Citric Acid‐Inhibited Phase Separation Assists In Situ Growth of Ternary Cationic Perovskite Single Crystal Thin Films Toward Efficient Solar Cells

ABSTRACT The in situ growth of ternary‐cation perovskite single crystal thin films (TC‐PeSCTFs) on charge‐transport layers is a promising route to high‐performance optoelectronics. However, such growth is plagued by intrinsic instability during crystallization; namely, the oxidation of iodide (I − to I 2 /I 3 − ) and deprotonation of organic cations (FA + and MA + ), which lead to phase separation and a high trap density that degrades device performance. Here, we introduce citric acid (CA) as a multi‐functional additive to suppress these deleterious reactions. We demonstrate that CA acts as a reducing agent, scavenging I 2 /I 3 − and regenerating I − , and as a proton source, mitigating cation deprotonation. Concurrently, CA passivates Pb‐related dangling bonds, further reducing defect density. This approach effectively stabilizes the precursor solution and suppresses phase separation during crystallization. Consequently, we fabricate perovskite solar cells that achieve a power conversion efficiency of 23.39% and a high open‐circuit voltage ( V OC ) of 1.14 V for FA‐based PeSCTFs. This work provides a molecular‐level strategy for regulating the growth of mixed‐cation perovskite single crystals, critical for their application in advanced optoelectronic devices.