Isoindigo-Based Terpolymers for Organic Solar Cells: Achieving a Thick (300 nm) and Larger-Area (1 cm²) Bulk Heterojunction with Minor Power Conversion Efficiency Drop

Enriching molecular structures with curious substituents has been considered as a potential strategy to build high-performing polymer semiconductors and boost the photovoltaic performance of organic solar cells (OPVs). In this work, difluoro and dicyano substituted terpolymers based on Isoindigo-sexithiophene π-conjugated system have been synthesized and characterized. The photovoltaic performances of the terpolymers were thoroughly examined by fabricating both binary and ternary bulk heterojunction (BHJ) polymer solar cells (PSCs). With a fine tuning of the device fabrication process, a high-power conversion efficiency (PCE) of 12.8% and 14.5% has been achieved for IT4F and Y6 based blends with a 300 nm BHJs, respectively. In a larger area (1 cm² nm) BHJ, these devices exhibited PCE as high as 12.4% and 14.0%, which is a minor decline of PCE compared with the 0.4 cm² BHJs and an encouraging result for PSCs under such thick and larger area active layer. Moreover, good shelf lifetime stability of the thick solar cells has been demonstrated. Comprehensive characterizations including AFM, TEM, 2D-GIWAXS, charge carrier mobility (SCLC), photoluminescence quenching, and charge recombination measurements have been performed to examine the structure-property relationships in the BHJs. Therefore, our work demonstrates the potential of fluorine and cyano-substituted Isoindigo-sexithiophene based terpolymers as a donor material to realize high performances in thick film and large area organic photovoltaics.