Enhanced photocurrent efficiency of a carbon nanotube p–n junction electromagnetically coupled to a photonic structure

2011

Phys. Rev. B

The band structure and size scaling of electronic properties in self-assembled cyclic oligothiophene nanotubes are investigated using density functional theory. In these unique tubular aggregates, the π -π stacking interactions between adjacent monomers provide pathways for charge transport and energy migration along the periodic one-dimensional nanostructure. In order to simultaneously describe both the π -π stacking interactions and the global electronic band structure of these nanotubes, we utilize a dispersion-corrected Becke three-parameter Lee-Yang-Parr-D (B3LYP-D) hybrid functional in conjunction with all-electron basis sets and one-dimensional periodic boundary conditions. Based on our B3LYP-D calculations, we present simple analytical formulae for estimating the fundamental band gaps of these unique nanotubes as a function of size and diameter. Our results on these molecular nanostructures indicate that all of the oligothiophene nanotubes are direct-gap semiconductors with band gaps ranging from 0.9 to 3.3 eV, depending on tube diameter and oligothiophene orientation. These nanotubes have cohesive energies of up to 2.43 eV per monomer, indicating future potential use in organic electronic devices due to their tunable electronic band structure and high structural stability.

Section: Discussionmentioning

confidence: 99%

Self-assembled cyclic oligothiophene nanotubes: Electronic properties from a dispersion-corrected hybrid functional

2011

Phys. Rev. B

“…Conjugated organic structures have attracted significant recent attention due to their potential applications in single-molecule transistors and organic photovoltaics. In the quest for smaller and more efficient electronics, organic semiconductors serve as a promising alternative to their silicon counterparts because of their increased electronic efficiency [1][2][3][4][5] and ease of chemical functionalization. [6][7][8][9][10] In this context, oligoacenes which are composed of linearly fused benzene rings (Figure 1) have high application potential since they possess large charge-carrier mobilities and tunable electronic band gaps.…”

Section: Introductionmentioning

confidence: 99%

Optoelectronic and Excitonic Properties of Oligoacenes: Substantial Improvements from Range-Separated Time-Dependent Density Functional Theory

J. Chem. Theory Comput.

Hsieh

2010

263

278

The optoelectronic and excitonic properties in a series of linear acenes (naphthalene up to heptacene) are investigated using range-separated methods within time-dependent density functional theory (TDDFT). In these rather simple systems, it is well-known that TDDFT methods using conventional hybrid functionals surprisingly fail in describing the low-lying La and Lb valence states, resulting in large, growing errors for the La state and an incorrect energetic ordering as a function of molecular size. In this work, we demonstrate that the range-separated formalism largely eliminates both of these errors and also provides a consistent description of excitonic properties in these systems. We further demonstrate that reoptimizing the percentage of Hartree−Fock exchange in conventional hybrids to match wave function-based benchmark calculations still yields serious errors, and a full 100% Hartree−Fock range separation is essential for simultaneously describing both of the La and Lb transitions. From an analysis of electron−hole transition density matrices, we finally show that conventional hybrid functionals over-delocalize excitons and underestimate quasiparticle energy gaps in the acene systems. The results of our present study emphasize the importance of both a range-separated and asymptotically correct contribution of exchange in TDDFT for investigating optoelectronic and excitonic properties, even for these simple valence excitations.

“…[1][2][3][4] In particular, the relative ease in functionalizing organic materials using various electron donor/acceptor groups [5][6][7][8][9] allows the possibility of designing polymers with small band gaps intrinsic to the material itself, negating the need for further electrostatic doping of the system. Consequently, the increasing drive towards fully-organic systems has resulted in significant technological progress in next-generation organic field-effect transistors (OFETs), [10][11][12] organic light-emitting diodes (OLEDs), 13,14 and flexible photovoltaic materials. [15][16][17] The conventional approach to developing novel conducting polymers is based on the chemical intuition of synthetic chemists which has had significant success in the past, but is ultimately timeconsuming due to the nearly limitless number of promising candidate materials.…”

Section: Introductionmentioning

confidence: 99%

Electronic Properties of Vinylene-Linked Heterocyclic Conducting Polymers: Predictive Design and Rational Guidance from DFT Calculations

Cordaro

2011

J. Phys. Chem. C

The band structure and electronic properties in a series of vinylene-linked heterocyclic conducting polymers are investigated using density functional theory (DFT). In order to accurately calculate electronic band gaps, we utilize hybrid functionals with fully periodic boundary conditions to understand the effect of chemical functionalization on the electronic structure of these materials. The use of predictive first-principles calculations coupled with simple chemical arguments highlights the critical role that aromaticity plays in obtaining a low band gap polymer. Contrary to some approaches which erroneously attempt to lower the band gap by increasing the aromaticity of the polymer backbone, we show that being aromatic (or quinoidal) in itself does not ensure a low band gap. Rather, an iterative approach which destabilizes the ground state of the parent polymer toward the aromatic ↔ quinoidal level crossing on the potential energy surface is a more effective way of lowering the band gap in these conjugated systems. Our results highlight the use of predictive calculations guided by rational chemical intuition for designing low band gap polymers in photovoltaic materials.