Despite the rapid development of perovskite solar cells (PSCs), their efficiencies still suffer from significant non-radiative recombination losses, undermining their full thermodynamic potential in terms of open-circuit voltage (Voc) and fill factor (FF).

In PSCs, defective perovskite grain boundaries (GBs) and/or surface, have limited the performance enhancement and aggravated the PSCs instability. Suppressing non-radiation recombination and maximizing quasi-Fermi level splitting in the polycrystalline perovskite films are still a hot topic in the field. 

In a study published in Advanced energy 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 developed a novel semiconductive silicon naphthalocyanine derivative (Cl-SiNcTI) to reduce the deep level trap states at the GBs and the surface of perovskite film via a newly proposed photon-relaxation mechanism. 

The researchers proposed a novel energy passivation strategy by applying ambipolar Cl-SiNcTI molecule via a simple antisolvent-assisted crystallization strategy. The titled conjugated molecule with closed-shell and diamagnetic silicon (IV) at the center displays rather low internal reorganization energy to favor electron delocalization and transport. 

They introduced Cl-SiNcTI into the perovskite film at a proper stage of crystallization, it is reasonable to expect synergetic chemical passivating effects from these functional groups. The carbonyl (C=O) helps with better perovskite crystallization and passivates the defects at GBs such as uncoordinated Pb²⁺, and the –Cl passivates iodine vacancy defects. The individual molecular orbital formed a high-lying band above the conduction band (CB) of perovskite to compete for the high-energy photocarriers and reduce the probability of photolysis and reduction of Pb²⁺ to Pb⁰. 

Besides, the researchers realized a much-decreased defect density, a more favorable energy alignment at the perovskite/spiro-OMeTAD interface, and a more increased photo-stability. With a synergistic contribution of the Cl-SiNcTI and 2-(2-Fluorophenyl)ethylamine iodide (oFPEAI), a 24.30% efficiency was achieved in a single cell with excellent operational stability. Under steady-state light illumination, 93% power output compared to its initial state can still be maintained after 250 h of continuous operation. 

Furthermore, the researchers provided an efficient and simple method for the surface polarity regulation between the bulk and surface of the perovskite film through reduced Fermi-level pinning after suppressing Pb⁰ formation. 

This study provides new insights into the mechanism of passivation via assisting the thermalization of hot carriers in excited perovskites, which signifies the importance of a rational design of conjugated passivators to improve the photo-stability of PSCs.