Sahar Sharifzadeh, Ariel Biller, Leeor Kronik, Jeffrey B. Neaton
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.
View original:
http://arxiv.org/abs/1110.4928
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