Charge-carrier injection via semiconducting electrodes into semiconducting/electroluminescent polymers

The indium-tin-oxide (ITO) electrode commonly used for hole injection in organic electroluminescent devices is replaced by a crystalline p-type Si electrode in order to improve the injection efficiency. Several conducting/electroluminescent polymers such as poly(9-vinylcarbazole), poly(9,9-dihexylfluorene) and polyhexylcarbazole are deposited via spin-coating onto the Si electrode, and an Al contact is evaporated on top. Current-voltage characteristics indicate that hole injection into these polymers is easier from p-Si than from ITO or Au. Surface effects hinder an even better performance, expected from naive energetics considerations. This major role of the surface is demonstrated by comparing the average photoconductivity decay time at the Si/polymer-interface with that at an Si surface, using spatially resolved microwave reflection. Also, various surface treatments such as hydrogenation, oxidation and methylation are applied to the Si substrate before polymer deposition. The results highlight the key role of the interface state density at the semiconductor/polymer interface, and the need for a surface state density as low as possible in order to minimise the operating voltage.