Development of new materials is the major driving force of the technological evolution. During the last two decades immense progress has been achieved in the fabrication and manipulation of nanoscale crystallites and manipulation of the nanoscale crystallites and understanding of the size and shape dependent properties of the nanomaterial’s [1,2].

Taking this fact into consideration, CIS based chalcopyrite thin films have found potential interest in the field of solar energy due to its absorbing property. The CuInSe₂ thin films possess characteristic properties such as high absorption coefficient, tunable band gap and high stability. Among the I-III-VI ternary chalcopyrite’s CuInSe₂ thin films have a calculated exciton Bohr radius of approximately 10.6 nm and have a lowest band gap of 1.04 eV with largest absorption coefficient. It has less toxicity than other semiconductor nanomaterial’s currently being used [3-6].

The electronic structure of these materials strongly relate with the [Cu]/[In] ratios there is a great need to precisely control their size, shape, surface and composition. Nowadays, great success has been achieved in the synthesis of ternary, quaternary nanomaterials via several routes. The methods for the synthesis are solvothermal approach, hot injection method, thermal decomposition, chemical bath deposition, electrode position etc. [7-11].

Considering all these facts we are reporting here a low cost arrested precipitation technique for the deposition of CuInSe₂ thin films. Further the optical, structural, morphological and compositional properties of CuInSe₂ thin films have been investigated.

All the chemicals were analytical reagent (AR) grade and used without further purification. The list of chemicals used to deposit CuInSe₂ thin films is given in Table 1. Herein the depositions of CuInSe₂ thin films were carried out onto glass substrates as well as FTO using APT at room temperature.

Table 1: Chemicals used to deposit CuInSe₂ thin films.

Deposition of CuInSe₂ thin films

The 0.05 M Cu-TEA and 0.05 M In-TEA complexes were used as a source of Cu²⁺ and In³⁺ ions respectively. They were prepared by dissolving the [CuSO₄.5H₂O] and In₂(SO₄)₃ salts separately with TEA complexing agent. A 0.25 M Na₂SeSO₃ solution was prepared by dissolving 99% pure selenium metal powder in saturated solution of Na₂SO₃ by refluxing the whole solution for 8 h at 60°C and used as a source of Se²⁻ ions. In order to deposit CuInSe₂ thin films an 8 mL of 0.05 M Cu-TEA and 2 mL of 0.05 M In-TEA complex solutions were taken in reaction bath container. The pH of the reaction mixture was adjusted to 9.5±0.5 by drop wise addition of aqueous ammonia. Then 10 mL of 0.25 M Na₂SeSO₃ solution was added to the reaction mixture with constant stirring. The total volume of reaction bath was made to 40 ml by adding double distilled water. The reaction mixture was stirred for 10 minutes on a magnetic stirrer to get homogeneous solution. The precleaned glass substrates were immersed vertically in the reaction bath and then kept at room temperature without stirring for 3 hrs. When the terminal growth is over films withdrawn from reaction bath and designated as CuInSe₂. In detailed optimized parameters are given in Table 2.

Table 2: Optimized preparative parameters for the deposition of CuInSe₂ thin films.

Characterizations of CuInSe₂ thin films

The synthesized thin films were characterized for their thickness, optical, structural, morphological, compositional properties. Thickness of the films was measured by using Ambios XP-1 surface profiler. The absorption spectrum was recorded in the range of 400-1100 nm by using a UV-Vis-NIR spectrophotometer (Shimadzu UV-1800). XRD patterns were obtained using X-ray diffractometer (Bruker AXS Model D8 Advance) in the 2θ range 20-80° with Cu Kα as a target.

The surface morphology of films was studied using scanning electron microscopy (SEM-S-4700, Hitachi). Chemical compositions of synthesized thin films were studied by energy dispersive X-ray spectroscopy (EDS) technique attached to SEM equipment.

Growth and reaction mechanism of CuInSe₂ thin films

The controlled and slow release of ions is the main beauty of APT. Hence we have used a TEA as complexing agent. The Na₂SeSO₃ solution added to the cationic alkaline reaction mixture play dual role as a source of Se²⁻ ions as well as a reducing agent due to presence of excess Na₂SO₃. At alkaline pH, Na₂SeSO₃ hydrolyses into Na₂SO₄, follows base catalyzed reaction and release Se²⁻ ions into the solution as per the reactions (1) and (2)

(1)

(2)

The excess Na₂SO₃ oxidises into Na₂SO₄ and releases electrons concomitantly, which are responsible for the reduction of Cu⁺² into Cu⁺¹ states during the deposition of CuInSe₂ and according to reactions (3) and (4)

(3)

(4)

The APT is based on the principle of Ostwald ripening law. Initially, there is a formation of seed nuclei take place. These nucleation centres form a monolayer on a substrate by ion by ion condensation. The number of smaller crystallites comes together to form a larger crystallites in accordance with the Ostwald ripening law.

Following chemical reactions (5) and (6) are proposed for the formation of CuInSe₂ thin films in this investigation:

Optical study

The absorption spectra of CuInSe₂ thin films are recorded in the range of 300 nm to 1100 nm on a UV-Vis spectrophotometer. The absorption spectra and band gap is shown in the Figure 1. The absorption is in the visible range of solar spectrum. The band gap of a thin film is determined by using the following equation,

Figure 1: TEM and SAED images of Chitosan magnetite nanocomposites CMN particles.

(7)

Where, ‘h’ is Planck’s constant, ‘E_g’ is Band gap of the material, ‘A’ is Parameter depends on the transition probability, ‘n’ is exponent. Exponent ‘n’ may have values such as 1/2, 3/2, 2 and 3 depending on the nature of electronic transition responsible for the absorption of light. The value of n=1/2 for allowed and direct transition, n=3/2 for direct forbidden transition n=2 for indirect allowed transition, n=3 for indirect forbidden transition. The nature of obtained data confirms that CuInSe₂ thin films shows a direct allowed type of transition with a band gap 1.38 eV. Such a band gap is highly beneficial for the solar cell applications.

Structural study

XRD spectra of CuInSe₂ thin films show the peaks of CuInSe₂ with mixed phases of Cu₂Se₃ and In₂Se₃ phases. The XRD pattern of CuInSe₂ thin film is shown in Figure 2. The observed peaks corresponding to the (101), (111), (210), (211), (002) and (311) planes are of Cu₂Se₃ phase, whereas (101), (205), (021) planes in the XRD belong to In₂Se₃. The peak observed (112), (103), (211), (105), (220), (301), (312), (323) and (316) are corresponds to CuInSe₂ (JCPDS file No.81-1936). CuInSe₂ have tetragonal crystal structure with lattice constants a=5.781 Å and c=2.0139Å. The crystallite size of CuInSe₂ thin film samples calculated from well-known Scherer’s formula.

Figure 2: a) Optical Absorption spectra of CuInSe2 thin film. b) Optical band gap from Tauos plot.

From calculated values of crystallite size the dislocation density (δ) and microstrain (ε) for all the samples were determined by using equation 8 and 9 respectively.

(8)

(9)

Where, D is crystallite size, θ is Bragg’s diffraction angle and β is full width at half maximum (FWHM). The obtained values of (δ) and () of CuInSe₂ thin film given in Table 3.

Table 3: Variation of crystallite size (D), dislocation density (δ) and microstrain (ε) CuInSe₂ thin film.

Morphological study

In order to study the morphological aspects of CuInSe₂ thin films, FESEM analysis is carried out. The low resolution and high resolution SEM images show a uniform, adherent, pin hole free thin film formation. The SEM images reveal nanocubic structure for thin films. Such a nanocubic structures are interconnected with each other. The interconnected nanocubic structure provides a better pathway for the conduction of electrons. Hence it is beneficial for the solar cell applications. The Figure 3 shows SEM images of CuInSe₂ thin films.

Figure 3: SEM images of CuInSe2 thin films.

Compositional study

The quantitative analysis of CuInSe₂ thin films is done using EDS analysis. The EDS spectrum shows a three principle peaks for Cu, In and Se respectively. The observed atomic percentage of CuInSe₂ thin films is in well agreement with the expected atomic percentage. Hence the presence of Cu, In and Se in the synthesized thin film is confirmed by EDS. The EDS spectrum is shown in the Figure 4.

Figure 4: EDS spectrum of CuInSe2 thin films.

The nanostructured CuInSe₂ thin films are successfully synthesized by APT at room temperature. The opt structural, morphological, compositional properties of CuInSe₂ thin films are studied. The optical study reveals a band gap 1.38 eV. The X-ray diffraction study confirms nano crystalline nature of CuInSe₂ thin films. The SEM images reveal uniform nanocubic thin film formation. The presence of Cu, In and Se in the synthesized thin film is also confirmed by EDS. All these parameters reveal that CuInSe₂ thin films are beneficial for the solar cell applications.

Today scientists all over the world are working on the solar cell technology. The chalcopyrite and chalcogenide thin films are considered as potential candidates for the solar cell applications. The chalcopyrite thin films possess characteristic properties such as high absorption coefficient, tunable band gap, high stability etc. Considering all these facts we are interested in the synthesis of CuInSe₂ thin films. Here we have used a simple and low cost arrested precipitation technique for the synthesis of CuInSe₂ thin films. CuInSe₂ thin films is the most widely studied material for their optostructural and optoelectronic properties. Hence, we have done the characterization of CuInSe₂ thin films to know their optical, structural, morphological and compositional properties.

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