Journal ofMaterials Chemistry A

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aKey Lab of Organic Optoelectronics & Molec

Department of Chemistry, Tsinghua Univ

chldwang@mail.tsinghua.edu.cn; Fax: +86bState Key Lab of Polymer Physics and Ch

Academy of Sciences, Beijing, 100190, P.R. C

Cite this: J. Mater. Chem. A, 2013, 1,11735

Received 10th June 2013Accepted 25th July 2013

DOI: 10.1039/c3ta12240a

www.rsc.org/MaterialsA

This journal is ª The Royal Society of

Post modification of perovskite sensitized solar cells byaluminum oxide for enhanced performance

Wenzhe Li,a Jiaoli Li,b Liduo Wang,*a Guangda Niu,a Rui Gaoa and Yong Qiua

Themethod of post-modification by aluminumoxidewas successfully introduced into perovskite sensitized

solar cells with a liquid electrolyte. Post-modification by Al2O3 could both protect the perovskite sensitizer

from corrosion by electrolyte and effectively suppressed electron recombination. The UV-vis spectra

revealed an enhanced absorption especially in the long wavelength range after modification. The XRD

results showed a disappeared peak of PbI2, demonstrating that the modification could effectively

protect the perovskite from dissolution in the electrolyte. Stability test showed that the remaining JSCimproved from 10% to 50% at a given period of time. The EIS results and dark current curves illustrated

that this modification increased the interface resistance in dark, confirming that the electron

recombination process was effectively restrained. Finally, the corresponding efficiency was largely

increased from 3.56 to 6.00% by 68%. The strategy using aluminium oxide to post-modify a perovskite

sensitized solar cell was therefore proved to be a useful tool for the optimization of perovskite

sensitized solar cells.

1 Introduction

The CH3NH3PbI3 perovskite and its derivative nanocrystals areused as light harvesters for solar cells.1–9 These compounds havesome physical properties such as 20 multiple roles of light-absorption, electroluminescence, organic-like mobility,magnetic properties, charge separation, transport of both holesand electrons in single material.7,10–12 In addition, the sensitizedsolar cells based on this kind of materials with the advantagesof low-cost and high efficiency have the potential to be the idealphotoelectric conversion device.1,13

The CH3NH3PbI3 perovskite solar cells were divided into twocategories: all-solid-state solar cells and liquid state solar cells.It was reported that in the all-solid-state sub-micron thin lmmesoscopic solar cells, the spiro-MeOTAD (2,20,7,70-tetrakis-(N,N-di-p-methoxyphenylamine)-9,90-spiro-biuorene) was usedas the hole transport material (HTM), and achieved a very highPCE of 12%.3 Aer the iodine in CH3NH3PbI3 was practicallychanged by chlorine, the device stability and incident-photon-to-carrier conversion-efficiency (IPCE) increased, the PCEimproved to 12.3%.5,7 On the other hand Kojima2 usedCH3NH3PbI3 perovskite as a sensitizer and the iodine redox aselectrolytes fabricated liquid state perovskite sensitized solarcells, and the power conversion efficiency (PCE) was 3.81%.Then Im6 optimized the precursor concentration of

ular Engineering of Ministry of Education,

ersity, Beijing 100084, China. E-mail:

10 62795137; Tel: +86 10 62788802

emistry, Institute of Chemistry, Chinese

hina

Chemistry 2013

CH3NH3PbI3 perovskite, the nal PCE was boosted to 6.5%. TheCH3NH3PbI3 perovskite nanoparticles are unstable in an iodide-contained liquid electrolyte due to rapid dissolution, the fastdegradation of performance was oen witnessed.6 The perov-skite sensitizer was therefore hardly applied to the most effi-cient liquid-cell-structure.

post-modication focused on a sensitized TiO2/electrolytesinterface was proposed by Gratzel14 in dye sensitized solar cells(DSCs). The 4-tert-butylpyridine (TBP) was used as the modi-cation agent so that the electron recombination from aconductive band of TiO2 to an electrolyte was retarded. The PCEimproved from 3.7 to 8.5%. Due to the problem of organicmodication agents that includes a low boiling point andsolubility in electrolytes, the inorganic materials with a widebandgap such as Al2O3

15–19 could also suppress electronrecombination. In quantum dots sensitized solar cells (QDSCs)post-modication materials, for instance ZnS,20,21 tetrabuty-lammonium iodide (TBAI),22 halogen ions23 were also used toimprove the photoelectric conversion efficiency and devicestability. However, the post-modication in CH3NH3PbI3perovskite solar cells has never been reported. Compared withall-solid-state solar cells, the liquid state solar cells were moresensitive to photovoltage performance through post-modica-tion by Al2O3.

Here we report a useful strategy to stabilize perovskite in aliquid cell by the method of post-modication. It was found inour former work that the Al2O3 overlayer could act as an insu-lator barrier to retard recombination between the TiO2 and thedye/quasi-solid state electrolyte interface.15,24 A similar effectwas observed in the perovskite sensitized liquid cell where the

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Al2O3 overlayer effectively improved the efficiency 1.71 times,and at the same time the stability from 10% remaining JSC to50% at a given period of time. The paper systematically dis-cussed the CH3NH3PbI3 post-modication and passivation,which potentially revealed a normally applied method forinterface modication in all-solid-state perovskite solar cells.

2 Experimental2.1 Materials synthesis

CH3NH3I was synthesized and puried based on the methodproposed by J. H. Im.6 To prepare perovskite CH3NH3PbI3, thesynthesized CH3NH3I (0.395 g) and PbI2 (1.157 g, Aldrich) weremixed in g-butyrolactone (2 mL, TCI) at 60 �C for 12 h withstirring. The nanocrystalline TiO2 colloid was synthesized withthe hydrothermal method which was well documented in aprevious report.25

2.2 Solar cell fabrication

The cleaned FTO glasses were coated with 0.15 M titaniumdii-sopropoxide bis(acetylacetonate) (75% Aldrich) in 1-butanol(Aldrich) solution by the spin-coatingmethod with 3000 rpm and30 s, which was heated at 125 �C for 5 min, and then raised to500 �C for 30 min. The nanocrystalline TiO2 paste was depositedon the pre-treated FTO substrate, which was followed by heatingat 500 �C for 1 h. The thickness of the annealed TiO2 lms was 5.4mm, determined by SEM (JEOL JSM-7401F). The perovskitecoating solution was spread on the annealed TiO2 lm and spunfor 60 s at a speed of 2000 rpm in air atmosphere. Then the lmswere heated at 100 �C for 15 min. The room temperature lmswere immersed into a 45 mM aluminium triethyl (Alfa, Al(C2H5)325% w/w in hexane) hexane solution for 30 s. The hydrolysis ratewas very fast in the air. Aluminium triethyl is ammable, there-fore it was reasonable to keep it in a glovebox. The redox elec-trolyte was prepared by dissolving 0.9M LiI (Alfa), 0.45M I2 (Alfa),0.5 M N-methylbenzimidazole (Aldrich, NMBI), 0.05 M urea(Aldrich) in ethyl acetate (Alfa). Here the NMBI was used aselectrolyte additives, considering the lower boiling point of thenormally used 4-tert-butylpyridine (TBP).26 Chemically platinizedconductive glass was used as the counter electrode. Whenassembling the DSCs, the sensitized TiO2 electrode and a counterelectrode were sandwiched using an adhesive tape with a thick-ness of 15 mm. Last, the electrolyte was introduced into the spaceof the sealed electrodes prior to measurement.

2.3 Characterization

The UV-vis absorption spectra were used for testing theabsorption of the perovskite sensitized TiO2 lm with a HitachiU-3010 spectroscope. X-Ray diffraction (XRD) patterns wereobtained with Smart LAB instruments Cu Ka beam (l ¼ 1.54 A).X-Ray photoelectron spectroscopy (XPS) was measured with aPHI 5300ESCA instrument. Photocurrent–voltage (J–V), incidentphoton-to-electron conversion efficiency (IPCE) and electro-chemical impedance spectros (EIS, ranged from 0.1 Hz to 105

Hz) were measured by a ZAHNER CIMPS electrochemicalworkstation, Germany.

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3 Results and discussion3.1 Characterization of the sensitized lms with Al2O3 post-modication

To investigate the generated material aer Al(C2H5)3 treatment,XPS was used to analyze the TiO2/CH3NH3PbI3 lm before andaer surface modication. All of the peaks were calibratedusing C1s (284.8 eV) as the reference. The elements of thecomposite lms could be identied in the XPS wide-scan surveyspectrum, as shown in Fig. 1a. A detailed analysis of the XPSspectra presented a clear evidence that the lms were chemi-cally modied and conrmed by high resolution spectra of O 1sand Al 2p based on a Gaussian spectral deconvolution. Fig. 1bshowed that aer the modication the peak of Al was located at74.5 eV. As shown in Fig. 1c, the O 1s spectrum for TiO2/CH3NH3PbI3 in line (A) could be partitioned to two peaks ofTiO2. The large peak at 530.0 eV was assigned to lattice oxygenin anatase TiO2, whereas the other peak located at 531.5 eVcould be attributed to the hydroxyl O atoms.27 Aer the modi-cation, a new peak appeared at 531.9 eV shown in line (B),which was the binding energy of Al2O3.28,29 It was thereforeproved that the Al2O3 layer was generated aer the Al(C2H5)3modication.

3.2 Photovoltage performance of perovskite solar cells byAl2O3 post-modication

The liquid state perovskite solar cells were fabricated usingCH3NH3PbI3 sensitized TiO2 lms as a photoanode and the J–Vcurves were shown in Fig. 2a. For electrode without post-modication by Al2O3, the open circuit voltage (VOC) increasedfrom 0.56 to 0.68 V. A similar phenomenon was observed in ourformer results about Al2O3 post-modication of a quasi-solidstate solar cell, where a 0.13 V increase was acquired.15 The llfactor was relatively high, 0.68 in comparison with 0.53 beforemodication. The Al2O3 coating between the sensitized TiO2

lm and the electrolyte served as an insulating barrier layer. TheAl2O3 coating depressed the recombination between the injec-ted electrons and the electrolytes. As a result, the overall effi-ciency of 6% was doubled for the device modied with Al2O3

layer. The post-modication thus proved to be an efficientmethod to improve the efficiency of the perovskite sensitizedliquid perovskite solar cells.

The photocurrent response in monochromatic was alsoinvestigated by IPCE, and the results were shown in Fig. 2b. Thenarrow IPCE curve revealed a maximum efficiency of 58% at 500nm, while having an efficiency above 50% in the range of 440–580 nm. Aer modication with Al2O3, the range was broadenedfrom 140 to 210 nm, which was 530–740 nm in detail. Thismight be due to a more efficient incident photon absorption inthe long wavelength range. However the photo-electron currentresponse of TiO2/CH3NH3PbI3/Al2O3 perovskite solar cells was10% lower than that of the device before modication. In thetesting process, TiO2/CH3NH3PbI3 perovskite solar cells withlower stability would generate more PbI2 than the devices aermodication, leading the higher photocurrent response at500 nm wavelength which was attributed to absorption from

This journal is ª The Royal Society of Chemistry 2013

Fig. 1 XPS wide-scan survey (a), O 1s and (Al) 2p patterns (b), (c) of the (A) TiO2/CH3NH3PbI3 and (B) TiO2/CH3NH3PbI3/Al2O3 films.

Fig. 2 (a) J–V curves of TiO2/CH3NH3PbI3 and TiO2/CH3NH3PbI3/Al2O3 perov-skite solar cells; (b) IPCE; (c) device stability.

This journal is ª The Royal Society of Chemistry 2013

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PbI2. Even though the IPCE was lower in the range from 400 to550 nm, the enhanced effect of Al2O3 layer in the range from 550to 760 nm played a dominant role. The corresponding shortcircuit current exhibited an increase of 12% aer modicationunder illumination of 1 sun, as shown in Fig. 2a.

The decay of the device performance was recorded bycontinually testing under 1 sun without the device packaged.The JSC decreased with time, as shown in Fig. 2c. The aging ofthe liquid cell happened very quickly, less than 10% values wasremained aer 15 min of illumination. In comparison, themodied cell maintained more than 50% values. The Al2O3

layer could perfectly protect the perovskite sensitizer fromdissolution by the electrolyte and erosion by the moisture in air.Although the overall stability was not so far ideal as solid stateperovskite solar cells, this work inspired a possible method forthe development of liquid cells by using perovskite as a sensi-tizer and provided a feasible method to further improve the PCEof the solid state.

3.3 Effect on suppressing the corrosion process by Al2O3

post-modication

The CH3NH3PbI3 perovskite sensitized TiO2 lm had a broadUV-vis spectrum until 775 nm, as shown in Fig. 3. This iscomparable to the traditional successful black dye. However fora liquid cell, the electrolyte decomposed the perovskite easily,which made the cell unstable. It was clearly seen that aercontact with the electrolyte, the spectrum of the sensitized TiO2

lm without Al2O3 layers was strongly narrowed to 520 nm.Fortunately, the electrolyte decay could be effectively restrainedby post-modication of the sensitized photoanode with Al2O3.The Al2O3 is traditionally formed by hydrolysis of aluminiumisopropoxide. However in our experiment one of the hydrolysisproducts isopropanol also affected the stability of CH3NH3PbI3perovskite. Here the Al(C2H5)3 was used as a modication agentwhose hydrolysis products do not decompose the CH3NH3PbI3.The absorption spectrum of the corresponding lm showed aslight blue shi, while retaining the enhanced absorption until789 nm as before the erosion. This was probably because theAl2O3 overlayer blocked the water in air, so the erosion of theperovskite was effectively retarded. The Al2O3 overlayer couldsuppress both electrolytes and vapour in the air corrosionprocess. A similar phenomenon was observed in Luo's work, the

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Fig. 3 UV-vis spectra of the sensitized TiO2 film.

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cell with an insulating coating showed better stability underboth low and high illumination.15

The crystal structure changes in the corrosion process werefurther veried by a XRD experiment. Fig. 4 shows the crystalinformation existing in the photoanode. The peaks at 28.30 and31.71� in curve B, corresponding to the (220) and (310) planes,conrmed the formation of a tetragonal perovskite structurewith lattice parameters of a ¼ b ¼ 8.883 A and c ¼ 12.677 A,which was well consistent with previous reports.3,4,13 Only thesensitized lm without modication showed a typical 002 and102 crystal face of PbI2 located in 25.8 and 34.2�, respectively,which was produced by the erosion procedure. The bandgap ofbulk PbI2 is 2.57 eV30 with absorption at 482 nm wavelengthwhich is similar to the ‘TiO2/CH3NH3PbI3 aer the erosioncurve in Fig. 3. That meant the CH3NH3PbI3 was dissolved anddecomposed to PbI2 and CH3NH3I in electrolyte conditions. Incomparison, the sensitized TiO2 lm aer modication main-tained a similar spectrum both before and aer erosion. Thistherefore proved the idea that the Al2O3 overlayer could effec-tively protect the perovskite sensitizer to obtain improvedstability.

Fig. 4 XRD spectra of the sensitized TiO2 films.

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3.4 Effect on retarding the recombination by Al2O3 post-modication

The chemical corrosion retarded by Al2O3 was revealed whilethe physical performance was also needed to be investigated. Inorder to reveal the role of Al2O3 for the improvement of thedevice, the EIS spectra were measured. Under full illumination,the Nyquist plot (Fig. 5a) showed typical semicircles in themeasured frequency range of 0.1 Hz to 100 kHz. As reported insuch perovskite solar cells, the resistance at the TiO2/sensitizer/electrolyte interface (the R2 in the equivalent circuit representedin Fig. 5a) can be determined by the middle frequency (10–100Hz) semicircle in the Nyquist plots.31 The values of R2 are 192 U,and 225 U for TiO2/CH3NH3PbI3 and TiO2/CH3NH3PbI3/Al2O3

Fig. 5 Impedance spectra of the studied device under illumination of AM 1.5G(a) and in the dark (b), and dark J–V curve of the investigated device (c).

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device, respectively. The interface resistance slightly increasedby 17%, reecting that the forward electron transmission wasimpeded. In theory, the JSC would be decreased, which wascontradicted with the increased JSC in the J–V curve (Fig. 2b).However, the Al2O3 in the devices protected CH3NH3PbI3 fromcorrosion by the electrolyte. The relatively stable CH3NH3PbI3contributed the higher JSC in modied cells which had beenillustrated above. The Nyquist plot of impedance spectra in thedark under 0.8 V bias voltage revealed an increased interfaceresistance at sensitized TiO2/electrolyte interface (Fig. 5b). Thevalues changed from 50 U before modication to 125 U aermodication 150% times. The interface resistance was reectedin the obstruction of electron transmission from TiO2 conduc-tive band (CB) to electrolytes. Normally an increased resistancein the dark indicated restrained electron recombination.32 Asshown in Scheme 1 the electron recombining process from TiO2

CB to the electrolyte was suppressed by the Al2O3 insulatinglayer. Considering both the interface resistance changes in theillustrated and dark conditions, the recombination resistanceincreased much more substantially than the resistance of theforward electron transport. Therefore the current density wasenhances, also reected in the J–V curves. In our case, thiswas veried by dark current measurements. The J–V curve wasrecorded using the fabricated device measured in the dark. Asshown in Fig. 5c, 2.8 times dark current was found for theunmodied device in comparison with the modied one, forexample at VOC ¼ 0.6 V. The recombination of the separatedexcitons was thus effectively restrained. VOC could be deter-mined using the formula below:33,34

VOC ¼�mRT

F

�ln

�ISC

I0� 1

�(1)

where ISC is the short-circuit photocurrent, I0 is the darkcurrent, m is the ideality factor, whose value is between 1 and 2for perovskite solar cells,35 and R and F are the ideal gas andFaraday constants, respectively. The formula proved that VOCincreased with decreasing I0. An increased photovoltageperformance and decreased dark current suggested that thedark reaction at the interface of the sensitized-TiO2 electrode/electrolyte in perovskite solar cells was reduced aer themodication of Al2O3 and that electron transfer from theconduction band of the TiO2 lm to the triiodide ions wassuppressed.

Scheme 1 Illustration of the interfacial charge transfer processes occurring atthe sensitized TiO2/electrolyte interface. Also shown is the Al2O3 overlayer asdeveloped in this study.

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4 Conclusions

An efficient method both improved the photovoltaic perfor-mance of perovskite sensitized solar cells and developed thedevice stability. The Al2O3 overlayer could act as an insulatorbarrier to protect the easily dissolved perovskite sensitizer andretard the recombination between the sensitized TiO2/liquidstate electrolyte interface. The corresponding efficiency wasimproved by 68%, and an overall efficiency of 6.00% wasobtained. Moreover, the remaining JSC improved from 10% to50% at a given period of time. At least two factors resulted in theimprovement: one was the enhanced absorption in the longwavelength range, coming from the protected perovskitesensitizer. This was evidenced by the XRD, where a disappearedpeak of PbI2 indicated that the modication could effectivelyprotect the perovskite from dissolution in the electrolyte;another was the increased interface resistance in the dark,which afforded restrained electron recombination. This workthus supplied a normally applicable strategy for interfacemodication in liquid state perovskite solar cells, which couldalso be applied in solid state perovskite sensitized solar cells.This part of the work was underway in our lab.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China under Grant no. 51273104, the NationalKey Basic Research and Development Program of China underGrant no. 2009CB930602, and the Ministry of Science andTechnology of China (no. 2012CB933200).

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