Solid-Phase Deposition: Conformal Coverage of Micron-Scale Relief Structures with Stretchable Semiconducting Polymers

There are a variety of methods available for forming thin films of electronic polymers, but very few of them are applicable to surfaces bearing relief structures on the micron scale. Of the methods that are capable of conformal coverage of such topography, most are applicable only to coatings that can be polymerized in situ—for example, by chemical vapor deposition—and are, thus, not amenable to polymers with complex molecular structures, such as π-conjugated (semiconducting) polymers. This Letter describes a method termed solid-phase deposition (SPD). The SPD process is a variant of water transfer printing whereby a thin film of a semiconducting polymer is suspended on water and taken up by a substrate bearing micron-scale relief structures (in this case, sharp pyramids etched in silicon). Under ambient conditions, solid films that are sufficiently compliant (thin, ductile, or of low modulus) can coat these surfaces readily. Stiffer films comprising higher modulus polymers can be made amenable to the process by the application of heat or solvent vapor. We successfully formed coatings from films of poly(3-alkylthiophenes) spanning the range from glassy (alkyl = butyl) to rubbery (alkyl = heptyl), along with the low-bandgap polymers DPP-DTT and PTB7-Th, on textured silicon and indium tin oxide (ITO) surfaces.