Michel Carrara
Organic light emitting devices (OLED) are among the most promising future technologies for the manufacture of flat and inexpensive displays. Although OLEDs provide already much higher brightness as compared to liquid crystals devices (LCD), they have a major drawback which limits their industrialisation; they are degrading too quickly. Among the factors acting on the performance of OLEDs, interfaces play an important role. The purpose of this work is to develop stable interfaces by using self-assembled monolayers at the device electrodes. Monolayers of para substituted benzoic acids, grown by the traditional method of self-assembly in solution, were characterised by Langmuir isotherms. The experiments showed that the layers are compact, but the binding between the surface and the adsorbed molecule is weak: -5,7 kJ·mole-1 on indium tin oxide (ITO) and -10,6 kJ·mole-1 on aluminium. These molecules allowed to increase the anode surface potential by 0,2 eV and to decrease the one of the cathode by -0,2 eV. Although giving encouraging results, this method has several disadvantages related to the use of a solvent. This is one reason why a new way of growing self-assembled monolayers was developed, the self-assembly from vapour phase. This new grafting method made it possible to increase the surface potential of ITO by 1,4 eV and to decrease the one of aluminium by 0,9 eV using the same molecules as in solution. Another advantage of this technique is the short growing time of the monolayers. Only one minute is necessary to graft the electrode which is considerably shorter than for the traditional technique where 12 hours are necessary. Moreover, it seems that under clean vacuum conditions, the effect on the surface potential depends only on the molecule and not on the surface composition, nor on the initial potential. Finally, this method allowed to discriminate between the different dipole contributions involved in the surface potential modification by the self-assembled monolayers. In a further step, the extended electrode interface consisting of the self-assembled layer covered by an electroactive material was studied. It appears that the organic material partly screens the monolayer dipole moment due to its polarisability. When the initial electrode surface potential of the modified electrode is far apart from the highest occupied or the lowest unoccupied molecular orbitals of the organic material, no charge transfer is expected and the variation in the surface potential comes only from the polarisability of the material. This variation is weaker in the case of a non-polar substituted diamine, α-NPD (the maximum is 0,3 eV) than in the case of the widely used aluminium complex, Alq3, which has a permanent electrical dipole and thus can induce shifts up to 0,7 eV. On the other hand, if the surface potential is close to the energy level of the occupied or unoccupied molecular orbitals, spontaneous charge transfer can occur between the electrode and the organic material. This increases considerably the measured potential variation during the deposition of the electroactive material. With α-NPD, variations of 0,6 eV are observed and in the case of Alq3 shifts are as important as 0,9 eV. Those negative effects can be decreased by the use of an aliphatic chain between the attachment group and the aromatic cycle in the adsorbed molecules. This positive effect is considerable: it can decrease the electrostatic screening of the adsorbed dipole layer by the electroactive material and prevent the charge transfer induced by the image force effect. Finally, homopolar and bipolar devices were studied to investigate the effect of self-assembled monolayers on the device characteristics. In particular, "electron-only" devices were studied using a grafted or a bare aluminium cathode. To study the injection of holes, homopolar and bipolar devices were conceived, having a grafted or a bare ITO anode. The results clearly demonstrate the decrease of the threshold voltage by using suitable self-assembled monolayers. The reasons for this decrease are numerous: improved alignment of the energy levels due to the permanent electrical dipole of the adsorbates, insulating spacer effect due to the aliphatic chains of some of the molecules and improvement of film morphology.
EPFL2002
