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  1. Photodetection spanning the short-, mid-, and long-wave infrared (SWIR-LWIR) underpins modern science and technology. Devices using state-of-the-art narrow bandgap semiconductors require complex manufacturing, high costs, and cooling requirements that remain prohibitive for many applications. We report high-performance infrared photodetection from a donor-acceptor conjugated polymer with broadband SWIR-LWIR operation. Electronic correlations within the π-conjugated backbone promote a high-spin ground state, narrow bandgap, long-wavelength absorption, and intrinsic electrical conductivity. These previously unobserved attributes enabled the fabrication of a thin-film photoconductive detector from solution, which demonstrates specific detectivities greater than 2.10 × 10 9 Jones. These room temperature detectivities closely approach those of cooled epitaxial devices. This work provides a fundamentally new platform for broadly applicable, low-cost, ambient temperature infrared optoelectronics. 
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  3. Abstract

    Supercapacitors have emerged as an important energy storage technology offering rapid power delivery, fast charging, and long cycle lifetimes. While extending the operational voltage is improving the overall energy and power densities, progress remains hindered by a lack of stable n‐type redox‐active materials. Here, a new Faradaic electrode material comprised of a narrow bandgap donor−acceptor conjugated polymer is demonstrated, which exhibits an open‐shell ground state, intrinsic electrical conductivity, and enhanced charge delocalization in the reduced state. These attributes afford very stable anodes with a coulombic efficiency of 99.6% and that retain 90% capacitance after 2000 charge–discharge cycles, exceeding other n‐dopable organic materials. Redox cycling processes are monitored in situ by optoelectronic measurements to separate chemical versus physical degradation mechanisms. Asymmetric supercapacitors fabricated using this polymer with p‐type PEDOT:PSS operate within a 3 V potential window, with a best‐in‐class energy density of 30.4 Wh kg−1at a 1 A g−1discharge rate, a power density of 14.4 kW kg−1at a 10 A g−1discharge rate, and a long cycle life critical to energy storage and management. This work demonstrates the application of a new class of stable and tunable redox‐active material for sustainable energy technologies.

     
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  4. Abstract

    This work examines an additive approach that increases dielectric screening to overcome performance challenges in organic shortwave infrared (SWIR) photodiodes. The role of the high‐permittivity additive, camphoric anhydride, in the exciton dissociation and charge collection processes is revealed through measurements of transient photoconductivity and electrochemical impedance. Dielectric screening reduces the exciton binding energy to increase exciton dissociation efficiency and lowers trap‐assisted recombination loss, in the absence of any morphological changes for two polymer variants. In the best devices, the peak internal quantum efficiency at 1100 nm is increased up to 66%, and the photoresponse extends to 1400 nm. The SWIR photodiodes are integrated into a 4 × 4 pixel imager to demonstrate tissue differentiation and estimate the fat‐to‐muscle ratio through noninvasive spectroscopic analysis.

     
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  5. Abstract

    Conductive polymers largely derive their electronic functionality from chemical doping, processes by which redox and charge‐transfer reactions form mobile carriers. While decades of research have demonstrated fundamentally new technologies that merge the unique functionality of these materials with the chemical versatility of macromolecules, doping and the resultant material properties are not ideal for many applications. Here, it is demonstrated that open‐shell conjugated polymers comprised of alternating cyclopentadithiophene and thiadiazoloquinoxaline units can achieve high electrical conductivities in their native “undoped” form. Spectroscopic, electrochemical, electron paramagnetic resonance, and magnetic susceptibility measurements demonstrate that this donor–acceptor architecture promotes very narrow bandgaps, strong electronic correlations, high‐spin ground states, and long‐range π‐delocalization. A comparative study of structural variants and processing methodologies demonstrates that the conductivity can be tuned up to 8.18 S cm−1. This exceeds other neutral narrow bandgap conjugated polymers, many doped polymers, radical conductors, and is comparable to commercial grades of poly(styrene‐sulfonate)‐doped poly(3,4‐ethylenedioxythiophene). X‐ray and morphological studies trace the high conductivity to rigid backbone conformations emanating from strong π‐interactions and long‐range ordered structures formed through self‐organization that lead to a network of delocalized open‐shell sites in electronic communication. The results offer a new platform for the transport of charge in molecular systems.

     
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  6. Abstract

    Infrared organic photodetector materials are investigated using transient absorption spectroscopy, demonstrating that ultrafast charge generation assisted by polymer aggregation is essential to compensate for the energy gap law, which dictates that excited state lifetimes decrease as the band gap narrows. Short sub‐picosecond singlet exciton lifetimes are measured in a structurally related series of infrared‐absorbing copolymers that consist of alternating cyclopentadithiophene electron‐rich “push” units and strong electron‐deficient “pull” units, including benzothiadiazole, benzoselenadiazole, pyridalselenadiazole, or thiadiazoloquinoxaline. While the ultrafast lifetimes of excitons localized on individual polymer chains suggest that charge carrier generation will be inefficient, high detectivity for polymer:PC71BM infrared photodetectors is measured in the 0.6 < λ < 1.5 µm range. The photophysical processes leading to charge generation are investigated by performing a global analysis on transient absorption data of blended polymer:PC71BM films. In these blends, charge carriers form primarily at polymer aggregate sites on the ultrafast time scale (within our instrument response), leaving quickly decaying single‐chain excitons unquenched. The results have important implications for the further development of organic infrared optoelectronic devices, where targeting processes such as excited state delocalization over aggregates may be necessary to mitigate losses to ultrafast exciton decay as materials with even lower band gaps are developed.

     
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  7. Abstract

    Organic semiconducting donor–acceptor polymers are promising candidates for stretchable electronics owing to their mechanical compliance. However, the effect of the electron‐donating thiophene group on the thermomechanical properties of conjugated polymers has not been carefully studied. Here, thin‐film mechanical properties are investigated for diketopyrrolopyrrole (DPP)‐based conjugated polymers with varying numbers of isolated thiophene moieties and sizes of fused thiophene rings in the polymer backbone. Interestingly, it is found that these thiophene units act as an antiplasticizer, where more isolated thiophene rings or bigger fused rings result in an increased glass transition temperature (Tg) of the polymer backbone, and consequently elastic modulus of the respective DPP polymers. Detailed morphological studies suggests that all samples show similar semicrystalline morphology. This antiplasticization effect also exists inpara‐azaquinodimethane‐based conjugated polymers, indicating that this can be a general trend for various conjugated polymer systems. Using the knowledge gained above, a new DPP‐based polymer with increased alkyl side chain density through attaching alky chains to the thiophene unit is engineered. The new DPP polymer demonstrates a record lowTg, and 50% lower elastic modulus than a reference polymer without side‐chain decorated on the thiophene unit. This work provides a general design rule for making low‐Tgconjugated polymers for stretchable electronics.

     
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