Organic electronics are at the forefront of new and emerging technologies. From solar cells
to photodetectors and OLED screens, these soft and versatile materials offer great
optoelectronic properties for realising efficient, lightweight, transparent, and flexible
devices. Furthermore, their rich chemistry can be harnessed to tune their absorption to the
infrared region by narrowing the optical gap, achieving absorption onsets below 1.1 eV. In
addition to surpassing the absorption cut-off of silicon benchmark, organic semiconductors
can be processed from solutions, making them excellent candidates for the new generation
of photodetectors.
This thesis presents spectroscopic investigations of narrow-bandgap materials for infrared
organic photodetectors. The work described herein focuses on novel donor:acceptor blends
which offer infrared light detection, while also maintaining solubility. The key approach to
bandgap narrowing is based on the push-pull nature of the proposed conjugated polymer
systems and atomic substitutions in small molecule acceptors to tune the energetics and
microstructure of the organic semiconducting materials.
Utilising ultrafast and steady-state spectroscopic methods, we aim to elucidate the
underlying mechanisms of charge photogeneration, separation, transport, and
recombination processes in these novel blends. Spectroscopic models involving the
processes contributing to the photodetector performance are proposed. Strategies to
improve infrared organic photodetector performance through dark current reduction and
morphology optimisation are discussed.
As well as presenting novel materials, the findings of this work have deepened our
understanding of the mechanisms limiting photodetector performance and, thus, served to
further unlock the potential of organic semiconductors in infrared imaging and sensing
applications.