Interface formation between metal and polymer in

Polymer Light Emitting Diodes

Frank Janssen

In collaboration with the group of applications of ion beams and

the Dutch Polymer Institute (DPI)

Introduction

For the last decays polymers (plastics) are widely used in daily life. One of the well-known properties of plastics is that they are electrical insulators (e.g. plastics are used to insulate electrical cables). However in the late 70 ’s a subclass of polymers (conjugated polymers) was discovered, which showed semi-conducting properties. In the early 90 `s it was found that some of the conjugated polymers emit can light, when a voltage is applied. These polymers have received much attention since, because they are candidates to be the basis for full colour, easy to process, cheap, flexible displays.

Polymer LEDs

The most simple Polymer LEDs consist of an emitting polymer layer (OC₁C₁₀ PPV), which is sandwiched between an anode and a cathode (figure 1). Devices are prepared by spin coating the polymer onto ITO substrates, subsequently the cathodes (calcium and aluminium) are evaporated in high vacuum.

Figure 1: PLEDs consisted of a polymer layer (PPV in this case), sandwiched between a cathode (calcium) an an anode (ITO). PPV is spincoated onto Glass/ITO substrates and subsequently calcium is thermally evaporated onto the PPV. Electrons are injected through the calcium cathode and holes through the ITO anode. Holes and electrons recombine in the PPV and light is emitted.

Research target

The cathodes (e.g. Ca,Ba, Al ) in PLEDs are applied by thermal evaporation of the metal onto the polymer layer. Electrons are injected from the cathode and holes from the anode. It is well known that holes are the majority charge carrier in these kinds of devices. Optimal devices performance is achieved if the charge carriers are more or less balanced and therefore it is important to optimise the injection of electrons. Thus it is important to study Ca/PPV interface formation to optimise PLED performance.

Studies on metal/PPV interface formation can be found in literature but they are mainly focussed on the electrical characterisation of the interface. The dynamics and stability of the Ca/PPV interface are not studied extensively (see figure 2).

Figure 2: We want to study how calcium behaves when evaporated on a PPV layer. Will calcium diffuse only into PPV during evaporation or will the cathode be also unstable after evaporation?

We want to study the formation of the Ca/PPV interface in order to understand the diffusion mechanisms and the effects influencing PLED performance.

This is done by performing LEIS (Low Energy Ions Scattering (see figure 3)) and XPS experiments on the Ca/PPV interface. Apart from surface techniques also devices were prepared and characterised by I-V characterisation and ac impedance spectroscopy.

Figure 3: Principles of Low Energy Ion Scattering. A He ion beam is directed onto the sample and the energy of the scattered He ions is measured. The energy is determined by the mass of the atom from which the He ion is scattered. He ions that penetrate the sample will be neutralised and because only ions are detected, surface sensitivity is reached.

Experiments

For surface characterisation PPV was spin coated on (cleaned) ITO in a glovebox set up (O₂ & H₂O < 1 ppm). Subsequently the ITO/PPV samples were transported in an airtight suitcase to the LEIS/XPS set up. In the high vacuum of the LEIS/XPS set up calcium(/barium) was deposited onto the ITO/PPV sample. In order to study the initial formation of the Ca/PPV interface only very small amounts of Ca were deposited. After deposition the calcium surface concentration is measured at consecutive times. From the time dependent LEIS signal, the diffusion coefficient of metal into PPV can be derived.

Preliminary results, Future

In figure 4 LEIS measurements on a Ca/PPV interface is shown. It is clear that Ca diffuses into the PPV, with XPS the same behaviour was observed.

The next step is to study interface formation under specific conditions (e.g. Oxygen ambient) and evaporated with different speed and the sample temperature. Furthermore, interface formation on chemically different PPV derivatives will be studied.

Figure 4: LEIS spectrum of Ca (scatter energy 2280 eV) on PPV (initial surface coverage ~50%). Measured 2, 45 and 105 minutes are calcium deposition. Calcium diffuses from the surface into the PPV after evaporation.