Understanding and controlling morphology evolution via DIO plasticization in PffBT4T-2OD/PC71BM devices

Zhang, Yiwei; Parnell, Andrew J.; Pontecchiani, Fabio; Cooper, Joshaniel F.K.; Thompson, Richard L.; Jones, Richard A.L.; King, Stephen M.; Lidzey, David G.; Bernardo, Gabriel

Abstract

We demonstrate that the inclusion of a small amount of the co-solvent 1,8-diiodooctane in the preparation of a bulk-heterojunction photovoltaic device increases its power conversion efficiency by 20%, through a mechanism of transient plasticisation. We follow the removal of 1,8-diiodooctane directly after spin-coating using ellipsometry and ion beam analysis, while using small angle neutron scattering to characterise the morphological nanostructure evolution of the film. In PffBT4T-2OD/PC71BM devices, the power conversion efficiency increases from 7.2% to above 8.7% as a result of the coarsening of the phase domains. This coarsening process is assisted by thermal annealing and the slow evaporation of 1,8-diiodooctane, which we suggest, acts as a plasticiser to promote molecular mobility. Our results show that 1,8-diiodooctane can be completely removed from the film by a thermal annealing process at temperatures ≤100 °C and that there is an interplay between the evaporation rate of 1,8-diiodooctane and the rate of domain coarsening in the plasticized film which helps elucidate the mechanism by which additives improve device efficiency.

Citation

Zhang, Y., Parnell, A. J., Pontecchiani, F., Cooper, J. F., Thompson, R. L., Jones, R. A., …Bernardo, G. (2017). Understanding and controlling morphology evolution via DIO plasticization in PffBT4T-2OD/PC71BM devices. Scientific Reports, 7, Article 44269. https://doi.org/10.1038/srep44269

• Journal Article Type: Article
• Acceptance Date: Feb 7, 2017
• Online Publication Date: Mar 13, 2017
• Publication Date: Mar 13, 2017
• Deposit Date: Mar 17, 2017
• Publicly Available Date: Mar 21, 2017
• Journal: Scientific Reports
• Publisher: Nature Research
• Peer Reviewed: Peer Reviewed
• Volume: 7
• Article Number: 44269
• DOI: https://doi.org/10.1038/srep44269
• Public URL: https://durham-repository.worktribe.com/output/1360955