Modelling charge transport in organic semiconductors

Modelling of the charge transport in different types of organic semiconductors (molecular crystals and semicrystalline polymers) is undertaken using a number of different methods. Using a polarisable force field we have determined the extent of the external reorganisation energy for hole transport in molecular crystals. We have found that this key parameter is very small for naphthalene, anthracene, tetracene, pentacene and rubrene and discussed the implications of this finding in the context of charge transport modelling. Charge trapping in organic crystals due to defect states is studied by computation of the rate of escape of a trapped charge carrier using non-adiabatic rate theory. Two cases are considered, the first is hopping between separate identical defect states and the second is hopping from a defect state into the bulk (delocalised) states. This latter process is determined as the more likely to occur for a realistic defect concentration. The necessity of the inclusion of an effective quantum mode of vibration for the accurate computation of the rate is also demonstrated. A linear scaling method based on the definition of a localised molecular orbital set is devised for the computation of the electronic structure of a conjugated polymer (poly(3-hexylthiophene) (P3HT)). Utilising a combined classical and quantum chemical approach we show that charge carriers are localised in correspondence with long-lived traps which are also present in the crystalline phase of the polymer. The regio-regularity of P3HT is shown to have little impact on the shape and tail of the density of states; we compute the 2D localised trap density as ~3.1×1013 cm-2, a value which is not found to significantly change with the regioregularity. The states in the tail of the density of states are found to be predominantly localised but energetically close to delocalised "quasi-band" states which are accessible under device operation.EThOS - Electronic Theses Online ServiceEngineering and Physical Sciences Research Council (EPSRC)European Research Council (ERC)GBUnited Kingdo