Viscoelastic Grain-Boundary Regulation in Blade-Coated Tin–Lead Perovskites Enables High-Efficiency Tandem Solar Cells

ACS Publications USA

Overview

This research developed a mechanically adaptive viscoelastic grain-boundary regulation strategy for blade-coated tin–lead (Sn-Pb) perovskite solar cells. By precisely controlling grain-boundary formation during rapid crystallization, the strategy improves structural coherence, reduces residual stress, and suppresses defect density within the perovskite film. This approach achieved 21.02% PCE for 1.25 eV mixed Sn-Pb single-junction cells and 26.94% for monolithic 2-terminal all-perovskite tandem devices, demonstrating a scalable manufacturing pathway for high-performance tandem solar cells.

In Depth

Potential and Challenges of Sn-Pb Perovskites

Perovskite solar cells are emerging as a next-generation technology with the potential to surpass silicon. Specifically, mixed tin–lead (Sn-Pb) perovskites, with their narrow bandgap (approx. 1.2–1.3 eV), hold significant promise for substantially increasing the theoretical efficiency limit when combined in tandem architectures with wide-bandgap perovskites. However, Sn-Pb perovskites have faced challenges in terms of low oxidative stability of tin and difficulties in controlling crystal quality during large-area film deposition, affecting both efficiency and stability. Particularly with scalable blade-coating methods, rapid solvent drying and crystallization tend to induce non-uniform grain boundaries and residual stress.

Novel Strategy with Viscoelastic Grain-Boundary Regulation

This study overcame these challenges by introducing a mechanically adaptive viscoelastic grain-boundary regulation strategy during the formation of Sn-Pb perovskite films using the blade-coating method. This strategy involves incorporating specific additives into the perovskite precursor solution to precisely control grain-boundary formation during the crystallization process. This led to the following significant technical improvements:

• Enhanced Structural Coherence: The viscoelastic additives alleviate stress during crystallization and suppress defect formation at grain boundaries, significantly improving the overall structural uniformity and coherence of the perovskite film.
• Reduced Residual Stress: Effectively reduces the residual stress within the film, which is commonly induced by rapid drying and crystallization. This enhances the mechanical stability and long-term reliability of the devices.
• Suppressed Defect Density: Improved grain boundary quality reduces the density of defects that act as carrier recombination sites, thereby increasing charge transport efficiency.

Achieved High Performance and Tandem Applications

As a result of applying this viscoelastic grain-boundary regulation strategy, the following excellent performances were achieved:

• 1.25 eV Single-Junction Sn-Pb Perovskite Solar Cells: A high power conversion efficiency of 21.02% was recorded for standalone Sn-Pb perovskite cells. This is one of the highest efficiencies worldwide for narrow-bandgap perovskites.
• Monolithic 2-Terminal All-Perovskite Tandem Devices: A monolithic 2-terminal tandem structure was constructed using this Sn-Pb perovskite as the bottom cell and a wide-bandgap perovskite as the top cell. This configuration achieved a high power conversion efficiency of 26.94%. This indicates that all-perovskite tandem solar cells, alongside silicon-based tandem cells, are competitive as next-generation ultra-high-efficiency solar cells.

Technical Significance and Future Outlook

This research establishes a groundbreaking method to simultaneously improve the efficiency and stability of Sn-Pb perovskites using a scalable blade-coating technique. This opens a practical manufacturing pathway for large-scale production of tandem solar cells. Notably, the novel concept of viscoelastic control can also be applied to control crystallization in other complex thin-film materials, potentially impacting the broader optoelectronic device sector. Future research will focus on further stability verification and optimization for application in actual manufacturing lines.