Excitons at Polymer Interfaces
Many of the interesting processes occurring in polymer electronic devices take place at interfaces between different materials. These include charge injection and extraction at electrodes, charge generation and recombination in photovoltaics, and exciton (energy) transfer in both LEDs and photovoltaics. Even the most basic process of charge transport involves intermolecular hopping between sites, and often this happens close to the interface with another material (e.g., the dielectric in a polymer transistor). These intermolecular processes are frequently challenging to model theoretically and to measure experimentally. Although crucial in determining device performance, the regions close to the interface often comprise only a small fraction of the total volume, and the optoelectronic transitions associated directly with the interface are always much weaker than those occurring within a single polymer chain.
The importance of interfaces is not unique to organic semiconductors. However, molecular semiconductors, and polymer semiconductors in particular, bring some unique advantages in studying interfacial processes. At the free surface of an inorganic semiconductor, there is inevitably a plane of dangling bonds, sometimes accompanied by a surface reconstruction of the crystal structure. This fundamentally alters the electronic structure, producing new electronic states within the bandgap that dominate the observed ...