A novel strategy has been successfully developed for highly efficient nanosheet-structured NiS counter electrodes. The NiS was deposited on FTO substrate with different deposition times using the simple and cost-effective chemical bath deposition technique. The NiS CEs were used to grow high quality thin films containing nanoparticles, nanosheets, or nanorods. The nanosheetstructured NiS CE in QDSSCs under one-sun illumination (AM 1.5, 100 Mw cm −2 ) yielded a high short circuit current density (J sc ) of 13.53 mA cm −2 , open circuit voltage (V oc ) of 0.570 V, fill factor (FF) of 0.450, and power conversion efficiency (η) of 3.47%. These values are much higher than those of the Pt CE (J sc = 7.85 mA cm −2 , V oc = 0.611, FF = 0.243, and η = 1.170%). The NiS was strongly adhered on the FTO substrate by acetic acid which acts as stabilizer and strong reagent in this one step preparation. The performance of NiS CE was improved by the surface morphology, which enable rapid electron transport and a lower electron recombination rate for the polysulfide electrolyte redox couple. In the present study NiS has obtained higher electrocatalytic activity which plays a crucial role in the QDSSC. Electrochemical impedance spectroscopy and Tafel-polarization measurements were used to investigate the electrocatalytic activity of the NiS and Pt CEs.
■ INTRODUCTIONIncreasing energy demands and global population over global warming led to focus on renewable energy sources. In the past few years quantum dot sensitized solar cells (QDSSCs) represents a one of the new photovoltaic devices that could emerged as a promising requirements of third generation solar cells. 1 The QDSSC device structure similarly to the dye sensitized solar cells (DSSC), 2−4 including a quantum dot (QD) loaded nonporous TiO 2 electrode, a polysulfide electrolyte, and platinum (Pt) counter electrode (CE) (except QD absorbers replacing the dye molecules that collect incident light in a DSSC). Advantages of inorganic semiconductor sensitizers over conventional dyes their low production cost, acceptable power conversion efficiency, 5,6 attractive photon harvesters due to their high molar extinction coefficient and the ability to tune their optical band gap through size control. 7,8 In addition, quantum dots (QDs) have large intrinsic dipole moment, 9 multiple exciton generation (MEG) 10 and hot carrier transfer, 11 grants QDs unique advantages as light absorbers that leads to rapid charge separation. However, the efficiency of QDSSC is still very low and the most recently reported conversion efficiency of QDSSCs (typically about 7%) 12 are still far below than DSSCs (12%) 3,13 and its theoretical efficiency value (44%)., 14−16 Poor charge transfer to the oxidized polysulfide redox species (S n 2− ) on CEs is considered to be a major hurdle in attaining a high fill factor and conversion efficiency in QDSSCs. 16−18 In DSSCs, Pt has exhibited excellent electrocatalytic activity due to its stability for the reduction of I 3 − , but Pt is very poor for the reductio...