Polyfluorenes electron injection

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. 

Brewer, P.J., et al. 2005. Role of electron injection in polyfluorene-hased light-emitting diodes containing PEDOT PSS. Phys Rev B 71 205209. 

Polyfluorene is one of the most attractive conjugated polymers for application in light-emitting devices [36-38], It is characterized by low electron affinity and, in consequence, several authors have inserted electron-deficient moieties into the aromatic chain to modify the position of the frontier orbital and increase the electron affinity in order to achieve a better balanced hole and electron injection in electroluminescent diodes. Among the electron-deficient groups, thienyl-5,5-dioxide and dibenzothienyl-5,5-dioxide have also been employed [16-18, 20-25, 27, 28],... 

ZHO 11] Zhong C., Liu S., Huang F. et al, Highly efficient electron injection from indium tin oxide/cross-linkable amino-functionalized polyfluorene interface in inverted organic light emitting devices , Chemistry of Materials, vol. 23, no. 21, pp. 4870-4876, 2011. 

T. van Woudenbergh, J. Wildeman, P.W.M. Blom, J.J.A.M. Bastiaansen, and B.M.W. Langeveld-Voss, Electron-enhanced hole injection in blue polyfluorene-based polymer light-emitting diodes, Adv. Fund. Mater., 14 677-683, 2004. 

Although the optical properties of polyfluorenes are attractive, the electrical properties are, if anything, even more so. Charge carrier mobilities are a key factor in determining the performance of polymer LEDs because of the requirement of both balanced injection and transport of electrons and holes. Charge carrier mo-... 

The first quantitative study of the device efficiency of polyfluorene LEDs was reported by Grice and co-workers in 1998.5 The LED used ITO-coated glass as the anode, a hole-transport layer of a polymeric triphenyldiamene (poly-TPD), an emissive layer of PFO, and a calcium cathode. A schematic energy-level structure for the device is inset in Fig. 10.14. The injection barrier for holes into poly-TPD is approximately 0.3 eV, much less than that for PFO (0.8 eV), and the injection barrier for electrons into PFO is approximately 0.1 eV. Poly-TPD had excellent film-forming properties, was transparent to PFO emission, and was insoluble in... 

The various functional layers in PLED devices are outlined in Figure 10.14. By applying a voltage across the device, electrons and holes are injected into the photoluminescent polymer, where they recombine to form excitons and emit light. Suitable, commonly used emissive polymers are conjugated materials such as poly(phenylenevinylene)s (PPVs) and polyfluorenes. 

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. 

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