Multifunctional ionic liquids dual-interface passivation for HTL-free carbon-based perovskite solar cells with fill factor exceeding 82%

Defects on the surface and at the grain boundaries of perovskite crystals are the main reasons for non-radiative recombination, reducing device efficiency and stability. This work conducts post-surface modification of CsPbI₃ perovskite by using acetylthiocholine chloride (ATCCl), a multifunctional ionic liquid containing positively charged ATC⁺ and negatively charged Cl⁻ for selectively passivation of uncoordinated Pb²⁺, I⁻ and the iodine vacancies (V_I⁻). Experimental results show that ATCCl can passivate perovskite surface defects and penetrate through grain boundaries to the bottom interface, achieving dual-interface passivation of perovskite. With the modified device, while the carrier lifetime was increased and the defect density was suppressed, resulting in an improved fill factor of 82.02%, and the hysteresis index was reduced to 0.07. The modified HTL-free carbon-based perovskite solar cells achieved a power conversion efficiency (PCE) of 18.08% and exhibit better stability.