Charge Delocalization in Oligomers of Poly(2,5-bis(3-alkyl­thiophene-2-yl)­thieno­[3,2‑<i>b</i>]thiophene) (PBTTT)

We investigate theoretically charge delocalization in radical cations, i.e., positive polarons, formed on oligomer chains of poly­(2,5-bis­(3-alkyl­thiophene-2-yl)­thieno­[3,2-<i>b</i>]­thiophene) (PBTTT). We use nonempirically tuned range-separated density functionals (TRS-DFT), including LC-ωPBE, LC-BLYP, and ωB97XD. We consider the evolution with oligomer length of the molecular geometric and electronic structures, optical absorption features, and spin densities. The TRS-DFT results indicate that a positive polaron can delocalize ideally over some 10 thiophene rings when the backbone is nonplanar and up to 14 rings for a backbone forced to be completely planar. Interestingly, up to six polarons can coexist side-by-side in a hexamer (which contains 24 thiophene rings), which is consistent with the highest degrees of doping (oxidation) experimentally achievable in polythiophene derivatives

Improving the Stability of Organic Semiconductors: Distortion Energy versus Aromaticity in Substituted Bistetracene

Polycyclic aromatic hydrocarbons (PAHs) have been widely explored as molecular semiconductors in organic electronic devices such as field-effect transistors or solar cells. However, their tendency to undergo photooxidation is a primary limitation to their practical applications. Bistetracene derivatives have recently been demonstrated to possess much larger photooxidation stability than the widely investigated pentacene and rubrene, while maintaining high charge-carrier mobilities. Here, using several levels of density functional theory, we identify the origin of the increased stability of bistetracene with respect to molecular oxygen by systematically investigating the [4 + 2] cycloaddition (Diels–Alder) photooxidation reaction mechanism. Importantly, our computational results indicate that endoperoxide formation in bis­(2-(trimethylsilyl)­ethynyl) bistetracene (BT) occurs not on the ring with least aromaticity, but rather on the ring with smallest distortion energy. This feature was subsequently confirmed by experimental NMR analyses. The oxidation activation barriers of bistetracene, pentacene, and rubrene are found to be 17.7, 13.6, and 14.4 kcal/mol, respectively, in agreement with the observed order of stability of these molecules with respect to oxidation reactions in solution. In the cases of BT and pentacene, the rates of electron transfer to create charged species (PAH⁺ and O₂^–) are at least two orders of magnitude lower than that of the charge recombination process (back to PAH and O₂); for rubrene, both of these processes are calculated to be of the same order of magnitude, in agreement with experimental electron paramagnetic resonance spectroscopy observations

Tuning the Charge-Transport Parameters of Perylene Diimide Single Crystals via End and/or Core Functionalization: A Density Functional Theory Investigation

Perylene tetracarboxylic diimide (PTCDI) derivatives stand out as one of the most investigated families of air-stable n-type organic semiconductors for organic thin-film transistors. Here, we use density functional theory to illustrate how it is possible to control the charge-transport parameters of PTCDIs as a function of the type, number, and positions of the substituents. Specifically, two strategies of functionalization related to core and end substitutions are investigated. While end-substituted PTCDIs present the same functional molecular backbone, their molecular packing in the crystal significantly varies; as a consequence, this series of derivatives constitutes an ideal test bed to evaluate the models that describe charge-transport in organic semiconductors. Our results indicate that large bandwidths along with small effective masses can be obtained with the insertion of appropriate substituents on the nitrogens, in particular halogenated aromatic groups

Impact of Organic Spacers on the Carrier Dynamics in 2D Hybrid Lead-Halide Perovskites

We have carried out nonadiabatic molecular dynamics simulations combined with time-dependent density functional theory calculations to compare the properties of the two-dimensional (2D) (BA)2(MA)­Pb2I7 and three-dimensional (3D) MAPbI3 (where MA = methylammonium and BA = butylammonium) materials. We evaluate the different impacts that the 2D-confined spacer layer of butylammonium cations and the 3D-confined methylammonium cations have on the charge carrier dynamics in the two systems. Our results indicate that, while both the MA+ and BA+ cations play important roles in determining the carrier dynamics, the BA+ cations exhibit stronger nonadiabatic couplings with the 2D perovskite framework. The consequence is a faster hot-carrier decay rate in 2D (BA)2(MA)­Pb2I7 than in 3D MAPbI3. Thus, tuning of the functional groups of the organic spacer cations in order to reduce the vibronic couplings between the cations and the Pb–I framework can offer the opportunity to slow down the hot-carrier relaxations and increase the carrier lifetimes in 2D lead-halide perovskites

Impact of Structural Defects on the Electronic Properties of Two-Dimensional Covalent Organic Frameworks

While structural defects appear to be unavoidable in two-dimensional covalent organic frameworks (2D COFs), little is known of their influence over the material electronic properties. Here, we investigate the impact of typical defects such as single vacancies, double vacancies, and ring defects on the electronic structures of 2D COFs, taking COF-C4N and TANG-COF as representative examples. To facilitate the modeling of extensive systems with defect densities ranging from 1.25% to 25%, i.e., values on the order of those found experimentally, we rely on efficient density functional-based tight binding methods. In the presence of vacancies and ring defects, the 2D COFs still exhibit semiconducting characteristics with bandgap values that remain close to those of the corresponding perfect configurations. However, the different types of defects bring different modifications to the electronic structure: Vacancy defect levels are found to be primarily located near the top of the valence bands and occupied, while ring defect levels are situated just below the bottom of the conduction bands and unoccupied. Our study of how defects impact the electronic properties of 2D COFs provides valuable insight for future explorations of defect engineering in COFs

Nucleation–Elongation Dynamics of Two-Dimensional Covalent Organic Frameworks

Homogeneous two-dimensional (2D) polymerization is a poorly understood process in which topologically planar monomers react to form planar macromolecules, often termed 2D covalent organic frameworks (COFs). While these COFs have traditionally been limited to weakly crystalline aggregated powders, they were recently grown as micron-sized single crystals by temporally resolving the growth and nucleation processes. Here, we present a quantitative analysis of the nucleation and growth rates of 2D COFs via kinetic Monte Carlo (KMC) simulations using COF-5 as an example, which show that nucleation and growth have second-order and first-order dependences on monomer concentration, respectively. The computational results were confirmed experimentally by systematic measurements of COF nucleation and growth rates performed via in situ X-ray scattering, which validated the respective monomer concentration dependencies of the nucleation and elongation processes. A major consequence is that there exists a threshold monomer concentration below which growth dominates over nucleation. Our computational and experimental findings rationalize recent empirical observations that, in the formation of 2D COF single crystals, growth dominates over nucleation when monomers are added slowly, so as to limit their concentrations. This mechanistic understanding of the nucleation and growth processes will inform the rational control of polymerization in two dimensions and ultimately enable access to high-quality samples of designed two-dimensional polymers

Nucleation–Elongation Dynamics of Two-Dimensional Covalent Organic Frameworks

Homogeneous two-dimensional (2D) polymerization is a poorly understood process in which topologically planar monomers react to form planar macromolecules, often termed 2D covalent organic frameworks (COFs). While these COFs have traditionally been limited to weakly crystalline aggregated powders, they were recently grown as micron-sized single crystals by temporally resolving the growth and nucleation processes. Here, we present a quantitative analysis of the nucleation and growth rates of 2D COFs via kinetic Monte Carlo (KMC) simulations using COF-5 as an example, which show that nucleation and growth have second-order and first-order dependences on monomer concentration, respectively. The computational results were confirmed experimentally by systematic measurements of COF nucleation and growth rates performed via in situ X-ray scattering, which validated the respective monomer concentration dependencies of the nucleation and elongation processes. A major consequence is that there exists a threshold monomer concentration below which growth dominates over nucleation. Our computational and experimental findings rationalize recent empirical observations that, in the formation of 2D COF single crystals, growth dominates over nucleation when monomers are added slowly, so as to limit their concentrations. This mechanistic understanding of the nucleation and growth processes will inform the rational control of polymerization in two dimensions and ultimately enable access to high-quality samples of designed two-dimensional polymers

Synergistic Use of Bithiazole and Pyridinyl Substitution for Effective Electron Transport Polymer Materials

The development of semiconducting conjugated polymers for organic field effect transistors (OFETs) has been the focus of intense research efforts for their key role in plastic electronics as well as a vision of solution processability leading to reduced costs in device fabrication relative to those of their inorganic counterparts. The pursuit of high-performance n-channel (electron-transporting) polymer semiconductors vital to the development of robust and low-cost organic integrated circuits has faced significant challenges, mainly for poor ambient operational stability and OFET device performance lagging far behind that of p-channel organic semiconductors. As an alternative to the ubiquitous donor–acceptor molecular design strategy, an all-acceptor (A–A) unipolar approach was implemented in the design of poly­(2-(2-decyltetradecyl)-6-(5-(5′-methyl­[2,2′-bithiaol]-5-yl)-3-(5-methylpyridin-2-yl)-5-(tricosan-11-yl)-2,5-dihydropyrrolo­[3,4-c]­pyrrole-1,4-dione) (PDBPyBTz). The n-channel copolymer allowed investigation of the impact of electron-withdrawing moieties on conjugated polymer device performance and the utility of the A–A molecular design strategy. As an analogue to benzene, the pyridines flanking the diketopyrrolopyrrole moiety in PDBPyBTz were strategically chosen to lower the energy levels and impart planarity to the monomer, both of which aid in achieving stable n-channel performance. Incorporation of PDBPyBTz into a bottom-gate/bottom-contact OFET afforded a device that exhibited unipolar electron transport. In addition to developing a high-performance n-channel polymer, this study allowed for an investigation of structure–property relationships crucial to the design of such materials in high demand for sustainable technologies, including organic photovoltaics and other solution-processed organic electronic devices

Nucleation–Elongation Dynamics of Two-Dimensional Covalent Organic Frameworks

Homogeneous two-dimensional (2D) polymerization is a poorly understood process in which topologically planar monomers react to form planar macromolecules, often termed 2D covalent organic frameworks (COFs). While these COFs have traditionally been limited to weakly crystalline aggregated powders, they were recently grown as micron-sized single crystals by temporally resolving the growth and nucleation processes. Here, we present a quantitative analysis of the nucleation and growth rates of 2D COFs via kinetic Monte Carlo (KMC) simulations using COF-5 as an example, which show that nucleation and growth have second-order and first-order dependences on monomer concentration, respectively. The computational results were confirmed experimentally by systematic measurements of COF nucleation and growth rates performed via in situ X-ray scattering, which validated the respective monomer concentration dependencies of the nucleation and elongation processes. A major consequence is that there exists a threshold monomer concentration below which growth dominates over nucleation. Our computational and experimental findings rationalize recent empirical observations that, in the formation of 2D COF single crystals, growth dominates over nucleation when monomers are added slowly, so as to limit their concentrations. This mechanistic understanding of the nucleation and growth processes will inform the rational control of polymerization in two dimensions and ultimately enable access to high-quality samples of designed two-dimensional polymers