Optical properties of nanoparticulate organic photovoltaics and pathways to implementation

Research Doctorate - Doctor of Philopsophy (PhD)This thesis begins by examining the optical characteristics of organic photovoltaics (OPV) as the thesis continues the focus shifts steadily to practical considerations around the implementation of the OPV technology. The first chapter demonstrates an implementation of transfer matrix methods to model absorption in the active layer. The calculation is separated into two distinct material components and used to deduce the exciton generation rates for the two species in the blend. The calculation of the individual component absorption is performed for bulk hetrojunctions (BHJ) that all use P3HT as the donor polymer and use three different types of fullerenes, PCBM, ICBA and PC₇₀BM. The second chapter examines the P3HT:PCBM case again but in this case the active layer has now be formed by the deposition of the blend particles in discrete nanoparticles (NP). The NP films are examined optically and dielectric models that accurately describe their behaviours are developed for a range of weight ratios of polymer to fullerene. The calculation in the first chapter that attributed absorption to each of the species in the blend is now performed for the NP case over the weight ratios examined. The optimisation curve that examines the relationship between thickness and absorbed photons is calculated for the nanoparticles and compared to an equivalent curve using a BHJ dielectric function. In the third chapter the angular dependent current and performance characteristics is examined for both BHJ and NP₋OPV devices. The fourth experimental chapter examines a solution processable material that improves device performance in NP-OPVs in a similar manner to the evaporated calcium layer. The goal of this work was to find a material that was solution processable, that would enable the scale up of the nanoparticle system to roll-to-roll processes, and allow for the electrical inversion of devices. Different formulations of the thoroughly researched ZnO interface are examined to find a interface material, with the best candidate to be found being zinc acetylacetonate Zn(ac)₂. While the material found did not function as well in this role as calcium it did provide a significant improvement over its absence from the interface. In the final experimental chapter efforts to coat organic layers are outlined. Initially the coating efforts began as a group project in organic solvents with the lessons learned from that process being transferred to the NP coating. The coating regimes for achieving uniformity from chloroform and water were found to be significantly different. Additional challenges such as dewetting of the substrate, low evaporative rate and viscosity made the process of finding optimal conditions to print uniform films harder. Suitable conditions were discovered and functional devices were manufactured using a drawbar coating technique that had been developed earlier for the coating of materials from solvent