Influence of alkyl chain length on the photovoltaic properties of dithienopyran-based hole-transporting materials for perovskite solar cells

A tailored design of asymmetric hole-transporting materials (HTMs) is reported with the synthesis of a family of new HTMs based on the use of the 5H-dithieno[3,2-b:2',3'-d]pyran (DTP) moiety endowed with donor p-methoxytriphenylamines. A complete experimental and theoretical characterization of the optoelectronic, electrochemical and thermal properties is presented, showing more marked differences in the latter prompted by the different length of the alkyl chains (ethyl, butyl or hexyl) attached to the DTP core. This chemical design plays an important role in the morphological behavior of the new HTMs, displaying a different ability for the deposition on the top surface of the perovskite layer in perovskite solar cells (PSCs), as evidenced by scanning electron microscopy. The photovoltaic performance of the new DTP-based HTMs is highly affected by this morphological behavior, resulting in a maximum power conversion efficiency (PCE) of 17.39% for the ethyl derivative (DTPA-Et) in planar devices in combination with the state-of-the-art triple cation perovskite [(FAPbI(3))(0.87)(MAPbBr(3))(0.13)](0.92)[CsPbI3](0.08). Otherwise, the hexyl derivative (DTPA-Hex) showed a decreased value of PCE of 15.04% due to its higher dispersity in chlorobenzene, resulting in a less uniform and lower quality film. In comparison, the reference cell using spiro-OMeTAD reaches a maximum PCE of 18.06%. This work demonstrates that DTP is a good candidate for the preparation of HTMs with high hole mobilities for exploitation in efficient and stable PSCs.