Modulation of terminal alkyl chain length enables over 15% efficiency in small-molecule organic solar cells

Abstract

In small-molecule organic solar cells (SM-OSCs), it remains a big challenge to obtain favorable bulk heterojunction morphology by donor material design. Herein, we design and synthesize three small-molecule donors BPF3T-C4, BPF3T-C6 and BPF3T-C8, with different terminal alkyl chains. Although they possess similar absorption profiles and molecular energy levels, their crystallinity gradually decreases with the chain length of the terminal alkyl chains. After blending with an electron acceptor of BO-4Cl, the crystallinity and packing orientations of these donors are suppressed and changed from edge-on to face-on. Simultaneously, the crystallinity of BO-4Cl gradually weakens with the chain length of the terminal alkyl chain of donor materials. Finally, The BPF3T-C6 with moderate crystallinity exhibits the best phase-separation morphology among these blend films. As a result, the BPF3T-C6:BO-4Cl-based SM-OSC shows an impressive power conversion efficiency of 15.1%.[Figure not available: see fulltext.]