Structural and Optical Investigations of Quasi‐0D Lead‐Free Hybrid Halide Crystals: (C ₆ H ₁₆ N) ₂ ZnI ₄ and (C ₆ H ₁₆ N) ₂ CdI ₄

With excellent optoelectronic behavior, organic‐inorganic hybrid perovskites have become leading contenders in the development of future photovoltaic and optoelectronic devices. Herein, two new lead‐free, quasi‐0D OIHPs, (C 6 H 16 N) 2 ZnI 4 and (C 6 H 16 N) 2 CdI 4 , are synthesized, and both crystallize in the monoclinic system (P2 1 /n space group) as confirmed via single‐crystal X‐ray diffraction analysis. Optical bandgaps are measured to be 4.595 eV for (C 6 H 16 N) 2 ZnI 4 and 4.255 eV for (C 6 H 16 N) 2 CdI 4 , which align well with those from first‐principles calculations. Photoluminescence studies on (C 6 H 16 N) 2 ZnI 4 and (C 6 H 16 N) 2 CdI 4 show peaks at 393 and 382 nm, respectively, which are attributed to self‐trapped exciton (STE) transitions induced by lattice distortion. Notably, (C 6 H 16 N) 2 CdI 4 exhibits unusual blue emission (quantum yield: 1.41%, lifetimes: 168.5 ps), while (C 6 H 16 N) 2 ZnI 4 shows blue luminescence with enhanced efficiencies (quantum yields: 19.64%, lifetimes: 473.7 ps). These findings broaden the application prospects of 0D OIHP materials in renewable energy and photonics.