Synthesis and Characterization of Novel Chiroptical Low Bandgap π-Conjugated Polymers for Circularly Polarized Near-Infrared-Sensing Organic Thin-Film Phototransistors

Chiral optoelectronics manipulating the circularly polarized (CP) light has received extensive attention over the past decades due to its promising opportunities in next-generation optoelectronics such as quantum computation, optical spintronics, encryption, and 3D displays. For achieving technological advances in such fields, various chiral organic semiconducting materials, that differentially absorb CP light according to the polarization direction, have been developed, but the chiroptoelectronic devices using them have not been rarely explored, which is still at a very early stage. Particularly, chiral semiconducting materials with optical activity in near-infrared (NIR, 700~1700 nm) spectral region have become increasingly important because it can fully extend their applicability including long-distance optical communication. Recently, in order to develop chiral material applicable to the chiroptical organic optoelectronic devices operating in NIR region, we synthesized novel chiral low bandgap π-conjugated polymers through four-step chemical reaction procedure, in which the chirality is induced by incorporating enantiopure (<i>R</i>)- or (<i>S</i>)-side chain substituents into donor-acceptor type polymer backbone showing excellent charge transport and NIR absorption properties. In spin-coated thin films, intrinsically chiral sidechain polymers brought about distinct circular dichroism in the NIR region at the local/molecular level, which was amplified by long-range structural chirality of polymer chains depending on the film thickness increment to 200 nm. We pursued to understand the origin of such chiroptical phenomenon of chiral sidechain polymer thin films by using a combination of molecular/spectroscopic analysis and peculiar morphological stacking features of supramolecular self-assembles of chiral polymers. Based on this, we applied the chiral π-conjugated polymer thin films to circularly polarized NIR-sensing photoactive channel layer in organic thin-film phototransistors for direct detection of CP light and efficient amplification of photosignals, and optimized their thickness for a high CP NIR-sensing efficiency. As a result, by distinguishing the left or right handedness of incident CP NIR lights with wavelength of 830 and 920 nm, they achieved excellent photoresponsivity of 31.6 and 26.1 A/W for each wavelength, along with a high photocurrent dissymmetry (g-factors &gt; 1) as well as a fast optical response time (&lt;100 ms) at film thickness below 100 nm. This presentation provides new approaches for the design of chiroptical organic semiconducting materials and devices that can pave the way to high-performance chiral optoelectronic applications.