Optimizing Composition and Morphology for Large ... - ACS Publications

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Chemistry of Materials

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＊†

†

‡

PCS, Chemistry Division, Los Alamos National Laboratory Los Alamos, New Mexico 87545, USA. MPA-11, Materials Physics and Application Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA. ABSTRACT: We report solid iodine as precursor additive for achieving purified final organo-metallic perovskite crystals. By adding iodine, we found the reaction can be pushed towards pure iodine phase rather than the kinetically favoured chlorine phase. This approach can be applied in large crystalline perovskite solar cell and obtained improved average efficiency from 9.83 % to 15.58 %.

INTRODUCTION Lead methylammonium tri-iodide (PbMAI3) hybrid perovskites based solar cell have been recognized as a promising material for next generation photovoltaic devices[1-5] due to their extraordinary high power conversion efficiency, over 1519 % in both mesoporous titania[3, 6, 7] and planar[8-12] devices architectures. The planar hybrid pe-rovskite thin films can be easily fabricated by spin coating with a mixture of lead iodide (PbI2) and methylamine hydroiodide (MAI) solution followed by post annealing to form a covalently bonded perovskite ABX3 structure. It has been found that the crystallinity,[13, 14] film coverage,[15-17] and grain size [10, 18, 19] are all key factors to achieve high efficiency perovskite solar cells. Recent studies have provided great insights into perovskite crystallinity improvements through a variety of techniques such as sequential two-step coating from solution,[10, 20] vacuum deposition,[14, 21] organic vapor-assisted crystal formation[13] and solvent additive assisted crystal growth.[22, 23] Among those efforts, a recipe for mixed halide system developed by Lee et al have successfully achieved chlorineassisted highly orientated crystalline perovskites and solar cells based on highly crystalline PbMAI3 with solar cell PCE have average performance between 14-16 %.[2, 17, 24, 25] Our pervious study using mixed halide perovskite has led to largescale perovskite crystal grains (up to mm-scale in size), which lead to dramatic improvements in power conversion efficiency (PCE) surpassing 18% as well as stable performances.[26] However, while mixed halide perovskites offer promising opto-electronic properties, small variations in the processing /reaction could lead to an incomplete conversion or structural heterogeneity across the bulk film which may be due to the presence of defects or different phases in perovskite thin films. Because of the complex nature of perovskite crystal formation, growing high quality metal-organic (hybrid) perovskite crystals does not merely rely on the initial processing conditions; it is also dominated by chemical reaction, such as stoichiochemistry and reaction rate. The resulting lack of understanding of

the chemical physical processes occurring during crystal formation makes general structure performance correlations difficult. Colella et al have reported that different starting precursor combinations (i.e. (PbI2+MACl)-type (PV1) and (PbCl2+MAI)-type (PV2)) can lead to PbMAI3 perovskites as observed through X-ray diffraction (XRD) and optical spectra.[27] But this difference in precursor combination also leads to a different level of an impurity phase PbMACl 3, which has dramatic impacts on solar cell efficiency.[2, 27] Recent studies have revealed the role of chlorine in mixed halide perovskite crystal formation best described by a balanced reaction equation[17, 24] and role of chlorine during the perovskite formation,[28-32]which relates the effect of stoichiometry on the final product. However, few studies to date have focused on understanding and control these different phases and overall impurity formation in mix-halide perovskite systems, which are crucial for growing high quality crystal and reproducible solar cell devices. Our previous efforts on growing mm-scale crystal grains suggests that device performance and reproducibility can benefit from high quality large crystal grains with minimal grain boundaries;[26] however, those crystals also have a certain level of PbMACl3 phase in the bulk has led to drastic variations in PCE due to use MACl salt.[27] In this work, we investigate the origin of PbMACl3 phase by studying the chemical reaction mechanisms and find a simple route to control stoichiochemistry and the final product of the large crystals using this recipe. We first found that PbMACl3 is the kinetically favored product and significant amount of pure chlorine phase still remains after thermal treatment in both mixed halide systems. [17] In order to minimize the formation of PbMACl3 phases in the final crystal, we have demonstrated a simple method to change the reaction favors the formation of highly crystalline PbMAI3 phase by incorporating iodide (I2 or MAI solution) into mix halide perovskite. Both XRD and energy-dispersive X-ray spectroscopy (EDX) analysis indicate a strong tendency for forming the PbMAI3 (110) phase when iodine is incorporated

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into precursor solution. Therefore, we propose a new chemical reaction that describes the perovskite crystal evolution in an iodide-rich precursor environment. We apply this strategy in large crystalline films with various I2 concentrations; the final products are purified while the crystal grain size gets compromised at higher I2 loading. The solar cell performance shows an average power conversion efficiency (PCE) of 15.58 % in a simple planar architecture, as compared to a pristine device (without I2) of 9.83 % due to reduced pure chlorine phase. Our results demonstrate a strong correlation between the iodide concentration and the final perovskite crystal structure/composition. By understanding the chemical reaction of mixed halide perovskite precursors, the final products can be well controlled over phase and composition, resulting in homogenous crystalline films ideal for reproducible solar cell device performance.

RESULTS AND DISCUSSION We first examined the perovskite crystal structure from two mix halide precursor combinations ((PbI2+MACl)-type (PV1) and (PbCl2+MAI)-type (PV2)) using XRD (Figure 1). The XRD spectra for the as cast film and after annealing at 100 ºC are shown in Figure 1a and Figure 1b, respectively. The as cast film using PV1 shows low angle peaks at 6.66º, 8.16º and 9.70º which can be assigned as the intermediate phase (▼) prior to perovskite formation, consistent with previously reported results.[7] The peaks at 15.2º and 31.48º are found in both PV1 and PV2 films, that are assigned to the pure chlorine perovskite phase (PbMACl3, ).[27] After annealing (Figure 1b), the peak at 14.18° and 28.46° (*) represent the PbMAI3 (110) and (220) respectively. Peaks at 12.5° () (or 12.8° ()) can be assigned to PbCl2 (001) (or PbI2 (001)) and are both (fully or partially) converted 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 () that has been recognized in other reports.[27, 33] Even though PbMAI3 starts to form in PV2, the PbMACl3 crystal is the dominant product in the bulk material. These results suggest that PbMACl3 may be the kinetically favored phase in both mixed-halide perovskite combinations. After post annealing (Figure 1b), the PbMAI3 (*) phase emerges. However, even under optimized annealing conditions (See Figure S1 for annealing time optimization), the kinetically favored PbMACl3 phase still dominates the final crystal composition in the reaction from PV1 combination and residual amounts of PbMACl 3 can also be found in PV2 combination.[2, 24] Note that the PbMACl3 peak is strong and sharp in the bulk along with the 2nd order diffraction peak at 31.48º indicating the phase is not an experimental artifact and it exists over several samples and several batches processed under identical conditions. The above results suggest that PbMAI3 is the thermodynamically favored product; the presence of I2 causes halide exchanges between Cl and I in the kinetically favored PbMACl 3 phase, prompting the formation of PbMAI3. Since PbMACl3 has a wide band gap (3.11 eV)[27, 33], the presence of this phase could

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Figure 1. GIXRD pattern for perovskite films: (a) before and (b) after post annealing for (PbCl2+MAI)-type and (PbI2+MACl)-type. The diamond() refers to PbCl2 and PbI2; the asterisk (*) refers to hybrid perovskite PbMAI3 main peaks; the solid circle ()refers to as PbMACl3 peaks; the triangle (▼) refers to as hybrid perovskite intermediate peaks. In both combinations (PbCl2+MAI) and (PbI2+MACl), XRD spectra show that starting materials cannot convert to PbMAI3 completely and always left with various amount of PbMACl3.

interrupt the structural homogeneity of PbMAI3 main phase and thus cause variations in their optical and electronic properties[27, 34, 35] as well as solar cell efficiency. Therefore, it is very important to get rid of such impurity from the crystal. The final composition of the products will be dominated by the reactants involved and their stoichiochemistry in the final balanced chemical equation. Therefore, this is not a simple change in reactant concentration to change equilibrium of the reaction with Le Chateliers Principle, instead a new reactant (I2) is introduced allowing change in the composition of final product through further conversion of insulating PbMACl 3 to PbMAI3. Since PV1 combination clearly leads to incomplete conversion and is dominated by the kinetically favoured PbMACl3, we used PV1 as a test bed to understand the effects of I2 towards the reaction process and final products. We start by studying the crystal structure of perovskites formed with and without I2 incorporation in precursor solution using XRD. Figure 2a shows XRD for the as cast PV1 film without I2 (lower); the low angle intermediate phase peaks along with a small amount of the kinetically favored PbMACl3 peaks dominate the spectrum. The addition of I2 (upper spectrum) results in the as cast film showing PbMAI3 peaks at 14.18º (110) and 28.46º (220). Figure 2b shows the post annealed perovskite film with (upper) and without (lower) I2. The intermediate phase is converted to higher angle peaks in both cases. However, the film from precursor solution with I2 shows an intense and sharp peak at 14.18º and 28.46º corresponding to the PbMAI3 perovskite. Moreover, the peak ratio of PbMAI3/PbMACl3 becomes 1:0.41 with I2 added in the solution compare to the pristine post-annealed film which is 1:1.59. We applied I2 in the (PbCl2+MAI)-type (PV2) also showed the similar effect on reducing PbMACl3 (see Figure S2).


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Figure 2. The XRD patterns of (a) as cast and (b) post annealed films of (PbI2+MACl)-type (PV1) after iodide treatment. (c) Proposed reaction mechanisms of adding I2 during the hybrid perovskite formation process. The diamond () peaks are assigned as PbCl2 and PbI2; the asterisk (*) are hybrid perovskite PbMAI3 main peaks; the solid circle () are assigned as PbMACl3 peaks; the triangle (▼) refers to as hybrid perovskite intermediate peaks.

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

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 reaction mechanism that helps understanding the involvement of iodide in the reaction. In Reaction (1), the PbI2 reacts with MACl (1:1 molar ratio) to form PbMAI3 and PbMACl3 with a 2:1 molar ratio, respectively. Whereas in Reaction (2), the present of additional I2 favors the formation of PbMAI3 as I2 will trigger ICl formation and the presence of Cl- is known to play an essential role for perovskite formation.[24, 25, 36] The ICl (bp =97.4°C) escapes during the high temperature annealing process further facilitate the product formation. These reactions happen simultaneously and the products can be experimentally monitored by XRD. In essence, the iodide involved in the chemical reaction favors the conversion of PbMACl3 to PbMAI3. To further validate our hypothesis, rather than using I2, we use Methylamine hydroiodide (MAI) as the iodine source for the formation of PbMAI3. Our results have shown MAI has a similar effect as I2 in facilitating the conversion of the intermediate and promoting the formation of PbMAI3 (see Figure S3). The addition of MAI has been shown to improve solar cell performance in planar device configurations (Figure S4 and Table S1). In our previous studies, we developed a “hot-cast” method to achieve large scale crystals,[26] but with significant amounts of the PbMACl3 phase in the final product (33%). Therefore, here we employ the same method with I2 addition to drive the overall reaction with significantly reduced PbMACl3 phase. Note, in this case, we adopt a moderate processing temperature of 150 °C, a temperature slightly below the boiling point of the reaction solvent DMF to prevent evaporation of the I2 before its reaction. We fabricated solar cell devices using perovskites with various composition and crystal sizes obtained by varying the amount of I2 in the precursor solution. Figure 3a shows XRD spectra of the hybrid perovskite films resulting from perovskite precursor with various volume percentages (v %) of iodide. Figure 3b plots the XRD peak intensity ratio of

PbMAI3/PbMACl3, extracted from Figure 3a, as a function of iodide loading v %. Figure 3c shows the solar cell structure and current density-voltage (J-V) curve under AM1.5 illumination. The corresponding external quantum efficiency (EQE) is shown in Figure 3d. It is clear from the XRD spectra that all of the perovskites films have identical peak positions. They only differ in the peak ratio between PbMAI3 and PbMACl3, illustrating the effect of iodide precursor concentration once cast into a film. This is consistent with our observation for the as cast vs. post-annealed films (see Figure 2a-b). The PbMAI3/PbMACl3 peak ratio extracted from Figure 3b reveals a monotonic increase in the peak intensity ratio from 1.8 to 2.9 as I2 v% increases from 0 % to 10 %, and suggests that I2 incorporation facilitates the conversion of PbMACl3 to PbMAI3 perovskite. The J-V curves in Figure 3c shows the solar cell performance with open circuit voltage (Voc), current density (Jsc), fillfactor (F.F.) and power conversion efficiency (PCE), summarized in Table 1. Note that we have fixed the film thickness of perovskite layer to about 400~450 nm in order to achieve an optimal Voc, Jsc and F.F and also rule out the effects of film thickness and absorption on the device perormance (the UVVis absorption spectra of all perovskites are shown in Figure S5). The device without additional I2 shows an average PCE of ~10 %, slightly lower than the average benchmark device reported in literature (~12 %) using PV2 or pure iodide perovskite. However, the comparison of grain size and performance with previous report,[26] the performance with ~50-60 µm grain size is comparable (with ~9.83% WO I2 and 13-15 % with I2). This slightly lower PCE may be due to the presence of the PbMACl3 phase that is identified to have a wide band gap (band gap 3.11 eV)[27, 33] which could interrupt continuous PbMAI3 crystal formation and reduced charge transport efficiency. The incorporation of I2 with various v %, has profound impacts on device properties. The Voc remains almost unchanged with I2 loading (870 mV to ~ 830 mV as the I2 v% changes from 0 % to 10 %) but slightly lower than the same



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Table 1. Parameters of perovskite solar cells based on iodide treatment under simulated 100 mW/cm2 AM 1.5 G illumination. Devices were shine under the solar simulator for 20 minutes prior to measurement. Devices

Voc [mV]

Jsc [mA/cm2]

F.F. [%]

PCE [%]

W/O I

870.0

16.02

70.51

9.83±0.34

873.8

20.16

74.43

13.11±0.22

877.1

22.31

79.70

15.58±0.2

833.8

21.60

73.52

13.22±0.15

830.6

20.30

73.21

12.34±0.24

2 a

1 v% I 2 v% I 5 v% I

2 2

a

2

10 v% I a

a

2

a

Volume percentage of solution

architecture with other report[37] due to unmodified the HTL and EML layers. However, the Jsc increases from 16.2 mA/cm2 up to 20 mA/cm2, which may be attributted to the reduced amount of PbMACl3 impurity phase. Among all the devices, the highest PCE (15.58%) is demonstrated with perovskites resulting from solution incorporating 2 v% I2 has a Jsc of 22.31 mA/cm2, VOC of 877 mV and nearly 80 % FF; as a result, the device has a PCE of 15.58 %. The similar effect on Jsc and F.F can also be found by incorporating MAI in precursor solution as alternative iodide source (Table S1). In addition, we have tested the hysteresis effect with different sweep direction and voltage scan rate using the best efficiency device (see Figure S6). Similar to our previous observation,[26] the hysteresis effect on the I2-added device is not noticeable. As shown in the Figure S6a, the J-V curves show a minimum difference between forward and reverse scans. Furthermore, we vary the scan rate from 3 ms to 100 ms as shown in Figure S6b, the J-V curves are nearly identical. The above results suggest that the perovksite materials treated with I2 results in high quality crystal grain with reproducible device performance. Figure 3d plots the external quantum efficiency (EQE) of devices fabricated from precursor solutions with 2 v% iodide (red line) and without iodide added (black line). Specifically, perovskites from solution with 2 v% I2 loading, its EQE spectrum has significantly enhanced shoulder bands at 440 nm and 650 nm as compared to that of the pristine perovskite (without I2). Since the measured film thickness for both of these perovskite films are identical (450 nm), the increase in EQE shoulder band is likely due to the optimized composition resulting from addition of I2 solution. However, a higher loading of I2 (> 4 v%) did not further improve device performance. This is possibly because of a change in film morphology; a higher number of crystal grains and smaller grain size negate the iodide improvements in device efficiency. We further examined the film morphology and composition by scanning electron microscope (SEM) images and use energy-dispersive X-ray (EDX) for crystal films obtained from solution without I2 and with 2 v% of 40mM I2 to determine the iodide and chlorine content in hybrid perovskite and the results are shown in Figure 4. The SEM images in Figure 4a and 4c show similar morphology except the perovskite resulting from solution with 2 V% I2 in precursor (Figure 4c) has a greater number and smaller size crystal grains. Figure 4b and Figure 4d are the EDX spectra of perovskite crystal in Figure 4a and Figure 4c, respectively. Insets of Figure 4b and 4d are EDAX mapping which show the iodine (red) and chlorine (yellow) distribution in the films. From the EDX mapping, one

Figure 4. (a,c) Scanning Electron Microsgraphs of perovskite crystal films w/wo iodide (Scale bar: 40 µm). (b) and (d) are energydispersive X-ray (EDX) spectra of (a) and (c), respectively. The insets are EDX mapping for iodine and chlorine; the EDX imaging shows the uniform distribution for iodine and chlorine.

can tell that both Cl and I are distributed uniformly throughout the whole film, which suggests that there is no obvious phase segregation w/wo I2 incorporated in the precursor solution. The molar ratio of I and Cl for films with and without I2 added increases dramatically from 2.23 without I2 to 4.05 with 2 v% I2, consistent with the XRD result (Figure 2b) showing an increased PbMAI3 phase or a decreased PbMACl3 phase. It is important to note that this I:Cl ratio of perovskite without I2 incorporation (2.23 :1) is consistent with Equation (1) in Figure 2C where the molar ratio between PbMAI3 and PbMACl3 is 2:1. Similarly, the I:Cl ratio of perovskite with 2 v% I2 incorporation is (4:1) reflects exactly the molar ratio between PbMAI3 and PbMACl3 in the Reaction (2). The above results clearly demonstrate how incorporation of I2 changes stoichiochemistry and the composition of final perovskite. In order to further confirm the element distribution is not just a surface phenomenon, we also run EDX on the cross-section of the same films, and the results again reveals a homogeneous distribution of I and Cl throughout the bulk (See Supplementary Information Figure S7). Therefore, based on the XRD spectra of bulk film along with the EDX analysis on the surface and cross section, we can conclude that by varying the I2 concentration in the precursor, one can change chemical reaction and thus control the final product formation, in particular the ratio between PbMAI3 and PbMACl3; A ratio that have shown to be critical to have the optimized hybrid solar cell efficiency[38] (The detailed composition of other elements can also be found in SI Table S2). In Figure 5 we present a series of optical microscopy images that illustrate the effect of iodide on the film morphology, and how this change can impact solar cell performance. Figure 5a shows an optical images without I2, Figure 5b, c, d & e show perovskite crystals with 2~10 % I2. The average domain size extracted from the OM images as a function of I2 concentration is shown in Figure 5f. The average domain size without I2 incorporation is ~28 µm in diameter. By adding a small amount (1 v% ~2 v%) of I2-DMF solution, we have observed variations of grain sizes of the same perovskite film with size distribution from 56 µm to 7 µm. The crystal grains produced from the 2 v% I2 solution is greater in number and smaller in


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size as compared to that of the crystals from solution without I2. This result suggests that further conversion of PbMACl3 to PbMAI3 leads to the smaller grain size. It can be fund in the article by Moore et al. which claimed that the crystal formation time is shorter in iodide than chloride.[39] When the concentration of I2 increases from 0 % to 10 %, the average grain size drops from 28µm to 11 µm. This is direct evidence that a small amount (< 2 v%) of I2 didn’t change the morphology dramatically and therefore the increased efficiency is due to the I2 facilitates the transformation of PbMACl3 to PbMAI3.[26]

CONCLUSIONS In conclusion, we have demonstrated an in-depth understanding of the perovskite phase formation mechanisms using mixed halide perovskite precursor. We found that the chlorine containing, mixed halide perovskite precursors produce a kinetically favored PbMACl3 phase, while the PbMAI3 phase is more thermodynamically stable. Employing a previously developed solution process route, we have shown control over the final perovskite composition (PbMACl3 vs PbMAI3) and morphology (large grain perovskite) through incorporation of I2 in precursor solution. Incorporation of I2 changes the chemical reaction and the stoichiochemistry of the final products. Both XRD and EDX showed that we have reduced the composition of and PbMACl3 and achieved homogeneously dispersed PbMACl 3 and PbMAI3 throughout the film without any sign of phase separation. Such control allows for optimization of final perovskites in composition and morphology, which lead to improvement in device efficiency from 9.83 % to 15.58 %.

EXPERIMENTAL SECTION

Methylamine (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 (PVP AI 4083) was purchased from Clevios™. Patterned ITO substrates (150 nm, 7 Ω/ ☐) were purchased from Thin Film Device Inc.. Synthesis of methylamine hydrochloride (MACl) and methylamine hydroiodide (MAI). Methylamine hydrochloride was synthesized by dissolving 10 mL of methylamine and 50 mL of diethyl ether in 100 mL round-bottomed flask and then immersed in ice bath for 30 minutes, followed by the addition of 12 mL of hydrochloric acid (37 wt.% in water) dropwisely. The resulted white precipitate was collected and washed with diethyl ether three times and then dried in a vacuum oven at 60 ºC overnight. The synthesis of methylamine hydroiodide was carried out following the same procedures by using hydroiodic acid as a halide source. Perovskite precursor solution preparation. For PV1-type precursor, PbI2 is mixed with MACl with a molar ratio of 1:1 in DMF with PbI2 concentration of 0.2169 M. The PV1 solution was stirred overnight at 65 ºC before spin casting on a substrate. After the precursor mixture was stirred overnight, 1~10 v % of 40 mM iodide solution was added to the precursor solution. As an alternative iodide source, MAI was added to the precursor solution by adjusting the molar ratio between MACl : MAI at 9:1, 8.5:1.5 and 8:2. For PV2-type precursor, it was used the molar ratio of PbCl2:MAI =1:3 with concentration of 40 wt % PbCl2 in DMF. Device fabrication. Patterned ITO glasses were cleaned using an ultra-sonication bath in DI water followed by sequential washing with acetone and isopropyl alcohol for 10 minutes. After drying on a hot-plate in air at 120 ºC for 30 min., the substrate surface was cleaned by oxygen plasma for 3 minutes under roughing vacuum. The PEDOT-PSS solution was spin coated on top of a FTO/glass substrate with a spin rate of 5000

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



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rpm for 45 sec; PEDOT-PSS serves as the hole transporting layer (HTL). The PEDOT-PSS film was then dried in air on a 120 ºC hot-plate for 30 minutes. After drying, the substrate is transferred to Argon-filled glove-box for further use. The hybrid perovskite layer was processed following the procedure from previous literature.[26] The perovskite mixture solution was heated at 70 ºC for 10 minutes and then spun cast on a substrate preheated to 150oC. After spin casting, the film color turned from yellow to dark-brown within 2 sec as the solvent evaporated. Then the [6,6]-Phenyl C61 butyric acid methyl ester (PCBM) solution (20 mg/mL in chlorobenzene) was spin-coated at room temperature on top of the perovskite film at 1000 rpm for 45 sec to form a 20 nm thick electron transporting layer (ETL). Finally, the whole device was transferred to an inbuilt thermal evaporation chamber. The chamber was pumped down to 1e-7 Torr for aluminum deposition. The aluminum top electrode (100 nm) was deposited through a shadow mask that defined the device active-area of 0.03 cm2 for the solar cells. Films characterization. Optical microscope images were collected using Olympus BX51M microscopy. The scanning electron micrographs, EDX spectra and EDX mapping data were obtained from FEI 400 F with 30 keV and spot sized 4. The average domain size was calculated with analyze function from Image J. Device characterization. The external quantum efficiency was measured with a NIST calibrated monochromator (QEX10, 22562, PV measurement INC.) in an AC mode. The light intensity was calibrated with a NIST calibrated photodiode (91005) as a reference before each measurement. The monochromater was chopped at a frequency of 150 Hz. The integrated software calculated the quantum efficiency using measured photocurrent for the perovskite device and the standard reference cell.

ASSOCIATED CONTENT Supporting Information. XRD patterns, SEM images, EDX and UV spectra. This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] and [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research is supported by the Laboratory Directed Research and Development (LDRD) program, under the auspices of Department of Energy (DOE). HT is partially supported by Basic Energy Science (BES), Biomaterials program, Materials Sciences and Engineering Division. WN, PC and PX are supported by Los Alamos Director Funded postdoctoral fellowship.

REFERENCES (1) 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.; Grätzel, M.; Park, N.-G. Lead Iodide Perovskite Sensitized AllSolid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591.

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