How can the long-term stability of perovskite solar cells be improved

The long-term stability of perovskite solar cells (PSCs)

can be significantly improved through a combination of intrinsic material modifications, interface engineering, and protective measures against environmental stressors. Below are the key strategies supported by recent research:

Material Composition and Structural Modifications

• Mixed A-cations and Halides: Replacing the organic “A-cation” in the perovskite ABX₃ structure, such as using a mixture of formamidinium (FA⁺), cesium (Cs⁺), and methylammonium (MA⁺), can enhance thermal and moisture stability. For example, mixed-halide perovskites (e.g., adding bromine to iodine) tend to form a more stable pseudo-cubic crystal structure, which improves durability. However, the halide ratio must be optimized to maintain high efficiency without sacrificing stability.
• Compositional Engineering: Substituting certain ions or doping the perovskite material can reduce defect density and suppress degradation pathways. This includes site-based substitution in the perovskite lattice and incorporation of stable dopants that enhance material robustness.

Interface and Surface Passivation

• Defect Passivation: Defects on the surface and grain boundaries act as nonradiative recombination sites and pathways for moisture and oxygen infiltration. Passivating these defects using conjugated organic molecules, zwitterions, or other passivation agents improves both efficiency and long-term stability by reducing trap states on the surface.
• Inter-layer Modification: Adding interface layers to reduce defects at the electron transport layer (ETL)/perovskite and hole transport layer (HTL)/perovskite interfaces can significantly enhance charge transport and stability. For instance, inter-layer modifications between SnO₂ ETL and perovskite absorber layers have achieved high power conversion efficiency (PCE) while maintaining nearly 90% of initial efficiency after extended light exposure.

Protective Coatings and Encapsulation

• Improved Protective Layers: Conventional ammonium-based surface coatings degrade under heat and moisture. Researchers at Northwestern University developed a more stable amidinium-based protective coating that is 10 times more resistant to decomposition and extends the T₉₀ lifetime (time to 90% efficiency drop) of PSCs by threefold under harsh conditions.
• Encapsulation Techniques: Extrinsic protection through encapsulation shields PSCs from environmental factors such as moisture, oxygen, UV light, and temperature fluctuations, dramatically slowing degradation.

Hole Transport Materials (HTM) and Charge Transport Layers

• HTM Optimization: The choice of HTM influences stability; substituting lithium-containing dopants with alternatives like spiro-OMeTAD reduces degradation related to hygroscopic and corrosive chemicals. HTM-free PSCs have also been explored to circumvent instability introduced by these materials.
• Bilayer HTMs: Employing bilayer HTMs (e.g., combinations of NiO and Cu:NiO) enhances hole extraction and suppresses electron recombination, leading to devices that maintain about 80% of their initial PCE after several hundred hours under operational conditions.

Processing and Fabrication Controls

• Controlled Atmosphere Fabrication: Perovskites are highly sensitive to oxygen and moisture, necessitating fabrication in inert environments (e.g., glove boxes) to ensure high-quality films and improved stability.
• Post-Treatment and Additives: Post-fabrication treatments with organic molecules or additives that bond to defects and strengthen the crystal lattice can reduce degradation and improve film quality.

Summary Table of Stability Improvement Strategies

Conclusion

Improving the long-term stability of perovskite solar cells requires an integrated approach combining compositional tuning, interface engineering, robust protective coatings, and optimized fabrication methods. Recent advances such as amidinium-based coatings have shown record improvements in thermal and moisture stability, bringing PSCs closer to practical, commercial viability while maintaining high efficiency around 25–26%.