Synthesis and Characterisation of Conjugated Polymers with High Electron Affinity

Doctoral thesis, 2002

This thesis describes synthesis and characterisation of conjugated polymers with high electron affinities. Polymers of this type are desired, for example, for use in light-emitting devices or as electron accepting material in photodiodes/solar cells. The high electron affinities for the polymers synthesized here were obtained from the incorporation of the nitrogen containing heterocycles pyridine, quinoxaline or pyridopyrazine. Most of the polymers synthesized were alternating copolymers of one of the mentioned heterocycles with vinylene units, i.e. analogues of poly(phenylene vinylene) (PPV). The highest electron affinity, measured with cyclic voltammetry, was obtained for a poly(pyridopyrazine vinylene) with phenyl substituted side-groups. The affinities of some of the polymers synthesized here are among the highest reported for conjugated polymers, higher than, for example, soluble cyano substituted PPV. One of the poly(pyridopyrazine vinylene)s synthesized was tested as the electron accepting material in a number of donor/acceptor solar cells. One of the devices had an efficiency which was amongst the highest reported for solar cells based on two conjugated polymers, 7% (555 nm, 0.21 mW/cm2). However, the efficiency of the device was decreased at higher light intensities, probably due to low charge mobility and poor stability. Furthermore, the photo-oxidation process was investigated for four of the polymers; poly(pyridopyrazine vinylene)s and poly(quinoxaline vinylene)s with different side-groups, and found to be quite complex. At least three factors can influence the stability towards photo-oxidation for this type of polymers; namely band-gap, the HOMO and LUMO levels of the polymer and whether the polymer contains reactive hydrogens such as benzylic hydrogens.