Improved performance of quantum dot-sensitized solar cells adopting a highly efficient cobalt sulfide/nickel sulfide composite thin film counter electrode

Abstract

Cobalt sulfide (CoS), nickel sulfide (NiS), and cobalt sulfide/nickel sulfide (CoS/NiS) were deposited onto fluorine-doped tin oxide (FTO) substrate using a facile chemical bath deposition method and utilized as counter electrodes (CEs) for polysulfide redox reactions in CdS/CdSe quantum dot-sensitized solar cells (QDSSCs). The thickness of 750 nm and 695 nm are optimized for NiS and CoS electrodes to prepare the CoS/NiS CE. Compared to a platinum (Pt) electrode, the CoS, NiS, and composite CoS/NiS electrodes provide higher electrocatalytic activity and lower charge-transfer resistance. The combination of a QDSSC with composite CoS/NiS CE shows an improved power conversion efficiency of 3.40% under the illumination of one sun (100 mW cm−2), which is higher than the CoS (2.53%), NiS (2.61%), and Pt (1.47%) CEs. This enhancement is mainly attributed to the NiS nanoparticles deposited on CoS film, due to which the composite structure exhibits a lower charge transfer resistance (7.61 Ω) at the interface of the CE and the electrolyte, along with superior electrochemical catalytic ability. This is well supported by the cyclic voltammetry, electrochemical impedance spectroscopy, and Tafel polarization measurements.