Quasiparticle and Optical Spectroscopy of Organic Semiconductors Pentacene and PTCDA from First Principles

TL;DR

This study uses first-principles calculations to accurately determine the spectroscopic properties of organic semiconductors pentacene and PTCDA, clarifying surface effects and exciton binding energies relevant for optoelectronic applications.

Contribution

The paper presents a parameter-free computational approach to interpret spectroscopic measurements of organic semiconductors, accounting for surface and bulk effects.

Findings

Computed transport gaps are 2.4 eV for pentacene and 3.0 eV for PTCDA.

Optical gaps are 1.7 eV for pentacene and 2.1 eV for PTCDA.

Exciton binding energies are approximately 0.5 eV for both materials.

Abstract

The broad use of organic semiconductors for optoelectronic applications relies on quantitative understanding and control of their spectroscopic properties. Of paramount importance are the transport gap - the difference between ionization potential and electron affinity - and the exciton binding energy - inferred from the difference between the transport and optical absorption gaps. Transport gaps are commonly established via photoemission and inverse photoemission spectroscopy (PES/IPES). However, PES and IPES are surface-sensitive, average over a dynamic lattice, and are subject to extrinsic effects, leading to significant uncertainty in gaps. Here, we use density functional theory and many-body perturbation theory to calculate the spectroscopic properties of two prototypical organic semiconductors, pentacene and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA), quantitatively comparing with measured PES, IPES, and optical absorption spectra. For bulk pentacene and PTCDA, the computed transport gaps are 2.4 and 3.0 eV, and optical gaps are 1.7 and 2.1 eV, respectively. Computed bulk quasiparticle spectra are in excellent agreement with surface-sensitive photoemission measurements over several eV only if the measured gap is reduced by 0.6 eV for pentacene, and 0.6-0.9 eV for PTCDA. We attribute this redshift to several physical effects, including incomplete charge screening at the surface, static and dynamical disorder, and experimental resolution. Optical gaps are in excellent agreement with experiment, with solid-state exciton binding energies of ~0.5 eV for both systems; for pentacene, the exciton is delocalized over several molecules and exhibits significant charge transfer character. Our parameter-free calculations provide new interpretation of spectroscopic properties of organic semiconductors critical to optoelectronics.