In current organic photovoltaic devices, the loss in energy caused by the inevitable charge transfer step leads to a low open circuit voltage, which is one of the main reasons for rather low power conversion efficiencies. A possible approach to avoid these losses is to tune the exciton binding energy below 25 meV, which would lead to free charges upon absorption of a photon, and therefore increase the power conversion efficiency towards the Shockley Queisser limit for inorganic solar cells. We determine the size of the excitons for different one-dimensional organic small molecules or polymers by electron energy loss spectroscopy (EELS) measurements and by DFT calculations. Using the measured dielectric constant and exciton extension, the exciton binding energy is calculated for the investigated molecules, leading to a lower limit of the exciton binding energy for ladder-type polymers. We discuss and propose potential ways to increase the ionic and electronic part of the dielectric function in order to further lower the limit of the exciton binding energy in organic materials. Furthermore, the influence of charge transfer states on the exciton size and its binding energy is calculated with DFT methods for the ladder-type polymer poly(benzimidazobenzophenanthroline) (BBL) in a dimer configuration.
The performance of novel organic devices such as organic light-emitting diodes or organic field-effect transistors is intimately connected to the nature and dynamics of the charge carriers in the device components. Carrying out intercalation studies of solid model oligomers, it is experimentally demonstrated that the low lying electronic excitations in p-type doped systems are significantly confined on the individual molecules due to polaronic effects and thus deserve the name polaron excitations. These results allow for a quantitative experimental estimate of the charge carrier (polaron) extension which is of the order of 20 Angstrom . In addition, it is shown that electron correlation effects play an important role in the determination of the band gap of molecular organic semiconductors. The implications of these results for organic devices are discussed.