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Impact of fullerene intercalation on structural and thermal properties of organic photovoltaic blends

Wade, J; Wood, S; Collado-Fregoso, E; Heeney, M; Durrant, J; Kim, J S (2017) Impact of fullerene intercalation on structural and thermal properties of organic photovoltaic blends. Journal of Physical Chemistry C, 121 (38). pp. 20976-20985.

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The performance of organic photovoltaic blend devices is critically dependent on the polymer:fullerene interface. A stable intercalated bimolecular crystal can result in efficient ultrafast charge photogeneration, where pure phases support fast change transport. These interfaces are expected to alter the structural and thermal properties of the polymer; with regards to the conjugated backbone planarity and transition temperatures during annealing/cooling processes. The properties of the polymer will play an important role in determining final blend morphology through intermixing, diffusion and phase separation with fullerene molecules during device processing, eventually determining device performance. Here we report the impact of fullerene intercalation on structural and thermal properties of poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b] (PBTTT), a highly stable material known to exhibit liquid crystalline behaviour. We undertake a detailed systematic study of the extent of intercalation in the PBTTT:fullerene interfaces considering the use of four different fullerene derivatives and loading ratios to control the extent of fullerene intercalation. Resonant Raman spectroscopy allows direct observation of the interfaces in situ during controlled heating and cooling. We find that small fullerene molecules readily intercalate into PBTTT crystallites resulting in a planarization of the polymer backbone, but high fullerene loading ratios or larger fullerenes result in non-intercalated domains. During cooling from melt, non-intercalated blend films are found to return to their original morphology, whereas intercalated blend films with high fullerene loading ratio form a distinct nano-ribbon morphology which exhibits a reduced geminate recombination rate. These results reveal that careful consideration should be taken when during device fabrication, as deposition thermal treatments significantly impact the charge generation and recombination dynamics.

Item Type: Article
Subjects: Advanced Materials > Photovoltaics
Divisions: Engineering, Materials & Electrical Science
Identification number/DOI: 10.1021/acs.jpcc.7b05893
Last Modified: 02 Mar 2018 15:25
URI: http://eprintspublications.npl.co.uk/id/eprint/7773

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