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Unravelling the problem of perovskite instability

ENERGY.GOV High Flux Beam Reactor Public Domain

Perovskite solar cells are a very promising technology as their performance has increased rapidly over the past years and currently exceeds 20%. It is also possible to produce them more cost-effectively than commercial solar cells. However, so far science has not been able to solve one of the main problems of perovskite cells: poor stability, which is caused by the perovskite layer when it comes into contact with water.

Now (2020), scientists at Brookhaven Lab have come one step closer to unravelling the problem that lies at the bottom of perovskite instability. With the help of single crystal X-ray diffraction, researchers discovered that the main reason for the thermodynamic instability in the halide perovskite caesium lead iodide (CsPbI3) was the inorganic caesium atom and its unstable behaviour in the crystal structure, which occupied a single site within the structure at temperatures below 150K and split into two sites above 175K. Furthermore, the scientists also found that the low number of caesium-iodine contacts within the crystal and the high degree of local octahedral distortion also contributed to the instability. To resolve this issue they replaced the organic cation with inorganic caesium, which they found to be less volatile. However, unlike methylammonium lead iodide, the perovskite phase of caesium lead iodide was found to be metastable at room temperature. Pair distribution function measurements revealed that on the length scale of the unit cell, the Pb–I octahedra also became greatly distorted, with one of the I–Pb–I angles approaching 82° and not the ideal 90°. All in all, the research yielded the result that the “rattling” of Cs, low number of Cs–I contacts, and high degree of octahedral distortion was what caused the instability of perovskite‐phase CsPbI3.

Scientists have long sought to tackle the problem of perovskite instability. In 2016, researchers found that rapid light–induced free-radical polymerization at ambient temperature generated multifunctional fluorinated photopolymer coatings that endowed the front side of solar devices with luminescent and easy-cleaning features, while at the same time forming a strong hydrophobic barrier against environmental moisture on the back contact side. The luminescent photopolymers could re-emit ultraviolet light in the visible range and boost perovskite solar cells efficiency to nearly 19% under standard illumination. The coated devices kept their full functional performance during prolonged operation, even after a series of severe aging tests carried out for more than 6 months.

In 2018, scientists invented the first Post-Device Ligand (PDL) treatment to improve the solar-to-power conversion efficiency (PCE) of perovskite solar cells from 18.7% to 20.13%. At the same time, the stability of the treated devices without any encapsulation also improved, with a PCE of 70% under ambient conditions after a 500-hour maximum-power-point tracking test. Furthermore, this post-device treatment exhibited a so-called healing effect, namely by mitigating the defects of the perovskite active region. Their approach greatly improved the production yield of high-quality PVSCs and their module performances as well as facilitating the reduction of lead-waste. Additionally, the treatment was a post-device approach that could be integrated into any existing perovskite device and offered a general strategy to improve the stability and performance of perovskite optoelectronic devices.

The advantages of the new measurement methods are that it presented the scientists with the opportunity to analyse what was happening at the atomic scale, as well as being able to have a look at material behaviour in general. The research also yielded detailed structural information and suggested methods to stabilize the perovskite phase of CsPbI3 in order improve the stability of cell structure. It also allowed for greater insight into the limitations of tolerance factor models concerning stability estimates for halide perovskites.

With problems concerning perovskite instability removed and their power conversion efficiencies currently reaching 25.2%, perovskite solar cells might finally become suitable for large-scale energy production.