Organic semiconductors are attractive materials for displays, solar cells, lighting and lasers because of their combination of desirable optoelectronic properties with simple processing and fabrication. An increasing trend is to be able to manipulate these properties through not only through molecular design but also through processing and the use of wavelength scale cavities. In this talk two examples of modifying exciton behaviour will be presented.
The first is engineering exciton diffusion length to enhance device efficiency in organic photovoltaics. The strong exciton binding energy means it is desirable to increase exciton diffusion length so that excitons can reach a heterojunction and charge separation can occur. We explore the effect of a range of processing methods on exciton diffusion and find that for the widely studied small molecules DR3TBDT and SMPV1, solvent vapour annealing leads to a doubling of the exciton diffusion length. It also leads to larger acceptor domains which would normally reduce charge separation. However, the increased exciton diffusion length means that exciton harvesting is still efficient even in larger domains. The larger domains lead to improved charge extraction and higher device efficiency.
The second aspect of controlling the properties of excitons is in cavities in which strong light-matter coupling can occur. Here we will present low threshold polariton lasing.