White organic light-emitting diodes with an ultra-thin premixed emitting layer

We described an approach to achieve fine color control of fluorescent White Organic Light-Emitting Diodes (OLED), based on an Ultra-thin Premixed emitting Layer (UPL). The UPL consists of a mixture of two dyes (red-emitting 4-di(4'-tert-butylbiphenyl-4-yl)amino-4'-dicyanovinylbenzene or fvin and green-emitting 4-di(4'-tert-butylbiphenyl-4-yl)aminobenzaldehyde or fcho) premixed in a single evaporation cell: since these two molecules have comparable structures and similar melting temperatures, a blend can be evaporated, giving rise to thin films of identical and reproducible composition compared to those of the pre-mixture. The principle of fine color tuning is demonstrated by evaporating a 1-nm-thick layer of this blend within the hole-transport layer (4,4'-bis[N-(1-naphtyl)-N-phenylamino]biphenyl (\alpha-NPB)) of a standard fluorescent OLED structure. Upon playing on the position of the UPL inside the hole-transport layer, as well as on the premix composition, two independent parameters are available to finely control the emitted color. Combined with blue emission from the heterojunction, white light with Commission Internationale de l'Eclairage 1931 color coordinates (0.34, 0.34) was obtained, with excellent color stability with the injected current. The spectrum reveals that the fcho material does not emit light due to efficient energy transfer to the red-emitting fvin compound but plays the role of a host matrix for fvin, allowing for a very precise adjustment of the red dopant amount in the device

Operating mechanism of the organic metal-semiconductor field-effect transistor (OMESFET)

International audienceOrganic metal-semiconductor field-effect transistors (OMESFETs) were fabricated with a polycrystalline organic semiconductor (pentacene) and characterized in order to systematically analyze their operation mechanism. Impedance measurements confirmed full depletion of the thick pentacene film (1 μm) due to the low doping concentration of unintentional doping (typically less than 10^14 cm^−3). The necessity of developing a specific device model for OMESFET is emphasized as the classical (inorganic) MESFET theory based on the depletion modulation is not applicable to a fully-depleted organic semiconductor. By means of joint electrical measurements and numerical simulation, it is pointed out that the gate voltage controls the bulk distribution of injected carriers, so that the competition between the gate and drain currents is critical for determining the operation mode. Finally, the geometrical effect is investigated with comparing a number of transistors with various channel widths and lengths

Phosphorus-based chromophores: Emitters for OLEDs

International audienceIn this paper, we present the development of white OLEDs (WOLEDs) using organophosphorus derivatives as fluorescent emitters. The structural changes that we have performed on and around the phosphole ring (functionalization of the phosphorus atom, nature of the substituents at the 2 and 5 position) allow the ability to tune the emission wavelengths and the color emitted by the OLEDs. Using this concept, we developed organophosphorus dopants emitting orange light, which can be mixed with a blue emitting matrix for the development of WOLEDs. The OLEDs structure allows the development of flexible OLEDs with good performance and CIE coordinates which are independent from the current density

Properties modulation of organic semi-conductors based on a donor-spiro-acceptor (D-spiro-A) molecular design: new host materials for efficient sky-blue PhOLEDs

International audienceFour high triplet org. semi-conductors based on the donor-spiro-acceptor design (D-spiro-A) have been synthesized. Their physicochem. and photophys. properties have been studied, compared and discussed in light of the nature of their resp. donor/acceptor units. The four compds. have been used as host materials in efficient sky-blue (EQE \textgreater 10% at 10 mA cm-2) phosphorescent org. light emitting diodes

Capacitive behavior of pentacene-based diodes: Quasistatic dielectric constant and dielectric strength

International audienceThe capacitive behavior of pentacene films was investigated in the metal-semiconductor-metal (MSM) diode structure. Impedance analysis of diodes with a thick pentacene layer up to 1012 nm showed a full depletion of the organic layer. This observation allowed us to regard the MSM diode as a parallel-plate capacitor in the reverse-bias regime without current flow. Under forward-bias, the diode was evaluated through frequency-dependent impedance measurements by using an equivalent circuit composed of a single parallel resistance-capacitance circuit. The analysis of the data in both the reverse and forward bias regime led us to electrical methods for quantifying dielectric properties of pentacene

Flexible organic–inorganic hybrid layer encapsulation for organic opto-electronic devices

In this work we produce and study the flexible organic–inorganic hybrid moisture barrier layers for the protection of air sensitive organic opto-electronic devices. The inorganic amorphous silicon nitride layer (SiNx:H) and the organic PMMA [poly (methyl methacrylate)] layer are deposited alternatingly by using hot wire chemical vapor deposition (HW-CVD) and spin-coating techniques, respectively. The effect of organic–inorganic hybrid interfaces is analyzed for increasing number of interfaces. We produce highly transparent (∼80% in the visible region) hybrid structures. The morphological properties are analysed providing a good basis for understanding the variation of the water vapor transmission rate (WVTR) values. A minimum WVTR of 4.5 × 10−5g/m2day is reported at the ambient atmospheric conditions for 7 organic/inorganic interfaces. The hybrid barriers show superb mechanical flexibility which confirms their high potential for flexible applications.The authors would like to thank Dr. J.C. Vanel for help in electrical characterizations used in this study. The first author (S.M) acknowledges the financial support from Direction des Relations Extérieures, Ecole Polytechnique during his thesis

Influence of low energy argon plasma treatment on the moisture barrier performance of hot wire-CVD grown SiNx multilayers

The reliability and stability are key issues for the commercial utilization of organic photovoltaic devices based on flexible polymer substrates. To increase the shelf-lifetime of these devices, transparent moisture barriers of silicon nitride (SiNx) films are deposited at low temperature by hot wire CVD (HW-CVD) process. Instead of the conventional route based on organic/inorganic hybrid structures, this work defines a new route consisting in depositing multilayer stacks of SiNx thin films, each single layer being treated by argon plasma. The plasma treatment allows creating smoother surface and surface atom rearrangement. We define a critical thickness of the single layer film and focus our attention on the effect of increasing the number of SiNx single-layers on the barrier properties. A water vapor transmission rate (WVTR) of 2 x 10-4 g/(m2 day) is reported for SiNx multilayer stack and a physical interpretation of the plasma treatment effect is given.Direction des Relations Extérieures, Ecole PolytechniquePICS (French–Portuguese No. 5336) projec

2-Substituted vs 4-substituted-9,9′-spirobifluorene host materials for green and blue phosphorescent OLEDs: a structure-property relationship study

Nanotek for organic synthesis, and organic synthesis for nanotekInternational audienceWe report a structure-property relationship study of four 9,9′-spirobifluorene (SBF) derivatives (4-5Pm-SBF, 2-5Pm-SBF, 4-Ph-SBF and 2-Ph-SBF), substituted with either phenyl or pyrimidine at the \C2\ or \C4\ position of the \SBF\ core. Structural, thermal, electrochemical and photophysical properties have been examined and correlated to theoretical calculations in order to study the influence of the nature and the position of the substituent. The emission properties of 4- versus 2-substituted \SBFs\ are noticeably different highlighting, in the excited state, the remarkable effect of substitution in ortho position of SBF. All compounds have been used as host material for green dopant in PhOLEDs with very high performances (2-5Pm-SBF: CE>58 cd/A, PE>35 lm/W, EQE>14%). More importantly, the two 4-substituted \SBFs\ have been used as host materials in blue PhOLEDs, displaying high performance and a decrease of \VTH\ for 4-5Pm-SBF due to the incorporation of the electron-withdrawing pyrimidine

Persistent photoexcitation effect on the poly(3-hexylthiophene) film: Impedance measurement and modeling

International audienceWe report on the equivalent circuit modeling of the relaxation behavior of an optically excited thick poly(3-hexylthiophene) (P3HT) film by means of impedance spectroscopy. Fabricated metal-semiconductor-metal devices with Au electrodes showed a nearly perfect ohmic behavior under ambient conditions. Impedance measurements on illuminated P3HT device showed a dramatical decrease of the impedance modulus under illumination and very slow relaxation to the initial state. Impedance-frequency data obtained during relaxation could not be explained by a simple parallel resistance-capacitance circuit but it could be best fitted by incorporating a constant-phase element instead of a normal capacitance. By observing the variation of the circuit parameters, it is found that the relaxation process is dominated by slow recombination (elimination) of the excess photogenerated carriers, which is confirmed by the time-varying photoconductivity of the device

Synthesis, electronic properties and OLED devices of chromophores based on λ5‐phosphinines

A new series of 2,4,6‐triaryl‐λ5‐phosphinines have been synthesized that contain different substituents both on the carbon backbone and the phosphorus atom of the six‐membered heterocycle. Their optical and redox properties were studied in detail, supported by in‐depth theoretical calculations. The modularity of the synthetic strategy allowed the establishment of structure–property relationships for this class of compounds and an OLED based on a blue phosphinine emitter could be developed for the first time