Synthesis and Characterization of CuIn 2 n + 1 S 3 n + 2 ( with n = 0 , 1 , 2 , 3 and 5 ) Powders

CuIn2n+1S3n+2 crystals were synthesized by horizontal Bridgman method using high purity copper, indium, sulfur elements. The phases and crystallographic structure of the CuIn2n+1S3n+2 crystals were analyzed by X-ray diffraction (XRD) and the composition of the material powders was determined using the energy dispersive X-ray analysis (EDX). Measurement data revealed that CuIn2n+1S3n+2 materials have not the same structure. In fact, CuInS2 and CuIn3S5 crystallize in the chalcopyrite structure whereas CuIn5S8, CuIn7S11 and CuIn11S17 crystallize in the cubic spinel structure.


Introduction
Developments of thin film solar cells based on CuInS 2 and related alloys have made considerable progress in recent years.Copper indium sulfide thin films are one of the most promising absorber materials in solar cells because he has a high optical absorption coefficient (10 5 cm −1 ), and an optical band gap of 1.5 eV [1] and that's why CuIn 2n+1 S 3n+2 materials attract much attention.In addition, the materials do not contain any toxic elements such as Ga or Se, and this may have an advantage in comparison with other ternary materials like CuIn 2n+1 Se 3n+2 and CuGa 2n+1 S 3n+2 .They belong to I-III 2n+1 -VI 3n+2 ternary materials which are receiving a great deal of attention as candidate materials for visible-light and IR emitters, high-efficiency solar cells, and other semiconductor and quantum-electronic devices [2].Many researchers tried to synthesize CuInS 2 material by various methods because of its important properties, a new green synthesis is described by some authors without using any organic solvent [3], a new strategy has been presented to the controllable synthesis of CuInS 2 hollow nanospheres based on the Cu 2 O solid nanospheres as the precursor in the absence of any surfactant [4], a facile and low-cost method was developed to prepare metastable wurtzite copper indium sulfide (CuInS 2 ) nanocrystals under atmospheric conditions [5] and luminescent CuInS 2 nanocrystals were synthesized in dodecanethiol precursors [6].I-III 2n+1 -VI 3n+2 ternary materials are called ordered vacancy compound (OVC).The formation of the OVC compound CuIn 3 Se 5 has already been explained as due to the presence of a single pair of the defect complex ( ) in every five units of CIS [7].In the present study, we have investigated the structural properties of CuIn 2n+1 S 3n+2 powders synthesized by the horizontal Bridgman method.

CuIn 2n+1 S 3n+2 Materials
In order to understand the formation of ternary compounds with chemical formula CuIn 2n+1 S 3n+2 , their phase equilibrate can be discussed in terms of temperature or composition.These compounds stabilize due to the ordering of the neutral defect pairs ( ) in the CuInS 2 phase and this is due to its huge tolerance to off-stoichiometry [8].In our knowledge, few papers [9] dealing with the physical properties of CuIn 2n+1 S 3n+2 compounds have been reported but not much is known about the fundamental properties of this system.The ternary compositional triangle is the basis for analyzing the composition phase behavior of these materials.In Figure 1, a schematic ternary diagram for CuIn 2n+1 S 3n+2 compounds is shown.This ternary diagram can be reduced in a pseudo-binary diagram along the interconnection line between Cu 2 S and In 2 S 3 binary materials.Indeed, by combining these two compounds, we can obtain all materials belonging to the family with chemical formula CuIn 2n+1 S 3n+2 with n = 0, 1, 2, 3 and 5.The bold points along the line connecting Cu 2 S and In 2 S 3 represent these materials.been prepared by Bridgman horizontal method growth.High purity elemental materials of copper, indium and sulfur (Balzers 99.999%) were taken in proportions corresponding to the stoichiometric composition of the compounds CuInS 2 , CuIn 3 S 5 , CuIn 5 S 8 , CuIn 7 S 11 and CuIn 11 S 17 and then loaded into five quartz ampoules.The growth of crystals was carried out in ampoules (20 cm in length with thickness 2 mm), that were pre-cleaned by chemical etching in concentrated acid HF, washed in distilled water then with acetone, and finally, dried in oven at 150˚C during 30 minutes.The ampoules were evacuated down to 10 −5 mbar and were sealed off.The sealed ampoules containing the pure elements were placed into a horizontal position in programmable furnace (Nabertherm-Allemagne).For the synthesis, the temperature of the furnace was increased from room temperature to 600˚C with a slow rate of 10˚C/hour in order to avoid explosions due to sulfur vapor pressure (2 atm at 493˚C and 10 atm at 640˚C).The temperature was kept constant at 600˚C for 24 hours.Then, the temperature was increased with a rate of 20˚C/hour up to 1000˚C.
A complete homogenization could be obtained by keeping the melt at 1000˚C for 48 hours.After that, the temperature was lowered to 800˚C at a rate of 10˚C/hour and the furnace was switched off until the tube reached room temperature.Then, the ampoules were removed from the furnace and were broken to retrieve the synthesized ingots.The resulting ingots are opaque and black in color.3 could be indexed as those of CuIn 2n+1 S 3n+2 (n = 0, 1, 2, 3 and 5) with tetragonal chalcopyrite structure for CuInS 2 (space group I-42d [10]) and CuIn 3 S 5 (space group P-42c [11]) and with cubic spinel structure (space group Fd3m [12][13][14]) for CuIn 5 S 8 , CuIn 7 S 11 and CuIn 11 S 17 .This transition in the crystal structure between n = 0 and 1 and n = 2, 3 and 5 in the CuIn 2n+1 S 3n+2 system can be explained by the migration of a part of In 3+ ions towards octahedral sites when the indium atoms increase in the structure.Indeed, the In 3+ ions can be stabilized in both tetrahedral and octahedral sites but tend to form bonding with octahedral coordinations.The spinel structure is favored by increasing the indium content in the CuIn 2n+1 S 3n+2 system [15].We also note that the XRD patterns of all compounds do not contain extra reflections corresponding to the elements or other secondary phases, which confirms the homogeneity of the synthesized materials.The lattice parameters (a) and (c) of CuInS 2 and CuIn 3 S 5 was calculated by using Equation (1) whereas Equation ( 2) was used to calculate the lattice parameter (a) of CuIn 5 S 8 , CuIn 7 S 11 and CuIn 11 S 17 [16].

Structural Characterization
Bragg angle 2θ˚ Bragg angle 2θ˚

Bragg angle 2θ˚
Bragg angle 2θ˚ Bragg angle 2θ˚ (where θ is Bragg angle) is calculated and the Nelson-Riley plot is represented for different reflections.In this method, the value of lattice parameter is determined by extrapolating Nelson-Riley functions to f(θ) → 0.
where d is interplanar spacing determined using Bragg's equation and h, k, l are the miller indices of the lattice planes.The corrected values of lattice parameters are estimated from Nelson-Riley [17] method.Consequently, Nelson-Riley function [18]:   1 cos cos ( ) 2 sin f

EDX Results
The atomic ratios of Cu, In and S elements and the chemical composition of the prepared powders have been determined using the energy dispersive X-ray analysis (EDX).The EDX analysis is made at several zones of the powders in order to obtain an average atomic concentration.The atomic ratios of the elements and the composition of powders are presented in Table 2.The uncertainty of the present measurements is about 5%.As is seen in Table 2, the compositions of CuIn 2n+1 S 3n+2 (n = 0, 1, 2, 3 and 5) powders is fairly close to the ideal theoretical values of the starting composition.We also note that all powders were deficient in sulfur.

Conclusion
In summary, CuIn 2n+1 S 3n+2 (n = 0, 1, 2, 3 and 5) materials were successfully synthesized using the horizontal Bridgman method.The XRD spectra of the powders indicate that the CuIn 2n+1 S 3n+2 powders can be formed in different structures.Indeed, for n = 0 and 1, the powders crystallize in the chalcopyrite structure with the preferential orientation along 112 plane.On the passage to n = 2, 3 and 5, CuIn 5 S 8 , CuIn 7 S 11 and CuIn 11 S 17 powders crystallize in the spinel structure with the preferential orientation along 311 plane.The compositions of CuIn 2n+1 S 3n+2

17 Figure 1 .
Figure 1.The ternary system Cu-In-S including the pseudo-binary section Cu 2 S-In 2 S 3 .

Figure 2 .
Figure 2. Photographs showing the CuIn 11 S 17 ingot.comparing the d-spacing with Joint Committee on Powder Diffraction Standard (JCPDS) data files.The composition of powders was determined by means of energy dispersive X-ray analysis (EDX) by a JEOL 6700F equipment which uses K-ray for Cu and L-ray for In and S as standards.

Figure 4
Figure 4 represents the Nelson-Riley plots for CuInS 2 and CuIn 7 S 11 powders.The calculated values of lattice parameters were collected inTable 1.
Figure 4 represents the Nelson-Riley plots for CuInS 2 and CuIn 7 S 11 powders.The calculated values of lattice parameters were collected inTable 1.

Figure 4 .
Figure 4. Nelson-Riley plots for calculation of corrected lattice parameters of CuInS 2 and CuIn 7 S 11 powders.
by EDX measurements and all powders are deficient in sulfur.