Organic photovoltaic devices are receiving extensive interest, with device efficiencies now exceeding 8%. There is increasing evidence that the efficiency of dissociation of excitons (bound electron-hole pairs) into free charge carriers is a key factor determining device performance. Dissociation of these excitons occurs at the interface between donor and acceptor molecules in the photoactive layer of the device and is driven by a favourable difference in electronic energy levels between the two materials. Several factors can potentially determine the efficiency of this process, including interfacial energetic energies, molecular structure, film microstructure and device electric fields. This thesis employs optical spectroscopic techniques, including photoluminescence quenching and transient absorption spectroscopy to assay the efficiency of charge separation for a range of donor / acceptor blend films and devices, thereby providing new insights into the factors determining the efficiency of this process.
The first results chapter focuses on the experimental technique, transient absorption spectroscopy, and how this can be employed to determine the yield of dissociated charges in donor / acceptor blend films. Three materials systems are studied, firstly looking at the effects of structure of the donor material, followed by a study into the effects of the temperature on charge generation and recombination.
The following two chapters investigate the effects of film morphology on charge generation, specifically focusing upon the fullerene acceptor. Aggregation of phenyl-C61-butyric acid methyl ester (PCBM) is shown to be a key factor in the generation of free charges in the blend film, leading to proposal of a model regarding the role of this aggregation in charge generation. This study has then been extended by using additives to modify the concentration threshold for PCBM aggregation by preventing the intercalation of the PCBM between the polymer side chains, and hence inducing PCBM aggregation at a lower concentration.
In the final chapter, the effects of an externally applied electric field on charge generation in devices have been studied. Such electric fields have been proposed to reduce geminate recombination losses, thereby increasing the dissociation of free charges, and thus leading to an increase in the device photocurrent. Polymer and small molecule blend systems have been studied and shown to exhibit different dependences of charge generation upon applied bias depending on the donor molecule and the morphology of the blend.