Page 30 - ILL Annual Report 2019

MATERIALS SCIENCE
Anthony Higgins. British
Swansea University, UK
'My research in the field of soft condensed matter investigates the physics of polymers near interfaces and in thin films. Understanding fundamental behaviour in such systems underpins many areas of life science,
physical science and engineering. My current focus is on understanding the thermodynamics of polymer-based, thin-film systems for organic electronics applications.'
Equilibrium behaviour in polymer/small-molecule mixtures of organic solar cell materials
Horizontal reflectometer FIGARO and vertical reflectometer D17
Horizontal reflectometer INTER (ISIS)
Organic photovoltaics (OPVs) are approaching the point where their use for building-integrated power generation is commercially viable [1]. However, challenges remain in terms of cost, efficiency and lifetime. The active layer of
an OPV consists of interconnected domains
of electron-donating and electron-accepting materials. The interfaces between domains within these ‘bulk heterojunctions’ are key locations at which photo-generated, bound electron-hole pairs (excitons) are separated into individual charges that can then be transported to the device electrodes. Our work investigates the applicability of equilibrium thermodynamic theories for describing the domain compositions and interfacial mixing within model binary mixtures containing OPV materials.
Figure 1
A schematic diagram illustrating the rapid diffusion of fullerene molecules into the top (polystyrene (PS)) layer. Two different PS/fullerene systems were investigated: PS/PCBM and PS/bis-PCBM.
AUTHORS
E.L. Hynes, D. Môn and A.M. Higgins (Swansea University, UK) J.T. Cabral (Imperial College London, UK)
A.J. Parnell and A. Dunbar (Sheffield University, UK)
P. Gutfreund (ILL)
R. Welbourn (ISIS, UK) ARTICLE FROM
Commun. Phys. (2019)—doi: 10.1038/s42005-019-0211-z
REFERENCES
[1] S. Strohm, F. Machui, S. Langner, P. Kubis, N. Gasparini, M. Salvador, I. McCulloch, H.J. Egelhaaf and C.J. Brabec, Energy Environ. Sci. 11:8 (2018) 2225
[2] L. Ye, H. Hu, M. Ghasemi, T. Wang, B.A. Collins, J.H. Kim,
K. Jiang, J.H. Carpenter, H. Li, Z. Li, T. McAfee, J. Zhao, X. Chen, J.L.Y. Lai, T. Ma, J.L. Bredas, H. Yan and H. Ade, Nat. Mater.
17:3 (2018) 253
[3] J.A. Bartelt, Z.M. Beiley, E.T. Hoke, W.R. Mateker, J.D. Douglas,
B.A. Collins, J.R. Tumbleston, K.R. Graham, A. Amassian,
H. Ade, J.M.J. Fréchet, M.F. Toney and M.D. McGehee, Adv. Energy Mater. 3:3 (2013) 364
[4] D. Môn, A.M. Higgins, D. James, M. Hampton, J.E. Macdonald, M.B. Ward, P. Gutfreund, S. Lilliu and J. Rawle, Phys. Chem. Chem. Phys. 17:3 (2015) 2216
Bulk heterojunctions are most commonly made from a binary mixture of a semiconducting polymer and a small molecule that de-mix into separate (typically non-pure) phases during device fabrication. Recent work has uncovered a strong correlation between OPV performance and equilibrium parameters
in polymer/small-molecule systems using bilayer mixing measurements [2]; equilibrium theory was used here to interpret domain compositions and quantitatively link the Flory–Huggins interaction parameter, χ (characterising the miscibility), to a
key measure of the efficiency of a solar cell (the fill-factor),
for an amorphous-polymer/fullerene and a range of polymer/ small-molecule devices. Of key significance here, for a wide range of amorphous and semi-crystalline polymers, is the existence of mixed amorphous polymer/small-molecule phases, whose composition plays an important role in charge transport and charge recombination within devices [3].
Understanding equilibrium behaviour in these systems is important, as i) one needs to be sure that the system is properly equilibrated to extract χ correctly, and ii) the domain compositions can evolve towards their equilibrium compositions given sufficient mobility during operation (e.g. as provided by the elevated temperatures that are encountered under solar illumination). Domain composition evolution can lead to significant degradation of performance during operation if the as-fabricated domain compositions are optimised but not equilibrated.
ANNUAL REPORT 2019