Designing Novel Poly(oxyalkylene)-Segmented Ester-Based Polymeric Dispersants for Efficient TiO2 Photoanodes of Dye-Sensitized Solar Cells.

A family of new polymeric dispersants, branched poly(oxyethylene)-segmented esters of trimellitic anhydride adduct (polyethylene glycol-trimethylolpropane-trimellitic anhydride, designated as PTT), were synthesized and utilized to homogeneously disperse TiO2 nanoparticles. The weight fraction of poly(oxyethylene)-segment in the dispersants and the molecular architecture in favoring the branched shape are two predominant factors for designing the effective dispersants. In particular, the poly(oxyethylene) block of 1000 g/mol from PEG1000 as the starting material and a total molecular weight of 12 000 g/mol have constituted the polymeric dispersants for the best performance for homogenizing TiO2 nanoparticles. The dispersant structures were characterized by using Fourier-transform infrared spectroscopy, acid value determination, and gel permeation chromatography. The TiO2 dispersibility was evaluated by dynamic light scattering and transmission electron microscopy. The synthesized dispersants were utilized to homogenize the as-prepared TiO2, further fabricated into films of photoanodes for dye-sensitized solar cells (DSSCs). The ultimate performance of DSSC was measured to be 8.17 ± 0.13% for the device efficiency (η) which was significantly higher than the conventional TiO2 photoanode at η = 7.14 ± 0.12%. The photoanode film was characterized by X-ray diffraction, Brunauer-Emmett-Teller surface area, and dye-loading amount measurements. The kinetics of photogenerated electron in the photoanode, including electron lifetime and electron transit time of the film, was studied via electrochemical impedance spectroscopy, intensity-modulated photocurrent spectroscopy, and intensity-modulated photovoltage spectroscopy.

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 (VOC) 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 VOC 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.

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%.