Understanding and improving the tolerance of perovskites

Perovskite solar cells

In the quest for future high-efficient electronic devices, perovskites have emerged as the most promising material candidates by far. The main reason for this is their inbuilt tolerance towards material defects. This favourable property, however, is not consistent, but hampered at boundaries between different crystallites in the material. Recently, researchers at the University of Ferrara investigated the behaviour of defects in perovskites with both experiments and simulations using PRACE resources and provided valuable insights for the fabrication of future perovskite devices.

Perovskites are peculiar materials: Because of a special characteristic of their electron orbitals, they are strongly tolerant towards defects — meaning that they can contain a lot of defects without losing their excellent optical and electronic properties. This defect tolerance makes it possible to fabricate perovskite-based electronic devices with simple and cost-efficient procedures, which is the main reason why perovskites are considered such promising key materials for future diodes, photodetectors and, especially, high efficiency solar cells. In fact, in the comparatively short time these materials have been investigated, scientists were able to create perovskite solar cells with lab efficiencies above 25%, quickly exceeding those of the much longer studied silicon solar cells.

“However, the inbuilt defect tolerance of perovskites is not consistent throughout the material”, says Simone Meloni, an Assistant Professor in Chemical Sciences at the University of Ferrara. In general, a perovskite film to be used in a solar cell or any other electronic device will consist of several crystalline grains arising in production, and it became apparent to Meloni and other researchers that the boundaries between these different grains play a determining role in defect tolerance. How exactly the defects at these grain boundaries behave, however, and to what effect, was not well understood. Scientific results were contradictory: According to some indications, as defects travelled through the material, they also managed to pass through grain boundaries, which would lead to a continuously favourable defect tolerance. “Other findings led to the opposite assumption that defects get stuck and accumulate at the boundaries which would make them detrimental to the material’s properties,” Meloni explains.

Perovskite solar cells

By now, perovskite solar cells show lab efficiencies above 25%, exceeding those of silicon solar cells.
Image source: Wikimedia Commons / Stanford ENERGY, Video by Mark Shwartz

Clarifying contradictory assumptions

To figure out what was really going on, the chemist studied the behaviour of defects in the vicinity of grain boundaries more closely. On the one hand, he and his co-workers used photoluminescence (PL) microscopy experiments to investigate lab-made samples of the archetypal perovskite methylammonium lead iodide (MAPbI3). Such PL microscopy measurements can be used to locally perturb a material with a light beam and thereby introduce defects. Simultaneously, the scientists can observe the dynamics and effects of these newly created defects — in particular, when and where they quench the material’s electronic properties.

On the other hand, Meloni performed molecular dynamics simulations using PRACE supercomputing resources to investigate the defect dynamics at grain boundaries. He created computational equivalents of different MaPbI3 grain boundaries and probed different kinds of ionic defects, such as the vacancy of an iodide-ion, which is a positively charged defect, or the presence of an additional iodide, which makes for a negatively charged defect.

CH3NH3PbI3 structure

The Structure of the probed perovskite material methylammonium lead iodide CH3NH3PbI3 (MAPbI3): a methylammonium ion (in the centre) is surrounded by octahedra of six iodides (purple) around a lead ion.
Image credit: Wikimedia commons, Christopher Eames et al.

With these simulations, Meloni and his colleagues showed that defects are, in fact, accumulating at the grain boundaries. And that the cause of this accumulation are the electrostatic forces resulting from net electric charges present at grain boundaries, as they create energy minima from which the defects can hardly escape. “This result was rather unexpected, since the chemical composition of grain boundaries suggested that they should be neutral so that structural strains and resulting strain fields were hypothesized to be the likely cause,” Meloni explains.

A graph showing energy minima at grain boundaries

Energy profile and entrapment: A crystal grain with two boundaries marked PbI (left) and MAI (right) features an absolute energy minimum (green column) and a local energy minimum (blue column) that both capture ionic defects. This leads to a reduced defect migration throughout the material and therefore to impaired optical and electronic properties.
Image credit: Phung et al. Adv. Energy Mater. (2020)

A tsunami causing no harm

The practical consequence of this hindering behaviour became visible in the PL experiments. Here, Meloni and his colleagues compared two perovskite samples: a film that contained many small crystalline grains — and therefore many grain boundaries — and a sample consisting of one single crystal — and therefore no grain boundary at all. The two samples behaved quite differently in the experiments: In the single crystal, the light-induced defects rapidly spread out — “like a tsunami,” comments Meloni — and quenched the photoluminescence properties in large parts of the material. But already after 30 minutes, the material had recovered from this perturbation and its properties had returned to the original level.

In contrast, in the poly-crystalline film, the light-induced defects did not propagate and remained confined within the crystal grains in which they were produced. But they were also strongly persistent: In the 12 hours the scientists observed the material, the photoluminescence properties hardly recovered at all. “Perovskite films to be used in solar cells should, therefore, not consist of too many crystalline grains to avoid the grain boundary’s unfavourable effect,” concludes Meloni.

photoluminescence images showing the effect of grain boundaries

The photoluminescence (PL) images demonstrate the effect of grain boundaries: The sample in the upper row (a) made of one single crystal with no grain boundaries recovers within 30 to 60 minutes after light-induced perturbation. Whereas the multi-crystalline sample containing several grain boundaries in the lower row (b), has only partially recovered even after 12 hours.
Image credit: Phung et al. Adv. Energy Mater. (2020)

Perovskites in the mist

In addition, grain boundaries within the material may influence another vital property, namely its stability against environmental stresses like moisture. In fact, some of the most efficient and well-studied perovskites are strongly susceptible to humidity and have to be covered by a protective glass layer in practice. “Ongoing research efforts look for ways to dispose of this additional layer by creating an inbuilt protection on the molecular level of the materials,” says Meloni. “For this, we need to better understand where the materials’ vulnerability against humidity comes from.”

Curiously enough, fabricating perovskite films in a humid environment can sometimes even enhance their efficiency, as has been shown for the prototypical perovskite MAPbI3. In his respect, Meloni and his colleagues recently investigated another perovskite, CsPbBr3. “This all-inorganic perovskite is expected to tolerate humidity better than other, organic compounds,” explains Meloni. In a set of experiments, he and his co-workers discovered that a limited and controlled use of humidity can indeed enhance CsPbBr3 films. An exposure to either 60% humidity for 48 hours or 80% humidity for 8 minutes improved the films’ crystalline quality: The procedure both brought molecular by-products and small crystallites into the active perovskite layer and stimulated growth of pre-existing crystallites. “The humidity treatment thus increased the size of crystalline grains and reduced the number of grain boundaries,” says Meloni. This improved the material’s performance as well: Treated films showed higher photoluminescence intensity, meaning a higher efficiency.

While a longer exposure to humidity again had detrimental effects, it also showed that larger crystallites have a better chance of surviving the wet treatment. “Overall, we have demonstrated that both the stability and the efficiency of CsPbBr3 films can be manipulated by controlled treatment with humidity during fabrication,” Meloni points out. He recommends including similar procedures in perovskite film fabrication to further enhance their potential for future electronics.

Project title:
17th call: ADRENALINE – hAliDe peRovskites sEqueNtiAL deposItioN mEchanism (by ab initio rare events simulations)
19th call: PROVING-IL – PeROVskite Interface eNgineerinG with Ionic Liquids

Resources awarded:
17th call: 78 million core hours on Piz Daint hosted by CSCS, Switzerland
19th call: 41 million core hours on MARCONI (KNL) hosted by CINECA, Italy

Research field: Chemical Sciences & Materials

References:

N. Phung, A. Al-Ashouri, S. Meloni, A. Mattoni, S. Albrecht, E.L. Unger, A. Merdasa and A. Abate: The Role of Grain Boundaries on Ionic Defect Migration in Metal Halide Perovskites. Adv. Energy Mater (2020). DOI: https://doi.org/10.1002/aenm.201903735

A. Mattoni and S. Meloni: Defect Dynamics in MAPbI3 Polycrystalline Films: The Trapping Effect of Grain Boundaries. Helv. Chim. Acta (2020). DOI: https://doi.org/10.1002/hlca.202000110

D. Di Girolamo, M.I. Dar, D. Dini, L. Gontrani, R. Caminiti, A. Mattoni, M. Graetzel and S. Meloni: Dual effect of humidity on cesium lead bromide: enhancement and degradation of perovskite films. J. Mater. Chem. A (2019). DOI: https://doi.org/10.1039/C9TA00715F

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