Influence of donor units on spiro[fluorene-9,9′-xanthene]-based dopant-free hole transporting materials for perovskite solar cells

Three spiro[fluorene-9,9'-xanthene] (SFX)-based organic molecules with different donor units were synthesized and used as dopant-free hole-transport materials (HTMs) in perovskite solar cells (PSCs). The end-capping donor units on the SFX core are two methoxydiphenylamine-substituted diphenylamine (MDDPA), two and four methoxydiphenylamine-substituted carbazole (MDCZ) for SFX-2DPA, SFX-2CZ and SFX-4CZ, respectively. The structure-function relationship is systematically investigated as the impact of the different donor units upon the optical, electrochemical and photophysical properties of SFX-based HTMs. The PSCs based on SFX-2CZ attain the higher power conversion efficiency (PCE) of 15.83% than that based on SFX-2DPA (14.25%), due to higher molecular planarity of SFX-2CZ with enhanced hole transporting ability and the downshifted HOMO level. On other hands, SFX-4CZ has the lowest efficiency (12.19%) among three HTMs mainly due to its inferior film forming ability. The results provide not only the new molecular design of dopant-free organic HTMs based on SFX core, but also a promising strategy to develop efficient HTMs with both the molecular planarity and number of donor groups being considered.

Automated summary

Three spiro[fluorene-9,9'-xanthene] (SFX)-based organic molecules with different donor units were synthesized and utilized as dopant-free hole-transport materials (HTMs) in perovskite solar cells (PSCs). The PSCs based on SFX-2CZ achieved the highest power conversion efficiency (PCE), while SFX-4CZ exhibited the lowest efficiency due to its inferior film forming ability. This study provides insights into the design of efficient HTMs based on SFX core with considerations of molecular planarity and number of donor groups.