Perovskites can help measure hard and soft x rays simultaneously

In modern medical x-ray imaging, x rays pass through a patient before hitting a flat semiconductor panel typically made of amorphous selenium. At the panel’s surface, the x-ray photons are then converted to electron–hole pairs, and the measured electric charges are proportional to the radiation intensity. (For more on x rays and medical imaging, see the article by John Rowlands and Safa Kasap, Physics Today, November 1997, page 24 , and the article by Sol Gruner, Physics Today, December 2012, page 29 .)

Detectors are often tuned to measure so-called hard x rays, with an energy of 10–100 keV. That’s ideal for imaging bones and other high-density materials but not, for example, malignant tumors or other low-density tissues. To better measure lower-energy x rays, researchers have studied dual detection methods that produce two images of the same object but at different x-ray energies. In practice, conventional amorphous selenium has its limits: Because of its weak attenuation of radiation energies beyond 30 keV, a detector requires a thick film of the material, which restricts its resolution and sensitivity to soft x rays.

By using perovskite materials, Babar Shabbir of Monash University in Australia and his colleagues built a large-area x-ray detector that avoided those limitations. The initial tests of their prototype show that the perovskites are 10³ to 10⁴ times as sensitive as amorphous selenium.

Perovskites are a type of material that takes the chemical form ABX₃, in which A and B are different cations and X is an anion. Because of the ease with which they can be manufactured, metal halide perovskites have been studied intently for their capabilities in solar-cell technology (for example, see Physics Today, May 2014, page 13 ). The affordability of perovskites has also attracted the attention of x-ray scientists.

In addition, halide perovskites have other properties that make them good materials for an x-ray detector. They include a high tolerance to defects, large bulk resistivity, and high density, which translate to a strong radiation-stopping ability. Some previous studies reported on early successes in using the material in detectors limited to measuring hard x rays.

The detector developed by Shabbir and his colleagues, however, works for a broad energy range, from 0.1 to 20 keV. After some initial optimization, they settled on a metal halide perovskite with a cesium–formamidinium cation and a lead anion, Cs_0.1FA_0.9PbI₃. The researchers sandwiched the perovskite between a gold electrode and an indium tin oxide electrode, creating a configuration that supports efficient charge collection and is similar to a high-efficiency perovskite solar cell.

The total thickness of the optimized perovskite-based x-ray detector is 800 nm, which is a compromise that allows for it to sufficiently measure hard and soft x rays. The sensitivity of typical amorphous-selenium detectors is about 20 µC Gy⁻¹ cm⁻², but the perovskite one is about 1000 times as sensitive for soft x rays and 10 000 times as sensitive for hard x rays. (B. Shabbir et al., Adv. Mater., 2023, doi:10.1002/adma.202210068.)