Study of Physical Properties of Chloro Aluminum Phthalocyanine Thin Films and their Photovoltaic Applications

Abstract

Organic semiconducting materials are of particular interest, since they possess a prosperous optoelectronic, electrical and processing properties for designing and fabrication of electronic devices [1]. Among these materials, a series of phthalocyanines represent a large family of heterocyclic conjugated molecules with high chemical stability. Phthalocyanines as a class of organic materials are generally thermally stable and can easily be deposited as thin films with high quality by thermal evaporation without dissociation. Metal phthalocyanines (MPc’s) have gained considerable attention in recent years because they have been successfully applied in many applications such as gas sensors [2, 3], solar cells [4– 6] and light emitting diodes [7, 8]. Optical absorption studies of MPc’s thin films have attracted the researchers over the last few years [9, 10]. Relatively few studies have focused on the halogenated MPc’s although there is evidence that they may exhibit properties suitable for gas sensing applications [11]. It has also been shown that the halogenated Pc’s exhibit remarkable morphological and thermal stability over a larger temperature range compared to unhalogenated Pc’s [12]. One of the most halogenated MPc’s derivatives is aluminum phthalocyanine chloride (AlPcCl), which is the focus of our study. It has a chemical formula of (C32H16AlClN8). Thin films of aluminum phthalocyanine chloride (AlPcCl) were fabricated, with different thicknesses (109–518 nm) using thermal evaporation technique. Structural and optical properties of AlPcCl thin films were investigated. The material in powder form showed a polycrystalline nature with monoclinic structure. Miller indices, hkl, values for each diffraction line in X-ray diffraction (XRD) spectrum were calculated. The field emission scanning electron microscope (FESEM) showed the nanostructure property of the as- deposited thin films. The optical properties of the thin films were studied by the spectrophotometric measurements of the transmittance, T, and the reflectance, R, at the normal incidence of the light in the spectral range 200–2500 nm. The refractive and absorption indices of the thin films were calculated and were found to be independent on film thickness in the range 109-518 nm. The films exhibited indirect allowed inter-band transitions. The optical band gaps transitions were calculated as 1.42, 2.87 and 3.69 eV, respectively. Also, different dispersion and absorption parameters were determined for thin films of AlPcCl. AC conductivity and dielectric behavior for bulk aluminum phthalocyanine chloride (AlPcCl) have been studied over a temperature range (303– 403K) and frequency range (42–106 Hz). The frequency dependence of ac-conductivity (ω) has been investigated by the universal power law (ω) = Aωs. The variation of the frequency exponent (s) with temperature was analyzed in terms of different conduction mechanisms, and it was found that the correlated barrier hopping (CBH) model is the predominant conduction mechanism. The barrier height was calculated by using (CBH) model, and was found to be 1.41 eV. The temperature dependence of (ω) showed a linear increase with the increase in temperature at different frequencies. The density of states N ( ) was calculated to be equal 4.11 x 1019 cm-3 using Elliott model. The ac- activation energy was determined at different frequencies. Dielectric values were analyzed using complex permittivity and complex electric modulus for bulk AlPcCl at various temperatures.