Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV16)
Publication date: 28th March 2016
Control of fullerene crystallization and mixing is of considerable importance for optimization of organic photovoltaics. Several studies of fullerene and fullerene/polymer films have observed different types of [6,6]-phenyl C61-butyric acid methyl ester (PCBM) crystals, including nanoscale crystals and larger (micron-sized) crystals. This presentation will discuss the use of model fullerene/polymer bilayers to provide significant insight into; (i) the degree to which the growth of these two forms of PCBM crystal impact on one another, and (ii) the influence of the polymer on these potentially competitive crystal-growth mechanisms.1 In polymer/fullerene blends there can be a complex evolution in film morphology and structure during film-formation and on annealing, including crystallization (often of both components), surface/substrate segregation of one component and coarsening of the nanoscale morphology. The motivation of this work is to reduce this complexity and complement device optimisation strategies with an in-depth study of structure and kinetics in fullerene-polymer systems, aimed at increasing the fundamental understanding of the materials science within fullerene/polymer films. The simplified film morphology of bilayers (compared to blends) enables excellent characterization of local composition and structure, allowing us to clearly establish the impact of the polymer layer on both nanocrystal and micron-sized-crystal growth.
Investigating the interaction between these two forms of crystal using a bilayer geometry enables; (i) characterization of the composition-profile during crystallization, and (ii) a study of the role of polymer-film-thickness on PCBM crystal growth. This presentation will report some findings for PCBM/poly(3-hexylthiophene) bilayers, but will mostly focus on the well-studied amorphous polymer, atactic polystyrene (PS).
The main findings of this work are that the growth-kinetics of PCBM crystals, on two different length-scales, is shown to be controlled by the thickness of the polymer layer within the bilayer. Evidence will be presented, from in-situ optical microscopy and grazing-incidence x-ray diffraction (GIXD), showing that an increased growth-rate of nanoscale crystals impedes the growth of micron-sized, needle-like PCBM crystals. A combination of neutron reflectivity and GIXD measurements, also allows observation of the establishment of a consistent liquid-liquid (equilibrium) composition-profile between the PCBM layer and a polymer-rich layer, before PCBM crystallization occurs. While the interfacial composition-profile is independent of polymer-film-thickness, the growth-rate of nanoscale PCBM crystals is significantly larger for thinner polymer films. A similar thickness-dependence is observed for different molecular weights of entangled polymer, suggesting that the behaviour may be related to enhanced local-polymer-chain-mobility in these nanocomposite thin-films.
1 Môn et al Phys. Chem. Chem. Phys., (2015), 17, 2216-2227
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