Tuning of PCDTBT:PC71BM blend nanoparticles for eco-friendly processing of polymer solar cells

- Aqueous processing exposed for low bandgap polymers requiring high boiling solvents.
- Synthesis of PCDTBT:PC71BM nanoparticles (NPs) with excellent composition control.
- A solar cell reaching a PCE of 1.9% was achieved using the water-based blend NPs.
- Analytical electron microscopy study reveals fine mixing at the molecular level.
- Nanomorphology of the photoactive layer reflects the morphology of individual NPs.

We report the controlled preparation of water processable nanoparticles (NPs) employing the push-pull polymer PCDTBT and the fullerene acceptor PC71BM in order to enable solar cell processing using eco-friendly solvent (i.e. water). The presented method provides the possibility to separate the formation of the active layer blend and the deposition of the active layer into two different processes. For the first time, the benefits of aqueous processability for the high-potential class of push-pull polymers, generally requiring high boiling solvents, are made accessible. With our method we demonstrate excellent control over the blend stoichiometry and efficient mixing. Furthermore, we provide visualization of the nanomorphology of the different NPs to obtain structural information down to ~2 nm resolution using advanced analytical electron microscopy. The imaging directly reveals very small compositional demixing in the PCDTBT:PC71BM blend NPs, in the size range of about <5 nm, indicating fine mixing at the molecular level. The suitability of the proposed methodology and materials towards the aspects of eco-friendly processing of organic solar cells is demonstrated through a processing of lab scale NPs solar cell prototypes reaching a power conversion efficiency of 1.9%.

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