Edward H. Sargent's team at Northwestern University published a research paper titled "Amidination of ligands for chemical and field-effect passivation stabilizes perovskite solar cells" in the journal Science. Team members Yang Yi, Chen Hao, and Liu Cheng are first authors, with Edward H. Sargent, Mercouri G. Kanatzidis, and Bin Chen serving as co-corresponding authors.

Key Highlights: This paper developed a library of amidinyl ligands for field-effect and chemical passivation. The N-H bonds of these amidinyl ligands are resonantly strengthened, effectively resisting deprotonation and significantly improving the thermal stability of the perovskite surface passivation layer. This strategy reduced the ligand deprotonation equilibrium constant by more than 10 times and improved the photoluminescence quantum yield maintenance by a factor of 2 after aging at 85°C in air. Implementing this approach, they achieved a certified quasi-steady-state PCE of 26.3% for inverted PSCs. Furthermore, after 1100 hours of continuous operation at 85°C in air, the PCE retention was ≥90%, demonstrating stable performance.

Perovskite solar cells (PSCs) have experienced rapid development in recent years. A key driver of this progress has been the implementation of surface passivation techniques, including the use of low-dimensional perovskites, aromatic amines, and ammonium ligands. However, current state-of-the-art PSCs use organic ammonium ligands to address surface defects and reduce nonradiative recombination at the perovskite-charge transport layer interface. While various ammonium ligands can achieve both chemical and field-effect passivation, they also tend to deprotonate to volatile amines and halogens, particularly under light and thermal stress. Ammonium deprotonation leads to a loss of passivation effectiveness and creates vacancy defects on the perovskite film surface during long-term device operation, thereby reducing device stability.

To address this issue, the research team proposed that by changing the head group from ammonium to amidine in chemical and field-effect passivators, they could address the instability caused by deprotonation of each molecule at high temperatures, thereby improving the thermal stability of the surface passivation layer in PSCs. This strategy reduced the ligand deprotonation equilibrium constant by more than tenfold and tripled the photoluminescence quantum yield retention of the perovskite film after light aging at 85°C. By implementing this approach, the researchers achieved a certified quasi-steady-state PCE of 26.3% in inverted PSCs. Furthermore, after 1100 hours of continuous operation at 85°C in air, the PCE retention was ≥90%, demonstrating stable performance.

The study demonstrated that changing the anchoring group of the passivating ligand from ammonium to amidine prevented ligand deprotonation and prolonged the stability of the passivation layer at high temperatures, while maintaining the passivation effect. The research team believes that combining ligand amidine with expanded functionality represents a promising approach for developing next-generation passivation strategies, potentially further improving the durability of high-efficiency perovskite optoelectronic devices.

Fig. 1. Stability of amidinium ligands. (A) Molecular structures of ligands used in this study. (B) The N–H dissociation energies (E_D) of ammonium ligands and their corresponding amidinium ligands obtained by DFT calculations and acid dissociation constant (pK_a) values measured by titration of a 0.05 N ligand solution with 0.05 N NaOH. (C) PCE comparison of PSCs using different ligand passivation. Twelve devices were evaluated at each condition, and data are presented as mean ± standard deviation.

Fig. 2. Stability of amidinium passivation layers. (A) N 1s XPS depth profile of fresh and aged perovskite films treated with PDAI₂ and PDII₂, and F 1s depth profile of fresh and aged perovskite films treated with 4FBAI and 4FBII. The white color represents the highest intensity, while the red and blue represent the lowest intensity. (B) ToF-SIMS of C₃H₁₂N₂ ²⁺, C₇H₉FN⁺, C₃H₁₀N4²⁺, and C₇H₈FN²⁺ for fresh and aged perovskite films treated with PDAI₂, PDII₂, 4FBAI, and 4FBII, respectively. a.u., arbitrary units. 

Fig. 3. Passivation effect of amidinium ligands. (A) The electron density in the conduction band (n) near the surface of perovskite films with different treatments. (B) TRPL spectra of perovskite films with different treatments. (C) PLQY of the control, PDAI₂/3MTPAI-based, and PDII₂/4FBII-based perovskite films with and without C₆₀ deposition. (D) PLQY of the control, PDAI₂/3MTPAI-based, and PDII₂/4FBII-based perovskite films without C₆₀ before and after aging under 85°C, 1-sun-equivalent light illumination, and 50% RH in air. 

Fig. 4. Device performance. (A) Cross-sectional SEM image of the device structure. (B) PCE statistics for control, PDAI₂/3MTPAI-passivated, and PDII₂/4FBII-passivated devices. The center line indicates the median, the box limits represent the upper and lower quartiles, the whiskers denote the minimum and maximum values, and the vertical curved lines illustrate the data distribution. (C) Current density–voltage (J-V) curves of the best PDII₂/4FBII-passivated device. (D) The stabilized power output of the PDII₂/4FBII-passivated device. (E) MPP stability tracking of glass-encapsulated devices under 1-sun illumination at 85°C under 50% RH in air. 

Source：DOI: 10.1126/science.adr2091