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A structurally unstable semiconductor stabilized and enhanced by strain

Semiconductors known as halide perovskites have remarkable optoelectronic properties, but their structural instability limits practical applications. A solution has been found that involves squeezing the compounds’ crystal lattices.
Jian Shi is in the Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, USA.
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The surprising discovery1 in 2009 that compounds known as halide perovskites can convert sunlight into electricity triggered a revolution in photovoltaics (solar cells), inspiring new cell designs that will enable solar energy to be harnessed efficiently and at low cost. However, tuning the properties of these crystals to enable practical applications has been a long-standing challenge. Writing in Nature, Chen et al.2 report that a solution to this problem has finally been found.

Discovering new materials and identifying appropriate applications can sometimes be achieved serendipitously, but often takes decades, and has historically required centuries or even millennia. The first halide perovskite was discovered in the 1890s3, but its potential remained untapped until a decade ago1, when it took photovoltaics by storm. As its name suggests, halogen atoms in the compound enable the formation of a cubic (or pseudo-cubic) array known as a perovskite structure.

Harnessing the full potential of halide perovskites for technological applications has been difficult. A major obstacle is the tendency of one of the best-performing perovskite crystals, α-formamidinium lead iodide (HC(NH2)2PbI3, known as α-FAPbI3), to assume a hexagonal structure at room temperature (Fig. 1a) — the approximate temperature at which photovoltaic devices operate. This hexagonal structure cannot respond to most of the frequencies of light in solar radiation, and hence is not of interest for technological applications. It would therefore be helpful to stabilize the structure of α-FAPbI3.

Figure 1 | The stabilization and strain engineering of a semiconductor. a, The semiconductor α-formamidinium lead iodide (HC(NH2)2PbI3, known as α-FAPbI3) belongs to the halide perovskite family of compounds, and has potential applications in solar cells. However, its cubic perovskite lattice is unstable, and converts to a hexagonal form that is unsuitable for practical applications. The octahedra represent subunits of the lattices: iodine atoms at the vertices surround a central lead atom; (HC(NH2)2)+ ions fill the gaps between octahedra, but are not shown. b, Chen et al.2 report that the structure of α-FAPbI3 can be stabilized by growing it on a stable halide perovskite (the substrate) that has an analogous structure, so that the atoms in the two lattices align. Because the lattice dimensions of the substrate are smaller than those of α-FAPbI3, the crystal lattice of the latter is squeezed (put under strain). This strain increases the mobility of electrical charge carriers in the material.

Several strategies can be used to engineer a material’s properties. Two of the most effective are to alter the material’s composition or to use it at high or low temperatures, but the costs involved for commercial applications can be tremendous. Scientists have therefore developed a simple but extremely useful approach known as strain engineering, which has been used to tune the electronic properties of semiconductors.

When a crystal is compressed or stretched, the resulting deformation is called strain. Strain is calculated by dividing the change of length of a deformed object by the object’s original length, and is expressed as a percentage of the original length. In semiconductors, strain can alter the mobility of charge carriers — a property that characterizes how fast a charge carrier, such as an electron, can travel in a crystal subjected to an electric field. By changing the charge mobility, the electronic properties of a semiconductor can be altered.

Semiconductors in modern electronic devices are most commonly used as thin films, and the maximum sustainable strain of a film is less than a few per cent4. Nevertheless, strain modulation can be a highly effective tool for enhancing electronic properties. One such success story is that of lasers based on layered semiconductor structures known as quantum wells. By straining the quantum-well layers, the electrical current needed to power a laser can be reduced by as much as ten times, thus improving the energy efficiency of the devices5,6. Most commercial quantum-well lasers are therefore strained5.

One of the most interesting but problematic features of halide perovskites is that only a few, including α-FAPbI3, have high charge mobility and absorb light strongly, but the technologically useful crystal structures of these few compounds are unstable — some can spontaneously transform into other phases in less than a second. In photovoltaic devices, fast-moving charge carriers and strong light absorption are both needed to convert solar energy into electricity with high efficiency. Unfortunately, structural stability, high charge mobility and the ability to absorb light strongly don’t seem to coexist in perovskite halides.

Chen et al. have therefore used strain engineering to tackle this problem. They grew crystalline α-FAPbI3 from a solution so that it formed on another, more stable halide perovskite (the substrate). The FAPbI3 atoms in the growing crystal align with the cubic structure of the atoms in the substrate, thereby forming a pseudocubic structure themselves (Fig. 1b). The alignment of atoms in different materials is called epitaxy. This epitaxy locks α-FAPbI3 into the pseudocubic structure as a result of the strong chemical forces between it and the substrate, preventing its transformation into the undesirable hexagonal structure. The authors find that the pseudocubic structure remains stable for at least a year at room temperature.

A compressive strain is imposed on the α-FAPbI3 film because the dimensions of the cubic array of the substrate are different from those of the natural atomic array of α-FAPbI3. Chen and colleagues were therefore able to control the strain of α-FAPbI3 from 0 to 2.4% compressive deformation by growing FAPbI3 on substrates that have different lattice dimensions. The authors found that this squeezing of the α-FAPbI3 crystal increases the mobility of positively charged quasiparticles called holes, which correspond to the absence of electrons in the crystal. The authors attribute this increased mobility to the modification of the electronic structure of the crystal under compressive strain: compression leads to faster oscillations of the holes’ wavefunctions, speeding up the movement of charge wavepackets (superpositions of wavefunctions) and thus producing higher charge mobility.

Previous work on the strain engineering of halide perovskite films lacked strain control7 or involved straining methods that are harder to use8,9. By contrast, Chen and colleagues’ study provides an extremely accessible and practical avenue through which to explore and use the physical properties of strained halide perovskites.

Questions remain about how the authors’ findings could be used in solar cells. Currently available halide perovskite photovoltaic devices do not contain a genuinely epitaxial substrate, and so new cell designs will be needed to make use of the reported discovery. But a range of halide perovskite compounds are available that have similar atomic arrays to α-FAPbI3, and which exhibit many different technologically important electronic properties. Chen and co-workers’ study therefore suggests that there is plenty of scope for designing and developing epitaxial quantum-well devices using these materials, by mimicking the way in which quantum-well devices were developed using semiconductors from the III–V family of materials. This might bring down the cost of manufacturing these devices.

Finally, it will be interesting to see whether crystals of halide perovskites can be grown with sufficient atomic precision to make superlattices — periodic structures that contain multiple layers of two or more materials. The use of halide perovskites in superlattices could open up otherwise inaccessible electronic band structures, thereby allowing a rich array of physics to be explored, and emerging quantum-well devices to be further developed.

Nature 577, 171-172 (2020)

doi: 10.1038/d41586-019-03918-1

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    Wang, Y. et al. Phys. Rev. Mater. 2, 076002 (2018).

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