Abstract
We report on the fabrication and properties of single layer blue LEDs based on conjugated polymer blends. We have used PFO as the host and a hole transport triarylamine/fluorene copolymer as the guest. Despite the fact that the photoluminescence (PL) quantum efficiency of the blend is lower compared than that of the host and guest polymers on their own, an enhancement in both the electroluminescence (EL) quantum and power efficiency is seen for the blend. This observation indicates that the hole transport material leads to a significant improvement in hole injection and transport and a greatly improved charge carrier balance factor. A careful comparison of the PL and the EL spectra reveals that more emission originates from the guest polymer for EL than for PL. This proceeds from the expectation that both Foerster transfer and charge transfer from the host to the guest occur under electrical operation of the device. Only Foerster transfer is expected for optical excitation. A much higher brightness and lower turn-on and operating voltages are achieved for the blend. (Author)