Narrow-Wide Copolymer for Strong Red-Color-Selective Absorption

Full-color image sensors for retinal prosthesis and artificial vision would require organic RGB-color-selective photodiode components. The most challenging components are those for red-selective absorption. Molecular dyes such as phthalocyanine and squaraine require vacuum deposition, and red-light absorption achieved by solution-processed push-pull copolymers is often accompanied by higher-energy absorption in green and blue regions. Push-pull copolymers designed to suppress such high-energy absorption show their low-energy absorption in the IR region rather than in the red region. Herein, we define red selectivity (RS) of a polymer as the ratio of its red-region absorption (the area under the absorption spectrum between 625 and 800 nm) to its total absorption in the visible and near-IR regions (the area between 400 and 1000 nm) and propose a narrow-wide (rather than push-pull) design rule for RS-enhancing copolymers, (1) highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) localized in the narrow-band-gap unit and HOMO-1 and LUMO+1 localized in the other wide-band-gap unit and (2) hybridization between the two units minimized by introducing a twist in the backbone. Doping them with red-selective nonfullerene acceptors would enhance charge transport and sensitivity without sacrificing RS. Such polymers tuned for strong absorption of skin-penetrating red light would be also useful for spatially controlled wireless (noninvasive) power supply to photovoltaic-coupled organic (conformable/biocompatible) electronic implant.