Adding potassium to perovskite films can increase the efficiency of solar cells, shows new research published inNature (doi:10.1038/nature25989).

Perovskite solar cells suffer from nonradiative losses: the recombination of light-generated charge carriers without generating a photon, when carriers get trapped at crystal defects. Efficiency increases when more carriers recombine radiatively, producing a photon that can again generate charge carriers. Another challenge is the movement of ions in the material, which creates spatial distribution of bandgaps and reduces the solar-cell performance.

Atomic-scale view of perovskite crystal formation with added potassium. Credit: Matthew Klug (University of Oxford).

An international team of researchers led by Samuel Stranks at the University of Cambridge added potassium iodide to a precursor solution for a cesium-formamidinium-methylammonium lead halide perovskite thin film. The potassium iodide forms a layer on the surface and at the grain boundaries of the perovskite, healing the traps and preventing ions from moving.

The films had a very high luminescence yield—important to maximize efficiency—exceeding 95%, as well as excellent charge transport, with mobilities of more than 40 cm²/V∙s. Solar cells made with the films had an efficiency of 21.5%.

The presence of lead in high-performance perovskites raises serious concern. Unfortunately, lead-free perovskites based on tin halide compounds have come up short in terms of efficiency and stability. A new type of three-dimensional (3D) hollow perovskite might be the answer, say Northwestern University researchers.

Mercouri Kanatzidis and his colleagues first reported the hollow hybrid halide perovskites in 2017. They have now used a suite of physical and spectroscopic methods to study the material’s chemical nature and structural properties. Their analysis appears in the Journal of the American Chemical Society (doi:10.1021/jacs.8b01034).

Hollow perovskites incorporate the ethylenediammonium (en) cation into 3D ASnX₃ perovskites. Using a range of techniques such as x-ray diffraction, H-NMR (hydrogen-1 nuclear magnetic resonance), and gas pycnometry, researchers confirmed that the en cation creates large vacancies in the metal-halide framework. Density functional theory calculations showed that this disruption leads to a widening of the material’s bandgap, which is important for tandem solar cells in which perovskites sit atop silicon cells. The en cation also makes the material more stable in air and improves its photoelectric properties.

Hollow perovskites present “a new platform of highly promising light absorbers that can be utilized in single junction or tandem solar cells,” the authors say.

Just as for humans, allowing perovskites to soak up sun helps them to relax and makes them more efficient. Constantly illuminating a triple-cation hybrid perovskite thin film expands its crystal lattice, which relaxes strain, researchers report in Science (doi: 10.1126/science.aap8671). This, in turn, aligns the material’s crystal planes and repairs defects, as well as lowers the energy barrier at the perovskite-contact interface, improving the material’s power-conversion efficiency.

Recent studies have shown that light-induced structural changes play an important role in the optoelectronic properties and stability of devices. But such studies on mixed-cation halide perovskites are lacking. Wanyi Nie, Aditya Mohite, and their colleagues at Los Alamos National Laboratory illuminated formamidinium-methylammonium cesium lead iodide perovskite thin films using a standard 1-sun source for 180 minutes.

The efficiency of solar cells made with the films went up from 18.5% to 20.5%. Light soaking did not compromise the cells’ stability: they worked with minimal degradation for 1500 hours under standard full-spectrum solar illumination.

While halide perovskites are making strides toward commercial devices, their inorganic perovskite oxide counterparts have also attracted attention for solar cells. Devices made from these ferroelectric perovskites are highly stable and have high open-circuit voltages, but they have shown limited efficiencies.

In a Nature Photonics (doi:10.1038/s41566-018-0137-0) paper, Canadian researchers demonstrate an alternative route to making high-performance solar cells from perovskite oxides. They made a device from a composite bismuth-manganese-oxide thin film with two different crystal phases—BiMnO₃ and BiMn₂O₅—which had a power-conversion efficiency of around 4.2%. Inorganic perovskite oxides are ferroelectric. They do not conduct charge carriers well, leading to low photocurrent density and hence low efficiency. But in the mixed material, ferroelectric BiMnO₃ grains are incorporated into semiconducting BiMnO₅, which boosts photocurrent density.