One of the biggest hurdles perovskite solar cells still face is their ability to weather moisture and ultraviolet rays.
Using a novel method to synthesize a lanthanum-doped barium tin oxide (LBSO) electrode, Korean researchers have made solar cells that are highly stable under light exposure and boast the highest reported efficiency for devices made of the perovskite methylammonium lead iodide (CH3NH3PbI3). Devices reported in a recent issue of Science (doi:10.1126/science.aam6620) demonstrate an efficiency of 21.2% and were stable for 1000 hours under full sun.
Efficient perovskite solar cells typically use titanium oxide as electron-transporting layers. However, titanium oxide reduces the stability of the devices under light. LBSO is an ideal replacement because it has a high electron-conveying ability and crystalline structure compatible with perovskites. But the material crystallizes at temperatures over 1000°C, making it hard to apply on glass and plastic surfaces.
(a) Structure of BaSnO3. Green, gray, and red spheres indicate Ba, Sn, and O ions, respectively. (b) Perovskite solar-cell performance with lanthanum-doped barium tin oxide (LBSO). Adapted with permission from Science 356, 167 (2017).
Sang-Il Seok, at the Ulsan National Institute of Science and Technology, and his colleagues came up with a solution-based route to prepare a LBSO electrode at temperatures below 300°C. They first made a colloidal solution of LBSO nanoparticles by the reaction of BaCl2, SnCl2, La(NO3)3, and H2O2 in an NH4OH aqueous solution at 50°C. LBSO perovskite crystals developed from the colloidal solution, which the researchers coated on a fluorine-doped tin oxide substrate to create the electrode.
A research team led by Zachary Holman of Arizona State University and Michael McGehee of Stanford University has made tandem perovskite/silicon solar cells with a record-high efficiency of 26.3% that are also highly stable.
Silicon/perovskite tandem cells typically contain perovskite layers deposited on top of crystalline silicon. They suffer from low stability as well as unwanted light absorption in the top window layer through which light enters the solar cell. This light absorption reduces efficiency, so the window layer needs to be highly transparent. Indium tin oxide (ITO) is the ideal transparent electrode used in displays and solar cells, but the sputtering method used to deposit it can damage perovskite layers.
As reported recently in Nature Energy (doi:10.1038/nenergy.2017.9), the researchers used a bottom silicon cell tuned to absorb infrared light. The top cell was made of the perovskite cesium formamidinium lead halide [Cs0.17FA0.83Pb(Br0.17I0.83)3]. This stable perovskite allows researchers to deposit a tin oxide buffer layer, on top of which they could sputter-deposit an ITO electrode. The performance of the highly efficient devices did not degrade for more than 1000 hours of testing at 85°C and 85% relative humidity.
Understanding the mechanism of perovskite degradation is key to designing more stable perovskite solar cells. In a recent Nature Communications (doi:10.1038/ncomms15218) study, researchers reported insights on why methylammonium lead iodide perovskites undergo degradation when exposed to oxygen and light.
The team from Imperial College London and the University of Bath previously found that in the presence of light, oxygen molecules scavenge photogenerated electrons to form highly reactive superoxide species, which can quickly degrade CH3NH3PbI3.
For the study, they combined experimental and computational methods for a microscopic assessment of the mechanism. They exposed a 500-nm-thick CH3NH3PbI3 film to dry air for 20 minutes while recording its weight. The film saturated with oxygen within 10 minutes. Time-of-flight mass spectrometry showed the gas was uniformly distributed in the film.
Films made of 100 nm crystals had much more superoxide yield than those with larger 250 nm crystals, so they degraded within two days as opposed to nine for the latter. Further, ab initio simulations showed that vacant iodine sites acted as traps for oxygen molecules and electrons, facilitating superoxide formation.
When the researchers coated the perovskite film with iodide salts, the films remained stable for more than three weeks of oxygen and light exposure, because the salts filled iodide defects in the film, which suppressed the superoxides.
An ink developed for perovskite thin films reported recently in Nature Energy (doi:10.1038/nenergy.2017.38) should allow the manufacture of perovskite films on large areas at high volumes, important for producing solar photovoltaic modules commercially.
Most researchers make perovskite films using spin coating, which involves depositing a precursor solution on a fast-spinning substrate, evaporating the solvent, and then heating the film. This is hard to accomplish quickly on a large scale. Antisolvents are typically added during spin coating to rapidly saturate the perovskite crystals out of the solution, which gives uniform films. But the window for this processing step is only a few seconds, making it difficult for large-scale manufacturing.
Kai Zhu and Maikel van Hest, at the National Renewable Energy Laboratory, formulated an ink of CH3NH3I and PbI2 in a N-methyl-2-pyrrolidinone: N,N-dimethylformamide solvent and added an excess of methylammonium chloride.
The ink can be deposited on a substrate using blade coating, an easily scalable processing method used to make large-area films on rigid or flexible surfaces in which a blade spreads liquid on a moving substrate. Instead of seconds, the precursor ink film can be processed for up to eight minutes. Further, it only needs a minute of heat treatment. All of this would be attractive for manufacturing. The researchers used the process to make 1.2 cm2 cells that had an efficiency of 17.33%.
