All‐In‐One Additive Enabled Efficient and Stable Narrow‐Bandgap Perovskites for Monolithic All‐Perovskite Tandem Solar Cells

Abstract Hybrid tin‐lead (Sn‐Pb) perovskites have garnered increasing attention due to their crucial role in all‐perovskite tandem cells for surpassing the efficiency limit of single‐junction solar cells. However, the easy oxidation of Sn²⁺ and fast crystallization of Sn‐based perovskite present significant challenges for achieving high‐quality hybrid Sn‐Pb perovskite films, thereby limiting the device's performance and stability. Herein, an all‐in‐one additive, 2‐amino‐3‐mercaptopropanoic acid hydrochloride (AMPH) is proposed, which can function as a reducing agent to suppress the formation of Sn⁴⁺ throughout the film preparation. Furthermore, the strong binding between AMPH and Sn‐based precursor significantly slows down the crystallization process, resulting in a high‐quality film with enhanced crystallinity. The remaining AMPH and its oxidation products within the film contribute to improves oxidation resistance and a substantial reduction in defect density, specifically Sn vacancies. Benefiting from the multifunctionalities of AMPH, a power conversion efficiency (PCE) of 23.07% is achieved for single‐junction narrow‐bandgap perovskite solar cells. The best‐performing monolithic all‐perovskite tandem cell also exhibits a PCE of 28.73% (certified 27.83%), which is among the highest efficiency reported yet. The tandem devices can also retain over 85% of their initial efficiencies after 500 hours of continuous operation at the maximum power point under one‐sun illumination.