Defect passivation is the most effective strategy to improve the photoelectric conversion efficiency and stability of perovskite solar cells. Lewis bases are one of the commonly used passivation additives for perovskite solar cells, which are defined as small organic molecules with electron-withdrawing functional groups. They are electron-pair donors that can give electrons to the other components.
Although molecules with protonic functional groups and conventional Lewis-base groups exhibit a more efficient passivation effect than pure Lewis bases, the detailed passivation mechanism has not yet been thoroughly studied and proven. Theoretical calculations have proved that surface-active oxygen (superoxide ion, O2-) can impair the stability of perovskite films and devices. However, there is rare experimental research on the passivation molecules to prevent the process of superoxide degradation of perovskite.
In a study published in Advanced Materials, the research group led by Prof. GAO Peng from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences reported a new type of polyaromatic passivator named 4OH-NMI that combines chemical passivation (e.g., Lewis-base functional groups and protonic functional groups) and energetic passivation (e.g., creating benign midgap states for the excited perovskite) effects for fabricating high-performance perovskite solar cells.
The researchers combined protonic and Lewis-base functional groups on 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 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.
Combined 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.
Moreover, 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. Consequently, 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.
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 (C=O, S=O, P=O, or -CN) and protonic groups (-OH, -NH, or -SH) 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.
