Aphotoelectrochemical “artificial leaf” device with perovskite electrodes uses sunlight to produce syngas, a mixture of hydrogen and carbon monoxide, from water and carbon dioxide. “This demonstration is a first step to develop solar technologies that may enable a circular carbon economy in the long term,” says University of Cambridge chemistry professor Erwin Reisner, who led the study reported in Nature Materials (doi:10.1038/s41563-019-0501-6).

Syngas, which is widely used to produce fuels, chemicals, and fertilizers, is typically made by reforming fossil fuels. Solar syngas production is a cleaner alternative.

An artificial leaf device with perovskite and bismuth vanadate electrodes produces syngas from water and carbon dioxide. Oxygen evolution occurs at the front BiVO₄ anode, while a perovskite cathode reduces CO₂ and protons to CO and H₂. WOC, water oxidation catalyst; FTO, fluorine-doped tin oxide; PCBM, [6,6]-phenyl-C₆₁-butyric acid methyl ester; PEI, polyethylenimine; FM, field’s metal. Credit: Nature Materials.

The new device contains a bismuth vanadate anode and a cesium formam-idinium methylammonium mixed-halide perovskite cathode, with special cobalt catalysts in each. When the device is immersed in water, the anode produces oxygen, and the perovskite cathode reduces CO₂ and protons into CO and H₂, respectively. The perovskite cathodes even reduce aqueous CO₂ for one day at light intensities as low as 0.1 Sun, which means the device would work on cloudy days. The perovskite’s high-efficiency sunlight absorption “ultimately allows catalysis with good performance,” Reisner says.

Plants take up lead from perovskites in the ground 10 times more effectively than other contamination sources, a study in Nature Communications shows (doi: 10.1038/s41467-019-13910-y). The authors call for a need to reassess the safety levels of lead in perovskite solar cells.

Lead content in perovskite modules is typically less than 0.1% by weight, which is the safety limit imposed by many countries. To find out if it can still have an environmental and health impact, Jian Lü of Fujian Agriculture and Forestry University, Antonio Abate of Helmholtz-Zentrum Berlin für Materialien und Energie and colleagues grew mint, chili, and cabbage—which have high, medium, and low lead accumulation ability, respectively—in perovskite-laced soil.

Natural soil samples had a lead level of 36 mg/kg, in line with agricultural lands worldwide. The team added 5 mg/kg of lead to simulate a solar module leaching its entire perovskite content into the ground. Increasing lead concentration by only 10% boosted the lead measured in plants by more than 100%. All the plants blackened and rotted with larger perovskite amounts, which added 250 mg/kg lead to the soil.

The researchers recommend keeping the safety level for lead in perovskites lower than that in other lead-containing electronics. They also “encourage replacing lead completely with more environment-friendly metals to deliver safe perovskite technologies.”

Researchers report a new strategy to make high-quality perovskite quantum dots (QDs), giving QD solar cells with a record certified power-conversion efficiency of 16.6%. Perovskite QD solar cells could be more stable than their thin-film counterparts, but making high-quality QDs with desirable optoelectronic properties has been a challenge.

“This substantial improvement of efficiency and stability paves the way for addressing high-value applications in niche markets,” says Yang Bai of The University of Queensland in Australia and an author of the Nature Energy paper (doi:10.1038/s41560-019-0535-7).

Bai and colleagues made cesium and formamidinium lead triiodide perovskites (Cs_1–xFA_xPbI₃) QDs by first preparing CsPbI₃ and FAPbI₃ QDs using a modified hot-injection method. Before mixing them together, they intentionally retained excessive oleic acid (OA) ligands in the parent CsPbI₃ and FAPbI₃ QD solutions, forming an OA-rich environment. The surface ligands promoted a cation exchange reaction of the Cs and FA cation and suppressed surface defects, boosting efficiency.

The best material, Cs_0.5FA_0.5PbI₃, gave solar cells a remarkable power-conversion efficiency of 16.6% and negligible hysteresis. The cell retained 94% of its efficiency under continuous 1-Sun illumination for 600 hours, a stability comparable to that of thin-film materials.

Infrared (IR) light-emitting diodes (LEDs) used in night vision, optical communications, and medical applications are expensive. Embedding lead sulfide QDs into perovskite films could give low-cost, efficient IR LEDs. These devices combine the tunability of QDs to precise wavelength emissions with excellent charge-transport properties of perovskites. But the QDs tend to aggregate, causing inhomogeneity because of imbalanced charge accumulation.

To overcome these issues, a team led by Jiang Tang of the Huazhong University of Science & Technology and Edward Sargent of the University of Toronto turned to low-dimension layered perovskites as a matrix for QDs. They altered the surface of the QDs with perovskite cations, which caused them to disperse evenly through the matrix.

In the LEDs reported in NaturePhotonics (doi:10.1038/s41566-019-0577-1),energy flowed in the form of excitons, tightly bound electron–hole pairs that traveled together, from the perovskite into the QDs. The devices exhibited a high external quantum efficiency of 8.1% at 980 nm at a radiance of up to 7.4 W Sr⁻¹m⁻².