A group of researchers led by Nanjing University in China has developed a perovskite solar cell based on a dipolar passivation strategy that reportedly reduces defect density at the buried interface of mixed narrow-bandgap tin lead (Sn-Pb) perovskites, while enabling precise energy-level alignment at the interface between the hole transport layer (HTL) and the perovskite absorber.
“This dipole-induced passivation strengthens the ohmic contact, promotes efficient hole injection into the HTL, and repels electrons from the HTL/Pb–Sn perovskite interface,” the research's lead author, Renxing Ling, told pv magazine. “With this approach, the carrier diffusion length increases to 8.3 μm as characterized by terahertz probing.”
The scientists used, in particular, a dipolar passivation molecule known as sulfanilic acid (SA), which is commonly utilized as a cross-linking agent and a dopant in chemical synthesis, to reduce the defect density of the mixed Pb-Sn perovskite surface. Moreover, they used a sulfonic acid group at the interface between the perovskite absorber and the hole transport layer based on PEDOT-PSS.
This strategy is claimed to enable efficient hole injection into the HTL while pushing apart electrons from the HTL/Pb-Sn perovskite
interface, thus suppressing non-radiative recombination and avoiding carrier transport losses. “The dipolar passivation strategy
facilitates a trade-off between minimizing carrier recombination and enhancing carrier transport,” they further explained.
Image: Nanjing University
The group fabricated the cell with a substrate made of glass and indium tin oxide (ITO), the PEDOT:PSS HTL, the dipolar passivation layer, the perovskite absorber, an electron transport layer (ETL) based on buckminsterfullerene (C60) and tin(IV) oxide (SnO2) deposited through atomic layer deposition (ALD), and a copper (Cu) metal contact.
Under standard illumination conditions, the mixed Pb–Sn perovskite cell achieved a power conversion efficiency of 24.9%, an open-circuit voltage of 0.911 V, a short-circuit current density of 33.1 mA/cm², and a fill factor of 82.6%.
The researchers integrated the cell into two all-perovskite tandem devices with areas of 0.049 cm² and 1.05 cm², respectively. The larger device reached an efficiency of 29.4%, and the smaller one achieved 30.1%, with both results being certified by the Japan Electrical Safety and Environment Technology Laboratories (JET).
“These results suggest that dipolar passivation suppressed non-radiative carrier recombination while maintaining good electrical contact quality,” they further explained, noting that the cell was also able to retain around 87% of its initial efficiency after 1,025 h. “After 216 h of thermal stress, we observed that degradation proceeded more slowly in dipolar-passivation-based devices than in control devices.”
Looking forward, the team intends to improve the proposed dipolar passivation with new stabilization strategies and a better understanding of the degradation mechanisms that could affect the long-term stability of tandem devices.
The new cell design was introduced in “All-perovskite tandem solar cells with dipolar passivation,” which was recently published in nature.
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