Ling-Yu Chang

Facile fabrication of PtNP/MWCNT nanohybrid films for flexible counter electrode in dye-sensitized solar cells

A platinum nanoparticle/multi-wall carbon nanotube (PtNP/MWCNT) hybrid counter electrode (CE) based on a flexible substrate, Ti foil, was prepared for a high performance dye-sensitized solar cell (DSSC) via a facile fabricating route. This flexible nanohybrid CE was established by using a requisite homemade dispersant, consisting of poly(oxyethylene) segment and imide linkage functionalities. MWCNTs were well suspended in an ethanol/water solution in the presence of copolymer dispersant and PtNPs. The solution containing the PtNP/MWCNT (2/1 weight ratio) hybrid was further coated into a thin film on the Ti foil by the doctor blade technique, followed by annealing at 390 °C to obtain the flexible PtNP/MWCNT hybrid CE. The solar-to-electricity conversion efficiency (η) of a DSSC with the flexible PtNP/MWCNT hybrid CE gave a higher value of 9.04% in comparison to that of the cell with a conventional Pt CE (η = 7.47%). A rougher surface morphology of the nanohybrid film precisely controlled by the configuration of MWCNT was obtained, with reference to that of a Pt-sputtered film. The PtNP/MWCNT hybrid film was physically characterized by scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. Cyclic voltammetry, incident-photon-to-current efficiency, and electrochemical impedance spectra were examined for confirming the high electro-catalytic ability of this flexible PtNP/MWCNT hybrid CE.

Dye‐Sensitized Solar Cells with Reduced Graphene Oxide as the Counter Electrode Prepared by a Green Photothermal Reduction Process

Highly conductive reduced graphene oxide (rGO) with good electrocatalytic ability for reducing triiodide ions (I3(-)) is a promising catalyst for the counter electrode (CE) of dye-sensitized solar cells (DSSCs). However, hazardous chemical reducing agents or energy-consuming thermal treatments are required for preparing rGO from graphene oxide (GO). Therefore, it is necessary to find other effective and green reduction processes for the preparation of rGO and to fabricate rGO-based DSSCs. In this study, GO was prepared using a modified Hummers method from graphite powder, and further reduced to rGO through a photothermal reduction process (to give P-rGO). P-rGO shows better electrocatalytic ability due mainly to its high standard heterogeneous rate constant for I3(-) reduction and in part to its considerable electrochemical surface area. The corresponding DSSC shows a higher cell efficiency (η) of 7.62% than that of the cell with a GO-based CE (η=0.03%). When the low-temperature photothermal reduction process is applied to all-flexible plastic DSSCs, the DSSC with a P-rGO CE shows an η of 4.16%.

Facile Fabrication of Robust Superhydrophobic Epoxy Film with Polyamine Dispersed Carbon Nanotubes

Nanocomposite films of superhydrophobic surface are fabricated from the dispersion of unmodified carbon nanotubes (CNTs) and hydrophobic poly(isobutylene)-amine (PIB-amine). The PIB-amine prepared from the amidation of poly(isobutylene)-succinic anhydride and poly(oxypropylene)-amines is essential for dispersing the originally entangled CNTs into the debundled CNTs as observed by TEM. A robust CNTs/epoxy nanocomposite film with high dimensional stability is made by subsequent curing with epoxy resin. The self-standing film exhibits a superhydrophobic property, with water droplet contact angle > 152° due to the CNTs controlled alignment on the surface forming micrometer-size plateaus, as observed by SEM. The preparation of PIB-amine/CNTs dispersion and subsequently curing into a superhydrophobic CNTs/epoxy film is relatively simple and can potentially be applied to large surface coating.

Efficient titanium nitride/titanium oxide composite photoanodes for dye-sensitized solar cells and water splitting

Efficient titanium nitride/titanium oxide (TiN/TiO2) composite photoanodes have been proposed for the use not only in dye-sensitized solar cells (DSSCs) but also for water splitting. When the TiN precursor films were sintered at 500 °C for 0.5 to 4 h, they were partially converted to crystalline TiO2 containing both rutile and anatase phases. For the DSSCs, higher TiN content in the photoanodes resulted in a more negative flat-band potential and higher conductivity but lower surface area for the dye adsorption; therefore, the increase in VOC and FF but the decrease in JSC value were observed. The best DSSC, with TiN/TiO2 composite photoanode annealed for 1 h, exhibited a power conversion efficiency of 7.27%, while the cell without TiN, i.e., the cell with a standard P25 photoanode, showed an efficiency of 7.02%. For the water splitting, higher TiN content in the photoanodes resulted in better triggering of the H2O electrolysis, but less photo-induced current response at UV light illumination. Considering the water splitting performance measured at AM 1.5G, the TiN/TiO2 composite photoanode annealed for 1 h showed the best photo-induced current density (Jpho) of 0.12 mA cm−2, as compared with that of the standard P25 film. With the TiN/TiO2 composite photoanode annealed for 1 h, both DSSCs and water splitting electrochemical devices achieved their best performance independently.

Dye-sensitized solar cells with low-cost catalytic films of polymer-loaded carbon black on their counter electrode

Dye-sensitized solar cells (DSSCs), consisting of counter electrodes (CEs) with composite films made of carbon black (CB) and conducting polymers such as polypyrrole (PPy) and polyaniline (PANI), were fabricated. A slurry was first prepared with 99 wt% of conducting polymer-loaded CB using the requisite 1 wt% of poly(oxyethylene)-segmented imide (POEM) as the dispersant in ethanol. The conducting polymer-loaded CB was a commercial product with a composition of about 80 wt% of CB and 20 wt% of conducting polymer. The POEM dispersant was required to generate a homogeneous dispersion of the carbon material in the ethanolic solution. The dispersive ability of POEM for CB was demonstrated both visually and through UV–vis absorption spectra and the dynamic light scattering method (DLS). The slurries with PPy/CB and PANI/CB were coated on FTO glasses to prepare CEs for DSSCs. The DSSCs with CEs containing PPy/CB and PANI/CB have shown power conversion efficiencies (η) of 5.85 ± 0.23% and 6.77 ± 0.13%, respectively, while the cells with CEs containing bare CB and bare platinum have shown ηs of 5.35% and 7.24 ± 0.11%, respectively. The dispersive function of POEM for CB to impart high electrocatalytic abilities on the corresponding CEs was responsible for the better performance of the cells for both PPy/CB and PANI/CB than that of the cell with bare CB. Hydrogen-1 nuclear magnetic resonance (1H-NMR), gel permeation chromatography (GPC), scanning electronic microscopy (SEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), incident photo-to-current conversion efficiency (IPCE) spectra, Tafel polarization plots, and rotating disk electrode measurements (RDE) were used to substantiate the explanations.

Synthesis of a novel amphiphilic polymeric ionic liquid and its application in quasi-solid-state dye-sensitized solar cells

A novel polymeric ionic liquid (PIL), poly(oxyethylene)-imide-imidazole complex coupled with iodide anions (coded as POEI-II), was synthesized for preparing the gel electrolyte for quasi-solid-state dye-sensitized solar cells (QSS-DSSCs). Herein, POEI-II, which acts simultaneously as a redox mediator in the electrolyte and a polymer for the gelation of an organic solvent-based electrolyte, was used to improve cell durability. In the structure of POEI-II, the presence of the POE segment can chelate lithium cations (Li+) within the electrolyte to improve the open-circuit voltage (VOC) of a QSS-DSSC, and enable a strong dipole–dipole lone-pair electron interaction with iodide ions (I−) of electrolyte composition rendering the high ionic conductivity and the diffusivity of a redox couple within the gel electrolyte. Consequently, the QSS-DSSC with the POEI-II gel electrolyte reaches a high cell efficiency of 7.19%. In addition, 5 wt% of multi-wall carbon nanotubes (MWCNTs) are incorporated into the POEI-II gel electrolyte as the extended electron transfer material (EETM) to facilitate charge transfer from the counter electrode to the redox mediator, which benefits the dye regeneration more efficiently. Meanwhile, the POE segments in POEI-II can prevent the MWCNTs from aggregation, which makes the well dispersed MWCNTs largely exposed to redox mediators. The highest cell efficiency of 7.65% was achieved by using the POEI-II/MWCNT gel electrolyte and it showed an unfailing durability of greater than 1000 h under 50 °C. This properly designed PIL paves a promising way for developing highly efficient and durable QSS-DSSCs.

Enhanced performance of a dye-sensitized solar cell with an amphiphilic polymer-gelled ionic liquid electrolyte

A novel amphiphilic copolymer, poly(oxyethylene)-amide–imide (POEM), was synthesized and utilized for gelling an ionic liquid electrolyte. The polymer-gelled ionic liquid (PG-IL) electrolyte containing 1-methyl-3-propylimidazolium iodide (PMII), 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) and POEM has been employed as a quasi-solid-state electrolyte in DSSCs. By adding the POEM to a room temperature ionic liquid (RTIL) electrolyte up to 5 wt%, the binary electrolyte exhibited a quasi-solid-state. The structure of the PG-IL electrolyte with POE segments, which have lone pair electrons, could cause the ionic pair separation and enhance mobility of the counter I− ions. When fabricated into a quasi-solid-state dye-sensitized solar cell, the cell performance exhibited a high power conversion efficiency of 6.28% and long-term durability over 1000 h.

Control of morphology and size of platinum crystals through amphiphilic polymer-assisted microemulsions and their uses in dye-sensitized solar cells

A film of platinum nanostructure crystals is synthesized for the counter electrode (CE) of a dye-sensitized solar cell (DSSC) via an approach of water-in-oil microemulsions in the presence of an amphiphilic polymer, poly(oxyethylene)-segmented imide (POEM). Phase diagrams are constructed in mapping the compositions of the microemulsions to guide the synthesis of platinum nanostructure crystals. The conventional cationic surfactant, cetylmethyl ammonium bromide (CTAB), and anionic surfactant, docusate sodium salt (AOT), are compared with the newly developed POEM for preparing the emulsions. Transmission electron microscopic images reveal that the synthesized Pt nanostructure crystals have diversified morphologies, including nanospindle, nanosquare and nanosphere in correlation with the surfactants of CTAB, AOT, and POEM, respectively. X-ray diffraction patterns of the three Pt crystals indicates the presence of (111), (200) and (220) lattice planes of Pt. The Pt with different morphologies and structures are proved to be influential for catalytic ability when using them as Pt CEs in DSSCs. The DSSC with the CE containing the film of Pt nanospheres demonstrates a significantly improved power conversion efficiency of 9.34%, compared to 7.20% and 8.17% for the corresponding Pt nanosquare and Pt nanospindle films. Fourier transform spectroscopy (FTIR), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) are also used to substantiate the explanation for the DSSC performance.

Tailoring Pigment Dispersants with Polyisobutylene Twin-Tail Structures for Electrowetting Display Application

We have designed a class of highly hydrophobic dispersants for finely dispersing carbon black and organic pigment nanoparticles in apolar mediums. The synthesis involved the use of polyisobutylene-g-succinic anhydride (PIB-SA) and judiciously selected amines by amidation and imidation. The structures were characterized by infrared spectroscopy for anhydride functionalities in the starting materials and amide/imide linkages in the products. These polymeric forms of dispersants were structurally varied with respects to their PIB molecular weight, twin-tails, and linkages. Their relative performance for dispersing six different pigments in decane was evaluated against solution homogeneity, viscosity, stability, and particle size. The fine dispersion was achieved at particle sizes of ca. 100 nm, with the viscosity as low as 2-3 cP. The measurement of zeta potentials, which varied from -39.8 to -5.1 mV with pigment addition, revealed a strong surface-charge interaction between pigment and PIB dispersant molecules. Examination by TEM (transmission electronic microscope) showed the homogeneous dispersion of the primary structures of pigment particles at ca. 20 nm in diameter. The polymeric dispersants with different PIB tails and imide functionalities could be tailored for pigment stability in the oil phase, which is potentially suitable for the electrowetting devices.

Solid-State Ionic Liquid Based Electrolytes for Dye-Sensitized Solar Cells

The increasing global need for energy coupled with the depletion of easily accessible, hence cheap, fossil fuel reserves, poses a serious threat to the human global economy in the near future [1]. Considering in addition the harmful ecological impact of conventional energy sources, it becomes obvious that development of clean alternative energy sources is a neces‐ sity [2, 3]. Best renewable energy options must rely on a reliable input of energy onto the earth. Since the sun is our only external energy source, harnessing its energy, which is clean, non-hazardous and infinite, satisfies the main objectives of all alternative energy strategies. Mastering the conversion of sunlight to electricity or to a nonfossil fuel like hydrogen is without any doubt the most promising solution to the energy challenge. It is remarkable that a mere 10 min of solar irradiation onto the Earth’s surface is equal to the total yearly human energy consumption [4]. Therefore, solar power is considered to be one of the best sustaina‐ ble energies for future generations. To date photovoltaics has been dominated by solid-state junction devices, usually in silicon, crystalline or amorphous, and profiting from the experi‐ ence and materials availability resulting from the semiconductor industry. However, the ex‐ pensive and energy-intensive high-temperature and high-vacuum processes is needed for the silicon based solar cells. Therefore, the dominance of the photovoltatic field by such kind of inorganic solid-state junction devices is now being challenged by the emergence of a third generation solar cell based on interpenetrating network structures, such as dye-sensitized solar cells (DSSCs) [5].