The black-phase cesium lead iodide (CsPbI₃) perovskite is a highly promising material for next-generation optoelectronic devices due to its optimal bandgap, high carrier mobility, and excellent light absorption properties. However, its practical application remains significantly limited by its structural instability under ambient conditions, where it tends to rapidly convert into the non-perovskite yellow δ-phase. Various strategies have been explored to stabilize the black phase, including A-site and X-site doping, surface passivation, laser writing, and strain engineering. While these approaches have shown partial success, they also present significant limitations. A-site doping often involves volatile organic cations, which can introduce chemical instability [1]. X-site halide substitution (e.g., Br^-, Cl^-) tends to alter the bandgap [2], which is undesirable for certain optoelectronic applications. Additionally, methods such as surface passivation [3] and laser-induced [4] stabilization can create by-products or defects that impair charge transport, limiting device performance.

Among the emerging approaches, strain engineering has demonstrated significant potential for stabilizing the black phase. Interface strain, typically induced by annealing thin films on substrates with differing thermal expansion coefficients, has been shown to suppress the phase transition to the yellow phase [5]. However, this effect is confined to the interface region and effective only in ultrathin films (<100 nm), which are insufficient for high-performance optoelectronic devices like solar cells and photodetectors that require thicker active layers (300–500 nm).

In this work, we propose a novel approach to induce bulk-localized nanoscale strain through partial B-site substitution in CsPbI₃. By incorporating a small amount of dopant cations at the Pb²⁺ site, we introduce localized lattice distortions throughout the bulk of the film. This nanoscale strain reduces the spontaneous orthorhombic distortion of the crystal lattice and increases the Pb–I–Pb bond angle, both of which are critical factors in stabilizing the black perovskite phase. Unlike interface-limited strain methods, our strategy enables stabilization throughout the entire film thickness, thereby overcoming the primary limitation of previous strain engineering techniques.

We demonstrate that the optimized B-site substitution by Sn²⁺, Bi³⁺, and Cd²⁺ (up to 20%) does not significantly alter the optical bandgap of CsPbI₃, preserving its desirable optoelectronic properties. Furthermore, this method enhances phase stability under ambient conditions without compromising film quality or charge transport. Devices fabricated using this strain-stabilized CsPbI₃ exhibit superior performance, as evidenced by enhanced photodetector metrics, including higher responsivity, faster response times, and improved operational stability.

This study establishes a alternative route to achieving long-term black-phase stability in thicker perovskite films by leveraging atomic-level strain engineering via targeted B-site doping. Our findings provide a pathway toward the practical deployment of CsPbI₃-based optoelectronic devices with improved reliability and performance.