review article perovskite solar cells: potentials...

Review ArticlePerovskite Solar Cells Potentials Challenges and Opportunities

Muhammad Imran Ahmed1 Amir Habib1 and Syed Saad Javaid2

1School of Chemical and Materials Engineering National University of Sciences and Technology Islamabad 44000 Pakistan2College of Aeronautical Engineering National University of Sciences and Technology Islamabad 44000 Pakistan

Correspondence should be addressed to Muhammad Imran Ahmed imranrahbarscmenustedupk

Received 16 March 2015 Revised 30 May 2015 Accepted 1 July 2015

Academic Editor Shuping Pang

Copyright copy 2015 Muhammad Imran Ahmed et al This is an open access article distributed under the Creative CommonsAttribution License which permits unrestricted use distribution and reproduction in any medium provided the original work isproperly cited

Heralded as a major scientific breakthrough of 2013 organicinorganic lead halide perovskite solar cells have ushered in a new eraof renewed efforts at increasing the efficiency and lowering the cost of solar energy As a potential game changer in the mix oftechnologies for alternate energy it has emerged from a modest beginning in 2012 to efficiencies being claimed at 201 in a spanof just two years This remarkable progress encouraging at one end also points to the possibility that the potential may still befar from being fully realized With greater insight into the photophysics involved and optimization of materials and methods thistechnology stands tomatch or even exceed the efficiencies for single crystal silicon solar cellsWith thin film solution processabilityapplicability to flexible substrates and being free of liquid electrolyte this technology combines the benefits of Dye Sensitized SolarCells (DSSCs) Organic Photovoltaics (OPVs) and thin film solar cells In this review we present a brief historic perspective to thisdevelopment take a cognizance of the current state of the art and highlight challenges and the opportunities

1 Introduction

Solar energy along with wind biomass tidal and geothermalenergy is emerging as an alternate source of energy for ourenergy-starved planet Out of the mix solar energy is themost abundant and clean form of energies offering an answerto the increasing concern of global warming and greenhousegases by fossil fuels Over the past four decades silicon solarcells have advanced tremendously both in terms of cost ofproduction and efficiency [1 2] In some locations of theworld they are delivering on grid power at competitive costscompared to that of fossil fuels Newer in the mix in theform of thin film and vapor deposited semiconductor basedtechnologies like CdTe or CIGS [3 4] and organicinorganicsolar cells hybrid composites or inorganic semiconductors[5ndash10] referred to as second and third generation solarcells are pushing the frontiers further in terms of ease ofprocessing cost efficiency and stability owing to sustainedresearch effort over the last decade This has resulted in theavailability of commercial products from this line of solarcells to select consumers in power electronics and low powerapplications in buildings However for their adoption in

larger markets the per watt cost has to be brought downto a level comparable to that of electricity generated fromfossil fuels This demands a substantial increase in efficiencyand cost reduction for these emerging technologies Recentadvances in the manufacturing of main stream silicon solarcell assure incorporation of photovoltaics in main streamenergy mix with a recent forecast anticipating a third of theglobal electricity demand being met by photovoltaics by 2030[1]

Silicon based solar cell technologies offering a combi-nation of properties like ease of surface passivation lowcost hardness and high temperature stability have madethemselves a favored option in photovoltaic applicationsTechnologies promising a combination of lower cost andease of fabrication with a better energy payback matrix offerexciting opportunities for replacement of silicon As newentrant in this field organometallic halide perovskites offercaptivation prospects [11ndash14] Solution processability broadspectrum solar absorption low nonradiative recombinationlosses and the potential to capitalize on over two decades ofresearch and development in the field of dye sensitized andorganic solar cells provide all the right ingredients for this

Hindawi Publishing CorporationInternational Journal of PhotoenergyVolume 2015 Article ID 592308 13 pageshttpdxdoiorg1011552015592308

2 International Journal of Photoenergy

technology to prove a viable alternate to the dominance ofsilicon The efficiency for this class of solar cells has jumpedfrom a meager 39 in 2009 to over 15 in 2013 on the backof properties like high absorption coefficient panchromaticabsorption low exciton binding energies longer diffusionlengths and lifetimes and the range of options as to compo-sition and preparation

In this review we present the current state of the artfor photovoltaic devices based on perovskites spelling outthe underlying phenomenon different device architecturesfabrication techniques comparison to other technologiesand future outlook and challenges This paper is aimed atrecollecting the recent developments in perovskite solar cellsproviding a broader context to the reviews already publishedon this topic [11 15ndash17]

2 The Phenomena

Perovskites are a family of materials with the crystal structureof calcium titanate that is ABX

3 There are numerous

materials which adopt this structure with exciting applica-tions based on thermoelectric insulating semiconductingpiezoelectric conducting antiferromagnetic and supercon-ducting properties [19] Perovskites are routinely synthesizedby solid-state mixing of constituent elements or compoundsat high temperatures in the range of gt1300K [20] Theycan also be synthesized by drying the solution of precursorsalts and the ones offering semiconductor properties findimportant applications in printable electronics due to solu-tion processability [21 22]

ABX3describes the crystal structure of perovskite class

of materials where A and B are cations and X is an anionof different dimensions with A being larger than X Crystalstructure of perovskites is illustrated in Figure 1 Considera-tions of tolerance factor t and octahedral factor 120583 [23] dictateprobable structure and crystallographic stability where t isdefined by the ratio of the bond length AndashX to that of BndashX in an ideal solid sphere model (t = (rA + rX)radic2(rB +rX) where rA rB and rX are the ionic radii involved) andthe ratio rBrX defines the octahedral factor 120583 081 lt t lt111 and 044 lt 120583 lt 090 [23] are the typical values forhalide perovskites (X = F Cl Br and I) Narrower rangeof t values form 089 to 10 dictates cubic structure whilelower values of t stabilize less symmetric tetragonal andorthorhombic structures In organic inorganic halide per-ovskite of interest in photovoltaic applications A is a largerorganic cation typically methylammonium (CH

3NH3+)

with rA = 018 nm [24] though good results have alsobeen reported for ethylammonium (CH

3CH2NH3+) (rA =

023 nm) [25ndash28] and formamidinium (NH2CH=NH2

+) (rAis estimated in the range 019ndash022 nm) [29ndash31] Anion Xis a halogen typically iodine (rX = 0220 nm) though Brand Cl are also increasingly being reported (rX = 0196 nmand 0181 nm) typically in a mixed halide configurationPb (rB = 0119 nm) and Sn (rB = 0110 nm) as cation Bhave exclusively been used for high efficiency perovskitesolar cells because of lower and theoretically ideal band gaps[29] Lower stability associated with Sn is due to ease ofoxidation of Sn to SnI

4while relativistic effects afford greater

oxidation protection in Pb [29]Thus the standard compoundis methylammonium lead triiodide (CH

3NH3PbI3) with

mixed halides CH3NH3PbI3minusxClx and CH

3NH3PbI3minusxBrx

also being important

3 Perovskite-Sensitized Solar Cells

DSSCs [7 32] are the forerunners of perovskite solar cellsConstituent of a typical DSSC is mesoporous n-type titaniasensitized with a light absorbing dye in a redox activeelectrolyte Porous titania affords greater internal surfacearea to the sensitized dye for efficient absorption of incidentphotons though thicker films of the order of 10 120583m arerequired for complete absorption over the absorbing rangeof the dye [32] The requirement of 10 120583m thick active layer isimpractical for solid-state DSSCs where a number of factorslimit the active layer thickness to less than 2120583m [33] As analternate thin film semiconductor active layers and quantumdots enable complete light absorption in much thinner layerswhile at the same time pushing the photosensitivity furtherinto near infrared (NIR) [34ndash38] In the backdrop of findingmore efficient light sensitizers for DSSCs Miyasaka andcoworkers reported the first perovskite-sensitized solar cellsbetween 2006 and 2008 CH

3NH3PbI3and CH

3NH3PbBr3

absorbers were employed with an iodide triiodide redoxcouple or a polypyrrole carbon black composite solid-statehole conductor Full sun power conversion efficiency varyingbetween 04 and 2 was measured for solid-state and liquidelectrolyte cells respectively [39 40]

The first peer-reviewed report for perovskite-sensitizedsolar cell was published in 2009 CH

3NH3PbI3absorber in an

iodidetriiodide redox couple achieved an efficiency of 35[41] Figure 2 illustrates the schematic of an organometallichalide sensitized titania solar cell and the associated spectralresponse Retaining the liquid electrolyte reported byKim and coworkers achieved an improved efficiency of65 by optimizing the titania surface morphology andperovskite processing [41 42] The tumbling block in theliquid electrolyte based perovskite-sensitized solar cellwas the dissolution and decomposition of perovskite inthe liquid electrolyte Resultantly the solar cells exhibitedpoor stability and would degrade within minutes [41 42]The solution to this problem lied in adoption of solid-state hole transport medium in place of electrolyte as wasoriginally tried by Kojima and coworkers in 2006 [40]Methylammonium trihalogen plumbates being relativelyinsoluble in nonpolar organic solvents paved the way forrealizing first perovskite sensitization and made subsequentinfilling with the organic hole conductor possible Thismade T N Murakami and T Miyasaka and N G Parkin collaboration with M Gratzel and coworkers developsolid-state perovskite solar cells employing (22(77)-tetrakis-(NN-dipmethoxyphenylamine)99(-spirobifluorene))(spiro-MeOTAD) as the hole transporter [43] withmaximumfull sun power conversion efficiencies of between 8 and 10employing CH

3NH3PbI3minusxClx mixed halide perovskite and

CH3NH3PbI3 respectively [18 44]

This achievement marked a considerable jump in perfor-mance over the best reported solid-state (ss) DSSCs with a

International Journal of Photoenergy 3

X

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X XX

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X

XXX

X

X XX

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X

BB

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A

A

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A

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A

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AAA

A

A

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XX

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XB

XXXXX

X

X XXX

XXBX

X

XX

XXXX

XXXXXXXXXXXXXXB

X

X

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A

BX

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X

X

X

A

A

A

A

A

A

A

AA

A A

XXXXXX

XXBX

X

XX

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AA

Figure 1 Perovskite crystal structure

Perovskites

Perovskites

Perovskites

Perovskites

eminusTiO2

Visible l

ight70

60

50

40

30

20

10

0

400 500 600 700 800

Wavelength (nm)

IPCE

()

CH3NH3PbBr3CH3NH3PbI3

Figure 2 Schematic of perovskite-sensitized TiO2undergoing photoexcitation and electron transfer (left) The incident photon-to-electron

conversion efficiency (IPCE) spectra for perovskite-sensitized solar cells [18]

recorded efficiency of over 7 [45] This is made possibleby the advantages perovskites afford over conventional dyesin the form of stronger absorption over a broader rangeresulting in complete absorption in film thickness as low as500 nm This is a considerable advantage for solid-state cellswhere light absorption and photocurrent generation havetypically been limited by the requirement of film thicknessof around 2 120583m [33]

4 Mesoporous TiO2

Scaffold

First reported high efficiency and stable perovskite solarcell [46] was synthesized by spin coating 120574-butyrolactone

solution containing equimolar CH3NH3I and PbI

2in 2012

This resulted in deposition of semispherical nanodots ofCH3NH3PbI3on TiO

2 Since the surface was not fully cov-

ered with nanodots it was evident that photoexcited electronswere transferred from CH

3NH3PbI3to TiO

2 This cell with

a 06 120583m titania film achieved a PCE of 97 and a Voc of888mV Fill factor of 062 was indicative of poor pore fillingwith spiro-MeOTAD and resulting poor interface betweenHole Transport Material (HTM) active layer and ElectronTransport Material (ETM)

A device architecture of TiO2CH3NH3PbI3Au [41]

experimentally confirmed the hole transport properties ofCH3NH3PbI3 It achieved a PCE of 55 with FTO100 nm


thick TiO2as hole blocking and electron-transporting mate-

rial Increase of TiO2to 100 nm improved PCE to 8 [47]

Mott-Schottky analysis was used to confirm depletion zonebetween TiO

2and CH

3NH3PbI3 and the structure was

referred to as depleted heterojunction perovskite solar cellLarge shunt resistance caused a poor fill factor (FF) and alower ExternalQuantumEfficiency (EQE) at longwavelengthrange of 540minus800 nm

Ambipolar characteristics of CH3NH3PbI3were reported

by Laban and Etgar [48] based on thin film transistor (TFT)analysis with slightly stronger p-type nature Static and tran-sient photoluminescence (PL) decay studies established thatcharge carriers are injected into TiO

2with some charge sepa-

ration taking place at CH3NH3PbI3poly(triarylamine)HTM

interface as well Superior PCE over spiro-MeOTAD systemis attributed to thin (sim30 nm) HTM layer as compared tospiro-MeOTAD HTM (sim500 nm) resulting in reduced seriesresistance Devices without HTM exhibited poor perfor-mance necessitating a physiochemical study of CH

3NH3PbI3

systemsPerovskite light harvesters have also been reported with

one-dimensional nanostructures Rutile TiO2(sim06 120583m long)

with photoactive CH3NH3PbI3achieved PCE of 94 [49]

Increasing the length from 06 to 16 120583m decreased thephotovoltaic performance With an increase in the lengthof nanorods the tilted nanorods resulted in problems inpore filling with spiro-MeOTAD A change from nanodotdeposition of active layer to pillared structure can overcomethis issue BrminusI mixed perovskite CH

3NH3PbI2Br with TiO

2

achieved PCE of 487 slightly higher than that achievedfor CH

3NH3PbI3owing to the higher Voc of CH

3NH3PbI2Br

mixed halide perovskite [50]CH3NH3PbBr3in a mesoporous TiO

2with poly[N-9-

hepta-decanyl-27-carbazole-alt-36-bis-(thiophen-5-yl)-25-dioctyl-25-dihydropyrrolo[34-]pyrrole-14-dione] (PCBTDPP)as HTM layer generated a Voc of 12 V [51] With P3HT usedas HTM Voc reduced to 05 V When considering electroninjection from the photoactive layer to mesoporous titaniaVoc is determined by difference in the Fermi level of TiO

2

and the HOMO level of the HTM Such large differencein Voc while the HOMO levels are only 002 eV apart inPCBTCP and P3HT (PCBTDPP 54 eV and P3HT 52 eV)was attributed to the efficiency of light filtering and degree ofchemical interaction [52] PCBTDPP based device affordedenhanced light filtering and stronger chemical interactioneffecting charge recombination and an up-shift of Fermilevel resulting in a high Voc Faster recombination in P3HTresulted in electron lifetime being one order of magnitudelower than spiro-MeOTAD in TiO

2CH3NH3PbI3HTM

system [53] These results suggest that factor other thanenergy levels must be born in mind for selecting HTMs

5 Mesosuperstructured PSCs Based onNonelectron Injecting Oxides

CH3NH3PbI2Cl mixed perovskite coated alumina layer in

a photovoltaic cell resulted in a PCE of 109 [54] Thisdevice structure was called ldquomesosuperstructured solar cellrdquo(MSSC) as the photogenerated electrons are not transferred

to alumina because of the difference in band edges of aluminaand perovskite active layer which acts only as a scaffold forcarrying the photoactive layer Spin coated CH

3NH3PbI2Cl

was more stable in air than CH3NH3PbI3 Smaller size of Clminus

ion presumably stabilizes the anion in perovskite structureAl2O3ndashCH3NH3PbI2Cl system generated higher Voc than

TiO2ndashCH3NH3PbI2Cl Photoinduced spectroscopic study

confirmed that the difference is due to the differing behaviorof photogenerated electrons in the two systems Scaffoldlayers afford processing at lower temperatures by excludinghigh temperatures annealing step as neither are generatedelectrons injected into themesoporous layer nor transportedPCE value of 123 [33] was achieved for the low temperatureprocessed mesosuperstructured solar cell (as 150∘C-driedAl2O3mesoporous layers) Varying the thickness of the

Al2O3scaffold layer from 0 (no scaffold) to ca 400 nm a

perovskite CH3NH3I3minusxClx overlayer formed on the 80 nm

thick alumina film though no capping layer formed on the400 nm thickAl

2O3filmThehighest PCEwas observed from

the 80 nm Al2O3layer resulting in highest Jsc while 400 nm

Al2O3layer exhibited improved Voc and FF These results

suggest that enhanced crystallinity in perovskite active layerenhances Jsc while pinhole-free perovskite results in highVoc and FF Codeposited alumina nanoparticle suspension(5wt Al

2O3) and perovskite precursors in DMF resulted in

a PCE of 716 [55]CH3NH3-PbBr

3with a larger bandgap (119864

119892= 23 eV)

than CH3NH3PbI3was employed as a light harvester and

electron transporter employing the mesosuperstructure con-cept Voc of 13 V [56] was achieved with device structure ofAl2O3CH3NH3PbBr3NN1015840-dialkylperylene diimide (PDI)

This large Voc could be explained due to the large differ-ence between LUMO (42 eV) of CH

3NH3-PbBr

3and the

HOMO (58 eV) of PDI and the noninjection of photoex-cited electrons into the alumina scaffold Mesosuperstructureconcept was also evaluated with ZrO

2mesoporous scaf-

fold with CH3NH3PbI3light harvester exhibiting significant

photovoltaic activity Voc of sim900mV though lower thantitania [57] Impedance spectroscopic study employing three-electrode electrochemical cell verified that zirconia scaffoldwas not charged up to a bias of 09 V in contrast to tita-nia mesoporous layer indicating that the photogeneratedelectrons are not injected into zirconia scaffold [57] Thiscomparative study determined that CH

3NH3PbI3was able to

accumulate charges owing to large density of states (DOS)A higher efficiency of 108 was reported for CH

3-NH3PbI3

[51] sensitizing ZrO2scaffold with a Voc of 107V

6 Planar Heterojunction Structured Cells

Thermally coevaporated CH3NH3I and PbCl

2and deposited

CH3NH3PbI3minusxCl3 onto FTO with a thin TiO

2layer resulted

in a PCE of 154 [58] This proves the bifunctional propertyof perovskites that is photoexcitation and charge trans-port Mesoporous oxide layer thus may not be requiredVapor deposited perovskite layer showed an enhancedPCE over solution processed active layer due to increasedmorphology control and formation of homogeneous flatpinhole-free active layer While it is easier to obtain a flat


FTO

Blocking layer

FTO

Blocking layer

FTO

Blocking layer

FTO

Blocking layer

HTMTiO2 TiO2Al2O3

CH3NH3PbX3

120578 = 97 120578 = 109 120578 = 15 120578 = 154

Figure 3 Progress in perovskite solar cells from PCE of 97 through 109 15 and 154 using device architectures ranging from titaniasensitized with active layer to mesosuperstructure inert scaffold and planar pndashn junction

CH3NH3PbI3minusxClx active layer by solution processing it is

difficult to form a uniform active layer of CH3NH3PbI3by

solution processing Schematics of device architecture aresketched in Figure 3

7 Hybrid Perovskite Solar Cells

P3HTPCBM blend has been extensively explored inorganic photovoltaic applications P3HT was replaced withCH3NH3PbI3as photoactive layer in planar heterojunction

solar cell achieving a PCE of 39 [59] Better wettabilitywas achieved with PEDOT PSS coated ITO substrate withDMF solution rather than coating perovskite active layerwith 120574-butyrolactone solution A device structure based onTiO2CH3NH3PbI3minusxClxP3HT achieved better photovoltaic

performance when the ITO substrate was treated with C60self-assembled monolayer PCE was improved from 38 to67 with the incorporation of self-assembled monolayerThis was achieved with a significant increase in both Jscand Voc [60] Concept of hybrid planar heterojunctioncell incorporating 285 nm thick layer of CH

3NH3PbI3

was investigated recently Active layer was sandwichedbetween hole-transporting poly(NN1015840-bis(4-butylphenyl)-NN1015840-bis(phenyl)-benzidine) (poly-TPD) layer (10 nm) andelectron accepting PCBM layer (10 nm) resulting in a PCE of12 [61] Charge collecting layers were solution-processedin spin-coated chlorobenzene while the active layer wasvacuum-deposited by heating of reagents CH

3NH3I to 70∘C

and PbI2to 250∘C

8 Flexible Perovskite Solar Cells

Low temperature solution processability of perovskite solarcells makes it possible to fabricate cells on flexible substratesAbility to conform to the contours of the platform holdsobvious promises for incorporation of this technology in

diverse applications CH3NH3PbI3minusxClx as active layer with

PEDOT PSS and PCBM as hole-transporting and electronselective contacts respectively have been investigated inregular and inverted device architecture on an ITO coatedPET substrate achieving a PCE of 64 [62] A higher PCEof 102 was achieved using device structure of ITOZnO(25 nm)CH

3NH3PbI3spiro-MeOTADAg fabricated using

low temperature solution processing techniques [63] ITOcoated PEN substrate has been used to demonstrate a wear-able perovskite based energy source achieving a PCE of 122with only 5 loss over 1000 bending cycles of radius 10mm[64]

9 Hybrid Multijunction Solar Cells

With the current state of the art perovskite solar cells can beused effectively as a top cell in a tandem cell configuration[65] with existing technologies like crystalline silicone andthin film solar cells like CZTSSe CIGS [66] and CIS Witha reasonable estimate of achieving 20mA cmminus2 and Voc of11 V at the top perovskite cell a silicon cell generating 075VVoc [67] will lead to a FF of 08 and an efficiency of 296[68]This all is possible with the existing technologies at handwith little optimization in terms of band gap widening FFenhancement and integration into tandem cell architecture[69 70]

10 Balanced Electron- andHole-Transporting Properties

Determination of diffusion lengths of electrons and holes inan active layer is vital parameter which dictates the optimaldevice architecture If the diffusion length of the charges issmaller than the absorption depth amesostructured device issuitable for obtaining optimal charge collection On the con-trary planar junction devices are suited for efficient charger


collection and transport without significant recombinationfor larger diffusion lengths

Transient photoluminescence (PL) and transient absorp-tion spectroscopy measurements were used by Xing et al[71 72] and Stranks et al [73] to report transportproperties of CH

3NH3PbI3and CH

3NH3PbI3minusxClx Dif-

fusion lengths for electrons and holes were reported at129 nm (130 nm) and sim105 nm (90 nm) respectively inCH3NH3PbI3and at sim1069 nm and sim1213 nm respectively

in CH3NH3PbI3minusxClx [71 73] Thus it is evident that optimal

architecture for CH3NH3PbI3is mesostructured cell while

for CH3NH3PbI3minusxClx planar geometry will yield the highest

PCE Comparable decay time of photoinduced absorptionat sim550 nm in transient absorption spectroscopy and tran-sient PL has indirectly confirmed the generation of weaklybound photogenerated excitons in CH

3NH3PbI3minusxClx Data

from dielectric constant transient absorption spectroscopyimpedance spectroscopy and transient photoluminescencefor perovskite materials suggest that the excitons generatedare Wannier-type [74]

11 Band Gap Engineering ofPerovskite Materials

Semiconductors with a band gap of 11 V are consideredmost suitable for harvesting the solar spectrum from visibleto NIR Strength of the potential developed is dependenton this band gap with low potential leading to drawingof extra current at the expense of reduced voltage Thebalancing of these two parameters dictates the optimal bandgap in the range of 14 eV for cells made of single material[75] Research has indicated that band gap in perovskitesdecreases with increase in dimensionality of MO(X)

6net-

work [76] increase in the angle of MndashO(X)ndashM bonds [77]decrease in electronegativity of anions [78ndash82] and decreasein difference of electronegativity between metal cation andanion Perovskite structure incorporating mixed iodide andbromide [31 49 83] or bromide and chloride [84] halide hascontinuously tuneable band gap allowing for light harvestingover the complete solar spectrum Mixed chloride iodidehalide perovskite renders no obvious band gap tuning owingto difficulty in incorporating Clminus ion into PbI

6octahedron

[33 85] Energy levels of typical absorbers hole selectivecontacts and electron selective contacts are illustrated inFigure 4

12 Improving Fill Factor

Low conductivity of HTM is the major reason for low FFof perovskite solar cells which can be remedied by dopingthe HTM with p-type codopant Lithium salt Li-TFSIadded to spiro-MeOTAD increases the hole conductivityin spiro-MeOTAD [86] Photoelectron spectroscopy com-bined with absorbance measurements revealed that theFermi level in spiro-MeOTAD is shifted toward HOMOdue to the oxidation of 24 spiro-MeOTAD moleculesby the addition of Li-TFSI [87] Cobalt complexes basedp-dopants in spiro-MeOTAD have been used to enhance

the fill factor in ss DSSC [88] p-dopant coded FK209 (tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)-cobalt(III)-tris(bis(trifuoromethylsulfonyl)imide)) to the spiro-MeOTADtogether with Li-TFSI and tBP has been used to enhance thephotovoltaic properties of CH

3NH3PbI3activated solar cells

by increasing the FF and Voc [89] Protic ionic liquid has alsobeen used recently as p-dopant to HTMs [90] In additionto use of doping technique to improve the FF improving themorphology (defect- and pinhole-free) of the active layerenhances the FF by enhancing the conductivity of HTMand reducing the series resistance Establishing perfect pndashncontact between active layer and HTM also improves the fillfactor Chemical modification can be used to fine-tune thiscontact between organic and inorganic layers from HTMand perovskite respectively

13 Fabrication Techniques

Approaches reported for the synthesis of perovskite activelayers are one-step precursor solution deposition [72] two-step sequential deposition [91] dual-source vapor deposition[59] vapor assisted solution process [92] and sequentialvapor deposition [93] CH

3NH3PbX3perovskites have been

reportedwith both one-step and two-step coating techniquesIm et al [43] first reported the one-step coating method foriodide perovskite active layer by reacting equimolar CH

3NH2

andHI in the appropriate solventThewhite precipitates wereobtained by introducing ethyl ether to the resulting solutionfollowed by vacuum drying for about 12 h The synthesizedCH3NH3I was then mixed with PbI

2at a 1 1 molar ratio in

120574-butyrolactone at 60∘C and used as a coating solution Fortwo-step coating method lead iodide film is first formed ontitania surface through spin coating or vacuum depositionand the layer thus formed is immersed in CH

3NH3I [91 94]

containing solutionOne-step solution processing is the simplest of the solu-

tion processing technique for the growth of perovskite layerA precursor solution is spun-cast on substrate resulting inloss of excess solution drying of solvent and annealing ofthe solvent While these processes proceed simultaneouslyproperties of the solvent additives and the annealing andprocessing temperatures and environments have a profoundimpact on the final film quality

Solvents reported for optimum solution process are typ-ically 120574-butyrolactone (GBL) dimethylformamide (DMF)and dimethyl sulfoxide (DMSO) GBL gives a lower maxi-mum concentration of solute at 40wt [43] while DMF andDMSO give higher loading of 60wt at room temperatures[95] Mixture of solvents has also been reported for obtain-ing optimum concentrations for solution processing sincehigher concentrations lead to better film coverage and higherefficiency devices [72 73 96] Best results are reported fora combination of DMF and GBL (3 97 vol) owing to therapid evaporation of this combination of solvents [74 97]Application of solvents like toluene during spin coating at thesurface of the perovskite film leads to better concentrationand uniform morphology This is attributed to the rinsing ofexcess solvent by toluene leading to better crystallization ofperovskites [98]


Al 2O

3Ti

O2

ZnO

PCBM mdash mdash

N719

Sb2S

3

CH3N

H3P

bI3 mdash mdash

spiro

-MeO

TAD

CuSC

NP3

HT

PCPD

TBT

PTA

A

minus30

minus35

minus40

minus45

minus50

minus55

minus60

minus65En

ergy

ban

d le

vels

(eV

)

CH3N

H3P

bI3minus

xCl

x

Figure 4 Energy level illustration for different ETM (left) absorbers (center) and HTM (right) in solar cells

Perovskite film coverage is also dependent upon anneal-ing temperature during solution processing by one-stepsolutionmethod [99] Lower annealing temperature results inpoor film convergence while higher annealing temperatureslead to decomposition of active layer [100] Optimum anneal-ing conditions are reported to be slow annealing at 100∘Cforming particles of 100ndash1000 nm [101]

Improvement in film formation was made possible bytwo-step sequential deposition [91 94] within the realm ofsolution processing Two-step methods afford higher loadingof lead iodide by first spin casting of lead iodide followed bysolution processing or vacuum assisted deposition of MAI[93] It is a heterophase reaction resulting in conversionto MAPbI

3 This results in a more compact uniform and

reproducible film Alterations have been adopted by differentresearchers to optimize this process including prewetting[102] the lead iodide film in dispersion solvent and heatingthe substrate resulting in higher efficiencies [91 102 103] Suc-cessive spin coating method an improvement over two-stepmethod has also been used to improve the film formation[104] Use of DMSO a strong coordinated solvent for solu-tion processing of PbI

2results in amorphous film ensuring

smooth and uniform coverage and complete conversion toMAPbI

3[105]

A modification of two-step deposition method is thevapor assisted growth of MAI on the PbI

2film Compact

and uniform PbI2film obtained by solution processing is

exposed toMAI vapors under ambient conditions In contrastto vapor deposition this method does not require expensivevacuum equipment and environmental controls Combiningthe advantages of solution processing and low temperaturevapor deposition the films grown are pinhole-free offeringhigher efficiencies [93 106] Doctor blade [107] liquid dropletassisted two-step solution process [108] spray and brushsolution processing [109] and flash evaporation [110] havealso been recently reported to add to the versatility ofsynthesis techniques

14 Stability Studies

Two parameters important for commercial application ofperovskite solar cells are stability and efficiency Lots ofresearch effort has been directed at enhancing the efficiencyof these devices by adoption of various device architec-tures compositions and manufacturing techniques This hasresulted in substantial increase in efficiencies to a provenefficiency of 201 [111] The limiting factor to this successstory is the efficiency High efficiency devices reported aresynthesized under controlled environments and lose theirefficiencies rapidly For their commercial viability it is imper-ative that studies be undertaken on issues of stability andreproducibility to enhance the lifetime of these devicesDegradation in perovskite solar cells is a synergetic effectof exposure to humidity oxygen ultraviolet radiations andtemperatures

Niu et al have proposed a sequence of chemical reactionsconsidered responsible for the degradation of CH

3NH3PbI3

in the presence of moisture [16]

CH3NH3PbI3 (s) larrrarr PbI2 (s) +CH3NH3I (aq) (1)

CH3NH3I (aq) larrrarr CH3NH2 (aq) +HI (aq) (2)

4HI (aq) +O2 (g) larrrarr 2I2 (s) + 2H2O (l) (3)

2HI (aq) larrrarr H2 (g) + I2 (s) (4)

The equilibrium species in the presence of water oxygenand UV radiation are thus CH

3NH3I CH

3NH2 and HI

HI can either decompose by a one-step redox reaction (3)or by photochemical reaction under UV radiation to H

2

and I2 This sensitivity requires synthesis in a controlled

environment like a glove box [59 91] A humidity of 55is reported to deteriorate performance and is evident bya color change from dark brown to yellow [49] Devicesincorporating Al

2O3scaffold showed better stability in line

with the reports on stability of DSSC with this scaffold [112


113] CH3NH3PbBr3is reported to bemore stable to moisture

exposure andCH3NH3Pb(I1minusxBrx)3 based absorbers retained

good PCE on exposure to humidity of 55 for 20 days[49] Amoisture induced reconstructionmechanism has alsobeen proposed for controlled humidity synthesis in planargeometryThough the efficiency increased to 193 it rapidlydeteriorated to less than 5 of original performance whenstored in ambient conditions [110] Encapsulation techniquesdeveloped for CIGS based devices could prove effective inaddressing the moisture sensitivity of these devices [114] APCE of 1576 has been achieved for devices synthesized inambient conditions with humidity of 50 by incorporatinga substrate preheating step before spin-coating lead iodide[115]

UV sensitivity is attributed to use of TiO2as photoanode

inPSCWith a band gap of 32 eV it is typically a photocatalystfor oxidizing water and organicmatter [116] Proposed degra-dation mechanism for CH

3NH3PbI3under UV illumination

is [117]

2Iminus larrrarr I2 + 2eminus

[at the interface between TiO2 and CH3NH3PbI3](5a)

3CH3NH3+larrrarr 3CH3NH2 uarr + 3H

+ (5b)

Iminus + I2 + 3H++ 2eminus larrrarr 3HI uarr (5c)

Inclusion of Sb2S3layer at the interface between mesoporous

TiO2and perovskite layer was observed to increase the

stability attributed to the interruption of iodide couple atthe interface UV activated decay at the interface of TiO

2

is also reported to be arrested by the presence of oxygenwhich removes surface states and pacifies deep trap sites atthe interface titania being an n-type semiconductor [118119] Use of alumino silicate shell on titania nanoparticlescan increase stability [119] Al

2O3scaffold is a more stable

alternate to TiO2in MSSC [118 120]

Thermal stability studies are important as under solarillumination the temperatures are expected to be over thephase transition temperature of CH

3NH3PbI3 It changes

from tetragonal to cubic structure at 56∘C [121] Studies haveindicated that thermal stability can be affected by subtle vari-ations in synthesis routes and the precursors used [122] Poorthermal conductivity by monocrystalline and polycrystallineCH3NH3PbI3also creates internal stresses to the detriment of

the mechanical integrity of the films and hence the lifetime ofthe devices [123] Use of polymers with single-walled CNTs asa HTL has been reported to increase the thermal stability andretard moisture sensitivity of the perovskite solar cells [124]Issue of lead poisoning has been simulated by investigatingthe effect of rain with water of different pH values concludingthat the use is environmentally safe [125]

15 Hysteresis Effects

Ferroelectric polarization of perovskites has been under focusrecently Understanding the ferroelectric behaviour of thisclass of materials may be critical in increasing its efficiencyand stability as ferroelectricity may affect the photoexcited

electron hole pairing and separation [126 127] The originsof this effect are still postulated Ferroelectricity came underfocus on the reports of hysteresis behaviour in current voltagescans [128ndash130] dependent on the scan rate and direction[129] light soaking history [128] and contact material andinterfaces [131] Snaith et al have proposed capacitive effectsferroelectric behavior of absorber and defect densities tobe the source of hysteresis behaviour [131] This effect ifnot accounted for results in erroneous efficiency values incomparison to the stabilized output [132] Conflicting reportsthrough theoretical and experimental values have addedimpetus to the current research The observed hysteresis incurrent voltage (JV) curves was attributed to ferroelectricdomain under applied electric field while the same effectcan also be caused by ionic migration [131 133] or trappingdetrapping [131 134] of charge carriers Identification ofpolar I4 cm space group [135] instead of previously acceptednonpolar I4mcm [131 133] had added weight to the theoryof ferroelectric polarization Theoretical calculations havepostulated a polarization value of 38 120583Ccm2 [126]

Studies of polarization electric field [130] were usedto investigate the ferroelectric response of CH

3NH3PbI3

though they were heavily distorted by leakage currents andmay not be conclusive evidence to the ferroelectric responsePiezoresponse force microscopy [136] indicated ferroelectricdomains though no confirmation of local switching of ferro-electric domains was presented Concurrent evaluation of PEloops and piezoresponse force microscopy by Xiao et al [137]in a recent report did not detect ferroelectric polarization incontrast to the other reports Many reports have supporteda ferroelectric response [129 130 135 136 138] by thisclass of materials Understanding the physical phenomenonresponsible for this effect is vital for efficiency enhancementand is an open area of research Physical optimization by trialand error and theoretical simulations will lead to stabilizedefficiencies and will add to maturing of this technology

16 Comparison to Other Technologies

Solar cell has to absorb sun light and convert its incidentenergy into electrical power The energy of incident photonsis scattered over the entire spectrum of solar radiationsPhotons with energy equal to the band gap of the active layerin a PV cell only are absorbed while photons with energylower than the band gap are not absorbed at all while photonswith energy higher than the band gap generate electron abovethe conduction band which lose their energy in the form ofheat while relaxing back to valence bandThus themaximumvoltage solar cells generate is the open circuit voltage and itindicates the maximum electrical power derivable form theincident photons

Perovskites are unique as active layer in PV modulesowing to their ability to deliver high open circuit voltagesunder full sun illumination leading to light harvesting froma broad spectrum of incident solar radiation Voc is ameasure of the maximum energy that can be drawn fromthe incident photon by the solar cell The difference betweenVoc and the potential of the lowest energy photon generatinga charge is a measure of the fundamental loss in a solar


cell [139] Thermodynamic treatment limits this loss to thetune of 250minus300meV varying with the band gap basedon Shockley-Queisser treatment [75] Onset of the IPCEspectrumdetermines the lowest energy absorbed photon ForCH3NH3I3minusxClx perovskite the onset is 155 eV (800 nm) and

the best Voc of 11 gives a loss in potential of 450meVwhich islower than the reported loss of 059 eV for best commerciallyavailable PV technology CdTe at an efficiency of 196 [140]Thus perovskite solar cells at the current state of the art areat par with commercial technologies like CIGS GaAs andcrystalline silicon [141]

17 Future Outlook

In a span of a couple of years perovskites have demonstratedthat they possess the right mix of properties to offer asolution to our energy requirements Though they evolvedout of liquid electrolyte DSSCs they are now established asa class of their own with extensive research focus pushingthe efficiency limit beyond 20 Low temperature solutionprocessing assures low per watt cost and quick energypayback times Device architectures ranging from pin [5559 99] to mesoporous to mesosuperstructure configurationthrow wide open the possibilities for incorporation of novelmaterials and synthesis approaches The future may holdno scaffold pin planar configuration or the incorporation ofother semiconductormaterials for inert oxide scaffold Use ofmaterials with highmobility asHTMwill further improve theFF while optimization of the interfaces and selection of HTMand ETMmay push forward the efficiencies to higher valuesIncorporation of narrow band gap perovskites and plasmoniclight harvester may broaden the spectral response withbetter light harvesting Interface engineering and introduc-tion of self-assembled layers will reduce losses and improveefficiency Understanding of the underlying photophysicalphenomenon will further help improving device structuresand better selection of materials Exploration of tandem cellconfiguration with perovskite based cell as the top cell willpush forward the achievable efficacy limit further With theamount of research effort under way guided by the adherenceto the issue of best practices [142] this technology holdsgreat promise to addressing our energy concerns Addressingthe issues of stability and use of lead can go a long way inmaturing this technology for commercial application thoughin the present legal framework use of lead is not a problem asCdTe based solar cell has received wide acceptance despiteCd content Use of lead extensively in lead acid batteriesand its content at comparable levels in CIGS and siliconmodules to perovskites [143] suggest that in the short termthe concern may not be pressing but these technologies areincreasingly being phased out and alternatives are explored tominimize the environmental impacts of these heavy metalsReplacement of lead with tin in perovskite solar cell is alreadyunder investigation and may offer an environment friendlyalternative [144]

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] M A Green K Emery Y Hishikawa W Warta and E DDunlop ldquoSolar cell efficiency tables (version 40)rdquo Progress inPhotovoltaics Research and Applications vol 20 no 5 pp 606ndash614 2012

[2] M A Green ldquoSilicon solar cells evolution high-efficiencydesign and efficiency enhancementsrdquo Semiconductor Scienceand Technology vol 8 no 1 pp 1ndash12 1993

[3] J Britt and C Ferekides ldquoThin-film CdSCdTe solar cell with158 efficiencyrdquo Applied Physics Letters vol 62 no 22 article2851 1993

[4] I Repins M A Contreras B Egaas et al ldquo199-efficientZnOCdSCuInGaSe2 solar cell with 812 fill factorrdquo Progressin Photovoltaics Research and Applications vol 16 no 3 pp235ndash239 2008

[5] M Graetzel R A J Janssen D B Mitzi and E H SargentldquoMaterials interface engineering for solution-processed photo-voltaicsrdquo Nature vol 488 no 7411 pp 304ndash312 2012

[6] J J M Halls C A Walsh N C Greenham et al ldquoEfficientphotodiodes from interpenetrating polymer networksrdquoNaturevol 376 no 6540 pp 498ndash500 1995

[7] B OrsquoRegan and M Gratzel ldquoA low-cost high-efficiency solarcell based on dye-sensitized colloidal TiO

2filmsrdquo Nature vol

353 no 6346 pp 737ndash740 1991[8] C W Tang ldquoTwo-layer organic photovoltaic cellrdquo Applied

Physics Letters vol 48 no 2 article 183 1986[9] T K Todorov K B Reuter and D B Mitzi ldquoHigh-efficiency

solar cell with earth-abundant liquid-processed absorberrdquoAdvanced Materials vol 22 no 20 pp E156ndashE159 2010

[10] G Li V Shrotriya J Huang et al ldquoHigh-efficiency solutionprocessable polymer photovoltaic cells by self-organization ofpolymer blendsrdquo Nature Materials vol 4 no 11 pp 864ndash8682005

[11] M A Green A Ho-Baillie and H J Snaith ldquoThe emergence ofperovskite solar cellsrdquo Nature Photonics vol 8 no 7 pp 506ndash514 2014

[12] G Hodes and D Cahen ldquoPhotovoltaics perovskite cells rollforwardrdquo Nature Photonics vol 8 no 2 pp 87ndash88 2014

[13] S H Im J-H Heo H J Han D Kim and T Ahn ldquo181hysteresis-less inverted CH

3NH3PbI3planar perovskite hybrid

solar cellsrdquo Energy amp Environmental Science vol 8 pp 1602ndash1608 2015

[14] J H Heo D H Song H J Han et al ldquoPlanar CH3NH3PbI3

perovskite solar cells with constant 172 average power conver-sion efficiency irrespective of the scan raterdquoAdvancedMaterials2015

[15] P P Boix K Nonomura N Mathews and S G MhaisalkarldquoCurrent progress and future perspectives for organicinorganicperovskite solar cellsrdquo Materials Today vol 17 no 1 pp 16ndash232014

[16] G Niu X Guo and L Wang ldquoReview of recent progress inchemical stability of perovskite solar cellsrdquo Journal of MaterialsChemistry A vol 3 pp 8970ndash8980 2014

[17] S Luo andW A Daoud ldquoRecent progress in organic-inorganichalide perovskite solar cells mechanisms andmaterials designrdquoJournal of Materials Chemistry A vol 3 pp 8992ndash9010 2014

[18] A Kojima K Teshima Y Shirai and T Miyasaka ldquoOrgano-metal halide perovskites as visible-light sensitizers for photo-voltaic cellsrdquo Journal of the American Chemical Society vol 131no 17 pp 6050ndash6051 2009


[19] R F Service ldquoEnergy technology perovskite solar cells keep onsurgingrdquo Science vol 344 no 6183 article 458 2014

[20] J G Bednorz and K A Muller ldquoPossible high119879119888supercon-

ductivity in the BandashLandashCundashO systemrdquo Zeitschrift fur Physik BCondensed Matter vol 64 pp 189ndash193 1986

[21] X Xu C Randorn P Efstathiou and J T S Irvine ldquoA redmetallic oxide photocatalystrdquoNatureMaterials vol 11 no 7 pp595ndash598 2012

[22] Z Cheng and J Lin ldquoLayered organic-inorganic hybrid per-ovskites structure optical properties filmpreparation pattern-ing and templating engineeringrdquoCrystEngComm vol 12 no 10pp 2646ndash2662 2010

[23] D B Mitzi S Wang C A Feild C A Chess and A MGuloy ldquoConducting layered organic-inorganic halides contain-inglt110gt-oriented perovskite sheetsrdquo Science vol 267 no 5203pp 1473ndash1476 1995

[24] C Li X Lu W Ding L Feng Y Gao and Z Guo ldquoFormabilityof ABX

3(X = F Cl Br I) halide perovskitesrdquo Acta Crystallo-

graphica B vol 64 pp 702ndash707 2008[25] B N Cohen C Labarca N Davidson and H A Lester

ldquoMutations in M2 alter the selectivity of the mouse nicotinicacetylcholine receptor for organic and alkali metal cationsrdquoJournal of General Physiology vol 100 no 3 pp 373ndash400 1992

[26] J-H Im J Chung S-J Kim and N-G Park ldquoSynthesisstructure and photovoltaic property of a nanocrystalline2H perovskite-type novel sensitizer (CH

3CH2NH3)PbI3rdquo

Nanoscale Research Letters vol 7 article 353 2012[27] R S Sanchez V Gonzalez-Pedro J-W Lee et al ldquoSlow dynamic

processes in lead halide perovskite solar cells Characteristictimes and hysteresisrdquo Journal of Physical Chemistry Letters vol5 no 13 pp 2357ndash2363 2014

[28] N K McKinnon D C Reeves and M H Akabas ldquo5-HT3receptor ion size selectivity is a property of the transmembranechannel not the cytoplasmic vestibule portalsrdquo Journal ofGeneral Physiology vol 138 no 4 pp 453ndash466 2011

[29] S Pang H Hu J Zhang et al ldquoNH2CH=NH

2PbI3 an alter-

native organolead iodide Perovskite sensitizer for mesoscopicsolar cellsrdquo Chemistry of Materials vol 26 no 3 pp 1485ndash14912014

[30] S Lv S Pang Y Zhou et al ldquoOne-step solution-processedformamidinium lead trihalide (FAPbI

(3minus119909)CI119909) for mesoscopic

perovskite-polymer solar cellsrdquo Physical Chemistry ChemicalPhysics vol 16 no 36 pp 19206ndash19211 2014

[31] G E Eperon S D Stranks C Menelaou M B Johnston LM Herz and H J Snaith ldquoFormamidinium lead trihalide abroadly tunable perovskite for efficient planar heterojunctionsolar cellsrdquo Energy and Environmental Science vol 7 no 3 pp982ndash988 2014

[32] P Umari E Mosconi and F De Angelis ldquoRelativistic GWcalculations on CH

3NH3PbI3and CH

3NH3SnI3perovskites for

solar cell applicationsrdquo Scientific Reports vol 4 article 44672014

[33] M M Lee J Teuscher T Miyasaka T N Murakami andH J Snaith ldquoEfficient hybrid solar cells based on meso-superstructured organometal halide perovskitesrdquo Science vol338 no 6107 pp 643ndash647 2012

[34] H J Snaith and L Schmidt-Mende ldquoAdvances in liquid-electrolyte and solid-state dye-sensitized solar cellsrdquo AdvancedMaterials vol 19 no 20 pp 3187ndash3200 2007

[35] H J Snaith A Stavrinadis P Docampo and A A R WattldquoLead-sulphide quantum-dot sensitization of tin oxide based

hybrid solar cellsrdquo Solar Energy vol 85 no 6 pp 1283ndash12902011

[36] H Lee H C Leventis S-J Moon et al ldquoPbS and CdS quantumdot-sensitized solid-state solar cells lsquoold concepts new resultsrsquordquoAdvanced Functional Materials vol 19 no 17 pp 2735ndash27422009

[37] P V Kamat ldquoQuantum dot solar cells Semiconductornanocrystals as light harvestersrdquo Journal of Physical ChemistryC vol 112 no 48 pp 18737ndash18753 2008

[38] Y Itzhaik O Niitsoo M Page and G Hodes ldquoSb2S3-sensitized

nanoporous TiO2solar cellsrdquoThe Journal of Physical Chemistry

C vol 113 no 11 pp 4254ndash4256 2009[39] P V Kamat ldquoQuantum dot solar cells The next big thing in

photovoltaicsrdquo Journal of Physical Chemistry Letters vol 4 no6 pp 908ndash918 2013

[40] A Kojima K Teshima T Miyasaka and Y Shirai ldquoNovel pho-toelectrochemical cell with mesoscopic electrodes sensitized bylead-halide compounds (2)rdquo ECS Meeting Abstracts absrtactMA2006-02 p 397 2006

[41] H-S Kim C-R Lee J-H Im et al ldquoLead iodide perovskitesensitized all-solid-state submicron thin film mesoscopic solarcell with efficiency exceeding 9rdquo Scientific Reports vol 2article 591 2012

[42] A Kojima K Teshima Y Shirai and T Miyasaka ldquoNovel pho-toelectrochemical cell with mesoscopic electrodes sensitizedby lead-halide compounds (11)rdquo ECS Meeting vol 27 abstractMA2008-02 2008

[43] J-H Im C-R Lee J-W Lee S-W Park and N-G Parkldquo65 efficient perovskite quantum-dot-sensitized solar cellrdquoNanoscale vol 3 no 10 pp 4088ndash4093 2011

[44] B E Hardin H J Snaith andMDMcGehee ldquoThe renaissanceof dye-sensitized solar cellsrdquo Nature Photonics vol 6 no 3 pp162ndash169 2012

[45] U Bach D Lupo P Comte et al ldquoSolid-state dye-sensitizedmesoporous TiO

2solar cells with high photon-to-electron

conversion efficienciesrdquoNature vol 395 no 6702 pp 583ndash5851998

[46] J Burschka A Dualeh F Kessler et al ldquoTris(2-(1H-pyrazol-1-yl)pyridine)cobalt(III) as p-type dopant for organic semicon-ductors and its application in highly efficient solid-state dye-sensitized solar cellsrdquo Journal of the American Chemical Societyvol 133 no 45 pp 18042ndash18045 2011

[47] L Etgar P Gao Z Xue et al ldquoMesoscopic CH3NH3PbI3TiO2

heterojunction solar cellsrdquo Journal of the American ChemicalSociety vol 134 no 42 pp 17396ndash17399 2012

[48] W A Laban and L Etgar ldquoDepleted hole conductor-free leadhalide iodide heterojunction solar cellsrdquo Energy and Environ-mental Science vol 6 no 11 pp 3249ndash3253 2013

[49] J H Noh S H Im J H Heo T N Mandal and S ISeok ldquoChemical management for colorful efficient and stableinorganic-organic hybrid nanostructured solar cellsrdquo NanoLetters vol 13 no 4 pp 1764ndash1769 2013

[50] H-S Kim J-W Lee N Yantara et al ldquoHigh efficiency solid-state sensitized solar cell-based on submicrometer rutile TiO

2

nanorod and CH3NH3PbI3perovskite sensitizerrdquoNano Letters

vol 13 no 6 pp 2412ndash2417 2013[51] E Edri S Kirmayer D Cahen and G Hodes ldquoHigh open-

circuit voltage solar cells based on organic-inorganic leadbromide perovskiterdquo The Journal of Physical Chemistry Lettersvol 4 no 6 pp 897ndash902 2013


[52] J Qiu Y Qiu K Yan et al ldquoAll-solid-state hybrid solar cellsbased on a new organometal halide perovskite sensitizer andone-dimensional TiO

2nanowire arraysrdquo Nanoscale vol 5 no

8 pp 3245ndash3248 2013[53] B Cai Y Xing Z Yang W-H Zhang and J Qiu ldquoHigh

performance hybrid solar cells sensitized by organolead halideperovskitesrdquo Energy and Environmental Science vol 6 no 6 pp1480ndash1485 2013

[54] D Bi L Yang G Boschloo A Hagfeldt and E M J JohanssonldquoEffect of different hole transport materials on recombinationin CH

3NH3PbI3perovskite-sensitized mesoscopic solar cellsrdquo

Journal of Physical Chemistry Letters vol 4 no 9 pp 1532ndash15362013

[55] JM BallMM Lee AHey andH J Snaith ldquoLow-temperatureprocessed meso-superstructured to thin-film perovskite solarcellsrdquo Energy and Environmental Science vol 6 no 6 pp 1739ndash1743 2013

[56] M J Carnie C Charbonneau M L Davies et al ldquoA one-step low temperature processing route for organolead halideperovskite solar cellsrdquo Chemical Communications vol 49 no72 pp 7893ndash7895 2013

[57] H-S Kim I Mora-Sero V Gonzalez-Pedro et al ldquoMechanismof carrier accumulation in perovskite thin-absorber solar cellsrdquoNature Communications vol 4 article 2242 2013

[58] D Bi S-J Moon L Haggman et al ldquoUsing a two-stepdeposition technique to prepare perovskite (CH

3NH3PbI3) for

thin film solar cells based on ZrO2and TiO

2mesostructuresrdquo

RSC Advances vol 3 no 41 pp 18762ndash18766 2013[59] M Liu M B Johnston and H J Snaith ldquoEfficient planar

heterojunction perovskite solar cells by vapour depositionrdquoNature vol 501 no 7467 pp 395ndash398 2013

[60] J-Y Jeng Y-F Chiang M-H Lee et al ldquoCH3NH3PbI3

perovskitefullerene planar-heterojunction hybrid solar cellsrdquoAdvanced Materials vol 25 no 27 pp 3727ndash3732 2013

[61] A Abrusci S D Stranks P Docampo H-L Yip A K-YJen and H J Snaith ldquoHigh-performance perovskite-polymerhybrid solar cells via electronic coupling with fullerene mono-layersrdquo Nano Letters vol 13 no 7 pp 3124ndash3128 2013

[62] O Malinkiewicz A Yella Y H Lee et al ldquoPerovskite solar cellsemploying organic charge-transport layersrdquo Nature Photonicsvol 8 no 2 pp 128ndash132 2014

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[65] Z M Beiley and M D McGehee ldquoModeling low cost hybridtandem photovoltaics with the potential for efficiencies exceed-ing 20rdquo Energy and Environmental Science vol 5 no 11 pp9173ndash9179 2012

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[68] H J Snaith ldquoPerovskites the emergence of a new era for low-cost high-efficiency solar cellsrdquo Journal of Physical ChemistryLetters vol 4 no 21 pp 3623ndash3630 2013

[69] J P Mailoa C D Bailie E C Johlin et al ldquoA 2-terminalperovskitesilicon multijunction solar cell enabled by a silicontunnel junctionrdquo Applied Physics Letters vol 106 no 12 ArticleID 121105 2015

[70] H Uzu M Ichikawa M Hino et al ldquoHigh efficiency solar cellscombining a perovskite and a silicon heterojunction solar cellsvia an optical splitting systemrdquo Applied Physics Letters vol 106no 1 Article ID 013506 2015

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[73] S D Stranks G E Eperon G Grancini et al ldquoElectron-holediffusion lengths exceeding 1 micrometer in an organometaltrihalide perovskite absorberrdquo Science vol 342 no 6156 pp341ndash344 2013

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[75] W Shockley and H J Queisser ldquoDetailed balance limit ofefficiency of119901minus119899 junction solar cellsrdquo Journal of Applied Physicsvol 32 no 3 pp 510ndash519 1961

[76] Y Takeoka K Asai M Rikukawa and K Sanui ldquoSystematicstudies on chain lengths halide species and well thicknessesfor lead halide layered perovskite thin filmsrdquo Bulletin of theChemical Society of Japan vol 79 no 10 pp 1607ndash1613 2006

[77] J L Knutson J DMartin andD BMitzi ldquoTuning the band gapin hybrid tin iodide perovskite semiconductors using structuraltemplatingrdquo Inorganic Chemistry vol 44 no 13 pp 4699ndash47052005

[78] H W Eng P W Barnes B M Auer and P M WoodwardldquoInvestigations of the electronic structure of d0 transitionmetaloxides belonging to the perovskite familyrdquo Journal of Solid StateChemistry vol 175 no 1 pp 94ndash109 2003

[79] J Etourneau J Portier and F Menil ldquoThe role of the inductiveeffect in solid state chemistry how the chemist can use it tomodify both the structural and the physical properties of thematerialsrdquo Journal of Alloys and Compounds vol 188 pp 1ndash71992

[80] M Jansen and H P Letschert ldquoInorganic yellow-red pigmentswithout toxic metalsrdquo Nature vol 404 no 6781 pp 980ndash9822000

[81] E Knittle and R Jeanloz ldquoSynthesis and equation of state of(MgFe) SiO

3perovskite to over 100 gigapascalsrdquo Science vol

235 no 4789 pp 668ndash670 1987[82] R Marchand F Tessier A Le Sauze and N Diot ldquoTypical

features of nitrogen in nitride-type compoundsrdquo InternationalJournal of Inorganic Materials vol 3 no 8 pp 1143ndash1146 2001

[83] S A Kulkarni T Baikie P P Boix N Yantara N Mathewsand S Mhaisalkar ldquoBand-gap tuning of lead halide perovskitesusing a sequential deposition processrdquo Journal of MaterialsChemistry A vol 2 no 24 pp 9221ndash9225 2014

[84] N Kitazawa Y Watanabe and Y Nakamura ldquoOptical prop-erties of CH

3NH3PbX3(X = halogen) and their mixed-halide

crystalsrdquo Journal of Materials Science vol 37 no 17 pp 3585ndash3587 2002

[85] S Colella E Mosconi P Fedeli et al ldquoMAPbI3minus119909

CI119909mixed

halide perovskite for hybrid solar cells the role of chloride as


dopant on the transport and structural propertiesrdquo Chemistryof Materials vol 25 no 22 pp 4613ndash4618 2013

[86] H J Snaith and M Gratzel ldquoEnhanced charge mobility ina molecular hole transporter via addition of redox inactiveionic dopant Implication to dye-sensitized solar cellsrdquo AppliedPhysics Letters vol 89 no 26 Article ID 262114 2006

[87] R Scholin M H Karlsson S K Eriksson H Siegbahn EM J Johansson and H Rensmo ldquoEnergy level shifts in spiro-OMeTAD molecular thin films when adding Li-TFSIrdquo Journalof Physical Chemistry C vol 116 no 50 pp 26300ndash26305 2012

[88] J Burschka F Kessler M K Nazeeruddin and M GratzelldquoCo(III) complexes as p-dopants in solid-state dye-sensitizedsolar cellsrdquo Chemistry of Materials vol 25 no 15 pp 2986ndash2990 2013

[89] J H Noh N J Jeon Y C Choi M K Nazeeruddin M Gratzeland S I Seok ldquoNanostructured TiO

2CH3NH3PbI3hetero-

junction solar cells employing spiro-OMeTADCo-complex ashole-transporting materialrdquo Journal of Materials Chemistry Avol 1 no 38 pp 11842ndash11847 2013

[90] A Abate D J Hollman J Teuscher et al ldquoProtic ionic liquidsas p-dopant for organic hole transporting materials and theirapplication in high efficiency hybrid solar cellsrdquo Journal of theAmerican Chemical Society vol 135 no 36 pp 13538ndash135482013

[91] J Burschka N Pellet S-JMoon et al ldquoSequential deposition asa route to high-performance perovskite-sensitized solar cellsrdquoNature vol 499 no 7458 pp 316ndash319 2013

[92] H Zhou Q Chen G Li et al ldquoInterface engineering of highlyefficient perovskite solar cellsrdquo Science vol 345 no 6196 pp542ndash546 2014

[93] H Hu D Wang Y Zhou et al ldquoVapour-based processingof hole-conductor-free CH

3NH3PbI3perovskiteC

60fullerene

planar solar cellsrdquo RSC Advances vol 4 no 55 pp 28964ndash28967 2014

[94] K Liang D B Mitzi and M T Prikas ldquoSynthesis and charac-terization of organicminusinorganic perovskite thin films preparedusing a versatile two-step dipping techniquerdquo Chemistry ofMaterials vol 10 no 1 pp 403ndash411 1998

[95] B Conings L Baeten C De Dobbelaere J DrsquoHaen J Mancaand H-G Boyen ldquoPerovskite-based hybrid solar cells exceed-ing 10 efficiency with high reproducibility using a thin filmsandwich approachrdquo Advanced Materials vol 26 no 13 pp2041ndash2046 2014

[96] Y Zhao A M Nardes and K Zhu ldquoSolid-state mesostructuredperovskite CH

3NH3PbI3solar cells charge transport recom-

bination and diffusion lengthrdquo Journal of Physical ChemistryLetters vol 5 no 3 pp 490ndash494 2014

[97] J Seo S Park Y Chan Kim et al ldquoBenefits of very thin PCBMand LiF layers for solution-processed pndashindashn perovskite solarcellsrdquo Energy and Environmental Science vol 7 no 8 pp 2642ndash2646 2014

[98] N J Jeon J H Noh Y C KimW S Yang S Ryu and S I SeokldquoSolvent engineering for high-performance inorganic-organichybrid perovskite solar cellsrdquo Nature Materials vol 13 no 9pp 897ndash903 2014

[99] G E Eperon V M Burlakov P Docampo A Gorielyand H J Snaith ldquoMorphological control for high perfor-mance solution-processed planar heterojunction perovskitesolar cellsrdquoAdvanced FunctionalMaterials vol 24 no 1 pp 151ndash157 2014

[100] A Dualeh N Tetreault T Moehl P Gao M K Nazeeruddinand M Gratzel ldquoEffect of annealing temperature on filmmorphology of organic-inorganic hybrid pervoskite solid-statesolar cellsrdquo Advanced Functional Materials vol 24 no 21 pp3250ndash3258 2014

[101] M Saliba K W Tan H Sai et al ldquoInfluence of thermalprocessing protocol upon the crystallization and photovoltaicperformance of organicndashinorganic lead trihalide perovskitesrdquoJournal of Physical Chemistry C vol 118 no 30 pp 17171ndash171772014

[102] L Zheng Y Ma S Chu et al ldquoImproved light absorption andcharge transport for perovskite solar cells with rough interfacesby sequential depositionrdquo Nanoscale vol 6 no 14 pp 8171ndash8176 2014

[103] P Docampo F C Hanusch S D Stranks et al ldquoSolutiondeposition-conversion for planar heterojunction mixed halideperovskite solar cellsrdquoAdvanced Energy Materials vol 4 no 14Article ID 1400355 2014

[104] Y Zhou M Yang A L Vasiliev et al ldquoGrowth controlof compact CH

3NH3PbI3thin films via enhanced solid-state

precursor reaction for efficient planar perovskite solar cellsrdquoJournal of Materials Chemistry A vol 3 pp 9249ndash9256 2015

[105] Y Wu A Islam X Yang et al ldquoRetarding the crystallizationof PbI2 for highly reproducible planar-structured perovskitesolar cells via sequential depositionrdquo Energy and EnvironmentalScience vol 7 no 9 pp 2934ndash2938 2014

[106] Z Xiao C Bi Y Shao et al ldquoEfficient high yield perovskite pho-tovoltaic devices grown by interdiffusion of solution-processedprecursor stacking layersrdquo Energyamp Environmental Science vol7 no 8 pp 2619ndash2623 2014

[107] Y Deng E Peng Y Shao Z Xiao Q Dong and J HuangldquoScalable fabrication of efficient organolead trihalide perovskitesolar cells with doctor-bladed active layersrdquo Energy and Envi-ronmental Science vol 8 no 5 pp 1544ndash1550 2015

[108] K M Boopathi M Ramesh P Perumal et al ldquoPreparationof metal halide perovskite solar cells through a liquid dropletassisted methodrdquo Journal of Materials Chemistry A vol 3 no17 pp 9257ndash9263 2015

[109] A K Chilvery P Guggilla A K Batra D D Gaikwad and J RCurrie ldquoEfficient planar perovskite solar cell by spray and brushsolution-processing methodsrdquo Journal of Photonics for Energyvol 5 no 1 2015

[110] G Longo L Gil-Escrig M J Degen M Sessolo and H JBolink ldquoPerovskite solar cells prepared by flash evaporationrdquoChemical Communications vol 51 no 34 pp 7376ndash7378 2015

[111] efficiency chartjpg (4190times2456) httpwwwnrelgovncpvimagesefficiency chartjpg

[112] B Ma R Gao L Wang et al ldquoAlternating assembly structureof the same dye and modification material in quasi-solidstate dye-sensitized solar cellrdquo Journal of Photochemistry andPhotobiology A Chemistry vol 202 no 1 pp 33ndash38 2009

[113] W Li J Li LWang G Niu R Gao and Y Qiu ldquoPost modifica-tion of perovskite sensitized solar cells by aluminum oxide forenhanced performancerdquo Journal of Materials Chemistry A vol1 no 38 pp 11735ndash11740 2013

[114] M D Kempe A A Dameron and M O Reese ldquoEvaluation ofmoisture ingress from the perimeter of photovoltaic modulesrdquoProgress in Photovoltaics Research and Applications vol 22 no11 pp 1159ndash1171 2014

[115] H-S Ko J-W Lee and N-G Park ldquo1576 efficiency per-ovskite solar cells prepared under high relative humidity


importance of PbI2morphology in two-step deposition of

CH3NH3PbI3rdquo Journal of Materials Chemistry A vol 3 pp

8808ndash8815 2015[116] A Fujishima T N Rao and D A Tryk ldquoTitanium dioxide

photocatalysisrdquo Journal of Photochemistry and Photobiology CPhotochemistry Reviews vol 1 no 1 pp 1ndash21 2000

[117] S Ito S Tanaka K Manabe and H Nishino ldquoEffects ofsurface blocking layer of Sb

2S3on nanocrystalline TiO

2for

CH3NH3PbI3perovskite solar cellsrdquo Journal of Physical Chem-

istry C vol 118 no 30 pp 16995ndash17000 2014[118] T Leijtens G E Eperon S Pathak A Abate M M Lee

and H J Snaith ldquoOvercoming ultraviolet light instability ofsensitized TiO

2with meso-superstructured organometal tri-

halide perovskite solar cellsrdquo Nature Communications vol 4article 2885 2013

[119] S K Pathak A Abate T Leijtens et al ldquoTowards long-termphotostability of solid-state dye sensitized solar cellsrdquoAdvancedEnergy Materials vol 4 no 8 Article ID 1301667 2014

[120] S Guarnera A Abate W Zhang et al ldquoImproving the long-term stability of perovskite solar cells with a porousAl

2O3buffer

layerrdquoThe Journal of Physical Chemistry Letters vol 6 no 3 pp432ndash437 2015

[121] A Poglitsch andDWeber ldquoDynamic disorder inmethylammo-niumtrihalogenoplumbates (II) observed by millimeter-wavespectroscopyrdquo The Journal of Chemical Physics vol 87 no 11pp 6373ndash6378 1987

[122] A Dualeh P Gao S I Seok M K Nazeeruddin and MGratzel ldquoThermal behavior ofmethylammonium lead-trihalideperovskite photovoltaic light harvestersrdquo Chemistry of Materi-als vol 26 no 21 pp 6160ndash6164 2014

[123] A Pisoni J Jacimovic O S Barisic et al ldquoUltra-lowthermal conductivity in organic-inorganic hybrid perovskiteCH3NH3PbI3rdquo Journal of Physical Chemistry Letters vol 5 no

14 pp 2488ndash2492 2014[124] S N Habisreutinger T Leijtens G E Eperon S D Stranks

R J Nicholas and H J Snaith ldquoCarbon nanotubepolymercomposites as a highly stable hole collection layer in perovskitesolar cellsrdquo Nano Letters vol 14 no 10 pp 5561ndash5568 2014

[125] B Hailegnaw S Kirmayer E Edri G Hodes and D CahenldquoRain on methylammonium lead iodide based perovskitespossible environmental effects of perovskite solar cellsrdquo TheJournal of Physical Chemistry Letters vol 6 no 9 pp 1543ndash15472015

[126] J M Frost K T Butler F Brivio C H Hendon M Van Schil-fgaarde and A Walsh ldquoAtomistic origins of high-performancein hybrid halide perovskite solar cellsrdquoNano Letters vol 14 no5 pp 2584ndash2590 2014

[127] S Liu F ZhengN Z KoocherH Takenaka FWang andAMRappe ldquoFerroelectric domain wall induced band gap reductionand charge separation in organometal halide perovskitesrdquo TheJournal of Physical Chemistry Letters vol 6 no 4 pp 693ndash6992015

[128] E L Unger E T Hoke C D Bailie et al ldquoHysteresis andtransient behavior in current-voltage measurements of hybrid-perovskite absorber solar cellsrdquo Energy and EnvironmentalScience vol 7 no 11 pp 3690ndash3698 2014

[129] H-W Chen N Sakai M Ikegami and T Miyasaka ldquoEmer-gence of hysteresis and transient ferroelectric response inorgano-lead halide perovskite solar cellsrdquoThe Journal of PhysicalChemistry Letters vol 6 no 1 pp 164ndash169 2015

[130] J Wei Y Zhao H Li et al ldquoHysteresis analysis based on theferroelectric effect in hybrid perovskite solar cellsrdquoThe Journalof Physical Chemistry Letters vol 5 no 21 pp 3937ndash3945 2014

[131] H J Snaith A Abate J M Ball et al ldquoAnomalous hysteresis inperovskite solar cellsrdquo Journal of Physical Chemistry Letters vol5 no 9 pp 1511ndash1515 2014

[132] H-S Kim and N-G Park ldquoParameters affecting IndashV hysteresisof CH

3NH3PbI3perovskite solar cells effects of perovskite

crystal size and mesoporous TiO2layerrdquoThe Journal of Physical

Chemistry Letters vol 5 no 17 pp 2927ndash2934 2014[133] A Dualeh T Moehl N Tetreault et al ldquoImpedance spectro-

scopic analysis of lead iodide perovskite-sensitized solid-statesolar cellsrdquo ACS Nano vol 8 no 1 pp 362ndash373 2014

[134] Y Shao Z Xiao C Bi Y Yuan and J Huang ldquoOrigin andelimination of photocurrent hysteresis by fullerene passivationin CH

3NH3PbI3planar heterojunction solar cellsrdquo Nature

Communications vol 2 p 5748 2014[135] C C Stoumpos C D Malliakas and M G Kanatzidis

ldquoSemiconducting tin and lead iodide perovskites with organiccations phase transitions high mobilities and near-infraredphotoluminescent propertiesrdquo Inorganic Chemistry vol 52 no15 pp 9019ndash9038 2013

[136] Y Kutes L Ye Y Zhou S Pang B D Huey and N P Pad-ture ldquoDirect observation of ferroelectric domains in solution-processed CH

3NH3PbI3perovskite thin filmsrdquo The Journal of

Physical Chemistry Letters vol 5 no 19 pp 3335ndash3339 2014[137] Z Xiao Y Yuan Y Shao et al ldquoGiant switchable photovoltaic

effect in organometal trihalide perovskite devicesrdquo NatureMaterials 2014

[138] B Chen J Shi X Zheng et al ldquoFerroelectric solar cells basedon inorganic-organic hybrid perovskitesrdquo Journal of MaterialsChemistry A vol 3 no 15 pp 7699ndash7705 2015

[139] H J Snaith ldquoEstimating the maximum attainable efficiency indye-sensitized solar cellsrdquo Advanced Functional Materials vol20 no 1 pp 13ndash19 2010


[141] P K Nayak G Garcia-Belmonte A Kahn J Bisquert and DCahen ldquoPhotovoltaic efficiency limits and material disorderrdquoEnergy and Environmental Science vol 5 no 3 pp 6022ndash60392012

[142] J A Christians J S Manser and P V Kamat ldquoBest practicesin perovskite solar cell efficiency measurements Avoiding theerror of making bad cells look goodrdquo The Journal of PhysicalChemistry Letters vol 6 pp 852ndash857 2015

[143] J HWerner R Zapf-GottwickM Koch and K Fischer ldquoToxicsubstances in photovoltaic modulesrdquo in Proceedings of the 21stInternational Photovoltaic Science and Engineering ConferenceFukuoka Japan December 2011

[144] N K Noel S D Stranks A Abate et al ldquoLead-free organic-inorganic tin halide perovskites for photovoltaic applicationsrdquoEnergy and Environmental Science vol 7 no 9 pp 3061ndash30682014

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Inorganic ChemistryInternational Journal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal ofPhotoenergy


Carbohydrate Chemistry

International Journal of


Journal of

Chemistry


Advances in

Physical Chemistry

Hindawi Publishing Corporationhttpwwwhindawicom

Analytical Methods in Chemistry

Journal of

Volume 2014

Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

SpectroscopyInternational Journal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Medicinal ChemistryInternational Journal of


Chromatography Research International


Applied ChemistryJournal of



Theoretical ChemistryJournal of


Journal of

Spectroscopy

Analytical ChemistryInternational Journal of


Journal of


Quantum Chemistry


Organic Chemistry International

ElectrochemistryInternational Journal of



CatalystsJournal of


technology to prove a viable alternate to the dominance ofsilicon The efficiency for this class of solar cells has jumpedfrom a meager 39 in 2009 to over 15 in 2013 on the backof properties like high absorption coefficient panchromaticabsorption low exciton binding energies longer diffusionlengths and lifetimes and the range of options as to compo-sition and preparation

In this review we present the current state of the artfor photovoltaic devices based on perovskites spelling outthe underlying phenomenon different device architecturesfabrication techniques comparison to other technologiesand future outlook and challenges This paper is aimed atrecollecting the recent developments in perovskite solar cellsproviding a broader context to the reviews already publishedon this topic [11 15ndash17]

2 The Phenomena

Perovskites are a family of materials with the crystal structureof calcium titanate that is ABX

3 There are numerous

materials which adopt this structure with exciting applica-tions based on thermoelectric insulating semiconductingpiezoelectric conducting antiferromagnetic and supercon-ducting properties [19] Perovskites are routinely synthesizedby solid-state mixing of constituent elements or compoundsat high temperatures in the range of gt1300K [20] Theycan also be synthesized by drying the solution of precursorsalts and the ones offering semiconductor properties findimportant applications in printable electronics due to solu-tion processability [21 22]

ABX3describes the crystal structure of perovskite class

of materials where A and B are cations and X is an anionof different dimensions with A being larger than X Crystalstructure of perovskites is illustrated in Figure 1 Considera-tions of tolerance factor t and octahedral factor 120583 [23] dictateprobable structure and crystallographic stability where t isdefined by the ratio of the bond length AndashX to that of BndashX in an ideal solid sphere model (t = (rA + rX)radic2(rB +rX) where rA rB and rX are the ionic radii involved) andthe ratio rBrX defines the octahedral factor 120583 081 lt t lt111 and 044 lt 120583 lt 090 [23] are the typical values forhalide perovskites (X = F Cl Br and I) Narrower rangeof t values form 089 to 10 dictates cubic structure whilelower values of t stabilize less symmetric tetragonal andorthorhombic structures In organic inorganic halide per-ovskite of interest in photovoltaic applications A is a largerorganic cation typically methylammonium (CH

3NH3+)

with rA = 018 nm [24] though good results have alsobeen reported for ethylammonium (CH

3CH2NH3+) (rA =

023 nm) [25ndash28] and formamidinium (NH2CH=NH2

+) (rAis estimated in the range 019ndash022 nm) [29ndash31] Anion Xis a halogen typically iodine (rX = 0220 nm) though Brand Cl are also increasingly being reported (rX = 0196 nmand 0181 nm) typically in a mixed halide configurationPb (rB = 0119 nm) and Sn (rB = 0110 nm) as cation Bhave exclusively been used for high efficiency perovskitesolar cells because of lower and theoretically ideal band gaps[29] Lower stability associated with Sn is due to ease ofoxidation of Sn to SnI

4while relativistic effects afford greater

oxidation protection in Pb [29]Thus the standard compoundis methylammonium lead triiodide (CH

3NH3PbI3) with

mixed halides CH3NH3PbI3minusxClx and CH

3NH3PbI3minusxBrx

also being important

3 Perovskite-Sensitized Solar Cells

DSSCs [7 32] are the forerunners of perovskite solar cellsConstituent of a typical DSSC is mesoporous n-type titaniasensitized with a light absorbing dye in a redox activeelectrolyte Porous titania affords greater internal surfacearea to the sensitized dye for efficient absorption of incidentphotons though thicker films of the order of 10 120583m arerequired for complete absorption over the absorbing rangeof the dye [32] The requirement of 10 120583m thick active layer isimpractical for solid-state DSSCs where a number of factorslimit the active layer thickness to less than 2120583m [33] As analternate thin film semiconductor active layers and quantumdots enable complete light absorption in much thinner layerswhile at the same time pushing the photosensitivity furtherinto near infrared (NIR) [34ndash38] In the backdrop of findingmore efficient light sensitizers for DSSCs Miyasaka andcoworkers reported the first perovskite-sensitized solar cellsbetween 2006 and 2008 CH

3NH3PbI3and CH

3NH3PbBr3

absorbers were employed with an iodide triiodide redoxcouple or a polypyrrole carbon black composite solid-statehole conductor Full sun power conversion efficiency varyingbetween 04 and 2 was measured for solid-state and liquidelectrolyte cells respectively [39 40]

The first peer-reviewed report for perovskite-sensitizedsolar cell was published in 2009 CH

3NH3PbI3absorber in an

iodidetriiodide redox couple achieved an efficiency of 35[41] Figure 2 illustrates the schematic of an organometallichalide sensitized titania solar cell and the associated spectralresponse Retaining the liquid electrolyte reported byKim and coworkers achieved an improved efficiency of65 by optimizing the titania surface morphology andperovskite processing [41 42] The tumbling block in theliquid electrolyte based perovskite-sensitized solar cellwas the dissolution and decomposition of perovskite inthe liquid electrolyte Resultantly the solar cells exhibitedpoor stability and would degrade within minutes [41 42]The solution to this problem lied in adoption of solid-state hole transport medium in place of electrolyte as wasoriginally tried by Kojima and coworkers in 2006 [40]Methylammonium trihalogen plumbates being relativelyinsoluble in nonpolar organic solvents paved the way forrealizing first perovskite sensitization and made subsequentinfilling with the organic hole conductor possible Thismade T N Murakami and T Miyasaka and N G Parkin collaboration with M Gratzel and coworkers developsolid-state perovskite solar cells employing (22(77)-tetrakis-(NN-dipmethoxyphenylamine)99(-spirobifluorene))(spiro-MeOTAD) as the hole transporter [43] withmaximumfull sun power conversion efficiencies of between 8 and 10employing CH

3NH3PbI3minusxClx mixed halide perovskite and

CH3NH3PbI3 respectively [18 44]

This achievement marked a considerable jump in perfor-mance over the best reported solid-state (ss) DSSCs with a


X

X

XX

X

XX

X

X XX

X

X

XX

X

X

XXX

X

X XX

XX

X

X

BB

A

A

A

A

A

A

AA

A

A

AA

AAA

A

A

BB

XX

XX

XB

XXXXX

X

X XXX

XXBX

X

XX

XXXX

XXXXXXXXXXXXXXB

X

X

X

X

A

BX

X

X

X

X

A

A

A

A

A

A

A

AA

A A

XXXXXX

XXBX

X

XX

X

AA


Perovskites

Perovskites

Perovskites

Perovskites

eminusTiO2

Visible l

ight70

60

50

40

30

20

10

0

400 500 600 700 800

Wavelength (nm)

IPCE

()





4 Mesoporous TiO2

Scaffold



2in 2012




3NH3PbI3to TiO

2 This cell with








2and CH








3NH3PbI3





3NH3PbI2Br with TiO

2



3NH3PbI2Br


2with poly[N-9-


2


2CH3NH3PbI3HTM






3NH3PbI2Cl















119892= 23 eV)




3NH3-PbBr

3and the


2mesoporous scaf-



3NH3PbI3was able to


3-NH3PbI3




2and deposited


2layer resulted



FTO

Blocking layer

FTO

Blocking layer

FTO

Blocking layer

FTO

Blocking layer

HTMTiO2 TiO2Al2O3

CH3NH3PbX3

120578 = 97 120578 = 109 120578 = 15 120578 = 154









3NH3PbI3


3NH3I to 70∘C

and PbI2to 250∘C














3NH3PbI3and CH











6net-


6octahedron











3NH2




3NH3I [91 94]





Al 2O

3Ti

O2

ZnO

PCBM mdash mdash

N719

Sb2S

3

CH3N

H3P

bI3 mdash mdash

spiro

-MeO

TAD

CuSC

NP3

HT

PCPD

TBT

PTA

A

minus30

minus35

minus40

minus45

minus50

minus55

minus60

minus65En

ergy

ban

d le

vels

(eV

)

CH3N

H3P

bI3minus

xCl

x








3[105]


2film Compact






3NH3PbI3







3NH3I CH

3NH2 and HI


2












is [117]




+ (5b)





2





3NH3PbI3 It changes







3NH3PbI3








17 Future Outlook




References




































3CH2NH3)PbI3rdquo





2PbI3 an alter-







3NH3PbI3and CH



























2
















3NH3PbI3) for




































CI119909mixed








2CH3NH3PbI3hetero-






3NH3PbI3perovskiteC

60fullerene



































2for








2O3buffer











































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


X

X

XX

X

XX

X

X XX

X

X

XX

X

X

XXX

X

X XX

XX

X

X

BB

A

A

A

A

A

A

AA

A

A

AA

AAA

A

A

BB

XX

XX

XB

XXXXX

X

X XXX

XXBX

X

XX

XXXX

XXXXXXXXXXXXXXB

X

X

X

X

A

BX

X

X

X

X

A

A

A

A

A

A

A

AA

A A

XXXXXX

XXBX

X

XX

X

AA


Perovskites

Perovskites

Perovskites

Perovskites

eminusTiO2

Visible l

ight70

60

50

40

30

20

10

0

400 500 600 700 800

Wavelength (nm)

IPCE

()





4 Mesoporous TiO2

Scaffold



2in 2012




3NH3PbI3to TiO

2 This cell with








2and CH








3NH3PbI3





3NH3PbI2Br with TiO

2



3NH3PbI2Br


2with poly[N-9-


2


2CH3NH3PbI3HTM






3NH3PbI2Cl















119892= 23 eV)




3NH3-PbBr

3and the


2mesoporous scaf-



3NH3PbI3was able to


3-NH3PbI3




2and deposited


2layer resulted



FTO

Blocking layer

FTO

Blocking layer

FTO

Blocking layer

FTO

Blocking layer

HTMTiO2 TiO2Al2O3

CH3NH3PbX3

120578 = 97 120578 = 109 120578 = 15 120578 = 154









3NH3PbI3


3NH3I to 70∘C

and PbI2to 250∘C














3NH3PbI3and CH











6net-


6octahedron











3NH2




3NH3I [91 94]





Al 2O

3Ti

O2

ZnO

PCBM mdash mdash

N719

Sb2S

3

CH3N

H3P

bI3 mdash mdash

spiro

-MeO

TAD

CuSC

NP3

HT

PCPD

TBT

PTA

A

minus30

minus35

minus40

minus45

minus50

minus55

minus60

minus65En

ergy

ban

d le

vels

(eV

)

CH3N

H3P

bI3minus

xCl

x








3[105]


2film Compact






3NH3PbI3







3NH3I CH

3NH2 and HI


2












is [117]




+ (5b)





2





3NH3PbI3 It changes







3NH3PbI3








17 Future Outlook




References




































3CH2NH3)PbI3rdquo





2PbI3 an alter-







3NH3PbI3and CH



























2
















3NH3PbI3) for




































CI119909mixed








2CH3NH3PbI3hetero-






3NH3PbI3perovskiteC

60fullerene



































2for








2O3buffer











































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of





2and CH








3NH3PbI3





3NH3PbI2Br with TiO

2



3NH3PbI2Br


2with poly[N-9-


2


2CH3NH3PbI3HTM






3NH3PbI2Cl















119892= 23 eV)




3NH3-PbBr

3and the


2mesoporous scaf-



3NH3PbI3was able to


3-NH3PbI3




2and deposited


2layer resulted



FTO

Blocking layer

FTO

Blocking layer

FTO

Blocking layer

FTO

Blocking layer

HTMTiO2 TiO2Al2O3

CH3NH3PbX3

120578 = 97 120578 = 109 120578 = 15 120578 = 154









3NH3PbI3


3NH3I to 70∘C

and PbI2to 250∘C














3NH3PbI3and CH











6net-


6octahedron











3NH2




3NH3I [91 94]





Al 2O

3Ti

O2

ZnO

PCBM mdash mdash

N719

Sb2S

3

CH3N

H3P

bI3 mdash mdash

spiro

-MeO

TAD

CuSC

NP3

HT

PCPD

TBT

PTA

A

minus30

minus35

minus40

minus45

minus50

minus55

minus60

minus65En

ergy

ban

d le

vels

(eV

)

CH3N

H3P

bI3minus

xCl

x








3[105]


2film Compact






3NH3PbI3







3NH3I CH

3NH2 and HI


2












is [117]




+ (5b)





2





3NH3PbI3 It changes







3NH3PbI3








17 Future Outlook




References




































3CH2NH3)PbI3rdquo





2PbI3 an alter-







3NH3PbI3and CH



























2
















3NH3PbI3) for




































CI119909mixed








2CH3NH3PbI3hetero-






3NH3PbI3perovskiteC

60fullerene



































2for








2O3buffer











































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


FTO

Blocking layer

FTO

Blocking layer

FTO

Blocking layer

FTO

Blocking layer

HTMTiO2 TiO2Al2O3

CH3NH3PbX3

120578 = 97 120578 = 109 120578 = 15 120578 = 154









3NH3PbI3


3NH3I to 70∘C

and PbI2to 250∘C














3NH3PbI3and CH











6net-


6octahedron











3NH2




3NH3I [91 94]





Al 2O

3Ti

O2

ZnO

PCBM mdash mdash

N719

Sb2S

3

CH3N

H3P

bI3 mdash mdash

spiro

-MeO

TAD

CuSC

NP3

HT

PCPD

TBT

PTA

A

minus30

minus35

minus40

minus45

minus50

minus55

minus60

minus65En

ergy

ban

d le

vels

(eV

)

CH3N

H3P

bI3minus

xCl

x








3[105]


2film Compact






3NH3PbI3







3NH3I CH

3NH2 and HI


2












is [117]




+ (5b)





2





3NH3PbI3 It changes







3NH3PbI3








17 Future Outlook




References




































3CH2NH3)PbI3rdquo





2PbI3 an alter-







3NH3PbI3and CH



























2
















3NH3PbI3) for




































CI119909mixed








2CH3NH3PbI3hetero-






3NH3PbI3perovskiteC

60fullerene



































2for








2O3buffer











































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of




3NH3PbI3and CH











6net-


6octahedron











3NH2




3NH3I [91 94]





Al 2O

3Ti

O2

ZnO

PCBM mdash mdash

N719

Sb2S

3

CH3N

H3P

bI3 mdash mdash

spiro

-MeO

TAD

CuSC

NP3

HT

PCPD

TBT

PTA

A

minus30

minus35

minus40

minus45

minus50

minus55

minus60

minus65En

ergy

ban

d le

vels

(eV

)

CH3N

H3P

bI3minus

xCl

x








3[105]


2film Compact






3NH3PbI3







3NH3I CH

3NH2 and HI


2












is [117]




+ (5b)





2





3NH3PbI3 It changes







3NH3PbI3








17 Future Outlook




References




































3CH2NH3)PbI3rdquo





2PbI3 an alter-







3NH3PbI3and CH



























2
















3NH3PbI3) for




































CI119909mixed








2CH3NH3PbI3hetero-






3NH3PbI3perovskiteC

60fullerene



































2for








2O3buffer











































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


Al 2O

3Ti

O2

ZnO

PCBM mdash mdash

N719

Sb2S

3

CH3N

H3P

bI3 mdash mdash

spiro

-MeO

TAD

CuSC

NP3

HT

PCPD

TBT

PTA

A

minus30

minus35

minus40

minus45

minus50

minus55

minus60

minus65En

ergy

ban

d le

vels

(eV

)

CH3N

H3P

bI3minus

xCl

x








3[105]


2film Compact






3NH3PbI3







3NH3I CH

3NH2 and HI


2












is [117]




+ (5b)





2





3NH3PbI3 It changes







3NH3PbI3








17 Future Outlook




References




































3CH2NH3)PbI3rdquo





2PbI3 an alter-







3NH3PbI3and CH



























2
















3NH3PbI3) for




































CI119909mixed








2CH3NH3PbI3hetero-






3NH3PbI3perovskiteC

60fullerene



































2for








2O3buffer











































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of








is [117]




+ (5b)





2





3NH3PbI3 It changes







3NH3PbI3








17 Future Outlook




References




































3CH2NH3)PbI3rdquo





2PbI3 an alter-







3NH3PbI3and CH



























2
















3NH3PbI3) for




































CI119909mixed








2CH3NH3PbI3hetero-






3NH3PbI3perovskiteC

60fullerene



































2for








2O3buffer











































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of




17 Future Outlook




References




































3CH2NH3)PbI3rdquo





2PbI3 an alter-







3NH3PbI3and CH



























2
















3NH3PbI3) for




































CI119909mixed








2CH3NH3PbI3hetero-






3NH3PbI3perovskiteC

60fullerene



































2for








2O3buffer











































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of













3CH2NH3)PbI3rdquo





2PbI3 an alter-







3NH3PbI3and CH



























2
















3NH3PbI3) for




































CI119909mixed








2CH3NH3PbI3hetero-






3NH3PbI3perovskiteC

60fullerene



































2for








2O3buffer











































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of













3NH3PbI3) for




































CI119909mixed








2CH3NH3PbI3hetero-






3NH3PbI3perovskiteC

60fullerene



































2for








2O3buffer











































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of







2CH3NH3PbI3hetero-






3NH3PbI3perovskiteC

60fullerene



































2for








2O3buffer











































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of








2for








2O3buffer











































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of










Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of

review article perovskite solar cells: potentials...

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