Screening of the carbazole-based phosphonic acids in perovskite solar cells: impact of the substitution pattern on device performance

Abstract

Self-assembled monolayers (SAMs) have become the standard hole-selective layer in high-efficiency p–i–n perovskite single-junction and multi-junction solar cells. Since there is limited information about the structure–property correlation, it is important to determine how structural changes in the molecule impact performance or enhance layer properties. In this work, nine different carbazole-based phosphonic acids with methyl, methoxy, and phenyl functional groups were compared to investigate the correlation between the chemical structure and device performance. The results reveal that phosphonic acids with different functional groups share some structure–property correlations and that substituents at 3,6 positions (3,6-Me-2PACz, 3,6-Ph-PACz, and 3,6-MeO-2PACz) of carbazole have higher perovskite single-junction device fill factors, which is one of the factors determining the performance of the device, than molecules with substituents in 2,7 or 4 positions. These findings were supported by transient surface photovoltage (trSPV) measurements. In addition, correlation between ionisation potential and fill factor suggests the threshold value, after which the fill factor is reduced, possibly due to the energy level misalignment. Additionally, the impact of different functional groups in the molecule was investigated with optical, thermal, and electrophysical methods by combining experimental and simulation methods. This work provides insightful knowledge that can contribute to further investigations and design of phosphonic acid-based materials.