In recent years, power conversion efficiency (PCE) of organic-inorganic hybrid halide perovskite solar cells (PSCs) has been greatly improved. The current world certification efficiency has reached 25.5%. However, poor environmental stability is a major factor hindering the commercialization of PSCs. Besides, the pervasive defects prone to non-radiative recombination and decomposition exist at the surface and the grain boundaries (GBs) of the polycrystalline perovskite films.  

To date, tremendous results have indicated that defect-induced electronic trap states at the perovskite surface and GBs should be minimized for optimal device performance with a lower open-circuit-voltage loss. Furthermore, the presence of site vacancies can lead to the formation of superoxide (O₂^-) species, which will trigger the cascade photodegradation reaction of the perovskites. 

In a study published in Angew. Chem. Int. Ed., 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 comprehensive dual-passivation (DP) strategy to effectively passivate the defects at both surface and GBs, to enhance, at the same time, the performance and stability of the PSCs.  

The researchers used a solution of hydrophobic and semiconductive molecule N, N’-bis-(7,7,8,8,9,9,10,10,11,11,12,12,12-tridecafluoro dodecan-5-yl)-perylene-3,4,9,10-tetracarboxylic diimide (FPD) in chlorobenzene (CB) as the antisolvent, to implant the FPD inside the film during the nucleation of perovskites. This protocol ensures that the defects (e.g., noncoordinating Pb²⁺) at GBs be passivated by the carbonyl groups (C=O), and the strong hydrogen bonding between fluorine groups and MA can immobilize the cations, thus ensuring thermal stability.  

A hydrophobic 2D perovskite layer is formed in-situ on the 3D perovskite layer by successively depositing 2-(2-Fluorophenyl)ethylamine iodide (ortho-FPEAI) after the FPD incorporated antisolvent-treatment to passivate the defects on the surface. 

The concentration (mg/mL) of FPD in the CB was optimized by investigating its effect on the surface morphology and crystallinity of the 3D perovskite films. The conductive FPD molecule filling at GBs of perovskite film can passivate defects and retard crystallization rate for batter crystallization and higher film quality are confirmed.  

By performing X-ray diffraction (XRD), photoluminescence (PL), and confocal laser scanning microscope (CLSM) experiments, the researchers confirmed the formation and distribution of (oFPEA)₂PbI₄ crystals on top of the 3D perovskite. By the steady-state PL and high-resolution X-ray photoelectron spectroscopy (XPS) analysis, they found that the synergetic passivation effect of the two passivators on different defect sites on perovskites. 

As a result, the novel DP strategy prolongs carrier lifetime through defect passivation and improves the perovskite film quality, and thus lifts the V_OC from 1.10 V to 1.18 V, corresponding to a V_OC deficit of 0.39 eV. In the complete devices, the researchers achieved a champion stabilized PCE as high as 23.80%.  

Besides the excellent thermal and moisture resistant properties provided by the DP strategy, the effective passivation of iodine vacancy defects suppressed the generation of O₂^- species. Especially, the researchers observed the decrease of destructive O₂^- species by forming a 2D perovskite layer atop the 3D perovskites. Therefore, the DP strategy-based PSCs also showed remarkable long-term stability, retaining over 85% of the initial efficiency after heating on a hot plate at 100 °C for 30 h under relative humidity (RH) of 70% and over 97% of the initial PCE after 71 days under a 30% RH environment conditions without encapsulation.  

This study provides new insight into the strategies to enhance thermal, moisture, and oxygen stability of perovskite materials, and helps to develop new passivation methods in the future.