Solution processed low-cost and highly electrocatalytic composite NiS/PbS nanostructures as a novel counter-electrode material for high-performance quantum dot-sensitized solar cells with improved stability

Abstract

The key challenges in boosting the power conversion efficiency (η) of quantum dot-sensitized solar cells (QDSSCs) are efficiently achieving charge separation at the photoanode and enhancing the charge transfer, which is limited by the interface between the polysulfide electrolyte and the counter-electrode (CE). We designed and fabricated new catalytic electrodes by combining a PbS nanoparticle catalyst with NiS nanoparticles by a facile chemical bath deposition method and optimizing the reaction conditions. These were used as CEs for polysulfide redox reactions in CdS/CdSe/ZnS QDSSCs. The PbS nano-morphologies were tuned from nanoparticles to nanospheres by controlling the PbS deposition time on the NiS surface. As the deposition time is increased, the surface morphology, the ratio of Ni:Pb:S, and the thickness of NiS and PbS are affected. The increase in the amount of PbS deposited on the NiS surface could improve the charge transfer at the CE/electrolyte interface. The optimized NiS/PbS composite CE shows a charge transfer resistance (R_ct) as low as 10.06 Ω, which is an order of magnitude lower than those of bare NiS (39.65 Ω), PbS (42.12 Ω) and Pt (99.71 Ω) CEs. Therefore, the NiS/PbS composite CEs show much higher catalytic activity for the polysulfide electrolyte than NiS, PbS and Pt CEs. As a result, the QDSSC using this newly synthesized NiS/PbS as a CE achieves a higher power conversion efficiency of 4.52% than the one applying NiS (3.26%) or PbS (3.06%) or Pt (1.29%) CEs. There was no degradation of the efficiency over 10 h under room conditions. This enhancement is mainly attributed to the improved electrocatalytic activity and improved absorption of NiS/PbS, which resulted in the absorption of residual long-wavelength solar irradiation by the PbS CE. This irradiation may have excited the PbS and facilitated the injection of electrons from the conduction band into the polysulfide electrolyte, leading to higher V_oc, J_sc, and FF. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and Tafel polarization measurements revealed that the composite CEs had better electrocatalytic activity, which improved the rate of polysulfide reduction compared to bare NiS and Pt CEs.