Strategies for Controlling Bulk Heterojunction Morphology

Organic photovoltaic devices have been extensively studied as a means to produce sustainable energy. However, the performance of organic-photovoltaic (OPV) devices is dependent upon a number of factors including the morphology of the active layer, device architecture, and processing conditions. Recent research has indicated that fullerenes in the bulk heterojunction are entropically driven to the silicon and air interfaces upon crystallization of P3HT, which occurs during thermal annealing. The first chapter of this research focuses on investigating the structure and function of end-tethered poly(3-hexylthiophene) chains to a transparent electrode as an anode buffer layer. Neutron reactivity reveals that these P3HT brush layers have severe effects in the vertical distribution of PCBM across the depth of the BHJ films, the extent of which depends on the grafting density of the P3HT brush layer. These results are confirmed by energy-filtered transmission electron microscopy measurements.

Another emerging trend in the advancement of OPVs is through the addition of a third component to impose morphological or electronic benefits to BHJ-based devices. In Chapter 3 of this dissertation, three different low MW P3HTs are incorporated into BHJ films as additives to reveal fundamental aspects of their behavior as a function of size and loading level. The best performing loading levels for each additive are found to be inversely proportional to the MW of the P3HT additive and appear to be driven by a coarsening of BHJ film morphology. The incorporation of porphyrin-based additives into BHJ OPV devices has been an emerging trend in recent years due to their strong solar absorption and π - π interactions between PCBM nanoparticles and porphyrin centers. Building on these reports, and the investigations of low MW P3HTs, porphyrin-capped low MW P3HTs (PP-P3HT)s are synthesized and incorporated into BHJ films as additives. Although PP-P3HTs impose many substantial morphological benefits to BHJ films, these favorable properties are overshadowed by lackluster device performances, ostensibly due to the presence of the Si-O linkage between the porphyrin and P3HT chains.