Charge transfer processes in polymer light-emitting diodes

Optical characterisation of an organic soluble poly (paraphenylene vinylene) (PPV) derivative involving an alkyloxy pendant group on the phenylene ring has shown a shift of the absorption and emission towards longer wavelengths in comparison to PPV ones. The changes of the optical characteristics are ascribed to the redistribution of the π molecular orbital over the phenylene ring induced by the alkyloxy substituent. The comparison of the absorption and emission of the PPV derivative in solution or as a thin layer shows the role of the interchain interactions in condensed matter. Another example of the importance of charge transfer processes in PLED is provided by the indium tin oxide (ITO)/poly (vinyl carbazole) (PVK) hole transport layer interface. The weakening of the ITO absorption in the UV resulting from interband transitions has been interpreted by the depletion of the ITO free carriers at the contact of PVK. The shift towards longer wavelengths of the plasma frequency in the near infrared confirms the reduction of the free electron concentration in ITO, which has been estimated to 30% using the Drude free electron theory. The formation of a dipole layer at the interface can account for such charge transfers.