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

Abstract

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 highenergy 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.

Narrow–Wide Copolymers Designed for Red-Selective Absorption by Time-Dependent Density Functional Theory Calculations

“…The frontier MO diagram (Figure , center, green box) shows that PTB7-Th is rather a typical push–pull copolymer with a strong hybridization between the pushing unit BDT2T and the pulling unit TTF along the coplanar backbone. Thus, PTB7-Th significantly absorbs the green/blue light and shows an unimpressive RS around 55% . If the TTF pulling unit of PTB7-Th is replaced by the T-DPP-T narrow unit of (T-DPP-T-PCZ) 2 , the resulting hybrid copolymer (T-DPP-T-BDT2T) n (Figure c) would be close to a narrow–wide polymer (Figure , right).…”

“…(a) MO diagram (adapted with permission from ref ; copyright 2022 ACS) and TDDFT absorption spectrum of (T-DPP-T-PCZ) n and (b–d) slight modifications that increase its narrow–wide characteristic and in turn the RS (i.e., lowering the HOMO level of the PCZ wide unit, b,c, or raising the HOMO level of the T-DPP-T narrow unit, (d) while keeping the same DFT-optimized backbone geometries. [Color code: H (black), C (gray), N (blue), O (red), F (green), S (yellow), and Se (cyan).…”

“…Thus, PTB7-Th significantly absorbs the green/blue light and shows an unimpressive RS around 55%. 17 If the TTF pulling unit of PTB7-Th is replaced by the T-DPP-T narrow unit of (T-DPP-T-PCZ) 2 , the resulting hybrid copolymer (T-DPP-T-BDT2T) n (Figure 3c) would be close to a narrow−wide polymer (Figure 4, right). Its five-membered-ring-mediated connection via two T subunits as well as the identical HOMO levels (−5.2 eV) of BDT2T and T-DPP-T lead to a coplanar backbone, a strong hybridization especially in the HOMO level, 53 and in turn the lowest energy absorption peak near the NIR region, but (T-DPP-T-BDT2T) n still gives an improved RS value of 66% (both experiment and TDDFT on the dimer; 52 Figure 3c).…”

“…Hence, in our previous work, 17 we have defined the red selectivity (RS, %) of a polymer as the ratio of its red region adsorption (the area under the absorption spectrum between 625 and 800 nm, i.e., in the red box of Figure 1) over its total absorption in the visible and the first NIR regions (the area under the absorption spectrum between 400 and 1000 nm, i.e., in the whole range of Figure 1). Then, we have proposed a RSmaximizing narrow−wide copolymer design principle (Figure 2), which requests that (1) 26 where diketopyrrolopyrrole (DPP) flanked with T (thiophene) 27 or TB (benzothiophene), T-DPP-T(B), is a major narrow unit, while Bz (benzene), Np (naphthalene), Fl (fluorene), phenanthrocarbazole (PCZ), and thienothiophene (TT) are the wide units (Figure 1).…”

“…17 Therefore, the HOMO of the T-TP-T narrow unit has a significant contribution from the T units on the side, contrary to T-DPP-T, leading to a significant T-mediated hybridization with neighboring wide units 40 and a significant amount of green/blue region band II absorption of narrow−wide copolymers including it, e.g., (T-TP-T-Cz) n (Cz = carbazole). 17,41 This problem may be resolved by replacing the flanking T subunits by furan (Fu) 42−45 subunits, which have a slightly lower HOMO level than T while still keeping planar connections to the DPP or TP subunit. These Fu analogues, i.e., Fu-DPP-Fu and Fu-TP-Fu, are herein considered as promising narrow units but turn out to be not successful (see Section 3.2).…”