Mineral perovskite is shown in the foreground, while behind it is a fluorescence image of a thin film of perovskite. The image shows film areas that received more light became more purified, as revealed by brighter fluorescence. Credit: MIT News
In a classic science-fiction trope, a doctor beams energy onto a patient to heal an injury. In reality, researchers from several U.S. and British universities have used bright light to fix some defects in a type of perovskite film that has many promising applications in optoelectronics (Nat. Commun., doi:10.1038/ncomms11683).
Organic-inorganic metal halide perovskites such as methylammonium lead iodide (CH3NH3PbI3) could become highly efficient solar cells, but scientists must first understand the materials' shortcomings. For example, the photoluminescent properties of CH3NH3PbI3 vary from grain to grain due to the migration of ions. These tiny defects, or traps, can interfere with predictable device performance.
Scientists based at the universities of Washington, Oxford and Cambridge, and the Massachusetts Institute of Technology first measured the bulk increase in photoluminescence, due to light from a 507-nm-wavelength pulsed laser, of a thin film of the perovskite material on a glass substrate. Next, to see how the microstructure of the film correlated with its optical properties, the team compared the scanning electron microscope images of the perovskite with measurements of its photoluminescence.
The researchers then exposed the film to intense simulated sunlight and studied the aftereffects. According to the team, the light causes the negatively charged iodide ions to redistribute themselves in the crystal to reduce the number of traps, and this “photo-induced cleaning” effect was particularly strong in the regions of the film that had been dimmer in the original pulsed-laser probe.
In the future, the authors suggest, scientists should strive to create perovskite films with uniform iodide distributions and low trap densities for stable optoelectronic performance within devices.