Abstract
Good selective contacts are necessary for solar cells that are efficient and have long-term stability. Since 1998, with the advent of solid-state dye sensitized solar cells (DSSC), Spiro-OMeTAD has become the reference hole-transporting material. Yet, for efficient solar cells Spiro-OMeTAD must be partially oxidized with chemical dopants, which compromises the long-term stability of the solar cell. Alternatively, semiconductor polymers such as PTAA have been also studied, matching or improving the solar cell characteristics. However, PTAA-based devices lack long-term stability. Moreover, both Spiro-OMeTAD and PTAA are expensive materials to synthesize. Hence, approaches toward increasing the solar cell stability without compromising the device efficiency and decreasing the manufacturing cost are very desirable. In this work we have modified Spiro-OMeTAD, by an easy-to-use methodology, by introducing a carboxylic acid anchoring group (Spiro-Acid), thereby allowing the formation of self-assembled monolayers (SAMs) of the hole-transporting material in dopant-free p-i-n hybrid perovskite solar cells (iPSCs). The resulting device showed a champion efficiency of 18.15% with ultralow energy loss, which is the highest efficiency among Spiro-OMeTAD-based iPSCs, and a remarkable fill factor of over 82%, as well as excellent long-term illumination stability. Charge transfer and charge carrier dynamics are studied by using advanced transient techniques to understand the interfacial kinetics. Our results demonstrate that the Spiro-OMeTAD-based SAMs have a great potential in producing low-cost iPSC devices, due to lower material usage, good long-term stability, and high performance.