news & views
MATERIAL WITNESS
MAKING SPACE FOR SHAPE Science doesn’t quite know what to do with people like Graham Parkhouse, a British engineer who surely qualifies as a maverick. While shoehorned into a brief academic career at the University of Surrey — he now runs a civil engineering consultancy nearby — Parkhouse developed ideas on materials selection and design1 that proved influential on pioneers of the field such as Michael Ashby and that now seem rather prescient. (Sadly, the same cannot really be said for his published ideas on cosmic background radiation.) Parkhouse promoted the idea of structure as an interplay of material and shape that can now be seen to foreshadow notions of hierarchical materials and metamaterials. His unusual career trajectory is described in Donald Braben’s book on blue-skies thinking, Scientific Freedom: The Elixir of Civilization (Wiley, 2008). Parkhouse’s little-remembered contribution is brought to mind in a recent exploration by Barthelat and Mirkhalaf of how material and shape interact2. The authors have taken an unusual approach to a well-studied issue: how best to configure a composite to achieve an ideal compromise between the mechanical properties of its constituents. As Barthelat and Mirkhalaf point out, while it is well known that combining a hard and a softer material can engender a balance of stiffness and strength (from the hard component) with toughness and ductility (from the
soft), typically in engineering only a few microstructures are employed, such as fibre composites and laminates. Nature is similarly conservative with its own microstructural repertoire, favouring in particular the staggered layering seen in nacre and bone. But are these really the best, or even the only, options? The design of microstructure is typically conducted as an optimization process that begins with a certain topology and refines it. A more exhaustive search of the space of microstructural possibilities is generally thought computationally prohibitive. But Barthelat and Mirkhalaf describe a model for which this kind of blanket survey is tractable: a two-dimensional composite in which hard, rectangular inclusions are regularly stacked within a softer matrix, subjected to extensional stress. Simple parametrization of this geometry gives rise to just over 7,000 microstructures for a particular choice of the hard (brittle) and soft (elastic) phases. The stress–strain curves and failure of each of these solutions can then be calculated. Most (about 90 per cent) of the resulting composites are either very brittle — they contain continuous hard phases, which yield by brittle failure — or very ductile, failing by over-extension of the soft matrix. But the remaining structures look more useful. Half of these are ‘quasi-brittle’: stiff but ductile, failing at strains much greater than that supported by the hard
PHILIP BALL phase alone. The others are labelled ‘ductile strong’, and include staggered hard platelets comparable to nacre. The results confirm some expected general principles: stiffness and strength usually come together, whereas strength and toughness tend to be mutually exclusive. With the microstructural space fully mapped, however, it becomes possible to answer design questions rather precisely: to find exactly which shapes achieve a particular balance of properties (if, say, toughness were to be weighted more than stiffness). The approach could be extended to include, for example more degrees of freedom: dissipation by delamination, hierarchical structure and anisotropy. It might even answer questions about natural design: are nature’s composites truly optimal, or constrained by their history? ❐ References 1. Parkhouse, J. G. in Proc. 3rd Int. Conf. Space Structures (ed. Nooshin, H.) 367 (Elsevier Applied Science, 1984). 2. Barthelat, F. & Mirkhalaf, M. J. R. Soc. Interface 10, 20130711 (2013).
HYBRID SOLAR CELLS
Perovskites under the Sun
Mixed-halide organic–inorganic hybrid perovskites are reported to display electron–hole diffusion lengths over 1 μm. This observation provides important insight into the charge-carrier dynamics of this class of semiconductors and increases the expectations for highly efficient and cheap solar cells.
Maria Antonietta Loi and Jan C. Hummelen
M
aterials that have a generic chemical formula ABX3 and a cubic structure are defined as perovskites, named after the mineral CaTiO3. The A and B sites can accommodate inorganic cations of various
valency and ionic radius. Alternatively, suitable organic species can replace cation A and create organic–inorganic hybrid materials1 (Fig. 1). A number of exciting physical properties, like colossal magnetoresistance, ferroelectricity and
NATURE MATERIALS | VOL 12 | DECEMBER 2013 | www.nature.com/naturematerials
© 2013 Macmillan Publishers Limited. All rights reserved
superconductivity, have been discovered in this prolific family of compounds during the past century 1,2. Recently, organic–inorganic hybrid perovskites (in particular CH3NH3PbX3, where X = I, Cl, Br) came to the fore as a result of their 1087
news & views
H C N Pb
Cl
I
Figure 1 | Crystal structure. Possible structure of the hybrid perovskite CH3NH3PbI3−xClx. At present, crystallographic data on the precise position of the organic ligands are not available.
high performance in converting solar light into electrical power 3, with powerconversion efficiencies (PCE) exceeding 15% (refs 4,5). This result is even more impressive considering that the first perovskite solar cells were only reported in 2009, and displayed PCE values as low as 3.8% (ref. 6). These initial prototypes were based on the classical architecture of dye-sensitized solar cells, with the organic– inorganic compounds deposited on top of a mesoporous TiO2 structure (Fig. 2a). More recent works demonstrated that a simpler geometry — a perovskite layer sandwiched between a compact thin film of TiO2 and a hole-conducting organic compound (Fig. 2b) — is also able to convert light with efficiencies higher than 10%, provided that a uniform and dense morphology is achieved in the deposited layer 5,7. Such a steep learning curve in the design and processing of hybrid perovskites is certainly unprecedented in the field of photovoltaic research. However, the understanding of the mechanisms underlying such exceptional performance has not grown at the same pace. Writing in Science, the groups of Henry J. Snaith and colleagues8 and Nripan Mathews, Tze C. Sum and collaborators9 now independently report on diffusion-length measurements performed on hybrid perovskites, which shed light on the dynamics of photoexcited species (excitons or charge carriers) in these materials. Both teams used photoluminescence quenching experiments to measure the electron–hole diffusion length. They deposited on top of a perovskite thinfilm a layer of quenching molecules, which act as a sink for the photoexcited 1088
species that, travelling in the film, reach the interface between the perovskite and the quencher. The photoluminescence dynamics of the material under study are therefore dependent on the thickness of the thin film and on the diffusion length LD of the photoexcitations; this last parameter can be extracted by modelling the photoluminescence decay curves according to a simple one-dimensional diffusion equation. One of the most challenging aspects of this technique is the exact determination of the ‘travelling distance’ of the photoexcitations — in other words, the thickness of the tested layer must be precisely controlled to minimize the uncertainty of the extracted LD values. In this respect, efforts reported in previous works by Snaith and co-workers to optimize the deposition of the perovskite layers5,7 have been fundamental in allowing a reliable estimation of the electron–hole diffusion length. Both teams obtain LD of about 100 nm for electrons and holes in CH3NH3PbI3. Furthermore, the group of Snaith also investigated the mixed-halide perovskite CH3NH3PbI3−xClx, obtaining in a
this case a LD exceeding 1 μm; this high value reinforces hope for the future of hybrid perovskite solar cells, because it makes possible the fabrication of devices with thicker active layers, where the absorption of light can be increased without affecting the collection efficiency of the generated charges. That is to say, PCE values of more than 15% may be possible. But what is so exciting about these materials, given that their PCE are still far from those displayed by other common photovoltaic examples — such as singlecrystalline silicon devices (PCE of 25%) or thin-film copper indium gallium selenide cells (PCE of 20.4%)? The answer is the lower manufacturing costs expected for this future perovskite-based photovoltaic technology. In fact, these materials can be directly deposited from solution, a cheap and scalable approach that is also the main strength of alternative technologies such as organic photovoltaics, dye-sensitized solar cells and colloidal quantum dot-based solar cells. In contrast to these latter devices, which at the moment do not seem to be able to boost the efficiency much above 10%, perovskites are easy to fabricate and have a higher power-conversion performance, and this combination is likely to be because of their hybrid nature. Indeed, the organic component makes the perovskite soluble and facilitates its self-assembly, thus enabling its deposition from solution. At the same time, the inorganic component forms an extended framework bound by strong covalent and/or ionic interactions, which most likely preserves a precise crystalline structure in the deposited films and ensures a high carrier mobility 1. As shown by Snaith and collaborators, the structural order of mixed-halide hybrid perovskites leads to a carrier (or exciton) diffusion length above 1,000 nm, in stark contrast with the maximum values of about 10 nm reported for the diffusion length of excitons in solar cells based on inherently less-ordered polymer–fullerene solar cells10. The studies reported by the two teams highlight that, after the initial excitement b
Gold
Gold
Hole-transporting layer Hybrid perovskite TiO2
Hole-transporting layer Hybrid perovskite
TiO2 Fluorine-doped tin oxide Glass
TiO2 Fluorine-doped tin oxide Glass
Figure 2 | Architectures of perovskite solar cells. a, Hybrid perovskite solar cell on mesoporous TiO2. b, Planar hybrid perovskite solar cell. NATURE MATERIALS | VOL 12 | DECEMBER 2013 | www.nature.com/naturematerials
© 2013 Macmillan Publishers Limited. All rights reserved
news & views and surprise of the high efficiency of the solar cells, it is now time to investigate the physical properties that make hybrid perovskites so promising for solar-energy conversion. The next point to be addressed, as Snaith and colleagues put forward in their work, is whether the photoexcited species in this class of materials are excitons or free charges. We can further suggest other aspects deserving thorough analysis: what is the mobility of electrons and holes? What is the exact chemical structure of the hybrid perovskites and how does it influence the transport behaviour of the photoexcitations? What is the precise role of each interface in the device architectures proposed so far? Are perovskite solar cells stable? And finally, is it possible to reach analogous optical and electrical performance using lead-free
organic–inorganic compounds, thus reducing the toxicity and environmental impact of this future technology? Only a large multidisciplinary effort will be able to answer these questions, and create from this exciting research topic a new and less expensive photovoltaic technology. ❐
5. Liu, M., Johnston, M. B. & Snaith, H. J. Nature 501, 395–398 (2013). 6. Kojima, A., Teshima, K., Shirai, Y. & Miyasaka, T. J. Am. Chem. Soc. 131, 6050–6051 (2009). 7. Eperon, G. E., Burlakov, V. M., Docampo, P., Goriely, A. & Snaith, H. J. Adv. Funct. Mater http://dx.doi.org/10.1002/ adfm.201302090 (2013). 8. Stranks, S. et al. Science 342, 341–344 (2013). 9. Xing, G. et al. Science 342, 344–347 (2013). 10. Mikhnenko, O. et al. Energy Environ. Sci. 5, 6960–6965 (2012).
Maria Antonietta Loi 1 and Jan C. Hummelen1,2 are at 1Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, Groningen, 9747 AG, The Netherlands, 2Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, Groningen, 9747 AG, The Netherlands. e-mail: [email protected]; [email protected] References 1. 2. 3. 4.
Mitzi, D. B. Prog. Inorg. Chem. 48, 1–121 (1999). Polyakov, A. O. et al. Chem. Mater. 24, 133–139 (2012). Lee, M. M. et al. Science 338, 643–647 (2012). Burschka, J. et al. Nature 499, 316–319 (2013).
NATURE MATERIALS | VOL 12 | DECEMBER 2013 | www.nature.com/naturematerials
© 2013 Macmillan Publishers Limited. All rights reserved
1089
MATERIAL WITNESS
MAKING SPACE FOR SHAPE Science doesn’t quite know what to do with people like Graham Parkhouse, a British engineer who surely qualifies as a maverick. While shoehorned into a brief academic career at the University of Surrey — he now runs a civil engineering consultancy nearby — Parkhouse developed ideas on materials selection and design1 that proved influential on pioneers of the field such as Michael Ashby and that now seem rather prescient. (Sadly, the same cannot really be said for his published ideas on cosmic background radiation.) Parkhouse promoted the idea of structure as an interplay of material and shape that can now be seen to foreshadow notions of hierarchical materials and metamaterials. His unusual career trajectory is described in Donald Braben’s book on blue-skies thinking, Scientific Freedom: The Elixir of Civilization (Wiley, 2008). Parkhouse’s little-remembered contribution is brought to mind in a recent exploration by Barthelat and Mirkhalaf of how material and shape interact2. The authors have taken an unusual approach to a well-studied issue: how best to configure a composite to achieve an ideal compromise between the mechanical properties of its constituents. As Barthelat and Mirkhalaf point out, while it is well known that combining a hard and a softer material can engender a balance of stiffness and strength (from the hard component) with toughness and ductility (from the
soft), typically in engineering only a few microstructures are employed, such as fibre composites and laminates. Nature is similarly conservative with its own microstructural repertoire, favouring in particular the staggered layering seen in nacre and bone. But are these really the best, or even the only, options? The design of microstructure is typically conducted as an optimization process that begins with a certain topology and refines it. A more exhaustive search of the space of microstructural possibilities is generally thought computationally prohibitive. But Barthelat and Mirkhalaf describe a model for which this kind of blanket survey is tractable: a two-dimensional composite in which hard, rectangular inclusions are regularly stacked within a softer matrix, subjected to extensional stress. Simple parametrization of this geometry gives rise to just over 7,000 microstructures for a particular choice of the hard (brittle) and soft (elastic) phases. The stress–strain curves and failure of each of these solutions can then be calculated. Most (about 90 per cent) of the resulting composites are either very brittle — they contain continuous hard phases, which yield by brittle failure — or very ductile, failing by over-extension of the soft matrix. But the remaining structures look more useful. Half of these are ‘quasi-brittle’: stiff but ductile, failing at strains much greater than that supported by the hard
PHILIP BALL phase alone. The others are labelled ‘ductile strong’, and include staggered hard platelets comparable to nacre. The results confirm some expected general principles: stiffness and strength usually come together, whereas strength and toughness tend to be mutually exclusive. With the microstructural space fully mapped, however, it becomes possible to answer design questions rather precisely: to find exactly which shapes achieve a particular balance of properties (if, say, toughness were to be weighted more than stiffness). The approach could be extended to include, for example more degrees of freedom: dissipation by delamination, hierarchical structure and anisotropy. It might even answer questions about natural design: are nature’s composites truly optimal, or constrained by their history? ❐ References 1. Parkhouse, J. G. in Proc. 3rd Int. Conf. Space Structures (ed. Nooshin, H.) 367 (Elsevier Applied Science, 1984). 2. Barthelat, F. & Mirkhalaf, M. J. R. Soc. Interface 10, 20130711 (2013).
HYBRID SOLAR CELLS
Perovskites under the Sun
Mixed-halide organic–inorganic hybrid perovskites are reported to display electron–hole diffusion lengths over 1 μm. This observation provides important insight into the charge-carrier dynamics of this class of semiconductors and increases the expectations for highly efficient and cheap solar cells.
Maria Antonietta Loi and Jan C. Hummelen
M
aterials that have a generic chemical formula ABX3 and a cubic structure are defined as perovskites, named after the mineral CaTiO3. The A and B sites can accommodate inorganic cations of various
valency and ionic radius. Alternatively, suitable organic species can replace cation A and create organic–inorganic hybrid materials1 (Fig. 1). A number of exciting physical properties, like colossal magnetoresistance, ferroelectricity and
NATURE MATERIALS | VOL 12 | DECEMBER 2013 | www.nature.com/naturematerials
© 2013 Macmillan Publishers Limited. All rights reserved
superconductivity, have been discovered in this prolific family of compounds during the past century 1,2. Recently, organic–inorganic hybrid perovskites (in particular CH3NH3PbX3, where X = I, Cl, Br) came to the fore as a result of their 1087
news & views
H C N Pb
Cl
I
Figure 1 | Crystal structure. Possible structure of the hybrid perovskite CH3NH3PbI3−xClx. At present, crystallographic data on the precise position of the organic ligands are not available.
high performance in converting solar light into electrical power 3, with powerconversion efficiencies (PCE) exceeding 15% (refs 4,5). This result is even more impressive considering that the first perovskite solar cells were only reported in 2009, and displayed PCE values as low as 3.8% (ref. 6). These initial prototypes were based on the classical architecture of dye-sensitized solar cells, with the organic– inorganic compounds deposited on top of a mesoporous TiO2 structure (Fig. 2a). More recent works demonstrated that a simpler geometry — a perovskite layer sandwiched between a compact thin film of TiO2 and a hole-conducting organic compound (Fig. 2b) — is also able to convert light with efficiencies higher than 10%, provided that a uniform and dense morphology is achieved in the deposited layer 5,7. Such a steep learning curve in the design and processing of hybrid perovskites is certainly unprecedented in the field of photovoltaic research. However, the understanding of the mechanisms underlying such exceptional performance has not grown at the same pace. Writing in Science, the groups of Henry J. Snaith and colleagues8 and Nripan Mathews, Tze C. Sum and collaborators9 now independently report on diffusion-length measurements performed on hybrid perovskites, which shed light on the dynamics of photoexcited species (excitons or charge carriers) in these materials. Both teams used photoluminescence quenching experiments to measure the electron–hole diffusion length. They deposited on top of a perovskite thinfilm a layer of quenching molecules, which act as a sink for the photoexcited 1088
species that, travelling in the film, reach the interface between the perovskite and the quencher. The photoluminescence dynamics of the material under study are therefore dependent on the thickness of the thin film and on the diffusion length LD of the photoexcitations; this last parameter can be extracted by modelling the photoluminescence decay curves according to a simple one-dimensional diffusion equation. One of the most challenging aspects of this technique is the exact determination of the ‘travelling distance’ of the photoexcitations — in other words, the thickness of the tested layer must be precisely controlled to minimize the uncertainty of the extracted LD values. In this respect, efforts reported in previous works by Snaith and co-workers to optimize the deposition of the perovskite layers5,7 have been fundamental in allowing a reliable estimation of the electron–hole diffusion length. Both teams obtain LD of about 100 nm for electrons and holes in CH3NH3PbI3. Furthermore, the group of Snaith also investigated the mixed-halide perovskite CH3NH3PbI3−xClx, obtaining in a
this case a LD exceeding 1 μm; this high value reinforces hope for the future of hybrid perovskite solar cells, because it makes possible the fabrication of devices with thicker active layers, where the absorption of light can be increased without affecting the collection efficiency of the generated charges. That is to say, PCE values of more than 15% may be possible. But what is so exciting about these materials, given that their PCE are still far from those displayed by other common photovoltaic examples — such as singlecrystalline silicon devices (PCE of 25%) or thin-film copper indium gallium selenide cells (PCE of 20.4%)? The answer is the lower manufacturing costs expected for this future perovskite-based photovoltaic technology. In fact, these materials can be directly deposited from solution, a cheap and scalable approach that is also the main strength of alternative technologies such as organic photovoltaics, dye-sensitized solar cells and colloidal quantum dot-based solar cells. In contrast to these latter devices, which at the moment do not seem to be able to boost the efficiency much above 10%, perovskites are easy to fabricate and have a higher power-conversion performance, and this combination is likely to be because of their hybrid nature. Indeed, the organic component makes the perovskite soluble and facilitates its self-assembly, thus enabling its deposition from solution. At the same time, the inorganic component forms an extended framework bound by strong covalent and/or ionic interactions, which most likely preserves a precise crystalline structure in the deposited films and ensures a high carrier mobility 1. As shown by Snaith and collaborators, the structural order of mixed-halide hybrid perovskites leads to a carrier (or exciton) diffusion length above 1,000 nm, in stark contrast with the maximum values of about 10 nm reported for the diffusion length of excitons in solar cells based on inherently less-ordered polymer–fullerene solar cells10. The studies reported by the two teams highlight that, after the initial excitement b
Gold
Gold
Hole-transporting layer Hybrid perovskite TiO2
Hole-transporting layer Hybrid perovskite
TiO2 Fluorine-doped tin oxide Glass
TiO2 Fluorine-doped tin oxide Glass
Figure 2 | Architectures of perovskite solar cells. a, Hybrid perovskite solar cell on mesoporous TiO2. b, Planar hybrid perovskite solar cell. NATURE MATERIALS | VOL 12 | DECEMBER 2013 | www.nature.com/naturematerials
© 2013 Macmillan Publishers Limited. All rights reserved
news & views and surprise of the high efficiency of the solar cells, it is now time to investigate the physical properties that make hybrid perovskites so promising for solar-energy conversion. The next point to be addressed, as Snaith and colleagues put forward in their work, is whether the photoexcited species in this class of materials are excitons or free charges. We can further suggest other aspects deserving thorough analysis: what is the mobility of electrons and holes? What is the exact chemical structure of the hybrid perovskites and how does it influence the transport behaviour of the photoexcitations? What is the precise role of each interface in the device architectures proposed so far? Are perovskite solar cells stable? And finally, is it possible to reach analogous optical and electrical performance using lead-free
organic–inorganic compounds, thus reducing the toxicity and environmental impact of this future technology? Only a large multidisciplinary effort will be able to answer these questions, and create from this exciting research topic a new and less expensive photovoltaic technology. ❐
5. Liu, M., Johnston, M. B. & Snaith, H. J. Nature 501, 395–398 (2013). 6. Kojima, A., Teshima, K., Shirai, Y. & Miyasaka, T. J. Am. Chem. Soc. 131, 6050–6051 (2009). 7. Eperon, G. E., Burlakov, V. M., Docampo, P., Goriely, A. & Snaith, H. J. Adv. Funct. Mater http://dx.doi.org/10.1002/ adfm.201302090 (2013). 8. Stranks, S. et al. Science 342, 341–344 (2013). 9. Xing, G. et al. Science 342, 344–347 (2013). 10. Mikhnenko, O. et al. Energy Environ. Sci. 5, 6960–6965 (2012).
Maria Antonietta Loi 1 and Jan C. Hummelen1,2 are at 1Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, Groningen, 9747 AG, The Netherlands, 2Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, Groningen, 9747 AG, The Netherlands. e-mail: [email protected]; [email protected] References 1. 2. 3. 4.
Mitzi, D. B. Prog. Inorg. Chem. 48, 1–121 (1999). Polyakov, A. O. et al. Chem. Mater. 24, 133–139 (2012). Lee, M. M. et al. Science 338, 643–647 (2012). Burschka, J. et al. Nature 499, 316–319 (2013).
NATURE MATERIALS | VOL 12 | DECEMBER 2013 | www.nature.com/naturematerials
© 2013 Macmillan Publishers Limited. All rights reserved
1089
