FAPbI3 Solar Cells

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NH2CH=NH2PbI3 An Alternative OrganoleadIodide Perovskite Sensitizer for Mesoscopic Solar

Cells

ARTICLE in CHEMISTRY OF MATERIALS middot JANUARY 2014

Impact Factor 835 middot DOI 101021cm404006p

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10 AUTHORS INCLUDING

Jiliang Zhang

University of Liverpool

64 PUBLICATIONS 272 CITATIONS

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Feng Wei

General Research Institute for Nonferrous Methellip


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Tianshi Qin

The Commonwealth Scientific and Industrial hellip


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Zhihong Liu

Chinese Academy of Sciences


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Available from Jiliang Zhang

Retrieved on 23 March 2016



NH2CHNH2PbI3 An Alternative Organolead Iodide PerovskiteSensitizer for Mesoscopic Solar CellsShuping Pang dagger Hao Hudagger Jiliang ZhangDagger Siliu Lvdagger Yaming Yusect Feng Weiperp Tianshi Qin∥

Hongxia Xudagger Zhihong Liudagger and Guanglei Cui dagger

daggerQingdao Key Lab of Solar Energy Utilization and Energy Storage Technology Qingdao Institute of Bioenergy and BioprocessTechnology Chinese Academy of Sciences Qingdao 266101 Peoplersquos Republic of ChinaDaggerDepartment of Physics and Materials Science City University of Hong Kong Hong Kong SAR sectCollege of Materials Science and Engineering Huaqiao University Xiamen 361021 Peoplersquos Republic of ChinaperpGeneral Research Institute for Nonferrous Metals Beijing 100088 Peoplersquos Republic of China∥Ian Wark Laboratory CSIRO Materials Science amp Engineering Clayton South Victoria 3168 Australia

S Supporting Information

ABSTR ACT A new nearly cubic NH2CHNH2PbI3

(FAPbI3) perovskite was synthesized for the mesoscopicsolar cells The measured band gap of bulk FAPbI 3 is 143 eV and it is therefore potentially superior than the CH3NH3PbI3

(MAPbI3) as the light harvester A homogeneous FAPbI3

perovskite layer was deposited on the TiO2 surface by utilizingthe in situ dipping technology As a result a high efficiency of 75 was achieved using P3HT as the hole transport materialThe nearly cubic crystal structure and appropriate band gaprender this new FAPbI3 perovskite extremely attractive for nextgeneration high-efficiency low-cost solar cells

1 INTRODUCTION

It has been recognized that solar energy is one of the promisingresources of renewable energy to power our lives Compared with conventional crystalline silicon cell thin 1047297lm solar celltechnology promises to reduce the cost of light-to-electricity conversion b y using abundant materials and easy fabricationprocedures1 2 A variety of promising light absorbing materialshas been explored in both organic and inorganic systemsGenerally organic materials suff er from short lifetime for theirdegradation when exposed to light for a long time3 4 and theinorganic absorbers (eg quantum dots QD) require acomplicated synthetic procedure to satisfy the solutionprocessing requirement5minus7

Recently due to the high processability large optical

absorption cross section and excellent thermal stabilityorganolead halid perovskites (APbX 3) have become one of the most promising light harvesters for mesoscopic solarcells8minus22 More and more new efficiency records of perovskite based solar cells have been achieved at rather short intervalsUp to now a record efficiency of sim15 has been achieved by solution processed M APbI3 (MA CH3NH3

+) containingperovskite solar cells22 The optical absorption coefficient of MAPbI3 is about 15 times 104 cmminus1 at 550 nm and thermally stableup to 300 degC9 1123 In the MAPbI3 perovskite structure thelarger MA cation and the smaller Pb cation are respectivelycoordinated to twelve and six I anions and PbI6 octahedra arecorner-shared to form a three-dimensional framework The

electronic properties of Pb halide perovskites are strongly dependent on the geometry of the three-dimensional network of the PbX 6 octahedra23 The measured band gap of MAPbI3 is151 eV at room temperature which is higher than theoptimized band gap value (140 eV) according to theShockley minusQueisser limit curve for a single band gap solar cell24

To tune the band gap one attempt is to substitute I with Bror Cl anions resulting in the band gaps changed from 151 to222 or 362 eV respectively13 Such a modi1047297cation isunfavorable for a wide range of light harvesting Anotherstrategy is to substitute the A site cations because the band gapof the perovskite is very sensitive to any modi1047297cation(enlargement tilting andor deformation) of the octahedranetwork25 It is known that at room temperature MAPbI3

perovskite is a tetragonal rather than a cubic structure due to

the smaller size of methylammonium cation (CH3NH3+)according to the tolerance factor f = (r B + r X )radic 2(r A +r X )23 25 The substitution of A site with a larger cation is thusexpected to yield a higher symmetry of the perosvskite structureand a decrease of the band gap as well However thereplacement of CH3NH3

+ with a long chain ethylammoniumcation (CH3CH2NH3

+) produces a layered perov sk ite crystal-line structure with a band gap increased to 22 eV26 The failed

Received December 6 2013Revised January 13 2014Published January 21 2014

Article

pubsacsorgcm

copy 2014 American Chemical Society 1485 dxdoiorg101021cm404006p | Chem Mater 2014 26 1485minus1491



formation of the three-dimensional framework should be owedto the oversize CH3CH2NH3

+cation Therefore it is logically feasible to employ an organic amine cation that is smaller thanCH3CH2NH3

+ but larger than CH3NH3+ in size to 1047297ll the A

site A formamidinium cation (HC(NH2)2+) with three atoms

(excluding hydrogen) is smaller than that of CH3CH2NH3+ but

larger than CH3NH3+25 27

In this paper we have for the 1047297rst time employedHC(NH2)2

+ to substitute CH3NH3+ to synthesize the pure

FAPbI3 perovskite and subsequently study its photoelectro-chemical properties as sensitizer in mesoscopic solar cells It isdemonstrated that the synthesized FAPbI3 possesses a band gapof 143 eV and its corresponding absorption edge reaches 870nm which is superior to MAPbI3 (820 nm) to serve as a lightharvester11 It is noteworthy that the in situ dipping technology is more adoptable to synthesize the high-quality FAPbI3 crystalsupon a mesoporous TiO2 scaff old at a lower processingtemperature compared with a one-step spin-coating method With poly(3-hexylthiophene) (P3HT) as the hole transportmaterial (HTM) and a gold back contact a power conversionefficiency of 75 with an open circuit voltage of 084 V andshort circuit current of 183 mAcm2 was obtained Considering

its nearly cubic crystal structure and appropriate band gapthere still remains big room for further improvement of theFAPbI3 solar cells through optimizing the FAPbI3 thickness andcrystallinity and also employing new hole transport materials

2 EXPERIMENTAL SECTION

21 Synthesis of FAI FAI was 1047297rst prepared by directly mixing 3 gof formamidine acetate and 82 g of HI (45 wt in water) at 0 degC for2 h The precipitate was collected by rotary drying at 65 degC for 30 min

which was followed by washing with a mixture of ethanol and diethylether by air pump 1047297ltration The white solid was 1047297nally dried at 60 degCunder vacuum for 2 days

22 Synthesis of Bulk FAPbI3 Peroviskite FAPbI3 perovskitepowder sample was prepared by a solution method The mixture of PbI2 and FAI with a mole ratio of 11 was 1047297rst dissolved in γ -

butyrolactone (GBL) to form a 40 wt solution FAPbI3 perovskitepowder sample was prepared via evaporating the GBL solvent at 180degC for 10 min on a glass substrate

23 Electronic Structure Calculations The electronic structurecalculations were performed using the 1047297rst-principles pseudopotential

based density functional theory as implemented in the SIESTA package2829 This software employs a localized orbital basis in therepresentation of wave functions The double-ζ polarized numericalatomic orbital basis set is adopted to provide sufficient accuracy andthe core electrons are represented by norm-conserving pseudopoten-tials and the Perdew minusBerkeminusEmzerhof exchange correlation energy

within the generalized gradient approximation30 In the calculationHC(NH2)2

+ was ignored and a uniform background charge density was added to neutralize the charge

24 Synthesis of TiO2 Gel The TiO2 dense layer was preparedusing a solminusgel procedure31 10 mL of titanium(IV) isopropoxide wasmixed with 50 mL of 2-methoxyethanol and 5 mL of ethanolamine in athree-necked 1047298ask each connected with a condenser thermometerand argon gas inletoutlet Then the mixed solution was heated to 80degC for 2 h under magnetic stirring followed by heating to 120 degC for 1h The two-step heating was then repeated for two times to prepare a

viscous solution25 Solar Cell Fabrication and Characterization Fluorine-

doped tin oxide (FTO) coated glass was patterned by etching with Znpowder and 1 M HCl diluted in Milli-Q water The etched substrate

was then cleaned with ethanol saturated KOH solution in isopropanoland water and then dried with clean dry air Subsequently a 30 nmthick dense TiO2 layer was fabricated from TiO2 gel as describedabove After a treatment with heat at 500 degC for 30 min a 500 nmthick TiO2 porous layer was fabricated on the TiO2 dense layer from a

dilute commercial TiO2 paste (125 with ethanol by weight) at 3000rpm The layers were then sintered in air at 550 degC for 30 min For theone-step solution processed solar cells a thin FAPbI3 perovskite layer

was formed by spin-coating of 40 wt PbI2FAI mixture in N N -dimethylformamide (DMF) To get a pure FAPbI3 perovskite phasethe mole ratio of PbI2 to FAI was modi1047297ed to 115 For the solar cellsfabricated using the in situ dipping technology PbI2 powder was 1047297rstdissolved in DMF solution to form a 08 M solution and then spin-

coated (3000 rpm) on the mesoporous TiO2 1047297

lm to prepare a FTOTiO2PbI2 structure22 The in situ reaction was carried out by directly dipping the FTOTiO2PbI2 substrate into an isopropanol solutioncontaining 5 M FAI until the PbI2 1047297 lm became dark brown followed

by a heating treatment at 100 degC for 10 min The HTM (12-dichlorobenzene solution containing 10 mgmL P3HT) was cast ontothe perovskite coated substrate and spun at a rate of 1500 rpm for 60 sFinally a 60 nm Au electrode was thermally evaporated to completethe solar cells Jminus V characteristics were measured (2400 Series SourceMeter Keithley Instruments) under simulated AM 15G sunlight at100 mWcm2 The area of the perovskite solar cells were typically 015cm2 External quantum efficiency (EQE) measurements were carriedout on a Newport setup comprising a Xe lamp a monochromator acurrentminus voltage preampli1047297er and a lock-in ampli1047297er It is worth notingthat all the device fabrication and measurement is under ambientconditions at a relative humidity of 30minus40 The characterization of

the materials was summarized in the Supporting Information

3 RESULTS AND DISCUSSION

To synthesize FAPbI3 the formamidine iodide (HC(NH2)2IFAI) was 1047297rst prepared by directly mixing formamidine acetateand hydroiodic acid (HI) at 0 degC The elemental analysis showsthe weight ratio of CNHI in the as-prepared FAI is70016332997368 which is in very good agreement withtheoretical atomic ratio of 1251(CNHI)The 1H and 13Cnuclear magnetic resonance (NMR) of the white powder werefurther characterized to verify the purity of the FAI (seeSupporting Information Figure S1minus2) The FAI is very pure with no presence of any impurity Subsequently the black FAPbI3 powder sample was prepared by the solution method It

was found that the mixture of PbI2 and FAI with a mole ratio of 11 was soluble in DMF and GBL In this paper bulk FAPbI3

sample was prepared via drying GBL solution at 180 degC onglass or quartz substrates For the construction of the FAPbI3

solar cells thin FAPbI3 perovskite layers were formed by one-step solution processing and two-step dipping technologyrespectively which will be described in details later

The FAPbI3 powder on the glass substrate was characterized by powder X-ray diff raction (XRD) on a Bruker-AXSMicrodiff ractometer with Cu K α radiation at room temper-ature as shown in Figure 1 The sharp diff raction peaks indicate

Figure 1 Typical XRD pattern of the FAPbI3 1047297lm on a glass substrate

Chemistry of Materials Article

dxdoiorg101021cm404006p | Chem Mater 2014 26 1485minus14911486



the high crystallinity of the as-synthesized FAPbI3 compoundRietveld re1047297nement analysis of the X-ray powder diff ractionpattern indicates that the synthesized FAPbI3 is a pure phase with the space group of P 3m1 (Supporting Information FigureS3)32 which was in good agreement with the very recentreport33 A point worth emphasizing is under our experimentalcondition (section 2) that no decomposition of the

formamidine was found The high quality of the FAPbI3 yieldsthe possibility to exactly study its photoelectric properties inmesoscopic solar cells

In the ideal cubic Pm3 m structure of ABX 3 perovskites thesingle A atom (such as Cs) could occupy the position in thecenter of the cube of the perovskite structure In the case of HC(NH2)2

+ this is not possible in this space group because the

Figure 2 (a) Primitive unit cell of the trigonal crystal structure of FAPbI3 (b) The cubic structure with a slight distortion when ignoring thedisplacement and orientation of HC(NH2)2

+ cations (c) The schematic brillouin zone calculated band structure and electronic density of states forthe FAPbI3 respectively The Fermi level is chosen as the energy reference at 0 eV

Figure 3 Optical electronic and thermal properties of the FAPbI3 (a b) UV minus visNIR absorption spectrum and transformed KubelkaminusMunk spectrum of FAPbI3 1047297lm that was formed by drop casting on a quartz substrate of the GBL solution containing PbI 2 and FAI with a mole ratio of 11and then annealled at 180 degC for 10 min respectively (c) Photo electron spectroscopy in air (PESA) of FAPbI3 1047297lm on glass substrate (d) Weightloss as a function of temperature





HC(NH2)2+ cations tend to be disordered in the cuboctahedral

cages formed by 12 nearest-neighbor iodine atoms from thePbI6

4minus octahedra25 As a result the synthesized FAPbI3 adoptsthe P3m1 trigonal space group with lattice parameter of a =90008(8) Aring b = 90008(8) Aring c = 11012(2) Aring and α = 90deg β = 90deg γ = 120deg (Figure 2a) according to the Rietveldre1047297nement When the eff ect of HC(NH2)2

+ on the symmetry of crystal lattice was ignored the FAPbI

3 could then be regarded

as a nearly cubic structure as shown in Figure 2 b The tiny change of PbminusI bonds length (3169minus3181 Aring) in PbI6

octahedra also supports its close relationship with the cubicstructure whereas in the tetragonal distortion of cubic M APbI3 the PbminusI bonds length varies from 3156 to 3213 Aring33 It has been reported that the electronic properties of such compoundsare strongly dependent on the inorganic framework2325 At thispoint the band gap of FAPbI3 should be close to that of theirideal cubic MAPbI3 On the basis of the data obtained by theRietveld re1047297nement band structure and density of states(DOS) were theoreticall y calculated for FAPbI3 perovskite asshown in Figure 2c3034 The electronic band structure alongthe high-symmetry k lines and the projected density of statesshow that FAPbI3 has a well-de1047297ned direct band gap of 150 eV at A (0 0 12)

In Figure 3a the transmittance spectrum shows the strongabsorption of the light with the wavelength shorter than 870nm The diff use re1047298ectance spectrum (SSI) and the trans-formed KubelkaminusMunk spectrum for the FAPbI3 powder werepresented in Figure 3 b The optical absorption coefficient (α ) iscalculated using re1047298ectance data according to the KubelkaminusMunk equation F (R ) = α = (1 minus R )22R where R is thepercentage of re1047298ected light35 The incident photon energy (hν) and the optical band gap (Eg) are related to thetransformed KubelkaminusMunk function F (R )hνp = A(hν minus Eg) where A is the constant and p is the power index that is relatedto the optical absorption Similar to MAPbI3 there is a obviousldquocut-off rdquo feature in the UV minus visNIR absorption spectrum

indicating the FAPbI3 is also a direct band gap semiconductorThe measured band gap is sim143 eV from the extrapolation of the liner part of F (R )hν2 plot coinciding well with thetheoretical calculation The absorption edge reaches 870 nm which is about 30 nm wider than that of the MAPbI3 measuredunder the same condition The extended absorption edge could bring sim6 more light absorption according to the standardsolar spectrum The measured valence band (VB) of FAPbI 3

1047297lm on a glass substrate by Photo Electron Spectroscopy in air(PESA)36 was minus538 eV and from the observed optical bandgap the conduction band (CB) of FAPbI3 is calculated to beminus395 eV The thermogravimetric analysis result under anargon atomosphere in Figure 3d shows that the bulk sample was thermally stable up to 300 degC and he 26 weight loss was

consistent with the complete decomposition to PbI2 at 430 degCSimilar to the previous reports about the MAPbI3 solar cells

FAPbI3 thin layer absorber was 1047297rst fabricated by spin-coating a40 wt DMF solution of FAI and PbI2 with a modi1047297ed moleratio (see section 2) Unlike the MAPbI3 perovskite the one-step spin-coated PbI2FAI 1047297lm could not change to very dark under a 100 degC thermal annealing treatment as shown inFigure S4 (Supporting Information) With the increase of theannealing temperature from 100 to 160 degC the color of theFAIPbI2 changes to dark and then fade The corresponding XRD patterns of these samples are shown in Figure 4cminusf Forcomparison the XRD patterns of calculated FAPbI3 perovskiteand FTOTiO2 substrate are also presented in Figure 4ab

respectively As shown in Figure 4c with a 100 degC thermaltreatment only a low content of the FAPbI3 perovskite wasformed Besides the peaks from the FTOTiO2 substrate andthe FAPbI3 perovskite there are some additional diff ractionpeaks as marked with ldquordquo which attributes to the presence of anew yellow P 63mc FAPbI3 phase with la yered crystal structure(Supporting Information Figure S5)28 With the annealingtemperature increased to 120 degC there is a little increase of thecontent of the FAPbI3 perovskite according to the diff ractionintensity When the annealing temperature increased to 140 degCalmost all the diff raction peaks could be indexed to the FAPbI3

perovskite (Figure 4e) With the temperature increased to 160degC there is some PbI2 () appeared in the FAPbI3 1047297lm

The FAPbI3 based solar cell was fabricated with P3HT as theHTM The VB of FAPbI3 is minus538 eV below vacuum level which is lower than that of P3HT (Figure 5a) For the devicefabrication the spin-coated FAIPbI2 was thermally annealed at120 130 140 and 150 degC for 30 min to serve as the lightharvester and P3HT was spin-coated on top of the perovskitelayer It was found that the solar cell annealed at around 140 degCdelivered the highest efficiency as shown in Figure 5(b) Anopen-circuit voltage of 055 V and a short-circuit currentdensity of 166 mAcm2 were achieved The overall conversion

efficiency is 37 which is comparable to the MAPbI 3minusxClx

solar cell (38) and lower than M APbI3 solar cell (67) alsousing P3HT as the HTM19 37 There are many reasonsresponsible for the relatively poor performance such as theunoptimized 1047297lm thickness and spin-coating condition Another very important reason is proposed to the relatively highannealing temperature which may result in a poor crystalquality for the formation of more defects in the perovskitelayer The same result was also found in the literature9

In previous reports concerning the lead iodide perovskitesolar cells the perovskite quantum-dots were crystallized outfrom the solvents such as GBL and DMF resulting in low coverage on the TiO2 surface The two-step solid-solution

Figure 4 (a) Calculated XRD pattern of FAPbI3 (b) The XRDpattern of the FTOTiO2 substrate (cminusf) The XRD patterns of thespin coated FAIPbI2 solution on the mesoporsous TiO2 1047297lms andannealed at 100 120 140 and 160 degC for 30 min respectively





process for the MAPbI3 as shown in Figure 6 could obviously enhance the solar cell performance2238 Another advantage is

that such an in situ dipping technique could also decrease thereaction temperature This is especially important for theFAPbI3 based solar cells In a typical procedure a PbI 2 solution was 1047297rst spin-coated on a mesoporous TiO2 1047297lm to produce aFTOTiO2PbI2 structure with a bright yellow color Then theFTOTiO2PbI2 substrate was dipped in a dilute FAI solutionThe reaction of solid PbI2 and FAI solution was very fastDuring the reaction the color gradually changed from yellow todark red The complete reaction time highly depends on thethickness of the PbI2 1047297lm and the concentration of the FAI Inour experimental conditions a 1 min dipping time could get thepure phase as a nearly cubic FAPbI3 perovskite 1047297lm as shownin the optical images in Figure S6 (Supporting Information)and its corresponding XRD patterns in Figure S7 (SupportingInformation) With increased dipping times the color of the1047297lm changed from yellow (PbI2) to dark (FAPbI3 perovskite)and then faded This phenomenon is in agreement with the

published results28 Therefore the control of the dipping timeis of great importance for the FAPbI3 based mesoscopic solarcells

As above-mentioned such a two-step dipping reaction coulddirectly form a dark red layer even without additional thermaltreatment as shown in Figure 7a The transmission electronmicroscopy (TEM) image (Figure 7 b) of the TiO2FAPbI3

sample scratched from the mesoporous 1047297lm reveals theformation of the FAPbI3 perovskite thin 1047297lm on the TiO2

surface which is much diff erent from the MAPbI3 quantumdots structure prepared from the one-step spin-coatingtechnique from the GBL solution After the in situ dippingreaction as shown in Figure 7cd there are still many FAPbI3

domains on the mesoporous TiO2 surface The large FAPbI3

particles are of polyhedron structure with the size of sim05minus1 μm The cross-section scanning electron microscopy (SEM)image in Figure 7d and the EDS mapping for titanium leadand iodine in Figure S8aminusd (Supporting Information) furtherindicate that the homogeneous reaction of PbI2 and FAIthroughout the whole mesoporous TiO2 layer and the FAPbI3

are both located in the mesopores and top-surface of themesoporous TiO2 layer

The phase purity of the FAPbI3 formed via the two-stepdipping reaction was studied by XRD as shown in Figure 8

which is in very good agreement with the calculated XRDpowder pattern of FAPbI3 The two-step dipping reactionfollowed by a 100 degC thermal annealing treatment couldefficiently form a pure phase nearly cubic FAPbI3 perovskitecrystal structure The lower temperature processing feasibility isone of the huge advantages of the two-step dipping techniqueover the one-step spin-coating method

After the in situ dipping reaction and thermal annealingtreatment of FAPbI3 layer at 100 degC for 10 min 10 mgmL of P3HT in 12-dichlorobenzene was spin-coated on the FAPbI3

1047297lm at 1500 rpm Then a 60 nm thick Au electrode wasevaporated on the top of P3HT through a shadow mask Beforespin-coating P3HT layer the absorption spectra of FTOTiO2

Figure 5 (a) Schematic energy level diagram of TiO2 FAPbI3 and P3HT (b) The photocurrent density minus voltage (Jminus V) curve of the spin coatedFAPbI3 solar cells with an efficiency of 37

Figure 6 (a) Schematic illustration of in situ formation of FAPbI3 by in situ dipping reaction of PbI2 and FAI on the TiO2 surface

Figure 7 (a) Schematic illustration of in situ formation of FAPbI3 by in situ dipping reaction of PbI2 and FAI on the TiO2 surface (b)HRTEM image of the FAPbI3 continuously covered on the

mesoporous TiO2 1047297

lm (c d) Surface and cross-section SEM imagesof FAPbI3TiO2 1047297lm





FAPbI3 and FTOTiO2 1047297lms were measured as shown inFigure 9a The FAPbI3 perovskite in the mesoporous TiO2 1047297lmshows an absorption edge at sim820 nm The blue shift of absorption edges of the FAPbI3 perovskite in mesoporous TiO2

(820 nm) than that on the glass substrate (870 nm Figure 2a)is ascribed to the quantum size eff ect Figure 9 b presents the Jminus V curve for FAPbI3 based solar cell The power conversionefficiency of FAPbI3 based solar cell reaches 75 with anopen-circuit voltage of 084 V and a short-circuit currentdensity of 183 mAcm2 It is proposed that there are tworeasons responsible for the improvement of the power

conversion effi

ciency based on the in situ dipping reactionover the one-step spin-coating technique One is an improv edloading of the FAPbI3 in the holes of meso-porous layer22

Another reason is the relatively low annealing temperature of the FAPbI3 1047297lm Additionally compared with the traditionalMAPbI3 based solar cell the FAPbI3 solar cell exhibits a very high current density22 Such improvement is due to its widerabsorption range than the MAPbI3 as shown in Figure S9 TheFAPbI3 will provide new opportunities for improving theefficiency of the perovskite solar cells

4 CONCLUSIONS

In summary a new FAPbI3 adopts the P3m1 trigonal spacegroup was synthesized and studied The lattice parameters of the FAPbI3 are of a = 90008(8) Aring b = 90008(8) Aring c =11012(2) Aring and α = 90deg β = 90deg γ = 120deg Therefore thenew compound could be considered as a nearly cubicperovskite structure with a slight distortion when ignoring the

displacement and orientation of HC(NH2)2+

cations TheFAPbI3 based solar cells were subsequently fabricated by one-step solution processing and two-step dipping technologyrespectively It was found that the two-step dipping technology was more favorable to achieve a relatively high efficiency due tofeasibility of forming a phase-pure FAPbI3 perovskite atrelatively low temperaure (100 degC) The nearly cubic crystalstructure and extended light absorbance render this new FAPbI3 perovskite extremely attractive for the high efficientperovskite solar cells

ASSOCIATED CONTENT


NMR of the synthesized FAI Pawley 1047297t of the powder X-ray

diff raction pattern of FAPbI3 photographs of the spin-coatedand thermally annealed PbI2FAI 1047297lms XRD pattern of the yellow phase FAPbI3 photographs XRD patterns cross sectionSEM image EDS maps and of FAPbI3 1047297lms fabricated using thetwo-step method EQE of the FAPbI3 solar cells (PDF) Thismaterial is available free of charge via the Internet at httppubsacsorg

AUTHOR INFORMATION

Corresponding Author

G C E-mail cuiglqibebtaccn

Author Contributions All authors have given approval to the 1047297nal version of the

paper These authors contributed equallyFunding

This work was 1047297nancially supported by the National NaturalScience Foundation (No 51202266 21202178) NaturalScience Foundation of Shandong Province(NoZR2013FZ001)and National Program on Key Basic Research Project of China(973 Program) (No MOST2011CB935700)

Notes

The authors declare no competing 1047297nancial interest

Figure 8 (a) Calculated XRD pattern of FAPbI3 (b) The XRDpattern of the FTOTiO2 substrate (c) The XRD pattern of theFAPbI3 on the mesoporsous TiO2 1047297lm synthesized by the in situdipping technology and subsequently heated at 100 degC for 30 min

Figure 9 (a) UV minus visNIR absorption spectra of FTOTiO2 and FTOTiO2FAPbI3 obtained by in situ dipping technology respectively (b) The Jminus V curve of the as-fabricated FAPbI3 solar cells





ACKNOWLEDGMENTS

We thank Dr L Chen of Max Planck Institute for PolymerReasearch Prof R Yang of Qingdao Institute of Bioenergy andBioprocess Technology Chinese Academy of Sciences andProf G Qin of Peking University for their kind and valuablediscussion

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NH2CHNH2PbI3 An Alternative Organolead Iodide PerovskiteSensitizer for Mesoscopic Solar CellsShuping Pang dagger Hao Hudagger Jiliang ZhangDagger Siliu Lvdagger Yaming Yusect Feng Weiperp Tianshi Qin∥

Hongxia Xudagger Zhihong Liudagger and Guanglei Cui dagger

daggerQingdao Key Lab of Solar Energy Utilization and Energy Storage Technology Qingdao Institute of Bioenergy and BioprocessTechnology Chinese Academy of Sciences Qingdao 266101 Peoplersquos Republic of ChinaDaggerDepartment of Physics and Materials Science City University of Hong Kong Hong Kong SAR sectCollege of Materials Science and Engineering Huaqiao University Xiamen 361021 Peoplersquos Republic of ChinaperpGeneral Research Institute for Nonferrous Metals Beijing 100088 Peoplersquos Republic of China∥Ian Wark Laboratory CSIRO Materials Science amp Engineering Clayton South Victoria 3168 Australia


ABSTR ACT A new nearly cubic NH2CHNH2PbI3

(FAPbI3) perovskite was synthesized for the mesoscopicsolar cells The measured band gap of bulk FAPbI 3 is 143 eV and it is therefore potentially superior than the CH3NH3PbI3

(MAPbI3) as the light harvester A homogeneous FAPbI3

perovskite layer was deposited on the TiO2 surface by utilizingthe in situ dipping technology As a result a high efficiency of 75 was achieved using P3HT as the hole transport materialThe nearly cubic crystal structure and appropriate band gaprender this new FAPbI3 perovskite extremely attractive for nextgeneration high-efficiency low-cost solar cells

1 INTRODUCTION

It has been recognized that solar energy is one of the promisingresources of renewable energy to power our lives Compared with conventional crystalline silicon cell thin 1047297lm solar celltechnology promises to reduce the cost of light-to-electricity conversion b y using abundant materials and easy fabricationprocedures1 2 A variety of promising light absorbing materialshas been explored in both organic and inorganic systemsGenerally organic materials suff er from short lifetime for theirdegradation when exposed to light for a long time3 4 and theinorganic absorbers (eg quantum dots QD) require acomplicated synthetic procedure to satisfy the solutionprocessing requirement5minus7

Recently due to the high processability large optical

absorption cross section and excellent thermal stabilityorganolead halid perovskites (APbX 3) have become one of the most promising light harvesters for mesoscopic solarcells8minus22 More and more new efficiency records of perovskite based solar cells have been achieved at rather short intervalsUp to now a record efficiency of sim15 has been achieved by solution processed M APbI3 (MA CH3NH3

+) containingperovskite solar cells22 The optical absorption coefficient of MAPbI3 is about 15 times 104 cmminus1 at 550 nm and thermally stableup to 300 degC9 1123 In the MAPbI3 perovskite structure thelarger MA cation and the smaller Pb cation are respectivelycoordinated to twelve and six I anions and PbI6 octahedra arecorner-shared to form a three-dimensional framework The

electronic properties of Pb halide perovskites are strongly dependent on the geometry of the three-dimensional network of the PbX 6 octahedra23 The measured band gap of MAPbI3 is151 eV at room temperature which is higher than theoptimized band gap value (140 eV) according to theShockley minusQueisser limit curve for a single band gap solar cell24

To tune the band gap one attempt is to substitute I with Bror Cl anions resulting in the band gaps changed from 151 to222 or 362 eV respectively13 Such a modi1047297cation isunfavorable for a wide range of light harvesting Anotherstrategy is to substitute the A site cations because the band gapof the perovskite is very sensitive to any modi1047297cation(enlargement tilting andor deformation) of the octahedranetwork25 It is known that at room temperature MAPbI3

perovskite is a tetragonal rather than a cubic structure due to

the smaller size of methylammonium cation (CH3NH3+)according to the tolerance factor f = (r B + r X )radic 2(r A +r X )23 25 The substitution of A site with a larger cation is thusexpected to yield a higher symmetry of the perosvskite structureand a decrease of the band gap as well However thereplacement of CH3NH3

+ with a long chain ethylammoniumcation (CH3CH2NH3

+) produces a layered perov sk ite crystal-line structure with a band gap increased to 22 eV26 The failed

Received December 6 2013Revised January 13 2014Published January 21 2014

Article

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FAPbI3 Solar Cells