Constructing 2D/3D perovskite heterojunctions is an effective method for surface passivation of perovskite solar cells. However, previous studies have shown that merely depositing 2D perovskites physically on the 3D perovskite surface cannot eliminate the defects in the 3D perovskite bulk. Recently, a research team from a renowned university published a paper in Advanced Materials, stating that they proposed using chlorine-dissolved chloroform (Cl2-CF) as a multifunctional solvent to construct 2D/3D perovskite heterojunctions and induce secondary grain growth and passivation in the bulk. This study provides new insights into achieving high-efficiency and high-stability perovskite solar cells.

Firstly, the research team conducted a series of chemical analysis experiments to verify the composition of Cl2-CF. Specifically, they used barium hydroxide and silver nitrate reagents to confirm the presence of CO2 and Cl- in Cl2-CF; potassium iodide test paper to confirm the presence of Cl2. UV-vis absorption spectra showed the characteristic peak of chlorine gas in Cl2-CF, confirming that chloroform underwent photolysis to generate Cl2 under light illumination and humidity. To reveal the reactivity of Cl2-CF, the team observed the absorption peak changes of different perovskite precursors dissolved in Cl2-CF by UV-vis spectroscopy, proving the strong oxidizability of Cl2-CF which can oxidize iodide ions into iodine molecules.

Then, the researchers used XRD, GIWAXS, and other techniques to study the effect of Cl2-CF on the crystal structure of perovskites. The results showed increased diffraction peak intensity and peak shifts for perovskite films treated with Cl2-CF, indicating the occurrence of Cl doping and secondary grain growth during the treatment. XPS and EDS characterizations further verified that Cl2-CF treatment can introduce Cl- anions into the perovskite bulk and enable diffusion to the bottom side of the film, achieving Cl doping of perovskites.

Based on the above findings, the researchers proposed that the Cl2-CF solvent can both induce secondary grain growth of perovskites and serve as a post-treatment solvent to dissolve bulky cations for constructing 2D/3D heterojunctions. To verify this hypothesis, the team passivated perovskite films with hexahydrobromic ammonium. The results showed that hexahydrobromic ammonium dissolved in Cl2-CF can react with defective surfaces to form 2D passivation layers, while concurrently Cl2 induces secondary grain growth of perovskites, realizing a dual effect of surface passivation and bulk crystallization regeneration. SEM and AFM results displayed smoother perovskite film surfaces after the secondary growth. XRD and GIWAXS confirmed the formation of 2D perovskites.

Finally, through the study of redox reaction mechanisms, the team pointed out that the reaction between Cl2 and iodide ions in perovskites is the key to achieving the above effects. The generated Cl- diffuses into the perovskite bulk and interface regions, reducing defect densities and suppressing non-radiative recombination. Relying on this multifunctional solvent system, The research team used Enlitech’s solar simulator SS-X series and quantum efficiency tester QE-R to obtain the experimental results. the researchers eventually prepared perovskite solar cells with a high efficiency up to 24.21%, and significantly enhanced operational stability that maintained 80% of the initial efficiency for 905 hours under continuous one-sun illumination at the maximum power point.

This study provides brand new insights into realizing high-efficiency and high-stability perovskite solar cells. The integrated strategy of utilizing the oxidizing properties of chlorine-dissolved chloroform to induce perovskite defect passivation and secondary grain growth, as well as dissolving bulky cations to construct heterojunctions, may inspire new ideas for the device fabrication processes of perovskite solar cells. Of course, further process optimization to reduce costs and improve reproducibility is required for commercialization.