Publication date: 21st July 2025
Organic–inorganic hybrid perovskites have recently emerged as compelling candidates to replace conventional semiconductors, driven by a suite of remarkable optoelectronic properties. These include broadband light absorption, tunable band gaps, high charge-carrier diffusion lengths, solution processability at low cost, and intrinsic mechanical flexibility. However, their commercial viability is critically hindered by poor environmental stability—particularly their susceptibility to moisture, thermal stress, and photodegradation—as well as the toxicity associated with water-soluble lead compounds, posing significant ecological and health-related challenges. Substituting Pb²⁺ with homovalent (Sn²⁺, Ge²⁺) or heterovalent (Sb³⁺, Bi³⁺) cations effectively mitigates toxicity while preserving perovskite functionality. Notably, vacancy-ordered layered double perovskites (LDPs) (A₄M(II)M(III)₂X₁₂) emerged recently as a promising lead-free class, offering direct band gaps, enhanced stability, and tunable optoelectronic properties via precise divalent/trivalent cation engineering. In this study, we conducted a systematic investigation into M(III) cation engineering within the Cs₄CoIn₂Cl₁₂ LDP framework by substituting In³⁺ with Bi³⁺ and Sb³⁺. This strategy facilitated the first-ever colloidal synthesis of Cs₄CoBi₂Cl₁₂ and Cs₄CoSb₂Cl₁₂ NCs. We examined how the structural distortions arising from these substitutions influence the optoelectronic properties of these NCs, revealing that Cs₄CoBi₂Cl₁₂ exhibited superior performance, characterized by a stable photo response and enhanced photocurrent generation in photoelectrochemical (PEC) applications. Transient absorption analysis confirmed the highest population of self-trapped excitons (STEs) alongside the longest half-lifetime in Cs₄CoBi₂Cl₁₂ hosts, enabling it as a promising material for sustainable PEC applications. Additionally, all the NCs demonstrated remarkable air and compositional stability, preserving their structural and chemical integrity for over 100 days under ambient conditions. Organic–inorganic hybrid perovskites have recently emerged as compelling candidates to replace conventional semiconductors, driven by a suite of remarkable optoelectronic properties. These include broadband light absorption, tunable band gaps, high charge-carrier diffusion lengths, solution-processability at low cost, and intrinsic mechanical flexibility. However, their commercial viability is critically hindered by poor environmental stability—particularly their susceptibility to moisture, thermal stress, and photodegradation—as well as the toxicity associated with water-soluble lead compounds, posing significant ecological and health-related challenges. Substituting Pb²⁺ with homovalent (Sn²⁺, Ge²⁺) or heterovalent (Sb³⁺, Bi³⁺) cations effectively mitigates toxicity while preserving perovskite functionality. Notably, vacancy-ordered layered double perovskites (LDPs) (A₄M(II)M(III)₂X₁₂) emerged recently as a promising lead-free class, offering direct band gaps, enhanced stability, and tunable optoelectronic properties via precise divalent/trivalent cation engineering. In this study, we conducted a systematic investigation into M(III) cation engineering within the Cs₄CoIn₂Cl₁₂ LDP framework by substituting In³⁺ with Bi³⁺ and Sb³⁺. This strategy facilitated the first-ever colloidal synthesis of Cs₄CoBi₂Cl₁₂ and Cs₄CoSb₂Cl₁₂ NCs. We examined how the structural distortions arising from these substitutions influence the optoelectronic properties of these NCs, revealing that Cs₄CoBi₂Cl₁₂ exhibited superior performance, characterized by a stable photo response and enhanced photocurrent generation in photoelectrochemical (PEC) applications. Transient absorption analysis confirmed the highest population of self-trapped excitons (STEs) alongside the longest half-lifetime in Cs₄CoBi₂Cl₁₂ hosts, enabling it as a promising material for sustainable PEC applications. Additionally, all the NCs demonstrated remarkable air and compositional stability, preserving their structural and chemical integrity for over 100 days under ambient conditions.
M.L. and Yukta acknowledge the Royal Physiographic Society of Lund and Crafoord Foundation (No. 20240745) for funding. This work was partially supported by the Wallenberg Initiative Materials Science for Sustainability (WISE), funded by the Knut and Alice Wallenberg Foundation. The authors acknowledge the Swedish Research Council and SSF for access to nCHREM, which is a part of ARTEMI, the Swedish National Infrastructure in Advanced Electron Microscopy (2021-00171 and RIF21-0026). T.A.S., A.O.B., and K.R. thank Helmholtz-Zentrum Berlin for the allocation of instrument time at the THz beamline at BESSY II (Proposal number 242-12743-ST) and the Energy Materials In-situ Laboratory (EMIL) operated by HZB for granting access to its chemistry laboratory. This work was partially funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy─EXC 2008-390540038─UniSysCat to K.R., and A.O.B. thanks Einstein Foundation Berlin (ESB)—Einstein Center of Catalysis (EC2) for their scholarship and support. Q.S. and T.P. would like to acknowledge financial support from the Swedish Energy Agency, the Swedish Research Council, WISE, WASP, and the LU profile area Light and Materials.
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