The design and evaluation of amphiphilic block copolymers for organic photovoltaic active layers with enhanced self-assembly and solubility in green solvents

© 2017 Dr. Valerie MitchellOrganic photovoltaic (OPV) devices employ solution-processable organic molecules as the photovoltaic active layer. This technology could allow the inexpensive production of flexible solar cells via a roll-to-roll printing process. However, current high-performance materials rely on chlorinated or aromatic solvent for dissolution which are unacceptable in industry. Additionally, photocurrent generation in OPV relies on a precise thin-film structure composed of an intercalating network of an electron-rich ‘donor’ material and electron-deficient ‘acceptor’ material. This is because in OPV absorption of light generates a coulombically bound exciton, which is split into free charges at the interface between materials with differing electron affinities. The ideal thin-film morphology of small but well-defined donor and acceptor domains is difficult to reproducibly achieve and maintain over the lifetime of the OPV device. These factors limit the commercial applicability of OPV. Linear block copolymers (BCPs), in which two or more polymeric species are covalently linked, are well-known for their ability to adopt precisely defined morphologies. The covalent linkage of donor and acceptor materials into a block BCP should allow the exploitation of this self-assembly in OPV. The donor-acceptor BCP system provides a route to a theoretically ideal morphology. but has thus far failed to produce high-performance solar cells. It is often difficult to achieve clean phase separation in block copolymers composed of the fully-conjugated polymers which are ideal for OPV application, which can increase charge recombination and decrease photocurrent. Furthermore, the syntheses used to produce fully-conjugated BCPs often generate complex mixtures of donor and acceptor homopolymers in addition to the BCP. Homopolymer contaminants can have a large effect on thin-film morphology and subsequent performance. Additionally, previously reported donor-acceptor BCPs require chlorinated and aromatic solvents for processing and are thus subject to the same industrial limitations faced by traditional OPV blends. In this work, an amphiphilic BCP design is investigated as a means of addressing the problems facing the industrial adoption of OPV technology. Amphiphilic donor- acceptor BCPs incorporate hydrophilic side-chains into one block of the copolymer, introducing dissimilarity between blocks and enhancing phase separation between the materials and increasing solubility in polar solvents. A series of four novel amphiphilic BCPs were synthesized and investigated as active layer materials for organic photovoltaic devices. Poly(3-hexylthiophene) (P3HT) acted as the donor material, while the acceptor blocks employed either a poly(fluorene-alt-benzothiadiazole) (PFTBT) or poly(naphthalene diimide-alt-bithiophene) (PNDIT2) scaffold. Oligo(ethylene glycol side-chains) were incorporated in the acceptor block to confer hydrophilicity. This design allowed the development of a purification technique capable of separating homopolymer from BCP, an advantage over previously reported systems. As expected, the BCPS showed enhanced solubility in commercially acceptable solvents. The morphological behavior of these materials was investigated using a combination of spectroscopic and X- ray scattering techniques. The benefit of the amphiphilic system to microphase separation was demonstrated, as was the value of BCP purification to morphological reproducibility and control. This work clearly demonstrates the potential advantages of careful molecular design to the processing and commercial viability of BCP OPV materials, and demarcates a strategy for the optimization of these systems