Polyfluorene LEDs, 275 doped

Rhenium(I) tricarbonyl-2,2 -bipyridine moieties were used to cap both ends of a poly fluorine, yielding Re-capped Re(bpy)(CO)3(py)-X-(py)(CO)3(bpy)Re 2+ polymers, where X = polyfluorene [51, 52], The polymers with and without the Re caps were spin-coated from their solutions in CH2C12 onto an ITO surface previously modified with a layer of poly(styrene sulfonic acid), doped with poly(ethylenedioxythiophene). The LED (light-emitting device) was then topped with a layer of Ca/Al. The photoluminescence (PL) and electroluminescence seen were consistent with the presence of [Re(bpy)(CO)3(py)]+ [158],... 

Although the current density at a given bias is reduced by blending, the luminous efficiency of the devices sharply increases. The inset to Fig. 10.34 plots the EL intensity versus current density for all four devices. All three doped LEDs show much higher EL output at a given current density. Both of these phenomena can be attributed to better current balance. It has been proposed that trapping of holes will cause a space-charge field to develop under bias, which reduces the total electric field at the anode and increases the field at the cathode.69 This additional field will block hole injection and facilitate electron injection. As holes are the majority carrier, the overall current density will decrease, but the carrier balance is much improved. The device efficiency (Cd/A) consequently increases from 0.04 for the PF2/6 LED to 0.08 for ST 755, 0.15 for ST 16/7, and 0.87 for TPTE. Thus, a 22-fold improvement of device efficiency was obtained. The color purity of the blend LEDs was also observed to improve with a reduction of the polyfluorene excimer band at 560 nm. 

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