Charge Recombination with Fractional Reaction Orders in Water-Splitting Dye-Sensitized Photoelectrochemical Cells.

In water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs), charge recombination competes with catalytic water oxidation to determine the overall efficiency of the system. The kinetics of these processes have been difficult to understand because transient absorbance (TA) experiments typically show nearly complete charge recombination on the submillisecond time scale; in contrast, electrochemical measurements such as open circuit photovoltage decay suggest a charge recombination time scale that is 2-3 orders of magnitude longer. Here we explore these processes with dye-sensitized nanocrystalline TiO2 and TiO2/Ta2O5 core-shell photoanodes in aqueous electrolytes using TA spectroscopy, intensity-modulated photovoltage spectroscopy (IMVS), and photoelectrochemical impedance spectroscopy (PEIS). The fast recombination rates measured by TA result from strong laser excitation that leads to high electron occupancy in TiO2, whereas IMVS modulates the concentration of charge-separated states near solar irradiance levels. The recombination processes measured by electrochemical methods such as IMVS, PEIS, and transient photovoltage are the discharging of injected electrons in TiO2, as evidenced by the close agreement between the nearly first-order recombination rates probed by IMVS and the RC time constants derived from PEIS data. However, IMVS measurements at variable probe light intensity reveal that the reaction orders for the recombination of injected electrons with oxidized sensitizer molecules are far from unity. This kinetic analysis is relevant to understanding steady-state recombination rates in full WS-DSPECs in which molecular and nanoparticle catalysts are used to oxidize water.

Charged Polaron Polaritons in an Organic Semiconductor Microcavity.

We report strong coupling between light and polaron optical excitations in a doped organic semiconductor microcavity at room temperature. Codepositing MoO_{3} and the hole transport material 4, 4^{'}-cyclohexylidenebis[N, N-bis(4-methylphenyl)benzenamine] introduces a large hole density with a narrow linewidth optical transition centered at 1.8 eV and an absorption coefficient exceeding 10^{4} cm^{-1}. Coupling this transition to a Fabry-Pérot cavity mode yields upper and lower polaron polariton branches that are clearly resolved in angle-dependent reflectivity with a vacuum Rabi splitting ℏΩ_{R}>0.3 eV. This result establishes a path to electrically control polaritons in organic semiconductors and may lead to increased polariton-polariton Coulombic interactions that lower the threshold for nonlinear phenomena such as polariton condensation and lasing.

Large bipolaron density at organic semiconductor/electrode interfaces.

Bipolaron states, in which two electrons or two holes occupy a single molecule or conjugated polymer segment, are typically considered to be negligible in organic semiconductor devices due to Coulomb repulsion between the two charges. Here we use charge modulation spectroscopy to reveal a bipolaron sheet density >1010 cm-2 at the interface between an indium tin oxide anode and the common small molecule organic semiconductor N,N'-Bis(3-methylphenyl)-N,N'-diphenylbenzidine. We find that the magnetocurrent response of hole-only devices correlates closely with changes in the bipolaron concentration, supporting the bipolaron model of unipolar organic magnetoresistance and suggesting that it may be more of an interface than a bulk phenomenon. These results are understood on the basis of a quantitative interface energy level alignment model, which indicates that bipolarons are generally expected to be significant near contacts in the Fermi level pinning regime and thus may be more prevalent in organic electronic devices than previously thought.

Modulating short wavelength fluorescence with long wavelength light.

Two molecules in which the intensity of shorter-wavelength fluorescence from a strong fluorophore is modulated by longer-wavelength irradiation of an attached merocyanine-spirooxazine reverse photochromic moiety have been synthesized and studied. This unusual fluorescence behavior is the result of quenching of fluorophore fluorescence by the thermally stable, open, zwitterionic form of the spirooxazine, whereas the photogenerated closed, spirocyclic form has no effect on the fluorophore excited state. The population ratio of the closed and open forms of the spirooxazine is controlled by the intensity of the longer-wavelength modulated light. Both square wave and sine wave modulation were investigated. Because the merocyanine-spirooxazine is an unusual reverse photochrome with a thermally stable long-wavelength absorbing form and a short-wavelength absorbing photogenerated isomer with a very short lifetime, this phenomenon does not require irradiation of the molecules with potentially damaging ultraviolet light, and rapid modulation of fluorescence is possible. Molecules demonstrating these properties may be useful in fluorescent probes, as their use can discriminate between probe fluorescence and various types of adventitious "autofluorescence" from other molecules in the system being studied.