Review of electronic and optical properties of semiconducting π-conjugated polymers: Applications in optoelectronics
Résumé
A general overview of the optoelectronic properties of π-conjugated polymers is presented. Two types of polymer are discerned: interchangeable structures of the same energy (degenerate), such as polyacetylene; and non-degenerate polymers, such as poly(para-phenylene). The band structures of degenerate and non-degenerate polymers are related to their conductivities in doped and non-doped states. In both cases, disorder and impurities play an important role in conductivity. Polarons, bipolarons and excitons are detailed with respect to doping and charge transfers. Given the fibrillic nature of these materials, the variable range hopping (VRH) law for semiconducting polymers is modified to account for metallic behaviours. Optoelectronic properties-electroluminescence and photovoltaic activity-are explained in terms of HOMO and LUMO bands, polaron-exciton and charge movement over one or more molecules. The properties of H- or J-type aggregates and their effects on transitions are related to target applications. Device structures of polymer light-emitting diodes are explicitly linked to optimising polaron recombinations and overall quantum efficiencies. The particularly promising use of π-conjugated polymers in photovoltaic devices is discussed. © 2004 Society of Chemical Industry.