Effect of fluorine atoms on the dielectric constants, optoelectronic properties and charge carrier kinetic characteristics of indacenodithieno [3,2-b]thiophene based non-fullerene acceptors for efficient organic solar cells

In this work, the optical and electrochemical properties, charge mobility, the dielectric constants, photovoltaic performance of three non-fullerene acceptors (NFAs) named ITOR-IC, ITOR4F-IC, and ITOR-IC4F, in which 4-octyloxy-phenyl and 3-fluoro-4-octyloxy-phenyl side-chains were connected on the indacenodithieno[3,2-b] thiophene (IT) core and ended with 2-(3-oxo-indane-1-ylidene)-malononitrile or 2-3-(oxo-5,6-difluoro-indane-1-ylidene)-malononitrile end groups, were systematically investigated. It has been found that ITOR-IC4F which ended up with 2-3-(oxo-5,6-difluoro-indane-1-ylidene)-malononitrile groups, exhibited the narrowest bandgap and the highest dielectric constant of 5.10 as compared with that of ITOR-IC (4.03 at 1 kHz) and ITOR4F-IC (4.46 at 1 kHz). Moreover, the PM6:ITOR-IC4F based polymer solar cells (PSCs) presented the highest exciton disso-ciation probability, superior charge transport and extraction properties, combined with the longest carrier lifetime and the lowest recombination rate coefficient, eventually achieved the highest PCE of 11.1%. The results indicate that the introduction of fluorine atoms into end groups rather than side-chains of the NFAs is more conducive to increasing the dielectric constant of the IT-based NFA, more importantly, improving the photo-response and the morphology of the active layer, thus devoting to the improvement of the light harvesting, exciton dissociation, as well as the carrier lifetime in the active layer. This work brings forward a new aspect to understand the influence of fluorine atoms substituent position on the properties of the NFAs for the further designation and optimization of new NFAs for efficient PSCs.

Automated summary

This study systematically investigated the optical and electrochemical properties, charge mobility, dielectric constants, and photovoltaic performance of three non-fullerene acceptors (NFAs). It was found that the NFA ITOR-IC4F, with 2-3-(oxo-5,6-difluoro-indane-1-ylidene)-malononitrile end groups, exhibited the narrowest bandgap and highest dielectric constant. Moreover, polymer solar cells (PSCs) based on PM6:ITOR-IC4F showed the highest efficiency. The results suggest that introducing fluorine atoms into the end groups is more beneficial for improving the dielectric constant and performance of NFAs.