Fluorination of Star-Shaped Cyclopenta[2,1-b;3,4-b′]dithiophene Derivatives and Its Application as Hole-Transporting Materials in Scalable Perovskite Solar Cell Fabrication by Bar Coating

In this study, three novel star-shaped small molecules HYC-oF, HYC-mF, and HYC-H are designed and synthesized and are applied in perovskite solar cells (PSCs) as efficient hole-transporting materials (HTMs). Compared with non-fluorinated HYC-H, F-substituted analogs HYC-oF and HYC-mF exhibited enhanced hole mobility, suitable energy level alignment, improved interfacial contact and hole extraction/transport capability, better passivation effect, as well as increased hydrophobicity. PSC fabrication by spin-coating this series of HYC-oF, HYC-mF, and HYC-H molecules on a bar-coated perovskite layer yielded PCEs values of 19.64%, 18.38%, and 16.98%, respectively. In order to realize a fully scalable PSC preparation process, sequential coating of HYC-oF layer on perovskite layer all by thermal-assisted bar-coating (TABC) method was carried out on 10 × 10 cm² area. The TABC-based HYC-oF layer had a compact and uniform morphology with full surface coverage on the TABC-based perovskite film. Under optimized condition, the PSCs fabricated from such film exhibited a PCE of 18.49% as compared with a lower power conversion efficiency (PCE) of 17.51% from the reference cell using TABC-based spiro-OMeTAD. This work demonstrates the potential of fluorinated star-shaped cyclopenta[2,1-b;3,4-b′]dithiophene (CPDT)-based molecule as HTM for achieving efficient PSC in both small and large scale.