Effect of composition on the optical and electrical conductivity of CuIn(SexS1-x)2Abstract Optical and electrical conductivity of chalcopyrite CuIn(SexS1-x)2 (0 ‰¤ x ‰¤ 1) amorphous thin films are investigated. The transmission has been measured in a wavelength range (200 ‰¤” ‰¤ 2500 nm). The calculated optical energy band gap Eg and Urbach tail (band tail width) Eu decrease with increasing Se content. Refractive index, n, is calculated according to Wemple- DiDomenico single oscillator model. The oscillator energy (E—), dispersion energy (Ed), and the ratio of free carrier concentration to electron effective mass (N/m*) are calculated from optical data for all thin films.
Also, high frequency dielectric constant at infinite wavelength (µ€ћ), lattice high frequency dielectric constant (µL) and static refractive index (n—) are calculated. The electrical conductivity is investigated at room temperature.Key words: CuIn (SexS1-x)2, Amorphous thin films, optical properties, electrical properties.Introduction CuIn(SexS1-x)2 chalcopyrite semiconductors are wide band gap compounds and it is a promising material for photovoltaic and solar cell applications.
There are several methods for the preparation of this compound; such as solid state reaction at high temperature, Bridgman method, electrodeposition, spray pyrolysis, sputtering, coevaporation and organometallic precursors [1-8]. Also, CuInS2 is one of the promising chalcopyrite type semiconductors. CuInS2 has a direct band gap 1.53 eV which it can be suitable for photovoltaic’s application [9]. Moreover, CuInSe2and CuInxGa1-xSe2 chalcopyrite thin films have high potential for terrestrial and space applications [10]. On the other hand, CuIn(SexS1-x)2 thin films are considered as solar energy conversion material and have efficiencies up to 19.2% [ 11,12].
The aim of this work is investigating the effect of Se content on optical parameters of amorphous thin films, calculations of free carrier concentration to electron effective mass (N/m*), high frequency dielectric constant at infinite wavelength (µ€ћ), lattice dielectric constant (µL), and static refractive index (n—) from the optical data. Also, investigating the electrical conductivity of amorphous thin films at room temperature.3. Results and discussion3.1. Composition analysis CuIn (SexS1-x)2 thin films of compositions ( 0‰¤ x ‰¤ 1) were examined by energy dispersive X- ray analysis (EDAX). Calculated contents of Cu, In, Se and S wt% were comparable with wt% of the starting materials. Fig.1 displays representative EDAX curve of CuInSe2. The results of (EDAX) for CuInSe2 thin film are shown in table 1. The thin films have approximately stoichiometric compositions as obvious from table1. The obtained data for all compositions revealed an excess of In.3.2. Structural The structure of both powder and thin films were investigated by X-ray diffraction. Fig.2 represents XRD pattern of powder and thin films evaporated at room temperature for CuInSe2 as example. It is clear that from this pattern the thin films are amorphous in nature. The powder diffraction peaks was investigated by an ICCD database [13]. It was found that the compound matched with the card no.00-040-1487 of CuInSe2 tetragonal phase and no secondary phase. 3.3. Optical properties The optical transmittance spectra were measured in the wavelength range 200-2500 nm by using UV-Vis spectrophotometer. Fig.3 shows the transmission spectra of CuIn (SexS1-x)2 thin films with different compositions (0 ‰¤ x ‰¤ 1). The interference phenomenon is responsible for the variation of transmission [14]. The absorption coefficient ± was estimated from optical transmission data (T). The curve of absorption coefficients ± vs. photon energy h… of thin films with different compositions is shown in Fig.4. All thin films have a high absorption coefficient in the range (104 to 105 cm-1) and increasing with increasing Se content. This value of absorption coefficient is close to the reported values [15-18]. The absorption coefficient can determine the nature of electron transition if the values of absorption coefficient are low (±
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