Nanoporous Metal Thin Films: Multifunctional Platforms for Influencing the Performance of Organic Light-Emitting Devices
To expand their applications in commercial products, methods to increase organic optoelectronic device efficiency are attracting a lot of attention. Many studies focusing on tuning the molecular morphology, photophysics and electrical properties of organic materials to improve internal quantum efficiency of the devices or improving the light extraction/trapping ability of the device structure have been conducted to improve device efficiencies . In our study, we investigate nanoporous metallic thin films as multifunctional platforms for influencing molecular organization, photophysics and light-management in organic conjugated polymer semiconductor thin films for optoelectronic applications.
Nanoporous Ag (NPAg) thin films (>1.5 cm2) with different pore sizes and porosities are fabricated using the thermally-assisted dewetting method . Grazing-incidence wide-angle X-ray scattering data show that conjugated polymer chain organization can be affected by NPAg in different ways. For example, NPAg can increase the fraction of edge-on oriented poly(3-hexylthiophene) (P3HT) chains and decrease the intermolecular π-π stacking distance relative to P3HT on planar Ag. For smaller pore widths and larger porosity, the changes are more pronounced. However, for polyfluorene-based polymers (poly(9,9-dioctylfluorene), PFO and poly(9,9-dioctylfluorene-alt-benzothiadiazole, F8BT) only NPAg with larger pore size is found to alter chain orientation. These observed changes in molecular organization with pore size and porosity could allow the electrical properties of organic active layers in thin-film optoelectronic devices to be tuned .
Moreover, significant photoluminescence (PL) enhancements were achieved for PFO (up to 22), F8BT (up to 18) and P3HT (up to 26) on NPAg films relative to that on glass. Four mechanisms are propose to contribute to the large PL enhancement: 1) redistribution of emission by Ag; 2) redirection of emission by nanopores; 3) local electromagnetic field effects; and 4) polymer chain morphology changes caused by NPAg. Both redistribution and redirection of emission by Ag and the nanopores are believed to be the most important emission enhancement factors in polymers with high intrinsic quantum efficiencies (IQE) (i.e., PFO and F8BT). While redistribution of emission by Ag and local electromagnetic field effects were believed to be the most important factors in polymers with low IQE (i.e., P3HT) . Angle-resolved PL measurements are under way to study the redirection ability of nanopores together with single-pore spectroscopy and time-resolved absorption and emission measurements to investigate local electromagnetic field effects of NPAg.
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