Yi-June Huang

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

UNLABELLED 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.

Hierarchical TiO1.1Se0.9-wrapped carbon cloth as the TCO-free and Pt-free counter electrode for iodide-based and cobalt-based dye-sensitized solar cells

A titanium oxide-selenide (TiO1.1Se0.9) composite material was successfully obtained on a flexible substrate of carbon cloth (CC); this TiO1.1Se0.9/CC electrode was used as the counter electrode (CE) in a dye-sensitized solar cell (DSSC). Each carbon fiber in the CC was wrapped with a composite layer of TiO1.1Se0.9, and the composite layer contained TiO1.1Se0.9 nanospheres and one-dimensional (1D) nanorods. Thus, each carbon fiber with its TiO1.1Se0.9 layer has a hierarchical core–shell structure, in which the carbon fiber acts as a one-dimensional conductive core for electron transfer and the TiO1.1Se0.9 layer around the fiber functions as an electro-catalytic shell. In iodide-based electrolyte, the DSSC with the TiO1.1Se0.9/CC exhibits a power conversion efficiency (η) of 9.47%, which is higher than cells with CEs of Pt/CC (7.75%) and pristine TiO2/CC (4.90%). Under dim light conditions, the best TiO1.1Se0.9/CC electrode rendered its cell an η of 10.00% at 0.5 sun and the best η of 10.39% at 0.1 sun. In cobalt-based electrolytes, a DSSC with the TiO1.1Se0.9/CC shows an attractive η of 10.32% at 1.0 sun, the best η of 10.47% at 0.5 sun, and an impressive η of 10.20% at 0.1 sun. The earth abundant and low-cost TiO1.1Se0.9 is a promising alternative to Pt for the CE of a DSSC in both iodide and cobalt electrolytes.

Electrospun membranes of imidazole-grafted PVDF-HFP polymeric ionic liquids for highly efficient quasi-solid-state dye-sensitized solar cells

Three novel polymeric ionic liquids (PILs), denoted as PFII-F, PFII-E, and PFII-S, are successfully synthesized by grafting different molar ratios (one-fourth, one eighth, and one sixteenth, respectively) of 1-butylimidazolium iodide onto poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). These PFII PILs are fabricated as polymer membranes via a simple electrospinning technique, which are used as the electrolyte for quasi-solid-state (QSS) dye-sensitized solar cells (DSSCs). The PFII membranes have multiple functions including: (1) encapsulation of the liquid electrolyte with good charge transfer and ionic conductivity properties, (2) chelation of Li+ through the lone pair electrons on their fluoride atoms, and (3) filling the dye-uncovered TiO2 surface with their imidazolium segment. Function (1) endows DSSCs with prominent long-term stability, while functions (2) and (3) suppress the dark current. The best QSS-DSSC with the PFII-F membrane shows a larger open-circuit voltage (VOC), comparable short-circuit current density (JSC), better power conversion efficiency (η) of 9.26%, and superior long-term stability (up to 97% of its initial η) over 1500 h compared to the cell with standard liquid electrolyte (8.63%).

A zeolitic imidazolate framework-derived ZnSe/N-doped carbon cube hybrid electrocatalyst as the counter electrode for dye-sensitized solar cells

Owing to its excellent electrocatalytic performance, zinc selenide (ZnSe) is regarded as a promising counter electrode (CE) material for dye-sensitized solar cells (DSSCs). In this study, a zeolitic imidazolate framework (ZIF-7) derived ZnSe nanocomposite is introduced as the electrocatalyst for the CE in a DSSC. First, ZIF-7 powder was subjected to carbonization and then transformed into ZIF-7/N-doped carbon (ZIF-NC). To further increase the electrocatalytic ability of the materials, ZIF-7-NC was selenized and transformed into a ZIF-7 derived ZnSe/N-doped carbon cube (ZIF-ZnSe-NC). After optimizing the weight percentage of the ZIF-ZnSe-NC film, the DSSCs with a CE of ZIF-ZnSe-NC-11 wt% rendered a photovoltaic conversion efficiency (η) of 8.69 ± 0.13%, which is higher than that of the cell with a Pt CE (8.26 ± 0.02%). This study concludes that the CE based on the ZIF-ZnSe-NC is a prospective substitute for Pt and could provide new opportunities for advancing high-efficiency DSSCs under in-house conditions with dim light illumination.