C70-based aqueous-soluble fullerene for the water composition-tolerant performance of eco-friendly polymer solar cells

Abstract

Eco-friendly polymer solar cells (eco-PSCs) based on aqueous-soluble conjugated materials have recently received great research attention. In this work, we report the development of an aqueous-processable C₇₀-based fullerene derivative (PC₇₁BO₁₅) for eco-PSCs processed by water/ethanol co-solvents. The desirable aggregation behavior and enhanced light absorption ability of PC₇₁BO₁₅ have enabled the fabrication of PPDT2FBT-A:PC₇₁BO₁₅-based eco-PSCs with power conversion efficiencies (PCEs) of up to 2.51%, which is the highest value reported to date for aqueous-processed PSCs. These PPDT2FBT-A:PC₇₁BO₁₅ eco-PSCs exhibit significantly higher PCEs than those of the reference PPDT2FBT-A:PC₆₁BO₁₅ devices over all device-processable water/ethanol compositions. At the optimal water/ethanol composition (v/v = 15 : 85), the PCE of the PPDT2FBT-A:PC₇₁BO₁₅ eco-PSCs is 73% higher than that of the PC₆₁BO₁₅-based counterparts, as a result of enhanced light absorption. Importantly, the PC₇₁BO₁₅-based eco-PSCs show much higher tolerance in their PCEs to the water/ethanol composition. For example, the PCEs of the PPDT2FBT-A : PC₇₁BO₁₅ eco-PSCs at a 30 : 70 water/ethanol ratio maintain 89% of the optimal performance at a 15 : 85 ratio, whereas the PC₆₁BO₁₅-based devices only maintain 45%. This large difference in terms of water-tolerant behavior is mainly related to the different aggregation behaviors between PC₇₁BO₁₅ and PC₆₁BO₁₅ in blend films, which are carefully investigated using electrical, optical and morphological characterizations. This high water composition-tolerance affords an excellent reproducibility of the PC₇₁BO₁₅-based eco-PSCs with eco/human-friendly aqueous processing under ambient conditions.