Design rules for high mobility xanthene-based hole transport materials

Tunable and highly conductive hole transport materials are crucial for the performance of organic electronics applications such as organic light emitting diodes and perovskite solar cells. For commercial applications, these materials\u27 requirements include easy synthesis, high hole mobility, and highly tuned and compatible electronic energy levels. Here, we present a systematic study of a recently discovered, easy-to-synthesize class of spiro[fluorene-9,9′-xanthene]-based organic hole transport materials. Systematic side group functionalization allows us to control the HOMO energy and charge carrier mobility. Analysis of the bulk simulations enables us to derive design rules for mobility enhancement. We show that larger functional groups (e.g. methyl) decrease the conformational disorder due to steric effects and thus increase the hole mobility. Highly asymmetric or polar side groups (e.g. fluorine), however, increase the electrostatic disorder and thus reduce the hole mobility. These generally applicable design rules will help in the future to further optimize organic hole transport materials

Derivatization of Bichromic Cyclometalated Ru(II) Complexes with Hydrophobic Substituents

The syntheses and physical properties of cyclometalated Ru­(II) complexes containing a triphenylamine (TPA) unit bearing aliphatic groups are reported. Each member of the series consists of an octahedral Ru­(II) center coordinated by a tridentate polypyridyl ligand and a tridentate cyclometalating ligand. One of the chelating ligands contains electron-deficient methyl ester groups, while a TPA unit is attached to the central ring of the adjacent chelating ligand through a thiophene bridge. This study builds on our previous work (<i>Inorg. Chem</i>. <b>2011</b>, 50, 6019–6028; <i>Inorg. Chem</i>. <b>2011</b>, <i>50</i>, 5494–5508) by (i) outlining a synthetic protocol for installing aliphatic groups on the TPA substituents, (ii) examining the role that terminal −O-hexyl and −S-hexyl groups situated on the TPA have on the electrochemical properties, and (iii) demonstrating the potential benefit of installing the TPA on the <i>neutral</i> chelating ligand rather than the <i>anionic</i> chelating ligand. The results reported herein provide important synthetic advances for our broader goal of developing bis-tridentate cyclometalated Ru­(II) complexes for light-harvesting applications

Atomic Level Resolution of Dye Regeneration in the Dye-Sensitized Solar Cell

Two donor–acceptor organic dyes have been synthesized that differ only by a two-heteroatom change from oxygen to sulfur within the donor unit. The two dyes, (<i>E</i>)-3-(5-(4-(bis­(4-(hexyloxy)­phenyl)­amino)­phenyl)­thiophen-2-yl)-2-cyanoprop-2-enoic acid (<b>Dye-O</b>) and (<i>E</i>)-3-(5-(4-(bis­(4-(hexylthio)­phenyl)­amino)­phenyl)­thiophen-2-yl)-2-cyanoprop-2-enoic acid) (<b>Dye-S</b>), were tested in solar cell devices employing both I₃^–/I^–-based and [Co­(bpy)₃]^3+/2+ redox mediators. Power conversion efficiencies over 6% under simulated AM 1.5 illumination (1 Sun) were achieved in both electrolytes. Despite similar optical and redox properties for the two dyes, a consistently higher open-circuit voltage (<i>V</i>_oc) was measured for <b>Dye-S</b> relative to <b>Dye-O</b>. The improved efficiency observed with <b>Dye-S</b> in an iodide redox mediator is against the commonly held view that sulfur atoms promote charge recombination attributed to inner-sphere interactions. Detailed mechanistic studies revealed that this is a consequence of a 25-fold enhancement of the regeneration rate constant that enhances the regeneration yield under open circuit conditions. The data show that a high short circuit photocurrent does not imply optimal regeneration efficiency as is often assumed

Electrolysis of CO₂ to Syngas in Bipolar Membrane-Based Electrochemical Cells

The electrolysis of CO₂ to syngas (CO + H₂) using nonprecious metal electrocatalysts was studied in bipolar membrane-based electrochemical cells. Electrolysis was carried out using aqueous bicarbonate and humidified gaseous CO₂ on the cathode side of the cell, with Ag or Bi/ionic liquid cathode electrocatalysts. In both cases, stable currents were observed over a period of hours with an aqueous alkaline electrolyte and NiFeO_<i>x</i> electrocatalyst on the anode side of the cell. In contrast, the performance of the cells degraded rapidly when conventional anion- and cation-exchange membranes were used in place of the bipolar membrane. In agreement with earlier reports, the Faradaic efficiency for CO₂ reduction to CO was high at low overpotential. In the liquid-phase bipolar membrane cell, the Faradaic efficiency was stable at about 50% at 30 mA/cm² current density. In the gas-phase cell, current densities up to 200 mA/cm² could be obtained, albeit at lower Faradaic efficiency for CO production. At low overpotentials in the gas-phase cathode cell, the Faradaic efficiency for CO production was initially high but dropped within 1 h, most likely because of dewetting of the ionic liquid from the Bi catalyst surface. The effective management of protons in bipolar membrane cells enables stable operation and the possibility of practical CO₂ electrolysis at high current densities

Electrolysis of Gaseous CO₂ to CO in a Flow Cell with a Bipolar Membrane

The conversion of CO₂ to CO is demonstrated in an electrolyzer flow cell containing a bipolar membrane at current densities of 200 mA/cm² with a Faradaic efficiency of 50%. Electrolysis was carried out by delivering gaseous CO₂ at the cathode with a silver catalyst integrated in a carbon-based gas diffusion layer. Nonprecious nickel foam in a strongly alkaline electrolyte (1 M NaOH) was used to mediate the anode reaction. While a configuration where the anode and cathode were separated by only a bipolar membrane was found to be unfavorable for robust CO₂ reduction, a modified configuration with a solid-supported aqueous layer inserted between the silver-based catalyst layer and the bipolar membrane enhanced the cathode selectivity for CO₂ reduction to CO. We report higher current densities (200 mA/cm²) than previously reported for gas-phase CO₂ to CO electrolysis and demonstrate the dependence of long-term stability on adequate hydration of the CO₂ inlet stream