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Science and Engineering Applications 1(3) (2016) 1-8 ISSN-2456-2793(Online)

©JFIPS, India http://www.jfips.com/

Composition dependence structural and optical

properties of the CuInGaSe nanocrystals

Abhay Kumar Singha *, P. Senthamaraib, R. Ganesana

aDepartment of Physics, Indian Institute of Science, Bangalore-560012, India

bDepartment of Physics, Anna University of Technology, Trichy, India

Email: [email protected]

ABSTRACT

This report demonstrates colloidal route synthesis of the Cu24In16Ga4Se56 and Cu24In16Ga7Se53 nanocrystals at

temperature ~ 150 0C. 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 Ga7 Se53 composition, while the FT-IR transparency is unchanged in these materials.

Keywords: CIGS, Sole gel, Nanomaterials, Semiconducting materials, Crystal morphology, Optical properties.

Received on: 27/8/2016 Published online on: 5 /9/2016

1. INTRODUCTION

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 [5]. 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) [6]. 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 [7]. 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. [8] 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 [12], hot injection route [13],

mechano chemical synthesis [14], green synthesis [15], modified

polyol route [16], ambient pressure diethylene glycol based

solution process [9], 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

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Abhay Kumar Singh et al, Science and Engineering Applications 1(3) (2016) 1-8


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nano confined crystals size and their physical properties. The

synthesized nanocrystalline Cu24In16Ga4Se56 and

Cu24In16Ga7Se53 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.

2 SYNTHESIS ROOT AND

CHARACTERIZATION

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◦C (±10) under a continuous stirring it was changed into

blackish brown colour. The final solution was allowed to cool

down to room temperature. Further, 20 ml methanol and 10 ml

water were slowly incorporated into the solution and again allow

for continuing stirring upto next 12 h at a 60 0C 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 0C 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.

Figure.1. Schematic solution color representation of the

synthesis process

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 0.5cm-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.

3 RESULTS

3.1. Structural analysis

Crystallographic structure of the described materials is exhibited

in Figure.2 (a, b). XRD pattern of the Cu24 In16 Ga4Se56 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 [112]

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




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prime CIGS characteristic peak is appeared at 27.00 along the

crystallographic plane [112]. Other weak characteristic peaks

are also appearing with the noticeable feature the main CIGS

crystallographic peak [112] 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.

Figure.2 (a, b). XRD patterns of the Cu24 In16 Ga4Se56 and

Cu24In16Ga7Se53 nanocrystals

Materials micro-morphology can be visualized from the

TEM analysis. TEM image of the Cu24 In16 Ga4Se56 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

Cu24 In16 Ga4Se56 composition. Existence of the every alloying

elemental peak in their compositional ratio (± 2 %) reveals the

appropriate material configuration.

Figure.3 (a, b) TEM images for the Cu24 In16 Ga4Se56 and

Cu24In16Ga7Se53 nanocrystals

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.

Figure. 4. EDS patterns and elemental mapping for the

Cu24In16Ga4Se56 nanoparticles

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




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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.

Figure. 5. EDS patterns and elemental mapping for the

Cu24In16Ga7Se53 nanoparticles

3.2. XPS Interpretation

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 Cu24 In16 Ga7Se53

composition (See Figure. 6 b). The valance states for the

selenium can be accessed with the help of past reports [17]. 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 Cu24

In16 Ga7Se53 (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 Cu24 In16 Ga7Se53 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.

3.3. UV-visible interpretation

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 UV-

visible 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




5

Figure. 6. XPS core level spectra, (a) Copper (b) Selenium (c) Indium (d) Gallium for the Cu24 In16 Ga4Se56 and Cu24 In16 Ga7Se53

nanocrystals

Figure. 7 (a, b). UV-visible absorption spectrum (solid lines), Lorentz fit (circle lines) along with fitting parameter table, optical

energy band gap (Eg) Tuac plot (inside profile); (c). reflectance spectra for the Cu24 In16 Ga4Se56 and Cu24 In16 Ga7Se53 nanocrystals.




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Figure. 8 (a, b, c). Photoluminescence (PL) and FT-IR transmission spectrum for the Cu24 In16 Ga4Se56 and Cu24 In16 Ga7Se53

nanocrystals

Figure. 9. Schematic of the reaction mechanism for the synthesized CuInGaSe nanocrystals

(c)




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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.

3.4. Photoluminescence and FT-IR interpretation

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. 8 c ). 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.

4. DISCUSSION

Nanocrystals formation in aforesaid materials could be

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 [20]. 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, 30, 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.

5. CONCLUSIONS

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 Cu24 In16 Ga4Se56 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




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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 nano-

materials. Hence Cu24In16Ga7Se53 composition nanocrystalline

material can have a superior prospect for the photovoltaic

application in comparison to Cu24 In16 Ga4Se56 composition.

ACKNOWLEDGEMENT

AKS thankful to the Centre for Nano Science and Engineering

(CeNSE)-IIsc, for the materials characterizations.

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