Viktor Rehm

We introduce ligand‐assisted epitaxial growth (LAEG) for lead‐halide perovskites, enabling the formation of high‐quality, single‐crystalline epitaxial microstructures. In this approach, ligands fulfill multiple critical functions: they regulate crystal growth by slowing crystallization kinetics, allowing the formation of epitaxial microcrystals with smooth surfaces and well‐defined morphologies; they protect the perovskite from environmental degradation; and they passivate surface states. Near‐ambient‐pressure X‐ray photoelectron spectroscopy confirms well‐preserved surface stoichiometry of CsPbBr ₃ enabled by ligand protection. The LAEG method is broadly applicable to different ligands, perovskite compositions, and substrates, consistently yielding higher material quality than non‐ligand‐assisted solution epitaxy. CsPbBr ₃ epitaxial microcrystals exhibit a high photoluminescence quantum yield of ∼50% and nearly temperature‐independent photoluminescence decay dynamics up to 80°C, demonstrating strong thermal robustness relevant for optoelectronic devices. Lasing is observed up to 90°C, and improved thermal management enhances operational lifetime. Micro‐resonator lasers grown on mica substrates exhibit record‐low threshold excitation powers among microcrystal‐based perovskite lasers, outperforming previously reported CsPbBr ₃ devices and those grown on PbS substrates due to favorable refractive‐index contrast enabling total internal reflection. Overall, LAEG provides a simple yet powerful route to high‐quality perovskite epitaxy and is expected to be extendable to lower‐dimensional perovskite systems with enhanced and tunable optical properties.

Flexible lead‐halide perovskite solar modules with carbon electrodes (C‐fPSMs) are promising for printed photovoltaics due to their low cost and compatibility with roll‐to‐roll (R2R) fabrication. However, the mechanical fragility of microcrystalline perovskite films, arising from their high grain‐boundary density, limits their practical application. Here, we report a vapor‐phase grain‐boundary anchoring strategy via thiol vapor annealing to molecularly toughen printed perovskite layers. Thiol molecules interact with intermediate species and preferentially anchor at surfaces and grain boundaries, reprogramming crystallization across the film. This chemomechanical conditioning promotes grain growth, reduces trap density, and transforms the deformation behavior from brittle fracture to ductile strain accommodation, as evidenced by reduced Young's modulus, increased yield strain, and significantly enhanced fracture toughness. As a result, treated devices retain 93% of their initial performance after 25 000 bending cycles, representing a new benchmark for flexible perovskite cells. In addition, improved film uniformity and interfacial contact enable power conversion efficiencies of 15.7% for R2R‐printed cells and 12.1% for modules (20.25 cm ² ), with scalability demonstrated up to 900 cm ² (17.26%). This work provides a scalable strategy to simultaneously enhance mechanical robustness and photovoltaic performance.

Flexible lead‐halide perovskite solar modules with carbon electrodes (C‐fPSMs) are promising for printed photovoltaics due to their low cost and compatibility with roll‐to‐roll (R2R) fabrication. However, the mechanical fragility of microcrystalline perovskite films, arising from their high grain‐boundary density, limits their practical application. Here, we report a vapor‐phase grain‐boundary anchoring strategy via thiol vapor annealing to molecularly toughen printed perovskite layers. Thiol molecules interact with intermediate species and preferentially anchor at surfaces and grain boundaries, reprogramming crystallization across the film. This chemomechanical conditioning promotes grain growth, reduces trap density, and transforms the deformation behavior from brittle fracture to ductile strain accommodation, as evidenced by reduced Young's modulus, increased yield strain, and significantly enhanced fracture toughness. As a result, treated devices retain 93% of their initial performance after 25 000 bending cycles, representing a new benchmark for flexible perovskite cells. In addition, improved film uniformity and interfacial contact enable power conversion efficiencies of 15.7% for R2R‐printed cells and 12.1% for modules (20.25 cm ² ), with scalability demonstrated up to 900 cm ² (17.26%). This work provides a scalable strategy to simultaneously enhance mechanical robustness and photovoltaic performance.