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-cost manufacture of 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%). We synthesize regioisomeric NFAs (C4-In and C4-Out) in which the positions of the methyl substituents of their end groups differ. Interestingly, the molecular orientation of the regioisomers in thin film is found to change from edge-on (C4-Out) to face-on (C4-In) orientation, allowing a model study exploring the impact of the microstructure of NFAs on the performance of P3HT-based OSCs. We find that face-on oriented C4-In facilitates both electron and hole transport in a P3HT-based OSC along the vertical direction. In addition, C4-In shows reduced trap densities and suppressed charge recombination in the P3HT:C4-In OSC 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 demonstrates the importance of molecular orientation of NFAs for developing high-performance P3HT-based OSCs.