Dopant-free hydrophobic fluorene-based hole transport materials: impact of methoxy-substituted triphenylamine and carbazole peripheral groups on the performance of perovskite solar cells

Hole-transporting materials (HTMs) are crucial for charge separation in perovskite solar cells (PVSCs). Besides possessing suitable HOMO/LUMO energies, HTMs should ideally be hydrophobic to protect the perovskites from atmospheric moisture to enhance device stability. We designed two fluorene-core D-π-D-type organic HTMs (V1 and V2), consisting of either 4,4′-methoxy triphenylamine (V1) or N-phenyl-3,6-methoxy carbazole (V2) as the peripheral donor moiety. Optoelectronic characterization and density functional theory calculations confirmed the intramolecular charge transfer within these new HTMs. UPS and REELS analyses revealed favorable HOMO-LUMO energy level alignment of V1 and V2 with the work functions of MAPbI₃ and gold electrode for effective charge extraction. TRPL and transient absorption studies commendably explained better quenching of perovskite's luminescence by V1 over V2, suggesting a better interfacial contact of V1 with the perovskite layer. Accordingly, the PVSCs with V1 and V2 as HTMs in an architecture ITO/SnO₂/MAPbI₃/HTM(V1 or V2)/Au demonstrated power conversion efficiency (PCE) of 14.05% and 12.73% respectively. Also, the device with V1 retains 75% of its initial efficiency for more than 480 hours. The contact angle measurements revealed the strong hydrophobicity of both alkylated fluorene molecules (V1 and V2), and impedance spectroscopy measurements further revealed higher R_rec values for these HTMs, indicating improved charge transport and reduced recombination losses. These findings demonstrate the potential of the newly developed hydrophobic fluorene-based HTMs for achieving long-lasting performance in PVSCs.