Tuning the Surface Energy of Hole Transport Layers Based on Carbazole Self-Assembled Monolayers for Highly Efficient Sn/Pb Perovskite Solar Cells

Recently, carbazole-based self-assembled monolayers (SAMs) have been utilized as hole transport layers (HTLs) in perovskite solar cells. However, their application in Sn or mixed Sn/Pb perovskite solar cells has been hindered by the poor wettability of the perovskite precursor solution on the carbazole surface. Here a self-assembled bilayer (SAB) comprising a covalent monolayer (Br-2PACz) and a noncovalent wetting layer (4CzNH₃I) as the HTL in a Cs_0.25FA_0.75Sn_0.5Pb_0.5I₃ perovskite solar cell is proposed. It is demonstrated that the wetting layer completely solves the problem due to the higher polarity of the surface and, furthermore, the ammonium groups help in the passivation of trap states at the buried SAB/perovskite interface. The introduction of the SAB enhances the device reproducibility with an average efficiency of 18.98 ± 0.28% (19.45% for the best device), compared to 11.54 ± 9.36% (19.34% for the best device) for the SAM-only devices. Furthermore, the improved perovskite processability on the SAB helps to increase the reproducibility of larger size device, where, a 12.5% efficiency for a 0.8 cm² active area device compared to 0.68% for the best SAM-based solar cell is demonstrated. Finally, the device's operational stability is also improved to 358 hours (T_80%), compared to 220 hours for the SAM-based solar cell.