Development of Gradient Composition Lead Halide Perovskite Single Crystals for Scintillation

In recent years, the demand for fast, high-quality radiographic imaging and sensing technology has been steadily increasing across diverse fields such as medical diagnostics, security, and research applications, including Time-of-Flight Positron Emission Tomography (TOFPET). However, current radiographic imaging technology relies on expensive and environmentally sensitive single crystal scintillators grown from melts in high temperature furnaces. Metal halide perovskites (MHPs) are an emerging class of semiconductors that offer promising alternatives to current scintillator materials due to their ease of fabrication, low cost, and desirable optoelectronic properties. However, self-absorption, or reabsorption of emissions from within the crystal, is a key issue facing MHP single crystal scintillators due to the increased fraction of nonradiative recombination. This work approaches the problem of self-absorption by utilizing the unique property of compositional flexibility in MHPs to create a halide gradient heterojunction within a perovskite single crystal. This gradient is achieved using inverse temperature crystallization, an already accepted solution-based growth method for MHPs, to create a heterojunction between CH₃NH₃(MA)PbBr₃ and CH₃NH₃(MA)PbCl₃. We propose that the gradient in composition causes a gradient in band structure, allowing emissions to pass through the bulk of the crystal without being reabsorbed, as well as enabling carriers funneling from higher to lower band gaps. These factors lead to enhanced efficiencies compared to bulk single crystal scintillators.