Tuning and probing charge transfer and charge transport in conjugated polymers and their bi-layer devices.

There is currently a great deal of interest in exploring the application of conjugated polymers in devices, e.g., sensors, electroluminescent devices (LEDs), thin film transistors (TFTs), and photodiodes. Conjugated polymers possess many advantages, such as ease of preparation, low processing costs, and a nearly unlimited variability. In photocells, high efficiency photogeneration is required; thus, tuning and probing photogeneration related issues (charge separation and transport) become a focus and is the subject of this thesis. The synthesis and characterization of functional groups incorporated into poly(3-alkylthiophene)s, for example vinylene ferrocene, cyanovinylene ferrocene or ethylenedioxythiophene (EDOT), are described. These materials were used to fabricate bi-layer organic photocells with the polymer as the p-type layer and perylene-3,4,9,10-tetracarboxylic acid diimide (PV) as the n-type layer. These materials and devices demonstrated that (a) there exist charge-transfer (CT) effects between the P3BT main chain and the pendant Fc groups, (b) a significant increase (70%) in short-circuit current density (Jsc) as compared to a similar ITO/P3BT/PV/Ag device when the Fc concentration reached ∼40 mol%, (c) Jsc depended on the band-gap of polymers, and (d) Jsc increased when polymer was doped with NOBF4. Photo carrier generation, transport, and recombination processes in these organic bi-layer photocells at steady-state and under transient illumination were modeled successfully. Action spectra at steady-state illumination demonstrated that all the interfaces (polymer p/n junction and organic/electrode) are actively involved in the photogeneration of carriers. The decay rate constant of the transient photoconductance observed in organic bi-layer devices after pulsed illumination is independent of the applied electric field, light intensity, and temperature. However, the peak photo response was found to be strongly affected by the applied electric field, temperature, and light intensity, indicating that the transient current in these devices is highly dependent on the carrier mobility and that the decay times are dominated by carrier trapping.Ph.D.Applied SciencesMaterials sciencePhysical chemistryPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/123296/2/3068978.pd