Solution-Processed Cu₉S₅ as a Hole Transport Layer for Efficient and Stable Perovskite Solar Cells

Area of Science:

• Materials Science
• Renewable Energy
• Photovoltaics

Background:

• Organic-inorganic perovskite solar cells (PSCs) are rapidly advancing.
• 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD) is a common hole transport material (HTM) in n-i-p PSCs.
• The dopant lithium bis((trifluoromethyl)sulfonyl)amide (Li-TFSI) in Spiro-OMeTAD can degrade perovskite films due to its hydrophilicity.

Purpose of the Study:

• To develop a more stable and efficient hole transport layer (HTL) for perovskite solar cells.
• To improve the long-term operational stability of PSCs by eliminating Li-TFSI.
• To reduce hysteresis in PSCs by optimizing the electron transport layer (ETL).

Main Methods:

• A solution-processed organic-inorganic-integrated HTL comprising undoped Spiro-OMeTAD and Cu9S5 was fabricated.
• Cu9S5 was chosen for its superior p-type transport properties and favorable energy level alignment.
• Nitrogen implantation was used to modify the lowest unoccupied molecular orbital (LUMO) level of SnO2 (N:SnO2) to improve electron extraction.

Main Results:

• The integrated HTL, free of Li-TFSI, significantly enhances the long-term stability of the solar cells.
• Nitrogen implantation tuned the LUMO of SnO2 from -4.33 eV to -3.91 eV, matching well with the LUMO of CH3NH3PbI3 (-3.90 eV), effectively reducing hysteresis.
• The fabricated n-i-p perovskite solar cells achieved a maximum power conversion efficiency (PCE) of 17.10% and retained 96% of their initial PCE after 1200 hours of operation in air without encapsulation.

Conclusions:

• The combination of an undoped Spiro-OMeTAD/Cu9S5 HTL and N:SnO2 ETL offers a promising strategy for highly stable and efficient perovskite solar cells.
• Eliminating Li-TFSI doping is crucial for preventing perovskite degradation and ensuring device longevity.
• The energy level engineering of the ETL via nitrogen implantation effectively mitigates hysteresis, leading to improved device performance.