this chapter involves structure and optical analysis of the prepared CuI samples. The second part contains SEM analysis and J-V, V-t characteristics of the solar cells.
X-Ray Diffraction pattern shows all diffraction peaks of CuI prepared samples are well indexed to the standard chart of γ-CuI face centered cubic phase [JCPDS card No. 82-2111, space group: F-43m]. Also, no other phases were detected which ensures the absence of any impurities in samples under study and reflecting their high purity. The high intensity of reflection peaks proved the high crystallinity of CuI synthesized by methods. Moreover, by applying modified Scherrer equation we obtained the average particle size which is in rage from 25 nm to 56 nm. On the other hand by using by applying Williamson- Hall formula we calculate the average strain in the sample which in range 0f 0.11 to 0.26Furthermore, transmission electron microscope confirmed that the prepared CuI samples are in the nanoscale range where three samples (t26, w29 and t55) were choice for TEM analysis. The average diameter of the observed particles was given as 9 nm, 13 nm and 19 nm for samples t26, w29 and t55, respectively. These values not only confirm the formation of the prepared CuI samples in the nanoscale, but also reflect the same sequence of the calculated particle size by XRD analysis.
The absorption spectrum of prepared CuI samples shows low absorption in the visible and NIR regions and high absorption in UVA region. This is expected for wide band gap nature of CuI and high transmittance in visible region which required for solid state electrolyte. By applying Davis-Mott equation we found the transition is direct and from the linear extrapolation of values for each sample to zero absorption we determined the optical band gaps. The obtained optical band gaps were in range 3.36 to 3.52 eV. These values of energy gaps reflect the quantum size effect. Moreover, we employed Brus equation to calculate the particle size from the optical data. The calculated particle sizes are agreed with TEM values.
After employing CuI as a solid state electrolyte in SSDSSC, we took images by SEM technique to investigate the diffusion of CuI in pores of TiO2 and to determine the thickness of FTO, compact layer, TiO2 paste, and CuI layers. from the photoanode image we determine the average thickness of FTO, compact layer and paste that were 0.25 μm, 0.25 μm and 15 μm respectively. On the other hand, the effect of TMED and particle size is obvious on the penetration process of CuI within TiO2 pores.
The current-voltage measurements showed that using TMED as a crystal growth inhibitor increased the efficiency ten-fold more TMED free cell. Furthermore, particle size of CuI samples appears on the cell parameters that the cell with small particle size shows high current and also high efficiency Also, the thickness of CuI layer studied where efficiency enhanced when thickness increased which made a good contact between CuI and counter electrode. This dependence of particle size on cell parameters was found to agree with SEM images.
Finally, from open-circuit voltage decay curves we able to extract information about recombination mechanism in SSDSSCs.
According to Bisquert et al. we found all three regions which indicates that the recombination mechanism exist. The first region (at high Voc) has a constant life time indicates recombination via free electrons in the conduction band of TiO2. The second region (at mid Voc) indicates trapping and detrapping of electrons in the bulk trap state. The third region of inverted parabola (at the low Voc) indicates trapping via surface states. Also, from life time-Voc curves for T26-and T55-based SSDSSCs we found that the life time of T26 is smaller than T55-based cell which agree with Rsh values.