Organic solar cell materials represent a better sustainable alternative relative to inorganic materials in terms of lower cost, ease of large-scale processing, better absorptivity, ease of tunability, and unique flexibility so as to be used on different kinds of surfaces. With the knowledge of the fundamental molecular structure of an organic solar cell material, scientists and engineers can predict the electronic and optically-excited properties. Hence, understanding the structure-property-function relationships is paramount in optimizing the solar cell device performance. This thesis is structured into two sections. The first section focuses on electron acceptors in the active layer of bulk heterojunction (BHJ) device architectures, and the second section discuses organic and hybrid electron donors.
In the first section, the optical and photophysical properties of several structural variations of a special class of non-fullerene acceptor compound – perylene diimide (PDI), are described in substantive details with a number of studies. In the active layer, this material functions as the electron acceptor, and acts in tandem as an excellent light-harvester. The first study on intramolecular singlet exciton fission (iSEF) in PDI trimers – the generation of two triplet charges from one photogenerated singlet, seeks to elucidate the important structural features required to obtain high yield triplet formation as a result of multiexciton generation and subsequent separation in multichromophoric PDI systems. Time-resolved spectroscopic measurements were used to show how the flexibility of the π-bridge connections in these multichromophoric PDI systems, strongly affect the triplet yield and triplet formation rate. The results obtained showed that the weak electronic coupling observed in the twisted PDI trimer is necessary to activate iSEF in multichromophoric systems. The next chapter is about the effect of ring-fusion on the optically-excited properties of N-annulated thiophene π-bridged PDI dimers. The results of this study show that ring-fusion favors ultrafast photoinduced intramolecular charge transfer (CT) and opens up the triplet excited state deactivation pathway that was absent in the unfused dimer. The triplets were formed via spin-orbit CT intersystem crossing pathway owing to the strong electronic coupling present in the fused-ring thiophene π-bridged dimer. The final chapter about PDIs involves two analogous positional isomers, exhibiting twisted vs planar geometries. The results confirm an efficient and faster intramolecular CT mechanism (symmetry-breaking) taking place in the planar PDI dimer, leading to better device performance despite strong aggregation effects.
The second section involves the electron-donating portion of the active layer of BHJ devices. For donor polymers, the influence of furan vs thiophene π-bridge heterocycles and linear vs bulky sidechains on the optical properties of donor–π-bridge–acceptor polymers, was investigated. The results showed that the furan π-bridge polymer displayed better solar absorptivity and showed a more planar polymeric backbone that correlated to better CT and longer excitonic lifetimes. Also, the linear sidechain polymers showed improved solution-processability leading to better photophysical properties, like longer fluorescence lifetime, relative to the bulky sidechain polymers in BHJs. The final chapter of this dissertation covers three structural variations (Cube, Half and Corner) of hybrid silsesquioxanes, functionalized with donor chromophore(s) at the edge(s) of the cage unit. Improved excitation energy transport and ultrafast CT was observed in the Cube compound relative to the Half and Corner systems attributed to strong electronic coupling. A detailed summary of this dissertation alongside a set of molecular geometry guidelines for structure-function relationships and future direction, was provided.PHDChemical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/167973/1/kizmadu_1.pd
