Electrocatalytic Zinc Composites as the Efficient Counter Electrodes of Dye-Sensitized Solar Cells: Study on the Electrochemical Performances and Density Functional Theory Calculations

Highly efficient zinc compounds (Zn₃N₂, ZnO, ZnS, and ZnSe) have been investigated as low-cost electrocatalysts for the counter electrodes (CE) of dye-sensitized solar cells (DSSCs). Among them, Zn₃N₂ and ZnSe are introduced for the first time in DSSCs. The zinc compounds were separately mixed with a conducting binder, poly­(3,4-ethylene-dioxythiophene):poly­(styrenesulfonate) (PEDOT:PSS), and thereby four composite films of Zn₃N₂/PEDOT:PSS, ZnO/PEDOT:PSS, ZnS/PEDOT:PSS, and ZnSe/PEDOT:PSS were coated on the tin-doped indium oxide (ITO) substrates through a simple drop-coating process. In the composite film, nanoparticles of the zinc compound form active sites for the electrocatalytic reduction of triiodide ions, and PEDOT:PSS provides a continuous conductive matrix for fast electron transfer. By varying the weight percentage (5–20 wt %) of a zinc compound with respect to the weight of the PEDOT:PSS, the optimized concentration of a zinc compound was found to be 10 wt % in all four cases, based on the photovoltaic performances of the corresponding DSSCs. At this concentration (10 wt %), the composites films with Zn₃N₂ (Zn₃N₂-10), ZnO (ZnO-10), ZnS (ZnS-10), and ZnSe (ZnSe-10) rendered, for their DSSCs, power conversion efficiencies (η) of 8.73%, 7.54%, 7.40%, and 8.13%, respectively. The difference in the power conversion efficiency is explained based on the electrocatalytic abilities of those composite films as determined by cyclic voltammetry (CV), Tafel polarization plots, and electrochemical impedance spectroscopy (EIS) techniques. The energy band gaps of the zinc compounds, obtained by density functional theory (DFT) calculations, were used to explain the electrocatalytic behaviors of the compounds. Among all the zinc-based composites, the one with Zn₃N₂-10 showed the best electrocatalytic ability and thereby rendered for its DSSC the highest η of 8.73%, which is even higher than that of the cell with the traditional Pt CE (8.50%). Therefore, Zn₃N₂ can be considered as a promising inexpensive electrocatalyst to replace the rare and expensive Pt

Multifunctional Iodide-Free Polymeric Ionic Liquid for Quasi-Solid-State Dye-Sensitized Solar Cells with a High Open-Circuit Voltage

A polymeric ionic liquid, poly­(oxyethylene)-imide-imidazolium selenocyanate (POEI-IS), was newly synthesized and used for a multifunctional gel electrolyte in a quasi-solid-state dye-sensitized solar cell (QSS-DSSC). POEI-IS has several functions: (a) acts as a gelling agent for the electrolyte of the DSSC, (b) possesses a redox mediator of SeCN^–, which is aimed to form a SeCN^–/(SeCN)₃^– redox couple with a more positive redox potential than that of traditional I^–/I₃^–, (c) chelates the potassium cations through the lone pair electrons of the oxygen atoms of its poly­(oxyethylene)-imide-imidazolium (POEI-I) segments, and (d) obstructs the recombination of photoinjected electrons with (SeCN)₃^– ions in the electrolyte through its POEI-I segments. Thus, the POEI-IS renders a high open-circuit voltage (<i>V</i>_OC) to the QSS-DSSC due to its functions of b–d and prolongs the stability of the cell due to its function of a. The QSS-DSSC with the gel electrolyte containing 30 wt % of the POEI-IS in liquid selenocyanate electrolyte exhibited a high <i>V</i>_OC of 825.50 ± 3.51 mV and a high power conversion efficiency (η) of 8.18 ± 0.02%. The QSS-DSSC with 30 wt % POEI-IS retained up to 95% of its initial η after an at-rest stability test with the period of more than 1,000 h