The researchers combined protonic and Lewis-base functional groups with the polyaromatic conjugated molecules, and obtained the pure Lewis-base system molecule 9CN-PMI and the Lewis-base/protonic system molecule 4OH-NM, which were introduced into the perovskite precursor to fabricate perovskite solar cells. By comparing the characterization of related samples, they proved that 4OH-NMI with Lewis- base/polyaromatic conjugate/protonic structure can provide better chemical passivation for both shallow- and deep-level defects.  

Furthermore, the researchers found that the C=O/-OH system of 4OH-NMI effectively passivates the positive and negative charged defects and lead clusters, and the -OH groups interact with I- through hydrogen bonds, thereby promoting the interaction between the C=O group and the antisite Pb²⁺ defect, giving a maximized passivation chemical effect. 

Theoretical and experimental studies showed that the 4OH-NMI can bind more firmly with perovskite and the polyaromatic backbones create benign midgap states in the excited perovskite. These midgap states are above the trap states produced by O₂ so that they can capture the photoexcited electrons on perovskite prior to oxygen to avoid the formation of superoxide anions and thereby enhance the system stability and charge carrier lifetime. The polar and protonic nature of 4OH-NMI facilitates band alignment and regulates the viscosity of the precursor solution for thicker perovskite films with better morphology. The 4OH-NMI-passivated perovskite films exhibit reduced grain boundaries and nearly three times lower defect density, boosting the device efficiency to 23.7% with excellent stability. 

Besides, the researchers found that the electronic push-pull configuration provides strong dipole moments and increased non-adiabatic electron-phonon coupling. A judiciously chosen π-system to generate midgap states above the trap states produced by O₂ so that they can break the energetic degradation circle of the perovskite under the light. 

This study provides new insight into the design of multifunctional passivators. The combination of Lewis-base functional groups and protonic groups in the passivator is essential in realizing effective passivation of deep- and shallow-level defects in perovskite films. The unexpectedly found viscosity regulation by such compound deserves further study in perovskite tandem solar cells due to its ability to control the thickness of the perovskite layer for better current matching.