Authors: Eric G. Carlson
Source: CU Scholar (University of Colorado Boulder) (article, 2019-01-01)
Field: Materials Science — Electronic, Optical and Magnetic Materials

Molecular Alignment and Morphology Control Techniques for Liquid Crystal Organic Semiconductors Since the dawn of the digital age that began with the discovery of semiconductors in 1947, digital electronics have changed the way humanity operates. As technology continues to mature, the need for new, multifunctional, and affordable electronics grows stronger. Organic semiconductors (OSCs) fill this need as low cost, highly-flexible, easily-processible, and biocompatible materials for a new wave of flexible, wearable, and biosensing electronics. One major barrier to commercialization of organic electronics, however, is the relatively poor electrical performance of current versions of these devices. Because device performance primarily depends on the electronic structure of the material it is important to develop and understand techniques to improve the molecular order and alignment, and thus, electronic structure in OSC materials. In this work we explore the use of soft lithography and solvent processing of thin films to control the molecular alignment and morphology in two liquid crystal (LC) OSC systems.\nFirst, we use the soft lithography techniques of capillary force molding and template-assisted self-assembly with an elastomeric polydimethylsiloxane (PDMS) microchannel mold to align the exotic helical nanofilament (HNF) LC phase. We find that the use of PDMS preferentially aligns the highly crystalline HNF “nanowires”, and after removal of the mold, leaves exposed well-aligned, freestanding microchannels of HNFs. We also studied the relationship between thin film morphology and solvent composition in spin coated thin films of the monoalkyl-[1]benzothieno[3,2-b][1]benzothiophene (BTBT) LC OSC derivatives Th-BTBT-C8 and Th-BTBT-C10. In thin films of both Th-BTBT-C8 and Th-BTBT-C10 spin coated from chlorobenzene (CB), terraced mounds are observed, to the best of our knowledge, for the first time in an OSC material. We also find that mixing CB and chloroform (CF) at a ratio of 20% to 80% produces the highest quality thin films of Th-BTBT-C10. Thin films made from the same solvent mixtures are shown to have improved charge transport over CB, CF and other solvent mixture ratios in organic field-effect transistors (OFETs). The use of solvent mixture engineering to improve electronic properties of OSC thin films extends to another monoalkyl-BTBT derivative, Ph-BTBT-C8, where the 20% CB to 80% CF solvent mixture produces films with superior charge transport over films made from either CB or CF solutions alone. This trend is not observed in two other dialkyl-BTBT derivatives. Overall, we find that the novel use of PDMS microchannels to align HNFs, and solvent engineering to improve thin film morphology and electrical properties of LC OSCs, can be used to help move the field of organic electronics towards commercialization. Organic semiconductor ; Morphology (biology) ; Materials science ; Semiconductor ; Nanotechnology ; Crystallography ; Optoelectronics ; Chemistry ; Geology Liquid Crystal Research Advancements ; Organic Electronics and Photovoltaics ; Nonlinear Optical Materials Research