Integrated Calendar

Pi-conjugated organic molecules and polymers now provide a set of well-performing semiconductors that support a wide range of devices, including light-emitting diodes (LEDs) as used in smart-phone displays, field-effect transistors (FETs) and photovoltaic diodes (PVs). These are attractive materials to manufacture, particularly for large-area applications where they be processed by direct printing.
In this talk I will illustrate those aspects of the physics of their electronic properties that distinguish them from inorganic semiconductors, and that have required specific engineering of material and device design. In particular, these materials have low dielectric constants, and the consequently poor screening of Coulomb interactions causes electron-hole excitations (excitons) to be strongly bound. This often gives very high luminescence efficiency, as required for use in LEDs. For PVs, splitting of excitons to form free electrons and holes can be achieved efficiently at heterojunctions formed between materials with different electronegativities, which act as electron ‘donor’ and ‘acceptor’, and PVs now show up to solar cell 10% efficiency.
Strong Coulomb interactions also give rise to large exchange interactions, so that spin triplet excitons lie generally around 0.5 eV below singlet excitons. Triplet excitons can be formed by electron-hole capture both in LEDs and in PVs, and compromise device efficiency. However triplet-triplet fusion to form a singlet exciton can enhance LED efficiency and singlet exciton fission to triplet exciton pairs can be used to enhance PV efficiency, potentially beyond the Shockley-Queisser single junction limit.