Nanoporous silver thin films: Multifunctional platforms for influencing chain morphology and optical properties of conjugated polymers

Disordered nanoporous silver (NPAg) thin films fabricated by a thermally assisted dewetting method are employed as a platform to influence chain alignment, morphology, and optical properties of three well-known conjugated polymers. Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements show that the porous structure of the metal induces close π-π stacking of poly(3-hexylthiophene) (P3HT) chains and extended, planar chain conformations of poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) and poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT). A greater degree of vertically-oriented P3HT chains are found on NPAg compared with planar Ag. However, PFO and F8BT chain alignment is only affected when pore size is large. The optical properties of NPAg films are investigated by transmission and back-scattering spectroscopies. Strong back-scattering is observed for all NPAg morphologies, especially for NPAg with small pore sizes. Photoluminescence spectroscopy of conjugated polymer layers on NPAg showed pronounced emission enhancements (up to factors of 26) relative to layers on glass. The enhancements are attributed primarily to: 1) redistribution of conjugated polymer emission by Ag; 2) redirection of emission by polymer-filled nanopores; and 3) local electromagnetic field effects. This work demonstrates the potential of NPAg-thin films to influence molecular chain morphology and to improve light-extraction in organic optoelectronic devices. Nanoporous metal films are multifunctional platforms that affect both the morphology and emission properties of conjugated polymer layer coatings. Nanoporous silver can reorient and/or planarize a fraction of conjugated polymer molecules in the pores. Photoluminescence emission enhancements of up to 26 are possible from conjugated polymer layers using nanoporous silver instead of bare glass substrates.