Effect of incorporating iridium in conjugated polymers on light-emitting and photovoltaic properties

Chemical and structural characteristics of conjugated polymers give rise to their light-emitting and photovoltaic properties. This thesis reports on the synthesis and characterization of iridium-bound conjugated polymers. Suzuki polycondensation was used to make host-guest, alternating copolymers with iridium content varying from 0 to 40 mol %. Structure-property relationships were investigated using a wide variety of techniques including NMR, UV-vis absorption, X-ray fluorescence, and photoluminescent spectroscopies, cyclic voltammetry, gel permeation chromatography, light emitting devices and photovoltaic devices. Energy transfer in the host-guest systems from the polymer backbone to the iridium complex was studied in dilute solutions and thin films, and investigated with respect to iridium content. The relative rates of photophysical processes were used to explain the observed emission properties and correlated to polymer structure. Exchanging the 2,5-linked pyridine group with a 3,4-linked thiophene in the polymer backbone of polyfluorene copolymers, increased the polymer triplet energy level, which resulted in decreasing the back energy transfer from the iridium triplet state to the polymer triplet state. The work presented in Chapter 2 demonstrates the importance of the relative position of the triplet energy level of the polymer in comparison to the position of the triplet level of the iridium complex. The effect of triplet excitons on charge generation in photovoltaic devices was investigated. Incorporation of iridium into a polyfluorene backbone resulted in a 10-fold increase in the ratio of incident photons to converted electrons (also known as external quantum efficiency). However, when iridium was incorporated into a poly(fluorene-alt-bithiophene) polymer, photovoltaic performance decreased, thus revealing the complexity of the requirements necessary for efficient photon to electron conversion. In the case of poly(fluorene-co-bithiophene-co-iridium) it is believed that the morphology of the polymer:[6,6]-phenyl C61 butyric acid methyl ester blend plays a dominate role in determining photovoltaic properties. The structure-property relationships identified in this work build on the current understanding of iridium-bound conjugated polymers, and may be relevant to other conjugated polymer systems. The relationships drawn from this work should prove useful for the design of new polymers for light-emitting and photovoltaic applications.