M(III) Site‐Driven Structural Engineering on Lead‐Free Layered Double Perovskite Nanocrystals with Enhanced Photoelectrochemical Activity

Over the past decade, organic–inorganic hybrid perovskites have revolutionized next‐generation semiconductors, driving unprecedented advancements in cost‐effective optoelectronics. While lead‐based perovskite semiconductors exhibit exceptional optoelectronic properties, their inherent toxicity and vulnerability to environmental degradation remain significant barriers to widespread commercialization. Vacancy‐ordered layered double perovskites (LDPs) offer a viable alternative with direct bandgaps, reduced toxicity, superior stability, and tunable properties, while their divalent and trivalent cation integration enables precise control over electronic and photophysical characteristics for efficient optoelectronics. Herein, the M(III) cation site within the previously reported Cs 4 CoIn 2 Cl 1 2 LDP system by substituting In 3 + with Bi 3 + and Sb 3 + is systematically modified, achieving the first‐ever colloidal synthesis of Cs 4 CoBi 2 Cl 1 2 and Cs 4 CoSb 2 Cl 1 2 nanocrystals (NCs). A detailed investigation of their optoelectronic properties reveals significant structural distortions induced by different M(III) cations. Stability assessments demonstrate that Cs 4 CoSb 2 Cl 1 2 exhibits exceptional air and compositional stability, maintaining its compositional integrity for over 100 days under ambient conditions. Furthermore, the photoelectrochemical (PEC) performance of these NCs in benzoquinone oxidation is explored, identifying Cs 4 CoBi 2 Cl 1 2 as the most efficient candidate, with a stable photoresponse and enhanced photocurrent generation. Transient absorption studies further confirm that Cs 4 CoBi 2 Cl 1 2 sustains the largest self‐trapped exciton population and longest half‐lifetime, highlighting its potential for sustainable, high‐performance PEC devices.