Page 59 - Plastics News June 2018

teChnoLogy

Polymer crystals hold key to record-breaking energy transport

cientists from the universities of Bristol and distances of 200 nanometres -- 20 times further than
SCambridge have found a way to create polymeric was previously possible. 200 nanometres is especially
semiconductor nanostructures that absorb light significant because it is greater than the thickness of
and transport its energy further than previously material needed to completely absorb ambient light
observed.This could pave the way for more flexible thus making these polymers more suitable as "light
and more efficient solar cells and photodetectors.The
researchers, whose work appears in the journal Science,
say their findings could be a "game changer" by allowing
the energy from sunlight absorbed in these materials
to be captured and used more efficiently.
Lightweight semiconducting plastics are now widely
used in mass market electronic displays such those
found in phones, tablets and flat screen televisions.
However, using these materials to convert sunlight into
electricity, to make solar cells, is far more complex.

The photo-excited states -- which is when photons of
light are absorbed by the semiconducting material -- harvesters" for solar cells and photodetectors. Dr
need to move so that they can be "harvested" before George Whittell from Bristol's School of Chemistry,
they lose their energy in less useful ways. These explains: "The gain in efficiency would actually be for
excitations typically only travel ca. 10 nanometres two reasons: first, because the energetic particles
in polymeric semiconductors, thus requiring the travel further, they are easier to "harvest," and second,
construction of structures patterned on this length- we could now incorporate layers ca. 100 nanometres
scale to maximise the "harvest." thick, which is the minimum thickness needed to
absorb all the energy from light -- the so-called optical
absorption depth. Previously, in layers this thick, the
particles were unable to travel far enough to reach
the surfaces."
Co-researcher Professor Richard Friend, from
Cambridge, added: "The distance that energy can be
moved in these materials comes as a big surprise and
points to the role of unexpected quantum coherent
transport processes."The research team now plans to
prepare structures thicker than those in the current
study and greater than the optical absorption depth,
In the chemistry labs of the University of Bristol, Dr Xu- with a view to building prototype solar cells based on
Hui Jin and colleagues developed a novel way to make this technology.
highly ordered crystalline semiconducting structures
They are also preparing other structures capable of
using polymers.While in the Cavendish Laboratory in
Cambridge, Dr Michael Price measured the distance using light to perform chemical reactions, such as the
that the photo-exited states can travel, which reached splitting of water into hydrogen and oxygen.

59 June 2018 Plastics News