Green synthesis of CuO nanoparticles with leaf extract of Calotropis gigantea and its dye-sensitized solar cells applications Jitendra Kumar Sharma a , M. Shaheer Akhtar b,⇑ , S. Ameen c , Pratibha Srivastava a,⇑ , Gurdip Singh a a Department of Chemistry, D.D.U. Gorakhpur University, Gorakhpur, UP, India b New & Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea c Energy Materials & Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea article info Article history: Received 29 August 2014 Received in revised form 8 December 2014 Accepted 4 January 2015 Available online 30 January 2015 Keywords: Green synthesis CuO Calotropis gigantea Counter electrode Dye sensitized solar cells abstract Green synthesized copper oxide (CuO) nanoparticles (NPs) were employed as electrocatalytic materials for the fabrication of counter electrode in dye sensitized solar cells (DSSCs). Uniform CuO NPs were syn- thesized by the leaves extract of Calotropis gigantea plant in aqueous medium through green synthesis. The synthesized CuO NPs were extensively characterized in terms of morphology, crystalline nature, structural, electrochemical and photovoltaic properties using various experimental tools. The synthesized CuO NPs possessed a well crystalline nature which was perfectly matched to monoclinic structure of bulk CuO. For DSSC application, a thin film of synthesized CuO NPs was prepared by the paste of CuO NPs and coated onto FTO glass using glass rod. The cyclovoltametry measurement revealed that CuO NPs based thin film showed reasonably good surface for the reduction of triiodide ions in redox electrolyte, suggest- ing its good electrocatalytic activity toward the iodide ions. Moderately high solar to electrical energy conversion efficiency of 3.4% along with high short circuit current density (J SC ) of 8.13 mA/cm 2 , open circuit voltage (V OC ) of 0.676 V and fill factor (FF) of 0.62 was recorded in the DSSC fabricated with syn- thesized CuO NPs based counter electrode. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Extensive studies on the development of photovoltaic technol- ogy especially, dye sensitized solar cell (DSSC) have become of increasing importance and interest due to the growing demand for energy, especially from the view point of depletion of the fossil fuel resources, global warming and so on [1]. The DSSCs have been known as promising renewable energy devices compared to com- mercial silicon based solar cells owing to their easy, low manufac- turing cost and reasonably high overall light-to-electricity conversion efficiencies [2–4]. Generally, nanocrystalline TiO 2 has been used as an anode material and expensive platinum (Pt) based electrode as counter electrode for the fabrication of DSSCs. However, the recent research advances on DSSCs have been focused on improving the electron transport and reducing the recombination rate using semiconductor materials other than TiO 2 [5,6]. Meanwhile the Pt counter electrode is generally acted as a catalyst for the redox reaction of the iodide ions in the redox electrolyte [7]. The high cost Pt could be replaced with the other cheaper materials to fabricate the less expensive counter electrode for DSSCs. In this regard, cupric oxide (CuO) is a narrow band gap (E g 1.2 eV) p-type semiconductor and shows an excellent material for the fabrication of various efficient electronic and optoelectronic devices [1]. CuO NPs are being considered as an alternative counter electrode material for the fabrication of DSSCs. Additionaly, CuO NPs forms the basis for several high temperature superconductors [8,9], gas sensors [10,11] and giant magneto resistance materials [12,13]. Applications include as an antimicrobial, anti-fouling, anti-biotic and anti-fungal agent when incorporated in coatings, plastics and textiles [14]. Copper and copper-based compounds, due to their potent biocidal properties [15], are now routinely used in pesticidal formulations [16] and several health related areas applications are being explored and/or implemented. There are several routes through which CuO NPs can be synthe- sized, like Sonochemical [17], microwave irradiations [18], alkox- ide based route [19], sol–gel technique [20], one step solid-state reaction method at room temperature [21], electrochemical meth- ods [22], precipitation–pyrolysis [23], thermal decomposition of precursor [24] or by combination of electro deposition and self-cat- alytic mechanism etc. In chemical methods, toxic chemical (e.g. hydrazine hydrate, sodium borohydride, dimethylformamide, http://dx.doi.org/10.1016/j.jallcom.2015.01.172 0925-8388/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding authors. Tel.: +82 10 4028 2313 (M.S. Akhtar). Tel.: +91 9807751297 (P. Srivastava). E-mail addresses: shaheerakhtar@jbnu.ac.kr (M.S. Akhtar), pratibhasri1980@ yahoo.co.in (P. Srivastava). Journal of Alloys and Compounds 632 (2015) 321–325 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom