Molecular Orientation Control of Regioisomeric Small-Molecule Acceptors for High-Performance Poly(3-hexylthiophene)-Based Organic Solar Cells with 9.3% Efficiency

Use of poly-(3-hexylthiophene) (P3HT) enables low-costmanufactureof organic solar cells (OSCs). However, the power conversion efficiencies(PCEs) of the P3HT-based OSCs should be further enhanced. Herein,we report the development of noncovalently fused nonfullerene acceptors(NFAs) that realize high-performance P3HT-based OSCs (>9%). Wesynthesizeregioisomeric NFAs (C4-In and C4-Out) in which the positions of themethyl substituents of their end groups differ. Interestingly, themolecular orientation of the regioisomers in thin film is found tochange from edge-on (C4-Out) to face-on (C4-In) orientation, allowinga model study exploring the impact of the microstructure of NFAs onthe performance of P3HT-based OSCs. We find that face-on orientedC4-In facilitates both electron and hole transport in a P3HT-basedOSC along the vertical direction. In addition, C4-In shows reducedtrap densities and suppressed charge recombination in the P3HT:C4-InOSC due to its lower energetic disorder relative to C4-Out. As a result,the P3HT:C4-In OSC (PCE = 9.35%) outperforms a P3HT:C4-Out OSC (PCE= 8.12%) owing to simultaneous enhancement in open-circuit voltage,short-circuit current density, and fill factor. This study demonstratesthe importance of molecular orientation of NFAs for developing high-performanceP3HT-based OSCs.