Electronic structure and optical absorption spectra of poly(3,4-ethylenedioxythiophene) (PEDOT) for different oxidation levels were studied using density functional theory (DFT) and time-dependent ...DFT. It is shown, that the DFT-based predictions for the polaronic and bipolaronic states and the nature of corresponding optical transitions are qualitatively different from the widely used traditional picture based on semi-empirical pre-DFT approaches that still dominate the current literature. On the basis of the results of our calculations, the experimental Vis/NIR absorbance spectroscopy and the electron paramagnetic resonance spectroscopy are re-examined, and a new interpretation of the measured spectra and the spin signal, which is qualitatively different from the traditional interpretation, is provided. The findings and conclusions concerning the nature of polaronic and bipolaronic states, band structure and absorption spectra presented for PEDOT, are generic for a wide class of conducting polymers (such as polythiophenes and their derivatives) that have a similar structure of monomer units.
In this article, we report on advanced molecular dynamics simulations of the atomistic DMASnBr3–water interface, which, coupled with a grand-canonical formulation of adsorbates and defects, elucidate ...the surface chemistry and reactivity of this novel water-stable perovskite and highlight the role of small electron bipolarons in photocatalytic hydrogen production. We find that the extremely acidic nature of the surface Br atoms does not allow for significant adsorption of protons at the interface under charge-neutral conditions. However, when electrons are accumulated on the surface, the formation of a small electron bipolaron in the form of a Sn-Sn dimer provides the required electron localization to drive adsorption of H, which is assimilated on surface Sn atoms as hydride. Finally, we estimate a favourable alignment between the bipolaron energy level and the H+/H2 redox level, which suggests the occurrence of a feasible route for hydrogen evolution, bypassing the common reaction mechanism.
When an electron is removed from a conjugated polymer, such as poly(3‐hexylthiophene‐2,5‐diyl) (P3HT), the remaining hole and associated change in the polymer backbone structure from aromatic to ...quinoidal are referred to as a polaron. Bipolarons are created by removing the unpaired electron from an already‐oxidized polymer segment. In electrochemically‐doped P3HT films, polarons, and bipolarons are readily observed, but in chemically‐doped P3HT films, bipolarons rarely form. This is explained by studying the effects of counterion position on the formation of polarons, strongly coupled polarons, and bipolarons using both spectroscopic and X‐ray diffraction experiments and time‐dependent density functional theory calculations. The counterion positions control whether two polarons spin‐pair to form a bipolaron or whether they strongly couple without spin‐pairing are found. When two counterions lie close to the same polymer segment, bipolarons can form, with an absorption spectrum that is blueshifted from that of a single polaron. Otherwise, polarons at high concentrations do not spin‐pair, but instead J‐couple, leading to a redshifted absorption spectrum. The counterion location needed for bipolaron formation is accompanied by a loss of polymer crystallinity. These results explain the observed formation order of single polarons, coupled single polarons, and singlet bipolarons in electrochemically‐ and chemically‐doped conjugated polymers.
At high doping concentrations, polarons can spin‐pair to form bipolarons or become strongly J‐coupled depending on the counterion locations. The spectroscopic signatures of these two species are identified using time‐dependent density functional theory calculations and spectroscopic and X‐ray diffraction experiments on chemically‐ and electrochemically‐doped P3HT films. Coupled polarons have a redshifted absorption relative to the polaron, while bipolarons have a blueshifted absorption.
In this article, we report on advanced molecular dynamics simulations of the atomistic DMASnBr3–water interface, which, coupled with a grand-canonical formulation of adsorbates and defects, elucidate ...the surface chemistry and reactivity of this novel water-stable perovskite and highlight the role of small electron bipolarons in photocatalytic hydrogen production. We find that the extremely acidic nature of the surface Br atoms does not allow for significant adsorption of protons at the interface under charge-neutral conditions. However, when electrons are accumulated on the surface, the formation of a small electron bipolaron in the form of a Sn-Sn dimer provides the required electron localization to drive adsorption of H, which is assimilated on surface Sn atoms as hydride. Finally, we estimate a favourable alignment between the bipolaron energy level and the H+/H2 redox level, which suggests the occurrence of a feasible route for hydrogen evolution, bypassing the common reaction mechanism.
•Poor adsorption of protons at the DMASnBr3–water interface rules out common reaction mechanisms for H2 production.•Small electron bipolarons alter the reactivity of the surface, inducing H adsorption as surface hydride.•An alternative route for photocatalytic H2 production is proposed from the alignment of energy levels at the interface.
Previous reports indicate that cove-type graphene nanoribbons (CGNR) may present high intrinsic charge mobility of almost 15,000 cm2/Vs. Still, with experimental estimates varying from 150 to ...15,000 cm2/Vs. Typically, theoretical mobilities are obtained from methods such as the Drude-Smith model, which tends to neglect the electron-phonon coupling mechanism, or the Boltzmann transport equation, that considers only acoustic phonons. As such, more thorough approaches are needed. In this work, we simulated charge transport in 4-CGNR by explicitly contemplating the lattice collective behavior. The nanoribbon is simulated by a two-dimensional Su-Schrieffer-Heeger (SSH) tight-binding model with electron-phonon coupling and considering all phonon modes. Results show the rise of two quasiparticles: polaron and bipolaron. We probed their dynamical properties by including the presence of an external electric field. Findings indicate that each carrier has a characteristic transport regime that is deeply related to phonon collision interactions. Model derived mobilities for polarons and bipolarons reach up to 18,000 cm2/Vs and 1500 cm2/Vs, respectively. Furthermore, calculations reveal the carriers to be highly efficient charge transporters, with a field independent low effective mass and notable mobility, delivering a better performance than other narrow GNRs. All presented features place the CGNR as a potential base material of future high-quality organic-based optoelectronic devices. The work also contributes to the theoretical understanding of transport physics in highly confined materials.
•Polarons and bipolarons are responsible for the charge transport in the 4-CGNR.•Their transport regime is deeply related to phonon collision interactions.•The formalism adequately describes charge transport in highly confined materials.•The effective masses and mobilities are provided.
In this work, three n‐type donor–acceptor copolymers consisting of glycolated naphthalene tetracarboxylicdiimide (gNDI) coupled with variable donating companion moieties are reported. Using ...2,2′‐bis(3,4‐ethylenedioxy)bithiophene, 2,2′‐bithiophene, 3,3′‐difluoro‐2,2′‐bithiophene (FBT), the donating strength of the donor units is systematically functionalized. These copolymers are used as a platform for aqueous‐based electrochemical devices, including energy‐storage devices, electrochromic devices (ECDs), and organic electrochemical transistors (OECTs). It is found that the electrochemical redox stability and electron mobility of copolymers are significantly improved via weakening the electron‐donating strength of donor units. gNDI coupling with FBT (gNDI‐FBT) exhibits a charge‐storage capacity exceeding 190 Fg−1, which is the highest value reported to date for NDI‐based polymer electrodes in aqueous media. For ECDs, gNDI‐FBT remains 100% of initial electrochromism contrast (∆%T = 20%) up to 1200 s. In addition, gNDI‐FBT outperforms its two analogs in OECTs, including lower threshold voltage (0.19 V), faster response time (45.5 ms), and higher volumetric capacitance (197 F cm−3). Moreover, gNDI‐FBT with fluorine atoms leads to the bipolarons delocalization along the polymer backbone and favorable molecular packing for ion–electron transport. Through such weak donor functionalization strategy, this work provides ways for n‐type copolymers tuning to access desirable performance metrics in optical, electrochemical, and bioelectronic applications.
Three glycolated n‐type naphthalene tetracarboxylicdiimide‐based copolymers coupled with variable electron‐donating companion moieties are used for aqueous‐based electrochemical devices. Weakening the electron‐donating strength of donor units leads to improved electrochemical redox stability, electron mobility, delocalized bipolarons distribution, and favorable molecular packing for ion–electron transport. Through weak donor strategy, n‐type copolymers can access desirable performance metrics in optical, electrochemical, and bioelectronic applications.
Molecular dopants are often added to semiconducting polymers to improve electrical conductivity. However, the use of such dopants does not always produce mobile charge carriers. In this work, ...ultrafast spectroscopy is used to explore the nature of the carriers created following doping of conjugated push–pull polymers with both F4TCNQ (2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane) and FeCl3. It is shown that for one particular push–pull material, the charge carriers created by doping are entirely non‐conductive bipolarons and not single polarons, and that transient absorption spectroscopy following excitation in the infrared can readily distinguish the two types of charge carriers. Based on density functional theory calculations and experiments on multiple push–pull conjugated polymers, it is argued that the size of the donor push units determines the relative stabilities of polarons and bipolarons, with larger donor units stabilizing the bipolarons by providing more area for two charges to co‐reside.
A chemically doped donor–acceptor conjugated polymer is studied using ultrafast spectroscopy, showing conclusively that the carriers created by doping are exclusively bipolarons and not single polarons at all doping levels. DFT calculations show that the physical size of the donor unit is what determines the relative stability of polarons and bipolarons in push–pull semiconducting polymer systems.
PDF
