Synthesis of nickel sulfides of different phases for counter electrodes in dye-sensitized solar cells by a solvothermal
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Guang Lia) Anhui Key Laboratory of Information Materials and Devices, Hefei 230601, China (Received 21 November 2013; accepted 24 March 2014)
Two phases of nickel sulfide (a-NiS and b-NiS) nanoarchitectures were successfully and controllably synthesized by a facile solvothermal method with two different solvents of alcohol and water, respectively. The products were characterized by x-ray diffraction, scanning electron microscopy, transmission electron microscopy, and UV-vis diffuse reflectance spectrophotometer. The sphere-like shape for a-NiS and cross-like shape composed of nanorods for b-NiS are uniform and well distributed as well as their size. Both a-NiS and b-NiS powders were used as counter electrodes (CEs) in dye-sensitized solar cells (DSSCs). It is found that the DSSC with an a-NiS CE performs much better than the one with a b-NiS CE. The energy conversion efficiency of the former was 5.2%, whereas the latter was 4.2%, about 20% increment.
I. INTRODUCTION
Since O’Regan and Grätzel’s first report in 1991, DSSCs have attracted considerable interest as potential candidates to replace conventional Si-based solar cells in specialized applications because of low manufacturing cost and easy fabrication.1–4 As well known, the metal platinum is widely used as counter electrode (CE) in the traditional dye-sensitized solar cells (DSSCs). However, its high cost restricts the large-scale production of DSSCs although to date the highest energy conversion efficiency of 12.3% has been reported.5 Hence, searching for low-cost and flexible conductive material acted as CE to substitute for platinum has been a crucial issue in photovoltaic research. Inorganic compound semiconductors, such as CoS,6 NiS2,7 SnSx,8 PbS,9 MoS2,10 as well as some carbon materials,11 have potential applications as cathodes for DSSCs. Among various metal sulfides, nickel sulfide (NiS) is more favorable in terms of abundant resource, low toxicity, and excellent electric conductivity.12 Nickel sulfide has many polymorphs, such as the alpha form with a hexagonal unit cell and the beta form with a rhombohedral cell. Although NiS nanoparticles were reported to have a band gap of 3.53 eV measured via UV-visible spectroscopy,13 it is still black at room temperature and has a low metal/insulator transition temperature (252 K)14 and the high temperature phase of
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.74 J. Mater. Res., Vol. 29, No. 8, Apr 28, 2014
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NiS is a metal.14 In addition, the raw materials used for the preparation of NiS are accessible to get. Furthermore, NiS owns high energy conversion efficiency and its preparation method is simple.15,16 Controlling the shape, size, and structure of inorganic nanomaterials is of fundamental importance because of the strong correlation between these parameters and physical/chemical properties.17,18 The formation of nanocrystal is a kinetically controlled process that can yield nanorods and other complicated
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