Lead-free coating improves heat stability and durability of flexible perovskite solar cells for portable and building-integrated applications.
(Nanowerk Spotlight) Thin, lightweight solar cells that can bend and flex open the door to power sources built into clothing, curved surfaces, and portable devices. But flexibility comes at a cost. Under strong sunlight, surfaces can heat up, and in flexible perovskite solar cells that heat can quickly damage the light-absorbing layer. Once the crystal structure begins to break down, performance drops and the device’s useful life shortens.
Perovskites are a family of materials known for their high light absorption and low-cost processing. They have shown promise for flexible photovoltaics. Yet their sensitivity to heat and environmental stress has slowed progress toward real-world use. Conventional stabilizing additives often contain lead or other toxic elements, raising environmental concerns.
Researchers from Nanjing University of Posts and Telecommunications and collaborators have found a way to make flexible perovskite solar cells more resistant to heat without using hazardous additives. Their method adds a thin, lead-free protective layer that strengthens the perovskite against thermal stress while keeping efficiency high.
The team uses potassium hexafluorotitanate, or K₂TiF₆, a compound containing potassium and titanium but no heavy metals. When applied to the perovskite surface, it bonds with the crystal and fills in defect sites — small imperfections that can trap charges and allow moisture or oxygen to penetrate. By passivating these defects, the coating makes the perovskite more stable under heat.
a) Infrared thermal imaging during the thermal annealing of the perovskite thin films. PL mapping images of the b) control and c) target perovskite thin films. d) Time-resolved PL decay spectra. Top-view SEM images of the e) control and f) target perovskite thin films. High-resolution TEM images of the g) control and h) target perovskite thin films. (Image: Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
Flexible perovskite films are typically prepared from solutions, which can leave behind solvent residues and uneven crystal edges. These imperfections can change under heat, disrupting the material’s structure. The K₂TiF₆ treatment reduces these weak points, keeping the crystal lattice intact for longer periods.
The researchers built flexible solar cells on polyethylene naphthalate, a plastic substrate that tolerates processing temperatures but can bend without damage. They compared untreated cells with those treated using K₂TiF₆. Both started with similar efficiencies, but after prolonged storage at 85 degrees Celsius, the treated devices retained much more of their initial performance.
Tests showed that the treatment also reduced ion migration — the movement of charged atoms or molecules within the perovskite layer. Ion migration can shift the internal structure and cause gradual performance loss. Measurements using X-ray diffraction and spectroscopy confirmed that the protective layer slowed these changes.
Mechanical flexibility was unaffected by the treatment. The coated devices endured repeated bending with minimal loss in output, meeting a core requirement for wearable and portable applications. Because the method relies on a simple coating step and a widely available, non-toxic compound, it could be integrated into scalable manufacturing.
The researchers note that their approach could be combined with other stabilizing strategies, such as encapsulation layers or mixed-cation perovskite compositions, to further extend device lifetimes. Using a benign additive also helps lower the environmental footprint of perovskite technology, which is a priority for commercial adoption.
This work highlights how targeted surface engineering can address one of the main weaknesses of flexible perovskite solar cells. By adding a thin, lead-free coating that protects against heat-induced degradation, the team has advanced the durability of a technology poised for applications ranging from rollable solar panels to integrated building materials.
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