Polyfluorenes electronic properties

J. Cornil, I. Gueli, A. Dkhissi, J.C. Sancho-Garcia, E. Hennebicq, J.P. Calbert, V. Lemaur, D. Beljonne, and J.L. Bredas, Electronic and optical properties of polyfluorene and fluorene-based copolymers a quantum-chemical characterization, J. Chem. Phys., 118 6615-6623, 2003. 

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... 

Abstract This account provides a state of the art overview of polyfluorene structure and phase behaviour in solutions and the solid state. This review covers key aspects of the hierarchical intra- and intermolecular self-assembly starting at the molecular level and extentding up to larger length scale structures. This includes crystallization, alignment on surfaces and texture. Many Central ideas are highlighted via structural archetypes. Recent theoretical treatments for understanding these structural properties are discussed and the implications for opto-electronics and photophysics are described. 

In Section 2.1.4, the electronic and photophysical properties of donor-acceptor or type-II heterojunctions between two semiconductor polymers were investigated. Exciplex formation was found to be a general phenomenon for type-II heterojunctions between polyfluorene copolymers. These are among the most widely used materials in polymer optoelectronics, but the presence of exciplexes had not yet been fully appreciated. This is because the exciplex population is often depopulated via efficient endothermic transfer towards the exciton, which leads to bulk exciton instead of exciplex emission. However, via time- and temperature-depen-dent photoluminescence (PL) spectroscopy the exciplex states can be identified. Employing a relationship known from small-molecule solution systems that relates the relative HOMO and LUMO levels of the molecules to the exciplex emis-... 

---