  Achieving an active manipulation of colours has huge implications in
optoelectronics, as colours engineering can be exploited in a number of
applications, ranging from display to lightning. In the last decade, the
synergy of the highly pure colours of 1D photonic crystals, also known as Bragg
stacks, with electro-tunable materials have been proposed as an interesting
route to attain such a technologically relevant effect. However, recent works
rely on the use of liquid electrolytes, which can pose issues in terms of
chemical and environmental stability. Here, we report on the proof-of-concept
of an electrolyte free and solution-processed electrochromic Bragg stack. We
integrate an electro-responsive plasmonic metal oxide, namely indium tin oxide,
in a 1D photonic crystal structure made of alternating layers of ITO and TiO2
nanoparticles. In such a device we observed 15 nm blue-shift upon application
of an external bias (5 V), an effect that we attribute to the increase of ITO
charge density arising from the capacitive charging at the metal
oxide/dielectric interface and from the current flowing throughout the porous
structure. Our data suggest that electrochromism can be attained in all-solid
state systems by combining a judicious selection of the constituent materials
with device architecture optimisation.
