The Research Of Organic Light-emitting Devices Treated By Electric Field

Organic light emitting diodes (OLEDs) is one of potential candidate for flat panel display. Efficiency is one of the key parameters in the performance of OLEDs. To improve brightness and efficiency in OLEDs has attracted more and more attention.When fabricating the film with the presence of electric field which induced the orientation of polymer molecules, the carrier transport in the polymer layer will be changed and as a result the brightness and efficiency will be influenced.In our works, devices were fabricated in the electric field. When the prepared polymer solution was spin-coated to film an emission layer, a strong electric field would be applied through the film in the vertical position of ITO anode. Then we studied the influence of electric field on the electric and optic properties of OLEDs.Single layer MEH-PPV devices were prepared under strong electric field. The current and brightness of the device has been enhanced. Under the presence of electric field, the polymer chain orientation has been changed. By the direction of the electric field, the probability of the carrier transmission along the chain increases, and the probability of the carrier transmission between the chains reduced. Therefore, the presence of the electric field increases the rate of the carrier transmission, as a result the current at the same operating voltage had increased, and at the same time, the carrier balance in the emission layer (EML) was improved and thus the current efficiency was enhanced.Devices with Ir(ppy)₃ doped PVK films with a weight ratio of 1:0.1% EML were fabricated at different electric field. From the electroluminescence spectra, it could be indicated that the electric field has no influence on the characteristic emission of luminescent material. The current-voltage characters were tested to study the electric properties in different devices. When the devices EML was processed under electric field, the current at the same operating voltage had decreased which is quite different with fluorescent devices. However, the brightness was enhanced and the current efficiency was also increased.Ir(ppy)₃ and DCJTB doped PVK films were fabricated under the presence of electric field. The results showed that the electroluminescence efficiency increased.Ir (ppy)₃ doped PVK films were fabricated under the presence of electric field, and the excited lifetime were tested. The result showed that the excited lifetime of the film with the presence of electric field become shorter. The electric field may increased triplet-triplet annihilation which induced the decrease in lifetime.From the above studies in fluorescent devices, phosphorescent devices and fluorescent-phosphorescent co-doped devices, the electric field had improved the devices brightness and current efficiency, so electric field processed devices is a simple and efficient way to enhance device brightness and current efficiency.