An efficient electrocatalyst for the reduction of polysulfide electrolyte is vital to the construction of quantum dot sensitized solar cells (QDSCs). Carbonaceous materials would be the promising candidates for the fabrication of counter electrodes (CEs) on account of the competitive prices, good conductivity and corrosion inertness for the reduction of polysulfide electrolyte. Metal-organic frameworks (MOFs), especially N-rich zeolitic imidazolate frameworks (ZIFs), recently have been considered to be versatile precursors/sacrificial templates for the preparation of various homogeneous hetero-doping mesoporous carbons with predetermined functionalities.

In view of the abundant doped active sites and inherent larger surface area of ZIF derived carbon materials, we prepared Co,N-bidoped carbon nanomaterials simply via the pyrolysis of bimetallic (Zn and Co) ZIFs. In light of the inherent characteristics of heteroatom doped carbons and the benign catalytic performance of Cu nanoparticles (NPs), the N-C@Cu composites with 2-4 nm Cu NPs uniformly distributing in mesoporous N doped carbons prepared by carbonization of ZIF-8 were also synthesized. The obtained Co,N-bidoped and N-C@Cu composites were utilized as electrocatalysts to develop CEs for QDSCs. The CEs based on both Co,N-bidoped and N-C@Cu composites exhibit superior catalytic activity for polysulfide reduction in QDSCs, resulting in a low charge transfer resistance at the interface of CE/electrolyte, an improved fill factor (FF) and a high short circuit current density (J_sc). The outstanding performances of the CEs can be ascribed to the uniform distributed catalytic active sites, large hydrophilic surface area, and good conductivity. When Co,N-bidoped carbons deposited on fluorine doped tin oxide (FTO) glass were used as CEs, an impressive power conversion efficiency of 9.12% (V_oc = 0.635 V, J_sc = 26.15 mA·cm^-2, FF = 0.549) under one sun illumination with 100 mW·cm^-2 intensity was observed on QDSCs using Zn-Cu-In-Se QDs as sensitizers. Under the same conditions, the optimized N-C@Cu based QDSCs present a promising power conversion efficiency of 9.23%, significantly superior to those of pristine N-C/FTO (7.20%) and Cu₂S/FTO (8.89%) CE based QDSCs. Consequently, there is a good chance to fabricate QDSCs of high efficiency with the CE catalysts derived from MOFs.