All-Conjugated Block Copolymers for Organic Photovoltaic Applications
Smith, Kendall Allen
Doctor of Philosophy
Conventional inorganic solar technologies are expensive due to the high cost of processing, while organic materials have significant cost advantages in the raw materials and ease of processing. Unfortunately, organic devices suffer from low efficiency due to difficulty in transporting charges to the electrodes. Typical devices mix the donor and acceptor components and anneal them to allow for phase separation. However, because the phase separation is uncontrolled, domains may be larger than optimal and isolated domains can be formed reducing efficiency. All-conjugated block copolymers have the potential to improve efficiency by creating an ordered structure with controlled domains and continuous pathways through self-assembly. In this work, the relationships between structure, optoelectronic properties, and processing conditions for these materials are systematically investigated using two routes to obtain the materials. In one route, functionalized catalysts are used to initiate controlled polymerizations of two different polymers. These well functionalized precursors are then joined together using copper catalyzed azide alkyne click chemistry. In a second route, a sequential polymerization route is employed where one polymer is synthesized with a well-defined end-group. The polymer is then used as a macroreagent to end-cap a Suzuki polycondensation reaction, yielding materials with direct conjugation between the blocks. The first route yields well-defined materials, whereas the second can access a broader variety of polymers. For all these materials, processing conditions are varied and the morphology of the all-conjugated block copolymers are analyzed by a combination of grazing-incidence X-ray scattering, neutron scattering and reflectivity, atomic force microscopy, and transmission electron microscopy. Materials are found to self-assemble into thermodynamically stable structures with well-defined length scales. It is found that crystallization of either block is predominant in all block copolymers studied, but at intermediate ratios crystallization of both blocks is observed. Processing conditions such as casting temperature, annealing duration, and speed of quenching to room temperature are found to have important effects on thin film crystallinity and orientation of the π-π stacking direction of polymer crystallites. By varying the annealing duration and quenching speed, crystallization of either or both block can be obtained.