news & views Shibata and colleagues report in situ Lorentz transmission electron microscopy measurements on a thin plate of FeGe with a thickness of about 150 nm. They show by analysis in real- and reciprocal-space that a uniaxial tensile strain on the FeGe sample leads to large deformations of both the skyrmion lattice and individual skyrmions (Fig. 1b,c). Anisotropic strain on the crystal lattice as small as 0.3% induces a distortion of the skyrmion lattice along the strain direction of up to 20%. The researchers propose that the crystal lattice deformation results in an anisotropic change of the DMI; furthermore, the change in the strength of the DMI is approximately 100 times larger than the lattice strain. The proposal is supported by theoretical analysis that explores a strain-induced anisotropic DMI. The fact that the DMI may be efficiently tuned by strain in the crystal lattice provides a mechanism to control magnetic structures, such as skyrmions, that are based on this interaction. The role of strain in the properties and control of magnetic skyrmions is particularly important in thin

films, which are, in general, prone to strain effects (for example due to interactions with the substrate) and are relevant for most potential applications. In particular, the work of Shibata and co-workers is relevant for applications based on skyrmion lattices, such as those involving microwave excitations, rather than for applications based on the manipulation of single skyrmions, such as concepts proposed for data storage9. When exposed to microwaves, skyrmion lattices exhibit collective spin excitations that are typically in the gigahertz range12,13. The microwave response can be tailored by changing the shape and material of the sample16. In combination with the control of the spin texture using magnetic fields or uniaxial strain, new microwave devices may be designed. For instance, one might think of a microwave diode that uses the large directional dichroism present in chiral magnets with magnetoelectric coupling 14, where the absorption may be fine-tuned via mechanical strain. The strain-induced properties of skyrmions provide an efficient control

parameter and furthermore represent another step towards the development of a new generation of spintronics devices based on skyrmions. ❐ Robert Ritz is in the Department of Physics, Technische Universität München, 85748 Garching, Germany. e-mail: [email protected] References

1. Mühlbauer, S. et al. Science 323, 915–919 (2009). 2. Yu, X. Z. et al. Nature Mater. 10, 106–109 (2010). 3. Tokunaga, Y. et al. Preprint at (2015). 4. Kézsmárki, I. et al. Preprint at (2015). 5. Heinze, S. et al. Nature Phys. 7, 713–718 (2011). 6. Romming, N. et al. Science 341, 636–639 (2013). 7. Jonietz, F. et al. Science 330, 1648–1651 (2010). 8. Nagaosa, N. & Tokura, Y. Nature Nanotech. 8, 899–911 (2013). 9. Fert, A. et al. Nature Nanotech. 8, 152–156 (2013). 10. Ritz, R. et al. Nature 497, 231–234 (2013). 11. Bauer, A. et al. Phys. Rev. B 82, 064404 (2010). 12. Mochizuki, M. et al. Phys. Rev. Lett. 108, 017601 (2012). 13. Onose, Y. et al. Phys. Rev. Lett. 109, 037603 (2012). 14. Okamura, Y. et al. Nature Commun. 4, 2391 (2013). 15. Shibata, K. et al. Nature Nanotech. 10, 589–592 (2015). 16. Schwarze, T. et al. Nature Mater. 14, 478–483 (2015). 17. Yu, X. Z. et al. Nature 465, 901–904 (2010).


Signs of stability

Studies on a perovskite photovoltaic device suggest that improved stability, one of the hurdles to large-scale applicability of perovskites in solar cells, can be achieved.

Karl Leo


hotovoltaic devices based on the inorganic–organic methylammonium lead halide perovskite absorbers (CH3NH3PbX3, X = Br, Cl, I)1 are attracting considerable attention. There are three main reasons why: the materials are inexpensive to produce; the fabrication methods are relatively simple; and the devices have a high power conversion efficiency 2–6. Indeed, improvements in the efficiency have been remarkable in the last 2–3 years, faster than that seen for any other photovoltaic material before. As Fig. 1 shows, in terms of efficiency, these materials are already better than other low-cost, thin-film technologies such as amorphous silicon, dye-sensitized solar cells, and organic solar cells. Efficiency is not the only factor that determines the suitability of a device for realistic applications however. The operational stability, which is determined by the time it takes before the device degrades, also needs to be taken into account. So far, 574

the issue of operational stability has not been addressed satisfactorily for perovskites. Unfortunately, the poor thermal stability of the basic perovskite material, which starts to decompose at temperatures around 70 °C, suggests that device stability might be a major issue (see ref. 7 for a recent review). Now, writing in Energy Technology, Michael Grätzel and colleagues, from EPF Lausanne, Switzerland, and King Abdulaziz University, Jeddah, Saudi Arabia, present a cell structure that displays encouraging stability data and that maintains a reasonably high efficiency close to 13%8. The solar cells do not contain the standard organic hole-transport layer and evaporated metal contact, but instead a porous zirconium oxide layer and a carbon back electrode. Small-area individual cells were tested outdoors under the harsh late summer conditions of Jeddah for one week. The parameters show fluctuations with time, which the authors attribute to

changes in solar illumination, but they remain almost constant overall. The devices were also subject to two indoor tests. The first was at 85 °C in the dark for 90 days to assess how the cells reacted to prolonged heat stress. Here, the cells lost about 10% of their initial efficiency, assuming a linear decay, this would correspond to a lifetime (usually defined as 20% efficiency loss) of 180 days. To pass the standard International Electrotechnical Commission damp heat test (85 °C, 85% relative humidity), photovoltaic modules would require a maximum 10% efficiency loss in 1,000 hours (just over 40 days), that is, the perovskite cells presented by Li et al. would pass at least the temperature part of the test. Furthermore, the authors performed a test under illumination at 45 °C for 44 days in an argon atmosphere. Under these conditions, the cells remained remarkably stable. The results show that the perovskites can withstand high temperatures and are


© 2015 Macmillan Publishers Limited. All rights reserved

news & views 20


Amorphous Si single layer Dye sensitized


Small-area OPV 2

Certified >1 cm OPV

Conversion efficiency (%)


Perovskite United Solar Sharp

United Solar




Mitsubishi Solarme


0 1985




Heliatek Mitsubishi Heliatek Mitsubishi

Heliatek/IAPP Konarka

SKKU Plextronics Heliatek/IAPP Konarka Konarka Princeton Univ. Univ. of Tokyo Univ. of Linz Sharp





KRICT Univ. of Oxford EPFL

Univ. of Cambridge







Figure 1 | Development of efficiencies for several thin-film photovoltaic technologies. Perovskites have more quickly grown in efficiency than any other technology in the history of photovoltaics. OPV, organic photovoltaic; EPFL, École polytechnique fédérale de Lausanne; IAPP, Institut für Angewandte Photophysik; KRICT, Korean Research Institute of Chemical Technology; NIMS, National Institute for Materials Science; SKKU, Sungkyunkwan University; UCLA, University of California, Los Angeles; UCSB, University of California, Santa Barbara.

stable under outdoor illumination. The results of Li et al. are thus an important step underlining the commercialization potential of organic–inorganic perovskite solar cells. The results on stability are encouraging. They, of course, do not solve other issues that will require substantial efforts. For example, it was recently reported that the perovskites strongly react to electric fields9: when applying high fields, the cells could

be efficiently polarized so that their polarity was completely reversed. In modules where many cells are connected in series, large potential differences can occur. Recently, such potential effects have caused serious problems even in crystalline silicon solar cells, a technology commonly believed to be almost unbreakable. Thus, further careful tests on the module level are needed to assess the stability of perovskite cells under


© 2015 Macmillan Publishers Limited. All rights reserved

real application conditions. Furthermore, the cell structure without a hole-transport layer should be optimized to achieve higher efficiencies and it should be carefully tested to see whether the good stability observed by Li and co-workers is indeed related to the omission of the hole-transport layer and the use of a carbon electrode. Finally, there is one other controversially debated issue that is crucial for a potential breakthrough of the perovskite technology — the acceptability of lead. While the total amount of lead is small (much less than a gram per square metre), it is questionable whether a ‘green’ technology containing lead is acceptable in times when it has been banned for electronic applications. Replacing lead so far has turned out to be challenging. The efficiencies obtained with alternatives are less than 10%, and the best replacement, tin, is environmentally questionable as well. ❐ Karl Leo is at the Institut für Angewandte Photophysik, Technische Universität Dresden, 01069 Dresden, Germany. e-mail: [email protected] References

1. Mitzi, D. B. Prog. Inorg. Chem. 48, 1–121 (1999). 2. Kojima, A., Teshima, K., Shirai, Y. & Miyasaka, T. J. Am. Chem. Soc. 131, 6050–6051 (2009). 3. Im, J.‑H., Lee, C.‑R., Lee, J.‑W., Park, S.‑W. & Park, N.‑G. Nanoscale 3, 4088–4093 (2011). 4. Liu, M. Z., Johnston, M. B. & Snaith, H. J. Nature 501, 395–398 (2013). 5. Burschka, J. et al. Nature 499, 316–319 (2013). 6. Jeon, N. J. et al. Nature 517, 476–480 (2015). 7. Niu, G., Guoa, X. & Wang, L. J. Mater. Chem. A 3, 8970–8980 (2015). 8. Li, X. et al. Energy Technol. 3, 551–555 (2015). 9. Xiao, Z. Nature Mater. 14, 193–198 (2015).


Perovskite photovoltaics: Signs of stability.

Perovskite photovoltaics: Signs of stability. - PDF Download Free
121KB Sizes 67 Downloads 11 Views