Halide perovskite single crystals (SCs) are gaining much attention owing to their optoelectronic properties. The ability to epitaxially integrate distinct halide perovskite SCs with pristine interface enables the creation of functional heterocrystals for both fundamental research and applied device designs. 

However, all those reported halide perovskite heterocrystals utilize Pb as core component, which raises concerns due to its toxicity. Thus, developing lead-free halide perovskite heterocrystal is challenging. 

In a study published in J. Am. Chem. Soc., a research group led by Prof. LUO Junhua from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences developed a high-quality centimeter-sized lead-free (BA)₂CsAgBiBr₇/Cs₂AgBiBr₆ heterocrystals for self-driven X-ray detection. 

By the solution-processed in-situ epitaxial method, the researchers obtained the 2D/3D heterocrystal as large as 10 × 7 × 6 mm³ with ultrahigh crystalline quality and a near-atomically sharp interface.  

They found that the (BA)₂CsAgBiBr₇/Cs₂AgBiBr₆ integration allows the formation of built-in electric potential in junction, which enables spontaneous charge separation/transport without external energy supply.  

Driven by the built-in electric field, detectors using the (BA)₂CsAgBiBr₇/Cs₂AgBiBr₆ heterocrystals exhibit outstanding X-ray response at zero bias with a high detecting sensitivity reaching up to 206 μC Gy^-1 cm^–2, an ultralow dark current, and a sizable on/off switching ratio. The sensitivity of 206 μC Gy^-1 cm^–2 significantly outperforms the Cs₂AgBiBr₆ SC X-ray detectors and is more than 10-times higher than that of the state-of-the-art commercial amorphous selenium X-ray detectors.  

Besides, owing to the self-driven operation mode, the present device shows long-term stable response current and negligible baseline drift under the continuous high-flux X-ray irradiation. 

This study not only widens the knowledge on chemistry involved in the synthesis of halide perovskite heterocrystals, but also creates new functionalities that are not accessible in the single-crystal system, which may enable optoelectronic devices beyond the conventional approaches.