Synchronously regulating the alkyl side-chain and regioisomer of polymerized small molecule acceptor enabling highly efficient all-polymer solar cells processed with non-halogenated solvent

The power conversion efficiencies (PCEs) of all-polymer solar cells (all-PSCs) have been rapidly boosted up to 15% recently due to the emerging polymerized small molecule acceptors (PSMAs). However, most of the all-PSCs are processed with halogenated solvents, which are not compatible with future massive production. In this work, non-halogenated solvent processable PSMAs are designed by integrating both side-chain and backbone regioisomer engineering. The long-branched alkyl side-chain is selected to enhance the solute-solvent interaction and enable the resulting polymers to be readily dissolved and processable in a non-halogenated solvent 2-methyl-tetrafuran (2-MeTHF). Besides, the regioisomer effect on the optical and electronic properties, molecular crys-talline and orientation, and change transport of these PSMAs with screened side chain are studied. Results showed that when blending with the polymer donor PTzBI-Si, the PSMA PRi-C39 and PRo-C39 with regular backbone can enable high-efficiency devices with PCEs of 14.4% and 13.4%, significantly higher than that for the irregular PSMA PIR-C39 (11.5%). Further investigations reveal that the higher PCEs are the result of superior molecular crystallinity and packing, higher charge mobility, and reduced recombination loss in device. Our results highlight the critical role of the alkyl-chain and regioisomer engineering in developing efficient PSMAs for all-PSCs processed with non-halogenated solvents.

Automated summary

The power conversion efficiencies of all-polymer solar cells have been significantly improved recently due to the usage of emerging polymerized small molecule acceptors. However, most of the processing methods for these solar cells involve halogenated solvents, which are not suitable for future mass production. In this study, non-halogenated solvent processable PSMAs are designed by integrating side-chain and backbone regioisomer engineering.