Platinum-free metal sulfide counter electrodes for DSSC applications: Structural, electrochemical and power conversion efficiency analyses

Abstract

Micro/Nanoscale binary (CoS, NiS, and ZnS) and ternary (CoNi2S4 and Zn0.76Co0.24S) metal sulfides were directly 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, CoNi2S4 and Zn0.76Co0.24S. HRSEM images of CoS, NiS and CoNi2S4 films exhibits hierarchical flower, hexagonal microcage and flake like particulates respectively. The ZnS and Zn0.76Co0.24S films show spherical micro-nanoparticles. The nanoflake morphological CoS and CoNi2S4 revealed high surface area measured by BET analysis. XPS result confirms the chemical states of CoNi2S4 and Zn0.76Co0.24S. A detailed electrochemical cyclic voltammetry, Tafel polarization and electrochemical impedance spectroscopy (EIS) analyses reveals the ternary metal sulfide (CoNi2S4) 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 CoNi2S4 and Zn0.76Co0.24S confirms the less recombination of electrons in ternary metal sulfides based DSSCs. The power conversion efficiency of ~4.03% achieved in Pt-free CoNi2S4 counter electrode is comparable to standard Pt based DSSC (~4.59%). The CoNi2S4 exhibiting good device performance and high electrocatalytic activity can serve as efficient low-cost counter electrode for Pt-free DSSCs.