Michel Schaer

Water Vapor and Oxygen Degradation Mechanisms in Organic Light Emitting Diodes

The degradation of organic light emitting diodes (OLEDs) due to the growth of dark spots can be attributed to the synergy of three external causes: dust particles deposited during the fabrication process, pollution by water vapor, and pollution by oxygen. On the basis of a set of new experiments performed on benchmark devices, we demonstrate that, for a given distribution of dust particles and a given concentration of the polluting agent, water is a thousand times more destructive than oxygen at room temperature. While the thermal diffusion of oxygen causes the oxidation of both the metal at the interface and the dye in the bulk of the device, water acts by an electrochemical process causing the delamination of the electrode.

The role of carbazole in organic light-emitting devices

Abstract New organic oligomers and polymers based on the carbazole molecule are explored for possible applications in light-emitting devices. In one case, (butyl- or octyl-) carbazole dimers and poly (N-butyl-3,6-carbazolylene) polymer were used as the hole-transporting and light-emitting layer in multilayer light-emitting diodes (LEDs). These devices yielded bright blue light (as much as about 6000 cd m−2) with high external quantum (about 10%) and luminance efficiencies (about 2 1m W−1). The other case involved ([3-octylthiophene]-[bis-(N-ethyl or octyl carbazolylene)]) multiblock copolymers as the active emitting layer in single-layer LEDs. Color tuning was achieved in these devices by changing the number of monomer units contained in the thiophene chain. We also observed an increase of the external quantum efficiency in diodes based on the copolymers with short thiophene segments that we attributed to a more balanced charge injection.

Effects of doping in polymer light‐emitting diodes

We observed a dramatic improvement in the performance of polymer light‐emitting diodes (LEDs) upon light doping of the organic layer. The LEDs betrayed symmetrical electrical and light‐emission characteristics. Their turn‐on voltage is lower and their external quantum and power conversion efficiencies are higher by nearly an order of magnitude when compared with devices that utilized a nominally undoped organic layer. We attributed these results to the modification of the tunneling barrier in metal–polymer–metal junctions due to the presence of an induced polarization electric field associated with the ionized dopant counterions and charged polymer chains.

The role of copper phthalocyanine for charge injection into organic light emitting devices

Abstract We investigate the charge injection efficiency of plasma treated indium tin oxide (ITO) anodes into copper phthalocyanine (CuPc) in single-layer diodes fabricated under inert conditions. Using electroabsorption and Kelvin p`robe surface potential measurements, we demonstrate that the effective ITO work function is pinned at the energy level of the highest occupied molecular orbital of CuPc. We ascribe this effect to oxygen doping from the ITO electrode. Such doping results in high-efficiency hole injection from ITO as inferred from the current–voltage characteristics. We find evidence for a time-dependent modification of the device characteristics particularly in reverse bias that we attribute to oxygen diffusion from ITO into bulk CuPc. Oxygen plasma treatment of ITO produces an oxide surface that is stable with respect to oxygen diffusion.

Thermopower measurements on pentacene transistors

We present the first thermoelectric measurements on pentacene field effect transistors. We report high values of the Seebeck coefficient at room temperature between 240 and 500 micro V/K depending on the dielectric surface treatment. These values are independent of the thickness of the channel and of the applied gate voltage. Two contributions to the Seebeck coefficient are clearly identified: the expected contribution that is dependent on the position of the transport level and reflects the activated character of carrier generation, and an unexpected intrinsic contribution of 265 plus minus 40 micro V/K that is independent of the temperature and the treatment of the oxide surface. This value corresponds to an unusually large lattice vibrational entropy of 3 kB per carrier. We demonstrate that this intrinsic vibrational entropy arises from lattice hardening induced by the presence of the charge-carrier. Our results provide direct evidence of the importance of electronic polarization effects on charge transport in organic molecular semiconductors.

Blue organic light emitting diodes based on bicarbazyle derivates: Device stability and multilayer configuration

The device stability of blue light emitting diodes based on N,N′-diethyl-3,3′-bicarbazyle, (EtCz)2, are substantially improved by coevaporation with N,N′-diethyl-3,3′-bicarbazyle-6,6′ dicarbaboxylic acid, (OcCzCOOH)2. We attribute this effect to the formation of a network of hydrogen bonds in the alloy related to the presence of carboxyl groups. A three layer device with transport layers for both electrons and holes improves the quantum efficiency: With an Al cathode we obtain a luminance of 100 cd/m2 at current density levels of 50 mA/cm2. We find that the current voltage characteristics of (EtCz)2 is well described by our recent microscopic model for space charge limited currents in an assembly of conjugated polymer segments. The incorporation of an electron transport layer splits and shifts the electroluminescent band to lower energies and we observe a significant additional downshift after a period of three weeks which we attributed to molecular diffusion at the interface between the emitting and elec...

Organic thin-film transistors: the passivation of the dielectric-pentacene interface by dipolar self-assembled monolayers

In organic thin-film transistors (OTFTs), the conducting channel is located near the interface between the organic semiconductor and the oxide dielectric; this interface is crucial for transistor performance. Self-assembled monolayers (SAMs) on the interface reduce the negative influences of the oxide dielectric surface by decreasing the coupling of the carriers at the gate and the role of the active surface defects on transfer. In this paper, we show that SAMs carrying a dipole moment determine the OTFT performance by controlling the charge transfer between the oxide dielectric and the semiconductor. The charges introduced into the semiconductor by this transfer (i.e., residual carriers) lead to a threshold shift to positive values, as well as a decrease in the contact resistance and an increase in the apparent mobility. In this study, other effects of the SAMs, such as the gate potential shift in the channel or a direct reaction between semiconductor and SAM molecules, can be excluded as dominant processes.

Ultrathin organic transistors on oxide surfaces

We have built a model organic field-effect transistor that is basically composed of a single layer of pentacene crystal in interaction with an oxide surface. Drain and source contacts are ohmic so that the pentacene layer can carry a current density as high as 3000 A cm−2 at a gate voltage of –60 V. Four-probe and two-probe transport measurements as a function of temperature and fields are presented in relation with structural near-field observations. The experimental results suggest a simple two-dimensional model where the equilibrium between free and trapped carriers at the oxide interface determines the OFET characteristics and performance.

Light-emitting diodes based on copolymer organic semiconductors

Light-emitting diodes (LED5) based on organic semiconductors have received much attention recently due to their promise as cheap, novel light sources for electro-optical applications. Unlike conventional diodes, light emission from these organic LEDs is achieved by double injection of electrons from a low-work-function electrode and holes from a high-work-function electrode into the organic polymer light-emitting layer. By appropriately engineering the polymer backbone, emission of various visible colors has been demonstrated. Our work in this field has concentrated on organic block copolymers as the active light-emitting materials. We demonstrate that the use of copolymer systems leads to large enhancement of device performance.

Blue light-emission from a nanostructured organic polymer semiconductor

We report on the enhancement of the photoluminescence (PL) intensity from a nanostructured organic material composed of nanosized aggregates of poly(para-phenylene) (PPP) in a polystyrene insulating matrix. In addition, the peak of the PL emission spectrum from these nanostructured organic semiconductors is blue-shifted from that of unstructured PPP. Furthermore, these PL characteristics correlate with the presence of an unusual sub-gap absorption feature that is present only in the nanostructured PPP. These results suggest the departure of the optical properties of random nanostructured organic polymers from their unstructured counterparts.