Synthesis of Nanocrystalline FeS2 with Increased Band Gap for Solar Energy Harvesting

https://doi.org/10.1016/j.jmst.2014.01.005Get rights and content

Highlights

  • FeS2 nanoparticle has been synthesized by hydrothermal route.

  • Morphology of high thermally stable FeS2 has been engineered by capping reagent.

  • Optical band gap energy of FeS2 pyrite has been improved to 2.75 eV.

  • FeS2 has been used in conjunction with polymer for solar energy harvesting.

In this paper, we have reported the synthesis of FeS2 of higher band gap energy (2.75 eV) by using capping reagent and its successive application in organic–inorganic based hybrid solar cells. Hydrothermal route was adopted for preparing iron pyrite (FeS2) nanoparticles with capping reagent PEG-400. The quality of synthesized FeS2 material was confirmed by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, Fourier transform infrared, thermogravimetric analyzer, and Raman study. The optical band gap energy and electro-chemical band gap energy of the synthesized FeS2 were investigated by UV–vis spectrophotometry and cyclic voltammetry. Finally band gap engineered FeS2 has been successfully used in conjunction with conjugated polymer MEHPPV for harvesting solar energy. The energy conversion efficiency was obtained as 0.064% with a fill-factor of 0.52.

Introduction

The FeS2 in pyrite phase is an important material because of its environmental compatibility and high stability towards photo corrosion[1]. It has attracted significant scientific interest and has numerous technological applications[2], [3]. Owing to their large potential capacities in application of devices, iron–sulfur based materials have been extensively studied by Kirkeminde et al.[4]. The photo-sensing behavior mainly depends on the absorbance and the direct band gap of the materials. The indirect band gap of FeS2 was measured as 0.95 eV by Ennaoui et al., which is suboptimal for single junction photovoltaic application[5]. To improve the band gap of pyrite FeS2, enormous efforts have been made. By controlling the environmental conditions of reaction, it is possible to synthesize high quality FeS2 nanoparticle with desired property. Improvement in absorbance of FeS2 can be done by tuning the nanomorphology. FeS2 nanoparticles with different morphology have been fabricated by using a variety of synthetic methods[6]. Sun and Ceder studied and tried the technique to tune the band gap of FeS2 by controlling the particle size[7]. As it had been reported so far, there are various phases of FeS2, such as mackinawite (tetragonal), troilite (hexagonal), pyrrhotite (monoclinic), smythite (hexagonal), pyrite (cubic), marcasite (orthorhombic) and greigite (cubic)[7]. The solids consisting only of iron and sulfur are known to occur naturally at lower temperature (below 200 °C). The structural growth strongly depends upon the reaction temperature and the vapor pressure of the solvent. Wadia et al.[8] reported that iron pyrite (FeS2) is significantly attractive in both cost and availability for the application of thin film technology. Like FeS2 there are very few nano-semiconducting materials, which can meet the large scale need, outside quantum confined systems[8]. As the pyrite phase has more extensive stability in natural environment in spite of its lower band gap, the use of this phase is enormous. Unfortunately, the iron pyrite based solar energy harvesting devices have been plagued by performance problems. The science behind its underperformance is still not well understood. After hard research work and a prolonged study it is identified that the various phases of FeS2 produce surface defects near the surface of thin film and grain boundaries that limit the open circuit voltage of the photovoltaic devices. Ennaoui et al. reported pyrite single crystal based photo electrochemical cells, which show low open circuit voltage of 187 mV and fill-factor (FF) of 0.5[9]. The phase purity was attributed to low open circuit voltage, which was reported by Thomas et al.[10]. Ganta et al.[11] successively synthesized superstrate type FeS2 and CdS ink-based solar cell with efficiency of 0.03% and open circuit voltage (Voc) of 565 mV. Several recent publications exhibited that successful and robust synthesis of pyrite nanocrystals and many efforts have been directed toward exploring their optoelectronic applications[12], [13], [14], [15], [16]. Most recently (in the year 2012) Kirkeminde et al. reported inorganic solar cell based on ITO/PEDOT:PSS/(TFB) poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4-(N-(4-s-butylphenyl)) diphenylamine)]/FeS2:CdS/Al structure, which showed more promising results with a PCE of 1.1% and Voc of 0.79 V[17], but the absorption and charge transportation highly depends on the CdS quantum dot, rather FeS2. Bi et al.[14] synthesized FeS2 NCs with improved stability in air and observed a photo response in an ITO/FeS2 NC/Al device, but the result is not upto the mark. Moreover, not surprisingly, no rectification behavior was observed, as the pyrite formed ohmic contacts at the ITO (indium tin oxides) and aluminum interfaces. Despite these recent works and renewed interest in pyrite FeS2 over the past several years, no high performance device has been made yet based on this material. Further investigation on the fundamental properties of pyrite FeS2 and deployment in alternative device architectures are required to explore the potential of pyrite FeS2 in photovoltaics.

Here we have studied a different technique to improve the band gap energy of FeS2 by changing the architectural growth. Capping reagent has been introduced to influence the vapor pressure of the solution, which can effectively control the morphology via controlling the growth rate. The optical absorption of the synthesized material was studied by UV–vis absorption spectroscopy. Depending upon the morphological growth, energy absorption of as-synthesized iron pyrite was noticed as very low compared to other inorganic semiconductors. The optical band gap energy estimated from UV–vis absorption data was significantly improved. This fact was in agreement with the absorbing behavior of iron pyrite, for its modified architecture and epitaxial growth. The electrochemical band gap energy was calculated from oxidation (corresponding to valence band) and reduction (corresponding to conduction band) states with the help of cyclic voltammetry (CV). Thermal stability of the FeS2 nanoparticle was studied with thermogravimetric analyzer (TGA). It is obvious that by controlling the growth of morphology with chemical surfactant, the band gap would successively tune up for application in high temperature semiconducting electronic devices. Thus our synthesized semiconducting material with higher band gap and thermal stability can be applied in photovoltaics at high temperature. It is noteworthy that it has still remained as a challenging unexplored arena for the researchers. In the fore step we have fabricated ITO/PEDOT:PSS/MEH-PPV:FeS2/Al based hybrid solar cell and characterized the device to estimate different cell parameters. The polymer MEHPPV has a wide research application for its higher absorption with band gap energy of 2.2 eV, as a semiconducting donor polymer in fabrication of efficient thin film hybrid solar cell. This was the early challenge to harvest solar energy by mostly abundant iron-pyrite conjugated with MEHPPV. The subject still remains unexplored. The synthesized FeS2 of higher band gap energy with corresponding energy levels demonstrates the successive charge transportation phenomena from MEHPPV donor to FeS2 acceptor and also explains the improvement of open circuit voltage of the device. This is the uniqueness of this work.

Section snippets

Experimental reagents

Hydrated ferric chloride (FeCl3·6H2O), hydrated sodium sulfide (Na2S·9H2O), ammonium hydroxide (NH4OH), ethanol, chloroform and polyethylene glycol (PEG-400) of AR grade were procured from Merck. Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV), poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) and sulfur powder were purchased from Sigma Aldrich. All these chemicals were used without further purification.

Synthesis of iron-pyrite nanoparticles

In this work FeS2 nanoparticles were synthesized by

Characterization

The characterization of blackish FeS2 nanopowder was done by recording powder X-ray diffraction (XRD) spectra with the help of Bruker D8-X-ray Diffractometer, Raman spectroscopy, field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) of JEOL make. To investigate the functional groups, which are responsible for homogenous dispersion in chloroform medium, Fourier transform infrared (FTIR) spectra were recorded with the help of FTIR-8400S Spectrophotometer

Results and Discussion

Fig. 1 represents the XRD spectra of the synthesized material. The XRD peaks from each responsible Bragg's (hkl) planes for diffraction were recorded as (111), (200), (210), (211), (220), (221), (311) and (222) at 2θ = 28.5, 33, 37, 40.7, 47.4, 51.9, 56.3, and 58.9 deg, respectively, which is related to FeS2 pyrite (cubic structure) with phase purity[13]. This phase is approved by JCPDS card no. 42-1340. Few extra peaks may be due to the attenuation and the presence of noise. Analysis of the

Conclusion

In this experimental study, the energy band gap of iron pyrite (FeS2) nanoparticle was successively improved to be 2.74 eV by improving the synthesis technique with capping reagent. The morphological growth of nanoparticle with capping reagent PEG-400 improved the surface to volume ratio and thermal stability of semiconducting FeS2. By incorporating synthesized FeS2 nanoparticle with MEHPPV we have succeeded to harvest solar energy. The energy conversion efficiency can be improved further by

Acknowledgments

This work was supported by University Grants Commission (UGC), Govt. of India under project 39-508/2010 (SR). The authors acknowledge Madhusudan Nandy of Department of Chemistry, Jadavpur University and Priyanka Das of Department of Chemistry, West Bengal State University, Barasat for their valuable advice and enormous technical assistance.

References (32)

  • A. Ennaoui et al.

    Sol. Energy Mater. Sol. Cells

    (1993)
  • J.M. Philias et al.

    Electrochem. Acta

    (1999)
  • G. Smestad et al.

    Sol. Energy Mat.

    (1989)
  • K. Sato

    Prog. Cryst. Growth Charact.

    (1985)
  • I. Bedja et al.

    Adv. OptoElectron.

    (2011)
  • C. Wadia et al.

    Chem. Mater.

    (2009)
  • A. Ennaoui et al.

    J. Electrochem. Soc.

    (1985)
  • A. Kirkeminde et al.

    ACS Appl. Mater. Interfaces

    (2012)
  • S. Disale et al.

    Adv. Sci. Lett.

    (2010)
  • R. Sun et al.

    Phys. Rev. B

    (2011)
  • C. Wadia et al.

    Environ. Sci. Technol.

    (2009)
  • A. Ennaoui et al.

    J. Electrochem. Soc.

    (1986)
  • B. Thomas et al.

    Solid State Phenom.

    (1996)
  • L.K. Ganta et al.

    MRS Proc.

    (2012)
  • Y.Y. Lin et al.

    Nanotechnology

    (2009)
  • J. Puthussery et al.

    J. Am. Chem. Soc.

    (2011)
  • Cited by (60)

    • The quaternary ammonium salts as corrosion inhibitors for X65 carbon steel under sour environment in NACE 1D182 solution: Experimental and computational studies

      2023, Colloids and Surfaces A: Physicochemical and Engineering Aspects
      Citation Excerpt :

      The ATR-FTIR spectrum was employed to identify the composition of corrosion products and film formation on the carbon steel surface. Fig. 18 demonstrates that in the blank solution (Fig. 18a), the band at 825 cm−1 corresponds to the Fe-S bond's stretching vibration, at 1090 cm−1 to the S-O stretching, and at 1166 cm−1 indicates the presence of the FeS bond [70,76,77]. In the presence of inhibitors (Fig. 18b and c), the peaks at 1000–1350 cm−1 are related to the stretching vibration of the C-N bond, 1375–1450 cm−1 corresponds to the bending of -CH3, and at 1450–1500 cm−1 attributes to the -CH2 bending state.

    View all citing articles on Scopus
    View full text