High-Performance Wearable Organic Photodetectors by Molecular Design and Green Solvent Processing for Pulse Oximetry and Photoplethysmography

White-light detection from the visible to the near-infrared region is central to many applications such as high-speed cameras, autonomous vehicles, and wearable electronics. While organic photodetectors (OPDs) are being developed for such applications, several challenges must be overcome to produce scalable high-detectivity OPDs. This includes issues associated with low responsivity, narrow absorption range, and environmentally friendly device fabrication. Here, an OPD system processed from 2-methyltetrahydrofuran (2-MeTHF) sets a record in light detectivity, which is also comparable with commercially available silicon-based photodiodes is reported. The newly designed OPD is employed in wearable devices to monitor heart rate and blood oxygen saturation using a flexible OPD-based finger pulse oximeter. In achieving this, a framework for a detailed understanding of the structure-processing-property relationship in these OPDs is also developed. The bulk heterojunction (BHJ) thin films processed from 2-MeTHF are characterized at different length scales with advanced techniques. The BHJ morphology exhibits optimal intermixing and phase separation of donor and acceptor moieties, which facilitates the charge generation and collection process. Benefitting from high charge carrier mobilities and a low shunt leakage current, the newly developed OPD exhibits a specific detectivity of above 1012  Jones over 400-900 nm, which is higher than those of reference devices processed from chlorobenzene and ortho-xylene.

In Situ Modification of Multi-Walled Carbon Nanotubes with Polythiophene-Based Conjugated Polymer for Information Storage

One-dimensional multi-walled carbon nanotubes (MWNTs) have unique electrical properties, but they are not solution-processable, which severely limits their applications in microelectronic devices. Therefore, it is of great significance to improve the solubility of MWNTs and endow them with new functions by chemical modification. In this work, MWNTs were in situ functionalized with poly[(1,4-diethynyl-benzene)-alt-(3-hexylthiophene)] (PDHT) via Sonogashira-Hagihara polymerization. The obtained material PDHT-g-MWNTs was soluble in conventional organic solvents. By sandwiching a PDHT-g-MWNTs film between Al and ITO electrodes, the fabricated Al/PDHT-g-MWNTs/ITO electronic device exhibited nonvolatile rewritable memory behavior, with highly symmetrical turn-on/off voltages, a retention time of over 10⁴ s, and durability for 200 switching cycles. These findings provide important insights into the development of carbon nanotube-based materials for information storage.

Impact of Side-Chain Hydrophilicity on Packing, Swelling, and Ion Interactions in Oxy-Bithiophene Semiconductors

Exchanging hydrophobic alkyl-based side chains to hydrophilic glycol-based side chains is a widely adopted method for improving mixed-transport device performance, despite the impact on solid-state packing and polymer-electrolyte interactions being poorly understood. Presented here is a molecular dynamics (MD) force field for modeling alkoxylated and glycolated polythiophenes. The force field is validated against known packing motifs for their monomer crystals. MD simulations, coupled with X-ray diffraction (XRD), show that alkoxylated polythiophenes will pack with a "tilted stack" and straight interdigitating side chains, whilst their glycolated counterpart will pack with a "deflected stack" and an s-bend side-chain configuration. MD simulations reveal water penetration pathways into the alkoxylated and glycolated crystals-through the π-stack and through the lamellar stack respectively. Finally, the two distinct ways triethylene glycol polymers can bind to cations are revealed, showing the formation of a metastable single bound state, or an energetically deep double bound state, both with a strong side-chain length dependence. The minimum energy pathways for the formation of the chelates are identified, showing the physical process through which cations can bind to one or two side chains of a glycolated polythiophene, with consequences for ion transport in bithiophene semiconductors.

Efficient energy transport in an organic semiconductor mediated by transient exciton delocalization

Efficient energy transport is desirable in organic semiconductor (OSC) devices. However, photogenerated excitons in OSC films mostly occupy highly localized states, limiting exciton diffusion coefficients to below ~10^-2 cm²/s and diffusion lengths below ~50 nm. We use ultrafast optical microscopy and nonadiabatic molecular dynamics simulations to study well-ordered poly(3-hexylthiophene) nanofiber films prepared using living crystallization-driven self-assembly, and reveal a highly efficient energy transport regime: transient exciton delocalization, where energy exchange with vibrational modes allows excitons to temporarily re-access spatially extended states under equilibrium conditions. We show that this enables exciton diffusion constants up to 1.1 ± 0.1 cm²/s and diffusion lengths of 300 ± 50 nm. Our results reveal the dynamic interplay between localized and delocalized exciton configurations at equilibrium conditions, calling for a re-evaluation of exciton dynamics and suggesting design rules to engineer efficient energy transport in OSC device architectures not based on restrictive bulk heterojunctions.

Conjugated Polymer Films Having a Uniaxial Molecular Orientation and Network Structure Prepared by Electrochemical Polymerization in Liquid Crystals

A new method for fabricating conjugated polymer films was developed using electrochemical polymerization in liquid crystals and magnetic orientation. A uniaxial main chain orientation and a crosslinked network structure were achieved with this method. By employing eight types of monomers, the influence of the crosslinking for the film was investigated. The crosslinking was found to improve the solvent resistance of the conjugated polymer films. This new method is expected to be useful in various applications, such as high-powered organic electronic devices with durability.

Effect of static external electric field on bulk and interfaces in organic solar cell systems: A density-functional-theory-based study

In this work, a detailed comparison of optical and electronic properties in bulk and interfaces of well-known organic semiconductor systems in presence of an external electric field is reported. We have used density functional theory (DFT) to model organic solar cell systems. The study promotes a deeper understanding of the connection between the chemical structures and the optical and electronic properties in the well-known organic solar cell systems based on thiophene and fullerene polymers. We have performed a vibration-mode analysis by simulating Raman spectra in presence of external electric fields. Time-dependent DFT has been used to investigate the effect of an external electric field on excited state properties. The charge-transfer rate controlled by the external electric field has been quantitatively extracted using the simulated excited state dipole moment, Gibbs free energy, and Marcus theory. Our results provide a detailed characterization of the effect of the external electric field on the neat polymers (bulk) and on the donor-acceptor heterojunctions (interfaces) in organic solar cell systems. This theoretical approach not only helps to understand the effect of an external field on bulk and interfaces in organic semiconductors, but it also supports the design of novel devices.

Approaching Crystal Structure and High Electron Mobility in Conjugated Polymer Crystals

Conjugated polymers usually form crystallized and amorphous regions in the solid state simultaneously, making it difficult to accurately determine their precise microstructures. The lack of multiscale microstructures of conjugated polymers limits the fundamental understanding of the structure-property relationships in polymer-based optoelectronic devices. Here, crystals of two typical conjugated polymers based on four-fluorinated benzodifurandione-based oligo(p-phenylene vinylene) (F₄ BDOPV) and naphthalenediimide (NDI) motifs, respectively, are obtained by a controlled self-assembly process. The strong diffractivity of the polymer crystals brings an opportunity to determine the crystal structures by combining X-ray techniques and molecular simulations. The precise polymer packing structures are useful as initial models to evaluate the charge transport properties in the ordered and disordered phases. Compared to the spin-coated thin films, the highly oriented polymer chains in crystals endow higher mobilities with a lower hopping energy barrier. Microwire crystal transistors of F₄ BDOPV- and NDI-based polymers exhibit high electron mobilities of up to 5.58 and 2.56 cm²  V^-1  s^-1 , respectively, which are among the highest values in polymer crystals. This work presents a simple method to obtain polymer crystals and their precise microstructures, promoting a deep understanding of molecular packing and charge transport for conjugated polymers.

Atomistic modelling of entropy driven phase transitions between different crystal modifications in polymers: the case of poly(3-alkylthiophenes)

Polymorphism and related solid-state phase transitions affect the structure and morphology and hence the properties of materials, but they are not-so-well understood. Atomistic computational methods can provide molecular-level insights, but they have rarely proven successful for transitions between polymorphic forms of crystalline polymers. In this work, we report atomistic molecular dynamics (MD) simulations of poly(3-alkylthiophenes) (P3ATs), widely used organic semiconductors to explore the experimentally observed, entropy-driven transition from form II to more common form I type polymorphs, or, more precisely, to form I mesophases. The transition is followed continuously, also considering X-ray diffraction evidence, for poly(3-hexylthiophene) (P3HT) and poly(3-butylthiophene) (P3BT), evidencing three main steps: (i) loss of side chain interdigitation, (ii) partial disruption of the original stacking order and (iii) reorganization of polymer chains into new, tighter, main-chain stacks and new layers with characteristic form I periodicities, substantially larger than those in the original form II. The described approach, likely applicable to other important transitions in polymers, provides previously inaccessible insight into the structural organization and disorder features of form I structures of P3ATs, not only in their development from form II structures but also from melts or solutions.

Crystallographic and Dynamic Aspects of Solid-State NMR Calibration Compounds: Towards ab Initio NMR Crystallography

The excellent results of dispersion-corrected density functional theory (DFT-D) calculations for static systems have been well established over the past decade. The introduction of dynamics into DFT-D calculations is a target, especially for the field of molecular NMR crystallography. Four (13) C ss-NMR calibration compounds are investigated by single-crystal X-ray diffraction, molecular dynamics and DFT-D calculations. The crystal structure of 3-methylglutaric acid is reported. The rotator phases of adamantane and hexamethylbenzene at room temperature are successfully reproduced in the molecular dynamics simulations. The calculated (13) C chemical shifts of these compounds are in excellent agreement with experiment, with a root-mean-square deviation of 2.0 ppm. It is confirmed that a combination of classical molecular dynamics and DFT-D chemical shift calculation improves the accuracy of calculated chemical shifts.

Flexible tactile sensor using the reversible deformation of poly(3-hexylthiophene) nanofiber assemblies

In this letter, we report a simple approach to fabricating scalable flexible tactile sensors using a nanofiber assembly of regioregular poly(3-hexylthiophene) (P3HT). Uniform P3HT nanofibers are obtained through a continuous electrospinning process using a homogeneous solution of high-molecular-weight P3HT. The P3HT nanofibers are oriented by collecting them on a rotating drum collector. Small physical inputs into the self-standing P3HT nanofiber assemblies give rise to additional contact among neighboring nanofibers, which results in decreased contact resistance in directions orthogonal to the nanofiber orientation. The P3HT nanofiber assemblies could detect pressure changes and bending angles by monitoring the resistance changes, and the sensor responses were repeatable.