Solid-liquid convertible fluorinated terthiophene as additives in mediating morphology and performance of organic solar cells

• The fluorinated terthiophene additive [2F]3T was designed and synthesized. • [2F]3T is a solid-liquid convertible additive as nucleation centers and plasticizers. • [2F]3T has the role of tailoring the nanoscale morphology of the BHJ active layer. • The organic solar cells with [2F]3T showed significantly improved PCE of 17.6%. Specifically, the efficient photovoltaic materials have bithiophene, thieno[3,2-b]thiophene or terthiophene sequences along conjugated backbones. In this contribution, a fluorinated terthiophene derivative ([2F]3T) is developed and used as additive to finely regulate the morphology of bulk heterojunction photoactive layers. Results show that the [2F]3T possess melting point of 75 °C and crystallization temperatures of 35 °C, and thus can serve as nucleation center at room temperature and plasticizer under thermal annealing to form favorable morphology with increased crystallinity and P-i-N like heterojunction. Therefore, the optimized PM6:Y6 based solar cells with [2F]3T additive reach efficiency of 17.6%, which is higher than that of the control devices with a modest efficiency of 16.3%. Moreover, the fill factor (FF) and short circuit current ( J sc ) increase significantly from 72.8% and 26.3 mA cm −2 for the control devices to 76.6% and 27.0 mA cm −2 for the devices with [2F]3T additive. Carrier dynamic studies reveal that the favorable morphology in [2F]3T-based devices translate into enhanced charge generation, transportation, and suppressed charge recombination. These findings make clear that the additive featuring with similar chemical building blocks as that of the organic photovoltaic materials has broader implications for further optimization of organic solar cells.