This report demonstrates colloidal route synthesis of the Cu24In16Ga4Se56 and Cu24In16Ga7Se53 nanocrystals at temperature ~ 1500C. Materials crystallographic phase formations analysis is performed using the X-ray diffraction, micro-morphology from the Transmission Electron Microscopy and stoichiometric homogeneity by the energy dispersive X-ray mapping. Modification in the bonds feature and the chemical environment are studied with the X-ray photoelectron spectroscopy. The optical properties of the materials are described with the help of UV/Visible, FT-IR transparency and photoluminescence spectroscopy. The synthesized nanocrystals/ particles size are obtained around ~ 40 nm to ~ 60 nm. The optical energy band gaps are evaluated 1.30 eV and 1.39 eV for the Cu24In16Ga4Se56 and Cu24In16Ga7Se53 compositions. A higher order photoluminescence is noticed for the Cu24In16 Ga 7 Se53 composition, while the FT-IR transparency is unchanged in these materials.
Keywords: CIGS; Sol gel; Nanomaterials; Semiconducting materials; Crystal morphology; Optical properties.
More than two decades, chalcogenide based photovoltaic (PV) modules have been extensively studied due to cost reduction and high efficiency performance [1-4]. To achieve a reliable, low cost and high performance PV modules investigators have been paid attention toward nanocrystalline CuInGaSe (CIGS) materials . Colloidal CIGS nanocrystals have also attracted much attention due to the tunability of the optical band gap and multiple electron-hole pair formation. These two critical parameters may play a crucial role to improve solar cell performance. To produce CIGS nano crystals, investigators preferred to mix Cu, In, Ga and Se elements in intended stoichiometric ratio with the high boiling point solvent oleylamine (OLA) . This gives inorganic nanocrystals readily dispersed within the ligand organic solvent and the final product is extracted from it. Nanocrystals produced this way can have a smaller grain size and relatively larger inter- interplaner spacing owing to presence of organic capping ligand on the surface. However the smaller grain size and larger interplaner spacing can limit high density of electrons and holes in the material. Therefore an appropriate growth of the nanocrystal-grains is a crucial parameter to get a higher power conversing from the CIGS material.
Over 15 % efficiency has been reported for the hydrazine synthesized CIGS nanocrystals modules . It was demonstrated that the CIGS nano crystals synthesized from this route can be an efficient absorber material with promising photovoltaic features. But the use of highly toxic and explosive hydrazine restricts their commercial utilization. S.H. Mousavi et al.  had demonstrated the colloidal CIGS nanocrystals around 20-40 nm size with the purity upto 99%.They were commented that choice of precursor can play an important role in CIGS pure phase formation. The metallic source precursors led to the formation mixed phases. While the chloride precursor (except selenium) could have less impurity phases in CIGS material. Thus the impurity mainly reflects from the copper selenide phases due to the fast reaction rate between copper with selenium [9-11].
Therefore, CIGS nanocrystals can also synthesized by using various solutions based techniques such as microwave assisted synthesis, solvo thermal synthesis , hot injection route , mechano chemical synthesis , green synthesis , modified polyol route , ambient pressure diethylene glycol based solution process , low temperature colloidal process [10-11].Here we have synthesized Cu24In16Ga4Se56 and Cu24In16Ga7Se53 (CIGS) nanocrystals with the aim to see the effect of gallium and selenium alloying concentrations on the nano confined crystals size and their physical properties. The synthesized nanocrystalline Cu24In16Ga4Se56 and Cu24In16Ga7Se5353 materials physical properties are studied using the X-ray powder diffraction (XRD), Field Emission Scanning Electron Microscope- Energy Dispersive X-ray Spectroscopy (FESEM-EDS) and Transmission Electron Microscopy (TEM) measurements. The binding energy core levels spectrum is recorded using X-ray Photoelectron Spectroscopy (XPS). The UV-Visible and Fourier Transform Infrared (FT-IR) spectroscopy are used to explain optical properties of the materials.
To prepare the CIGS nanocrystals, colloidal route was adopted under the heating up process in the normal environment. Precursors copper chloride (CuCl2), indium chloride (InCl3), gallium chloride (GaCl3) and selenium (Se) compositional amounts for 2 mg were dissolved into the 15 ml Oleylamine (OLA) solvent. In the beginning it was noticed prepared solution in dark blue colour. But with the slow heating (12 h) temperature upto 160 ℃ (±10) under a continuous stirring it was changed into blackish brown colour. The final solution was allo℃into the solution and again allow for continuing stirring upto next 12 h at a 60 ℃C temperature. Additional methanol and water were formed a gel like substance within the solution. The formed gel was filtered and washed several times with methanol and water to reduce byproduct impurity. The final product was dried at a 150 ℃ temperature. The obtained product was crushed in the powder form and it used for different characterizations. The different stage colours schematic of the colloidal synthesized CIGS materials is given in Figure 1.
The XRD measurement in the 2θ range 10 - 900 was performed from the Rigaku equipment, whereas, the Cu-Kα radiation ( 1.54 Å) source used. Existence of the alloying elementals and their particles distribution can be verified from the EDS and mapping. The EDS and elemental mapping were performed from the ULTRA 55 Karl Zeiss model equipment. To avoid the external contamination the powder samples were desiccated under at a vacuum 10-2 Torr and it maintained for the 12 h before performed the experiment. Further, to minimized the surface charging effect during the experiment 5 nm gold thin film was deposited on samples specimen using the ultrathin gold coater equipment.TEM measurement was performed from the TECHNI G2 T20 equipment. For the TEM grid preparation small amounts of the samples were dissolved in the 5 ml Dimethylformamide (DMF). The prepared solutions ultra-sonication was performed upto 5 min. A few drops of the ultra-sonicated solutions were put on top of the copper grid. Before performing the TEM experiment copper grid specimens were desiccated for the 24 h under at a vacuum 10-2 Torr. The XPS characterization was performed from the high energy resolution AXIS ULTRA-165 instrument. To produce the photoelectric effect 12 h desiccated samples were irradiated with a low -energy (~1.5 keV) X-rays under a ultra high vacuum 1010 torr. The core energy levels spectrum of the emitted photoelectrons was recorded by the high-resolution spectrometer.
UV/Visible and reflectivity spectroscopic measurement was performed from the Sepctro S-600 equipment. The fine powder of the materials was kept carefully in sample holders and recorded the UV/Visible absorption spectra in the wave length range 200 to 1000 nm under a reflectance mode. The PL characterization was performed in a wave length range 330 to 900 nm from the LabRAM HR equipment. The 325 nm wave length Argon LASER was used, whereas, the charge coupled detector (CCD) in backscattering geometry. Spectra were recorded with the resolution 0.5 cm-1. For the FT-IR measurement fine powders of the samples and KBR chemical were mixed into the ratio ~5: 95. Then 2.0 mm thick pellets were made under at a 4 ton load. The FT-IR measurement was performed in a wave number range 400 to 10000 cm-1 in the transmission mode by using the Perkin Elmer Spectrum GX. Spectrum was collected with a resolution of 4 cm-1 in interval of 1 cm-1.
Crystallographic structure of the described materials is exhibited in Figure 2 (a, b). XRD pattern of the Cu24In16Ga4Se56 material exhibits the mixed phases of copper indium gallium selenide (CuInGaSe) and copper selenide (CuSe2). The peak at 2θ (26.760) is representing CIGS chalcopyrite phase along the  plane. With other relatively weak mixed phase peaks at 29.828, 33.1201, 34.0795, 38.7825, 41.9618, 44.4262, 46.5649, 52.5718, 56.9174 and 60.6789. These weak peaks might be arises due to inhomogeneous phase mixing of the copper selenide. Figure 2 b represents XRD pattern of the Cu24In16Ga7Se53 material. The prime CIGS characteristic peak is appeared at 27.00 along the crystallographic plane . Other weak characteristic peaks are also appearing with the noticeable feature the main CIGS crystallographic peak  shifted toward higher 2θ value for the Ga 7% containing alloy. The higher intensity of the prime peak could be correlated to the improved crystalline behavior of the material. However, weak peaks at 2θ value 30.994, 45.8559 can be correlated with copper selenide secondary phase.
Materials micro-morphology can be visualized from the TEM analysis. TEM image of the Cu24In16Ga4Se56 is showing non uniform agglomerated nanocrystals (see Figure 3 (a)) with the distinguishable grain boundaries. While, the relatively smaller crystal size and high order agglomeration is appeared (See Figure 3 (b) ) for the Cu24In16Ga7Se53 composition. This might be due to higher order phase mixing of the alloying elements within the configuration. The TEM image analysis gives the overall particle size around 40 to 60 nm.
Moreover the alloying elemental presence and their distributions are demonstrated from the EDS and mapping. Figure 4 represents the EDS patterns and their elemental mapping for the Cu24In16Ga4Se56 composition. Existence of the every alloying elemental peak in their compositional ratio (± 2 %) reveals the appropriate material configuration.
The corresponding elemental mapping in surface cross sectional area 10 μm is demonstrating their distributions throughout the configuration. This composition mapping analysis gives the element copper and selenium particle distributions denser and uneven throughout the surface. The element gallium particles distribution is also relatively rarer with the uneven distribution. While, the element indium distribution seems to homogeneous throughout the surface area with the lest dense particle distribution appearance. It could be due to its high order reactivity and diffusivity with other alloying elements
On other hand Figure 5 is representing EDS pattern and elemental mapping for the Cu24In16Ga7Se53 composition. EDS pattern is showing presence of the alloying elements in their compositional ratio (± 2%).While the alloying elemental distribution for this composition is exhibiting relatively higher order diffusion and inclusion within the configuration. Predominantly the non metallic host selenium, metallic copper and semi metallic gallium homogenous particle distribution throughout the surface area reveals their high order reactivity to each other. This mapping result can also infer in terms of the high order reactivity of selenium and copper with the gallium, while, the indium can have a highest reactivity with the individual alloying element. Therefore, element indium particles distribution is fewer than other alloying elements even though less compositional amount of the gallium. Thus the EDS elemental mapping evidence is demonstrated the higher order elemental phase mixing for the Cu24In16Ga7Se53 than the Cu24In16Ga4Se56 nanocrystals.
The XPS measurement has been performed to know the impact of the alloying elements concentration on the core energy levels. Figure. 6 (a,b,c,d) is representing XPS spectra of the copper, selenium, gallium and indium core shell electrons bonding energies for the under test materials. The Cu24 In16 Ga7Se53 nanocrystals (See Figure 6 a) exhibits the Cu 2p core energy level splits into 2p3/2 (929.8 eV) and 2p1/2 (949.6 eV) two peaks having a shift toward higher binding energy side containing low count value. However, selenium broad strong 3 d5/2 core energy level peak is observed at 52.49 eV, with a noticeable counts reduction for the Cu24In16Ga7Se53 composition (See Figure 6 b). The valance states for the selenium can be accessed with the help of past reports . This XPS outcome is also supporting high order diffusion /or inclusion of the selenium within the alloys configuration. The unaltered indium core energy level peaks 3d5/2 and 2p3/2 at 442.3 and 449.9 eV (with a low counts value) is appeared for the Cu24In16Ga7Se53 (See Figure 6 c) composition. Moreover, the core energy levels binding energies at 1115.6 and 1143 are belongs to gallium 2P3/2,1/2 states. The gallium core energy level peak at 1143 is disappeared in the Cu24In16Ga4Se56 composition, while, it can easily recognize in Cu24In16Ga7Se53 composition XPS profile (See Figure 6 d). The XPS core energy level peak analysis of the materials has been performed with the reference of carbon (C1) peak ±2.
The optical energy band gap and reflectivity are the crucial parameter for the photovoltaic materials. Therefore, it is significant to define the UV-visible optical absorption and reflection properties. The obtained UV-visible absorption spectra (solid line) and corresponding Lorentzian fit (circle line) with optical energy band gap (Eg) Tuac plot (inside profile) for these compositions is given Figure 7 (a, b). These materials are exhibiting a broad UV-visible light absorbance peaks in the range 400 to 850 nm. The higher order light absorbance is observed for the Cu24In16Ga7Se53 in comparison to Cu24In16Ga4Se56 composition. The direct optical energy band gap (Eg) for the materials is evaluated around 1.30 and 1.39 eV [18-20].
Moreover, it is well recognized [21-24] that nano materials overall particle size can also determine from the UVvisible absorption spectrum. Predominantly, it is appropriate to describe materials particle size from this approach those nanoparticles having agglomerated non-homogenous distribution throughout the configuration [21-24]. Cause it reflects overall material property instead of the specified location (or area). Photovoltaic nanocrystalline materials usually do not possess well defined even grain boundaries. Therefore, it is customary to describe on-average particle size with the help of the UV-visible spectrum Lorentzian fit for the developed materials. Obtained Lorentzian fit R-square values are 0.815 and 0.699 for the Cu24In16Ga4Se56 and Cu24In16Ga7Se53 compositions. The obtained other parameters (y0, xc, w and A are in usual meaning) value is given in the table inside the respective figure. The particle size of the material can be directly related to inverse of the R value with the help of well established relationship [25-26]. Using this approach on average particle size values 64 nm and 55 nm are obtained for the described nanocrystalline materials. Here Figure 7 (c) is representing reflectivity measurement result; both materials have exhibited the high order reflectivity in the UV-visible wave length range. This is also consisting with the photovoltaic material essential criteria reflectivity should be low in UV-visible wave length range.
To know the optical quantization behaviuor in these materials, we were performed the Photoluminescence (PL)measurement in the wave length range 400 nm to 800 nm. Both materials are exhibiting a strong PL single (See Figure 8 (a, b)) at a room temperature. This reflects under test materials have a high order nanoparticles confinement within the quantized energy levels. Significantly it gives material Cu24In16Ga7Se53 has a stronger PL property than the Cu24In16Ga4Se56. Further, to explore the IR optical properties of these materials we have performed FT-IR measurement in the wave number range 400 cm-1 to 4000 cm-1 (see Figure 8c ). Both materials have exhibited IR transmission spectra with a low transparency percentage in the wave number range 600 to 3200 cm-1. Both materials IR spectrum are exhibiting a small multi phononic jump peak above the 3200 cm-1 wave number range. The two combined functional absorption groups are also appeared at 1699 and 2844 cm-1 . This may be due to adopted non vacuum drying process of the samples. Besides materials have a good IR transparency in the MID-IR range they could not be considered a promising candidate for the IR application, cause, low transparency percentage as compare to other existing materials.
interpreted with three steps mechanism under the followed heating up process. The schematic of the mechanism is given in Figure 9. Due to less reaction temperature requirement the CuSe2 phase can be formed first at the low temperature. It could be become only from the OLA– copper complex decomposition.Resulting it can be released the monomers containing Cu+ ions at low reaction temperature (~1000C) in a shorter time period. As well as the reactivity of the OLA–copper complex increases and reach at a much higher temperature then OLA–In and Ga complexes can react. Therefore, the monomers containing Cu+ ions can react with the Se ions. As a consequence dissolution of the Se in OLA can be increased with a reduction of Se powder in colloidal configuration and it results the CuSe2 nanocrystallites phase formation . It is well defined the reactivity of the Se depends on the reaction temperature and reduction ability of the ligand. Therefore, at a low reaction temperature (~1000C) materials may form the CuSe2 phase due to weak reduction ability of the OLA with the Se powder. In the second step it can deliver In+ ions monomer when the reaction temperature in between the 1000C to 1500C. Therefore the thermally enhanced decomposition of the OLA–In complex increases slowly and reacts with Se2 ions. Results the In-rich selenide shells on the surfaces of CuSe2 nanocrystallites through surface nucleation. Then CuSe2 nanocrystallites can react with the inner diffused In ions and form CuInSe2 phase. In the final step; where temperature ≥ 1500C with continuous stirring the thermal decomposition of the OLA-Ga complex is occurred resulting the CuInGaSe nanocrystals through the surface nucleation reaction mechanism [27-28].
Thus the physical property modifications in these materials could be correlated with these words; reduction and increment of the selenium and gallium alloying amounts alter the properties of the materials due to increase in the rate of reaction; by making the more metalloid and less non metallic bonds within the configuration [29-31]. The incorporation of the additional amount of gallium as cost of selenium can affect the chalcogen chains and rings within the configuration. During the long chemical processing alloying element particles interacts strongly through breaking the individual bonds and the combined effect reduced the crystals/particles size with an induced strong quantum confinement at the nano level. The colloidal interaction in between the gallium and copper can also play a key role in crystals/particles size modifications owing to their high metallic nature reactivity. However, alloying element indium can substantially contribute to homogenize the configuration structure due to its recombination blocking ability
In summary, authors have discussed the colloidal synthesis and physical properties of the Cu24In16Ga4Se56 and Cu24In16Ga7Se53 composition nanocrystals. The obtained experimental evidences have revealed low temperature (≥ 1500C) colloidal route synthesized materials physical properties is varying with the alloying elements concentration. TEM and Lorentzian fit have demonstrated the agglomerated phases with the nanocrystals average size ≤ 64 nm for the Cu24In16Ga4Se56 composition. While, the Cu24In16Ga7Se53 composition has a rather agglomerated homogeneous distribution of the nanocrystals with the reduced size around ≤ 55 nm. The reduction in the nanocrystalline size and homogeneous distribution has affected the binding energies of the elemental core levels, UV-visible absorption, reflectivity, optical energy band gap, PL and FT-IR transparency. Therefore, the experimental evidence demonstrates element gallium has optical energy band gap tunability aptitude through the physical modifications in Cu24In16Ga4Se56 and Cu24In16Ga7Se53 composition nanomaterials. Hence Cu24In16Ga7Se53 composition nanocrystalline material can have a superior prospect for the photovoltaic application in comparison to Cu24In16Ga4Se56 composition.