Controllable Sequential Deposition of Planar CH3NH3PbI3 PerovskiteFilms via Adjustable Volume ExpansionTaiyang Zhang,† Mengjin Yang,‡ Yixin Zhao,*,† and Kai Zhu*,‡

†School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China‡Chemistry and Nanoscience Center, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado80401, United States

*S Supporting Information

ABSTRACT: We demonstrate a facile morphology-controllable sequentialdeposition of planar CH3NH3PbI3 (MAPbI3) film by using a novel volume-expansion-adjustable PbI2·xMAI (x: 0.1−0.3) precursor film to replace pure PbI2.The use of additive MAI during the first step of deposition leads to the reducedcrystallinity of PbI2 and the pre-expansion of PbI2 into PbI2·xMAI with adjustablemorphology, which result in about 10-fold faster formation of planar MAPbI3 film(without PbI2 residue) and thus minimize the negative impact of the solventisopropanol on perovskites during the MAI intercalation/conversion step. Thebest efficiency obtained for a planar perovskite solar cell based on PbI2·0.15MAI is17.22% (under one sun illumination), which is consistent with the stabilizedmaximum power output at an efficiency of 16.9%.

KEYWORDS: Perovskite, solar cells, sequential deposition, volume expansion

Lead halide perovskite solar cells have become one of themost promising candidates for low-cost and high-efficiency

solar cell technologies as evidenced by their unprecedented cellefficiency progress during the past several years.1−6 High-performance perovskite solar cells have been demonstrated bymultiple groups using different deposition approaches.7−18

Most of the reported high-efficiency perovskite solar cells arebased on either a planar or planar/mesoporous hybridperovskite architecture. One of the key requirements forachieving high-performance perovskite solar cells is to have auniform, pinhole-free perovskite layer with controlled morphol-ogy and composition.19 The pinhole-free compact perovskitefilm was initially demonstrated by using vapor-phase deposition(coevaporation).9 Recent development in solution chemistrywith various engineering and synthetic controls has also led tothe fabrication of high-quality perovskite films by differentgroups.19 Among various solution deposition approaches, two-step sequential solution deposition has shown promise forfabricating high-efficiency CH3NH3PbI3 (or MAPbI3) perov-skite solar cells.8 In the standard two-step sequential solutiongrowth, the initially deposited PbI2 film undergoes anintercalation reaction with MAI in the isopropanol (IPA)solution to form MAPbI3. As a result, the density of thematerial decreases from about 6.16 g/cm3 for PbI2 to 4.29 g/cm3 for MAPbI3,

20 corresponding to about a two-fold volumeexpansion per formula.21 However, this volume expansionassociated with the intercalation processes could present severalissues for preparing a compact, planar MAPbI3 film. Inpreparation of planar perovskite films, space-expansion-inducedformation of a tight MAPbI3 surface layer would significantly

inhibit the complete conversion of PbI2 to MAPbI3 by blockingthe MAI diffusion to the deeper layer of PbI2. This issue hasbeen discussed in a recent study and has been associated withthe low reproducibility of the performance of planar perovskitesolar cells using two-step sequential solution deposition.15 Incontrast, it normally is relatively easier to have a complete PbI2-to-MAPbI3 conversion using a mesoporous scaffold (e.g.,nanocrystalline TiO2 film); this is due to the small PbI2 grainsize coupled with the porous structure to provide sufficientpathways for MAI diffusion, facilitating a rapid conversionprocess.8 To address the PbI2 residue issue for preparing planarperovskite films, a long intercalation/conversion time isrequired during the MAI-solution-dipping step. However, along solution-dipping time is expected to cause significantdamage to the MAPbI3 layer resulting from the possible backextraction (deintercalation) of MAI from MAPbI3.

21 Otherthan the issue of incomplete conversion, the morphologies ofmost reported MAPbI3 films fabricated by sequential depositionare relatively rough and difficult to control. A recent studyfound that carefully adjusting the synthetic conditions tocontrol the perovskite morphology is critical to the perform-ance of perovskite cells.12 Although this study is based onmesoporous cell architecture, the conclusion should also applyto the planar perovskite solar cells. Thus, it is highly desired forfuture perovskite-based device applications and fundamentalinvestigations to develop synthetic approaches via sequential

Received: March 2, 2015Revised: April 26, 2015Published: May 21, 2015

Letter

pubs.acs.org/NanoLett

© 2015 American Chemical Society 3959 DOI: 10.1021/acs.nanolett.5b00843Nano Lett. 2015, 15, 3959−3963

solution deposition to produce a planar MAPbI3 film that hascontrolled morphology, no PbI2 residue, and minimum damagefrom deposition solution.Here, we demonstrate a facile morphology-controllable

sequential deposition of planar MAPbI3 film by using a novelPbI2·xMAI (x: 0.1−0.3) precursor film. The addition of smallamount of MAI to the standard PbI2 precursor during the firststep of deposition leads to about 10-fold faster completeMAPbI3 formation without any PbI2 residue during the(second) MAI intercalation step, and much improved deviceperformance with higher reproducibility when 0.1−0.2 MAI isused. The best cell efficiency obtained is 17.22% using the PbI2·0.15MAI precursor with the maximum stable power output atabout 16.9% under one-sun illumination. The morphologies ofPbI2·xMAI precursor films and the corresponding MAPbI3films can be adjusted by tuning the relative amount of MAIused during the first-step deposition. The effects of addingxMAI on the perovskite film morphology, device characteristics,and electrical/optical properties are discussed with respect tothe standard two-step sequential solution deposition approach.Figure 1, panel a shows the ultraviolet−visible (UV−vis)

absorption spectra evolution of the pure PbI2 film after dipping

in a 10 mg/mL MAI IPA solution for various durations, from0−30 min. The characteristic absorption shoulder near 750 nmfor perovskite MAPbI3 increases slowly with the MAI dippingtime up to 20 min. The absorption spectrum increases verylittle after 20 min dipping, which indicates that most PbI2 hasbeen converted to MAPbI3. The baseline of the absorptionspectra also increases simultaneously with longer dipping time,which reflects the stronger light-scattering effect associated witha rougher perovskite morphology. A recent study shows thatthe grain size of perovskites near the top surface of theperovskite layer increases significantly with increasing MAIsoaking/dipping time.12 Although the MAPbI3 film grown with6 min dipping has already exhibited strong MAPbI3 absorbancespectrum, the X-ray diffraction (XRD) pattern of the same

sample (Figure 1b) still exhibits the characteristic PbI2 peaksalong with the MAPbI3 peaks, which indicates incompleteconversion of PbI2 to MAPbI3. The intensity of the PbI2diffraction peak is substantially reduced with longer dippingtime up to 20 min. The PbI2 peak disappears completely with30 min dipping. The typical scanning electron microscopy(SEM) images of planar PbI2 and MAPbI3 (20 min dipping/conversion) films are compared in Figure 1, panels c and d,respectively. The morphology of this 20 min-dipping MAPbI3film is similar to that of a film with shorter dipping time (6 min;Figure S1, Supporting Information). The planar PbI2 filmappears porous with a coarse top surface, which may be causedby the shrinkage of the precursor film during the dryingprocess.11,15,22 The resulting MAPbI3 film is also full of tinypinholes, and its surface is covered with rough perovskitenanocrystals; this appearance is similar to that of the perovskitecapping layer grown on a mesoporous substrate using two-stepsequential deposition.8,12,13,23 The increased roughness of theMAPbI3 film compared to the initial PbI2 film is attributable tothe space expansion during the MAI intercalation/reaction step.Taken together, the XRD and absorption results (Figure

1a,b) illustrate the difficulty in preparing planar MAPbI3 thinfilms from PbI2 using the two-step sequential solutiondeposition approach. This challenge has been discussed indetail elsewhere.24 The effect of varying MAI dipping time onthe device characteristics is shown in Figure S2 with thecorresponding photovoltaic parameters listed in Table S1. Thecell efficiency increases significantly with longer dipping time,from 3.15% at 0.5 min to 6.11% at 2 min to 9.30% at 6 min,which is consistent with the more complete conversion of PbI2to MAPbI3. However, when the dipping time is furtherincreased to 20 min, the device efficiency drops to only1.29% even though the MAPbI3 film with 20 min dipping hasmuch reduced PbI2 with stronger light absorption than theMAPbI3 film with 6 min dipping. Thus, it is evident that simplylengthening the reaction time to complete the PbI2-to-MAPbI3conversion presents a challenge for the typical two-stepsequential solution processing. The long dipping/reactionduration could cause damage to the MAPbI3 layer resultingfrom the possible back extraction of MAI from MAPbI3,

21 andthus it deteriorates the device performance. Therefore, it iscritical to develop a strategy to speed up the conversion process(or minimize the MAI solution dipping time) for preparingplanar MAPbI3 films.To address the challenges associated with the incomplete

conversion of PbI2 and rough perovskite surface discussedabove, we examined the effect of using a new precursor(mixture of PbI2 and xMAI with x varying from 0.1−0.3)during the first deposition step. The resulting precursor film isdenoted as PbI2·xMAI. Even in the presence of MAI, thedeposited PbI2·xMAI films still look yellowish, without anyindication of the formation of perovskite MAPbI3. The UV−visabsorption spectra (Figure 2a) of the PbI2·xMAI precursorfilms exhibit an absorption onset near 510 nm, which ischaracteristic for PbI2. The sharp absorption edge associatedwith PbI2 becomes less clear with increasing MAI. When x =0.3, the baseline of the absorption spectrum increasessubstantially, indicating an enhanced light-scattering effect.Figure 2, panel b compares the XRD patterns of PbI2·xMAI (x:0−0.3) precursor films. In the absence of MAI, the PbI2 filmdisplays a strong characteristic XRD peak at about 12.5°. Theaddition of xMAI into PbI2 during the first deposition step doesnot result in any detectable MAPbI3 diffraction peaks such as

Figure 1. (a) UV−vis absorption spectra and (b) XRD patterns ofMAPbI3 films grown via the typical two-step sequential solutiondeposition using pure PbI2 precursor film with different MAI solution-dipping times. SEM images of the (c) typical spin-coated PbI2 film and(d) converted MAPbI3 film with 20 min dipping (reaction) time inMAI solution.

Nano Letters Letter

DOI: 10.1021/acs.nanolett.5b00843Nano Lett. 2015, 15, 3959−3963

3960

the main (110) peak near 14°. This is consistent with theappearance of the yellow color and the correspondingabsorption spectra. The intensity of the main PbI2 diffractionpeak (∼12.5°) decreases substantially with increasing amountof MAI in the precursor. This observation suggests that a partialincorporation of MAI in the PbI2 film decreases the PbI2crystallinity in the PbI2·xMAI films (compared to the pure PbI2film) without forming the perovskite MAPbI3 phase. It isnoteworthy that there are some unknown peaks formed below10° when MAI is used in the precursor film. By comparing theXRD patterns of the PbI2·xMAI precursor films to those of theMAPbI3 and MAI (Figure S8), we can attribute these peaksbelow 10° to the formation of certain complexes, whichbecome stronger for PbI2·0.3MAI. Similar observation ofcomplex formation was discussed previously.11

In contrast to the slow (difficult) conversion of planar PbI2film into MAPbI3 via the typical two-step sequential deposition,using the new PbI2·xMAI precursor films is found to convert tored−brown planar MAPbI3 film much more quickly when usingthe same sequential deposition processes. It takes about 1−3min for all planar PbI2·xMAI (x: 0.1−0.3) films to convertcompletely into MAPbI3 without any PbI2 residue (Figure S3).This contrasts significantly to the 20−30 min required for thepure PbI2 film to have full conversion to MAPbI3 (Figure 1b).It is worth noting that the conversion time for making planarMAPbI3 film reported in literature varies over a wide range; forexample, it changes from 10 min15 (complete conversion) to 45min8 (incomplete conversion) depending on the precursorcomposition and processing conditions. Figure 2, panel c showsthe UV−vis absorption spectra of the planar MAPbI3 filmsusing various PbI2·xMAI (x: 0.1−0.3) precursor films. All ofthese films exhibit the same absorption shoulder near 750 nm,which is typical for MAPbI3. A noticeable increase of thebaseline is observed for the PbI2·0.3MAI sample, whichcorresponds to a larger light-scattering effect as discussed inconnection with Figure 2, panel a. By using the PbI2·0.15MAIprecursor film as an example, Figure 2, panel d demonstratesthe effect of MAI on the evolution of the absorption spectra ofthe precursor films. The rapid increase of absorbance near 750nm indicates a quick conversion process from the PbI2·0.15MAI precursor film to the final perovskite MAPbI3 film.Furthermore, there is no obvious increase of the baseline of theabsorption spectra during the entire conversion process, whichsuggests that the MAPbI3 film sequentially deposited fromPbI2·0.15MAI may have a similar surface roughness as theinitial PbI2·0.15MAI film.Figure 3 panels a−d show the typical SEM images of top

views of the PbI2·xMAI (x = 0.1, 0.15, 0.2, and 0.3) precursorfilms. These films have very different morphologies than thepure PbI2 film, and their morphologies vary with the amount ofMAI used in the precursor. When x ≤ 0.2, introducing MAIinto PbI2 leads to the formation of a smoother PbI2·xMAI filmwith fewer pinholes than the pure PbI2 film. However, themorphology of the PbI2·0.3MAI film becomes even coarserthan that of pure PbI2 film, which is consistent with the higherabsorption baseline of the PbI2·0.3MAI film (Figure 2a). Figure

Figure 2. (a) UV−vis absorption spectra and (b) XRD patterns ofPbI2·xMAI precursor films before the second conversion step (insetshows the magnified view of XRD patterns from 5−10°); (c) UV−visabsorption spectra of MAPbI3 films prepared from PbI2·xMAI filmsafter the second conversion step; (d) evolution of UV−vis absorptionspectra using the PbI2·0.15MAI precursor film with different dippingtimes in the MAI solution.

Figure 3. Typical SEM images of (a−d) PbI2·xMAI (x = 0.1, 0.15, 0.2, and 0.3) precursor films and (e−h) MAPbI3 films prepared from theirrespective precursor films as indicated.

Nano Letters Letter

DOI: 10.1021/acs.nanolett.5b00843Nano Lett. 2015, 15, 3959−3963

3961

3, panels e−h show the typical SEM images of the MAPbI3films prepared from PbI2·xMAI film after the second dipping/conversion step. It is worth noting that the MAPbI3 filmsfabricated from PbI2·xMAI (x = 0.1−0.2) are nearly pinhole-free and show a relatively uniform, smooth surface, especiallyfor the 0.15MAI sample, which is significantly different fromthe relatively rough MAPbI3 films prepared from the pure PbI2film. The grain sizes of the MAPbI3 films also show adependence on the amount of MAI used in the precursor.When x = 0.1, the MAPbI3 film mainly consists of relativelysmall MAPbI3 nanocrystals with sizes less than 200 nm. When0.15−0.2 MAI is used, the MAPbI3 films are composed of manyrelatively large (∼500 nm) crystals filled with ∼200 nm smallcrystals; such film morphology is comparable to that of thevapor-phase-grown perovskite films. This result is alsoconsistent with the absence of an absorption baseline increaseduring the sequential deposition, as shown in Figure 2, panel d.When started with the relatively coarse PbI2·0.3MAI film, theMAPbI3 film looks similar to that prepared from the pure PbI2film. These results clearly demonstrate that our new PbI2·xMAI-based two-step sequential deposition allows (to a certaindegree) the control of the morphology of the MAPbI3 film.In the typical two-step sequential solution deposition of

MAPbI3, the layer-structured PbI2 crystal is intercalated by MAIto form the MAPbI3, as shown in Figure 4, panel a. As

discussed above, the significant phase transformation normallyleads to about a factor of two volume expansion per formuladuring the second conversion step.12,21 To help understand theeffect of adding MAI (during the first step of deposition) on theMAPbI3 film formation, the thickness (or volume) expansionratios of converting PbI2·xMAI to MAPbI3 films were examinedby using a surface profiler. Figure 4, panel b shows the result ofthe expansion ratios for the films prepared with differentamounts (x) of MAI from 0−0.3. The ratio continuouslydecreases with increasing amount of MAI, which could beaccounted for by the following hypothesis. For the PbI2·xMAIfilms, the added MAI is likely incorporated into the PbI2 matrixand has partially pre-expanded the volume of the PbI2 matrix.The degree of this pre-expansion depends on the amount ofMAI used during the first step of deposition. This pre-expansion decreases the final film expansion ratio withincreasing MAI amount. Assuming the pre-expansion ratioper formula of reactant MAI is the same as the expansion ratio

in the standard conversion process with pure PbI2, theexpansion ratio of the final MAPbI3 film thickness (df) to thethickness of the initial PbI2·xMAI film (di) can be calculatedwith the expression df/di = m/[1 + (m − 1)x], where m is theexpansion ratio. Best fits of the data to this express yield m =2.02 ± 0.07; the best-fitted line is shown in Figure 4, panel b.The derivation of the expression for the expansion ratio is givenin the Supporting Information (eqs S1−S3). It is interestingthat the expansion is also about two, which suggests that thepre-expansion to form PbI2·xMAI and the standard expansionto form MAPbI3 have essentially the same volume expansionratio. It is also interesting to note that the XRD intensity of thePbI2 peak is much reduced even when the PbI2 phase stillconstitutes the majority of the PbI2·xMAI precursor film (e.g., x= 0.15; Figure 2b). Thus, the addition of a small amount ofMAI results in several features that favor the rapid conversionreaction during the second step. The features of using PbI2·xMAI matrix include the reduced PbI2 crystallinity, the pre-expansion of PbI2 into PbI2·xMAI, and the controllablemorphology by varying the amount of MAI in PbI2·xMAI.Moreover, using different amounts of MAI allows the control ofthe pre-expansion of the PbI2·xMAI precursor film, which givesrise to a distinctive morphology, as shown in Figure 3, and alsoaffects the final photovoltaic performance of perovskite solarcells based on the planar MAPbI3 films, as discussed next.Figure 5 shows the typical photocurrent density−voltage (J−

V) curves of planar MAPbI3 solar cells prepared using different

PbI2·xMAI (x: 0−0.3) under simulated one-sun illumination.The details of photovoltaic parameters of all these devicesincluding the statistical analysis (mean values and standarddeviations) for each type of device is given in Table S2 in theSupport Information. These PbI2·xMAI films were dipped inthe MAI solution for 2 min to form the MAPbI3 layers, whichwere adjusted to about 250 nm thick. In the absence of MAI(i.e., using the standard PbI2 precursor), the device shows ashort-circuit photocurrent density (Jsc) of 8.85 mA/cm2, open-circuit voltage (Voc) of 0.969 V, fill factor (FF) of 0.712, andoverall conversion efficiency (η) of 6.11%. The cell efficiencyincreases significantly when 0.1−0.2 MAI along with PbI2 isused during the first-step deposition, especially when 0.15 MAIis used. A typical efficiency for the 0.15 MAI-based device isincreased to 15.62% with a Jsc of 19.89 mA/cm2, Voc of 1.065,and FF of 0.738. The performance improvement is largelydetermined by the increased Jsc value associated with theenhanced PbI2 conversion process and less perovskite damage

Figure 4. (a) Schematic illustration of the transformation from PbI2 toMAPbI3 in the typical two-step sequential deposition process. (b) Plotof the film thickness (or volume) expansion ratios of converting PbI2·xMAI to MAPbI3 films as a function of the relative amount xMAIused. The solid line is the best fit as discussed in the text.

Figure 5. Typical J−V curves of planar MAPbI3 solar cells preparedfrom PbI2·xMAI precursor films.

Nano Letters Letter

DOI: 10.1021/acs.nanolett.5b00843Nano Lett. 2015, 15, 3959−3963

3962

resulting from the minimized interaction with the IPA solvent.The Voc and FF values for the devices based on 0.1−0.2 MAIare also significantly larger than those of the cells based on purePbI2 (Table S2). However, when 0.3 MAI is used, all deviceparameters are reduced, with a typical Jsc of 13.87 mA/cm

2, Vocof 0.967 V, and FF of 0.668, yielding an efficiency of 8.98%.This is presumably due to the coarser morphology of MAPbI3when 0.3 MAI is used because there is no PbI2 residue found inthe perovskite film. The significant drop in cell performance ofthe 0.3 MAI-based device is consistent with its much reducedrecombination resistance (or faster recombination) comparedto the 0.1−0.2 MAI-based cells (Figure S4). Using 0.15 MAI,the best cell efficiency obtained is 17.22% (Figure S5a) with itsJsc value consistent with the external quantum efficiency (EQE)spectrum (inset of Figure S5a). The maximum power output ofthis device stabilizes at an efficiency of about 16.9% (FigureS5b), which is in good agreement with the value obtained fromthe J−V measurement.In summary, we report the use of a new composition

precursor PbI2·xMAI consisting of mixed PbI2 and partial MAI(molar ratio 1:x, where x varies from 0.1−0.3) to replace thepure PbI2 used in the two-step sequential solution deposition ofMAPbI3. In comparison to the standard two-step approachusing pure PbI2, the use of additive MAI during the first step ofdeposition leads to about 10-fold faster MAPbI3 formationwithout any PbI2 residue during the (second) MAI intercalationstep and much improved device performance when 0.1−0.2MAI is used. The morphology of the MAPbI3 film depends onthe relative amount of MAI used in the PbI2·xMAI precursorfilms (during the first-step deposition), and it is generallysmoother when 0.1−0.2 MAI is used. The addition of a smallamount of MAI is found to lead to the reduced crystallinity ofPbI2 and the pre-expansion of PbI2 into PbI2·xMAI withadjustable morphology, which favor the complete conversionreaction within a short period of time and thus minimize thenegative impact of the IPA solvent on the integrity of theMAPbI3 film during the (second) dipping/conversion step. Ourresults suggest that this novel first-step precursor (PbI2·xMAI)is promising for facile preparation of high-quality planarMAPbI3 film with controllable morphologies via two-stepsequential solution deposition to fabricate high-performanceperovskite solar cells.

■ ASSOCIATED CONTENT*S Supporting InformationExperimental method, SEM, XRD, solar cell parameters,impedance measurement, and best device characteristics. TheSupporting Information is available free of charge on the ACSPublications website at DOI: 10.1021/acs.nanolett.5b00843.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: yixin.zhao@sjtu.edu.cn.*E-mail: Kai.Zhu@nrel.gov.Author ContributionsT.Z. and M.Y. contributed equally.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSY.Z. and T.Z. are thankful for the support of the NSFC (Grants51372151 and 21303103). K.Z. and M.Y. acknowledge the

support by the U.S. Department of Energy/National Renew-able Energy Laboratory’s Laboratory Directed Research andDevelopment (LDRD) program under Contract No. DE-AC36-08GO28308.

■ REFERENCES(1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. J. Am. Chem. Soc.2009, 131, 6050−6051.(2) Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.;Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J.E.; Gratzel, M.; Park, N.-G. Sci. Rep. 2012, 2, 1−7.(3) Green, M. A.; Ho-Baillie, A.; Snaith, H. J. Nat. Photonics 2014, 8,506−514.(4) Jung, H. S.; Park, N.-G. Small 2015, 11, 10−25.(5) Yin, W.-J.; Yang, J.-H.; Kang, J.; Yan, Y.; Wei, S.-H. J. Mater.Chem. A 2015, 3, 8926−8942.(6) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith,H. J. Science 2012, 338, 643−647.(7) Heo, J. H.; Im, S. H.; Noh, J. H.; Mandal, T. N.; Lim, C.-S.;Chang, J. A.; Lee, Y. H.; Kim, H.-J.; Sarkar, A.; Nazeeruddin, K.;Gratzel, M.; Seok, S. I. Nat. Photonics 2013, 7, 486−491.(8) Burschka, J.; Pellet, N.; Moon, S. J.; Humphry-Baker, R.; Gao, P.;Nazeeruddin, M. K.; Gratzel, M. Nature 2013, 499, 316−319.(9) Liu, M.; Johnston, M. B.; Snaith, H. J. Nature 2013, 501, 395−398.(10) Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.-b.; Duan, H.-S.;Hong, Z.; You, J.; Liu, Y.; Yang, Y. Science 2014, 345, 542−546.(11) Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S.I. Nat. Mater. 2014, 13, 897−903.(12) Im, J.-H.; Jang, I.-H.; Pellet, N.; Gratzel, M.; Park, N.-G. Nat.Nanotechnol. 2014, 9, 927−932.(13) Liu, D.; Kelly, T. L. Nat. Photonics 2014, 8, 133−138.(14) Seo, J. W.; Park, S.; Kim, Y. C.; Jeon, N. J.; Noh, J. H.; Yoon, S.C.; Seok, S. I. Energy Environ. Sci. 2014, 7, 2642−2646.(15) Wu, Y.; Islam, A.; Yang, X.; Qin, C.; Liu, J.; Zhang, K.; Peng, W.;Han, L. Energy Environ. Sci. 2014, 7, 2934−2938.(16) Lee, J.-W.; Seol, D.-J.; Cho, A.-N.; Park, N.-G. Adv. Mater. 2014,26, 4991−4998.(17) Ono, L. K.; Wang, S.; Kato, Y.; Raga, S. R.; Qi, Y. EnergyEnviron. Sci. 2014, 7, 3989−3993.(18) Zhao, Y.; Zhu, K. J. Am. Chem. Soc. 2014, 136, 12241−12244.(19) Zhao, Y.; Zhu, K. J. Phys. Chem. Lett. 2014, 5, 4175−4186.(20) Baikie, T.; Fang, Y.; Kadro, J. M.; Schreyer, M.; Wei, F.;Mhaisalkar, S. G.; Gratzel, M.; White, T. J. J. Mater. Chem. A 2013, 1,5628−5641.(21) Harms, H. A.; Tetreault, N.; Pellet, N.; Bensimon, M.; Gratzel,M. Faraday Discuss. 2014, 176, 251−269.(22) Xu, X.; Zhang, H.; Cao, K.; Cui, J.; Lu, J.; Zeng, X.; Shen, Y.;Wang, M. ChemSusChem 2014, 7, 3088−3094.(23) Docampo, P.; Hanusch, F.; Stranks, S. D.; Doblinger, M.; Feckl,J. M.; Ehrensperger, M.; Minar, N. K.; Johnston, M. B.; Snaith, H. J.;Bein, T. Adv. Energy Mater. 2014, 4, 1400355.(24) Chen, Q.; Zhou, H.; Hong, Z.; Luo, S.; Duan, H.-S.; Wang, H.-H.; Liu, Y.; Li, G.; Yang, Y. J. Am. Chem. Soc. 2014, 136, 622−625.

Nano Letters Letter

DOI: 10.1021/acs.nanolett.5b00843Nano Lett. 2015, 15, 3959−3963

3963