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

Abstract

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\ue0d5for example, by chemical vapor deposition\ue0d5and 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. The manufacturing of semiconductor devices is enabled to a large extent by techniques that produce uniform thin films of polymeric materials. For example, spincoating, slot-die coating, and various forms of chemical vapor deposition (CVD). However, just a small subset of these methods can coat substrates bearing topographic (relief) structures in a conformal manner. Meaning, few methods can form coatings in such a way that the thin film is uniform in thickness and the surface features are preserved with a high degree of fidelity. Moreover, methods that have this capability\ue0d5for example, polymer CVD\ue0d5are limited to materials that can be polymerized in situ, and thus, the diversity in the molecular structure of the deposited film is limited. That is, there are currently no methods available to form conformal coatings of materials, such as low-bandgap semiconducting polymers. Development of such a process would be useful, for example, in the field of photovoltaics, where silicon, perovskite, and organic semiconductors are commonly structured with relief as a means of trapping light. Here, we describe a process inspired by water-based transfer printing, whereby a solid polymer film initially supported atop the surface of water can then be taken up by a topographically patterned substrate. This process\ue0d5solidphase deposition (SPD)\ue0d5relies on plastic deformation of the polymer film to adhere to the relief structures in a conformal, defect-free way. It is applicable to a wide variety of polymers of varying complexity and function to form conformal coatings of nanometer-scale thicknesses on surfaces with micron-scale relief features. In this work, we used silicon textured with random pyramids with heights on the order of 10 μm. Such substrates were chosen both to demonstrate the capability of SPD to coat features, which are much larger than the coatings are thick, as well as because prior work has shown that the reflectance of textured silicon surfaces significantly increases if the pyramids are too small (less than approximately 2 μm). Thin polymeric films are ubiquitous in science and technology. Along with photoresists\ue0d5which have enabled Received: April 2, 2021 Accepted: June 7, 2021 Leter www.acsmaterialsletters.org © XXXX American Chemical Society 988 https://doi.org/10.1021/acsmaterialslett.1c00213 ACS Materials Lett. 2021, 3, 988−995 D ow nl oa de d vi a D ar re n L ip om i o n Ju ne 9 , 2 02 1 at 1 5: 15 :1 0 (U T C ). Se e ht tp s: //p ub s. ac s. or g/ sh ar in gg ui de lin es f or o pt io ns o n ho w to le gi tim at el y sh ar e pu bl is he d ar tic le s.