Bruno Jousselme

All solution-processed organic photocathodes with increased efficiency and stability via the tuning of the hole-extracting layer

Photoelectrodes based on solution-processed organic semiconductors are emerging as low-cost alternatives to crystalline semiconductors and platinum. In this work, the performance and stability of P3HT:PCBM\MoS3-based photocathodes are considerably improved by changing the hole-extracting layer (HEL). Oxides such as reduced graphene oxide, nickel oxide or molybdenum oxide are deposited via solution processes. With MoOx, a photocurrent density of 2 mA cm−2 during 1 h is obtained with the processing temperature lower than 150 °C – thus compatible with flexible substrates. Furthermore, we show that the performances are directly correlated with the work function of the HEL material, and the comparison with solid-state solar cells shows that efficient HELs are not the same for the two types of devices.

Noble metal-free hydrogen-evolving photocathodes based on small molecule organic semiconductors

Organic semiconductors have great potential for producing hydrogen in a sustainable and economically-viable manner because they rely on readily available materials with highly tunable properties. We demonstrate here the relevance of heterojunctions to the construction of H2-evolving photocathodes, exclusively based on earth-abundant elements. Boron subnaphthalocyanine chloride proved a very promising acceptor in that perspective. It absorbs a part of the solar spectrum complementary to α-sexithiophene as a donor, thus generating large photocurrents and providing a record onset potential for light-driven H2 evolution under acidic aqueous conditions using a nanoparticulate amorphous molybdenum sulfide catalyst.

Covalently functionalized carbon nanotubes as stable cathode materials of lithium/organic batteries

Lithium batteries are among the most promising systems for electrochemical energy storage. However, their capacity and cost-efficiency have to be improved for further applications, for instance in electric vehicles. In this context, lithium/organic batteries offer an interesting alternative to the classical Li-ion systems. Indeed, organic materials offer a high electrochemical activity depending on their functional groups and can be cost-effective if their synthesis is simple and well-controlled. The major problem of the lithium/organic systems developed so far is the progressive dissolution of the active molecules in the electrolyte upon cycling, leading to poor capacity retention. In this work, new positive electrode materials avoiding the dissolution of the active material in the electrolyte upon cycling are developed, by grafting anthraquinone diazonium salts to carbon nanotubes. Carbon nanotubes are used both to bring electronic conductivity to the positive electrode and to serve as a support for covalent immobilization of the active material. The resulting systems remain very stable over prolonged cycling (80% of the initial capacity retained after 500 cycles) and present a promising specific capacity in the range of 100 mA h gelectrode−1.

A red to blue series of push–pull dyes for NiO based p-DSSCs

Finding efficient dyes for NiO photocathodes either for inverse or tandem DSSCs is essential to developing these promising low-cost solar cells. This paper reports the design, synthesis and physical property characterization of four new triphenylamine–bithiophene push–pull dyes with acceptors of increasing electronic affinity, in order to shift their absorption to the red region of the visible spectrum. The dyes were tested in a p-type DSSC configuration with 850 nm NiO ink-jet printed photocathodes and their performances were compared with that of the reference dye P1. With an iodine electrolyte, one of the dyes, possessing a 1,3-diethyl-2-thiobarbituric acceptor, shows superior performance to P1, with PCE reaching 0.124% and a JSC of 4.32 mA cm−2.

Insights into the mechanism and aging of a noble-metal free H

Co-grafting of a cobalt diimine–dioxime catalyst and push–pull organic dye on NiO yields a photocathode evolving hydrogen from aqueous solution under sunlight, with equivalent performances compared to a dyad-based architecture using similar components.

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There is a high interest in the development of new push-pull dyes for the use in dye sensitized solar cells. The pronounced charge transfer character of the directly photoexcited state is in principle favorable for a charge injection. Here, we report a time-resolved fluorescence study of a triphenylamine-bithiophene-naphthalimide dye in four solvents of varying polarity using fluorescence upconversion. The recording of femtosecond time-resolved fluorescence spectra corrected for the group velocity dispersion allows for a detailed analysis discriminating between spectral shifts and total intensity decays. After photoexcitation, the directly populated state (S1/FC) evolves toward a relaxed charge transfer state (S1/CT). This S1/CT state is characterized by a lower radiative transition moment and a higher nonradiative quenching. The fast dynamic shift of the fluorescence band is well described by solvation dynamics in polar solvents, but less so in nonpolar solvents, hinting that the excited-state relaxation process occurs on a free energy surface whose topology is strongly governed by the solvent polarity. This study underlines the influence of the environment on the intramolecular charge transfer (ICT) process, and the necessity to analyze time-resolved data in detail when solvation and ICT occur simultaneously.

-evolving dye-sensitized photocathode

The amazing properties of 2D materials are envisioned to revolutionize several domains such as flexible electronics, electrocatalysis, or biosensing. Herein we introduce scanning electrochemical microscopy (SECM) as a tool to investigate molybdenum disulfide in a straightforward fashion, providing localized information regarding the electronic transport within chemical vapor deposition (CVD)-grown crystalline MoS2 single layers having micrometric sizes. Our investigations show that within flakes assemblies some flakes are well electrically interconnected, with no detectable contact resistance, whereas others are not electrically connected at all, independent of the size of the physical contact between them. Overall, the work shows how the complex electronic behavior of MoS2 flake assemblies (semiconducting nature, contact quality between flakes) can be investigated with SECM.