A fluorinated polythiophene hole-transport material for efficient and stable perovskite solar cells

Charge-transport materials for use in highly efficient and stable perovskite solar cells (PSCs) must exhibit energy levels appropriate for high charge selectivity, sufficiently high charge-transport ability for efficient charge collection, and high humidity resistance for long-term device stability. Polythiophenes are a promising class of hole-transport layer (HTL) materials that could satisfy these requirements. However, PSCs fabricated using conventional poly(3-hexylthiophene) (P3HT) HTLs show limited efficiencies of < 16% owing to the shallow highest occupied molecular orbital (HOMO) energy level and poor charge extraction ability of P3HT. Herein, we demonstrate that the fluorinated polythiophene derivative FEH is a suitable replacement for P3HT and a promising HTL material for perovskite solar cells. The FEH was found to have a deeper HOMO and exhibit more efficient charge-extraction ability at the perovskite/HTL interface than P3HT. This is attributed to the electron withdrawing nature of the fluorine atoms in FEH and its ability to form more uniform films on the perovskite layer. Thus, when FEH was employed as the HTL, the corresponding PSC showed an improved efficiency of 18.0% and an enhancement of all device parameters compared with control devices fabricated using P3HT (10.8%) and Spiro-OMeTAD (17.0%) HTLs. Moreover, fluorination on the conjugated backbone of the polymer increases its hydrophobicity, and the resulting hydrophobic surface of the FEH HTL prevents the ingress of water, resulting in an improvement of the long-term stability of the corresponding PSCs under air exposure.