Bifunctional spiro-fluorene/heterocycle cored hole-transporting materials: Role of the heteroatom on the photovoltaic performance of perovskite solar cells

Hole-transporting materials (HTMs) play crucial roles in protecting the perovskite layer, promoting charge extraction, as well as controlling the cost of Perovskite solar cells (PSCs). In order to reduce PSCs cost and simplify PSCs preparation process, more and more attention has been paid to develop multifunctional HTMs. In this work, two novel spiro-fluorene/heterocycle cored bifunctional HTMs, denoted as SFHc-O and SFHc-S respectively, are designed and facilely synthesized. The two HTMs have similar molecular structures, energy levels and thermal properties, but show quite different PSCs performance. Significantly, the heteroatoms in freshly developed spiro cores are demonstrated to have large contributions to device performance. In particular, the sulfur atoms in SFHc-S display a positive impact on both the hole extraction/transport and the defect passivation, which ultimately endow corresponding device with much better performance. The PSC based on SFHc-S obtains a high efficiency excess 21.5% with negligible hysteresis. Moreover, the device with SFHc-S displays enhanced stability, compared to the reference device incorporating spiro-OMeTAD. This work paves a way to develop multifunctional spiro-HTMs for highly efficient and stable PSCs.