الفهرس 
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Abstract
Organic semiconductors are a recent class of electronic materials, considered as perspective materials for low cost organic electronics. During the last two decades, they have attracted considerable attention of scientists who working in both fundamental and applied research. The aim of this work is to study the structural, optical and electrical transport properties of p-N,N-dimethylaminobenzylidenemalononitrile (DBM) organic dye in its bulk and thin film forms with the purpose of using it in the fabrication of solar cells.
DBM compound was synthesized by the condensation reaction. The synthesization was examined by the elemental analysis of the carbon, hydrogen and nitrogen (CHN) atoms and by proton-nuclear magnetic resonance (1H-NMR) measurements. The thermal properties of this compound were studied using differential scanning calorimetery (DSC) and thermogravimetric analysis (TGA) methods. The DBM compound is thermally stable up to 445 K and has a melting point at 458 K.
X-ray diffraction (XRD) pattern for DBM compound in its powder form reveals that this material has a polycrystalline nature with the crystal structure of triclinic system with P1 ̅ space group and the lattice parameters are given as: ; a=9.38 Å, b=11.45 Å, c=20.55 Å, α=61.09^0, β=91.91^0 and γ=121.25^0. Miller indices, hkl, for each diffraction peak were calculated using a computer program.
The molecular structure of DBM compound was studied theoretically and experimentally. Two levels of theories with two basis sets were used for this purpose. The calculated and observed vibrational wavenumbers in the spectrum were assigned to different modes of the DBM molecule.
Thin films of DBM compound were prepared using thermal evaporation technique. The molecular structural of these films was investigated by Fourier-transform infrared (FT-IR). FT-IR spectra of as-deposited and annealed film at 423 K for one hour reflected the thermal and chemical stability of DBM compound.
Thermal evaporation of DBM compound led to nano-crystalline films. The effect of annealing process on the nature and degree of crystallinity has been investigated. It was found that the crystallite size increased after annealing.
The electronic transitions of DBM thin films were investigated by studying the ultraviolet/visible (UV/Vis) spectra; some important optical parameters such as molar extinction (ε_mol), the oscillator strength (f) and the electric dipole strength (q^2) were also reported. Optical properties were investigated for as-deposited and annealed DBM thin films by using spectrophotometric measurement of transmittance and reflectance at normal incidence of light in the wavelength range of 240-2500 nm. The obtained spectra showed that the DBM thin films are transparent above λ=820 nm. The optical constants (refractive index (n) and absorption coefficient (α)) are decreased by annealing process.
The analysis of the optical spectral behavior of the absorption coefficient (α) in the fundamental absorption region was performed by applying the energy band theory of solids and revealed direct allowed transitions with a value of optical gap of 1.54 eV which doesn’t affected by annealing process. The optical dispersion parameters have been analyzed by single-oscillator and Drude models. The values of oscillator energy (E_0) and dispersion energy ( E_d) were found to be 4.47 and 9.63 eV, respectively for as-deposited film, and 3.40 and 8.57 eV, respectively for annealed film.
The hot point probe technique showed that the DBM material is a p-type semiconductor one. The dark electrical resistivity of DBM thin films were measured as a function of the film thickness. Using Tellier’s model, the values of the mean free path and the bulk resistivity of DBM film were found to be 1122 nm and 5.02×〖10〗^6 Ω .cm, respectively.
The temperature dependence of the electrical conductivity of DBM thin films was measured in the temperature range 320-420 K. It was found that the conduction is thermally activated process having one conduction mechanism with the average value of the activation energy to be 0.97±0.1 eV. The average value of this activation energy after annealing the DBM thin films is equal to 0.98±0.1 eV. The value of the activation energy is less than the half value of the estimated optical energy gap (HOMO-LUMO gap) of DBM material. This finding suggested that the conduction process through the DBM material is extrinsic conduction. It is also clear that the non variation in the value of ∆E after annealing process means the thermal stability of DBM material, and also the increase in the degree of crystallinity isn’t enough to make a change in it.
The current density-voltage (J-V) characteristics of Au/DBM/Au sandwich structure thin films of different thicknesses ranging from 114 to 358 nm have been investigated. At lower voltage region, the current conduction in these samples obeys Ohm’s law, while at higher voltage region, the charge transport phenomenon appears to be space-charge-limited current (SCLC) dominated by a discrete deep level located at 1.04 eV with the trap concentration of 1.31×〖10〗^21 cm^(-3). In addition, also various electrical parameters were determined.
The electrical and photovoltaic characteristics of thermally evaporated DBM thin film sandwiched between indium tin oxide (ITO) and aluminum (Al) electrodes has been investigated. The forward J-V characteristics indicated a thermionic-emission conduction, at lower voltage region, followed by a SCLC, at relatively higher voltage region, which dominated by a discrete deep level located at 0.45 eVabove the valence band having a concentration 3.28 ×〖10〗^19 cm^(-3). The reverse bias curves are interpreted in terms of Pool-Frenkel effect. The capacitance-voltage (C-V) characteristics at room temperature indicated that the junction is of abrupt nature. Under illumination, the junction exhibits photovoltaic characteristics when it was illuminated at room temperature through Al electrode. The typical photovoltaic parameters were estimated at room temperature and under illumination with an input power of 60 mW.cm-2. These parameters are the open-circuit voltage of 0.42 V, saturation current density of 70 nAcm-2 and fill factor of 0.32.
However, hybrid organic/inorganic solar cell was fabricated by thin film of DBM material deposited on positive type silicon (p-Si) substrate. The forward J-V characteristics indicated a thermionic-emission conduction, at lower voltage region, followed by a SCLC, at relatively higher voltage region, which dominated by a discrete deep level located at 0.11 eV above the valance band having a concentration 1.29 ×〖10〗^20 cm^(-3). The reverse bias curves are interpreted in terms of Pool-Frenkel effect. The capacitance-voltage (C-V) characteristics at different temperatures indicated that the junction is of abrupt nature. Under illumination, the junction exhibits photovoltaic characteristics when it was illuminated at room temperature through Au electrode. The typical photovoltaic parameters were estimated at room temperature and under illumination with an input power of 60 mW.cm-2. These parameters are the open-circuit voltage of 0.23 V, saturation current density of 30.89 mA.cm-2 , fill factor of 0.39 and a power conversion efficiency of 4.58 %.