Photovoltaics, the direct conversion of light energy to electrical energy, offers access to perhaps the best source of clean renewable energy: the sun. The cost to produce electrical energy using photovoltaics could be dramatically reduced by fabricating photovoltaic devices (solar cells) out of carbon-based (organic) semiconducting materials. This dissertation describes two new approaches for increasing the efficiency in organic photovoltaic systems.
The first approach uses magnetic fields to inhibit the recombination of electrons and holes and increase charge collection. Many magnetic field effects in room-temperature organic semiconductor devices can be understood by the magnetoeffects by the interconversion of singlets and triplets (MIST) model. According to the MIST model, magnetic fields split the degeneracy of triplet electron-hole spin states via the Zeeman effect. This splitting alters inter-system crossing between loosely bound singlets and triplets, which can change the overall recombination rate of electron-hole pairs. The organic magnetoresistance in polymer-based organic photovoltaic devices was measured to be positive and on the order of a few percent. In addition, the magnitude of the organic magnetoresistance decreased with increasing bias and with increasing concentration of electron-accepting fullerenes. This behavior is consistent with the MIST model. The MIST model was then extended to describe magnetic field effects in photocurrents produced by organic photovoltaic devices. Single-layer polymer devices showed an increase in photocurrent of 6-9% with the application of relatively weak magnetic fields (30 mT) due to a reduction in the recombination rate of non-geminate electron-hole pairs. However, in bulk-heterojunction devices the magnetic field effect on photocurrent was significantly diminished with just a few percent by weight of fullerenes. These results are shown to be consistent with the robust MIST model: electron-accepting fullerenes reduce the population of loosely bound electron-hole pairs, thereby quenching the magnetic field effects.
The second approach to increase charge collection in organic photovoltaics uses self-assembled perylene diimide nanostructures in a nanofabric heterojunction. Cyclic voltammetry, photoluminescence quenching and bilayer device measurement shows that perylene diimides are good electron acceptors and potential alternatives to fullerene-based acceptors. Perylene diimides are also excellent n-type conductors; the field-effect transistor mobility of bis(octyl)-perylenediimide (PDI-C8) was measured to be μe =0.05±0.01 cm2 V-1 s-1. In addition, flat perylene diimide molecules tend to π-π stack to form nanofibers and nanofabrics using a simple solvent mixture self-assembly procedure. PDI-C8 nanofibers were incorporated into a novel device architecture—the nanofabric hetero junction—to increase collection of electrons. Devices incorporating PDI-C8 nanofibers exhibited a 110% increase in the short circuit current density compared to devices without the nanofibers. This increase is attributed to the fibers increasing the donor-acceptor interfacial area, transporting electrons out of the device along dedicated conduction pathways and reducing the build up of space-charge.