Perovskites, a mineral that has a chemical composition in the form ABX3 and a structure similar to CaTiO3, have been of particular interest due to their application in solar electricity generation, telecommunication and LEDs. Mixed Halide Perovskites (MHPs), perovskites that contain a halide atom, like iodine or bromine, are particularly interesting for their economic viability and unique electrical properties. Like highly tunable colour palettes, the colours of light MHPs absorb and emit can be precisely controlled, simply by mixing different halogen elements (like iodine and bromine) within their crystal structure. This makes them ideal candidates for creating ultra-efficient tandem solar cells, where a stack of different layers of solar cells is used to capture more sunlight, and vibrant optoelectronic devices like LEDs.
However, these wonder materials have a significant Achilles' heel: when continuously exposed to light or electrical current, the carefully mixed halogens tend to separate, like oil and water. This phase segregation creates unwanted iodine-rich and bromine-rich zones, degrading the material's performance and stability, and ultimately limiting its applications. Researchers have been searching for ways to keep the mixture mixed, essentially finding a way to stabilise the perovskite structure under stress.
Now, a team from the Indian Institute of Technology (IIT) Kharagpur, Polish Center for Technology Development, Poland and North Carolina State University, USA, has demonstrated an effective solution to address the phase segregation and the resulting degradation.
They focused on a specific MHP, Cesium Lead Iodide Bromide (CsPbI₁․₅Br₁․₅), known to be prone to this light-induced segregation. They watched this segregation happen in real-time using photoluminescence (PL) spectroscopy, a technique that measures the light emitted by a material when illuminated. When they shone a continuous ultraviolet laser at 325 nanometers onto the MHP film, its initial single, sharp emission peak quickly broadened and split into two distinct peaks. This indicated the formation of separate bromine-rich areas emitting greener light and iodine-rich areas emitting redder light. They captured this colour change happening live on video.
The researchers' remedy involved treating the MHP film with a chemical called 1-dodecanethiol (DSH). DSH is a type of molecule known as a ligand, containing a sulfur atom that likes to bind to the lead atoms in the perovskite, like applying a protective chemical coating at the nanoscale. The results were striking. When the DSH-treated film was subjected to the same intense laser illumination, it remained remarkably stable. Its PL spectrum showed almost no change for nearly four minutes, compared to the immediate splitting seen in the untreated film. Only after four minutes did a slight shoulder peak indicating minor bromine-rich domain formation appear. The real-time video confirmed this stability, showing a consistent orange emission without the dramatic colour separation seen previously. Interestingly, the researchers also noted that even in the untreated samples, letting the film sit in the dark after illumination allowed the separated phases to remix and the original PL signal to mostly recover, demonstrating a reversible aspect to the segregation.
A simple DSH treatment is so effective because of the crystal's defects. Perovskite crystals, like everything else, aren't perfect; they often have missing atoms, particularly halide ions (iodine or bromine), creating what scientists call halide vacancies. These vacancies act like tiny pathways or escape routes, allowing other halide ions to move around within the crystal structure when energised by light. This ion migration is the root cause of phase segregation.
The DSH molecule, with its sulfur atom, acts like a molecular patch. It preferentially binds to the lead atoms at these vacancy sites on the perovskite surface. By effectively plugging these vacancy defects, DSH blocks the pathways for ion migration. To confirm this, the team performed computer simulations using density functional theory (DFT), a well-known mathematical tool for finding electron structures. These calculations showed that DSH binding to halide vacancies is energetically favourable and alters the material's electronic structure in a way that stabilises it, increasing the energy barrier required for ions to move and thus suppressing segregation.
The work significantly advances efforts to stabilise MHPs. While previous studies identified the problem and explored various strategies like modifying the perovskite composition, this research offers a relatively straightforward post-treatment method using a common ligand (DSH) that drastically enhances stability under illumination for a specific, problematic MHP composition. It directly tackles the defect-mediated ion migration pathway. While the DSH treatment dramatically improved stability for up to four minutes under the high-power laser used in the study, some minor segregation did eventually occur, indicating that while suppressed, the process isn't wholly eliminated under such harsh conditions. Further testing would be needed to assess long-term stability under more realistic operating conditions found in solar cells or LEDs.
Nevertheless, the success of DSH passivation in preventing light-induced phase segregation is a crucial step forward. By significantly enhancing the stability of these highly tunable materials, the research opens the door for their practical use in next-generation technologies. It brings us closer to realising the potential of mixed halide perovskites for creating more efficient and potentially cheaper tandem solar cells, which could boost the adoption of solar energy, as well as enabling more vibrant and energy-efficient LEDs for lighting and displays. This molecular patch approach could be a key ingredient in unlocking the power of perovskites.
This research article was written with the help of generative AI and edited by an editor at Research Matters.