Self-assembled monolayers as interface materials for optoelectronic devices

Responsibles : S. Suarez, F.D. Fleischli, Ph. Bugnon, and L. Zuppiroli

Project Description

A strategy is developed to engineer metal, metal-oxide electrodes and oxide gates for organic optoelectronic devices. The influence of self-assembled monolayers (SAMs) on charge injection, electrode wettability and device lifetime are studied. The role of water in the growth and the properties of these layers are also studied.

Main results

Charge injection from an electrode into an organic electroactive material strongly depends on the energy difference between the electrode workfunction and the frontier molecular orbitals energy level (HOMO and LUMO) of the electroactive material. Dipolar monolayers adsorbed on the electrode can induce work function shifts of more than 1 eV (see figure on the right), which can be used to control charge injection. The molecules composing the SAM may also own an aliphatic carbon chain of about 1 nm in length. This extremely thin electrical insulator greatly improves charge injection by inhibiting image force effects and by suppressing back diffusion currents.


Self-assembled molecular layers can also be used with profit on the oxide gate of organic field effect transistors (OFETs). In this case, their main role is to inhibit the effect of charge defects of the oxide surface.

Right panel : Aluminum electrode work function shifts DF induced by various SAMs of benzoic acid derivatives. The work function shift is plotted versus the electrical dipole moment of the substituted benzene.

Organic field effect transistors made from acene materials

Responsibles : M. Schär, F.D. Fleischli, S. Suarez, M. Longchamp, and L. Zuppiroli

Project Description

In recent years, thin films organic field-effect transistors (OFETs) have begun to be considered as a possible alternative to the hydrogenated amorphous silicon thin-film transistors used in active matrix flat panel displays and other large area electronic applications. Of the many organic materials available, pentacene, in particular, is one of the leading candidate for use in current thin-film OFET architectures. The project aims to optimize ultrathin organic transistors on oxide and polymer surfaces.

Left panel : Atomic force microscopy image (phase mode) of a polycrystalline pentacene thin film grown on sapphire, showing monomolecular steps.

Main results

The growth of polycrystalline films depends on various parameters such as deposition temperature, deposition rate, substrate surface as well as substrate treatment. Self-assembled monolayers are useful to increase the crystallinity of these films and they also enhance the device performance.

We have compared the nucleation of pentacene islands onto polymer, silicon dioxide and alumina substrates. Whereas a critical nucleus size of two pentacene molecules is observed on all investigated substrates, the activation energy for nucleation depends significantly on the dielectric, due to different molecule-substrate interactions. We were able to optimize growth temperatures and rates on all substrates in order to obtain smooth crystalline pentacene films with grain sizes of the order of 10 microns. The results show that the morphology, crystal structure and molecular ordering of the first organic monolayer(s) at the pentacene/dielectric interface are essential determinants of carrier transport phenomena. To further investigate these interface effects, we have built a model organic field-effect transistor which consists essentially of a single layer of pentacene on an oxide substrate. Four-probe and two-probe transport measurements as a function of temperature and fields were performed 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.

Collaborations : M. Brinkmann, D. Tsamados and A.M. Ionescu

Modeling charge transport and light emission in multilayer organic light emitting diodes

Responsibles : S. Konezny, and L. Zuppiroli

Project Description

Most of our experience on transport processes and recombination in organic semiconductors was concentrated in a single computer code, which is available in a public library or on our web site .

Main results

We present an example of calculation of the charge density (electrons, n and holes, p) in a multilayer OLED device. From such kind of results, light emission can be optimized by changing, for instance, layer thickenesses.

Right panel : Electron and hole density distribution inside the four layers device ITO/CuPc/a-NPD/Alq3/LiF-Al. Charge recombination and light emission do occur in the zone of the device where both electrons and holes reach a high concentration, here in the light emitting material Alq3.

Collaborations : E. Tutis, and M.N. Bussac

Modeling charge transport in organic semiconductors

Responsibles : S. Konezny, and L. Zuppiroli

Project Description

The project aims to understand all transport basic processes, both in disordered polymer electroactive materials and in crystalline organic semiconductors.

Main results

Amongst the issues which were considered we can mention:

- Polaron formation and transport

- Role of molecular polarization

- Sources of localization

- Interface effects in the channel of an organic field effect transistor.

Collaborations : M.N. Bussac

Tuning and trimming the optical properties of planar photonic crystals

Responsibles : P. El-Kallassi, R. Ferrini, M. Schär, Ph. Bugnon, and L. Zuppiroli

Upper panel : Typical GaAs-based planar photonic crystal structure consisting of a triangular lattice of air holes with a lattice period a of 200 to 400 nm and a typical diameter d of 100 to 200 nm. Organic molecules are infiltrated into these holes. Their refractive index can be tuned resulting in significant modifications of the photonic band gap.

Project Description

In recent years the intense investigation on photonic crystals (PhCs) (see figure above) has been largely driven by their potential applications in integrated optics. However, there are still many factors limiting their use in real devices, such as losses, fabrication imperfections, and temperature/polarization sensitivity. In this project we focus on the infiltration of PhCs with a synthetic organic material that allows one to overcome some of these limiting factors by trimming (non-reversible process) or tuning (reversible process) the optical properties of PhCs. We also investigate some possible future applications that may arise from infiltrated PhCs such as switches and polarization components. These applications rely heavily on the characteristics of the infiltrated material. Fortunately, advances in synthetic chemistry have resulted in a huge variety of materials with tailor made functionalities.

Main results

We present the effect of infiltration of liquid crystals (LCs) on the optical properties of the photonic crystals. The shift of high-energy (air) photonic band gap edge due to the refractive index change inside the holes is illustrated in the figure on the left. The experimental results are shown to be in excellent agreement with the predicted two-dimensional finite difference time domain (2D-FDTD) calculations. From the best fit of the experimental spectra both the in-filling efficiency and the refractive index of the infiltrated material are obtained (see figure on the left). The latter value yields the orientation of the molecules inside the holes: in the case illustrated here the LC molecules are perpendicular to the electric field, i.e. parallel to the hole axis.

Left panel : Experimental (black lines) and calculated (by a 2D-FDTD method: grey lines) transmission spectra through planar photonic crystals slabs: (a) without and (b) with liquid crystals. The fitted values for the air filling factor (f), the refractive index of the infiltrated material and the in-fill efficiency are reported in the figure.

Collaborations : R. Houdré, Swiss NSF NCCR - Quantum Photonics, EU-Network of Excellence ePIXnet, and EU-COST Action P11

Fabrication of organic light emitting diodes on special glass substrates and light extraction

Responsibles : J.-Y. Bengloan, Ph. Bugnon, R. Ferrini, and L. Zuppiroli

Left panel : Optoelectronic device fabrication facility - Deposition chamber. Right panel: 7 digits OLED structure.