Optimizing Composition and Morphology for Large-Grain PerovskiteSolar Cells via Chemical ControlHsinhan Tsai,† Wanyi Nie,‡ Pradeep Cheruku,† Nathan H. Mack,† Ping Xu,† Gautam Gupta,‡

Aditya D. Mohite,*,‡ and Hsing-Lin Wang*,†

†PCS, Chemistry Division, and ‡MPA-11, Materials Physics and Application Division, Los Alamos National Laboratory, Los Alamos,New Mexico 87545, United States

*S Supporting Information

ABSTRACT: We report solid iodine as a precursor additivefor achieving purified organometallic perovskite crystals. Byadding iodine, we found that the reaction can be pushedtoward pure iodine phase rather than the kinetically favoredchlorine phase. This approach can be applied in largecrystalline perovskite solar cells and improved the averageefficiency from 9.83% to 15.58%.

■ INTRODUCTION

Solar cells based on lead methylammonium triiodide (PbMAI3)hybrid perovskites have been recognized as a promisingmaterial for next generation photovoltaic devices1−5 due totheir extraordinarily high power conversion efficiency of over15−19% in both mesoporous titania3,6,7 and planar8−12 devicesarchitectures. The planar hybrid perovskite thin films can beeasily fabricated by spin-coating with a mixture of lead iodide(PbI2) and methylamine hydroiodide (MAI) solution followedby postannealing to form a covalently bonded perovskite ABX3structure. It has been found that the crystallinity,13,14 filmcoverage,15−17 and grain size10,18,19 are all key factors to achievehigh-efficiency perovskite solar cells. Recent studies haveprovided great insights into perovskite crystallinity improve-ments through a variety of techniques, such as sequential two-step coating from solution,10,20 vacuum deposition,14,21 organicvapor-assisted crystal formation,13 and solvent-additive-assistedcrystal growth.22,23

Among those efforts, a mixed-halide system developed byLee et al. has successfully achieved chlorine-assisted highlyorientated crystalline perovskites, and solar cells based onhighly crystalline PbMAI3 have an average solar cell powerconversion efficiency (PCE) between 14 and 16%.2,17,24,25 Ourprevious study using mixed-halide perovskite has led to large-scale perovskite crystal grains (up to millimeter-scale in size),which led to dramatic improvements in PCE, surpassing 18%,as well as stable performances.26 However, while mixed-halideperovskites offer promising optoelectronic properties, smallvariations in the processing/reaction could lead to anincomplete conversion or structural heterogeneity across thebulk film, which may be due to the presence of defects ordifferent phases in perovskite thin films. Because of the complexnature of perovskite crystal formation, growing high-quality

metal−organic (hybrid) perovskite crystals does not merely relyon the initial processing conditions; it is also dominated by thechemical reaction, as in the stoichiochemistry and reaction rate.The resulting lack of understanding of the chemical physicalprocesses occurring during crystal formation makes generalstructure−performance correlations difficult. Colella et al. havereported that different starting precursor combinations [i.e.,PbI2+MACl-type (PV1) and PbCl2+MAI-type (PV2)] can leadto PbMAI3 perovskites, as observed through X-ray diffraction(XRD) and optical spectra.27 But this difference in precursorcombination also leads to a different level of an impurity phase,PbMACl3, which has dramatic impact on solar cell efficiency.

2,27

Recent studies have revealed the role of chlorine in mixed-halide perovskite crystal formation best described by a balancedreaction equation17,24 and the role of chlorine during theperovskite formation,28−32 which relates the effect ofstoichiometry on the final product. However, few studies todate have focused on understanding and controlling thesedifferent phases and the overall impurity formation in mixed-halide perovskite systems, which are crucial for growing high-quality crystals and creating reproducible solar cell devices.Our previous efforts to grow millimeter-scale crystal grains

suggest that device performance and reproducibility can benefitfrom high-quality large crystal grains with minimal grainboundaries;26 however, those crystals also have a certain level ofPbMACl3 phase in the bulk, which has led to drastic variationsin PCE due to use of MACl salt.27 In this work, we investigatethe origin of the PbMACl3 phase by studying the chemicalreaction mechanisms and find a simple route to control

Received: May 8, 2015Revised: July 27, 2015Published: July 29, 2015

Article

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stoichiochemistry and to produce large crystals as the finalproduct using this recipe. We first found that PbMACl3 is thekinetically favored product and significant amount of purechlorine phase still remains after thermal treatment in bothmixed-halide systems.17 In order to minimize the formation ofPbMACl3 phases in the final crystal, we have demonstrated asimple method to change the reaction that favors the formationof highly crystalline PbMAI3 phase by incorporating iodide (I2or MAI solution) into mixed-halide perovskite. Both XRD andenergy-dispersive X-ray spectroscopy (EDX) analysis indicate astrong tendency for forming the PbMAI3(110) phase wheniodine is incorporated into precursor solution. Therefore, wepropose a new chemical reaction that describes the perovskitecrystal evolution in an iodide-rich precursor environment. Weapply this strategy in large crystalline films with various I2concentrations; the final products are purified while the crystalgrain size gets compromised at higher I2 loading. The solar cellperformance shows an average PCE of 15.58% in a simpleplanar architecture, as compared to 9.83% for a pristine device(without I2) due to reduced pure chlorine phase. Our resultsdemonstrate a strong correlation between the iodideconcentration and the final perovskite crystal structure/composition. By understanding the chemical reaction ofmixed-halide perovskite precursors, the final products can bewell-controlled over phase and composition, resulting inhomogeneous crystalline films ideal for reproducible solar celldevice performance.

■ RESULTS AND DISCUSSIONWe first examined the perovskite crystal structure from twomixed-halide precursor combinations (PV1 and PV2) usingXRD (Figure 1). The XRD spectra for the as-cast film and after

annealing at 100 °C are shown in parts a and b of Figure 1,respectively. The as-cast film using PV1 shows low-angle peaksat 6.66°, 8.16°, and 9.70°, which can be assigned as theintermediate phase (▼) prior to perovskite formation,consistent with previously reported results.7 The peaks at15.2° and 31.48°, found in both PV1 and PV2 films, areassigned to the pure chlorine perovskite phase (PbMACl3,●).27 After annealing (Figure 1b), the peaks at 14.18° and

28.46° (*) represent PbMAI3(110) and -(220), respectively.The peak at 12.5° or 12.8° (◆) can be assigned to PbCl2(001)or PbI2(001), respectively, and both (fully or partially) areconverted into the perovskite phase after thermal annealing.It should be noted that the as-cast crystal films from both

PV1 and PV2 have significant amounts of PbMACl3 (●), whichhas been recognized in other reports.27,33 Even though PbMAI3starts to form in PV2, the PbMACl3 crystal is the dominantproduct in the bulk material. These results suggest thatPbMACl3 may be the kinetically favored phase in both mixed-halide perovskite combinations. After annealing (Figure 1b),the PbMAI3 (*) phase emerges. However, even underoptimized annealing conditions [see Figure S1 of theSupporting Information (SI) for annealing time optimization],the kinetically favored PbMACl3 phase still dominates the finalcrystal composition in the reaction from the PV1 combination,and residual amounts of PbMACl3 can also be found in the PV2combination.2,24 Note that the PbMACl3 peak is strong andsharp in the bulk along with the second-order diffraction peakat 31.48°, indicating that the phase is not an experimentalartifact and exists over several samples and several batchesprocessed under identical conditions. The above results suggestthat PbMAI3 is the thermodynamically favored product; thepresence of I2 causes halide exchange between Cl

− and I− in thekinetically favored PbMACl3 phase, prompting the formation ofPbMAI3. Since PbMACl3 has a wide band gap (3.11 eV),27,33

the presence of this phase could interrupt the structuralhomogeneity of the PbMAI3 main phase and thus causevariations in the optical and electronic properties27,34,35 as wellas the solar cell efficiency. Therefore, it is very important to getrid of such an impurity from the crystal.The final composition of the products will be dominated by

the reactants involved and their stoichiochemistry in the finalbalanced chemical equation. Therefore, this is not a simple changein reactant concentration to change the equilibrium of the reactionper Le Chatelier’s principle; instead, a new reactant (I2) isintroduced to allow a change in the composition of the f inal productthrough further conversion of insulating PbMACl3 to PbMAI3.Since PV1 combination clearly leads to incomplete conversionand is dominated by the kinetically favored PbMACl3, we usedPV1 as a test bed to understand the effects of I2 toward thereaction process and final products. We start by studying thecrystal structure of perovskites formed with and without I2incorporation in precursor solution using XRD. Figure 2ashows XRD for the as-cast PV1 film without I2 (lower); thelow-angle intermediate phase peaks along with a small amountof the kinetically favored PbMACl3 peaks dominate thespectrum. The addition of I2 (upper spectrum) results in theas-cast film showing PbMAI3 peaks at 14.18° (110) and 28.46°(220). Figure 2b shows the postannealed perovskite film with(upper) and without (lower) I2. The intermediate phase isconverted to higher-angle peaks in both cases. However, thefilm from precursor solution with I2 shows an intense and sharppeak at 14.18° and 28.46° corresponding to the PbMAI3perovskite. Moreover, the peak ratio of PbMAI3/PbMACl3becomes 1:0.41 with I2 added in the solution compared tothe pristine postannealed film, which is 1:1.59. I2 applied in thePbCl2+MAI-type (PV2) film also showed a similar effect,reducing PbMACl3 (see Figure S2, SI).The above results suggest that the incorporation of iodide is

vital to reducing PbMACl3 formation in the final product.Therefore, as shown in Figure 2c, we propose a reactionmechanism that helps explain the involvement of iodide in the

Figure 1. GIXRD pattern for perovskite films: (a) before and (b) afterannealing for PV1 and PV2. The diamond (◆) refers to PbCl2 andPbI2, the asterisk (*) refers to hybrid perovskite PbMAI3 main peaks,the solid circle (●) refers to PbMACl3 peaks, and the triangle (▼)refers to hybrid perovskite intermediate peaks. In both combinations(PbCl2+MAI and PbI2+MACl), XRD spectra show that startingmaterials cannot convert to PbMAI3 completely and always left variousamounts of PbMACl3.

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reaction. In reaction 1, the PbI2 reacts with MACl (1:1 molarratio) to form PbMAI3 and PbMACl3 with a 2:1 molar ratio,respectively, whereas in reaction 2, the present of additional I2favors the formation of PbMAI3, as I2 will trigger ICl formationand the presence of Cl− is known to play an essential role forperovskite formation.24,25,36 The ICl (bp = 97.4 °C) escapesduring the high-temperature annealing process, furtherfacilitating the product formation. These reactions happensimultaneously, and the products can be experimentallymonitored by XRD. In essence, the iodide involved in thechemical reaction favors the conversion of PbMACl3 toPbMAI3. To further validate our hypothesis, rather than usingI2, we use methylamine hydroiodide (MAI) as the iodine sourcefor the formation of PbMAI3. Our results have shown that MAIhas a similar effect as I2 in facilitating the conversion of theintermediate and promoting the formation of PbMAI3 (seeFigure S3, SI). The addition of MAI has been shown toimprove solar cell performance in planar device configurations(Figure S4 and Table S1, SI).In our previous studies, we developed a “hot-casting” method

to achieve large-scale crystals26 but with significant amounts ofthe PbMACl3 phase in the final product (33%). Therefore, herewe employ the same method with I2 addition to drive theoverall reaction with significantly reduced PbMACl3 phase.Note, in this case, we adopt a moderate processing temperatureof 150 °C, a temperature slightly below the boiling point of thereaction solvent DMF to prevent evaporation of the I2 beforeits reaction. We fabricated solar cell devices using perovskiteswith various composition and crystal sizes obtained by varyingthe amount of I2 in the precursor solution. Figure 3a showsXRD spectra of the hybrid perovskite films resulting fromperovskite precursor with various volume percentages of iodide.Figure 3b plots the XRD peak intensity ratio of PbMAI3/PbMACl3, extracted from Figure 3a, as a function of iodideloading volume percent. Figure 3c shows the solar cell structureand current density−voltage (J−V) curve under AM1.5illumination. The corresponding external quantum efficiency

(EQE) is shown in Figure 3d. It is clear from the XRD spectrathat all of the perovskites films have identical peak positions.They only differ in the peak ratio between PbMAI3 andPbMACl3, illustrating the effect of iodide precursor concen-tration once cast into a film.This is consistent with our observation for the as-cast vs

postannealed films (see Figure 2a,b). The PbMAI3/PbMACl3peak ratio extracted from Figure 3b reveals a monotonicincrease in the peak intensity ratio from 1.8 to 2.9 as I2 volumepercent increases from 0% to 10% and suggests that I2incorporation facilitates the conversion of PbMACl3 toPbMAI3 perovskite. The J−V curves in Figure 3c show thesolar cell performance with respect to open circuit voltage(Voc), current density (Jsc), fill-factor (FF), and powerconversion efficiency (PCE), as summarized in Table 1. Note

that we have fixed the film thickness of the perovskite layer toabout 400−450 nm in order to achieve an optimal Voc, Jsc, andFF and also to rule out the effects of film thickness andabsorption on the device perormance (the UV−vis absorptionspectra of all perovskites are shown in Figure S5 of the SI). Thedevice without additional I2 shows an average PCE of ∼10%,slightly lower than the average benchmark device reported inthe literature (∼12%) using PV2 or pure iodide perovskite.

Figure 2. XRD patterns of (a) as-cast and (b) postannealed films ofPbI2+MACl-type (PV1) after iodide treatment. (c) Proposed reactionmechanisms of adding I2 during the hybrid perovskite formationprocess. The diamond (◆) peaks are assigned as PbCl2 and PbI2, theasterisks (*) are hybrid perovskite PbMAI3 main peaks, the solidcircles (●) are assigned as PbMACl3 peaks, and the triangle (▼) refersto hybrid perovskite intermediate peaks.

Figure 3. (a) The XRD patterns of hybrid perovskite from precursorwith 0%, 1%, 2%, 5% and 10% iodine solution (40 mM). (b) PbMAI3/PbMACl3 peak ratio from part a. (c) J−V curves of planar-type devicearchitecture and (d) EQE of hybrid perovskite solar cells.

Table 1. Parameters of Perovskite Solar Cells Based onIodide Treatment under Simulated 100 mW/cm2 AM 1.5 GIlluminationa

devices Voc (mV) Jsc (mA/cm2) FF (%) PCE (%)

no I2 870.0 16.02 70.51 9.83 ± 0.341 vol % I2

b 873.8 20.16 74.43 13.11 ± 0.222 vol % I2

b 877.1 22.31 79.70 15.58 ± 0.25 vol % I2

b 833.8 21.60 73.52 13.22 ± 0.1510 vol % I2

b 830.6 20.30 73.21 12.34 ± 0.24aDevices were exposed to the solar simulator for 20 min prior tomeasurement. bVolume percentage of solution.

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However, upon comparing the grain size and performance withthat in a previous report,26 the performance with ∼50−60 μmgrain size is comparable (∼9.83% without I2 and 13−15% withI2). This slightly lower PCE may be due to the presence of thePbMACl3 phase, which is identified to have a wide band gap(band gap 3.11 eV)27,33 that could interrupt continuousPbMAI3 crystal formation and reduced charge transportefficiency. The incorporation of various volume percents of I2has profound impact on device properties. The Voc remainsalmost unchanged with I2 loading (from 870 to ∼830 mV asthe I2 volume percent changes from 0% to 10%) but slightlylower than the same architecture from other report37 due tounmodified the HTL and EML layers. However, the Jscincreases from 16.2 to 20 mA/cm2, which may be attributtedto the reduced amount of PbMACl3 impurity phase. Among allthe devices, the highest PCE is demonstrated for perovskitesresulting from solution incorporating 2 vol % I2, with a Jsc of22.31 mA/cm2, VOC of 877 mV, and nearly 80% FF; as a result,the device has a PCE of 15.58%. A similar effect on Jsc and FFcan also be found by incorporating MAI in precursor solutionas an alternative iodide source (Table S1, SI). In addition, wehave tested the hysteresis effect with different sweep directionand voltage scan rate using the most efficienct device (seeFigure S6, SI). Similar to our previous observation,26 thehysteresis effect on the I2-added device is not noticeable. Asshown in Figure S6a (SI), the J−V curves show a minimumdifference between forward and reverse scans. Furthermore, wevaried the scan rate from 3 to 100 ms; as shown in Figure S6b(SI), the J−V curves are nearly identical. The above resultssuggest that the perovksite materials treated with I2 results inhigh-quality crystal grain with reproducible device performance.Figure 3d plots the external quantum efficiency (EQE) ofdevices fabricated from precursor solutions with 2 vol % iodide(red line) and without iodide added (black line). Specifically,for perovskites from solution with 2 vol % I2 loading, the EQEspectrum has significantly enhanced shoulder bands at 440 and650 nm as compared to that of the pristine perovskite (withoutI2). Since the measured film thickness for both of theseperovskite films are identical (450 nm), the increase in the EQEshoulder band is likely due to the optimized compositionresulting from addition of I2 solution. However, a higherloading of I2 (>4 vol %) did not further improve deviceperformance. This is possibly because of a change in filmmorphology; a higher number of crystal grains and smallergrain size negate the iodide improvements in device efficiency.We further examined the film morphology and composition

by scanning electron microscope (SEM) images and useenergy-dispersive X-ray (EDX) for crystal films obtained fromsolution without I2 and with 2 vol % of 40 mM I2 to determinethe iodide and chlorine content in hybrid perovskite, and theresults are shown in Figure 4. The SEM images in Figure 4a,cshow similar morphology, except that the perovskite resultingfrom solution with 2 vol % I2 in precursor (Figure 4c) has agreater number and smaller-sized crystal grains. Parts b and d ofFigure 4 are the EDX spectra of perovskite crystal shown inparts a and c of Figure 4, respectively. Insets of Figure 4b,d isthe EDAX mapping that shows the iodine (red) and chlorine(yellow) distribution in the films. From the EDX mapping, onecan tell that both Cl and I are distributed uniformly throughoutthe whole film, which suggests that there is no obvious phasesegregation with or without I2 incorporated in the precursorsolution. The molar ratio of I and Cl for films with and withoutI2 added increases dramatically from 2.23 without I2 to 4.05

with 2 vol % I2, consistent with the XRD results (Figure 2b)showing an increased PbMAI3 phase or a decreased PbMACl3phase. It is important to note that this I:Cl ratio of perovskitewithout I2 incorporation (2.23:1) is consistent with reaction 1in Figure 2c, where the molar ratio between PbMAI3 andPbMACl3 is 2:1. Similarly, the I/Cl ratio of perovskite with 2vol % I2 incorporation (4:1) reflects exactly the molar ratiobetween PbMAI3 and PbMACl3 in reaction 2. The aboveresults clearly demonstrate how incorporation of I2 changes thestoichiochemistry and the composition of the final perovskite.In order to further confirm that the element distribution is notjust a surface phenomenon, we also ran EDX on the crosssection of the same films, and the results again reveal ahomogeneous distribution of I and Cl throughout the bulk (seethe SI, Figure S7). Therefore, on the basis of the XRD spectraof bulk film along with the EDX analysis on the surface andcross section, we can conclude that by varying the I2concentration in the precursor, one can change the chemicalreaction and thus control the final product formation, inparticular the ratio between PbMAI3 and PbMACl3, a ratio thathas been shown to be critical to the optimized hybrid solar cellefficiency.38 (The detailed composition of other elements canalso be found in the SI, Table S2.)In Figure 5 we present a series of optical microscopy images

that illustrate the effect of iodide on the film morphology andhow this change can impact solar cell performance. Figure 5ashows an optical image without I2, and parts b, c, d, and e ofFigure 5 show perovskite crystals with 1, 2, 5, and 10 vol % I2,respectively. The average domain size extracted from the OMimages as a function of I2 concentration is shown in Figure 5f.The average domain size without I2 incorporation is ∼28 μm indiameter. By adding a small amount (1−2 vol %) of I2−DMFsolution, we have observed variations of grain sizes of the sameperovskite film with size distribution from 56 to 7 μm. Thecrystal grains produced from the 2 vol % I2 solution is greater innumber and smaller in size as compared to that of the crystalsfrom solution without I2. This result suggests that furtherconversion of PbMACl3 to PbMAI3 leads to the smaller grainsize. An article by Moore et al. claimed that the crystal

Figure 4. (a, c) Scanning electron micrographs of perovskite crystalfilms with and without iodide (scale bar: 40 μm). Parts b and d areenergy-dispersive X-ray (EDX) spectra of samples from parts a and c,respectively. The insets are EDX mapping for iodine and chlorine; theEDX imaging shows the uniform distribution for iodine and chlorine.

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formation time is shorter in iodide than chloride.39 When theconcentration of I2 increases from 0% to 10%, the average grainsize drops from 28 to 11 μm. This is direct evidence that asmall amount (<2 vol %) of I2 did not change the morphologydramatically, and therefore, the increased efficiency is due tothe I2 facilitating the transformation of PbMACl3 to PbMAI3.

26

■ CONCLUSIONSIn conclusion, we have demonstrated an in-depth under-standing of the perovskite phase formation mechanisms usingmixed-halide perovskite precursor. We found that the chlorine-containing, mixed-halide perovskite precursors produce akinetically favored PbMACl3 phase, while the PbMAI3 phaseis more thermodynamically stable. Employing a previouslydeveloped solution process route, we have shown control overthe final perovskite composition (PbMACl3 vs PbMAI3) andmorphology (large-grain perovskite) through incorporation ofI2 in the precursor solution. Incorporation of I2 changes thechemical reaction and the stoichiochemistry of the finalproducts. Both XRD and EDX showed that we have reducedthe proportion of PbMACl3 and achieved homogeneouslydispersed PbMACl3 and PbMAI3 throughout the film withoutany sign of phase separation. Such control allows foroptimization of final perovskites in composition and morphol-ogy, which led to improvement in device efficiency from 9.83%to 15.58%.

■ EXPERIMENTAL SECTIONMethylamine (33 wt % in absolute ethanol), hydrochloric acid (37 wt% in water), hydroiodic acid (57 wt % in H2O), lead(II) iodide (PbI2,99.99% purity), [6,6]-phenyl C61 butyric acid methyl ester (PCBM,>99.9%), and DMF (anhydrous, 99.8%) were purchased from Sigma-Aldrich and were used without further purification. PEDOT:PSS (PVPAI 4083) was purchased from Clevios. Patterned ITO substrates (150nm, 7 Ω/□) were purchased from Thin Film Device Inc.Synthesis of Methylamine Hydrochloride (MACl) and

Methylamine Hydroiodide (MAI). Methylamine hydrochloridewas synthesized by dissolving 10 mL of methylamine and 50 mL ofdiethyl ether in a 100 mL round-bottomed flask, and then the flask wasimmersed in an ice bath for 30 min, followed by the dropwise additionof 12 mL of hydrochloric acid (37 wt % in water). The resulting whiteprecipitate was collected and washed with diethyl ether three times

and then dried in a vacuum oven at 60 °C overnight. The synthesis ofmethylamine hydroiodide was carried out following the sameprocedures by using hydroiodic acid as the halide source.

Perovskite Precursor Solution Preparation. For PV1 pre-cursor, PbI2 is mixed with MACl in a molar ratio of 1:1 in DMF with aPbI2 concentration of 0.2169 M. The PV1 solution was stirredovernight at 70 °C before spin-casting on a substrate. After theprecursor mixture was stirred overnight, 1−10 vol % of 40 mM iodidesolution was added to the precursor solution. As an alternative iodidesource, MAI was added to the precursor solution by adjusting themolar ratio MACl/MAI at 9:1, 8.5:1.5, and 8:2. For PV2 precursor, themolar ratio of PbCl2/MAI = 1:3 with concentration of 40 wt % PbCl2in DMF was used.

Device Fabrication. Patterned ITO glasses were cleaned using anultrasonication bath in DI water followed by sequential washing withacetone and isopropyl alcohol for 10 min. After drying on a hot platein air at 120 °C for 30 min, the substrate surface was cleaned byoxygen plasma for 3 min under roughing vacuum. The PEDOT−PSSsolution was spin-coated on top of a FTO/glass substrate with a spinrate of 5000 rpm for 45 s; PEDOT−PSS serves as the hole-transporting layer (HTL). The PEDOT−PSS film was then dried inair on a 120 °C hot plate for 30 min. After drying, the substrate istransferred to an argon-filled glovebox for further use. The hybridperovskite layer was processed following a procedure from previousliterature.26 The perovskite mixture solution was heated at 70 °C for10 min and then spun-cast on a substrate preheated to 150 °C. Afterspin-casting, the film color turned from yellow to dark-brown within 2s as the solvent evaporated. Then the PCBM solution (20 mg/mL inchlorobenzene) was spin-coated at room temperature on top of theperovskite film at 1000 rpm for 45 s to form a 20 nm thick electron-transporting layer (ETL). Finally, the whole device was transferred toan inbuilt thermal evaporation chamber. The chamber was pumpeddown to 1 × 10−7 Torr for aluminum deposition. The aluminum topelectrode (100 nm) was deposited through a shadow mask thatdefined the device active area as 0.03 cm2 for the solar cells.

Films Characterization. Optical microscope images werecollected using an Olympus BX51 M microscope. The scanningelectron micrographs, EDX spectra, and EDX mapping data wereobtained from an FEI 400 F with 30 keV and spot size 4. The averagedomain size was calculated with the analyze function from ImageJ.

Device Characterization. The external quantum efficiency wasmeasured with a NIST-calibrated monochromator (QEX10, 22562,PV Measurement Inc.) in ac mode. The light intensity was calibratedwith an NIST-calibrated photodiode (91005) as a reference beforeeach measurement. The monochromator was chopped at a frequency

Figure 5. Optical micrographs of hybrid perovskite film (scale bar: 50 μm) from precursor with (a) 0%, (b) 1%, (c) 2%, (d) 5%, and (e) 10% iodinesolution (40 mM). (f) Avarage domain size as a function of iodide loading during fabrication. The average domain size decreases as the iodideloading increases.

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of 150 Hz. The integrated software calculated the quantum efficiencyusing measured photocurrent for the perovskite device and thestandard reference cell.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.chemma-ter.5b02378.

XRD patterns, SEM images, and EDX and UV spectra(Figures S1−S7 and Tables S1 and S2) (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*A.D.M. e-mail: amohite@lanl.gov.*H.-L.W. e-mail: hwang@lanl.gov.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis research is supported by the Laboratory Directed Researchand Development (LDRD) program, under the auspices ofDepartment of Energy (DOE). H.T. is partially supported byBasic Energy Science (BES), Biomaterials program, MaterialsSciences and Engineering Division. W.N., P.C., and P.X. aresupported by Los Alamos Director Funded PostdoctoralFellowship.

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Supporting Information

Optimizing Composition and Morphology for Large Grain Perovskite Solar Cell Efficiency via Controlling Chemical Reaction

Hsinhan Tsai, † Wanyi Nie, ‡ Pradeep Cheruku, † Nathan H. Mack, † Ping Xu,†

Gautam Gupta, ‡

Aditya D. Mohite＊,‡ and Hsing-Lin Wang

＊,†

Figure S1. XRD spectra for post annealing condition optimization films of (PbI2+MACl)-type hybrid perovskite. The films are annealed at 100ºC in argon-filled glovebox for various time lengths.

Figure S2. XRD spectra of (PV2) after annealed (a) and (PV2+I2) after annealed (b). It’s clear that the PbMACl3 (●) decreased after I2 added.

No

rma

lize

d X

RD

in

ten

sit

y (

a.u

)

5040302010

2 Theta (˚)

Post anneal - 30 min

Post anneal - 20 min

Post anneal - 10 min

Post anneal - 5 min

Figure S3. XRD spectra for as cast and post annealed films of (PbI2+MACl)-type hybrid perovskite with additional MAI.

Figure S4. Light J-V curves of hot-casted perovskite solar cell with (red) /without (black) alternative iodine source (MAI) treatment. The MAI treated (red line) showed the similar increase on device performance compare to iodide treated device described in main text.

-20

-10

0

10

20

Cu

rre

nt

De

nsit

y(m

A/c

m2)

0.80.40.0-0.4

Voltage(V)

W/O MAI W/ MAI

Table S1. Parameters of perovskite solar cells based on MAI added under simulated 100 mW/cm2 AM 1.5 G solar radiation.

Devices Voc [mV]

Jsc [mA/cm2]

F.F. [%]

PCE [%]

10% MAIa 870.93 20.31 75.22 13.31 ± 0.13

15% MAIa 850.80 16.64 77.69 11.00 ± 0.21

20% MAIa 833.85 16.67 76.63 10.65 ± 0.16

a Molar ratio of MAI to MACl

Figure S5. UV-Vis absorption spectra of spun casted perovskite films with different iodine loading.

1.0

0.8

0.6

0.4

0.2

0.0

No

rmali

ze

d U

V-V

is i

nte

ns

ity

(a

.u.)

800700600500400Wavelength (nm)

WO I2 1 v% I2 2 v% I2

5 v% I2 10 v% I2

Figure S6. Hysteresis effect on (a) reverse and forward sweep and (b) different voltage sweep speed.

Figure S7. (a) Cross-section SEM and (b) EDX spetra and elemental mapping (inlets) for chlorine and Iodine in hybrid perovskite film.

Table S2. Weight and atomic percentage of EDX spectra for as cast and post annealed films from (PbI2+MACl)-type hybrid perovskite. By analyzing the film composition change before and after involving I2 in the film cross-section, we obtained similar trend as what we have shown in the main manuscript. This confirms that the iodine treatment induced compositional change is not only on the surface, but in the bulk film.