Research into 'plastic' semiconductors has led to materials that can harvest the energy of the sun and convert it into electricity. Yet, current devices made with such semiconducting plastics are complex and require two materials to efficiently convert light into electricity. The current project explores whether eventually this structure could be simplified, possibly yielding devices based on a single material. For this, the effect of the local environment on the electronic and optical properties of the plastic semiconductor will be systematically investigated using model systems to test theories that predict that more efficient devices could be produced by making the local environment of the plastic semiconductor more polar. The fundamental insight achieved in this project will allow to design and produce new plastic semiconductors, improve the efficiency and potentially the stability of plastic-based solar cells, and will likely simplify their production so that they eventually could be printed similar to the way newspapers are printed. These all are factors that would improve the economic viability of plastic semiconductors in a range of applications and, in turn, would contribute to reduced greenhouse gas emission and facilitate implementation of concepts such as zero-energy housing and low-water-intensity farming based on solar cell integrations into greenhouses.
The planned work aims to significantly advance our fundamental understanding of how high-k environments affect the optoelectronic properties of polymer semiconductors. While the effective dielectric constant, k, of functional polymers can be manipulated via chemical design or the surrounding of the polymer, changes in molecular structure to create a high-k polymers also affect other features, including polarity, molecular packing, phase behavior, and electronic properties. As a consequence, few direct structure/function relations have been experimentally established to date. In this project, various current complications that have hindered the delivery of structure/function interrelations will be circumvented via i) separating the polymer self-assembly during processing from the introduction of the high-k material as much as possible, to allow for a clean before/after comparisons, and ii) pursuing new self-assembly strategies to create molecular hybrid materials and blends. These approaches allow to tune the lengthscales and specific interfacial areas of the self-assembly and, in turn, provide means to disentangle sought-after optoelectronic effects from other factors that might impact optoelectronic properties/materials performance. The knowledge gained during the project will be applied to polymer solar cells, which generally rely on donor:acceptor bulk heterojunctions to achieve efficient exciton splitting. A main goal of the project is to delineate if there is ever a pathway to significantly lower the exciton binding energy (and thus realize efficient exciton splitting in organic solar cells) by modifying k in the vicinity of relevant semiconducting polymer backbones or if one really needs to modify the backbone itself.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
