Conjugated Polymers Containing Sulfonic Acid Fluorene Unit for Achieving Multiple Interfacial Modifications in Fullerene-free Organic Solar Cells

Conjugated polymers (CPs) as interfacial layer materials have played a crucial role in improving the power conversion efficiency (PCE) of organic solar cells (OSCs). Sulfonic acid-functionalized fluorene units have showed promising applications in developing CP interlayers. However, the development of CPs containing sulfonic acid-modified fluorene unit into interlayers with multiple functions still remains a desired objective. In this work, three CPs named PFS, PFSF, and PFB are designed and synthesized on the basis of the sulfonic acid fluorene unit as interlayer materials for fullerene-free OSCs. The energy level of CPs is effectively tuned by copolymerizing sulfonic acid fluorene with different units, including bithiophene, difluorinated bithiophene, and benzene. The as-synthesized PFS can be used as the anode interfacial layer (AIL) and the corresponding OSC exhibits a PCE of 11.0%. Meanwhile, PFSF and PFB can serve as the cathode interfacial layer (CIL). More importantly, PFS can be used as a bifunctional interlayer for simultaneously modifying both the anode and the cathode in the same device. The fullerene-free OSC with a symmetric structure is fabricated by using PFS as both the AIL and the CIL, attaining a PCE of 9.54%. Furthermore, by using PFS as AIL and PFSF as CIL, an efficient OSC is fabricated with a PCE of 10.2%.