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Facile and Novel Chemical Synthesis, Characterization, and Formation Mechanism of Copper Sulfides (Cu2S, Cu2S/CuS, CuS) Nanostructures for Increasing the Efficiency of Solar Cells Mehdi Mousavi-Kamazani, Zabihullah Zarghami, and Masoud Salavati-Niasari J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11566 • Publication Date (Web): 11 Jan 2016 Downloaded from http://pubs.acs.org on January 12, 2016
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Facile and Novel Chemical Synthesis, Characterization, and Formation Mechanism of Copper Sulfides (Cu2S, Cu2S/CuS, CuS) Nanostructures for Increasing the Efficiency of Solar Cells
Mehdi Mousavi-Kamazani, Zabihullah Zarghami and Masoud Salavati-Niasari * Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P. O. Box. 87317-51167, I. R. Iran. *Corresponding author: Tel.: +98 31 55912383, Fax: +98 31 55913201. E-mail address:
[email protected] (Masoud Salavati-Niasari). ABSTRACT: This paper reports on the successful synthesis of various copper sulfides nanostructures via co-precipitation and hydrothermal route using new starting reagents such as Na2SO3 as a reducing agent for converting Cu2+ to Cu+ and PMP dyes-Cu(II) and carminic acid-Cu(II) complexes as new copper precursors for synthesizing quantum dots in aqueous medium. The as-synthesized products were extensively characterized by techniques including XRD, EDS, SEM, TEM, AFM and DRS. Effects of different parameters such as temperature, surfactant, solvent, concentration, copper precursor, sulfide source and etc. on morphology and particle size of as-synthesized nanostructures were investigated. Moreover, efficiency of various as-synthesized nanostructures in thin layer solar cells were evaluated. The results showed that particle size and morphology have salient effect on solar cells efficiency. Also, utilizing prepared Cu2S quantum dots as barrier layer in dye sensitized solar cells (DSSCs) presented a remarkable increase in efficiency of solar cells from 6.08% to 8.34% (~37% improvement). KEYWORDS: Copper(I) sulfide; Hydrothermal; Co-precipitation; Quantum dot; DSSC.
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1. INTRODUCTION: Semiconductor nanostructures which are regarded as promising building block for emerging active and integrated nanoscale devices are currently attractive field for researchers.1-3 The most important part in this field is to select a semiconductor material with an appropriate band gap. Moreover, it could be more preferable if it is an earth abundant, nontoxic, environmentally friendly and recyclable material, from which various nanostructures can be easily grown onto a variety of substrates using cost effective techniques. Copper(I) sulfide is one of the candidates satisfying the above requirements with a bulk band gap of 1.21 eV which is in the optimum range for solar-energy conversion.4-7 Copper sulfides can adopt a large number of crystallographic phases depending on their stoichiometric natures8 from copper rich Cu2S9 to copper poor CuS210 phases. Synthesis each phase without impurities especially about Cu2S can be important because among these copper sulfides, chalcocite (Cu2S) is a widely used material in solid state solar cells as a p-type absorber material11, nanoscale switches12 as well as in cold cathodes.13 Cu2S nanostructures such as nanocrystals14-16, nanowires17-19, nanorods20, nanodisks21-23 and nanoplatelets24 can be fabricated using a variety of techniques including hot injection moulding25, gas-solid reactions26 and suspension free thermolysis of copper based precursors.27 Synthesis of Cu2S among the other Copper sulfides is more difficult because reduction of Cu2+ to unstable Cu+ is a crucial challenge for synthesizing Cu2S. Therefore, the most reports about synthesis of Cu2S were performed in polyol solvents with high boiling point such as ethylene glycol due to their ability to reduce the Cu2+ to Cu+ at elevated temperatures. Nevertheless, these solvents are expensive and working with them is more difficult in comparison with water. Herein, we report a simple route for preparation of Cu2S with introducing Na2SO3 as a suitable reducing agent for reduction of Cu2+ to Cu+ in aqueous medium. Furthermore, by changing reaction terms such as surfactants, pH, temperature and etc. various Cu2S morphologies such as flower like, microsphere, nanoparticles and quantum dots will be achieved. Preparation of quantum dots in aqueous medium can be 2 ACS Paragon Plus Environment
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prominent because so far, the quantum dots have been synthesized by only expensive solvents and strong surfactants. For preparation of quantum dots, we prepared new copper precursors by using new dyes. For this purpose, phenolics, proanthocyanidins, flavonoids and carminic acid were extracted from pomegranate marc peel and cochineal by copper ions. Numerous natural dyes were applied as potential candidates in dye sensitized solar cells (DSSCs) since ruthenium dyes, like N719, are very expensive and environmentally toxic.28 Moreover, natural dyes have some other advantages such as improving preparation of quantum dots and single step preparation of triple layer FTO/TiO2/Cu2S-QD/dye solar cells.29,30 However, mildew, rot, and decay are big challenges in working with natural dyes. One way to solve this problem is to turn dyes such as pomegranate juice to the form of powder, which is a costly method. Also, dyes could be separated by using metallic ions or preparing complexes. Therefore, we used Cu2+ for synthesizing the PMP dyes-Cu(II) and carminic acid-Cu(II) complexes from the pomegranate marc peel and cochineal dye, respectively. Finally, the energy conversion efficiencies of obtained nanostructures and quantum dots were investigated through several solar cells including FTO/TiO2/Cu2S/Pt, FTO/TiO2/dye/Pt, FTO/TiO2/Cu2S-QD/dye/Pt and FTO/TiO2/Cu2S-QD/dye/Cu2S. Doctor’s blade and electrophoresis techniques were utilized to prepare the solar cells.31 2. EXPERIMENTAL 2.1. Materials and physical measurements All the chemicals used in this method were of analytical grade and used as-received without any further purification. The Pomegranate marc Peels (PMP) were collected and washed thoroughly with water to remove any impurities. After drying at room temperature, the samples were ground into powder by means of grinder. Cochineal dye was obtained from the dried bodies of female scale insects species, Dactylopius coccus, which feed on wild cacti. X-ray diffraction (XRD) patterns were recorded by a Philips-X’PertPro, X-ray diffractometer using Ni-filtered Cu Kα radiation at scan range of 10
