Solid-State Solvation and Enhanced Exciton Diffusion in Doped Organic Thin Films under Mechanical Pressure

Publication Type:

Journal Article

Source:

ACS Nano, ACS Publications, Volume 9, Issue 4, p.4412-4418 (2015)

ISBN:

1936-0851

Keywords:

2015, Förster radiative energy transfer, Onsager dielectric theory, organic semiconductor, pressure probing, solid-state solvation effect

Abstract:

Direct modification of exciton energy has been previously used to optimize the operation of organic optoelectronic devices. One demonstrated method
for exciton energy modification is through the use of the solvent
dielectric effects in doped molecular films. To gain a deeper appreciation
of the underlying physical mechanisms, in this work we test the
solid-state solvation effect in molecular thin films under applied
external pressure. We observe that external mechanical pressure increases
dipole-dipole interactions, leading to shifts in the Frenkel exciton
energy and enhancement of the time-resolved spectral red shift associated
with the energy-transfer-mediated exciton diffusion. Measurements are
performed on host:dopant molecular thin films, which show bathochromic
shifts in photoluminescence (PL) under increasing pressure. This is in
agreement with a simple solvation theory model of exciton energetics with
a fitting parameter based on the mechanical properties of the host matrix
material. We measure no significant change in exciton lifetime with
increasing pressure, consistent with unchanged aggregation in molecular
films under compression. However, we do observe an increase in exciton
spectral thermalization rate for compressed molecular films, indicating
enhanced exciton diffusion for increased dipole-dipole interactions under
pressure. The results highlight the contrast between molecular energy
landscapes obtained when dipole-dipole interactions are increased by the
pressure technique versus the conventional dopant concentration variation
methods, which can lead to extraneous effects such as aggregation at
higher doping concentrations. The present work demonstrates the use of
pressure-probing techniques in studying energy disorder and exciton
dynamics in amorphous molecular thin films.