Architecting Conjugated Molecules for Band Gap Engineering & Photostability

Abstract

Organic photovoltaic (OPV) modules are flexible, light weight, transparent and thin compared to other emerging photovoltaic technologies, making them well-suited for applications ranging from solar windows to fabrics. A large number of polymer semiconducting materials of Donor-Acceptor–Donor (D-A-D) architecture have been synthesized and used in OPV devices in recent times reaching remarkable power conversion efficiencies up to 13%. However, the diversity of monomeric units and the numerous available reports in the structural complexity of D-A-D conjugated p-type polymers indicate that there is scope for new materials which can further improve the performance of OPV devices based on D-A-D polymers. The first part of the thesis explores structural level architecturing of conjugated small molecules and polymers to modify the properties of the OPVs. Molecular level modification could tailor the energy levels of the polymers, planarity and broadened the absorption spectra. Enhancement of the material properties led to improved device performance. Additional to the development in power conversion efficiency of the OPVs, the cost of large area OPV device manufacture continues to decrease with improvements in roll-to-roll manufacturing processes. However, while these advances in efficiency and cost will ultimately the key to the success of the technology, the long term stability of OPV devices under illumination still remains an obstacle to their industrial viability. While the thermal and oxidative-stability of contacts and interfaces in an OPV device can contribute to a decrease in its performance with time, a leading contributor to device decay is photo-oxidation of the active layer itself. Hence a series of chalcogen based polymers have been synthesized and the photostability of the unencapsulated active layers and the device life time has been monitored over a period of time. The results showed, changing of hetero atom could improve the photostability. The thesis further investigated the photostability of several combinations of fluorinated and non-fluorinated high-performance donor polymers with both traditional and fluorinated fullerenes. The miscibility of the active layer component is probed with time-resolved photoluminescence (TRPL). These results are correlated with the photo conductance of the material. The miscibility of the polymers and the fluorinated fullerenes were improved by the strategic fluorination of the polymers, by new synthesis routes. These results ultimately suggest that appropriate fluorination strategies applied to both the donor and acceptor can be a viable route toward a new model of intrinsically photo- and phase-stable OPV active layers.