Polyfluorenes with Improved Charge Injection

Recently, Shu, Jen and coworkers [252] prepared a fluorene 152 with two electron-accepting oxadiazole substituents by nudeophihc substitution 

Mullen and co-workers have also prepared the blue-emitting (Xmax = 450 nm) polyketal 60, films of which can be converted by exposure to 

---

C. Ego, A.C. Grimsdale, F. Uckert, G. Yu, G. Srdanov, and K. Mullen, Triphenylamine-substi-tuted polyfluorene — a stable blue-emitter with improved charge injection for light-emitting diodes, Adv. Mater., 14 809-811, 2002. 

Polyfluorenes are an important class of LEP with high thermal, photo and environmental stability and efficient bright blue emission. This stimulated a number of researchers to develop fluorene-thiophene copolymers for light-emitting applications. In addition to an expected increase in PL quantum etSciency, such a combination of electron-rich thiophene units with relatively electron-deficient fluorene units should modify the bandgap of the material (and thus tune the emission) and improve the charge injection/transport balance, compared with fluorene homopolymers. 

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. 

---