Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Platinum-free metal sulfide counter electrodes for DSSC applications: Structural, electrochemical and power conversion efficiency analyses K. Subalakshmi a , K. Ashok Kumar a , Onuoha Prayer Paul a , S. Saraswathy a , A. Pandurangan b , J. Senthilselvan a, ⁎ a Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai 600 025, Tamil Nadu, India b Department of Chemistry, Anna University, Chennai 600 025, Tamil Nadu, India ARTICLE INFO Keywords: Metal sulfides Electrochemistry Pt-free counter electrodes DSSC Energy Photovoltaics ABSTRACT Micro/Nanoscale binary (CoS, NiS, and ZnS) and ternary (CoNi 2 S 4 and Zn 0.76 Co 0.24 S) metal sulfides were di- rectly grown on FTO substrates by a novel one step hydrothermal method for low-cost DSSC counter electrode applications. XRD study confirms phase pure formation of CoS, NiS, ZnS, CoNi 2 S 4 and Zn 0.76 Co 0.24 S. HRSEM images of CoS, NiS and CoNi 2 S 4 films exhibits hierarchical flower, hexagonal microcage and flake like parti- culates respectively. The ZnS and Zn 0.76 Co 0.24 S films show spherical micro-nanoparticles. The nanoflake mor- phological CoS and CoNi 2 S 4 revealed high surface area measured by BET analysis. XPS result confirms the chemical states of CoNi 2 S 4 and Zn 0.76 Co 0.24 S. A detailed electrochemical cyclic voltammetry, Tafel polarization and electrochemical impedance spectroscopy (EIS) analyses reveals the ternary metal sulfide (CoNi 2 S 4 ) possesses an excellent electrocatalytic activity and electrical conductivity than other counter electrodes for the reduction of tri-iodide to iodide. Photovoltaic performance of the devices was studied using standard solar simulator at 1 Sun intensity (AM 1.5 G) and dark conditions. The reduced dark current obtained in CoNi 2 S 4 and Zn 0.76 Co 0.24 S confirms the less recombination of electrons in ternary metal sulfides based DSSCs. The power conversion ef- ficiency of ~4.03% achieved in Pt-free CoNi 2 S 4 counter electrode is comparable to standard Pt based DSSC (~4.59%). The CoNi 2 S 4 exhibiting good device performance and high electrocatalytic activity can serve as ef- ficient low-cost counter electrode for Pt-free DSSCs. 1. Introduction Dye-sensitized solar cells (DSSCs) have attracted significant atten- tion as an alternative to conventional silicon solar cells since the breakthrough reported by Gratzel in 1991 owing to its cost effective- ness, simple fabrication and good performance (O'Regan and Gratzel, 1991). Typically, DSSC consists of dye-sensitized semiconductor oxide (nanocrystalline TiO 2 ) based photoanode, iodide/tri-iodide ( − − I I / 3 ) redox electrolyte and catalytic counter electrode (Grätzel, 2001). The function of counter electrode (CE) is to collect electrons from external circuit and subsequently catalyzes the reduction of − I 3 to − I ions (Grätzel, 2005). For efficient performance of DSSC, the CE material should possess desirable properties such as (i) high electrocatalytic activity for effective regeneration of electrolyte, (ii) high electrical conductivity for enhanced charge transportation, (iii) high surface area to produce more catalytic active sites and (iv) good adhesion and chemical stability for long-term usage (Iqbal and Khan, 2018; Yun et al., 2014). Generally, platinum (Pt) has been employed as CE material in DSSCs due to its high electrical conductivity and superior electrocatalytic ac- tivity to iodide/tri-iodide redox system (Calogero et al., 2011). How- ever, its application for large scale devices and commercialization for DSSC is severly restricted owing to its natural scarcity, high cost and easy decomposition to PtI 4 by − − I I / 3 electrolyte (Kay and Grätzel, 1996) large-scale and commercialization of DSSCs. Therefore, it is imperative to search low cost Pt-free CE materials. Up to date, various alternatives such as carbonaceous materials, conducting polymers and transition metal compounds have been investigated to replace the expensive Pt. The use of carbon materials in various forms like graphene (Zhang et al., 2011), carbon nanotubes (CNTs) (Lee et al., 2009a, 2009b; Mei et al., 2010), mesoporous carbon (Peng et al., 2013) and carbon black (Kim and Rhee, 2012) have exhibited comparable efficiency to that of Pt. However, the main problem associated with carbon materials are poor adhesion to the substrate surface. On the other hand the usage of various organic polymers like PEDOT (Ahmad et al., 2010), PANI (Hou et al., 2016) and PProDOT (Lee et al., 2009a, 2009b) with high https://doi.org/10.1016/j.solener.2019.09.075 Received 8 June 2019; Received in revised form 19 September 2019; Accepted 23 September 2019 ⁎ Corresponding author. E-mail address: jsselvan@hotmail.com (J. Senthilselvan). Solar Energy 193 (2019) 507–518 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved. T