Fine-Tuning the Optoelectronic Properties of Freestanding Borophene by Strain

Two-dimensional boron (borophene) is a promising, newly synthesized monolayer metal with promising electronic and optical properties. Borophene has only been recently synthesized on silver substrates, and displays a variety of crystal structures and substrate-induced strains depending on the growth conditions and surface orientation. Here, we present an extensive first-principles study of the structural and optoelectronic properties of the two proposed structures of borophene, β₁₂ and δ₆, under strain. With a density functional theory analysis, we determine that the optical absorbance and electronic band structure are continuously tunable upon application of few percent of strain. Although both structures remain metallic with moderate strains of up to 6% applied, key features of the band structure, as well as the inplane anisotropy of the complex dielectric function and optical absorption, can be significantly modified

Fine-tuning the Optoelectronic Properties of Borophene by Strain

<div> <div> <div> <p>Here, we present an extensive first- principles study of the structural and optoelectronic properties of the two proposed structures of borophene under strain. With a density functional theory analysis, we determine that the optical absorbance and electronic band structure are continuously tunable upon application of few percent of strain. While both structures remain metallic with moderate strains of up to 6%, key features of the band structure, as well as the in-plane anisotropy of the complex dielectric function and optical absorption can be significantly modified. </p> </div> </div> </div

Low-Energy Charge-Transfer Excitons in Organic Solids from First-Principles: The Case of Pentacene

The nature of low energy optical excitations, or excitons, in organic solids is of central relevance to many optoelectronic applications, including solar energy conversion. Excitons in solid pentacene, a prototypical organic semiconductor, have been the subject of many experimental and theoretical studies, with differing conclusions as to the degree of their charge-transfer character. Using first-principles calculations based on density functional theory and many-body perturbation theory, we compute the average electron–hole distance and quantify the degree of charge-transfer character within optical excitations in solid-state pentacene. We show that several low-energy singlet excitations are characterized by a weak overlap between electron and hole and an average electron–hole distance greater than 6 Å. Additionally, we show that the character of the lowest-lying singlet and triplet excitons is well-described with a simple analytic envelope function of the electron–hole distance

Relating Trends in First-Principles Electronic Structure and Open-Circuit Voltage in Organic Photovoltaics

Using first-principles density functional theory, and accounting for solid-state polarization effects and electron–hole interactions, we calculate excited electronic states at interfaces between C₆₀ and a series of functionalized boron(subphthalocyanine) molecules, a class of donor materials for organic photovoltaic (OPV) devices, and correlate energetics with their measured open-circuit voltages (<i>V</i>_oc). For isolated donor and acceptor molecules, a staggered (type-II) interface energy alignment is predicted with an energy offset of several tenths of an electron volt, capable of promoting charge separation. The solid-state charge transfer excited state energy, <i>E</i>_CT, obtained by including electronic polarization effects and electron–hole interactions, exhibits a near-quantitative linear relationship with <i>V</i>_oc. <i>E</i>_CT depends sensitively on interface morphology, resulting in a predicted 0.2–0.6 eV spread in energy for the geometries studied here. The agreement between theory and experiment provides insight into possible routes to higher <i>V</i>_oc OPVs, and suggests that our approximate approach can enable computational design of <i>V</i>_oc for a broad class of molecular-based OPVs

First-Principles Investigation of Borophene as a Monolayer Transparent Conductor

Two-dimensional boron is promising as a tunable monolayer metal for nano-optoelectronics. We study the optoelectronic properties of two likely allotropes of two-dimensional boron, β₁₂ and δ₆, using first-principles density functional theory and many-body perturbation theory. We find that both systems are anisotropic metals, with strong energy- and thickness-dependent optical transparency and a weak (<1%) absorbance in the visible range. Additionally, using state-of-the-art methods for the description of the electron–phonon and electron–electron interactions, we show that the electrical conductivity is limited by electron–phonon interactions. Our results indicate that both structures are suitable as a transparent electrode

Outer-valence Electron Spectra of Prototypical Aromatic Heterocycles from an Optimally Tuned Range-Separated Hybrid Functional

Density functional theory with optimally tuned range-separated hybrid (OT-RSH) functionals has been recently suggested [Refaely-Abramson et al. <i>Phys. Rev. Lett.</i> <b>2012</b>, <i>109</i>, 226405] as a nonempirical approach to predict the outer-valence electronic structure of molecules with the same accuracy as many-body perturbation theory. Here, we provide a quantitative evaluation of the OT-RSH approach by examining its performance in predicting the outer-valence electron spectra of several prototypical gas-phase molecules, from aromatic rings (benzene, pyridine, and pyrimidine) to more complex organic systems (terpyrimidinethiol and copper phthalocyanine). For a range up to several electronvolts away from the frontier orbital energies, we find that the outer-valence electronic structure obtained from the OT-RSH method agrees very well (typically within ∼0.1–0.2 eV) with both experimental photoemission and theoretical many-body perturbation theory data in the GW approximation. In particular, we find that with new strategies for an optimal choice of the short-range fraction of Fock exchange, the OT-RSH approach offers a balanced description of localized and delocalized states. We discuss in detail the sole exception founda high-symmetry orbital, particular to small aromatic rings, which is relatively deep inside the valence state manifold. Overall, the OT-RSH method is an accurate DFT-based method for outer-valence electronic structure prediction for such systems and is of essentially the same level of accuracy as contemporary GW approaches, at a reduced computational cost

<i>GW</i>100: Benchmarking <i>G</i>₀<i>W</i>₀ for Molecular Systems

We present the <i>GW</i>100 set. <i>GW</i>100 is a benchmark set of the ionization potentials and electron affinities of 100 molecules computed with the <i>GW</i> method using three independent <i>GW</i> codes and different <i>GW</i> methodologies. The quasi-particle energies of the highest-occupied molecular orbitals (HOMO) and lowest-unoccupied molecular orbitals (LUMO) are calculated for the <i>GW</i>100 set at the <i>G</i>₀<i>W</i>₀@PBE level using the software packages TURBOMOLE, FHI-aims, and Berkeley<i>GW</i>. The use of these three codes allows for a quantitative comparison of the type of basis set (plane wave or local orbital) and handling of unoccupied states, the treatment of core and valence electrons (all electron or pseudopotentials), the treatment of the frequency dependence of the self-energy (full frequency or more approximate plasmon-pole models), and the algorithm for solving the quasi-particle equation. Primary results include reference values for future benchmarks, best practices for convergence within a particular approach, and average error bars for the most common approximations