Materials scientists at the University of Oxford and Stanford University report high-performance perovskite–perovskite tandem cells using efficient and stable lead- and tin-based perovskites.

So far, researchers have made tandem cells by putting perovskite cells, which have a wide energy bandgap, on top of low-bandgap silicon cells. Perovskite-only tandem cells would cost less and be easier to produce.

The research team led by Henry Snaith (Oxford) and Michael McGehee (Stanford) used FA_0.75Cs_0.25Pb_0.5Sn_0.5I₃ as the low-bandgap (1.22 eV) and FA_0.83Cs_0.17Pb (I_0.5Br_0.5)₃ (1.8 eV) as the high-bandgap perovskite. By coupling solar cells made with these two materials, the researchers demonstrated a monolithic two-terminal all-perovskite tandem solar cell (see Figure) with a power-conversion efficiency of 17% and a four-terminal all-perovskite solar cell with an efficiency of 20.3%. The researchers reported their findings in Science (doi:10.1126/science.aaf9717).

(a) Scanning electron micrograph of the two-terminal (2T) all-perovskite solar cell with the schematic for the monolithic tandem cell operation; (b) current–voltage curves for the single junction and tandem solar cells. Credit: Pabitra Nayak.

Thin films of lead halide perovskites are remarkably efficient at converting light to electricity, even when they have defects. A research group has now uncovered the secret behind this trait. Charge carriers in hybrid organic–inorganic perovskites, they argue, acquire a dynamic cloaking screen that allows them to travel through the material without colliding with defects.

Researchers led by Xiaoyang Zhu of Columbia University compared charge dynamics in lead bromide perovskites with three different cations: organic methylammonium and formamidin-ium, and inorganic cesium. They made single-crystal samples of the materials and conducted time-resolved photoluminescence and time-resolved optical Kerr effect spectroscopy measurements.

Organic cations in the hybrid perovskites are dipoles that rotate freely as in a liquid. The dynamic reorientation of these dipoles creates an effective cloaking field for charged particles, keeping them from scattering by defect centers or optical phonon modes. The researchers report their work in Science (doi:10.1126/science.aaf9570).

University of Oxford researchers have shed light on how perovskites rapidly form macroscopic single crystals at high temperatures from solution. Using this knowledge, they were able to produce higher quality single crystals than ones made so far using the rapid crystallization technique.

Henry Snaith and his colleagues report in Nature Communications (doi:10.1038/ncomms13303) that perovskite crystallization is triggered by a change in the acid–base equilibrium of the solvent, which raises the concentration of the solute and results in the perovskite’s quick saturation out of the solution as crystals. Understanding the factors that influence and control crystallization is key to making high-performance perovskite optoelectronic devices.

Increasing the stability of perovskites is another key requirement for their commercial success. Michael Grätzel and co-workers at the École Polytechnique Fédérale de Lausanne, Switzerland have now made highly stable, efficient solar cells by integrating rubidium into lead-halide perovskite films.

The resulting solar cells have a power-conversion efficiency of 21.6%. They maintain 95% of their initial performance over 500 continuous hours under full sunlight at 85°C.

Earlier this year, the researchers reported a triple-cation (methylammonium, formamidinium, and cesium) perovskite, yielding a solar efficiency of 21.2% that remained stable for 250 hours. The new advance published in Science (doi:10.1126/science.aah5557) takes that work a step further. This time they added rubidium to the mix. The rubidium cations may help relax lattice strain, giving a more defect-free crystal, says the lead author Michael Saliba.

Researchers have used perovskite quantum dots in solar cells to get a relatively high power-conversion efficiency of 10.77%. This is comparable to efficiencies of quantum dot solar cells made of other materials, and higher than that of other reported all-inorganic perovskite solar cells.

Quantum dots are nanocrystals of semiconductor materials. The researchers, led by Joseph M. Luther at the National Renewable Energy Laboratory, made a thin film of nanocrystals of the perovskite cesium lead iodide (CsPbI₃) with good electronic coupling among the quantum dots. Low-bandgap all-inorganic perovskites such as CsPbI₃ were thought to be stable only at temperatures over 600°F. But the team discovered a method to keep nanocrystals of the material stable at room temperature, which they detail in Science (doi:10.1126/science.aag2700). They first mixed a Cs-oleate solution with a PbI₂ precursor. They then purified the nanocrystals using methyl acetate as an anti-solvent that removed excess unreacted precursors, which turned out to be critical to increasing their stability.