High brightness and efficiency green light-emitting diodes based on fluorene-containing conjugated polymers and associated blends

We report on the fabrication and properties of single layer green light-emitting diodes (LEDs) based on fluorene-containing conjugated polymers and associated blends. We have used a new green fluorene based conjugated polymer (namely 5BTF8) as the emissive material as well as the host in blends with a guest hole transport triarylamine/fluorene copolymer (namely BFB) to fabricate bright and efficient single layer green polymer light-emitting diodes (PLEDs). An enhancement in both the electroluminescence 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 thus to an improved charge carrier balance factor. A higher brightness and a lower turn on as well as operating voltage are also achieved for the blend. The emission from a green single layer LED with 5BTF8/BFB (4/1) as the emissive layer reaches a maximum brightness of 35000 cd/m2 with a maximum external quantum efficiency of 1.3% or 4.2 cd/A and a maximum power efficiency of 2.5 lm/W. Novel small area LEDs were also fabricated using a SiN insulating layer on top of the ITO that allowed much higher brightnesses to be achieved compared to the standard area LEDs due to the reduced heating and therefore to a better thermal management of the device. The emission from a PEDOT/5BTF8 small area LED reached a maximum brightness of 155,000 and 6,500,000 cd/m2 in DC and pulsed mode, respectively.