P. Le Rendu

Thermal and chemical treatment of ITO substrates for improvement of OLED performance

Abstract We have investigated thin film of Indium Tin Oxide (ITO) deposited on glass substrates by thermal annealing and by H 3 PO 4 chemical etching in order to improve their quality for use in polymer based light-emitting diodes. The morphology of the film surface was significantly changed by both treatments as observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The sheet resistivity of the annealed sample was found to be dependent on the annealing temperature, while it was practically not affected by the chemical treatment. On the chemically treated samples, analysis of the film surface by X-ray photoelectron spectroscopy (XPS) revealed that a thin ionic phosphorus layer was formed on the ITO surface. The electrical and optical characteristics of light-emitting diodes with poly(2-methoxy,5-(2′-ethylhexyloxy)-1,4-phenylene vinylene) or MEH-PPV as an active layer, deposited on treated and untreated ITO substrates were determined and compared. Subsequent enhancement of performance of the diodes was obtained with treated ITO substrates, proving the efficiency of the performed treatments.

An X-ray photoelectron spectroscopy investigation into the interface formed between poly(2-methoxy-5-(2′-ethyl-hexyloxyl)-p-phenylene vinylene) and indium tin oxide

Abstract One of the most studied anode materials is indium tin oxide (ITO), due to its properties which favour hole injection to the devices. However, little has been known about the contact formed between the ITO and the polymer film. In this study, we have investigated the interface formed between ITO and poly(2-methoxy-5-(2′-ethyl-hexyloxyl)- p -phenylene vinylene) (MEH-PPV) by using X-ray-photoelectron spectroscopy (XPS). Analysis of the XPS spectra recorded as a function of the polymer coverage reveals diffusion of the indium oxide deep into the MEH-PPV polymer. On the other hand, interaction of the oxide with the polymer occurred in the interface, resulting in the formation of a carbide or a metal–oxygen–carbon complex in the contact region. The partial decomposition of ITO surface may result from the cleaning process of the substrate, that favours the formation of hydroxy species on the surface, then the diffusion of oxygen and indium into the polymer layer upon its deposition.

Electrical properties of phenylene vinylene oligomer thin films

In this work, we report the results of electrical investigations on phenylene vinylene oligomer thin films with 5-phenyl, 4-vinyl compound (4PV). Synthesis of the oligomer powder was performed using a Wittig reaction of phosphonium salt with carbonyl group and thin films were deposited by conventional thermal evaporation under vacuum of the obtained powder. The conduction mechanisms of oligomer-based diodes were studied by measuring the current-voltage-temperature characteristics and the thermally stimulated current. In the high-temperature range, Poole-Frenkel emission is observed while, at lower temperatures, a hopping process probably occurs. Data from the thermally stimulated current measurements involve shallow traps (0.42 eV) which might originate from the 4PV/metal interface region.

Electrical and Optical Properties of Conducting Polymer/Porous Silicon Structures

Electrical and optical properties of diode structures based on porous silicon (PS) and thin films of phenylene vinylene oligomer (PVO) have been studied. Steady-state photoluminescence spectroscopy show that the structure of the luminescence band depends on the PS morphology. We assign the observed effect to the morphology-dependent penetration of PVO material into the pores. Current-voltage characteristics of the PVO/PS diodes are studied and interpreted assuming Schottky emission and hopping transport of carriers aspossible mechanisms of d.c. electrical conduction.

Influence of thermal and plasma treatments on the electrical properties of poly(para-phenylene vinylene)-based diodes

Abstract We have studied the influence of plasma and thermal treatments on the electrical properties of poly( para -phenylene vinylene) (PPV) thin films in metal-polymer-metal structures. Thermal heating of the samples modifies the diode characteristic by reducing the voltage threshold and by favouring the formation of the rectifying contact at the bottom side. Plasma treatments with oxygen or nitrogen drastically change the conductivity of the polymer with disappearance of the rectifying behaviour and increase the current by many orders of magnitude. Attempts to explain the modifications of the electrical property of the treated samples are given, using previous results from X-ray photoelectron spectroscopy (XPS) which highlight the role of the interfacial layer formed by chemical reaction between the polymer and metallic electrode.

Cellulose acetate and PVDC used as protective layers for organic diodes

Abstract In order to increase the lifetime of organic light emitting diodes (OLEDs), it is necessary to prevent the emitter from degradation induced by oxygen contamination from the contact with ambient air. Encapsulation of OLEDs by sealed glass is efficient but not adapted for flexible devices. In this study, we have investigated the behaviour of two polymeric materials used as a protecting layer: cellulose acetate (CA) and polyvinylidene chloride (PVDC). These materials were deposited onto active polymer layers such as poly( p -phenylene vinylene) (PPV) and poly(2-methoxy-5-(2′ethyl-hexyloxy) p -phenylene vinylene) (MEH-PPV). The optical properties (UV-Vis, Raman, IR) of the luminescent polymers were studied as a function of time, up to 21 or 28 days, and compared to those of bare films. The results clearly show the efficiency of the protecting layers. For PPV thin films, both protecting materials delay the effect of air contact, the better efficiency was obtained using CA films. In contrast, MEH-PPV is less sensitive to oxidation than PPV and no clear conclusion could be drawn on the efficiency of the encapsulating layers.

Poly(p-phenylene vinylene)/porous GaP composite materials

We report results obtained from physical characterization of thin composite films made by mixing poly(p-phenylene vinylene) (PPV) and porous gallium phosphide (GaP) particles with different GaP concentrations. Optical measurements including Raman, infrared and photoluminescence show that the GaP particles were progressively incorporated to the polymer film which fills the powder pores. The fabricated films possess properties of both materials with a weak contribution of GaP to the optical properties, the main interaction effect consists in a blue shift in the emission spectra of PPV. Diodes using composites as an active layer exhibit higher conductivity than that of PPV, leading to an enhancement of the light emission. The improvement in optical and electrical performance of the composite based light emitting diodes (LEDs) makes this new composite a good candidate for display applications.

Study of 4,4′-bis(4-dimethylaminostyryl) benzene based diodes

Abstract The electrical and optical characteristics of 4,4′-bis(4-dimethyl amino styryl) benzene (DMASB), a new derivative of poly( p -phenylene vinylene) (PPV) have been investigated for use as an active layer in light emitting diodes (LEDs). Analysis of photoluminescence and Raman spectra shows that the oligomer has properties close to those of PPV and the incorporated amino groups do not change significantly the optical absorption of PPV. The current–voltage characteristics of the DMASB based diodes show rectifying behavior, which is dependent on the nature of metals used for the electrodes. Analysis of the polymer–metal interfaces reveals that chemical interactions between the polymer and the metals have occurred and these results are related to the electrical behaviour of the diodes.

An X-ray photoelectron spectroscopy study of the interface formed between ITO and 4,4'-bis(4-dimethyl amino styryl)benzene based light emitting diode

Abstract In organic light emitting diodes (OLEDs), the structures of the metal–organic and organic–metal interfaces are crucial for their performance such as stability, quantum efficiency. In this paper, we analysed the 4,4′-bis(4-dimethyl amino styryl)benzene (DMASB)–indium tin oxide (ITO) interface by X-ray photoelectron spectroscopy (XPS). The DMASB is a new organic material containing amino groups and is expected to have high electroluminescent efficiency. The results showed a diffusion of indium and oxygen atoms into the organic layer, which induced the formation of metal–oxygen–carbon complex. The modification of the N 1s spectra suggested that formation of indium nitride would occur at the interface region. This formation already encountered may be favoured by the amino groups of the DMASB.

Characterization of 6PV phenylene vinylene oligomer thin films

Abstract Investigations of phenylene vinylene oligomer thin films with 7 phenyl, 6 vinyl units (6PV) have been performed by optical and electrical measurements. The optical characterizations including infrared, Raman and UV-VIS spectroscopies were discussed by comparing the results with those obtained in poly(p-phenylene vinylene) (PPV). The electrical properties of oligomer based diodes were studied by measuring the current-voltage-temperature characteristics and the thermally stimulated currents (TSC). At high fields, tunneling of carriers through the interfacial barrier would occur while at lower fields, two distinct temperature ranges were observed. In the high temperature range (> 250 K), a field assisted mechanism occurs involving localized states in the oligomer bulk whereas in the low temperature range, hopping conduction is probable. The TSC spectrum shows two relaxation peaks at 219 and 261 K which correspond to trap depths of 0.4 and 0.55 eV. The deep traps appeared as a characteristic of phenylenevinylene oligomers.