Bernard Geffroy

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:PCBMMoS3-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.

Modulation of the Physicochemical Properties of Donor–Spiro–Acceptor Derivatives through Donor Unit Planarisation: Phenylacridine versus Indoloacridine—New Hosts for Green and Blue Phosphorescent Organic Light-Emitting Diodes (PhOLEDs)

This work reports a detailed structure-property relationship study of a series of efficient host materials based on the donor-spiro-acceptor (D-spiro-A) design for green and sky-blue phosphorescent organic light-emitting diodes (PhOLEDs). The electronic and physical effects of the indoloacridine (IA) fragment connected through a spiro bridge to different acceptor units, namely, fluorene, dioxothioxanthene or diazafluorene moiety, have been investigated in depth. The resulting host materials have been easily synthesised through short, efficient, low-cost, and highly adaptable synthetic routes by using common intermediates. The dyes possess a very high triplet energy (ET ) and tuneable HOMO/LUMO levels, depending on the strength of the donor/acceptor combination. The peculiar electrochemical and optical properties of the IA moiety have been investigated though a fine comparison with their phenylacridine counterparts to study the influence of planarisation. Finally, these molecules have been incorporated as hosts in green and sky-blue PhOLEDs. For the derivative SIA-TXO2 as a host, external quantum efficiencies as high as 23 and 14u2009% have been obtained for green and sky-blue PhOLEDs, respectively.

Thioxanthene and dioxothioxanthene dihydroindeno[2,1-b]fluorenes: synthesis, properties and applications in green and sky blue phosphorescent OLEDs

We report herein the synthesis, and structural, electrochem. and photophys. properties of new dihydroindeno[2,1-b]fluorene-based semi-conductors, i.e. dispiro[thioxanthene-9,5-indeno[2,1-b]fluorene-7,9-thioxanthene] 2 and dispiro-[dioxothioxanthene-9,5-indeno[2,1-b]fluorene-7,9-thioxanthene] 3. These properties will be compared to those of the pure hydrocarbon deriv. dispiro[fluorene-9,5-indeno[2,1-b]fluorene-7,9-fluorene] 1 previously described. The incorporation of the sulfur atom within the bridged units allows tuning the HOMO and LUMO energy levels, nevertheless retaining the high ET of the dihydroindeno[2,1-b]fluorenyl core ([2,1-b]IF), the key feature of further device applications. This control of the properties has allowed incorporating 3 as the host material in blue and green phosphorescent OLEDs (PhOLEDs) with an EQE of 12.8% (at 1 mA cm-2 with the corresponding CE and PE of 48.6 cd A-1 and 29.5 lm W-1) for the green device and an EQE of 4.8% (at 1 mA cm-2 with the corresponding CE and PE of 14.8 cd A-1 and 8.2 lm W-1) for the sky-blue device. To the best of our knowledge, this work is the second example of incorporation of a dihydroindeno[2,1-b]fluorene based semi-conductor in a blue PhOLED and highlights the potential of this fragment in org. electronics.

Spirobifluorene Regioisomerism: A Structure–Property Relationship Study

The present works report the first structure-property relationship study of a key class of organic semiconductors, that is, the four spirobifluorene positional isomers possessing a para-, meta- or ortho-linkage. The remarkable and surprising impact of the ring bridging and of the linkages on the electronic properties of the regioisomers has been particularly highlighted and rationalised. The impact of the ring bridging on the photophysical properties has been stressed with notably the different influence of the linkages and the bridge on the singlet and triplet excited states. The first member of a new family of spirobifluorenes substituted in the 1-position, which presents better performance in blue phosphorescent OLEDs than those of its regioisomers, is reported. These features highlight not only the great potential of 1-substituted spirobifluorenes, but also the remarkable impact of regioisomerism on electronic properties.

9H-Quinolino[3,2,1-k]phenothiazine: A New Electron-Rich Fragment for Organic Electronics.

A new electron-rich fragment, namely the quinolinophenothiazine (QPTZ) is reported. The QPTZ fragment incorporated in spiroconfigured materials leads to higher performance in blue Phosphorescent OLEDs than structurally related phenylacridine and indoloacridine based materials (increasing the HOMO energy level, modulating the spin-orbit coupling, etc.) and leads to highly efficient blue phosphorescent organic light emitting diodes, indicating the strong potential of this new molecular fragment in organic electronics.

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.

Zinc oxide as a hole blocking layer for perovskite solar cells deposited in atmospheric conditions

Zinc oxide (ZnO) is a widely used transparent conductive oxide owing to its tunable optoelectronic properties, particularly in the field of solar energy conversion. In this study a thin layer of this compound has been employed as a hole-blocking layer in a planar perovskite based solar cell in substitution for the classical titanium dioxide (TiO2) bilayer. The introduction of intrinsic ZnO between the perovskite layer and the degenerate n-type doped contact leads to a large reduction of the recombination at this interface. Optimization of the ZnO layer properties together with that of the perovskite deposition parameters leads to the measurement of a power conversion efficiency of 14.2% measured at a reverse scan on a 0.175 cm2 cell. Moreover, the ageing cell performance has been investigated over more than two months for both encapsulated and non-encapsulated devices. When a decrease of the fill factor with time, possibly related to the degradation of the cell contact, is observed, the short circuit current (>18 mA cm−2) and the open circuit (>1 V) potential remain remarkably constant.