Perovskites under the Sun

8/11/2019 Perovskites under the Sun

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MATERIA

LWITNESS

Science doesnt quite know what to dowith people like Graham Parkhouse,

a British engineer who surely qualifiesas a maverick. While shoehornedinto a brie academic career at theUniversity o Surrey he now runs acivil engineering consultancy nearby Parkhouse developed ideas on materialsselection and design1that provedinfluential on pioneers o the fieldsuch as Michael Ashby and that nowseem rather prescient. (Sadly, the samecannot really be said or his publishedideas on cosmic background radiation.)Parkhouse promoted the idea ostructure as an interplay o material

and shape that can now be seen tooreshadow notions o hierarchicalmaterials and metamaterials. Hisunusual career trajectory is describedin Donald Brabens book on blue-skiesthinking, ScientificFreedom: Te Elixirof Civilization(Wiley, 2008).

Parkhouses little-rememberedcontribution is brought to mind in arecent exploration by Barthelat andMirkhala o how material and shapeinteract2. Te authors have taken anunusual approach to a well-studiedissue: how best to configure a composite

to achieve an ideal compromisebetween the mechanical properties o itsconstituents. As Barthelat and Mirkhalapoint out, while it is well known thatcombining a hard and a sofer materialcan engender a balance o stiffness andstrength (rom the hard component)with toughness and ductility (rom the

sof), typically in engineering only a ewmicrostructures are employed, such as

fibre composites and laminates. Natureis similarly conservative with its ownmicrostructural repertoire, avouring inparticular the staggered layering seen innacre and bone. But are these really thebest, or even the only, options?

Te design o microstructure istypically conducted as an optimizationprocess that begins with a certaintopology and refines it. A moreexhaustive search o the space omicrostructural possibilities is generallythought computationally prohibitive.But Barthelat and Mirkhala describe

a model or which this kind o blanketsurvey is tractable: a two-dimensionalcomposite in which hard, rectangularinclusions are regularly stacked withina sofer matrix, subjected to extensionalstress. Simple parametrization o thisgeometry gives rise to just over 7,000microstructures or a particular choiceo the hard (brittle) and sof (elastic)phases. Te stressstrain curves andailure o each o these solutions canthen be calculated.

Most (about 90 per cent) o theresulting composites are either very

brittle they contain continuoushard phases, which yield by brittleailure or very ductile, ailing byover-extension o the sof matrix. Butthe remaining structures look moreuseul. Hal o these are quasi-brittle:stiff but ductile, ailing at strains muchgreater than that supported by the hard

phase alone. Te others are labelledductile strong, and include staggeredhard platelets comparable to nacre.

Te results confirm some expected

general principles: stiffness and strengthusually come together, whereas strengthand toughness tend to be mutuallyexclusive. With the microstructuralspace ully mapped, however, it becomespossible to answer design questionsrather precisely: to find exactly whichshapes achieve a particular balance oproperties (i, say, toughness were tobe weighted more than stiffness). Teapproach could be extended to include,or example more degrees o reedom:dissipation by delamination, hierarchicalstructure and anisotropy. It might even

answer questions about natural design:are natures composites truly optimal, orconstrained 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. & Mirkhala, M.J. R. Soc. Interface

10,20130711 (2013).

MAKING SPACE FOR SHAPE

PHILIP BALL

Materials that have a genericchemical ormula ABX3and a cubic structure are

defined as perovskites, named afer themineral CaiO3. Te A and B sites canaccommodate inorganic cations o various

valency and ionic radius. Alternatively,suitable organic species can replacecation A and create organicinorganichybrid materials1(Fig. 1). A number oexciting physical properties, like colossalmagnetoresistance, erroelectricity and

superconductivity, have been discoveredin this prolific amily o compoundsduring the past century1,2. Recently,organicinorganic hybrid perovskites (inparticular CH3NH3PbX3, where X = I, Cl,Br) came to the ore as a result o their

HYBRID SOLAR CELLS

Perovskites under the SunMixed-halide organicinorganichybrid perovskites are reported to display electronhole diffusion lengths over 1 m.

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

2013 Macmillan Publishers Limited. All rights reserved


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high perormance in converting solarlight into electrical power3, with power-conversion efficiencies (PCE) exceeding15% (res 4,5). Tis result is even moreimpressive considering that the firstperovskite solar cells were only reportedin 2009, and displayed PCE values as lowas 3.8% (re. 6). Tese initial prototypeswere based on the classical architecture odye-sensitized solar cells, with the organicinorganic compounds deposited on top o amesoporous iO2structure (Fig. 2a). More

recent works demonstrated that a simplergeometry a perovskite layer sandwichedbetween a compact thin film o iO2anda hole-conducting organic compound(Fig. 2b) is also able to convert lightwith efficiencies higher than 10%, providedthat a uniorm and dense morphology isachieved in the deposited layer5,7. Sucha steep learning curve in the design andprocessing o hybrid perovskites is certainlyunprecedented in the field o photovoltaicresearch. However, the understandingo the mechanisms underlying suchexceptional perormance has not grown

at the same pace. Writing in Science, thegroups o Henry J. Snaith and colleagues8and Nripan Mathews, ze C. Sum andcollaborators9now independently reporton diffusion-length measurementsperormed onhybrid perovskites, whichshed light on the dynamics o photoexcitedspecies (excitons or charge carriers) inthese materials.

Both teams used photoluminescencequenching experiments to measure theelectronhole diffusion length. Teydeposited on top o a perovskite thin-film a layer o quenching molecules,which act as a sink or the photoexcited

species that, travelling in the film, reachthe interace between the perovskite andthe quencher. Te photoluminescencedynamics o the material under studyare thereore dependent on the thicknesso the thin film and on the diffusionlength LDo the photoexcitations; this lastparameter can be extracted by modellingthe photoluminescence decay curvesaccording to a simple one-dimensionaldiffusion equation. One o the mostchallenging aspects o this technique is

the exact determination o the travellingdistance o the photoexcitations in otherwords, the thickness o the tested layermust be precisely controlled to minimizethe uncertainty o the extracted LDvalues.In this respect, efforts reported in previousworks by Snaith and co-workers to optimizethe deposition o the perovskite layers5,7have been undamental in allowing areliable estimation o the electronholediffusion length. Both teams obtain LDoabout 100 nm or electrons and holes inCH3NH3PbI3. Furthermore, the group oSnaith also investigated the mixed-halide

perovskite CH3NH3PbI3xClx, obtaining in

this case a LDexceeding 1 m; this highvalue reinorces hope or the uture ohybrid perovskite solar cells, because itmakes possible the abrication o deviceswith thicker active layers, where theabsorption o light can be increased withoutaffecting the collection efficiency o the

generated charges. Tat is to say, PCE valueso more than 15% may be possible.

But what is so exciting about thesematerials, given that their PCE are still arrom those displayed by other commonphotovoltaic examples such as single-crystalline silicon devices (PCE o 25%) orthin-film copper indium gallium selenidecells (PCE o 20.4%)? Te answer is thelower manuacturing costs expected orthis uture perovskite-based photovoltaictechnology. In act, these materials can bedirectly deposited rom solution, a cheapand scalable approach that is also the main

strength o alternative technologies such asorganic photovoltaics, dye-sensitized solarcells and colloidal quantum dot-based solarcells. In contrast to these latter devices,which at the moment do not seem to be ableto boost the efficiency much above 10%,perovskites are easy to abricate and havea higher power-conversion perormance,and this combination is likely to be becauseo their hybrid nature. Indeed, the organiccomponent makes the perovskite solubleand acilitates its sel-assembly, thusenabling its deposition rom solution. At thesame time, the inorganic component orms

an extended ramework bound by strongcovalent and/or ionic interactions, whichmost likely preserves a precise crystallinestructure in the deposited films and ensuresa high carrier mobility1. As shown by Snaithand collaborators, the structural ordero mixed-halidehybrid perovskites leadsto a carrier (or exciton) diffusion lengthabove 1,000 nm, in stark contrast with themaximum values o about 10 nm reportedor the diffusion length o excitons in solarcells based on inherently less-orderedpolymerullerene solar cells10.

Te studies reported by the two teams

highlight that, afer the initial excitement

H

C

N

Pb

Cl

I

Figure 1 |Crystal structure.Possible structure of the hybrid perovskite CH3NH3PbI3xClx.At present,

crystallographic data on the precise position of the organic ligands are not available.

Figure 2 |Architectures of perovskite solar cells .a,Hybrid perovskite solar cell on mesoporous TiO2.

b,Planar hybrid perovskite solar cell.

TiO2

TiO2Fluorine-doped tin oxide

Hole-transporting layer

Gold

a b

Glass

Hybrid perovskite

TiO2Fluorine-doped tin oxide

Hole-transporting layer

Gold

Glass

Hybrid perovskite



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and surprise o the high efficiency o thesolar cells, it is now time to investigatethe physical properties that make hybridperovskites so promising or solar-energyconversion. Te next point to be addressed,as Snaith and colleagues put orward intheir work, is whether the photoexcited

species in this class o materials areexcitons or ree charges. We can urthersuggest other aspects deserving thoroughanalysis: what is the mobility o electronsand holes? What is the exact chemicalstructure o the hybrid perovskites andhow does it influence the transportbehaviour o the photoexcitations? Whatis the precise role o each interace in thedevice architectures proposed so ar? Areperovskite solar cells stable? And finally,is it possible to reach analogous opticaland electrical perormance using lead-ree

organicinorganic compounds, thusreducing the toxicity and environmentalimpact o this uture technology? Only alarge multidisciplinary effort will be ableto answer these questions, and create romthis exciting research topic a new and lessexpensive photovoltaic technology.

Maria Antonietta Loi1and Jan C. Hummelen1,2

are at 1Zernike Institute for Advanced Materials,

University of Groningen, Nijenborgh 4, Groningen,

9747 AG, Te Netherlands, 2Stratingh Institute for

Chemistry, University of Groningen, Nijenborgh 4,

Groningen, 9747 AG, Te Netherlands.

e-mail: [email protected];[email protected]

References

1. Mitzi, D. B. Prog. Inorg. Chem.48,1121(1999).

2. Polyakov, A. O. et al. Chem. Mater.24,133139 (2012).

3. Lee, M. M. et al.Science338,643647 (2012).

4. Burschka, J. et al.Nature499,316319 (2013).

5. Liu, M., Johnston, M. B. & Snaith, H. J. Nature

501,395398 (2013).

6. Kojima, A., eshima, K., Shirai, Y. & Miyasaka, .

J. Am. Chem. Soc.131,60506051 (2009).

7. Eperon, G. E., Burlakov, V. M., Docampo, P., Goriely, A. &

Snaith, H. J.Adv. Funct. Materhttp://dx.doi.org/10.1002/

adm.201302090(2013).

8. Stranks, S. et al. Science342,341344 (2013).

9. Xing, G. et al.Science342,344347 (2013).

10. Mikhnenko, O. et al.Energy Environ. Sci. 5,69606965(2012).

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Perovskites under the Sun