Polymer / Transparent Electrode Interface Studies with Applications for Organic Solar Cells
Knesting, Kristina Marie
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The field of organic photovoltaics continues to progress through a combination of improved materials selection and a deeper understanding of operational mechanisms. We contribute to advances in both areas through study of the interface between the semiconducting polymer and the transparent electrode. This interface is the site of charge injection and extraction, and this thesis specifically focuses upon nanoscale modification of interfacial properties resulting in measurable changes in bulk device performance and ultimately leading to a deepened understanding of device operation. The polymer / transparent electrode interface is modified through use of phosphonic acid self-assembled monolayers (PA SAMs). Using micro-contact printing, we deposit PA SAMs and spatially modulate the work function of the transparent conductive oxide, indium tin oxide (ITO). The contact potential difference on and off the SAM shows strong contrast and high resolution despite only sub-nanometer variations in topography. We incorporate the patterned ITO into a light-emitting diode as a way to visually confirm changes in charge injection resulting from changes in ITO work function. The patterned electroluminescence is stronger in regions where the SAM is present, indicating locally improved charge injection. We also use monolayers of PA SAMs (on the order of approximately 1nm thick) to modify the work function of the hole-extracting contact, ITO, in polymer/fullerene bulk heterojunction solar cells. We observe a linear dependence of the open-circuit voltage of these organic photovoltaic devices on the modified ITO work function when using a donor polymer with a deep-lying highest-occupied molecular orbital (HOMO). We measure charge carrier lifetimes using transient photovoltage and charge extraction in a series of SAM-modified devices. As expected, these measurements show systematically longer carrier lifetimes in devices with higher open-circuit voltages; however, the trends provide useful distinctions between different hypotheses of how transient photovoltage decays might be controlled by surface chemistry. We interpret our results as being consistent with changes in band-bending at the ITO/bulk heterojunction interface which have the net result of altering the internal electric field to help prevent electrons in fullerene domains from recombining at the hole-extracting electrode.
- Chemistry