Modification of light absorption in thin CuInS2 films by sprayed Au nanoparticles
© Katerski et al.; licensee Springer. 2014
Received: 13 June 2014
Accepted: 7 September 2014
Published: 14 September 2014
The chemical spray pyrolysis method was used to deposit CuInS2 (CIS) thin films and Au nanoparticles (NPs) in two configurations: glass/Au-NP layer covered with CuInS2 film (Au-NP/CIS) and glass/CuInS2 films covered with Au-NP layer (CIS/Au-NP). According to X-ray diffraction (XRD), the spray of 2 mM HAuCl4 aqueous solution with a volume of 2.5 to 15 ml onto a glass substrate at 340°C results in metallic Au nanoparticles with a similar mean crystallite size in the range of 30 - 38 nm. The mean crystallite sizes remain in the range of 15 - 20 nm when grown onto a CIS film. The prepared films show plasmonic light absorption with increasing intensity in the spectral range of 500- 800 nm when increasing the volume of HAuCl4 solution sprayed. When compared to bare CIS on glass, the absorptance was increased ca. 4.5 times in the case of glass/Au-NP/CIS and ca. 3 times in the case of glass/CIS/Au-NP configuration. The glass/Au-NP/CIS configuration had an advantage since Au-NP could be embedded without chemically damaging the CIS.
Photovoltaics (PV) is a clean and sustainable resource for producing electricity from sunlight. The future success of PV electricity requires significant advances in materials research and advances in the structural design of solar cells to increase conversion efficiency of the cell and reduce manufacturing costs . In view of manufacturing costs, PV devices such as dye-sensitized solar cells (DSSC), solar cells that use organic absorber, and extremely thin inorganic absorber (eta) solar cells are promising as prepared using low-cost non-vacuum technologies. However, all of these solar cells are based on ultrathin absorber layers, which in turn, lead to poor absorption of sunlight and low conversion efficiency when compared with conventional thin film solar cells. The plasmonic solar cell concept has been proposed in which metallic nanoparticles are embedded in the solar cell to increase light absorption ability and by that to enhance the conversion efficiency in various types of solar cells [2, 3]. Increased attenuation of light can be achieved throughout the visible up to infrared light as long as the embedded nanoparticles are with suitable sizes [4, 5]. Therefore, the addition of metallic nanoparticles to a PV absorber offers a way of reducing the physical thickness of the absorber layers while keeping their optical thickness similar . Plasmon-enhanced light absorption, increased photocurrent, and increased efficiency have been demonstrated in solar cells that use ultrathin light absorbing material, such as DSSC [7, 8] and organic solar cells . The metal (Au, Ag) nanoparticles have been made by methods such as heat treatment and evaporation technique , colloidal dispersion , spin-coating [11, 12], electrodeposition , and spray pyrolysis [14–17].
Eta solar cells use an absorber with a thickness of few tens up to a few hundred nanometers. Eta cells with chemically sprayed CuInS2 absorber layer are based either on TiO2 nanoparticles  or ZnO nanorod layer [19, 20] and show light-to-electricity conversion efficiencies of ca. 7%  and ca. 4% [20, 21], respectively. To obtain Au nanoparticles by chemical spray pyrolysis (CSP), a solution of an Au salt such as HAuCl4 · nH2O is deposited onto a preheated substrate. Thermoanalytical study has shown that the decomposition of HAuCl4 · nH2O into pure gold and gaseous products is completed at 320°C in air . Recently, it has been shown that SnO2, ZnO , ZrO2, and TiO2 thin films with plasmonic nanoparticles can be prepared by CSP at a substrate temperature of 400°C or higher. The formation of Au nanoparticles on a glass substrate has been studied by varying the concentration of HAuCl4 in an aqueous solution using the ultrasonic spray method and a substrate temperature of 300°C .
In this study, we prepare Au nanoparticles by spray of HAuCl4 solution onto a glass and onto the surface of preliminarily grown thin CuInS2 film. We study the possibility to observe the surface plasmon resonance effect in two configurations of CuInS2 and Au nanoparticle composites in order to increase light absorption in CuInS2. To preserve the simplicity of preparation, we use the CSP method for deposition of both components in the composite.
A thin CuInS2 (CIS) film was grown by CSP of an aqueous solution containing CuCl2, InCl3, and SC(NH2)2 as Cu, In and S sources, respectively, at a molar ratio of Cu:In:S = 1:1:3 ([Cu2+]/[In3+] = 1.0, [Cu2+] = 2 mM). The surface temperature of the substrate was kept at 310°C. Other deposition parameters such as spray solution volume of 5 ml and feeding rate of 1 ml/min were kept constant for all samples. These deposition conditions resulted in CuInS2 thin films with thicknesses of ca. 150 nm.
The Au-NP layer, the CuInS2 film, the Au-NP layer covered with CuInS2 film (Au-NP/CIS), and the CuInS2 films covered with Au-NP layer (CIS/Au-NP), all on glass substrates, were characterized using optical transmittance and reflectance spectra, scanning electron microscopy (SEM), and X-ray diffraction (XRD) methods.
The total transmittance and the total reflectance spectra of the samples were measured in the wavelength range of 300-1,500 nm on a Jasco V-670 UV-vis-NIR spectrophotometer (Jasco Corporation, Ishikawa-cho, Hachioji-shi, Tokyo, Japan) that was equipped with an integrating sphere to collect the diffused light. The sizes of Au nanoparticles were evaluated from the film surface images and the film thicknesses from the cross-sectional images by SEM. SEM study was performed using EVO MA 15 Zeiss apparatus (Carl Zeiss, Inc., Oberkochen, Germany) at an operating voltage of 10 kV. XRD patterns were recorded in the 2θ range of 20° - 70° on a Rigaku Ultima IV diffractometer (Cu Kα radiation, λ = 1.5406 Å, 40 kV at 40 mA; Rigaku, Shibuya-ku, Tokyo, Japan) equipped with a silicon strip detector D/teX Ultra. The mean crystallite size of Au particles was calculated from the full width at half maximum (FWHM) of the (111) peak of the metallic Au (PDF 00-004-0784)  using the Scherrer formula and the Scherrer constant of 0.94.
Results and discussion
Au nanoparticle size and Au mean crystallite size in nanoparticles
HAuCl4solution volume (ml)
Nanoparticle size (nm)
Crystallite size (nm)
30 to 50
50 to 80
25 to 200
20 to 60
20 to 60
20 to 200
However, further studies are needed to prove this reaction.
Figure 3 shows the SEM images of Au nanoparticles grown on glass substrates using various amounts of Au precursor solution. As seen, the spray of 2.5 ml HAuCl4 solution resulted in Au-NPs with sizes of 30 to 50 nm (Figure 3a, Table 1). By increasing the HAuCl4 solution amount to 5 ml, Au-NPs with sizes of 50 to 80 nm are grown (Figure 3b, Table 1). Further increase in the Au precursor solution volume to 15 ml resulted in Au-NPs with the size of 25 to 80 nm and the formation of Au-NP agglomerates with the size of ca. 200 nm (Figure 3c, Table 1). According to the literature, the size of Au nanoparticles on the glass substrate deposited by ultrasonic spray has been found to increase from 17 to 47 nm when increasing the concentration of the HAuCl4 solution from 1 to 30 mM .
Nanocomposite layers composed of CuInS2 (CIS) thin film and Au nanoparticles (Au-NP) were prepared by in-line chemical spray pyrolysis technique in two configurations: Au-NP/CIS and CIS/Au-NP both on a glass substrate. According to XRD, the spray of 2mM HAuCl4 aqueous solution with a volume of 2.5 to 15 ml onto a substrate at 340°C results in metallic Au nanoparticles. Irrespective of the volume sprayed, the Au crystallite size was in between 30 and 38 nm when Au particles are grown onto glass and in between 15 and 20 nm when grown onto CIS film. Agglomerates of Au nanoparticles with a size up to 200 nm are formed at spray volumes of 15 ml. According to XRD, the spray of HAuCl4 solution onto CIS causes damage of the CuInS2 film. We presume that the thermal decomposition products of HAuCl4 such as Cl2 and HCl react destructively with CuInS2.
The Au-NP/CIS nanocomposite films show additional light absorption in the absorbing region of the CuInS2 absorber material, characterized by a distinctive absorption band in the spectral region of 500 to 800 nm. Presumably, the band is due to surface plasmon resonance effect.
In the Au-NP/CIS configuration, the use of 15-ml 2 mM Au precursor solution to obtain Au nanoparticles by spray at 340°C leads to a gain of absorptance of up to 4.5 times when compared to the CuInS2 film reference. Thus, the preparation of CuInS2/Au-NP nanocomposite absorber by simple chemical spray method is feasible, and enhancement of light absorption in a solar cell with such thin absorber layer could be expected.
This study was financially supported by the Estonian Ministry of Education and Research (IUT19-4), the Estonian Science Foundation (ETF9081), and the European Union through the European Regional Development Fund projects: ‘Efficient plasmonic absorbers for solar cells’ (3.2.1101.12-0023) and TK114 ‘Mesosystems: Theory and Applications’ (3.2.0101.11-0029).
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