First-principles GW calculations for fullerenes, porphyrins, phtalocyanine, and other molecules of interest for organic photovoltaic applications

Abstract: We evaluate the performances of ab initio GW calculations for the ionization energies and HOMO-LUMO gaps of thirteen gas phase molecules of interest for organic electronic and photovoltaic applications, including the C60 fullerene, pentacene, free-base porphyrins and phtalocyanine, PTCDA, and standard monomers such as thiophene, fluorene, benzothiazole or thiadiazole. Standard G0W0 calculations, that is starting from eigenstates obtained with local or semilocal functionals, significantly improve the ionization energy and band gap as compared to density functional theory Kohn-Sham results, but the calculated quasiparticle values remain too small as a result of overscreening. Starting from Hartree-Fock-like eigenvalues provides much better results and is equivalent to performing self-consistency on the eigenvalues, with a resulting accuracy of 2~4% as compared to experiment. Our calculations are based on an efficient gaussian-basis implementation of GW with explicit treatment of the dynamical screening through contour deformation techniques.

• The paper demonstrates that GW approximations significantly improve predictions of ionization energies and HOMO-LUMO gaps in organic molecules.
• It compares non-self-consistent G0W0 with self-consistent GW, showing self-consistency yields results within 2-4% of experimental benchmarks.
• The study suggests that using HF-like starting eigenvalues is a simple yet effective alternative for computationally demanding systems.

First-Principles GW Calculations for Molecules Relevant to Organic Photovoltaics

The paper by Blase, Attaccalite, and Olevano evaluates the performance of ab initio GW calculations, specifically targeting molecules that are indispensable in the field of organic electronics and photovoltaics. This study is oriented towards calculating the ionization energies and HOMO-LUMO gaps of thirteen gas-phase molecules including the C$_{60}$ fullerene, pentacene, and various porphyrins, all of which are crucial to the advancement of organic photovoltaic technologies.

Highlights of the GW Approach

The GW approximation, rooted in many-body perturbation theory, is deemed an effective tool for addressing quasiparticle properties and has been widely applied to bulk semiconductors. This paper assesses the potential of GW methods to accurately predict electronic properties of larger, complex molecules typically encountered in organic photovoltaic applications. The calculations were executed using an efficient Gaussian-basis set implementation, addressing dynamic screening through contour deformation methods.

Numerical Findings

The researchers employed several methods to derive the electronic structure: LDA Kohn-Sham, standard G$_0$W$_0$ using LDA eigenstates as a starting point, and a self-consistent GW approach where eigenvalue adjustments were incorporated into the process. Additionally, they explored a simplified G$_0$W$_0$ scheme using Hartree-Fock-like starting eigenvalues.

The study demonstrated that the LDA Kohn-Sham method substantially underestimates the ionization energies and HOMO-LUMO gaps. Non-self-consistent G$_0$W$_0$ calculations significantly improved upon these values but tended to undershoot the experimental benchmarks due to overscreening issues endemic to LDA-derived inputs.

The adoption of a self-consistent GW methodology, updating only the eigenvalues, notably enhanced accuracy, achieving results within 2-4% of experimental values. Moreover, the alternative non-self-consistent G$_0$W$_0$ method based on Hartree-Fock-like estimates also yielded comparably accurate results, suggesting it as a viable simpler alternative, especially for computationally demanding systems.

Implications and Speculations

The implications of these findings are significant for the computational design and optimization of organic photovoltaic materials. By refining the starting point of the quasiparticle calculations, researchers can expect more reliable predictions of electronic excitation—a crucial parameter for solar cell efficiency.

The research highlights the necessity of adapting computational strategies based on the properties of the molecular system in question. It suggests that for gas-phase organic molecules, the HF-like starting approximation is almost as effective as intricate self-consistency processes, potentially streamlining computational workflows.

Future Directions

Looking forward, the practical application of these methods could expand to include larger, more diverse organic complexes and hybrid interfaces characteristic of real-world devices. Moreover, integrating these insights into a broader range of quantum chemical and electronic structure methodologies could foster significant advancements in the predictive power of ab initio calculations for organic materials, aiding in the design of next-generation organic photovoltaic cells.

This study stands out for accurately quantifying the emergent properties of organic photovoltaic molecules, providing a critical step towards optimizing the computational tools necessary for future technological advancements in this area.